1148 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 68, NO. 2, FEBRUARY 2021

Solid-State Pulsed Power Modulator for9.3 GHz 1.7 MW X-Band Magnetron

Su-Mi Park , Seung-Ho Song , Hyun-Bin Jo, Woo-Cheol Jeong , Sung-Roc Jang , Member, IEEE,and Hong-Je Ryoo , Member, IEEE

Abstract—This article describes a modification and anexperiment conducted on a solid-state pulsed power mod-ulator (SSPPM) for medical linear particle accelerator(LINAC) applications. Depending on the operating condi-tion of the magnetron, an existing design of an insulated-gate bipolar transistor (IGBT)-based modulator is modified,from a positive pulse generator to a negative pulse gener-ator. The design of the gate driver circuit of the pulse dis-charging IGBTs is also slightly changed to secure reliabilityagainst the arc condition. The developed modulator hasthe following maximum output—pulse voltage of −40 kV,pulse current of −100 A, pulsewidth of 5 µs, and pulserepetition rate of 250 Hz. By adapting the proposed designto the pulsed power modulator for driving the magnetron,high peak and average power densities of 43 kW/L and48.9 W/L, respectively, can be achieved. The experimentalresults from various tests conducted using a resistor load,including a rated operation and an arc protection, verify therobustness and utility of the developed SSPPM for medicalLINAC applications. An experiment using a 9.3-GHz 1.7-MWX-band magnetron is also conducted. The developed modu-lator achieved a successful arc protection operation for thearc generated in the waveguide during the magnetron drivetest.

Index Terms—Linear particle accelerator (LINAC), mag-netron, medical LINAC, solid-state pulsed power modulator(SSPPM), X-band.

I. INTRODUCTION

L INEAR particle accelerators (LINACs), in which chargedparticles are accelerated in a straight line using

Manuscript received July 31, 2019; revised October 29, 2019 and De-cember 2, 2019; accepted December 24, 2019. Date of publication Jan-uary 23, 2020; date of current version October 30, 2020. This work wassupported in part by the National Research Foundation of Korea (NRF),Korea Government (MSIT), under Grant NRF-2017R1A2B3004855 andin part by the Human Resources Program in Energy Technology of theKorea Institute of Energy Technology Evaluation and Planning (KETEP),Ministry of Trade, Industry and Energy, South Korea, under Grant20184030202270. (Corresponding author: Hong-Je Ryoo.)

S.-M. Park, S.-H. Song, H.-B. Jo, and W.-C. Jeong are withthe Department of Energy Systems Engineering, Chun-Ang Uni-versity, Seoul 06974, South Korea (e-mail: smi1715@nate.com;songho310@naver.com; dlood2007@naver.com; dncjf94@naver.com).

S.-R. Jang is with the Electric Propulsion Research Center,Korea Electrotechnology Research Institute and the University ofScience and Technology, Changwon 641120, South Korea (e-mail:scion10@keri.re.kr).

H.-J. Ryoo is with the School of Energy Systems Engineering, Chung-Ang University, Seoul 06974, South Korea (e-mail: hjryoo@cau.ac.kr).

Color versions of one or more of the figures in this article are availableonline at https://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2020.2967728

electromagnetic fields, have been studied in various researchfields such as physics, industrial applications, medicine, andnational security [1]–[3]. In medical applications, LINAC isused for radiation therapy by generating an electron beam toirradiate targeted cancer cells. Because a radio frequency (RF)electromagnetic field must be formed for the medical LINAC togenerate an electron beam, an RF power source such as a klystronor a magnetron is required. The RF power source is drivenby a high-voltage pulsed power modulator, and pulsed powermodulators using multistage pulse-forming networks (PFNs) aregenerally used [4]–[6].

However, recent studies on medical LINACs requiredlightweight modulators of small size to be mounted on 3-D sur-gical imaging systems such as an O-arm or a C-arm. In general,modulators using PFNs require additional cooling considera-tions owing to their low efficiency between 40–60% [6]–[8],and there may be solutions such as the use of an external chilleror linkage with the cooling system of the hospital. In contrast, theproposed solid-state pulsed power modulators (SSPMs) basedon semiconductor devices can achieve high power density withrelatively few considerations for cooling, owing to their highefficiency [9], [10]. Therefore, SSPPMs that are characterized bylong lifetime, high efficiency, and small size, have been recentlyintroduced for medical LINAC applications. The requirementsof modulators that are mounted on the O-arm of medical LINACsinclude a small volume, light weight, and reliable arc protectionfunction, which is required to address the arc conditions that canoccur in the waveguide due to insufficient insulation.

