Journal of the Korean Physical Society, Vol. 64, No. 9, May 2014, pp. 1267∼1271

Solid-state Pulse Modulator for a 1.7-MW X-band Magnetron

Jaegu Choi, Yong-Moon Shin, Young-Wook Choi and Kwan-Ho Kim∗

Korea Electrotechnology Research Institute (KERI), Ansan 426-170, Korea

(Received 27 December 2013, in final form 1 April 2014)

Medical linear accelerators (LINAC) for cancer treatment require pulse modulators to generatehigh-power pulses with a fast rise time, flat top and short duration to drive high-power mag-netrons. Solid-state pulse modulators (SSPM) for medical LINACs that use high power semicon-ductor switches with high repetition rates, high stability and long lifetimes have been introduced toreplace conventional linear-type pulse generators that use gaseous discharge switches. In this paper,the performance of a developed SSPM, which mainly consists of a capacitor charger, an insulated-gate bipolar transistor (IGBT) - capacitor stack and a pulse transformer, is evaluated with a dummyload and an X-band magnetron load. A theoretical analysis of the pulse transformer, which is acritical element of the SSPM, is carried out. The output pulse has a fast rise time and low droop,such that the modulator can drive the X-band magnetron.

PACS numbers: 07.50.-e, 52.70.Ds, 89.20.-aKeywords: Pulse modulator, Pulse transformer, Solid state, Medical LINAC, MagnetronDOI: 10.3938/jkps.64.1267

I. INTRODUCTION

Medical linear accelerators (LINACs) for cancer treat-ment require pulse modulators to generate short durationpulses to drive microwave tubes such as high power mag-netrons [1–3]. The modulators usually used in LINACsystems have been linear-type pulse generators, whichconsist of a high-power DC supply, a charging reactor, ablocking diode, a pulse-forming network (PFN), a thyra-tron switch and a pulse transformer. The thyratronswitch is one of the most critical elements in producingan accurate pulse in the operation of high-power mod-ulators. However, the performance of the gaseous dis-charge switch relies on the condition of the sealed gas andelectrodes, which deteriorate with aging, resulting in in-creased maintenance costs and downtime of the LINAC.

The recent development of high-power semiconductorswitches with excellent characteristics have enabled high-power pulse operation and the use of repetitive pulsedpower in various applications requiring a high repetitionrate, high stability and long lifetime. Consequently, thisallows for the introduction of a solid-state pulse modu-lator (SSPM) for a medical LINAC.

In this paper, the circuit topology of a SSPM, mainlyconsisting of a capacitor charger, an IGBT-capacitorstack and a pulse transformer, is newly adopted for amedical LINAC. The SSPM, with a peak power of 3.2MW, 4-μs pulse width and a repetition rate of 300 pps, isfabricated to drive a high-power X-band magnetron. The

∗E-mail: jgchoi@keri.re.kr

Fig. 1. Schematic diagram of the solid-state pulse modu-lator for an X-band magnetron. The modulator consists of acapacitor charger, a 12-stage IGBT-capacitor stack and a 1:6step-up pulse transformer.

performance of the SSPM is evaluated with a dummyload and an X-band magnetron load.

II. SYSTEM DESCRIPTION

The SSPM is used to drive a 1.7-MW X-band mag-netron for a medical LINAC system. The schematicdiagram of the SSPM is shown in Fig. 1. The SSPMconsists of a capacitor charger, a 12-stage insulated-gate

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Fig. 2. (Color online) Appearance of the developed pulsemodulator and the winding configuration of the pulse trans-former. The SSPM generates output pulses with a 4-μs pulsewidth, a 3.2-MW peak power and a 300-pps repetition rate.

Fig. 3. (Color online) Simplified equivalent circuit of thepulse transformer.

bipolar transistor (IGBT) - capacitor stack and a 1:6step-up pulse transformer. In this system, each capac-itor of 10 nF in the IGBT-capacitor stack is chargedto 600 V by using the capacitor charger. The capaci-tor charger’s power is provided from the 3-phase 220-Vpower line. With the capacitor fully charged, the IGBTsin the stack are simultaneously switched on by using thegate-trigger circuit to produce 4-μs pulses with a mag-nitude of 7.2 kV and a repetition rate of up to 300 pps.Then, the high-power pulse transformer steps up the pri-mary voltage, theoretically up to 43 kV, and is capableof matching the load impedance to the source impedancefor the maximum transfer of energy from the SSPM tothe magnetron. The modulator and the high-power pulsetransformer developed in this study are shown in Fig. 2.The dimensions of the modulator are 1.2 × 0.57 × 0.5 m3,resulting in a total volume of 342 l. The core (2605SA1,Metglas Alloy, USA) used for the pulse transformer hasa mean magnetic path length of 75 cm and a net cross-sectional area of 20.5 cm2.

