Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03K—PULSE TECHNIQUE
  • H03K3/00—Circuits for generating electric pulses; Monostable, bistable or multistable circuits
  • H03K3/02—Generators characterised by the type of circuit or by the means used for producing pulses
  • H03K3/53—Generators characterised by the type of circuit or by the means used for producing pulses by the use of an energy-accumulating element discharged through the load by a switching device controlled by an external signal and not incorporating positive feedback
  • H03K3/55—Generators characterised by the type of circuit or by the means used for producing pulses by the use of an energy-accumulating element discharged through the load by a switching device controlled by an external signal and not incorporating positive feedback the switching device being a gas-filled tube having a control electrode
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/097—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser

Definitions

• This invention relates to electrical pulsing sys- terns. More particularly, the invention relates to pulsed electrical power circuits for high repetition rate gas lasers. Specifically, the invention relates to circuits in which high power, high voltage, fast rise time, narrow electrical pulses provide electrical energy for exciting a gas mixture, thereby producing laser operation.
• Electronic transition lasers such as rare gas excimer, di er, and charge transfer lasers, offer scal ⁇ able high energy photon sources in the ultraviolet and visible wavelengths. These lasers can be scaled to high pulsed output energies by increasing the volume, pres ⁇ sure, and energy deposition into a high pressure rare gas halide mixture contained within the laser cavity.
• Rare gas halide electronic transition lasers operate on several fundamental principles.
• a selected rare gas is a dominant component of the mixture because the electron excitation is initially deposited into ionization and excitation of the rare gas.
• the electron energy in the discharge should be high enough to produce sufficient rare gas ions and metastables. There should also be a sufficiently high current density in order to produce a sufficient number of excited rare gas species in a short time period which is less than the time required to react all of the molecular additive. These three criteria require a high power, high voltage discharge circuit incorporating some method for stabilizing the discharge to prevent arcing.
• High voltage charging circuits typically have too large an inductance to provide either a rapid voltage rise time or a suffi ⁇ ciently low output impedance for optimum energy transfer to the laser load. Consequently, a key problem associ ⁇ ated with these lasers is the development of an effi- cient, long-lived, nondestructive, nonablative reliable and inexpensive method of electron energy deposition into the laser load.
• the pulse r_ e time shaping, pulse width compression, and impedance matching electrical excitation circuit of the present invention provides an effective solution to the problem.
• U.S. Patent 4,275,317 discloses a circuit for the purpose of efficient energy transfer from a relatively slow high power, high voltage charging circuit to a laser load.
• the circuit comprises one or more saturable inductor switches, each of which has an associated dis ⁇ tributed capacitance energy storage device.
• Energy is provided to a distributed capacitance energy storage device by a voltage source and is contained therein by a saturable inductor switch.
• the saturable inductor switch becomes saturated, thereby allowing the energy to flow therethrough and into either a next intermediate capacitance energy storage device or the laser load.
• the discharge loop inductance (neglecting the inductance of the saturable inductor L_) is typically a few nanohenries, and, therefore, the saturable inductor L_ represents a significant increase in total loop inductance.
• an electrical excitation circuit is provided which avoids any added inductance in the discharge loop.
• U.S. Patent 4,275,317 discloses that distributed capacitance energy storage devices must be utilized in order to provide the high voltage, narrow pulses required by electronic transition lasers.
• Distributed capacitance energy storage devices which can be used include coaxial lines, multiple coaxial lines, parallel plate transmis ⁇ sion lines, or two or more parallel-connected capacitors having an associated natural or added inductance for creating a pulse shaping network.
• U.S. Patent 4,275,317 further discloses that in order to achieve efficient operation of the laser, a pulse shaping network providing less than 10-nanosecond rise time pulses with durations in the hundred nanoseconds region must be used, and, therefore, the saturable inductor switch must have characteristics, and be constructed, in a manner differing from that of con ⁇ ventional saturable inductor switches. That is, the saturable inductor switch must be formed of a material having a very high permeability and a cross-sectional thickness on the order of the skin depth of the material at a frequency corresponding to the desired rise time of the pulse.
• mag ⁇ netic material films of this thickness can be obtained by deposition on a plastic insulator backing. These back- ings can be formed into a tape which is then wound around a suitable nonmagnetic core material, thereby creating the saturable inductor switch.