This article presents the design for a small and compactSSPPM with high efficiency and reliability for application toa medical LINAC. The reduced volume and light weight of themodulator are achieved by adopting an air-cooling method with-out the use of any insulating oil. The solid-state negative pulsegenerator for driving the magnetron was developed through cer-tain modifications to the design of the pulse modulator developedin [11]. A small size of 430 mm × 370 mm × 580 mm, and ahigh power density with a peak of 43 kW/L and an average of48.9 W/L were achieved. In the case of commercialized SSPPMsfor medical LINAC applications, these values of power densitiescan only be achieved when a large external water-cooling systemis not considered. Moreover, the design of the gate driving circuitwas modified to ensure more reliable operation. The developedpulsed power modulator can generate negative output pulseswith high reliability when an arc condition occurs; the maximumvalues of the negative output pulse voltage and the current of

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PARK et al.: SSPPM FOR 9.3 GHZ 1.7 MW X-BAND MAGNETRON 1149

Fig. 1. Simplified schematic diagram of configuration of the medicalLINAC system.

the designed SSPPM are 40 kV and 100 A, respectively. Thedevelopment of an SSPPM and an experiment conducted on itare also discussed in this article. The experiment was performedusing not only a resistor load, but also a megawatt-scale realmagnetron load. The high performance of the arc protection ofthe developed SSPPM was verified using a 1.7-MW X-bandmagnetron.

II. SSPPM FOR MEDICAL LINEAR

ACCELERATOR APPLICATION

A. Requirements of Pulse Modulator used in MedicalLINAC Applications

Fig. 1 shows a simplified schematic of the medical LINACsystem. The magnetron is powered by a heater power supply anda high-voltage pulse modulator to generate the RF electromag-netic fields. The electron gun and accelerator tube accelerate theelectrons to generate the electron beam. Although a simplifiedschematic of a single power modulator system is shown in Fig. 1,the actual system consists of separate high-voltage power mod-ulators and auxiliary heater power sources for each component.Among these various power modulators, this article describes ahigh-voltage pulse modulator used to drive the magnetron, whichis a key component of the medical LINAC system. The detailedhigh-voltage connections of the magnetron, pulse modulator,and heater power supply are shown in Fig. 2. The specificationsdescribed in Fig. 2 are the real values of the SSPPM developedin this article. The heater power supply applies a relatively lowvoltage to a cathode terminal of the magnetron to heat the fila-ment and release thermal electrons. Simultaneously, the negativeoutput pulse of the pulse modulator is connected to the cathode ofthe magnetron. For this reason, the heater power supply requireshigh-voltage isolation. In this practical experiment, a powersupply that can achieve a maximum voltage and current output of20 V and 20 A, respectively, and 50 kV isolation was used as theheater power supply. The high-voltage pulse modulator should

Fig. 2. High-voltage connection among the magnetron and powersupply used to drive the magnetron.

TABLE IDESIGN REQUIREMENTS OF SSPPM IN MEDICAL LINAC APPLICATION

generate a pulse with negative polarity to drive the magnetron,while maintaining a small size and lightweight characteristics.Therefore, the SSPPM, which can be used in a medical LINAChas the design requirements shown in Table I.

B. Design of SSPPM for Driving the Magnetron

Fig. 3 shows an overall simplified scheme of an SSPPM thatwas designed and developed for implementation in a medicalLINAC. The design of the SSPPM was modified based on amodulator developed in [11] such that it could meet the designrequirements shown in Table I. Because a pulse modulatorwith negative output pulses is required to drive the magnetron,the designed SSPPM can generate positive or negative pulsesdepending on the high-voltage wiring scheme. In the wiringscheme in Fig. 3, the dotted blue lines indicate the generationof positive output pulses, and the solid red lines indicate thegeneration of negative output pulses. In addition, since thesensing circuits “Vsense for Feedback” and “Isense for Arc”should be implemented to be closest to the ground terminal forhigh-voltage isolation, the connection of the sensing circuits alsochanges depending on the polarity of the output pulse.