The simplified equivalent circuit of the pulse trans-former is shown in Fig. 3 [4,5]. vG(t), vP (t) and vS(t)are the electric voltages in the modulator, primary and

secondary circuits, respectively. RP and LL are the pri-mary winding resistance and leakage inductance, respec-tively. RC , LP and CD are the core resistance, primaryinductance and distributed capacitance, respectively. CL

and RL are the load capacitance and load resistance, re-spectively. In particular, the circuit parameters LL, LP

and CD are critical for the best performance of the pulsetransformer, i.e., a fast rise time and a low pulse droop.These parameters are related to the geometrical quan-tities of the pulse transformer and the characteristics ofthe core material. Thus, the rise time and the pulsedroop can be improved by using an appropriate wind-ing arrangement and core material. LL, LP and CD aregiven as follows, [6,7]:

LL∼= 2π(a + b + 4Δ12)N2

S

(Δ12

lc

)[nH] (1)

LP∼= 4πμeN

2P

A

lm[nH] (2)

CD∼= 1

2

(8.854εrUclc

Δ12

) (n − 1

n

)(3)

(4)

where NP and NS are the number of turns in the pri-mary and the secondary coils, respectively, a and b arethe height and the width of the primary winding, respec-tively, Δ12 is the insulation distance between layers, Uc

is the average circumference of the layers in cm, lc is thewinding length in cm, εr is the relative dielectric con-stant of the insulating material between the layers, μe

is the effective pulse permeability of the core, A is thecross-sectional area of the core, lm is the mean magneticpath length of the core, and n is the step-up ratio.

By analyzing the response of the circuit shown inFig. 3, the rise time tr and the pulse droop Dr can bedetermined by using [8,9]

tr ∝√

LLCD (5)

Dr =R · tpLP

× 100% (6)

(7)

where R is RP ||RL, and tp is a pulse width. These areimportant equations that highlight some of the most im-portant relationships among the parameters of the pulsetransformer, namely, that the rise time depends on theleakage inductance LL of the transformer and the ca-pacitance CD and that the pulse droop depends on theprimary inductance.

From the preceding analysis, for a fast rise time, LL

and CD must be minimized. For a low droop, LP shouldbe kept as high as possible. Therefore, if the numberof the turns of the primary coil is increased to get alarge primary inductance, the leakage inductance is alsoincreased. Thus, a tradeoff between the requirementsfor a fast rise time and a low pulse droop is required toproduce the optimum output pulse.

Solid-state Pulse Modulator for a 1.7-MW X-band Magnetron· · · – Jaegu Choi et al. -1269-

Fig. 4. (Color online) Typical output voltage pulse wave-form of the simulated circuit for the pulse transformer. In thecircuit, a rectangular pulse was applied as an input pulse, anda resistive load was used as shown in Fig. 3.

Table 1. Specifications of the pulse transformer.

Primary voltage 7 kV

Primary current 550 A

Secondary voltage 36 kV

Secondary current 88 A

Output impedance 400 Ω

Flat top pulse width 3.5 μs

Pulse droop < 5%

Rise time < 900 ns

Turns ratio 1:6

Pulse repetition rate ≤ 300 pps

III. RESULTS

1. Circuit Simulation of the Pulse Transformer

The performance of the equivalent circuit of the pulsetransformer shown in Fig. 3 were checked using thePSpice program before the fabrication of the pulse trans-former. The simulated results with a rectangular inputpulse and a constant resistive load are shown in Fig. 4.The rise time can be seen to be longer than 1 μs, and aconsiderable pulse droop can be seen to take place withthe resistive load.

2. Fabrication of the Pulse Transformer

The pulse transformer has been carefully designed andoptimized through model simulations and experimen-tation. Parallel primaries and secondaries are woundon each leg, providing low leakage flux. The core is

Fig. 5. (Color online) Output pulse waveforms at the sec-ondary terminal of the pulse transformer with a dummy loadthat consists of 15 electric bulbs of 200 W in series.

Table 2. Circuit parameters for the pulse transformer.

Item Unit Measured value

Primary inductance (LP ) mH 1.75

Secondary inductance (LS) mH 63

Leakage inductance (LL) μH 11.8

Distributed capacitance (CD) nF 1.66

made up of two subcores strapped together. Each sub-core is wound from 0.025-mm-thick silicon steel ribbon.Table 1. shows the specifications of the pulse trans-former, which depend on the specifications of the X-band high-power pulse magnetron (PM1110X, L-3 Elec-tron Devices, USA). The primary inductance, secondaryinductance, leakage inductance and the distributed ca-pacitance between the primary and secondary windingswere measured with an LCR meter (4284A, Agilent Tech-nologies, USA). The measured parameters for the pulsetransformer are summarized in Table 2.

