JP4223989B2 - Plasma gun - Google Patents

Abstract

A high pulse repetition frequency (PRF) plasma gun is provided, which gun inlets (74) a selected propellant gas into a column (16) formed between a center electrode (12) and a coaxial outer electrode (14), utilizes a solid state high repetition rate pulse driver (130) to provide a voltage across the electrodes (12, 14) and provides a plasma initiator (82) at the base of the column (16), which is normally operative when the driver is fully charged. For preferred embodiments, the initiator (82) includes a solid state simulated RF driver (130), the output is applied to the electrodes (91) affixed in an insulator (72) and producing a high voltage field at a surface of the insulator (72) which forms part of the base end of the column (16).

Description

  The present invention relates to a plasma gun, particularly suitable for use as a space thruster, ie selectable including extreme ultraviolet (EUV), vacuum ultraviolet (VUV) and / or soft x-ray radiation in the high pulse repetition frequency band The present invention relates to an improved plasma gun suitable for producing radiation at various wavelengths. The present invention also includes a method utilizing such a plasma gun.

  The improved plasma gun disclosed in U.S. Pat. No. 5,866,871 (patent) has only been implemented with relatively large and expensive equipment that could not be implemented in the past, could not be implemented sufficiently It finds applications in various environments to perform functions that were not possible. These functions include thrusters for satellites or other space stations that maintain and perform applications, and controlled generation of radiation at selected frequencies, typically within the extreme ultraviolet (EUV) band. The plasma gun disclosed for such applications is particularly advantageous in that it provides high reliability and pulse repetition frequency (PRF), and PRF above about 100 Hz, preferably 5,000 Hz, for space applications. This was especially the case with plasma guns having a PRF of more than 10 and a PRF of at least 500 Hz, preferably 1,000 Hz, for lithography or other applications requiring radiation generation.

To achieve these objectives, the plasma gun of the above patent has two general embodiments, one of which is for space applications or other thrust applications, while the other or second. This embodiment was for radiation generator applications. In both cases, the plasma gun has a central electrode and an outer electrode substantially coaxial with the central electrode, and a coaxial column is formed between these electrodes. The selected gas was introduced into the column via the inlet mechanism and a plasma starter was provided at the base end of the column. Finally, a solid-state high repetition rate pulse driver is provided that can be operated to deliver a high voltage pulse across the electrode at the start of a pulse at the base of the column, and the plasma expands from the base end of the column and its end. Get out of the department. For the thruster embodiment, the voltage of each pulse decreases over the duration of the pulse, and the pulse voltage and electrode length are such that when the plasma exits the column, the voltage across the electrode reaches a substantially zero value. Selected. For this embodiment, the inlet mechanism preferably introduces gas radially from the central electrode at the base end of the column to improve the uniformity of the plasma velocity across the column, relative to this embodiment. The plasma emerges from the column at a discharge rate that is generally in the range of approximately 10,000 to 100,000 meters per second, and the discharge rate varies somewhat depending on the application.

  For the radiation source embodiment of the present invention, the pulse voltage and electrode length are selected such that the current for each voltage pulse is substantially at its maximum as the plasma exits the column. . The outer electrode in this embodiment of the invention is preferably a cathode electrode and may be solid or may take the form of a plurality of substantially equally spaced rods arranged in a circle. . The inlet mechanism in this embodiment of the present invention provides a substantially uniform gas filling into the column so that the plasma is first emitted from the central electrode and the plasma magnetically as it exits the column. When tightened, an extremely high temperature is generated at the end of the central electrode. Selected gases / elements sent to the clamp through the central electrode or otherwise as part of the gas are ionized by the high temperature at the clamp to provide radiation at the desired wavelength. The wavelength is achieved by careful selection of various parameters of the plasma gun including the selected gas / element delivered to the clamp, the current from the pulse driver, the plasma temperature in the region of the clamp and the gas pressure in the column. . The patent shows a combination of parameters for generating radiation at a wavelength of approximately 13 nm, for example using lithium vapor as the gas sent to the clamping part.

For the present invention, in order for any of the above applications to function effectively, it is important that the pre-ionization of the gas by the starter provides an absolutely uniform pre-ionization of the gas. In the above patents, this has been accomplished by forming equally spaced holes around the column and introducing gas through or directing gas through the holes. Trigger electrodes are provided, which are preferably mounted in the holes, or preferably mounted at the base of the column outside or close to the column, these trigger electrodes being energized to start the plasma It was done. The trigger electrodes are preferably equally spaced around the base end of the column and are energized substantially simultaneously to provide uniform starting of the plasma at the base end, and a DC signal to energize the trigger electrode. Was used. This mechanism provides a more uniform plasma start than is possible with any conventional configuration and is suitable for most applications, but a more uniform plasma start is desirable, especially when using a plasma gun as a radiation source. There are such applications. This more uniform plasma start can be provided by using an RF signal to energize the trigger electrode. However, currently available RF signal sources such as magnetrons, klystrons or RF amplifiers are relatively expensive to operate, cost approximately $ 1 per peak power watt, and are relatively large, for example 20 kilovolts at 8 megawatts To generate cabinet dimensions. Therefore, it generates power at a lower cost and uses a small solid-state circuit that provides a much lower heat removal burden on the device in addition to lower cost and substantially smaller dimensions, and generates RF signals . It was desirable to generate an RF signal that was used to energize the trigger electrode in such a way that it could be made. Although R F signal source of the type described above are particularly useful for plasma gun applications of the present invention, such R F signal source does not exist currently in the art it is also useful for other applications.

It is also desirable that the trigger electrode used for plasma start provides a high voltage field over as large an area as possible at the base of the column between the center electrode and the outer electrode , and the wires are routed into the vacuum environment of the column. It is desirable to be able to energize the trigger electrode to produce the required high voltage field at the base of the column without having to bring it in. This is because maintaining a vacuum around the wire increases the cost of the plasma gun.

Another problem with plasma guns is providing the necessary gas / material to the clamp where the material should be ionized to produce the desired radiation. So hold such material,
An improved technique for releasing this into the column to the clamp is desirable.

In addition, a plasma gun of the type described above can act as a radiation source and provide useful radiation at the desired wavelength, but this is the case with a fast plasma driven down the column and out of the central electrode. Can cause problems that significantly limit their usefulness as a useful radiation source. In particular, a temperature in the range of 100 eV (ie about 11,000 ° C.) to 1000 eV at the clamp is sufficient to drive the plasma to a speed of a few centimeters per microsecond, depending on the desired frequency of radiation. Requires a magnetic compression field. Plasma moving down the central electrode at such a speed and exiting from the end forming the clamp has a tendency to continue to move into the space away from the end of the central electrode , and the plasma sheath eventually Loss of electrical connection to the clamp. This ends early after a period as short as 100 nanoseconds, and causes a large voltage transition in the thousands of volts range, causing a reblow that can severely damage the electrode.

Since the discharge can last for a few microseconds, if the early loss of electrical connection between the plasma sheath and the electrode can be eliminated, the life of the clamp can be greatly extended and possible damage The blow can be eliminated. As a result, the output efficiency for the plasma gun is greatly increased, the plasma gun electrode life is greatly expanded, for example, reduce downtime and maintenance of the plasma gun which may be expensive in lithography applications. Therefore, very good performance can be obtained at a low cost.

  Finally, it is desirable to achieve breakdown as uniformly as possible, and in particular techniques to improve such uniformity of breakdown using improved drive signals.

Therefore, it provides a more uniform plasma start at a lower cost than is possible with conventional devices, facilitates the introduction of the material to be ionized at the clamping part into the column, Or) There is a need for an improved plasma gun and method of use that prevents re-striking and provides more uniform destruction when a high voltage is applied across the central and outer electrodes .
US Pat. No. 5,866,871

In accordance with the foregoing , the present application relates to a high PRF plasma gun (10, 10 ', 90) comprising a central electrode (12); an outer electrode (14) substantially coaxial with respect to the central electrode, An outer electrode in which a column (16) is formed between the central electrode and the outer electrode, the column having a closed base end and an open outlet end; a selected gas into the column Inlet mechanism for introduction (70, 72, 74); a solid state high repetition rate pulse driver operable to deliver high voltage pulses across the central and outer electrodes during plasma start-up at the base of the column (32, 34, 36) a solid state high repetition rate pulse driver in which plasma expands from the base end of the column and exits from the exit end of the column Connected to the at least one trigger electrode (82) at the base end of the column (16); and the at least one trigger electrode (82) to select an RF signal to the at least one trigger electrode (82) Providing a radio frequency (RF) signal source (130), wherein the trigger electrode is fed across the central and outer electrodes by the solid state high repetition rate pulse driver (32, 34, 36). A plasma gun having an RF signal source adapted to undergo at least two oscillation cycles for each high voltage pulse .

