US3508157A

US3508157A - High-voltage pulse generator

Info

Publication number: US3508157A
Application number: US663820A
Authority: US
Inventors: Ralph C Mobley
Original assignee: RALPH C MOBLEY
Current assignee: RALPH C MOBLEY
Priority date: 1967-08-28
Filing date: 1967-08-28
Publication date: 1970-04-21
Anticipated expiration: 1987-04-21

Description

April 2l, 1970 R. c. MoEaLEY,

HIGH-VOLTAGE PULSE GENERATOR 2 Sheets-Sheet 1 Filed Aug. 28. 1967 IAAIAAL vlvvvvv l n I I I INVENTOR. RALPH c. MQBLEY .EDUK DmZDF O IL w.

! BY GRAY, MASE & DUNSON ik ATTORNEYS Em 472mm April 21,1910 R. C. MOBLEY C 3,508,157

HIGH-VOLTAGE PULSE GENERATR Filed Aug. 28. 1967 2 Sheets-Sheet 2 I6 2 n/ I I v 2 3 4 33 SELF SECONDARY INVERTER AMPLIFIER ExCITATIoN EMISSION AND TUBE CIRCUIT TUBE CIRCUIT AM PLIFIER CIRCUIT SPLIT B, IDI-:LAY 'u LINE, 34 *200W I'I 'I (A, 'v' n'.

SELF SECONDARY INVERTER AM PLI FI ER EXCITATION EMISSION AND TUBE CIRCUIT TUBE CIRCUIT AMPLIFIER CIRCUIT IL T FIO. 2

FIG. 3

E 2 INVENTOR RALPH c; M o B LEY BY GRAY, MASE & DU NSON ATTORNEYS United States Patent O M 3,508,157 HIGH-VOLTAGE PULSE GENERATOR Ralph C. Mobley, 2585 Stoodleigh Drive, Rochester, Mich. 48063 Filed Aug. 28, 1967, Ser. No. 663,820 Int. Cl. H03k 5/159 U.S. Cl. 328-56 22 Claims ABSTRACT F THE DISCLOSURE An excitation circuit provides one narrow low-voltage positive pulse per microsecond. The pulses are amplified and fed to one end of a delay line. A voltage source is connected to a radio frequency choke at the other end of the delay line where pulses are reected in phase with subsequent pulses, thus reinforcing them to substantially higher voltage than that of the voltage source. The circuit can be duplicated and connected in push-pull to opposite ends of the delay line.

Cross-reference to related application Apparatus according to this invention is especially useful for supplying pulses needed in charged particle bunchers as disclosed and claimed in the copending United States patent application of the present inventor, Ser. No. 542,361, filed Apr. 13, 1966.

Background of the invention This invention relates to pulse generators for supplying short high-voltage pulses at high repetition rates. Such pulses, suitable for the charged particle bunchers mentioned above, were not obtainable from pulse generators heretofore available in a form sufficiently light, compact, and eicient, to be useful in the high voltage terminal of electrostatic particle accelerators.

Summary of the invention Typical apparatus according to this invention for providing short, high-voltage, periodic pulses comprises:

(a) means for generating short periodic pulses of lower voltage,

(b) means for amplifying the pulses, and

(c) a delay line, connected at one end to a choke and a high-voltage power supply and at the other end to the amplified pulses, the amplified pulses being refiected in phase from each end of the line and reinforced to substantially higher voltage.

The means (a) typically may include a self excitation circuit to which the amplified pulses are fed back, a secondary emission tube circuit, and a pulse inverter; and may also amplify the pulses.

The means (b) typically comprises an amplifier tube operated with approximately zero grid bias, `and provides positive pulses to the delay line.

The duration of each pulse preferably is a small fraction, such as about 0.1 to 0.15 of the period between successive pulses.

The length of the delay line should be such that a pulse moving along the line returns to its point of entry to the line simultaneously with the next pulse to enter the line. The choke 'at the end of the line typically is a radio frequency choke. An inductance may be connected between the amplifying means (b) and the other end of the delay line.

A preferred form of the delay line comprises a coil of wire wound on a rod of polystyrene or other non-magnetic insulating material and encased in a tube of polyethylene or other non-magnetic insulating material, having an outer sheathing of individual axially oriented wires that are insulated from each other, except at one end Patented Apr. 2l, i197@ Brief description of the drawings FIG. 1 is a schematic diagram of typical apparatus according to the present invention.

FIG. 2 is a similar diagram of another such apparatus.

FIG. 3 is a largely schematic view, partially cut away, of a delay line and associated structure according to this invention.

