Pulsed magnet system considerations for repetitive operation of high-power magnetrons

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 38. NO. 4. APRIL 1991 79 I

Pulsed Magnet System Considerations for Repetitive Operation of High-Power Magnetrons

Daun Bhasavanich, Member, IEEE, and H. George Hammon, I11

Abstract-A tradeoff evaluation has been performed to specify a compact, low-loss pulsed electromagnet system for a repetition rate application of high-power magnetrons. Required magnetic field is 5 to 15 kG with 1% uniformity over the magnetron volume, and pulse repetition rate is of the order of 5 pps. To drive the magnets, a recirculating driver scheme which transfers energy back and forth between two capacitor banks has been identified as a viable low power loss approach. The circuit comprises a IO-kW switching power supply, a high- voltage switch network, 400-kJ capacitor banks, and magnet coils. Several recirculating drive systems operating in the voltage range of 10 to 50 kV and output currents with a pulse duration of = 10 ms are compared relative to their power losses, system size and weight, and their limitations at high pulse repetition rates.

I. INTRODUCTION IGH-POWER magnetrons are short-pulse, gigawatt-range H microwave sources operating in the frequency range of ap-

proximately 0.4 to 10 GHz. A uniform axial magnetic field of = 10 kG over the magnetron cavity volume is required for their operation. High-power magnetrons are, at present, single-shot or very-low repetition rate devices [ 11-[3]. Projected systems require operation at several pulses per second (pps) or higher. In this study we examine the magnet driver system with a pulse repetition rate on the order of 5 pps. The objective is to identify a magnet system capable of producing 5 to 15 kG with 1 % uniformity over the magnetron volume.

Although pulsed, dc, or superconducting magnets are suitable approaches for the production of high magnetic fields, superconducting and dc systems, with their attendant refrigeration and cooling requirements, are not considered in this study. A standard pulsed magnet driver employing a “crowbar” tech- nique on an energy storage capacitor bank is compared to po- tential lower loss pulsed systems. We focus on a “recirculation” magnet driver scheme which transfers energy back and forth between two capacitor banks to drive the magnet coils, minimizes the energy loss per pulse, and recovers most of the stored energy for the subsequent pulses.

The recirculating pulsed power schemes were examined with the aim of achieving low average power dissipation as well as low system size and weight. Selected systems are presented and discussed relative to the complexity and risk for the switching network.

11. TRADEOFF STUDY APPROACH AND REQUIREMENTS Required magnetic field intensity and volume, system power

consumption, capacitor bank sizes, switching voltages, and pulsed currents are the major variables that determine the size

Manuscript received July 9, 1990; revised September 21, 1990. This work was performed under contract to McDonnell Douglas Technologies, Inc. (MDTI), San Diego, CA.

The authors are with Physics International Company, San Leandro, CA 94571-0599,

IEEE Log Number 9042205.

I ’< *’ I

Waveguide Pori

+ +loons

Magnetron Output: Microwave I I

I ’ -1OkG fi,- time + Pulsed Magnet: BLor I magnet

Fig. 1. Magnet coil configuration and requirements.

TABLE I MAGNETRON MAGNET REQUIREMENTS

Nominal E, 10 kG

Spatial uniformity Magnetron volume

Range 5-15 kG 1 % over magnetron volume cylindrical: 7.5-cm diameter, 10-cm length in the axial direction

(approximated by a hollow cylinder shown in Fig. 1 center, on the axis of the Helmholtz coils)

1 % over the 100-ns microwave output duration

Temporal uniformity

Repetition rate 5 PPS Coil dimensions 20-cm minimum loop inner diameter;

2 coils, IO-cm minimum separation.

and weight of the overall system. For this study, magnetic field calculations were based on circular current loop formulations for a Helmholtz coil configuration. The tradeoff design objective was to minimize the combined size, weight, and power consumption of the system consisting of the power supply, pulsed power system, and the electromagnet. Cooling requirements were identified, but were not directly included in the study.

Fig. 1 shows a simplified sketch of a magnet coil configuration. The requirements for the axial magnetic field B, are given in Table I. The requirements on the size and the placement of the magnet coils reflect typical high-power magnetron housing bore and waveguide port dimensions.