• the invention provides an electrical excitation cir- cuit including a pulse forming network for generating pulses of a given energy level.
• This network when charged, is capable of discharging the charge energy within a preselected pulse width interval. Charging of the network is done over a charging period in excess of the pulse width sought and with a charging current lower than the pulse discharge current.
• the slow-charge energy is rapidly discharged to obtain the high-energy pulse desired by means of a switching device in the pulse forming network in the form of a saturable inductor switch which is not connected in the discharge loop with the laser load.
• the invention provides an electrical excitation cir ⁇ cuit which can efficiently produce a high voltage pulse of very short rise time and duration for a laser load.
• the electrical excitation circuit in accordance with the invention includes a charging circuit means connected in series with a pulse forming network between a power source and the laser load.
• the pulse forming network includes at least one capacitor, preferably a discrete ceramic capacitor, connected to the laser load.
• a satur- able inductor switch is connected to the at least one capacitor so that as the capacitor is charged, only a small amount of the energy is absorbed by the saturable switch until the switch becomes saturated. At that time, the inductance of the saturable inductor switch decreases, and the energy contained within the at least one capacitor is switched into the laser load.
• an electrical excitation circuit for a gas laser connected between a power source and a laser load comprising: charging circuit means connected to the power source; and a pulse forming network connected to the charging circuit means and the laser load, the pulse forming network com ⁇ prising (a) a saturable inductor switch alternatively having an unsaturated state and a saturated state, the saturable inductor switch being shunted across the charging circuit means and (b) at least one capacitor shunted across the saturable inductor switch and con ⁇ nected to the laser load, the at least one capacitor being charged by the charging circuit means when the saturable inductor switch is in the unsaturated state and being discharged through the laser load when the satur ⁇ able inductor switch transposes to the saturated st ⁇ e.
• the operating point of the saturable inductor switch is preferably controlled by a bias circuit, thereby allowing adjustment of the operating point for a given laser load, as well as enabling the saturable inductor switch to be constructed from readily available components.
• the saturable inductor switch is not in the discharge loop of the laser load. This enables the electrical excitation circuit to meet the principal requirements for use with gas lasers, low inductance and ability to switch high voltage and high currents.
• the pulse forming network further com ⁇ prises a second capacitor connected between the at least one capacitor and the laser load and a magnetic diode charging inductor shunted across the laser load, the second capacitor being charged by the charging circuit means through the magnetic diode charging inductor when the saturable inductor switch is in the unsaturated state and being discharged through the laser load in series with the at least one capacitor when the saturable inductor switch transposes to the saturated state.
• the magnetic diode charging inductor provides a charging path for the second capa ⁇ citor and at the same time prevents prepulse (breakdown) of the laser gas.
• the operating point of the magnetic diode charging inductor is preferably controlled by a bias circuit, thereby allowing adjustment of the oper ⁇ ating point for a given laser load, as well as enabling the magnetic diode charging inductor to be constructed from readily available components.
• the saturable inductor switch is not in the discharge loop; and, also, the magnetic diode charging inductor is like ⁇ wise not in the discharge loop.
• the charging circuit means included in the electrical excitation circuit can com ⁇ prise a power source capacitor shunted across the power source and connected by a series-connected choke and . ⁇ -
• charging diode to a parallel circuit comprising, as one branch, a triggerable thyratron and, as the other branch, a charging capacitor and an inductor, the charging capa ⁇ citor and inductor being in series with the saturable inductor switch included in the pulse forming network of the electrical excitation circuit.
• the at least one capacitor included in the pulse forming network is charged when the thyratron is triggered.
• the charging circuit means included in the electrical excitation circuit can com ⁇ prise a series-connected choke, primary winding of a saturable step-up transformer, and power source capacitor in a parallel circuit with a triggerable silicon con- trolled rectifier connected across the power source.
• the operating point of the saturable step-up transformer is preferably controlled by a bias circuit, thereby allowing adjustment of the operating point for a given laser load, as well as enabling the saturable step-up transformer to be constructed from readily available components.
• a charging capacitor and a charging diode are connected in series across the secondary winding of the step-up trans ⁇ former, and at least one saturable inductor switch cir ⁇ cuit comprising a capacitor and a saturable inductor switch is connected between the charging capacitor and the pulse forming network, the at least one saturable inductor switch circuit being shunted across the satur ⁇ able inductor switch included in the pulse forming net ⁇ work of the electrical excitation circuit.