The RF signal source can operate at a frequency in the range of 10 MHz to 1,000 MHz, for example, and can be used alone or in combination with a DC signal source.

The plasma gun of the present invention also has a plurality of trigger electrodes attached to an insulator and spaced substantially uniformly around the column in addition to or instead of having a solid state RF signal source These trigger electrodes create a high voltage field at the surface of the insulator at the base end of the column. For at least one embodiment of the present invention, the insulator surrounds the central electrode at the base end of the central electrode, and the trigger electrode is attached to the insulator near the base end of the column. For another embodiment, the insulator forms the base of the column, and the trigger electrode is mounted in the insulator on the side of the insulator outside the column, separated by a small distance from the column by the insulator, and the trigger The biasing of the electrodes creates a high voltage field on the side of the insulator in the column.

Another feature of the present invention that can be utilized in connection with or independently of the previous features is that at least one of the central electrode and the outer electrode is formed of a sintered powder refractory metal, and for preferred embodiments, Both electrodes are made of such a sintered powder refractory metal. When the plasma gun operates as a radiation source at selected wavelengths, at least one of the electrodes can be saturated with a fluid (ie, liquid or gas) material suitable for generating radiation at such wavelengths. For certain embodiments of the invention, the fluid is liquid lithium. A preferred embodiment of the present invention includes a mechanism for providing liquid lithium to at least one of the center electrode and the outer electrode during operation .

Another feature of the invention that can also be used alone or in combination with one or more of the previous features is that the solid state high repetition rate pulse driver provides a high voltage spike, at a lower voltage than the high voltage spike and A duration signal longer than the high voltage spike is followed by the high voltage spike, and most of the driver energy is provided by the duration signal. Solid-state high repetition rate pulse driver may have a first non-linear magnetic pulse compression circuit for generating a high voltage spike, and a second nonlinear magnetic pulse compression circuit for generating a persistent signal. The second nonlinear magnetic pulse compression circuit has at least two stages, and the last of these stages is a saturable reactor from the first nonlinear magnetic pulse compression circuit to the second nonlinear magnetic pulse compression circuit. Normally biased to prevent the entry of high voltage spikes, the high voltage spikes will reduce the initial flow from the second nonlinear magnetic pulse compression circuit until the reactor is re-saturated to pass the sustained signal. The reactor is partially desaturated to inhibit.

The plasma gun of the present invention may further include a DC blocking capacitor connected between the RF signal source and the at least one trigger electrode. An RF signal source can be configured to generate the RF signal at a frequency above the critical frequency. At lower critical frequencies, electrons in the selected gas are swept across the entire gap between opposing conductors of the at least one trigger electrode in each half cycle of the RF signal. This frequency has such a time. A solid state high repetition rate pulse driver generates high voltage pulses across the central electrode and long term outer electrode at a rate of at least 5000 pulses per second. A resonant circuit that oscillates at a resonant frequency corresponding to the frequency of the RF signal generated by the RF signal source when energy is transferred; a large amount of energy is transferred from the energy source to the resonant circuit; A switch for selectively connecting the resonance circuit to the energy source so that the resonance circuit oscillates at the resonance frequency; and a vibration signal from the resonance circuit to the at least one trigger electrode. An output coupling circuit connected between a resonant circuit and the at least one trigger electrode, wherein for each vibration cycle of the resonant circuit, only a small amount of energy transferred from the resonant circuit is transferred to the at least one trigger electrode; And an output coupling circuit coupled to the trigger electrode.

  The above and other objects, features and advantages of the present invention will become apparent from the following more particular description of the preferred embodiment of the invention as illustrated in the accompanying drawings and otherwise discussed herein.

First, referring to FIG. 1, a plasma gun 10 as a thruster (hereinafter referred to as “thruster 10”) has a central electrode 12 which is a positive electrode or anode electrode in this embodiment, a concentric cathode, and ground or feedback. It has the outer electrode 14 which is an electrode, and the column 16 which has the substantially cylindrical shape formed between the two electrodes 12 and 14 . The column 16 is defined at its base end by an insulator 18 in which the central electrode 12 is mounted. The outer electrode 14 is attached to a conductive housing member 20 connected to a grounding portion via a conductive housing member 22. The center electrode 12 is mounted in an insulator 24 at its base end, and this insulator 24 is mounted in an insulator 26. Cylindrical outer housing 28 surrounds the outer electrode 14, central electrode 12, in the open拡端portion 30 is a region beyond the front or exit end of the outer electrode 14 expansively opened. The center electrode 12 and the outer electrode 14 can be made of, for example, tria tungsten, titanium, or stainless steel.

Positive voltage dc-dc (direct current / direct current) inverter 34, a nonlinear magnetic compressor 36 and can be through a terminal 38 to be connected to the central electrode 12, it is supplied from the DC voltage source 32 to the central electrode 12. The DC / DC inverter 34 includes a storage capacitor 42, which can be a single large capacitor or capacitor string, a control transistor 44, a pair of diodes 46, 48, and an energy recovery inductor 50. The control transistor 44 is preferably an insulated gate bipolar transistor. A DC / DC inverter 34 is utilized in a manner known in the art to transfer power from the DC voltage source 32 to the non-linear magnetic compressor 36. As will be described later, the DC / DC inverter 34 also functions to recover wasted energy generated from improper loads, particularly from the center electrode 12 and the outer electrode 14, and to improve pulse generation efficiency.

Nonlinear magnetic compressor 36 is shown as having two stages, with the first stage including a storage capacitor 52, a silicon controlled rectifier 54, and an inductor or saturable inductor 56. The second stage of the non-linear magnetic compressor 36 includes a storage capacitor 58 and a saturable inductor 60. If necessary, additional compression stages can be provided to obtain shorter and faster rising pulses and higher voltages. A method for achieving non-linear magnetic compression in this type of circuit is disclosed in US Pat. No. 5,142,166. Basically, the circuit of the non-linear magnetic compressor 36 uses a core that can be saturated in the resonant circuit as an inductor. Each stage core saturates before a significant portion of the energy stored in the previous stage capacitor is transferred. Nonlinear saturation increases the resonant frequency of the circuit by the square root of the decrease in transmittance when the core is saturated. Energy is combined more and more rapidly from one stage to the next. It should be noted that the non-linear magnetic compressor 36 is effective in transferring power in both directions. The reason is that this circuit not only acts to upshift the frequency in the forward direction but also downshifts the frequency and cascades for chain backup when the voltage pulse is reflected. Because. The energy resulting from the improper load / electrode can be cascaded for chain backup so that it appears as a reverse voltage stored in the storage capacitor 42 and is added to the next pulse. In particular, current begins to flow into the energy recovery inductor 50 when the reflected charge is switched back to the initial energy storage capacitor 42. The combination of the storage capacitor 42 and the energy recovery inductor (coil) 50 forms a resonant circuit. After a half-point [here, t = π / (L ₅₀ C ₄₂ ) ^1/2 ],
The polarity of the voltage on the storage capacitor 42 is reversed and this energy reduces the energy required to recharge this capacitor from the DC voltage source 32.

The drive circuit shown in FIG. 1 can also be tuned to a very low impedance load and can produce complex pulse shapes if necessary. The drive circuit is also designed to operate at a very high PRF and can be adjusted to provide a voltage in excess of 1 kV .

The propellant gas is delivered from line 64 through valve 66 under control of the signal on line 68 to manifold 70 that supplies a number of inlet ports 72 in housing 28 as shown in FIG. It is shown. For example, four to eight inlet ports 72 can be provided that are substantially equally spaced around the periphery of the housing 28 near the base of the housing. Inlet port 72 is a gas supply into the hole 74 formed in the outer electrode 14, the angle pickled to guide the radial and propellant gas inward toward the base of the column 16 in the vicinity of these holes 74 is a central electrode 12 It is done. Propulsion gas can also be supplied from the rear of the column 16.