00l8-9383/91/0400-0791$01 .OO 0 1991 IEEE

792 IEEE TRANSACTIONS ON ELECTRON DEVLCES. VOL. 38. NO. 4. APRIL 1991

switch on-- SP sc s1 SPSC

Fig. 2. Crowbar scheme for pulsed magnet

111. PULSED POWER APPROACHES

Figs. 2-4 show conceptual pulsed power approaches for repetitive magnet systems. The magnet coils are shown in the top portion of all the figures as two inductor coils and a resistor. The pulsed power circuit is represented by capacitor bank(s) and ignitron o r silicon-controlled rectifier (SCR) switch and diode networks. The switch networks are referred to in the figures as "SP"or " SN." The power supply is depicted as a bat- tery at a voltage V,,. The power supply is connected and dis- connected via switches ' ' S1 " or " S 2 " to charge the capacitor banks. Expected voltage and current waveforms are sketched below each of the circuits. Switching schedules for connection and disconnection of the power supply to the capacitor banks, and of the banks to the magnet coils are also shown at the bot- tom of the figures.

A "crowbar" circuit commonly used for a single-pulse operation of a magnetron's magnet is shown schematically in Fig. 2 . The power supply initially charges the capacitor bank to voltage V, . As the switch SP is closed, a sinusoidal current flows through the magnet coils. When the current reaches its peak value in a quarter of the period, and the voltage in the capacitor bank correspondingly approaches zero, the circuit is crowbarred by closing of the switch SC. The magnet coils' current then decays exponentially with a time constant given by the L / R ratio characteristic of the crowbarred circuit. L is predom- inantly the coils' inductance and R is the effective resistance of the coils and the crowbarred circuit. The magnet coils are therefore pulsed with a current of a rising sinusoid waveshape for one quarter period followed by an exponential decay. The crowbar circuit is lossy, as the stored energy in the capacitor bank is totally consumed in every pulsed operation.

High-power magnetron requires a relatively uniform magnetic field in a short pulse duration of tens of nanoseconds. The magnet driver timing is generally set such that the coils' current reaches its peak value at approximately the same time the electron beam is injected into the magnetron. The exact timing must take into consideration the diffusion time for the magnetic field through the walls of the magnetron's drift chambers, which are generally made of conducting hollow cylinders.

IV. RECIRCULATION C I R C U I T CONCEPT

The recirculation circuit, shown in Fig. 3, is a form of an energy recovery scheme in which the energy from one capacitor bank is transferred to the magnet coils and recovered in the sec-

switch on &--xPI( SP 52 SN S1 SP

Fig. 3. Half-wave energy recirculation pulsed magnet scheme.

s1 V

U

T T

switchon SP i - x A-x n--x S N M a--*

SI- n-

Fig. 4. Full-wave energy recirculation pulsed magnet scheme

ond capacitor bank for use in subsequent pulses. The power supply is reconnected between pulses to restore the energy-re- covering capacitor bank to the required energy. In repetitive operations, one capacitor bank is initially charged to V,. When the switches are fired, most of the initial energy is transferred through the inductance of the magnet coils to the second capacitor bank.

Following the first pulse, the voltage on the second capacitor bank rises to Vo - AV,,,,, where AV,,,, is the voltage drop due to losses in the magnet coils, switches, and connecting cables. The power supply is reconnected to replenish the second capac-

BHASAVANICH AND HAMMON: PULSED MAGNET SYSTEM

itor bank to the full voltage V,. The pulsed circuit is now ready for the next operation. An energy loss during the pulse of less than 5 % of the stored energy is easily realizable. In contrast to the recirculating scheme, the residual energy in a more conven- tional crowbar circuit, shown in Fig. 2 , is dissipated entirely in every pulse operation. This is, of course, not suitable for a low- loss repetitive system.

A . Two Capacitor Banks Circuit

Fig. 3 shows the concept, current and voltage waveforms, and switching schedule for a recirculating circuit. The concept is basically a capacitor-inductor-capacitor (CLC) network transferring energy between two capacitor banks, energizing the magnet during the transfer. Due to the change in direction of current flow each pulse, the axial magnetic field B, reverses direction at every pulse. This may be an advantage in that the debris should be swept off evenly inside the magnetron housing. Debris buildup has been noticed when the axial magnetic field operates only in one direction.