• the operating point of each saturable inductor switch circuit is pre ⁇ ferably controlled by an associated bias circuit, thereby allowing adjustment of the operating point for a giv-an laser load, as well as enabling each saturable inductor switch circuit to be constructed from readily available components.
• a plurality of saturable inductor switch circuits can be cascaded between the charging capacitor and the pulse forming network, each additional saturable inductor switch circuit providing a further degree of pulse compression and shaping.
• a pre-ionization means is preferably included in the electrical excitation circuit in accordance with the invention for conditioning the laser gas mixture so that a uniform discharge occurs across the electrodes of the laser.
• the pre-ionization means is an x-ray circuit.
• a corona pre-ionization circuit can be included. Both efficiency and lifetime of a gas laser are improved by the electrical excitation circuit of the invention in the laser's electrical discharge system. The electrical excitation circuit improves the laser's efficiency by speeding the deposition of electrical energy into the laser gas.
• Incorporation of the elec ⁇ trical excitation circuit between the power source and a gas laser reduces the voltage rise time from hundreds of nanoseconds, for example, 200 nanoseconds, to tens of nanoseconds, for example, 30 nanoseconds. This short rise time is required to produce a uniform (i.e., arc- less) discharge in the gas, and such a discharge is required to convert a large portion of the electrical input to optical output.
• Fig. 1 is a schematic diagram of a known pulse shaping network connected to a charging circuit for exciting a laser load useful in understanding the fea- tures and advantages of the electrical excitation circuit in accordance with the invention
• Fig. 2 is a schematic diagram of one embodiment of an electrical excitation circuit in accordance with the invention for connecting an electrical power source to a laser load
• Fig. 3 illustrates a B-H curve for the purpose of facilitating an understanding of the operation of the saturable magnetic elements included in the circuit of Fig. 2;
• Fig. 4 is a partial schematic diagram of another embodiment of an electrical excitation circuit in accor ⁇ dance with the invention for connecting an electrical power source to a laser load;
• Fig. 5 is a perspective view which shows a preferred configuration for a saturable magnetic element
• Fig. 6 is a cross-sectional view along line 6-6 of Fig. 5;
• Fig. 7 is a schematic diagram of an x-ray pre-ion ⁇ ization circuit preferably included in the circuit of Fig. 2; and Fig. 8 is a schematic diagram of a corona pre-ion ⁇ ization circuit alternatively included in the circuit of Fig. 2.
• Fig. 2 is a simplified schematic diagram of one embodiment of the electrical excitation circuit in accordance with the invention, generally indi- ⁇ " cated by the numeral 10.
• the electrical excitation circuit 10 provides an electrical interface between a high voltage, high impedance power source 12 and a rela- tively low impedance laser load 14.
• the electrical exci ⁇ tation circuit 10 includes a charging circuit 16, which charges a pulse forming network 18. The pulse forming -u-
• the power source 12 is preferably a direct current power source.
• the power source 12 for example, can be a 12 kV direct current rectified power supply.
• the charging circuit 16 included in the electrical excitation circuit 10 shown in Fig. 2 includes a power source capacitor C, shunted across the power source 12.
• the charging circuit 16 also includes a charging trans- former or choke L, and an isolating or charging diode D.. connected in series between the power source 12 and a first terminal or node TP..
• the choke L., and charging diode D. isolate the power source 12 from the pulse forming network 18.
• a thyratron S is included in the charging circuit 16 connected between the first node TP, and ground or common, the control electrode of the thyra ⁇ tron being connected to a source of an input trigger signal generated by a pulse generator circuit 20.
• the charging circuit 16 includes a charging capacitor C ⁇ series-connected with an inductor L- between the first node TP, and a second terminal or node TP-.
• the capacitor C is significantly larger than the capa ⁇ citor C_.
• the pulse forming network 18 included in the elec- trical excitation circuit 10 shown in Fig. 2 includes a saturable inductor switch S_ connected between the second node TP- and common.
• a biasing circuit 22 for the satur ⁇ able inductor switch S- is included in the pulse forming network 18 and comprises a S 2 bias power source and a choke L. connected in series with the bias winding of the saturable inductor switch.
• the S ⁇ bias power source is an adjustable direct current power supply connected so that bias current flows through the bias winding of the saturable inductor switch S_ in the direction from top to bottom in Fig. 2.
