GB2178203A - Regulating electron beam emission - Google Patents

Abstract

A system for regulating the electron beam emission of an accelerator has a filament whose temperature is controlled by pulse width modulation of the current flowing to the filament (8). A variable impedance transformer (4) serially connected between the filament and RF power source (4) is electronically switched between high and low impedance at a rate faster than the thermal time constant of the filament whereby the variations of current flowing in the filament are averaged by the thermal response time of the filament so that the temperature and electron emission level of the filament remain substantially constant. The impedance switching signal is generated in response to changes in current flowing in the high voltage coils of the accelerator. The pulse width can be modulated over a wide range to vary the electron current over a range greater than 2:1. The switching signal is transmitted across the high voltage gradient of the accelerator by an optical link including an LED coupled to a phototransistor (15) by a light pipe (16). <IMAGE>

Description

SPECIFICATION Pulse width modulated beam control with optical link The present invention relates to control of electron beam emission systems. More particularly, the present invention relates to control of electron emission by pulse width modulation of an impedance to the flow of current to a thermonic filament.

The emission beam of an electron accelerator needs to be controlled in order to irradiate at the proper desired irradiation rate.

Typically the beam emission level is adjusted by varying a resistance in series with the filament heating current.

The prior art regulation, as illustrated in Figure 6, comprises an electron gun filament connected across a secondary widing of a transformer. RF current is supplied to a primary winding of the transformer through two variable resistors connected in series. A choke and zener diode are connected in parallel across the input terminals of the RF supplied to the system.

Both variable resistors are operated by slow speed reversible control motors located at the ground end (low negative voltage end) of the power supply. Mechanical action is transmitted across the high voltage gradient of the accelerator by means of dielectric drive shafts between the motors and their respective variable resistors. One variable resistor is used for control of the emitted electron beam intensity.

The second variable resistor is used for setting the range of operation of the first, variable resistor in order to provide the greatest utilization of variable resistor adjustment, hence better resolution.

A shortcoming of this system exhibits itself when one attempts to operate and control beam emission over a wide range of energy (acceleration voltage). The current level across the choke varies with machine energy. The current level also varies with changes of operating parameters (at any operating energy) as load (beam current) is raised or lowered due to power supply regulation characteristics.

The zener assembly is only a clipping device and cannot guarantee a constant RMS source of heating power since its operating point varies from little or no clipping to a total square wave output due to severe RF drive levels at high energy and high beam.

Due to this rather unstable power source and the slowness of the variable resistor control motor's reaction time, (approx. 1 RPM), there are some accelerators which exhibit a tendency to have beam "run away" conditions. This occurs when the RF operating level automatically increases (due to D.C. regulation needs) at a rate faster than is controllable by the rheostat-motor drive system.

Further beam current is variable only over a 2:1 ratio with this form of control, whereas in some instances ratios of 3:1 are required.

Runaway problems have been eliminated and a wider operating range of beam current is attained by the present invention wherein a fast acting control loop regulates filament heating current.

Some other prior methods of regulation varied an impedance in series with the thermonic filament heating circuit. These methods often relied on linear variation of the impedance as seen in patents to M.R. Coe &num;2,617,045 and to Clapp and Bernstein &num;2,810,838. Linear control devices have inherent drawbacks. Linear control devices waste control power in the control element directly, and in the amplifier driving them, due to the fact that the devices have potential drop present with simultaneous load current flowing.

These systems therefore consume power whenever current is flowing even if no corresponding level of electron emission is produced.

The present invention eliminates these problems by eliminating the current flow when the impedance is pulsed in the high impedance state thereby eliminating power dissipation.

The circuit of Lauritzen & Boucher patent 3,567,995 uses a form of pulse modulation, but examination of the circuit and graphical data indicate a system which has a gain or transfer characteristic which is nonlinear, that is one that utilizes a sine function. This nonlinear gain caused by the sine-wave summing can lead to oscillation and instability in many applications especially where the curve tends to run into the peak area.