Regardless of the polarity of the output pulse, the charginginverter charges all 48 storage capacitors in four power stagesby supplying the charging current through the power loop; asingle power stage consists of six power cells. Each power cellconsists of two rectifier diodes, two storage capacitors, two mainpulse discharging insulated gate bipolar transistors (IGBTs) andtheir gate driving circuits, and two bypass diodes. Forty-eightstorage capacitors are charged by the charging inverter through

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1150 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 68, NO. 2, FEBRUARY 2021

Fig. 3. Overall configuration of the proposed pulse modulator that cangenerate positive or negative pulses.

Fig. 4. Gate driving circuit for the main pulse-discharging IGBT.

the power loop transformer, which has multisecondary windings.The charging inverter is designed with the modified series-loaded resonant converter topology, where secondary resonantcapacitor Cr2 (in this article, 48 secondary resonant capacitorsCr2_1 to Cr2_48 were used) with small capacitance values areused as parallel resonant capacitors. Accordingly, lower crestfactor and conduction losses are achieved because the chargingcurrent can rapidly increase owing to the use of Cr2 [12].Furthermore, a charging loop isolation transformer is used tominimize the unexpected effect due to the parasitic componentsof the power loop transformer. An additional third winding isadded for the voltage unbalance compensation of each powercell. The pulse control inverter transfers the gate driving signaland power through the control loop to the gate driving circuitsfor the main IGBTs. Through the control loop scheme shownin Fig. 3, it is possible for 48 isolated and synchronized gatesignals to be applied to each gate driving circuit for the mainIGBT [13].

Fig. 4 shows the circuit of the gate driver for each main IGBT.First, the gate-ON and -OFF pulses described in the figure as“Gate Pulses” are transmitted to the gate driver through trans-former TX1 (the control loop). This circuit has three operatingmodes; turn-ON pulse mode, turn-ON hold mode, and turn-OFF

pulse mode. After the gate pulses pass through the gate drivingcircuit, a voltage VGE is applied to the main IGBT with thesethree modes. The previously designed gate driving circuit in[11] uses the arc protection algorithm, which detects the risein voltage across the collector and emitter of the IGBT, andautomatically cuts off the gate driving circuit of the IGBT whenan arc condition occurs. However, this method suffers from aproblem; when a capacitive load is used, a malfunction in thearc protection may be caused by the fast rise in pulse even ifan arc condition has not occurred. In addition, as described inSection I, a reliable performance of arc protection is required fordriving the magnetron to minimize the effect of the arc conditioninside the waveguide on the switching devices of the modulator.To improve the reliability of the arc protection performance,which is one of the requirements of the pulsed power modulatorin the medical LINAC, the elements used for arc protection withthe Miller capacitor of the IGBT were removed in this article.Therefore, in the modified arc protection algorithm, the pulse

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PARK et al.: SSPPM FOR 9.3 GHZ 1.7 MW X-BAND MAGNETRON 1151

Fig. 5. Pulse ON/OFF signal and gate signal for normal conditions andtwo types of arc protection algorithms.

TABLE IISPECIFICATIONS OF THE DEVELOPED PULSED POWER MODULATOR

TABLE IIIDEVICES AND ELEMENTS USED FOR THE DEVELOPED

PULSED POWER MODULATOR

ON/OFF controller generates an additional turn-OFF pulse with afault signal when the arc current is detected. As shown in Fig. 5,for the developed modulator in this article, an additional OFF

pulse is generated and the main IGBTs are turned OFF when anarc condition occurs.

III. DEVELOPMENT AND EXPERIMENTAL VERIFICATION

OF THE DESIGNED SSPPM

The specifications of the SSPPM developed according to therequirements of a medical LINAC are shown in Table II. Inaddition, Table III summarizes devices and elements such asIGBTs and diodes, and also the inductor and capacitors that were

Fig. 6. Waveforms of output pulse voltage and current with resistiveload. (a) Without additional inductor. (b) With additional inductor (outputvoltage: 5 kV/div., output current: 20 A/div., and time: 1 µs/div.).

used to develop the SSPPM. In this article, various experimentson the developed SSPPM were conducted using a resistor loador a real load, from the rated and light loaded conditions to thearc conditions.