• the choke is an adjustable direct current power supply connected so that bias current flows through the bias winding of the saturable inductor switch S_ in the direction from top to bottom in Fig. 2.
• the pulse forming network 18 provides isolation of the S_ bias power source from high voltage pulses produced by transformer action on the bias winding of the saturable inductor switch S ⁇ .
• the pulse forming network 18 also includes a first pulse forming network capacitor C_ shunted across the saturable inductor switch S-.
• a second pulse forming network capacitor C. is included in the pulse forming network 18 connected between the second node TP_ and a third terminal or node TP_.
• the pulse forming network 18 includes a magnetic diode charging inductor L-, connected between the third node TP_ and common.
• the pulse forming network 18 includes a biasing circuit 24 for the magnetic diode charging inductor L-, which comprises a - bias power source and a choke L_ connected in series with the bias winding of the magnetic diode charging inductor.
• the L., bias power source is an adjustable direct current power supply connected so that bias current flows through the bias winding of the magnetic diode charging inductor L_ in the direction from bottom to top in Fig. 2.
• the choke L_ provides isolation of the L_ bias power source from high voltage pulses produced by transformer action on the bias winding of the magnetic diode charging inductor L... As shown in Fig. 2, the laser load 14 is connected between the third node TP_ and common.
• the inductance L represents the distributed inductance of the electrode structure 26 of the laser load 14.
• a pre-ionization cir- cuit 28 is preferably included in the laser for condi ⁇ tioning the gas mixture so that there is a uniform discharge and not arc discharges which constrict to streamers when the energy stored in the pulse forming network 18 is released and deposited into the gas mixture between the electrodes 26.
• the electrical excitation circuit 10 performs four relatively separate operations: a slow resonant charge of the charging capacitor C_, a medium speed charge of the pulse forming network 18, an inversion of the voltage on half of the pulse forming network, and finally the laser discharge. These operations will be described in sequence later. The description first treats saturable - i i -
• magnetic elements such as the saturable inductor switch S_ and the magnetic diode charging inductor L, which are either in a high inductance (unsaturated) condition or a low inductance (saturated) condition as determined by current flow and bias.
• S_ the saturable inductor switch S_ and the magnetic diode charging inductor L
• the functioning of these satur ⁇ able magnetic elements is described in some detail in order to facilitate an understanding of the operation of the electrical excitation circuit 10.
• FIG. 3 A typical B-H curve for ferromagnetic material is shown in Fig. 3. Several points are plotted on the curve and will be discussed. First, a brief description of inductance will be presented.
• N is the number of conductor turns about the core
• A is the cross-sectional area of the core
• E is the inte ⁇ grated applied voltage
• ⁇ B is the available change in magnetization. For maximum flexibility, it is desir- able to be able to vary the applied voltage E without changing the switching time T. Since the number of turns N and the area A are fixed for a given core, it is neces ⁇ sary to adjust ⁇ B.
• ⁇ B for a given material is determined by the quiescent operating point. Without bias, this value will be B- K-,_,.. (or B_K__,__-___! if the core is reset).
• a variable current supply can be used to set the operating point anywhere between +_ B ⁇ A * ⁇ his permits the value of ⁇ B to be adjusted to account for different applied voltages, as well as to allow for tolerances in materials.
• the initial phase of operation of the electrical excitation circuit 10 shown in Fig. 2 is a slow resonant charge of the charging capacitor C- included in the charging circuit 16.
• the laser discharge sequencing starts when the 12 kV DC appearing on the capacitor C. from the power source 12 is doubled in a resonant charging circuit comprising the capacitor C,, the choke L., the isolating diode D. , the capacitor C_, the inductor L-, and the saturable inductor switch S ⁇ .
• th voltage at the second node TP_ is clamped to near zero by the low impedance of the saturable inductor switch S- which is biased into a low inductance (saturated) condition by current flowing from the S- bias power source through the choke L. and the bias winding of the saturable inductor switch in the direction, from top to -bottom in Fig * . 2.
• the sinusoidal charging current of 1.9 amperes peak also flows through the saturable inductor switch S- from top to bottom.
• the next phase of oper ⁇ _Lion of the electrical exci ⁇ tation circuit 10 shown in Fig. 2 is a medium speed charge of the pulse forming network 18.