Also, the circuit of Lauritzen et al. infers line frequency operation. Line frequency pulse width modulation produces modulation of the filament temperature in X-ray tubes (and accelerator guns). While the average temperature and milliampere meter readings may appear acceptable, oscillograph readings indicate beam intensity modulation ripple approaching 15 to 20%. This type of operation is totally unusuable in an electron beam system of high power levels with beam current variations of greater than 2:1.

The present invention also avoids these difficulties by enabling a higher switching frequency to be utilized, thereby stabilizing the filament temperature due to smoother averaging of the higher frequency current by the thermal time constant of the filament and hence the electron emission level is stabilized.

By implementing a pulsed switching instead of sine wave summing the present invention further avoids oscillation and instability of the electron beam.

The present invention comprises a control circuit and system for better regulation of the emission level of an accelerator. In the preferred embodiment of the present invention the quantity of electrons which are emitted by a thermonic filament is regulated by pulse width modulation of the current supplied to heat the filament. The temperature of the filament is maintained essentially constant by regulated pulses of current flowing through the filament at a frequency high enough to allow the thermal time constant of the filament to average the pulses of current to maintain an essentially constant filament temperature.

The current to the filament is regulated by changing the impedance of the primary winding of a transformer placed in series between the filament and its power source. By passing pulses of current through the secondary winding of the transformer the impedance of the transformer is switched rapidly between high and low allowing either no current or full current to flow respectively.

A nominal 100 KHZ is utilized as the carrier or power source for the filament, and a switcher frequency in the 300 Hertz or greater region is utilized. The thermal time constant of the average filament is long enough to essentially average the heating effect of the pulses of current whereby little thermal ripple is evident at these high frequencies and therefore uniform electron beam current is produced.

The switching frequency used is important because the thermal emissivity characteristics of typical turgsten emitters is such that the emission varies as the temperature to the 4th or higher power in most cases.

The utilization of pulsed current provided by the switching of an impedance from high to low at a significantly high frequency greatly enhances the performance of the accelerator in many ways. By establishing a stable beam condition, beam current ranges previously unattainable of 3:1 or greater are accomplished by the beam current regulation of the present invention. The level of the emitted electron beam can now be varied over a wider range, which is essential for many applications, without beam runaway or development of unstable or inconsistent beam conditions.

The switching from high to low impedance is accomplished by the following sequence. A phototransistor is switched on by light pulses and completes the circuit from a DC power source to the base of an NPN transistor.

When the phototransistor conducts, current from a DC power source saturates the base of the NPN transistor whose collector and emitter complete the circuit of the DC side of a full wave bridge. The AC side of the bridge energizes the secondary winding of the transformer at a given set rate, thereby switching the primary winding of the transformer between high and low impedance.

The pulses of light which activate the phototransistor are generated by summing a feedback signal from the output current of the high positive voltage terminal with a symmetrical triangular wave. The feedback signal is an analog signal derived at the ground end of the power supply and represents the output current by means of a "viewing" resistor voltage drop in the positive (ground) end of the supply. The grounded viewing resistor therefore provides a true representation of the terminal output current. The analog signal is cable connected as a voltage source to the pulse generating circuit.

An even clock pulse of 50% on 50% off in conjunction with a square wave buffer amp generates a square wave. The square wave is integrated to produce a symmetrical triangular wave which is then summed with the feedback voltage and the sum is biased to oscillate between 0 volts and a set negative voltage.

Any change in the feedback voltage causes the triangular wave to peak above zero for a period of time proportionate to the amount of change in voltage. While the peak is maintained, current is supplied to a light emitting diode which emits light through a light tube across the high voltage gradient of the accelerator. The light impinges on the phototransistor resulting in the change in inductance described earlier.

The frequency of the triangular wave remains constant in the 300 Hertz range. The length of time during each cycle for which the LED is excited is proportional to the change in beam current detected from the analog feedback signal. The impedance is therefore switched high or low for a period of time directly proportional to the change in beam current detected. The degree of inpedance is not varied linearly with the change in beam current only the length of time in a high or low state is dependant on beam emission levels. The circuit of the present invention produces a linear change in pulse width across the entire operating range of the feedback signal in order to maintain a fixed gain system.