In a “transfer” of half-wave circuit approach, two capacitor banks are needed, compared to only one in the crowbar scheme. In the crowbar scheme, the energy initially stored in the bank is all transferred (ignoring early-time losses) into the magnet coil. Therefore, the capacitor bank must store an energy equal to that required in the magnetic field at its peak field. In the case used for this study, the bank for the crowbar-type circuit must store 200 kJ. For the half-wave transfer case, each of the two banks must be a 400-kJ bank. At the time of peak current in the magnet when there is 200 kJ in the magnetic field, each of the two banks is at a voltage equal to one-half of the initial charge voltage, with 100 kJ in each bank. As a result, the basic capacitor bank size and weight is considerably greater for the half-wave circuit than for the crowbar circuit. On the other hand, power requirements for the crowbar circuit power supply are much more substantial.

B. One Capacitor Bank Circuit

A full-wave circuit version of the magnet driver, shown in Fig. 4 can be constructed with a slight change to the switching schedule of the half-wave circuit. The magnetron is restored to the one-direction magnetic field operation. The circuitry would be similar to that of the half-wave circuit with two exceptions:

1) The pulsed power system can now consist of a single capacitor bank, rather than two.

2) With essentially a 100% voltage reversal, the capacitors used in the banks will probably have to be of a significantly lower energy density to avoid severe reduction in the life of the capacitors.

A 200-kJ capacitor bank for a crowbar circuit, or two 400-kJ capacitor banks for a half-wave circuit, are needed to deliver the required 200 kJ to the magnet coils. For a full-wave circuit, as can be seen from Fig. 4, peak current in the magnet occurs when the energy in the capacitor bank has fallen to zero. That is, the bank needs to store 2200 kJ for this application. With 100% capacitor voltage reversal, the life expectancy would be unacceptably low.

Capacitors for the full-wave circuit have to be operated at a reduced voltage to give an improved, reasonable life. For in- stance, operation at ( 1 /&) of the rated voltage is equivalent to utilizing only one-half of the rated energy of the capacitor bank. Operating at = 70% of the rated voltage, a 400-kJ capacitor bank would be required. With the restriction of operating

8 10

6 10

l o 4

l o 2

Fig. 5 . Trends for the switch requirements as a function of capacitor bank voltage. Power consumption and magnet coil’s current pulse duration are fixed.

the capacitors at half the energy density or less, the full-wave circuit capacitor bank is effectively one-half or greater in size and weight of the half-wave circuit.

V. SELECTION OF OPERATING PARAMETERS A tradeoff study was performed using a spreadsheet program

to calculate the electromagnet and pulsed power parameters for the recirculating circuit using two capacitor banks. Design trends were obtained for a fixed power-consumption system and for a fixed voltage system.

Fig. 5 shows the calculated trends for the switch duty (current, s l* d t , dZ/dt, and charge transfer) and the storage bank capacitance. The magnet coils’ power consumption per pulse, P,,,, is fixed, as well as the magnet current pulse duration At,,,. Benefit in operating at high voltages is indicated: the higher the charging voltage for the capacitor banks, the more the switch duty is relaxed. We selected an operating voltage of 2 10 kV, but less than 50 kV, in view of available high-energy capacitors and switches, such as SCR’s.

The second trend is shown in Fig. 6. Here the capacitor bank voltage V,, is fixed at 10 kV. It is clearly seen that as the current pulsewidth At,, is reduced, P,,,,, decreases rapidly. Since the capacitance is determined by the voltage and energy, and is thus fixed, the only way to adjust At,,, is to adjust the inductance of the magnet. As the inductance is reduced, peak current in- creases. As a result, the current flow period is shorter and the resistance of the coil is decreased. The net effect is reduced loss.

Since the At<,” period is directly related to the period of the magnetic field, we recommend a lower limit for A[,,, of approximately 10 ms, implying a = 100 pps system, or so, for the following reasons:

1) The skin depth limitation causes the coil wire cross-sec- tional area to be underutilized, leaving a high portion of the coil’s weight unused. For copper, the skin depth is only several millimeters for a magnetic field with a < IO-ms period.

2) Eddy currents limit diffusion of the magnetic field into the magnetron. Eddy currents create magnetic fields in the mag-

IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 38. NO 4. APRIL 1991 7 94

l o 8

6 10

4 10

2 10

Fig. 6. Trends for pulsed magnet system requirements as a function of magnet coil’s current pulse duration. Capacitor bank voltage is fixed.

netron structure that oppose and cancel the applied field. These currents may not have a sufficient time to damp away resistively compared to the duration of At<,”.