• the capa- citor C 2 charged to 24 kV
• the thyratron S is then switched on by a trigger signal from the pulse generator circuit 20.
• the 24 kV charge on the capacitor C- causes a sinusoidal current of 3200 amperes peak to flow out of the capacitor C-.
• a fraction of the current flows in the direction from bottom to top in Fig. 2 through the satur ⁇ able inductor switch S- forcing it out of saturation and into a high inductance (unsaturated) state.
• the bulk of the current flows into the capacitor C-, and the series combination of the magnetic diode charging inductor L_ and the capacitor C . This charges the capacitors C- and C. to a voltage of approximately 24 kV with negative polarity on the second node TP- in one microsecond.
• the portion of this current flowing through the mag ⁇ netic diode charging inductor L- produces a prepulse voltage across the electrodes 26 proportional to the inductance of the magnetic diode charging inductor. This prepulse must be held low enough to prevent breakdown of the laser gas.
• the magnetic diode charging inductor L- is magnetically biased into a low inductance (saturated) state by current flowing from the L- bias power source through the choke L_ and the bias winding of the magnetic diode charging inductor in the direction from bottom to top in Fig. 2. Furthermore, the current flow from common toward the third node TP- during the charging of the capacitor C. adds to the bias current which als holds the magnetic diode charging inductor L- in a low inductance state. This is adequate to avoid any prepulse causing a breakdown of the laser gas.
• the voltage from common to the second node TP_ increases (as a 1 - cos ( ⁇ t) function), and at the peak of the voltage, the saturable inductor switch S_ trans- poses to a saturated state, that is, saturates in the reverse direction, which allows current to flow through the saturable inductor switch in the direction from bottom to top in Fig. 2 in opposition to the bias current which is swamped out.
• the time required for the satur- able inductor switch S_ to reverse saturate coincides with the peak of the voltage and is determined by the voltage across the saturable inductor switch and the bias current through the bias winding of the saturable inductor switch.
• Adjustment of this bias current permits operation over a range of voltages with saturation always occurring at the peak.
• the next phase of operation of the electrical exci ⁇ tation circuit 10 shown in Fig. 2 is the inversion of the voltage on half of the pulse forming network 18.
• the saturable inductor switch S- reverse saturates and switches to a low inductance state, current flows out of the capacitor C... A fraction of the current flows back through the inductor L_ and charges the capacitor C- to a low reverse voltage, which aids recovery of the thyratron S,. Part of the current attempts to flow through the capacitor C. and the magnetic diode charging inductor L_, which would discharge the capacitor C 4 and not produce laser output.
• the final phase of operation of the electrical exci ⁇ tation circuit 10 shown in Fig. 2 is the laser discharge.
• a fraction of the current from the series combination of the capacitors C. and C. flows through the magnetic diode charging inductor L- which is still in a high inductance state and is lost.
• current reverses in the satur- able inductor switch S- forcing it out of saturation and back into a high inductance state, which minimizes the current lost through the saturable inductor switch.
• the bulk of the current flows through the laser gas for 0.1 microsecond, thereby producing a laser pulse.
• the voltage rise time is proportional to the inductance of the discharge circuit switch (saturable inductor switch S_ saturated) .
• the discharge time is pro ⁇ portional to the inductance of the discharge loop (L - Consequently, the electrial excitation circuit 10 has a fast voltage rise time and a comparable length discharge time in the load, as is desired in many lasers.
• the pulse forming network 18 included in the elec ⁇ trical excitation circuit 10 includes a saturable inductor switch S- as opposed to a spark gap or thyratron switch.
• the low inductance saturable inductor switch S- inverts the voltage on the capacitor C-, a function usu- ally Implemented using a spark gap or thyratron switch, both of which have a relatively high inductance.
• the inversion of the voltage on the capacitor C- is the result of the sinusoidal current through the switch which reverses at the peak voltage, diverting much of the load current through the switch.
• the saturable inductor switch S- rather than a spark gap avoids loss since the saturable inductor switch unsaturates when the current reverses, switching to a high inductance value. This results in a greater frac ⁇ tion of the stored energy being deposited into the laser load 14.
• the saturable inductor switch S_ has additional advantages over conventional spark gap switches. Advan ⁇ tages include longer lifetime, high pulse rate operation, and simplicity.