Thruster 10 is designed to operate in space or some other low pressure close to a vacuum environment, particularly at pressures where breakdown occurs on the low pressure side of the Paschen curve. This factual pressure curve varies somewhat depending on the gas used and other parameters of the thruster, but this pressure is typically in the range of 0.01 to 10 Torr, which is the preferred embodiment. On the other hand, it is about 1 Torr. When the pressure in the region increases with respect to the pressure in this range, the breakdown potential in the region decreases, and therefore the possibility that breakdown occurs in such a region is improved. Thus, theoretically, a propellant gas can be introduced at the base of the column 16 and, therefore, simply increasing the pressure at this point can cause destruction / plasma starting as desired at this point. However, in practice, to control the gas pressure sufficiently to cause destruction predictable, and, not in the segment which is selected in column 16, the column in order to produce a uniformly disrupted within column 16 It is difficult to make the pressure sufficiently uniform around the periphery of 16.

In order to ensure that the plasma start occurs uniformly at the base of the column 16 and such breakdown occurs at the desired time, at least two things can be done. In order to understand how such destruction enhancement is achieved, the plasma gun of the present invention typically operates at a pressure between 0.01 Torr and 10 Torr, in particular the Paschen curve. It should be understood that it operates at such a pressure that failure occurs on the low pressure side. For the preferred embodiment, the pressure in column 16 is approximately 1 Torr. In such a low pressure discharge, there are two important criteria that determine gas breakdown or starting:
1. The electric field in the gas must exceed the gas breakdown field depending on the gas used and the gas pressure. The breakdown field is the electron source at the cathode or outer electrode 14 known as the Paschen reference. In the low pressure region where the plasma gun operates, the breakdown electric field decreases with increasing pressure (which occurs on the low pressure side of the Paschen curve) relative to the size of the device. For this reason, destruction occurs in the column 16 at the point where the gas pressure is maximum.

  2. Second, an electron source must be provided. If the average electric field exceeds the breakdown field, nothing happens until the negative surface starts emitting electrons. In order to extract electrons from the surface, one of two conditions must occur. As a first condition, a cathode drop or a potential difference exceeding the cathode potential must be generated in the vicinity of the surface. Cathode fall / cathode potential is a function of gas pressure and is a function of surface composition and geometry. The higher the local gas pressure, the smaller the required voltage. Inwardly facing geometries, such as holes, provide a significantly improved level of surface area relative to volume and reduce cathode fall. This effect, in which the hole preferentially acts as an electron source with respect to the adjacent surface, is called the hollow cathode effect. Under the second condition, the electron source can be formed by a surface flash trigger source. These conditions can be met individually or both can be used. However, to prevent spurious starting, the voltage across the electrode should be less than the sum of the gas breakdown potential and the cathode fall potential.

Thus, in FIG. 1, a plurality of holes 74 are formed in the outer electrode 14 is a cathode, the gas through the holes 74 is guided to the base of the column 16, the hole 74 terminates at the base near the column 16. For the preferred embodiment, a plurality of such holes 74 are equally spaced around the periphery of the column 16. Enters through these holes 74, the gas is combined with hollow cathode effect from the presence of these holes 74, greatly increasing the pressure in the region of these holes 74 at the base near the column 16, Thus, a plasma start occurs at this location in the column 16 . Although this method of plasma starting is sufficient for plasma starting in certain applications, for most applications of the plasma gun of the present invention, especially for high PRF applications, a trigger electrode is provided in the manner described in the following embodiment, Both conditions are preferably met to ensure both uniformity and timeliness of plasma start.

If thruster 10 is to be utilized, valve 66 is first opened to allow propellant gas from the gas source to flow through manifold 70 into hole 74 leading to column 16. Since the valve 66 operates relatively slowly compared to the other components of the apparatus, the valve 66 is sufficient to allow a sufficient amount of propellant gas to flow into the column 16 to deploy the desired thrust over multiple plasma starts. It will remain open for a certain length. For example, the solenoid valve cycle time available as valve 66 is 1 millisecond or more. Because plasma combustion can occur in 2 to 3 microseconds and the gas is typically about 1/4000 seconds down the length of the 5 to 10 cm electrode used in the preferred embodiment thruster. Since it can flow, if there is only one pulse for each valve cycle, only about 1/10 of the propellant gas will be utilized. Thus, to achieve high propulsion efficiency, multiple (eg, at least 10) bursts or pulses are generated during a single valve opening. During each individual burst of pulses, the peak power is on the order of a few hundred kilowatts to produce the required force. The peak PRF is determined by two criteria. The impulse time must be long enough for the plasma from the previous pulse to leave the thruster exit or recombine. Moreover, the impulse time must be shorter than the time required for the cold propellant gas to travel the length of the electrode. The latter criterion is determined by the gas used. For argon, for a typical length of 5 cm column 16, the time period for the propellant gas to spread across the electrode surface of thruster 10 is only 0.1 milliseconds, while for heavier gases such as xenon. The time increases to approximately 0.2 milliseconds. As such, a high thruster pulse repetition rate (ie, about 5,000 pps or more) allows the plasma gun to achieve a high propulsion efficiency approaching 90%. The burst length of a pulse during a single valve operation of the fluid can vary from a few pulses to a few million, during which time some fuel is consumed, thus achieving lower propulsion efficiency for short burst lengths It will be. Therefore, if possible, the burst cycle is provided during the minimum time cycle of valve 66.
It should be long enough to allow at least full use of the gas .

Before the propellant gas reaches the end of the column 16, the gate transistor 44 is enabled or opened and the capacitor 58 is fully charged to provide a high voltage across the electrodes (400 to 800 volts for the preferred embodiment). This voltage, alone or in combination with the trigger electrode energization in the manner described below, causes a plasma start at the base of the column 16. As a result, the plasma sheath which connects the center electrode 12 and outer electrode 14, a current flows radially between the center electrode 12 and outer electrode 14 through the plasma sheath, to generate a magnetic field. The resulting magnetic pressure axially pushes the plasma sheath and provides a J × B Lorentz force that accelerates the plasma mass as it moves along the central electrode 12 and the outer electrode 14 . This results in a very fast plasma velocity, and the electrode length and initial charge first decrease with time across the central electrode 12 and the outer electrode 14 which increases with time and then decreases to zero, and decreases when the storage capacitor 58 is discharged. Both voltages are selected so that they are just zero when the plasma is emitted from the tip of the electrode. When the plasma reaches the end of the coaxial structure, substantially all of the gas has been entrained or drawn into the plasma and is released from the end of the electrode. This produces the maximum gas mass and hence the maximum momentum / thrust for each pulse. If the length of the structure is chosen so that the capacitor is fully discharged as the plasma exits the electrode, the current and voltage will be zero and the ionized slug of gas will leave the thruster at high speed. By operating the thruster in this manner and optimizing the discharge speed utilized for a given thruster application, discharge speeds in the range of 10,000 to 100,000 meters per second can be achieved, for example. Open拡端portion 30 of the thruster 10, by facilitating the expansion of controlled exit gases, but some of the residual thermal energy through the isentropic thermodynamic expansion to be converted to thrust this effect has been found to considerably negligible, diverging end 30 tapering is not commonly used. In fact, the weight of the thruster 10 can be reduced by completely eliminating the housing 28, except for the protection of the central electrode 12, which is not generally required in space. The burst of pulses can be terminated by disabling the control transistor 44 or otherwise disconnecting the DC voltage source 32 from the circuit of the nonlinear magnetic compressor 36.

FIG. 2 shows a thruster 10 ', which is another embodiment of a plasma gun that differs from that shown in FIG. First, instead of the non-linear magnetic compressor 36, a single storage capacitor 80 is used, which in practical applications is typically a capacitor string that achieves a capacitance of approximately 100 microfarads. Second , the outer electrode 14, which is the cathode , tapers slightly toward its exit end. Third, a spark plug-like trigger electrode 82 is shown positioned within each hole 74 with a corresponding drive circuit 86 for the trigger electrode, and an internal gas manifold 72 ′ formed by the housing member 77. Are provided for delivering propulsion gas to the holes 74, gas inlet holes (not shown) are provided in the housing member 77, and gas outlet holes 84 are formed in the insulator 24 and the central electrode 12. It is shown. With respect to the embodiment of FIG. 1, there are typically a plurality of, for example, four to eight holes 74 spaced equally around the periphery of the outer electrode 14 and a trigger electrode 82 is located within each hole 74. However, the gas outlet (s) 84 preferably faces each hole 74 and directs gas therethrough. For reasons discussed below, most of the gas at the inlet to the column 16 is from a suitable source that can be the same source as that for the manifold 72 ′ and the hole 74, through the gas outlet 84, and the central electrode 12. The gas flowing into the nearby chamber and flowing through the hole 74 primarily facilitates triggering by the trigger electrode 82 .