The present invention uses a form of "Class-D" switching, whereby the load current flows during the time that the switching device is "closed", having little voltage drop, hence little dissipation. Conversely, when the switcher is "open", full voltage drop is present, but no load current flows, hence little dissipation. This great power saving allows a terminal located, RF driven power supply to be utilized as a filament current source.

An important design feature of the present invention is the use of the switching transformer between the RF load circuit and the bridge rectifier and transistor base/emitter circuit for shielding and ease of logic coupling, but also, the transformer isolates the transistor from fault currents which can occur when the accelerator tube or power supply experiences a high voltage discharge or spark.

The transformer also (due to its step-down ratio) reduces the transistor applied voltage which makes the transistor very insensitive to over voltage transients.

For a further understanding of the nature and objects of the present invention, reference should be had to the following detailed description, taken in conjunction with the accompanying drawings in which like parts are given like reference numerals and wherein: Figure 1 is a diagrammatic view of the complete system of the preferred embodiment of the present invention.

Figure 2 is a schematic diagram of the filament heating control circuit.

Figure 3 is the first segment of a schematic diagram of the pulse generating circuit.

Figure 4 is the second segment of a schematic diagram of the pulse generating circuit.

Figures 5 A, B & C illustrate exemplary patterns of electrical pulses fed through the light emitting diode.

Figure 6 is a schematic diagram of a previous method of regulation of filament heating current.

An accelerator with the control circuit of the present invention is illustrated in Figure 1. The preferred location of components is shown where the filament heating control circuit, as better illustrated in Figure 2, is located within the high negative voltage dome 35 of the accelerator vessel. The light emitting diode (LED) 31 is located at the ground end of the accelerator and receives a pulsed signal from the pulse generating circuit, Figures 3 and 4, located outside the accelerator vessel.

Referring to Figure 2, the filament transformer 1 is connected across the RF source 2 through the primary 3 of the impedance transformer 4 which presents a high impedance to current flow when there is no base drive on the switching transistor 5.

The RF current limited source 2 provides a voltage drop across the RF choke 11. The zener regulator 10 clips or limits the available voltage across the choke when the impedance transformer 4 presents a high impedance to the available current. Capacitor 9 provides crude current adjustment, actuated by remote motor 12.

The impedance transformer secondary 6 is connected through a full wave bridge 7 to the collector and emitter of switching transistor 5.

Whenever no base drive is present in switched ing transistor 5, the collector-emitter junction is effectively open circuit. The load presented to secondary 6 of impedance transformer 4 through the bridge 7 is also effectively zero or open circuit. The primary 3, therefore, presents a high inductive reactance to the RF excitation source 2 applied through it to the filament transformer 1. The filament 8 is not energized.

When saturating base drive is present on switching transistor 5, the reflected impedance on the impedance transformer 4 is very low, which now enables relatively high excitation current to flow into the filament transformer 1. The filament 8 in the acceleration tube 20 now glows very brightly, emitting a very intense electron beam into the acceleration field of the acceleration tube 20.

With proper logic, a series of repetitive rectangular pulses of constant frequency, as typically illustrated in Figure 5 is generated and fed into the base driving circuit of transistor 5. By adjusting or controlling the duty cycle of these pulses, the average temperature of the filament 8, and hence the emitted beam intensity is controlled. This method of control of the time duration of the positive portion of the pulse is pulse width modulation. A logic circuit, illustrated in Figures 3 and 4, is used in the preferred embodiment of the present invention to create electric pulses which drive LED 31 which emits pulses of light to energize phototransistor 15, which in turn base drives switching transistor 5 in equivalent pulses.

The duration of the positive portion of the pulse (the pulse width W) as shown in Figure 5, is directly proportional to the amount of filament 8 heating required to achieve the proper filament 8 temperature for the desired level of electron emission. The width of the pulse W determines the length of time current flows to the filament and therefore determines filament average temperature. The level of impedance has only two values and therefore the degree of impedance can not be varied to regulate the current flowing to the filament 8.

The current is only regulated by the duration in time of the positive portion of the pulses.

RF energy 2 is used to energize the regulated power supply 13 over the operating range of the accelerator. This power supply 13 is used to power the phototransistor 15 (mechanically engaged in the interior of hoilow light pipe 16), and the pre-amp-driver 14, for the saturating base drive of switching transistor 5.