Description of the preferred embodiments FIG. 1 shows a typical embodiment of the invention as designed for generating high-amplitude pulses at a highrepetition rate. Some typical voltages are included for convenience to the reader, but the figure is largely schematic. Pulses at the point 1 from either an external 1 mc./ sec. sine wave signal generator (not shown) or the self excitation circuit 2, trigger the secondary emission tube trigger circuit 3 to provide short positive pulses of 0.1 microsecond or somewhat shorter duration and about volts peak. Each pulse is amplified and inverted by an amplifier 4 to produce a negative pulse of several hundred volts and of about 0.1 microsecond duration. The pulse travels through a small D.C. blocking capacitor 5 to the control grid of an amplifier tube 6. The grid is grounded through a radio frequency choke 7 for operation at approximately zero Ibias. The pulse applied to the control grid of the amplifier tube 6 drives the tube firmly into cutoff causing a positive pulse at the plate 31. The lamplifier tube 6 is normally operated in a zero bias condition, as indicated, with current limited by a rather low voltage at the screen 9. This is necessary to avoid excessive plate dissipation. The voltage drop across the tube y6 While on (between pulses) is low (ideally zero), and the plate dissipation is therefore also low.

The positive pulse at the amplifier tube plate 31 is sent through an inductance 10 and down a delay line 11. The inductance 10 offsets the output capacitance of tube 6 and substantially increases the peak Voltage of the amplified pulse. The pulse is reiiected at the other end 12 of the delay line 11 without change of sign. This is because the impedance of the radio frequency choke 8, connected thereto, is very high (about 100,000 ohms) compared to that of the delay line 11 (about 1000 ohms) at the approximately l to 10 mc./ sec. basi-c frequency components of the pulse. The delay line 11 therefore appears to be open ended at the RF choke 8 as far as the pulse is concerned, and reflection does not change the polarity of the pulse. The reflected pulse travels back toward the lamplifier tube 6 arriving there just as the next pulse from the drive circuit again cuts off the tube 6. The new pulse from the tube 6 reinforces the refiected pulse until the output pulse is larger than the B+ voltage at the point 13, ideally by the ratio of the trigger circuit repetition period (about 1 microsecond) to the pulse period (about 1A; microsecond) or 8 2000 volts=16,000 volts. This assumes square topped pulses and ideal components. In practice a multiplication of 6 (12,000 volts for a B+ of 2000 volts) has been achieved. The actual observed pulse 14 is about s microsecond long and in the 1/s microsecond interval is reasonably well approximated by one-half cycle of sin w. The waveform shown is a limiting one for obtaining maximum pulse output with minimum pulse width from the pulser. If the driver pulse is made less than ls microsecond, a reduced pulse height output results without substantial change in waveform, although it does become somewhat shorter than 1/s microsecond. If the driver pulse is |widened somewhat beyond 1A microsecond, a flat top develops in the output pulse giving a wider total pulse of substantially the same height, but with the rise and fall of the pulse remaining unchanged from that shown in FIG. 1. These variations are a function of the line-tube combination and are imposed by the coarseness of the winding of the delay line 11. Nearly rectangular pulses can be obtained (i.e., a delay line ten times as long amplifying 1 microsecond pulses at 100 kc./sec.), but in this embodiment an essentially half sinusoidal wave form is obtained. The theoretical maximum amplitude of the pulse is arrived at by assuming the system to be lossless, including the amplifier tube 6 which is assumed to be a perfect switch (i.e., when it is on there is no voltage drop across it and when it is off it looks like an open circuit). Under these conditions the RF choke 8 between the end point 12 and the voltage at 13 is just an energy storage element, storing energy when the tube y6 is on and delivering it to the pulse during reflection at the end point 12. In a lossless RF choke 8 these energies must be equal. If we assume a square topped pulse of duration t (Ms microsecond), the period between pulses to be T (1 microsecond), and the pulse amplitude to be P, then the current in the choke 8 (assuming that it starts at zero) will reach the value =V(T-t)/L from the end of one pulse until the arrival of the next pulse at the end point 12, where T-t=% micro-second, V is the power supply Ivoltage (B+) and L is the inductance of the choke. The energy E stored in the choke 8 will be (Lz'2) /2 or E: V2(Tt)2/2L. Since the choke current will again drop to zero, at the end of the pulse all of this energy will be delivered to the pulse. Assuming the system has a high Q (i.e., ratio of inductive reactance to resistance at the pulse repetition rate) and that the pulse amplitude does not change significantly during reflection at the point 12, then in terms of the pulse parameters this energy would be E=[-P-V]2t2/2L (by substituting pulse parameters in the immediately preceding equation). Equating these two expressions,

Solving for P using the binomial theorem:

P=Vi vwrugy-z] V2 In a typical pulser, T/ t-8 so PQjSV would be the theoretical pulse amplitude output. Since the current drops linearly to zero by the end of the pulse and rises linearly to i=V(T-t)/L between pulses, the average current will be:

. WT-t) ELV- For a choke 8 of 10 millihenries and a Voltage V at point 13 of 2Q00 volts, this gives Current in this ideal case would be set by the magnitude of the choke inductance and the voltage supplied at the point 13. Pulse power delivered through a very small capacitor 15 to an external circuit would be iav V, or in this case 87x10-3 amps 2000 volts, equals 175 watts.