In some applications, a storage capacitor 80 can be used in place of the non-linear magnetic compressor circuit 36 to store voltage and provide a high voltage drive pulse, but such a configuration is typically much lower. Used for applications that require PRF and / or lower voltage. The reason is that the non-linear magnetic compressor 36 is suitable for providing both shorter and higher voltage pulses. The non-linear magnetic compressor 36 also provides a pulse at a time determined by the voltage across the storage capacitor 58 and the saturation of the inductor 60 as a non-linear coil , which breaks at the base of the column 16 and discharges the capacitor. This is a time that can be predicted more easily than can be achieved with a storage capacitor 80 that is basically charged until it is allowed.

The trigger electrode 82 is energized by a separate drive circuit 86 that receives a voltage from the DC voltage source 32, but is otherwise independent of the DC / DC inverter 34 and the nonlinear magnetic compressor 36 or storage capacitor 80. . The drive circuit 86 has two non-linear compression stages and can be energized in response to an input signal to the SCR 87 to initiate energization of the trigger electrode 82 . The signal to the SCR 87 can respond, for example, to the detection of a voltage or charge across the storage capacitor 80 and the start of energization when this voltage reaches a predetermined value, or when the storage capacitor 80 begins to charge. It can respond to the timer that is started when the capacitor has allowed enough time to reach the desired value, biasing occurs. The nonlinear magnetic compressor 36 can be timed so that energization occurs when the inductor 60 is saturated. Controlled starting at the base of column 16, the geometry that is towards the inside of the hole 74, and, more narrow in the column 16 is the base end portion, further increasing the pressure in this area, therefore, For the reasons mentioned above, this is improved by the fact that it guarantees the onset of destruction in this area.

Each trigger electrode 82 is a spark plug-like structure having a screw section that fits into an opening 89 in the housing 77 and is screwed into the opening 89 to secure the trigger electrode 82 in place. The front end of the trigger electrode 82 has a diameter that is narrower than the diameter of the opening 89 , so that propellant gas can flow through the hole 74 around the trigger electrode 82 . For example, the hole 74 can be 0.44 inches (about 11.18 mm) in diameter, while the trigger electrode 82 is 0.40 inches (about 10.16 mm) at its lowest point. The trigger element 91 of the trigger electrode 82 extends near the end of the hole 74 adjacent to the column 16, but preferably into the column 16 to protect the trigger electrode 82 against plasma forces that develop in the column 16. Does not extend. The end of the trigger electrode 82 can be separated from the end of the hole 74 by a distance approximately equal to the diameter of the hole 74 (7/16 inch), for example.

Both the trigger electrode 82 and the central and outer electrodes 12, 14 are powered from a common DC voltage source 32, but the drive circuitry for the central electrode 12 and the outer electrode 14 is independent and substantially Generate different voltages and power while operating simultaneously. For example, the center and outer electrodes 12, 14 typically operate from 400 to 800 volts, while the trigger electrode 82 can have a voltage of 5 Kv across it. However, this voltage exists for a very short period of time, for example 100 ns (nanoseconds), so that the energy is very small, for example 1/20 Joule.

Another possible problem with the type of thruster shown in FIGS. 1 and 2 is that the Lorentz force across the column 16 is not equalized and is maximized in the vicinity of the central electrode 12, from which the outer electrode is the cathode outward. 14 to decrease somewhat uniformly. As a result, the gas plasma flows out at an angle and the gas first exits the central electrode 12 and later extends towards the outer electrode 14 . Thus, the outer electrode 14 can be made shorter in order to facilitate the uniform exit of gas across the thruster from the thruster, but this is not done for the preferred embodiment. The taper of the outer electrode 14 is provided for the same reason as the taper at the widened end 30 of the housing 28 and is optional for the same reason as described in connection with this taper.

The problem of non-uniform velocity within the column 16 is also that most of the gas passes through the hole 89 to the central electrode 12.
2 and / or in the vicinity thereof is handled in FIG. 2 by making the gas mass at the central electrode 12 greater than the gas mass at the outer electrode 14 so as to enter the column 16. If this is done carefully so that the greater mass in the vicinity of the central electrode 12 cancels out the greater acceleration force there, a more uniform velocity can be achieved radially across the column 16, so that the gas / plasma Flows out uniformly from the end of the thruster (ie, the front surface is perpendicular to the electrode). This modification is one reason why a shorter outer electrode 14 is virtually unnecessary.

Except for the above-mentioned differences, the thruster 10 'of FIG. 2 operates in the same manner as the thruster 10 of FIG. Furthermore, although the figure shows a single thruster, in space or other applications, multiple thrusters can be used, such as 12 thrusters, each thruster operating at less than 1 joule per pulse. 1 kg or less. All thrusters are operated by a central power source, have a central controller, and receive propellant gas from a common source. The latter does not depend on the fuel supply for the most frequently used thrusters, as in the case of solid fuel thrusters where the operational life of the spacecraft utilizing the thrusters is the same, but only by the total propulsion gas onboard the spacecraft. It is particularly advantageous for the thrusters of the present invention in that it depends on

Figure 3 shows another embodiment of the plasma gun in accordance with the teachings of the present invention, the plasma gun 90 is suitable for use as a radiation source rather than as a thruster. This embodiment of the invention uses a driver similar to that shown in FIG. 1 with a DC / DC inverter 34 and a non-linear magnetic compressor 36, and gas around the trigger electrode 82 through a hole 74 in the outer electrode 14. Has a manifold 72 '. However, for this embodiment, no propellant gas is input from the central electrode 12. The outer electrode 14 is also not tapered and has approximately the same length as the central electrode 12. For this embodiment of the invention, the length of the central electrode 12, the outer electrode 14 is also shorter than that for the thruster embodiment, and when the discharge current is maximum, the gas / plasma is Reach the end of the electrode / column. Typically, the capacitor approaches the half voltage point at this point. Further, for radiation source applications, the outer electrode 14 may be solid or perforated. It has been found that the best results are achieved with the outer electrode 14 consisting of a collection of rods that are typically evenly spaced to form a circle. According to the shape described above, when the plasma is emitted from the end of the central electrode 12 , the magnetic field generates a force that drives the plasma into the clamp and significantly increases its temperature. The higher the current and hence the magnetic field, the higher the final plasma temperature. Moreover, you are not necessary to perform the effort to profile the gas to achieve a more uniform velocity across column 16, static, uniform gas fill is typically used. Therefore, it is not necessary to introduce gas at the base end of the column 16, which is more preferable. The unprofiled gas produces a velocity at the central electrode 12 that is faster than that at the outer electrode 14. The capacitance, gas concentration and electrode length at the driver are adjusted to ensure that the plasma surface is emitted from the end of the central electrode 12 when the current is near its maximum value.

When the plasma is emitted from the end of the central electrode 12 , the plasma surface is pushed inward. The plasma forms an umbrella or fountain shape. The magnetic field of current flowing through the plasma column directly adjacent to the tip of the central electrode 12 provides an inward pressure that is maintained until the gas pressure reaches equilibrium with the inward magnetic pressure. Tighten inward.