• the pulse forming network 18 included in the elec- tricl excitation circuit 10 includes a magnetic diode charging inductor -. as opposed to a linear inductor. As described above, the magnetic diode charging inductor L- is across the laser load 14 to allow charging of the capacitor C . . The inductance of the magnetic diode charging inductor L_ must be low enough during charging of the capacitor C. to prevent laser gas breakdown (pre- pulse) .
• the ideal inductor would have zero inductance during charging of the capacitor C . and infinite inductance during laser discharge. A low inductance during laser discharge diverts current from the discharge, reducing laser efficiency. A single value linear inductor is a compromise at best between too high and too low an inductance.
• the magnetic diode charging inductor L has the pro ⁇ perty of high inductance when the ferromagnetic material is unsaturated and low inductance when the material satu ⁇ rates. Typically, there is a ratio of 100:1 between high and low inductance.
• the current reversal in the magnetic diode charging inductor - between charging of the capa ⁇ citor C. and laser discharge drives the magnetic diode charging inductor from a saturated (low inductance) state to an unsaturated (high inductance) state, thereby approximating an ideal inductor.
• a cascaded mag- netic charging circuit 30 shown in Fig. 4 replaces the charging circuit 16 described above in conjunction with Fig. 2.
• the cascaded magnetic charging circuit 30 is connected to a power source 12'.
• the power source 12' is a relatively low voltage power supply compared with the power source 12 described in conjunction with Fig. 2.
• the power source 12' for example, can be a 1 kV direct current rectified power supply.
• the cascaded magnetic charging circuit 30 shown in Fig. 4 also includes a charging transformer or choke L. ' , the primary winding of a saturable step-up transformer XFMR. , and a power source capacitor C, ' connected in series between the power source 12' and common.
• the saturable step-up transformer XFMR. can be a 1:25 transformer.
• the cascaded magnetic charging cir ⁇ cuit 30 additionally includes a biasing circuit 31 for the saturable step-up transformer XFMR, , which comprises an XFMR.1 bias power source and a choke Lo- connected in series with the bias winding of the saturable step-up transformer.
• the XFMR is a biasing circuit 31 for the saturable step-up transformer XFMR, which comprises an XFMR.1 bias power source and a choke Lo- connected in series with the bias winding of the saturable step-up transformer.
• bias power source is an adjust ⁇ able direct current power supply connected so that bias current flows through the bias winding of the saturable step-up transformer XFMR. in the direction from bottom to top in Fig. 4.
• the choke L fi provides isolation of the XFMR. bias power source from high voltage pulses produced by transformer action on the bias winding of the satur- able step-up transformer XFMR..
• a silicon controlled rectifier SCR, included in the cascaded magnetic charging circuit 30 is shunted across the choke L- ' / the primary winding of the saturable step-up transformer XFMR., and the capacitor C, ' , the gate of the silicon controlled rectifier being connected to a source of an input trigger signal generated by a pulse generator circuit 20'.
• the cascaded magnetic charging circuit 30 further includes a capacitor C-* connected between the secondary winding of the saturable step-up transformer XFMR. and a first terminal or node TP. ' .
• An isolating or charging diode D. ' included in the cascaded magnetic charging cir ⁇ cuit 30 is connected between the first node TP. ' and common.
• the cascaded magnetic charging circuit 30 finally includes at least one saturable inductor switch circuit 32, such as a saturable inductor switch circuit 34, com ⁇ prising a capacitor C- connected between the first node TP, ' and common, as well as a saturable inductor switch S- connected between the first node TP ' and the second node TP_ which corresponds to the second node TP- shown in Fig. 2.
• the saturable inductor switch circuit 34 also preferably comprises a biasing circuit 35 for the satur- able inductor switch S-, which comprises a S- bias power source and a choke L_ connected in series with the bias winding of the saturable inductor switch.
• the S- bias power source is an adjustable direct current power supply connected so that bias current flows through the bias winding of the saturable inductor switch S- in the direc ⁇ tion from left to right in Fig. 4.
• the choke L ⁇ provides isolation of the S_ bias power source from high voltage pulses produced by transformer action on the bias winding of the saturable inductor switch S,.
• a second saturable inductor switch circuit 36 comprising a capacitor C fi con ⁇ nected between the saturable inductor switch S 3 and common, as well as a saturable inductor switch S 4. con- nected between the saturable inductor switch S 3 and the second node TP_.