Before the magnetron was driven with the developed mod-ulator, the resistive load test was performed to verify stablepulse output. Fig. 6 shows the waveforms of the output pulsevoltage and current for the resistive load test. When using only anoninductive resistor, the waveform of the output pulse currentshows an initial peak value, as observed in Fig. 6(a). To changethe waveform of output pulse current into the flat-top version,an air-core inductor was added to the load. The waveforms withflat-top pulse current are shown in Fig. 6(b). The results of bothexperiments, in which a noninductive resistor was used and anadditional inductor was added to the resistor to obtain a flatpulse current, confirmed an output pulse with approximately−38 kV pulse voltage and −80 A pulse current. In addition, theoperation of the arc protection circuit was verified, as shownin Fig. 7. As an arc condition occurs during the pulse, the arccurrent increases rapidly, up to approximately 400 A. However,the output pulse is removed owing to the arc protection function,and the current thus decreases to zero. After confirming the stablepulse output and the arc protection operation of the developedmodulator through the resistive load tests, an experimental setupfor driving the magnetron was constructed, as shown in Fig. 8.The magnetron used for the real load test in this article is a

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1152 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 68, NO. 2, FEBRUARY 2021

Fig. 7. Waveforms of output pulse voltage and current for the arcprotection test (output voltage: 5 kV/div., output current: 100 A/div., andtime: 2 µs/div.).

Fig. 8. Experimental setup for driving the real load (magnetron) usingthe developed modulator.

TABLE IVRATINGS OF THE MAGNETRON USED IN THE REAL LOAD TEST

9.3-GHz 1.7-MW X-band magnetron. More detailed ratings ofthe magnetron are summarized in Table IV. The output RFpeak power of the magnetron was measured according to themagnitude of the negative pulse voltage of the modulator, asshown in Fig. 9, where the magnitude of the pulse currentflowing through the anode of the magnetron was also specifiedon the x-axis. It can be confirmed that the RF peak power ofthe magnetron was measured at approximately 1.7 MW, whichis the rated value of the magnetron, when a pulse voltage of–36 kV was applied. By using the experimental results shown inFig. 9, the load characteristics of the magnetron which was used

Fig. 9. Measured output RF peak power of the magnetron (L6170)according to the magnitude of the negative output pulse voltage of themodulator.

Fig. 10. Voltage–current characteristics of the magnetron (L6170)model between the magnitudes of pulse voltage of 30–36 kV.

in this article can be explained. The voltage–current character-istic curve of the magnetron L6170 model, shown in Fig. 10,represents the load characteristics of L6170 in an oscillationarea, where the magnitude of anode current changes significantlyeven for small changes in the anode–cathode voltage [14], [15].In a nonoscillation area, the magnetron can be modeled as aload with a small capacitance and does not generate RF power.Fig. 11 presents the experimental results when a pulse voltageof –25 kV was applied to the magnetron. The RF power of themagnetron under this condition was measured at almost zero.In addition, the waveforms of the output voltage and currentfor the initially designed modulator were measured accordingto the load characteristics of L6170 in an oscillation area, asshown in Fig. 12(a). When a negative pulse voltage of –38 kVwas applied to the magnetron, the magnitude of the initial pulsecurrent increased by approximately 150 A, and the voltage droopof the output pulse greatly affected the decrease of the pulsecurrent during the pulse period. Fig. 12(b) shows the waveformswhen the air-core inductor was added to reduce the peak valueof the pulse current and achieve flat-top operation, as in theresistive load test. Compared with the results in Fig. 12(a), the

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PARK et al.: SSPPM FOR 9.3 GHZ 1.7 MW X-BAND MAGNETRON 1153

Fig. 11. Waveforms of output pulse voltage and current for real mag-netron load test under the pulse voltage of –25 kV condition (outputvoltage: 10 kV/div., output current: 40 A/div., and time: 2 µs/div.).

Fig. 12. Waveforms of output pulse voltage and current for realmagnetron load test. (a) Without additional inductor (output voltage:10 kV/div. with two different probes, output current: 50 A/div., and time:2 µs/div.). (b) With additional inductor (output voltage: 10 kV/div., outputcurrent: 40 A/div., and time: 1 µs/div.).

peak value of the output current was decreased and the variationin the output voltage during the pulse period was improved, sothe output current was reduced to become relatively small. Ingeneral, the faster the rise time of the pulse voltage applied tothe magnetron, the lesser the influence of unnecessary operationmodes is, and more effective pulsewidth can be ensured. The

recommended value of the pulse rise time is below 500 ns. Inthis article, the output pulse of the modulator was tuned for theappropriate rise time and voltage droop by using the additionalair-core inductor, and finally, the modulator was designed witha rising time of less than 500 ns and a maximum pulse width of4 µs when driving the magnetron load.