Using this technique, temperatures at least 100 times higher than the surface of the sun can be achieved in the clamp. Radiation of the desired wavelength is obtained from the plasma gun by introducing a generally gaseous element having a spectral line at that wavelength at the clamp. This can be achieved by a plasma gas functioning as an element or by an element introduced into the clamp in some other way, but for the preferred embodiment the element is a central channel formed in the central electrode 12. 92. The central electrode 12 is preferably cooled at its base end by a flow of cooling water, gas or other material in contact with it over a portion of the housing. This provides a large temperature gradient with the cathode or tip of the outer electrode 14 , which can be approximately 1200 ° C. when plasma tightening occurs. In particular, at high temperatures, the radiation intensity is inversely proportional to the fourth power of the wavelength (ie, intensity≈1 / λ ⁴ = (f / c) ⁴ ; where λ is the desired radiation wavelength and f is the desired radiation frequency. , C is the speed of light). Thus, for certain gases / elements that are sent through the central channel 92 to the clamp or otherwise delivered to the clamp, the maximum intensity is the shortest emitted from the element during the decay from the 2P → 1S state. Obtained for a wavelength signal, this signal is for an elemental atom in a single electronic state (ie, an atom that has risen to a high energy state such that most of the atoms have been removed from the molecule). can get. For atoms in the single electronic state, the wavelength λ is given by λ = 121.5 nm / N ^2, where N is the atomic number of the element in the evaporating central channel 92. Using this equation, the wavelength with the highest energy for the first six elements of the periodic table is shown in Table 1 below.

To the extent that the supplied gas is not completely converted to its single electron state through the central channel 92, moreover, at a temperature that is present in the clamping unit, in case the majority of the gas is not substantially ionized in this state Even radiation is also output at other spectral wavelengths of the element. However, as is apparent from the above equation, these radiations have very low intensity, which is only a fraction of the intensity of the single electronic state. Thus, for example, atomic number 54 xenon has a small value of 0.04 nm single electron wavelength, but also has energy at a useful 13 nm wavelength, as outlined. However, the energy at 13 nm is 1/10 ¹⁰ of the energy at the single-electron wavelength of the temperature at the optimum clamp for the single-electron state, and substantially lower orders at lower laser clamping temperatures. It becomes the size of. The reason is that it is impossible to forcibly emit a small amount of energy (≤1 / 4) or more alone at 13 nm because of the shape of the black body radiation curve that depends on determining the relative line size. This is because the temperature changes greatly.

  Thus, in order to use radiation at a wavelength other than the optimum single electron wavelength of an element, it is necessary to filter the shorter wavelength of higher intensity being emitted for that element. FIG. 3 shows one way of doing this, in which the radiation 94 emitted from the plasma gun 90 absorbs all wavelengths of radiation except the desired wavelength that is reflected towards the desired target. Is supplied to a mirror 96 of a type known in the art. Other filters that are at least high-pass filters for the desired and higher wavelengths can also be used.

Thus, if possible, it is desirable to use elements for the gas or other elements supplied to the central channel 92 that generate radiation at the desired wavelength in the single-electron state of maximum energy. However, in the absence of any element that emits radiation at the desired wavelength in a single electronic state, and from Table 1, a very low wavelength of about 7.6 nm or more is actually available for the element in the maximum energy state. Is found, an appropriate filter must be found, such as the element that emits radiation at the desired wavelength and the mirror 96 as a filter used to obtain radiation at the desired wavelength. Since this radiation is much less intense than radiation at a wavelength in the single electronic state, a larger, substantially more expensive device or plasma gun 90 is generally required to obtain sufficient energy at the lower intensity. The intensity of radiation at a given wavelength is given in units of watts / (meters) ² / hertz and varies as a function of the frequency or wavelength of radiation, temperature and emissivity. The emissivity is a function having a maximum value of 1, and it is important to select the gas having the maximum emissivity at the desired output frequency / wavelength. The optimum clamping temperature (T _OPT ) for a given wavelength λ can be determined from the Wien displacement law T _OPT = 0.2898 cm × K ° / λ (where K ° is the temperature of the plasma in Kelvin). Since only a very small amount of gas is ionized to generate radiation during each clamp, xenon can flow through the central channel 92 at a relatively slow rate to obtain 13 nm radiation. However, as described above, when xenon is used, the output radiation at 13 nm has a relatively weak intensity, and a filter such as a mirror 96 is required to obtain useful radiation at this wavelength. For this reason, lithium, which is known from Table 1 to have a wavelength of maximum intensity at substantially the desired wavelength (ie at 13.5 nm), is the preferred element for radiation at this wavelength.

FIG. 4 shows the central electrode 12 for an embodiment that utilizes lithium vapor to produce the desired radiation. Referring to this figure, a solid lithium core 98 is held in a tube 100 of material such as stainless steel, the tip of the tube 100 being near the tip along the central electrode 12 and about 900 ° C. during plasma clamping. The temperature reaches a temperature, and lithium vapor is generated from the end of the lithium core 98 at a pressure of about 1 Torr. The lithium vapor flows out of the opening 102 at the end of the central electrode 12 at a flow rate that replaces argon or other plasma gas at the tip, and this required flow rate is, for the illustrated embodiment, The range is about 1-10 grams per year. Tube 100 can be slowly advanced in any suitable manner to hold the front end of lithium core 98 in place. When the lithium core 98 is used up, it can be replaced. A small amount of helium gas is preferably supplied around the tube 100 and flows out of the opening 102 to ensure that only lithium and helium are present in the clamping area. The reason is that argon, although a small amount, produces a high energy, short wavelength line that, if not filtered, will interfere with 13 nm radiation at the desired target.

Another method of providing the clamp with lithium or other suitable material is a sintered powder refractory metal saturated with liquid lithium or other suitable material in a fluid (ie, liquid or gas) state, with the central electrode 12 and the outer It is to form at least one of the electrodes 14 . By pressing a powdered refractory material such as tungsten with a suitable binder and then sintering the resulting mass at high temperature, a metal such as tungsten or molybdenum can be produced in the desired electrode shape. The resulting porous refractory metal matrix can be impregnated with liquid lithium or other desired material to provide another means of introducing improved lifetime and lithium / material into the discharge. If desired, liquid lithium can always be supplied to the metal matrix of the electrode during operation so as to provide a virtually infinite life of the process without having to replace the radiation generating material. One limitation in selecting powdered refractory metals is to ensure that the metal does not dissolve in the radiation-generating material that is burning in it.

If xenon is used to obtain 13 nm radiation, xenon must be confined in the immediate vicinity of the clamp because xenon has significant absorption at that wavelength. If the radiation used is at a wavelength other than the single electron wavelength for the element / gas in the central channel 92, as in xenon, a small amount of the element is ionized to its single electronic state and further The temperature at the clamp can be controlled to provide a large amount of radiation at a long wavelength and a small amount of radiation at a shorter wavelength (but stronger radiation).

  In addition, it is desirable that the cone angle of the emitted radiation is as small as possible. A small cone angle is achieved when the stimulated emission of radiation from the radiating gas at the clamping part is greater than the spontaneous emission, and the natural emission is more dispersive. In particular, assuming that the Boltzmann constant k × temperature at the tightening portion is higher than the radiation frequency f × Planck's constant h, the ratio of spontaneous radiation B to stimulated radiation A is given by (B / A = kT / hf). For example, if this ratio is equal to 20 (ie, the plasma temperature is 20 times the photon energy in question), the half cone angle will be about 25 °. The higher the plasma temperature, the smaller the cone angle. However, the shorter the wavelength of radiation, the more difficult it is to achieve a small cone angle. However, cone angle is one of the factors to consider when choosing current and other parameters to achieve the desired temperature at the clamp.

FIG. 5 shows a book that can be used as a thruster, radiation source or other function utilizing a plasma gun, depending on factors such as electrode length and whether or not a radiation emitting element / gas is introduced through the central electrode 12. Another embodiment of the invention is shown. The plasma gun is shown as being driven by a main solid state driver 110. For the preferred embodiment, the main solid state driver 110 includes a DC voltage source 32, a DC / DC converter 34, a non-linear magnetic compressor ( NMC ) 36. However, this embodiment utilizes a trigger electrode 82 in hole 74 for plasma starting, which includes a resonant coaxial line 116 where the trigger electrode or other electrode functions as a DC blocking capacitor 114 and a matching transformer. Via the pulse RF signal source 112, and is different from the previous embodiment. For the preferred embodiment, the RF signal is at a frequency between 10 MHZ and 1,000 MHZ and is activated for about 1 to 10 microseconds prior to activation of the main solid state driver 110. FIG. 5 also shows an optional DC bias source 118 connected to the center electrode 12 via an AC filter coil 120. DC bias source 118, Ki out to a DC voltage source 32 which is substantially powered via such shaping and control circuit of the driver circuit 86, or may be a different voltage source depending on the application.