• the saturable inductor switch circuit 36 also preferably comprises a biasing circuit 37 for the saturable inductor switch S., which comprises a S. bias power source and a choke L- connected in series with the bias winding of the saturable inductor switch. The S.
• bias power source is an adjustable direct current power supply connected so that bias current flows through the bias winding of the saturable inductor switch S 4 in the direction from left to right in Fig. 4.
• the choke o provides isolation of the S. bias power source from high voltage pulses produced by transformer action on the bias winding of the saturable inductor switch S..
• the various saturable inductor switch circuits 32 are cascaded to gradually narrow and sharpen the pulse fed to the pulse forming network 18 shown in Fig. 2.
• Each saturable inductor switch, S-, S., etc., shown in Fig. 4 is used as a "hold off” device. After a “hold off” period elapses, that is, the saturable inductor switch saturates, the discharge current from the associ- ated capacitor, C 5 , C g , etc., respectively, flows as though the saturable inductor were no longer there, and the capacitor in the following saturable inductor switch circuit 32 is charged. The output of the last saturable inductor switch circuit 32 is connected between the second node TP- and common.
• the power source 12' and cascaded magnetic charging circuit 30 shown in Fig. 4 can be substituted for the power source 12 and the charging circuit 16 shown in Fig.. 2.
• the output of the last saturable inductor switch circuit 32 shown in Fig. 4 is then connected in shunt across the saturable inductor switch S- shown in Fig. 2 in order to accomplish the substitution.
• the inclusion of the cascaded magnetic charging cir- cuit 30 provides an electrical excitation circuit 10 in which no spark gaps or thyratron switches are included. That is, the electrical excitation circuit 10 includes only a silicon controlled rectifier SCR. and saturable inductors as switches and therefore has an extended life- time. Furthermore, the saturable step-up transformer XFMR. provides isolation of the relatively low voltage power source 12' from the high voltages elsewhere in the electrical excitation circuit 10.
• the power source capacitor C. ' is charged from the 1 kV power source 12' through the choke L ' and the primary winding of the saturable step-up transformer XFMR. by current flowing through the primary winding in the direction from top to bottom in Fig. 4.
• the primary and secondary wind ⁇ ings of the saturable step-up transformer XFMR. are con ⁇ figured so that the direction of current flowing in the secondary winding is opposite to that in the primary winding.
• the charging diode D. ' blocks any significant charging of the charging capacitor C ' during the charging of the power source capacitor C. ' . After the power source capacitor C.
• the pulse generator circuit 20' produces a trigger signal which fires the silicon controlled recti ⁇ bomb SCR..
• Current flows out of the power source capa- citor C ' through the primary winding of the saturable step-up transformer XFMR. in the direction from bottom to top in Fig. 4.
• the current flowing through the primary winding of the saturable step-up transformer XFMR induces a voltage across the secondary winding of the saturable step-up transformer which causes a current to flow through the secondary winding in the direction from bottom to top in Fig. 4, as well as through the charging capacitor C ' and charging diode D 1 ' .
• the discharge of the power source capacitor C. ' takes 60 microseconds adjusted by the level of the bias current flowing through th bias winding of the saturable step-up transformer XFMR., at the end of which time the charging capacitor C-' is charged to a peak voltage of 25 kV.
• the saturable step-up trans ⁇ former XFMR saturates. Consequently, the charging capa ⁇ citor C-' discharges through the secondary " winding of the saturable step-up transformer XFMR and charges the capa ⁇ citor C- included in the initial saturable inductor switch circuit 34 with negative polarity on the first node TP. ' .
• the saturable inductor switch S, included in the satur- able inductor switch circuit 34 saturates, thereby trans ⁇ ferring the voltage on the capacitor C- to the capacitor Cg included in the next saturable inductor switch circuit 36.
• the saturable inductor switch S. included in the saturable inductor switch circuit 36 saturates, thereby transferring the voltage on the capacitor C g to the pulse forming network 18 shown in Fig. 2.
• Each cascaded saturable inductor switch circuit 32 shown in Fig. 4 provides pulse rise time sharpening and pulse width compression.
• the pulse width of the voltage transferred from the capacitor C ⁇ to the pulse forming network 18 shown in Fig. 2, for example, is 800 nanoseconds as compared to the 60-microsecond pulse width of the voltage transferred to the charging capacitor C- ' shown in Fig. 4.