Light pipe 16 is a tube or pipe with a reasonably smooth, highly reflective internal bore which has a dimension slightly larger than the O.D. of the phototransistor 15 and the light emitting diode 31 LED 31 is a light emitting diode with optical wavelength of transmission closely matching the phototransistor 15 peak sensitivity frequency. This enhances the signal to noise rejection characteristics.

Electrically energizing LED 31, produces a source of energy. This light (photon) energy radiates along the highly reflective interior of the light pipe 16 and travels the length of the light pipe 16 assembly. At the high negative voltage terminal end of light pipe 16 the infrared photon energy impinges on the photo transistor 15, and provides sufficient base drive to close the collector emitter junction of the phototransistor 15. The phototransistor 15 emitter output drives amplifier 14 which in turn produces saturation base drive on switching transistor 5.

The flow of current through the collector/emitter junction of switching transistor 5 across the impedance transformer secondary 6, through the full wave bridge 7 causes the primary 3 of transformer 4 to appear as a low impedance and the accelerator filament heating circuit is energized to full emission levels for a period of time equal to the positive pulse width. Control of the excitation time of LED 3 controls the "on" time of the filament heating circuit.

Referring now to Figures 1, 3 and 4, the pulse generating circuit consists of clock pulse subcircuit 25 producing a voltage pulse of 50% duty cycle, at a frequency of approximately 350 Hertz. This frequency is utilized so that the heating and cooling time constant of the electron gun filament assembly would integrate the on-off pulse heating signals and effectively produce a smoothly controllable ripple free average temperature determined by the input signal at 23 set by the control logic.

This form of heating current control also allows the beam current to be varied over a wider range of operating currents without beam runaway or fluctuation. The present invention allows the emitted electron beam current to be set over a 3:1 or greater ratio while maintaining a supply of current at a substantially high frequency to maintain a substantially constant filament temperature.

The 50% duty cycle pulse from IC1 is adjusted for symmetry by clock pulse subcircuit 25 and fed to a buffer amp subcircuit 26 which converts the clock pulse peak to a level square wave 36.

Buffer amp AR4 of buffer subcircuit 26 is furnished with a "back-to-back" series zener Z1 to guarantee equal, bipolar swing and symmetry of wave shape at the output of subcircuit 26.

The buffer subcircuit 26 is fed to integrator subcircuit 27 which is designed to produce a symmetrical triangular wave 37 and is used to drive summing subcircuits 28 and 28'.

AR4 and AR5 are furnished with gain and trimming potentiometers R20, R24 and R26 for bipolar balance and setting signal levels.

AR6 and AR7 are furnished with trimming potentiometers R27 and R37 which are used to set the triangular wave 38 at the output of the summing subcircuits 28 and 28' entirely below the zero base line. With the triangular output 38 entirely below zero base line, diode D-3 is in the non-conducting state. With D-3 nonconducting there is no drive of the final amp subcircuit 40. This results in no current to energize the LED 31 and therefore no beam current is generated.

Any change of input control voltage to summing subcircuit 28, coming from the error amplifier (AR3), will cause a shift upward in the triangular wave 38 at the output of the summing subcircuit 28'. As the peak of the triangular wave 38 elevates above zero, the conduction of D-3 occurs. A pulsed output such as shown in Figures 5A, B and C is produced by final amp subcircuit 40 where the pulse width lasts for a time period determined by the portion of triangular wave 38 which lies above zero. It is possible to produce from zero to 100% duty cycle pulses across D-3 into the final amp subcircuit 40 by the bias shift at AR6 as determined by error amp AR3 output conditions.

The input control voltage to summing subcircuit 28 is generated from a reference signal at 23 generated by control logic combined with a feedback signal 39 from the high positive voltage ground end of the accelerator high voltage coils 18. An initial beam-level-setting voltage generated by control logic set by the accelerator operator is input at 23. This causes a positive input to summing subcircuit 28. Depending on the value of this positive voltage, a given portion of triangular wave 38 peaks above zero and produces a pulse of given pulse width W thereby elevating the filament to a predetermined temperature and emission level.