In FIG. 5, only two trigger electrodes or spark plugs 82, 91 located on either side of the column 16 are shown, but the plasma gun is preferably at least four trigger electrodes spaced equidistantly around the periphery of the column 16. And can have 6 or 8 (or more if possible) trigger electrodes. For four trigger electrodes, RF signals supplied to the trigger electrode of the shown next to the first phase, RF signals supplied to the trigger electrode in the position of 90 ° with respect to those illustrated with respect to the first phase The second phase is 90 ° out of phase. For a plasma gun having six trigger electrodes, a three-phase RF signal is used and each phase is fed to a pair of trigger electrodes on both sides of the column 16. For eight trigger electrode, suitable 2-phase signal is available, the one phase is supplied to every other electrode, the second phase is supplied to the trigger electrode between these trigger electrode, four-phase signal Can also be used. The reason for using an RF signal rather than a DC signal for plasma start is that the RF signal applied to the trigger electrode causes more uniform and almost completely uniform volumetric ionization or start in column 16. Preferably the line (s) 1 DC bias from DC bias source 118 to the RF signal to be simultaneously supplied from an RF signal source 112 in response to control signals on 22 may further especially in the vicinity of the center electrode 12 This contributes to uniform ionization and reduces the required power at the RF signal source 112. DC bias may be supplied to the central electrode 12 as shown, or, for example, can be RF signal is supplied to the RF signal series or central electrode 12 in parallel to modulate the DC bias.

FIG. 6 shows the connection of the RF signal source 112 to, for example, two trigger electrodes ( spark plugs ) 82, 82 ′ located at an angle of 90 ° to each other. In in the plasma gun there are two additional trigger electrodes, the second trigger electrode 82 is located at an angle of 180 ° with respect to the trigger electrodes 82 shown, the illustrated for the trigger electrodes 82 METHOD Connected, the second trigger electrode 82 'is located at an angle of 180 ° from the illustrated trigger electrode 82' and is connected in the same manner as this electrode. The RF signal source 112 is connected to a point near the shortened ends of the coaxial lines 126, 126 'via the quarter-waveguide coaxial lines 124, 124', but at a distance from the shortened ends, respectively. They are separated by L1 and L2. Coaxial line 126 has a quarter wavelength length and has a trigger electrode 82 at its non-shortened end, while coaxial line 126 'has a half-wavelength and its non-shortened end. And a trigger electrode 82 '. For quarter-wavelength coaxial line 126 and half-wavelength coaxial line 126 ′, the desired phase difference for the RF signal is achieved at trigger electrodes 82, 82 ′. The coaxial line also provides a large voltage boost and, if the coupling positions / distances L1, L2 are correctly selected, rely on the RF signal source as a matched load until breakdown is achieved. If a good quality coaxial line is used, a voltage step-up ratio of about 10-20: 1 can be easily achieved. When breakdown is achieved, the line becomes like a short circuit at position L1. At the input coupled to the RF signal source 112 λ / 4 away from the position L1, the apparent impedance looks like an open circuit. Furthermore, if the position L2 is correctly selected, this line will look like a matching load after the break has begun. Although it is desirable to keep the coaxial lines 126, 126 'as short as possible, the desired phase and impedance matching can be substantially achieved for the lines at respective lengths of (2M-1) λ / 4, Mλ / 2. Therefore, the RF signal source 112 always monitors the matched load and first causes a voltage boost at the pair of trigger electrodes , then the voltage is started at the second pair of trigger electrodes 82 'after the plasma is started. Provides a fall, but provides a current ramp. The following Table 2 provides parameters for the RF signal source 112 of FIG. 6 for the illustrated embodiment.

The RF frequency and voltage from the RF signal source 112 alone or from both the RF signal source 112 and the DC bias source 118 are determined from dimensions and operating pressure to provide maximum uniformity. In general, the RF frequency must be selected to be above the critical frequency, at which point the electrons in the gas are swept across the entire electrode gap in each half cycle at lower frequencies. The frequency has a long time and therefore disappears. Above the critical frequency, the electrons oscillate back and forth between the electrodes, facilitating gas ionization. The critical frequency for a given plasma gun design is determined by first calculating the fluidity.

Here, ν _c is a collision frequency, ω = 2πf (where f is a frequency of radiation), q is an electron charge, E is an electric field, and m is an electron mass. Therefore, the time for gas transition is

Given in. Here, d is the distance between the electrodes.
For the thruster embodiment, the entire plasma gun 90 needs to be maintained in a near-vacuum environment (approximately gas pressure ≦ 10 Torr), which is further necessary because the radiation in the EUV band is easily absorbed. This is because it cannot be used to perform useful work except in a near vacuum environment. Propulsion efficiency is not very important for this embodiment, so there may be a single burst of radiation for each valve operation or valve operation period, and multiple pulses to provide radiation only for the desired period. / Burst can be selected.

Magnetron, RF signal source 112, such as a standard high voltage klystron or RF amplifiers, as described above, can be utilized as an RF signal source with respect to the previous embodiment, such a standard RF signal Sources are expensive to purchase and use, are large and generate significant heat that increases the thermal management burden of the equipment used. Therefore, such a RF signal source is significantly cheaper to purchase and operation, it is preferable to be replaced with a more compact RF signal source generates significantly less heat. Figure 7A shows the RF signal source solid state to meet these requirements. In particular, the circuit of the RF signal source 130 generates RF power at a cost of about 1% of the cost of a standard RF signal source, not a large cabinet, but a circuit board space that is, for example, “6” or “8” times smaller. It turned out to occupy.

Referring to FIG. 7A, the RF signal source 130 includes a capacitor 132 that is charged in a standard fashion from a voltage source, such as the DC voltage source 32 described above. Solid state switch 134, which can be, for example, an SCR, IGBT, or MOSFET, allows capacitor 132 to discharge to the input of a multistage nonlinear magnetic pulse compression circuit 136 of the type described above when closed or energized. . The multi-stage nonlinear magnetic pulse compression circuit 136 can include multi-stages and / or transformers, an example of such a shape is shown and terminates in a specialized output section 138. The output section 138 forms a saturable resonant shunt for ground, and the resonant circuit of the output section 138 includes a capacitor C _R and a saturable inductor L _R. Capacitor C _R is resonantly charged from n-th stage capacitor C _N of multistage nonlinear magnetic pulse compression circuit 136 . Capacitor C _N is selected such that the capacitance is much smaller than capacitor C _R , so that capacitor C _R is inverted during charging of capacitor C _N. Alternatively, inductor L _R can be selected to saturate before the transfer of charge from capacitor C _N to capacitor C _R is complete. When one or both of these conditions are met, a reverse voltage is created across capacitor C _N before inductor L _R saturates and capacitor C _R reaches its peak charge. Under these conditions, the sequential saturation of the inductor L _R, inductor L _R vibrates the capacitor C _R as shown in FIG. 7C. For plasma start-up applications of the present invention, the RF signal
Although only three or four cycles of the signal source are required as shown in FIG. 7C, the parameters of the RF signal source 130 can be selected to provide the desired number of cycles, depending on the application. The resonant frequency F of the output section 138 is determined by the values of the capacitor C _R and the inductor L _R , and either one of these values can be made adjustable to allow circuit tuning. An output coupling circuit 140 comprising a resistive element R _O and a capacitive element C _O is provided, each of which can be formed from a number of appropriately interconnected elements. Output coupling circuit 140 is coupled to the output terminal 142 a portion of the energy from the capacitor C _R, the output coupling circuit
Impedance 140 only part of the energy stored in the capacitor C _R for each cycle (e.g., 20% per cycle) is selected to remove. Furthermore, although particularly suitable for use in the plasma gun of the RF signal source 130 present invention shown in FIG. 7A, RF signal source solid state with the performance characteristics of the circuit of the RF signal source 130 shown in FIG. 7A is not currently present Thus, such an RF signal source 130 can also find use in other applications. Therefore, this RF signal source 130 is also part of the present invention.

Two possible problems in delivering the RF signal to the plasma gun are that a high voltage field occurs over a relatively large uniform area at the base of the column 16 and breakdown occurs in this area, and the column 16 The RF field is obtained at this point with minimal vacuum breakdown required. The latter is not a problem in space applications, but can be a problem in more common applications of plasma guns as radiation sources. FIG. 8A shows one way to achieve both objectives, while FIG. 8B shows a way to achieve only the second objective.