The self-excitation circuit 2 shown in FIG. 1 receives a small sample of the output pulse 14, fed back through a very small condenser 16 (approximately 1jr/lf.) and uses it to excite a resonant circuit made up' of a variable capacitor 17 and an inductance 18. The resonant circuit is tuned to approximately 1 mc./sec., and has an output of several hundred volts. Because of the bias developed across the resistor 19 and the capacitor 20 in the grid circuit of tube 21 due to grid current drawn when the tube 21 is subjected to such a large drive, the plate current of the tube 21 flows in short sharp spikes. The negative plate swing of tube 21 is limited by a catcher diode 22 to about 10 volts. This prevents overdriving the tube 23 and results in a clean rapidly rising positive pulse on the plate of the tube 23 suitable for properly triggering the following secondary emission tube 24. The pulse also is constant in amplitude for any output from the pulse generator over a few hundred volts. The tuning condenser 17 adjusts the triggering phase of the secondary emission tube 24. Proper adjustment is indicated by a maximum output from the pulse generator. The pulse generator as here described is not self-starting. Changing the bias at the point 25 from -30 volts to about 10 volts momentarily, however, will make the secondary emission tube a simple amplier for that time and hence let oscillation build up. This can be done either manually or automatically with suitable circuitry (not shown).

An alternate method of excitation is to insert about 10 volts of 1 mc./sec. R-F from an external signal generator (not shown) at the point 1. Adjustment of the signal generator frequency so that its period is equal to the time for the pulse to travel down the line and back is critical, however. 1n the case of self excitation it is not, and this is a very desirable advantage of self excitation.

DELAY LINE, FIG. 3

A critical part of the pulser is the delay line 11. As an example, for a 1 mc./sec. repetition rate the delay time from the plate 31 of the amplifier tube 6 to point 12 and back must be one microsecond. The effective length of the line must therefore be 0.5 l06 second. It must also be a high-impedance, high-voltage and high Q delay line. A suitable delay line as shown in FIG. 3 was constructed by taking a polystyrene rod 27 about 3 cm. in diameter, and winding it with a layer 28 of closely spaced turns of number 18 enameled wire. A coil 28 of Wire 39.5 inches long was thus formed over the polystyrene rod 27. Next a tube of polyethylene 29 with its wall onequarter inch thick was slipped over the rod 27 making close contact with the coil 28 and extending out past the ends of the coil 28. Because current traveling down the coil 28 generates a magnetic field reaching out from the coil 28 and capable of linking any closed circumferential conducting loop to produce currents and hence losses in that loop, the ground plane 30 needed to provide capacitance to ground from the coil 28 through the surrounding polyethylene insulation 29, was constructed of a sheath of insulated axial wires 30 completely covering the outside of the polyethylene tube 29. The wires were insulated from each other at one end 31 and connected together only at the other end 32, at a point removed from the reach of the magnetic field of the coil 28. This construction provides a very satisfactory high-voltage delay line having a high Q. Of course, various other non-magnetic insulating materials could be used instead of polystyrene and polyethylene.

PUSH-PULL CIRCUIT, FIG. 2

FIG. 2 shows how the single ended design of FIG. 1 can be converted into a double ended push-pull arrangement with a consequent doubling of the power output over that of the single ended circuit. For some applications such as the charged particle buncher, it can be used to supply two balanced outputs with pulses from the two outputs alternating 180 out of phase. Since the power output capability of the system is limited by the plate dissipation of the amplifier tube 6 in the amplifier tube circuit 32, two amplifier tubes 6 give twice the power handling capability of one.