VI. PULSE MAGNET SYSTEM COMPARISON

The spreadsheet program was used to minimize power loss, weight, and size for specific systems. Three cases are selected for comparison in Table 11: a baseline case serving as a refer- ence followed by the “low-loss” and “low-weight” system options. The baseline parameter approximates a crowbar-type driver for an existing magnet. Parameters for the coils, power supply, and pulsed power circuit are listed in the left rows of the table. Subsystem size and weight estimates are displayed, and average power losses for the coils are given in the top row in the power supply category. In all cases the magnet coil radii and the coil separation are kept essentially fixed. Significant observations from Table I1 are as follows.

1 ) Conversion from the crowbar baseline toward a lowest loss system (“low-loss’’ case in Table I ) incurs two penalties. First, the magnet coil weight rises sharply due to an increase in the wire size to reduce the effective resistance. Secondly, to keep the circuit current and associated losses low, high inductance is required in the magnet, requiring high voltage in the capacitor bank. Several switches in series may be required to handle the high voltage. Voltage gradient control for the stacked switches may be difficult.

2 ) A “lowest” loss system is not necessarily the lowest weight system. The overall “low-weight’’ option, while still being fairly low-loss, is shown in the last column of the table. Losses are higher than those for the “low-loss” case and a heavier power supply is thus needed. Fewer switches are needed to operate in series, due to the lower voltage. Parallel switches may be needed to handle the high peak currents. Selection of the “low-weight” option running at 5 pps implies magnet coils driven by = 40 kW power supply and = 400 kJ capacitor banks operating at near 10 kV.

Estimates of the system weight in Table I1 do not include the weight of the cooling systems for the magnet coils. It should be

TABLE I1 SAMPLE PULSED MAGNET SYSTEM COMPARISON: MAGNET COILS,

POWER SUPPLY, A N D PULSED POWER CIRCUITRY

Baseline Crow bar System

Low - Loss Low - Weight Option Option

I. Magnet Coils

Weight, kg ( 2 Cu coils) 184 Number of turns 100 Wire radius ( m ) 0.005 Loop separation ( m ) 0.38 AT per pulse ( “C) 2.610

918 104 100 14 0.010 0.010 0.38 0.38 0.003 0.180

Weight (kg) 200 920 1 IO Size O55mOD O55mOD O44mOD

Weight bdaed on Cu wire dssumed

11. Power Supply (at 5 pps)

Power loss average (kW) 1000 6 39 Charging voltage ( V ) 10 000 50 000 7150 Voltage losq per pulse ( V ) 10 000 77 66 At,,, ( ma 1 44 + decay 4 4

240 1 . 1 1 Weight (kg) 6000 50 Size ( m’) 32 0.26

Weight based on 40 kg/5 kW power supplies.

111. Pulsed Power

A . Cupuciior Bunk: System voltage (V ) 10 000 50 000 7150 Stored energy ( 1 bank) 200 40 1 428 ( kJ )

Weight ( k g ) 700 2800 2800 Size (m’ ) 1.3 5.2 5.2

Weight baaed on the present ”50 k J ” ( 0 . 3 kJ/kg) technology. Advanced capacitors (3kJ/kg) could reduce these capacitor bank weights by about an order of magnitude. B. Switches: Current average (A) I 89 665 Current peak (A) 5750 5755 42 915 d l / d t ( A / w 1 0.8 4. I 30.9 s 1pt (C) 179 18 133 j I & &(A’ . s ) 7.3 x IOr 3.6 x 10‘ 2.0 x 10’

Weight (kg) 30 120 15 0.054* 0.22’ 0.027’ I

*Using two ignitrons. ‘Ten SCR’s in series, plus diodes. iThree SCR’s in series, plus diodes.

noted that the “low-weight’’ option demands over six times the power of the “low-loss” option. The added weight of the cooling system to the “low-weight’’ option may well be substantial.

VII. SUMMARY AND CONCLUSION Recirculating circuitry offers an attractive alternative for re-

petitive pulsing of electromagnets for high-power magnetrons. Average power consumption is much less than that for a crowbar system. The two capacitor banks needed for the recirculation schemes, compared to one for the crowbar system, con- tribute to a major portion of the overall system weight. In terms of the overall system loss, weight, and size, the recirculating schemes can be considered superior to the crowbar system for a low repetition-rate magnet system of the order of 5 pps.