First, referring to FIG. 8A, a ceramic dielectric 150 is provided between the center electrode 12 and the outer electrode 14 at the base of the column 16. The plurality of trigger electrodes 152 are mounted on the surface of the ceramic dielectric 150 outside the column 16 and are separated from the surface 154 of the ceramic dielectric 150 in the column 16 by a small distance by the ceramic dielectric 150. The thickness of the ceramic dielectric 150 between the electrode 152 and the surface 154 can typically be 1/8 inch or less so that the ceramic dielectric 150 does not crack or break. It is chosen to be as thin as possible while guaranteeing that. When an RF signal and / or a DC signal is applied to the trigger electrode 152, a high voltage field appears on the surface 154 to initiate the desired plasma breakdown.

The device of FIG. 8B differs from that of FIG. 8A in that the ceramic dielectric 150 ′ is formed as a collar over the bottom portion of the central electrode 12 and extends a small distance into the column 16. The trigger electrode 152 is mounted on the outer surface 154 ′ of the ceramic dielectric 150 and a high voltage field is formed on the surface 154 ′ when an RF signal and / or a DC signal is supplied to the trigger electrode 152. For applications where the entire plasma gun is not in a vacuum environment, the shape of FIG. 8A is preferred in that it is not necessary to bring electrical leads into the vacuum column 16, which is in contrast to the embodiment of FIG. 8B. Lead 156 is brought into the column 16.

It has also been found that more uniform destruction can be achieved in the plasma gun by supplying an initial high voltage spike to the central electrode 12 and the outer electrode 14 after startup. FIG. 9A shows a circuit for achieving the desired waveform shown in FIG. 9B. In particular, the waveform has an initial spike signal 160, duration signal 162 follow. The initial spike signal 160 may be an about 10 times the magnitude of the voltage of the sustaining signals 162, but those of shorter duration, the central electrode 12, a small enough 1/10 of energy supplied to the outer electrodes 14 Deliver energy.

Referring to FIG. 9A, the circuit includes a first nonlinear magnetic pulse compression circuit 164 (only its final stage is shown in FIG. 9A) and a second nonlinear magnetic pulse compression circuit 166 (only its final stage is also shown in the figure). And have. The second non-linear magnetic pulse compression circuit 166 generates a high voltage short duration spike signal 160, while the first non-linear magnetic pulse compression circuit 164 is a longer period low voltage signal that occurs at the end of the spike signal 160. A continuous signal 162 is generated. Reactor 168 for the last stage of the first linear magnetic pulse compression circuit 164 is being saturated in a direction so as to permit the flow of signals from the first non-linear magnetic pulse compression circuit 164, the second linear magnetic A normal bias is applied by a bias signal supplied via the bias winding 170 to block the reverse signal flow from the pulse compression circuit 166. Therefore, this signal is not supplied to the first non-linear magnetic pulse compression circuit 164, particularly to the capacitor 172 in its final stage, and protects the first non-linear magnetic pulse compression circuit 164 from voltage spikes, all of this signal being center electrode. 12, to be supplied to the outer electrode 14. The spike signal 160 begins to reverse the bias of the saturable reactor 168, partially overcoming the bias supplied thereto, and at the same time causes avalanche breakdown at the central electrode 12 and the outer electrode 14. This allows the selection of the optimum voltage and drive impedance level for the main discharge chain without concern over the breakdown voltage. The reversible bias of the saturable reactor 168 provides a delay for the sustained signal 162 from the first nonlinear magnetic pulse compression circuit 164 until the reactor 168 resaturates and provides a smooth transition between the two signals. .

For example, one problem with the type of plasma gun shown in FIG. 3 is that the plasma can be centimeters per microsecond to achieve a desired clamping temperature in the range of 100 eV to 1000 eV, depending on the desired frequency of radiation. A magnetic compression field on the order of Tesla sufficient to drive at speed is required. Due to these high velocities, the plasma is driven down the central electrode 12 and emitted from the end of the central electrode 12 , and the plasma sheath continues to move into the space away from the end of the central electrode 12 . As a result, the plasma sheath eventually loses its electrical connection to the clamping part, terminating the clamping and causing a large voltage transition. This voltage transition can cause a high voltage respike that can severely damage the electrode. The loss of electrical contact with the plasma sheath also results in a substantial decrease in power efficiency from the plasma gun , and the tightening can be several microseconds (eg, 2-4 microseconds). It lasts only about 100 nanoseconds, not a much longer period.

In accordance with the teachings of the present invention, this problem of plasma separation by providing a blast shield i.e. focusing device 194 adjacent the outlet end of the central electrode 12 to guide again back the plasma sheath towards the central electrode 12 Overcome by FIGS. 10A-10C show three possible embodiments of such shields or focusing devices (hereinafter collectively referred to as shields) 194A, 194B, 194C, which differ mainly in the shape of the focusing cavities 196A, 196B, 106C, respectively . Indicates. In particular, the cavity 196A has a generally spherical shape, and the cavity is mounted to the outer electrode 14 or a suitable housing component of the plasma gun by a suitable mounting component (not shown) so that the wall of the cavity 196A is the tip of the central electrode 12. Away from it by a certain distance. This certain distance is sufficient to prevent contact between the shield and the central electrode 12 , but short enough to produce plasma radiation returning to the central electrode 12 prior to plasma separation. Is. These objects are achieved by a distance in the range of approximately R to 2R, where R is the radius of the central electrode 12. However, such a distance can vary to some extent depending on other parameters of the plasma gun 10. The cavity 196B has a conical shape, and the cavity 196C has a parabolic shape. The parameters described above for the distance of the cavity from the end of the central electrode 12 apply to all three cavity shapes.

While it is desirable to prevent separation of the plasma sheath and contain the plasma sheath within the shield 194, it is important that the shield 194 does not conflict with the desired radiation outflow from the plasma gun 10. Thus, each shield 194 has a central opening 198A, 198B, 198C formed at the top of the corresponding cavity and having a center coaxial with the centerline of the central electrode 12 . The aperture 198 is preferably circular and has a diameter sufficient to allow radiation emitted at the tip of the central electrode 12 at an angle of ± 15 ° (approximately the angle of the emitted radiation) from the clamp to pass through the aperture 198 unobstructed. Have. The upper portion of each opening 198 tapers outward to facilitate radiation outflow while substantially limiting any escape of the plasma sheath.

The material of the shield 194 must be a high temperature non-conductive material that can withstand temperatures in the range of approximately 1000 ° C. and above. Various high temperature ceramics have desirable properties, and for the illustrated embodiment, Al ₂ O ₃ (aluminum oxide) is utilized. Various glasses, quartz and sapphire also have desirable properties that serve as the material for the shield 194.

In the above description, the shield 194 for redirecting the plasma has been shown to be used with a particular shaped radiation source, but this shield 194 may be any arbitrary where plasma separation may be a problem. Suitable for use with any radiation source, and therefore the invention is in no way limited by the particular radiation source shape of FIG. Similarly, although three cavity shapes for redirecting radiation to the cathode or outer electrode 14 are shown in FIGS. 10A-10C, other cavity shapes suitable for performing this function may be utilized. The specific materials described above are also merely exemplary.

Furthermore, while the parameters for generating radiation at 13 nm have been described above, by controlling various parameters of the plasma gun 90 as a radiation source, and in particular from the element / gas utilized, the high voltage source. By carefully selecting the maximum current, the plasma temperature in the clamping region, the gas pressure in the column 16 and, in some cases, the radiation filter utilized, other wavelengths within the EUV band or in some cases outside this band. The radiation at can be obtained.

Many types of gases can be used as the plasma gas for the plasma gun described above, but inert gases such as argon and xenon are often preferred. Other gases that can be used include nitrogen, hydrazine, helium, hydrogen and neon. As described above, when using a plasma gun as a radiation source as in the embodiment of FIG. 3, various elements / gases may be used to achieve a selected EUV or other wavelength, In the case, the plasma and the radiation gas are the same gas. For example, hydrogen gas can be selected to effectively obtain radiation at 121.5 nm within the VUV band. Furthermore, although various embodiments have been described above, it is obvious that these embodiments are merely illustrative and do not limit the present invention. For example, although the driver shown is advantageous for various applications, other high PRF drivers that have the appropriate voltage and rise time and do not require high voltage switching can also be used. Similarly, although description of various plasma starting mechanism using the electrode trigger being driven by an RF signal source preferably iso lid state, other methods for initiating plasma breakdown can also be used in appropriate applications. The shape of the electrodes and the applications given for the plasma gun are also exemplary. Thus, while the invention has been particularly shown and described in terms of preferred embodiments, those skilled in the art can make the above and other modifications which constitute details, while remaining within the spirit and spirit of the invention. Is defined only by the claims.