FIG. 2 shows two amplifier tube circuits 33' and 33" and their associated electronic circuits. The same reference numeral used in FIG. l for each component is used also in FIG. 3 with a prime or double prime added. The associated electronic circuits driving the two amplified

tube circuits

33 and 33" consists of two self-

excitation circuits

2 and 2", two secondary emission

tube trigger circuits

3 and 3" and two

amplifiers

4 and 4, respectively. The current drawn from the two amplifier tube circuits 33' and 33 is twice that of one tube alone. With twice the current supplied to the split delay line circuit 34 the inductance of the RF choke 35 must be one-half that used in the single system of FIG. 1. The analysis of this system is exactly the same as that for the single-ended system in FIG. 1, except that With the choke 35 in the middle of the delay line the pulse passes the choke 35 twice as often as the pulse reaches the choke 8 in FIG. 1; once on the way from 33 to 33" and once on the way back, for every complete cycle. The

inductance

10 and 10 are the same as the inductance 10 of FIG. 1, and are inserted to offset the output capacitance of each amplifier tube 6 and substantially increase the amplitude of the output pulse.

While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. It is not intended herein to mention all of the possible equivalent forms or ramifications of the invention. It is to be understood that the terms used are merely descriptive rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention.

I claim:

1. Apparatus for providing short, high-voltage, periodic pulses comprising:

(a) means for generating short periodic pulses of lower voltage,

(b) means for amplifying said pulses, and

(c) a delay line connected at one end to a choke and a high-voltage supply and at the other end to the amplified pulses, said amplified pulses being refiected in phase from each end of said line and reinforced to substantially higher voltage.

2. Apparatus as in claim 1, wherein means (a) includes a self-excitation circuit.

3. Apparatus as in claim 2, wherein said amplified pulses are fed back to said self-excitation circuit.

4. Apparatus as in claim 1, wherein means (a) includes a secondary emission tube circuit.

5. Apparatus as in claim 1, wherein means (a) includes a pulse inverter.

6. Apparatus as in claim 1, wherein means (a) also amplifies the pulses.

7. Apparatus as in claim 1 wherein means (b) comprises an amplifier tube operated with approximately zero grid bias.

8. Apparatus as in claim 1, wherein the amplified pulses provided yby means (b) are positive pulses.

9. Apparatus as in claim 1, wherein the duration of each said pulse is a small fraction of the period between successive pulses.

10. Apparatus as in claim 1, wherein the duration of each said pulse is about 0.1 to 0.15 of the period between successive pulses.

11. Apparatus as in claim 1, wherein the length of said delay line is such that a pulse moving along said line returns to its point of entry to the line simultaneously with the next pulse to enter said line.

12. Apparatus as in claim 1, wherein said choke is a radio frequency choke.

13. Apparatus as in claim 1, wherein an inductance is connected between said amplifying means (b) and said other end of said delay line.

14. Apparatus as in claim 1 wherein said delay line comprises a coil of wire wound on a non-magnetic insulating rod, said coil being encased in a non-magnetic insulating tube having an outer sheathing of individual axially oriented wires, said wires being insulated from each other except at one end where they are connected together, the connected wire ends being located outside the effective magnetic field created by said coil.

15. Apparatus as in claim 14, wherein said rod comprises essentially polystyrene.

16. Apparatus as in claim 14, wherein said tube cornprises essentially polyethylene.

17. Apparatus as in claim 14, wherein the Q of said delay line is at least about at the repetition rate of said pulses.

18. Apparatus for providing short, high-voltage periodic pulses comprising:

(a) means for generating a first series of short periodic pulses of lower voltage,

(b) means for generating a second series of short periodic pulses of lower voltage,

(c) means for amplifying said first series of pulses,

(d) means for amplifying said Second series of pulses,

and

(e) a delay line connected in the center to a choke and high-voltage supply, at one end to the first series of amplified pulses, and at the other end to the second series of amplified pulses, said amplified pulses being reected in phase from each end of said line and reinforced to substantially higher voltage.

19. Apparatus as in claim 18, wherein each means (a) and (b) includes a self-excitation circuit.

20. Apparatus as in claim 19, wherein said amplified pulses are fed back to said self-excitation circuits.

21. Apparatus as in claim 18, wherein each means (a) and (b) includes a secondary emission tube circuit.

22. Apparatus as in claim 18, wherein the length of said delay line is such that a pulse moving along said line reaches an end of the line simultaneously with a pulse entering said line.

References Cited UNITED STATES PATENTS 2,587,741 3/1952 Libois 328-56 XR 2,820,909 1/1958 Plouffe 307-106 FOREIGN PATENTS 809,580 2/ 1959 Great Britain.

JOHN S. HEYMAN, Primary Examiner S. T. KRAWCZEWICZ, Assistant Examiner U.S. Cl. X.R.

Po-1o5o (5/69) Patent No.

REC'NN Dated April 2l. 1970 Inventor(s) Ralph C. Moblev It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 50, that portion of the formula reading- /LT T 2 it should read :E

UN UW 3EME@ 1 ST9- (SEL rm E. o. 5% @nung f m W; one@ of W