• the pulse forming network 18 shown in Fig. 2 operates in the manner described earlier for imparting electrical energy to the laser load 14.
• the electrical excitation circuit 10 shown in Fig. 2 was adapted for exciting a xenon chlo- ride excimer laser load.
• the values and types for the various circuit elements were selected as shown in Table I.
• the capacitors are preferably discrete ceramic capa ⁇ citors.
• water line capacitors can be used, although high voltage arcs and corrosion can occur in water line capacitors.
• the saturable inductor switch S- and the magnetic diode charging inductor L_ shown in Fig. 2 can be struc ⁇ tured as illustrated in Fig. 5 and include an inductance element 38 which surrounds a nonconductive tube 40 sup ⁇ ported by a housing 42.
• the nonconductive tube 40 is constructed from insulative material, such as polyvinyl- chloride.
• the housing 42 can contain the remainder of the circuit elements of the pulse forming network 18 shown in Fig. 2 and is constructed from insulative material, such as polyvinylchloride Bias current flows through a conductor 44 which is disposed in the interior of the nonconductive tube 40 as shown in Fig. 5.
• Fig. 6 is a cross-sectional view along the line 6-6 shown in Fig. 5 and illustrates the structure of the inductance element 38 in greater detail.
• the inductance element 38 includes a plurality of cores 46 contained within an outer length of copper tubing 48. There can be, for example, 24 cores, such as 3C8 Ferroxcube cores included in the inductance element 38.
• a sheet of insu ⁇ lative material 50 such as polypropylene sheet, is wrapped around the cores 46. Kapton tape insulation is preferably applied to the end core 46, as indicated gen ⁇ erally by the numeral 52.
• An inner length of copper tubing 54 lies between the cores 46, which are wrapped with the insulative material 50, and the nonconductive tube 40.
• the outer copper tubing 48 is connected to the inner copper tubing 54 at one end (i.e., the left end in Fig. 6).
• the outer copper tubing 48 is connected at the other end (i.e., the right end in Fig. 6) to one of the nodes TP_ or TP.. of the pulse forming network 18 shown in Fig. 2.
• the inner copper tubing 54 is connected at the other end (i.e., the right end in Fig. 6) to common.
• the bias winding in Fig. 6 is illustrated as a .single turn bias supply conductor 44.
• the saturable inductor switches S- and S. shown in Fig. 4 can also be structured as shown in Figs. 5 and 6.
• the electrical excitation circuit 10 as indicated earlier preferably includes a pre-ionization means 28 for conditioning the laser gas as shown in Fig. 2.
• the pre- ionization means is preferably an x-ray circuit as illus- trated in Fig. 7.
• the x-ray circuit comprises a length of anodized aluminum tubing 60 plated on the interior with a layer of gold, indicated by the numeral 62, as the anode, and carbon felt 64 as the cathode.
• a high voltage pulsed power source 66 is connected across the anode and cathode for producing x-rays which ionize the laser gas • between the -electrodes 26.
• the aluminum tubing 60 is evacuated to a pressure of 5 x 10 mm Hg during opera ⁇ tion.
• the pre-ionization means 28 shown in Fig. 2 can be a corona pre-ionization circuit as illus ⁇ trated in Fig. 8.
• the corona pre-ionization circuit com ⁇ prises a corona element 68, which is formed by an insu- lated conductor contained within a quartz tube, disposed near the uppermost of the electrodes 26, as well as an insulated conductor 70, which connects the conductor within the corona element 68 to common.
• the corona pre- ionization circuit does not require an additional power source.
• a laser incorporating an electrical excitation cir ⁇ cuit as provided by the present invention has many advan ⁇ tages over conventional pulse shaping networks which include switches such as multichannel arc switches (rail gaps), thyratrons, spark gaps, or ignitrons.
• the elec ⁇ trical excitation circuit according to the present inven ⁇ tion can be operated at high repetition rates and at high powers and voltages and will have a very long lifetime compared with conventional laser discharge circuits.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)
• Lasers (AREA)
• Generation Of Surge Voltage And Current (AREA)

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• 1985
  • 1985-10-18 WO PCT/US1985/002019 patent/WO1987002517A1/fr not_active Application Discontinuation
  • 1985-10-18 JP JP86502065A patent/JPS63501183A/ja active Pending
  • 1985-10-18 EP EP19860902263 patent/EP0243374A4/fr not_active Ceased

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