1 is a semi-schematic, semi-cut side view of a first exemplary thruster embodiment of a plasma gun of the present invention. FIG. FIG. 5 is a semi-schematic half cut side view of another thruster embodiment of the plasma gun of the present invention. It is a semi-schematic half cut side view of the embodiment of the radiation source of the plasma gun of the present invention. 4 is an enlarged (not to scale) cutaway view of the center electrode of FIG. 3 for one embodiment of the plasma gun of the present invention. FIG. Depending on the relative dimensions, other factors can be used as a thruster or radiation source, and a semi-schematic side cutaway view of an embodiment of the plasma gun of the present invention having a trigger electrode driven with an RF signal according to the teachings of the present invention It is. FIG. 6 schematically illustrates another implementation for obtaining an RF signal source in the plasma gun of the present invention. 7A is a schematic diagram of an RF signal source solid state suitable for use as an RF signal source for driving the trigger electrode, 7B and 7C a voltage across a certain capacitors in the circuit of Figure 7A FIG. 8A and 8B are cut-away partial side views of a portion of the plasma gun showing two different starter electrode shapes suitable for supplying a starting voltage to the plasma gun. FIG. 9A is a schematic diagram of a pulse driver circuit suitable for use to drive a plasma gun of the present invention according to another embodiment, and FIG. 9B shows an output signal from the circuit of FIG. 9A. FIG. FIG. 6 is an enlarged side cross-sectional view showing the end of the central electrode and the shield for a spherical embodiment of the present invention. FIG. 6 is an enlarged side cross-sectional view showing the end of the central electrode and the shield for the conical embodiment of the present invention. FIG. 6 is an enlarged side cross-sectional view showing the end of the central electrode and the shield for the parabolic embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Plasma gun 12 Center electrode 14 Outer electrode 16 Column 82 Trigger electrode 130 Circuit 132 Capacitor 134 Solid state switch 136 Multistage nonlinear magnetic pulse compression circuit 138 Output section 140 Output coupling circuit

Claims (16)

In the high PRF plasma gun (10, 10 ', 90)
Central electrode (12);
An outer electrode (14) substantially coaxial with the central electrode, wherein a coaxial column (16) is formed between the central electrode and the outer electrode, the column being closed and open with a base end; An outer electrode having an outlet end;
An inlet mechanism (70, 72, 74) for introducing the selected gas into the column;
With a solid state high repetition rate pulse driver (32, 34, 36 ; 110; 164 , 166 ) operable to deliver a high voltage pulse across the center and outer electrodes during plasma start-up at the base of the column A solid-state high repetition rate pulse driver in which plasma expands from the base end of the column and exits from the exit end of the column;
The plurality of trigger electrodes (82 ; 152 ); and the plurality of trigger electrodes (82 ; 152 ) at the base end of the column (16) are connected to the plurality of trigger electrodes (82 ; 152 ). A radio frequency (RF) signal source (112, 130) that selectively provides a signal, wherein the trigger electrode is driven by the solid state high repetition rate pulse driver (32, 34, 36 ; 110; 164, 166 ). An RF signal source adapted to undergo 3 or 4 oscillation cycles for each high voltage pulse delivered across the central and outer electrodes;
A plasma gun characterized by comprising:   The plasma gun of any preceding claim, wherein the RF signal source (112, 130) generates an RF signal at a frequency in the range of 10 MHz to 1000 MHz. The plurality of trigger electrodes comprises a plurality of trigger electrodes (152) attached to a ceramic dielectric (150; 150 ') and spaced substantially uniformly around the column (16), the plurality of triggers The plasma gun of claim 1, wherein the electrode produces a high voltage field at the surface of the ceramic dielectric at the base end of the column. Claims the ceramic dielectric (150 ') surrounds the central electrode at the base end portion of the center electrode, the plurality of trigger electrodes (152) is characterized in that it is attached to the upper Symbol ceramic dielectric (150') Item 4. The plasma gun according to Item 3. The ceramic dielectric (150) forms the base of the column (16), the plurality of trigger electrodes (152) is, the ceramic dielectric (150), said ceramic dielectric from the opposite side to the said column attached to, the ceramic dielectric by (150) is between one et away the column, the column-side surface of the ceramic dielectric by supplying the RF signal to the plurality of trigger electrodes 152 150 The plasma gun of claim 3, wherein a high voltage field is generated in (154) .   The plasma gun according to claim 1, wherein at least one of the central electrode (12) and the outer electrode (14) is formed of a sintered powder refractory metal. The plasma gun is operable as a radiation source at a selected wavelength;
The at least one formed of sintered powder refractory metal of the central electrode (12) and the outer electrode (14) is saturated with a fluid material suitable for generating radiation at the selected wavelength;
7. The plasma gun according to claim 6, wherein the fluid material is liquid lithium. Plasma gun according to claim 7, wherein the at least additional lithium into one of the upper Symbol central electrode during operation (12) and the outer electrode (14) is characterized in that it is supplied. 7. The plasma gun according to claim 6, wherein both the central electrode and the outer electrode are formed of the sintered powder refractory metal. The solid state high repetition rate pulse driver ( 164 , 166) provides a high voltage spike (160), a duration signal (162) at a voltage lower than the high voltage spike (160) and longer than the high voltage spike. 2. The method of claim 1, wherein following the high voltage spike (160), most of the energy stored in the solid state high repetition rate pulse driver (164, 166) is provided by the sustain signal. Plasma gun. The solid state high repetition rate pulse driver has a first non-linear magnetic pulse compression circuit (166) for generating the high voltage spike (160) and a second non-linear for generating the sustain signal (162). 11. A plasma gun according to claim 10, comprising a magnetic pulse compression circuit (164). The second nonlinear magnetic pulse compression circuit (164) has at least two stages;
The saturable reactor (168) in the last of the stages causes the high voltage spike from the first nonlinear magnetic pulse compression circuit (166) to enter the second nonlinear magnetic pulse compression circuit (164). Usually biased to prevent that,
The high voltage spike is applied to the reactor (168) to inhibit initial flow from the second non-linear magnetic pulse compression circuit (164) until the reactor (168) is saturated again to pass the sustained signal. The plasma gun according to claim 11, wherein the plasma gun is partially desaturated.   The plasma gun according to claim 1, further comprising a DC blocking capacitor (114) connected between the RF signal source (112, 130) and the plurality of trigger electrodes (82). The RF signal source (112, 130) generates the RF signal at a frequency above a critical frequency;
The critical frequency is lower than that at which the electrons in the selected gas have a full gap between the opposing central electrode (12) and outer electrode (14) in each half cycle of the RF signal. The plasma gun of claim 1, wherein the plasma gun has a frequency that has a time such that it is wiped across. The plasma of claim 2, wherein the solid state high repetition rate pulse driver (32, 34, 36 ; 110; 164, 166 ) generates a high voltage pulse across the central electrode and the outer electrode. gun. The RF signal source (130) is
A resonant circuit (138) that oscillates at a resonant frequency corresponding to the frequency of the RF signal generated by the RF signal source (130) when energy is transferred;
A large amount of energy is transferred from the energy source (32, 132) to the resonant circuit (138), thereby causing the resonant circuit (138) to oscillate at the resonant frequency to the energy source (32, 132). A switch (134) for selectively connecting a resonant circuit (138);
An output coupling circuit (140) connected between the resonance circuit (138) and the plurality of trigger electrodes (82) so as to supply a vibration signal from the resonance circuit (138) to the plurality of trigger electrodes (82). ) comprising first vibration cycle per the resonant circuit (138), an output coupling circuit part of the energy is transferred from the resonant circuit (138) is coupled to the plurality of trigger electrodes (82) and (140) ;
The plasma gun according to claim 1, further comprising:

Images