3. High Voltage Test

High-voltage tests of the pulse transformer connectedto a dummy load and a magnetron load have been per-formed. The dummy load consists of 3 modular loadsin parallel, each of which consists of 15 electric bulbsof 200 W in series to make a full load of 3.2-MW peakpower. The magnetron load forms between the anodeand the cathode of the X-band coaxial pulse magnetronthat delivers 1.7 MW peak output power at a duty cycleof up to 0.0008 and a pulse width of up to 4.0 μs. Thetransformer was fed by the solid state pulse modulator,as shown in Fig. 1. The output pulse waveforms at theprimary and the secondary terminal of the pulse trans-

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Fig. 6. (Color online) Energy conversion efficiency of thepulse transformer from the primary winding to the dummyload. The energy conversion efficiency is found to be 95.1%.

former with a dummy load are shown in Fig. 5. Theoutput pulse shows a rise time of less than 600 ns and apulse width of 4 μs. The slight difference in waveformsfrom the simulated waveforms in Fig. 4 can be mainlyattributed to the input pulse waveforms, the winding ge-ometry and the characteristics of the core and the load.

We should note that the primary and the secondarywaveforms before waveform compensation have consid-erable droops, but that the flatness of the secondarywaveform after the compensation was improved by awaveform compensator, which is based on the effect of aband-stop filter [10]. The compensator, which consists ofan inductor and a capacitor in parallel, was inserted infront of the primary terminal of the pulse transformer,as shown in Fig. 7(a). The inductance and the capaci-tance of the compensator range from 5 μH to 30 H andfrom 400 to 600 nF, respectively. According to the re-sults of the fast Fourier transform (FFT) analysis in thefrequency domain as shown in Fig. 7(b), the considerablefrequency components around 200 kHz, corresponding tothe parameter of the compensator, are decreased, conse-quently improving the flatness of the pulse in the timedomain, as shown in Fig. 4. Thus, we conclude thatthe compensator module can be used for improving theflatness of the output waveform with resistive loads. Theenergy conversion efficiency of the fabricated transformerfrom the primary winding to the dummy load is foundto be 95.1%, as shown in Fig. 6.

The output pulse waveforms at the secondary termi-nal of the pulse transformer with the magnetron load areshown in Fig. 8. The output pulse shows a rise time of320 ns and a flat top of 3.7 μs. We note that the wave-form has a fast rise time and a low droop such that themodulator can drive the X-band magnetron with a highperformance. The faster rise time and lower droop thanthose with the dummy load are attributed to the dy-namic impedance of the magnetron load, as indicated in

Fig. 7. (Color online) Effect of the waveform compensatorin the frequency domain. The flatness of the output pulseis improved by inserting a compensator in front of the pri-mary terminal of the pulse transformer. The reduction of thelow-frequency components around 200 kHz leads to betterflatness. Refer to Fig. 5.

Fig. 8. Output pulse waveform at the secondary terminalof the pulse transformer with a magnetron load. The outputpulse shows a rise time of 320 ns and a flat top of 3.7 μs, whichare suitable for driving high-power X-band magnetrons.

Ref. 11. Magnetrons can be simulated by using the ap-proximation R = ∞ from time t = 0 until the thresholdvoltage of the magnetron is reached; then, R = the dy-namic impedance of the magnetron whereas the dummyload has a constant resistance value.

Solid-state Pulse Modulator for a 1.7-MW X-band Magnetron· · · – Jaegu Choi et al. -1271-

IV. CONCLUSION

A new circuit topology of a SSPM, which mainly con-sists of a capacitor charger, an IGBT-capacitor stack anda pulse transformer, has been adopted to drive a high-power X-band magnetron for a medical LINAC. Theprincipal specifications of the output pulse of the SSPMare a peak power of 3.2 MW, a pulse width of 4 μs anda repetition rate of 300 pps. The performance of theSSPM has been evaluated with a dummy load and witha magnetron load. The compensator module based ona band-stop filter was found to be able to improve theflatness of the output waveform with resistive loads andthe fabricated modulator with a fast rise time of 320 nsand a low droop was found to be suitable for drivinghigh-power X-band magnetrons.

ACKNOWLEDGMENTS

This research was supported by the Korea Electrotech-nology Research Institute (KERI) Primary ResearchProgram through the Korea Research Council for Indus-trial Science & Technology (ISTK) funded by the Min-istry of Science, ICT and Future Planning (MSIP) (No.13-12-N0101-01).

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