The electron beam causes a change in the current within the high voltage coils 18 of the accelerator, which causes a corresponding change in feedback input 39. A change in feedback input 39 causes a corresponding change in the output of error amp subcircuit 24 and input into summing subcircuit 28. Consequently, a shift in triangular wave 38 and therefore a change in the pulse width of the pulses utilized to heat the filament. The system quickly achieves equilibrium and maintains a constant electron beam current through this feedback process.

The feedback signal is derived from a viewing resistor 33 connected between the high positive voltage end of the accelerator coils 18 and the accelerator local positive ground.

The terminal output current therefore flows through viewing resistor 33. The nongrounded end of resistor 33 is cable connected to the inverter subcircuit 22 where the polarity of the analog signal 39 is reversed.

This negative signal 41 from inverter subcircuit 22 is summed against the positive reference 23 supplied by the typical computer beam set point.

Error amp subcircuit 24 is comprised of amplifiers AR2 and AR3. Error amps AR2 and AR3 are used as high gain amplifiers for summing the signal developed at the input of AR2 by the combination of command or reference signal 23 representing electron beam intensity set point, and the feedback signal 41 arriving from inverter subcircuit 22.

Inverter stage AR1 is fitted with a "lead" circuit in order to enable the error amp AR2 and AR3 to "anticipate" rate changing summing conditions. Potentiometer R5 and capacitor C1 afford a convenient means to stabilize the SERVO control system with the least amount of overshoot. The 5.0 MFD Integration capacitor C2 and potentiometers R14 and R15 around AR3 provide sufficient adjustment to prevent oscillation in the control system.

The control motor 12 which adjusts variable capacitor 9 is computer controlled by the selected operating energy range of the accelerator, in order to bypass unusable RF current around the filament circuit when operating over a wide range of operating energy. This presents optimum operating conditions to the pulse width modulator transformer, by preventing core saturating currents at high R.F.

levels.

Claims (10)

1. A current stabilization system for an electron emission device comprising: a thermonic cathode filament, means for suppling current to said filament, feedback means monitoring variations in said electron emission level and producing a pulsed control signal of constant frequency and variable pulse duration, wherein the cycle time of said signal is shorter than the heat loss time constant of said filament, and means for receiving said control signal connected to means for varying said filament current in response to said signal.

2. A current stabilization system for an electron emission device comprising: a thermonic cathode filament, means for suppyling current to said filament, feedback means monitoring variations in said electron emission level and producing a pulsed control signal of constant frequency and variable pulse duration, wherein the duration of each of said pulses is linearly proportional to said monitored variations in said emission level, and means for receiving said control signal connected to means for varying said filament current in response to said signal.

3. The system of Claim 1 wherein, the duration of each of said pulses is linerly proportional to said monitored variations in said emissions level.

4. The system of Claim 1, or Claim 2, or Claim 3 further comprising; means for transmitting said control signal from its origin across said high voltage gradient to said current varying means, comprising light emitting, light transporting and light receiving means.

5. The system of Claim 1, or Claim 2, or Claim 3 wherein said feedback means comprises; feedback means monitoring current changes in said high voltage coils corresponding to electron emission level changes and producing a control signal which is a function of said variations in said coil current,

6. The system of Claim 1, or Claim 2, or Claim 3 wherein; said current varying means comprises a transformer wherein a primary winding of said transformer is serially conneceted between said current supply means and said filament, and wherein a secondary winding of said transformer communicates with said control signal.

7. The system of Claim 6, wherein; said primary winding is switched between high and low impedance by pulses of electricity in said secondary of said transformer corresponding to said control signal pulses.

8. A method for regulating the output of an electron emission device having a thermonic filament comprising the steps of: monitoring variations in the current of said electron emission, generating a pulsed control signal of a pulse duration proportional to said variations and cycle time shorter than the heat loss time constant of said filament, and varying the current to said filament in response to said signal.

9. The method of Claim 8 wherein; the step of varying the current to said filament comprises switching an impedance to said current between high and low.

10. A current stabilization system for an electron emission device, substantially as described with reference to Figures 1 to 5C of the accompanying drawings.