BHASAVANICH AND HAMMON: PULSED MAGNET SYSTEM 19.5

TABLE 111 TOTAL SYSTEM WEIGHT COMPARISON: CURRENT A N D ADVANCED HIGH

ENERGY DENSITY CAPACITOR

Crowbar-Type Low-Loss Low-Weight System Option Option

Weight (kg) Weight (kg) Weight (kg)

[21

131

S. T. Spang e t a l . , “A repetitively pulsed, mobile, HPM system,” in Proc. 1989 IEEE Int. Con& on Plastna Science (May 22-24. 1989). S. T. Spang er al . , “Relativistic magnetron development for use in a lightweight, repetitively pulsed, portable HPM transmitter,” IEEE Trans. Plasma Sci., vol. 18, no. 3, pp. 586-593, 1990.

SO-kJ capacitors 6900 3900 3200

Advanced capacitors 6300 1300 570 (0.3 kJ/kg)

( 3 kJ/kg)

*

At a higher repetition rate, the switching duty within the pulsed power circuit and the power supply becomes more com- plex, and the recirculating scheme is no longer competitive with a dc magnet. The upper limit for the recirculating scheme is equivalent to 100 pps due to the skin depth for pulsed magnetic fields in the magnet coil and the magnetron housing. Power supply ratings will be similar for both systems. The one needed for a pulsed magnet system at 100 pps, with output currents running at near 100% duty factor, consumes essentially the same power as the dc power supply. The dc magnet system is simpler as it will not require the two large capacitor banks. However, the magnet coils will need adequate cooling.

For the recirculating system, we recommend specifying advanced high-energy density capacitors, as opposed to the 50-kJ capacitors for the options assumed in Table 11. A significant reduction in the total system weight can be realized, as illus- trated in Table 111. System pulse repetition rate must be considered a key factor in the selection of capacitors. Capacitor life for single-pulse operation must be appropriately derated for repetition rate applications. A system specified for high repetition duty would probably require longer life capacitors and thus even a lower energy density design.

ACKNOWLEDGMENT

The authors wish to thank Dr. H. Goede of MDTI and Dr. R. R. Smith of Physics International Co., for valuable discus- sions.

Daun Bhasavanich (S’71-M’78) was born in Bangkok, Thailand, on October 2, 1951. He received the B . S . and M.E. degrees in electric power and electrical engineering from Rensse- laer Polytechnic Institute, Troy, NY, in 1974, the Ph.D. degree in electrical engineering from University of Liverpool, England, in 1977, and the MBA degree from the University of Pitts- burgh, Pittsburgh, PA, in 1982.

From 1978 to 1989, he worked at the West- inehouse R&D Center in Pittsbureh. PA. His

principal interests included a study of high current discharges and switching in gases and vacuum, high temperature lead vapor Raman cell for excimer laser frequency conversion, thallium-cesium atomic line filter as a narrow-band receiver for blue-green laser communica- tions with submarines. He joined Physics International Co., San Lean- dro, CA, in 1989. His current activity is in the design and analysis of switching components for electromagnetic launchers, electromagnets for high power microwave, and high repetition rate modulator for compact linear induction accelerator.

*

H. George Hammon, 111 (“Jud”) received the A.B. degree in physics from the University of California at Berkeley, in 1969, the M.S. degree in physics from the University of Wash- ington, Seattle, in 1970, and the D.A. degree in physics in 1975, also from the University of Washington.

He is Manager of the Pulsed Power Engi- neering Department at Physics International Company, San Leandro, CA. His responsibil- ities include technical and orogrammatic su-

REFERENCES

111 H. Sze, B. Harteneck, J . Benford, and T. S. T. Young, “Oper- ating characteristics of a relativistic magnetron with a washer cath- ode,” IEEE Trans. Plasma Sci., vol. PS-15, no. 3, pp. 321-334, 1987.

. - pervision of the development and systems for domestic and international customers. In his thirteen years of experience at Physics International, he has developed a broad range of advanced pulsed power systems, ranging from spark gap and magnetic switches to compact, low-jitter trigger generators, and on to very large systems providing the pulsed electrical energy for high-power lasers, high-power micro- waves, electromagnetic guns, and nuclear weapon effects simulation.

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Pulsed magnet system considerations for repetitive operation of high-power magnetrons