IV. CONCLUSION

This article presented the design and development of a com-pact SSPPM applicable for a medical LINAC system to drivethe magnetron, and the results of an experiment using thisSSPPM were also described. The design of the pulsed powermodulator was based on a semiconductor switching device toachieve compactness and high power density with an air-cooledmethod for implementation with an O-arm or a C-arm. Themodulator design was based on a modular structure with fourpower stages consisting of six power cells for each stage. Fordriving the magnetron, the modulator adopted a modified wiringscheme to generate negative pulses, and the gate driving circuitof the main IGBT was modified to offer arc protection. Theresults showed that despite its small size (430 mm × 370 mm ×580 mm), a high power density, with a peak of 43 kW/L and anaverage of 48.9 W/L, was achieved without an additional coolingsystem. The stable pulse output with a maximum voltage andcurrent of −40 kV and −100 A, respectively, was obtained, andthe reliability of the arc protection function of the developedmodulator was verified through the various experiments usingthe resistive and magnetron loads.

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Su-Mi Park received the B.S. degree in en-ergy systems engineering in 2017, from Chung-Ang University, Seoul, South Korea, whereshe is currently working toward the IntegratedM.S. and Ph.D. degrees in energy systemsengineering.

Her research interests include high-voltagepower converters, bidirectional dc/dc convert-ers, and solid-state pulsed power modulators.

Seung-Ho Song received the B.S. degreein electrical engineering from Kwang-WoonUniversity, Seoul, South Korea, in 2016. He iscurrently working toward the M.S. and Ph.D.degrees in energy systems engineering fromChung-Ang University, Seoul.

His research interests include soft-switchedresonant converter applications and high-voltage pulsed-power supply systems.

Hyun-Bin Jo received the B.S. degree inelectronic engineering from Catholic University,Bucheon, South Korea, in 2016. He is currentlyworking toward the M.S. and Ph.D. degrees inenergy systems engineering from Chung-AngUniversity, Seoul, South Korea.

His research interests include high-voltagepulsed-power supply systems.

Woo-Cheol Jeong received the B.S. degreein energy systems engineering in 2019 fromChung-Ang University, Seoul, South Korea,where he is currently working toward the Inte-grated M.S. and Ph.D. degrees in energy sys-tems engineering.

His research interests include soft-switchedresonant converter applications and high-voltage pulsed-power supply systems.

Sung-Roc Jang (Member, IEEE) was born inDaegu, South Korea, in 1983. He receivedthe B.S. degree in electrical engineering fromKyungpook National University, Daegu, SouthKorea, in 2008, and the Integrated M.S. andPh.D. degrees in electrical engineering from theUniversity of Science and Technology (UST),Deajeon, South Korea, in 2011.

He is currently a Senior Researcher withthe Electrophysics Research Center, KoreaElectrotechnology Research Institute (KERI),

Changwon, South Korea since 2011. He became an Assistant Professorwith the Department of Energy Conversion Technology, UST in 2015.His current research interests include high-voltage resonant convertersand solid-state pulsed power modulators and their industrial applica-tions.

Dr. Jang was the recipient of the Young Scientist Award at the ThirdEuro-Asian Pulsed Power Conference in 2010, and the IEEE NuclearPlasma Science Society (NPSS) Best Student Paper Award at the IEEEInternational Pulsed Power Conference in 2011.

Hong-Je Ryoo (Member, IEEE) receivedthe B.S., M.S., and Ph.D. degrees fromSungkyunkwan University, Seoul, South Korea,in 1991, 1995, and 2001, respectively all in elec-trical engineering.

From 2004 to 2005, he was a VisitingScholar with Wisconsin Electric Machines andPower Electronics Consortium, University ofWisconsin-Madison, Madison, WI, USA. From1996 to 2005, he was with the Electric Propul-sion Research Division as a Principal Research

Engineer and with the Korea Electrotechnology Research Institute,Changwon, South Korea, where he was a Leader with the PulsedPower World Class Laboratory and a Director of the Electric PropulsionResearch Center. From 2005 to 2015, he was a Professor with theDepartment of Energy Conversion Technology, University of Scienceand Technology, Deajeon, South Korea. In 2015, he joined the Schoolof Energy Systems Engineering, Chung-Ang University, Seoul, wherehe is currently a Professor in major of Electrical Energy Engineering. Heis an Academic Director of the Korean Institute of Power Electronics,a Planning Director and Editorial Director of the Korean Institute ofElectrical Engineers, and the Vice President of the Korean Institute ofIlluminations and Electrical Installation Engineers. His research inter-ests include pulsed-power systems and their applications, as well ashigh-power and high-voltage conversions.

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