Anami, S

Baba, H

Hanaki, H

Kihara, M

Kubo, T

Kubota, C

Kudo, K

Ono, M

Takasaki, E

Takenaka, T

CERN Document Server

Particle Accelerators, 1990, Vol. 32, pp. 51-56Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaDEVELOPMENT OF L-BAND lllGH-POWER RF SOURCE FOR THEJAPANESE HADRON PROJECf PROTON LINEAR ACCELERATORMASAAKI ONO, SHOZO ANAMI, mROBUMI HANAKI, TADASlll KUBO,CIllKASlll KUBOTA, KIKUO KUDO, EIICHI TAKASAKI, TATERU TAKENAKA,HITOSHI BABA and MOTOHIRO KIHARANational Laboratory for High Energy Physics(KEK)1-1 Oho,Tsukuba-shi,lbaraki-ken,305)apanAbstract A high-power and high-duty RF source for an intense proton linear accelerator of theJapanese Hadron Project (JHP) is under development at KEK. The construction of a long pulsemodulator and a pulse transformer designed to produce pulses with a 400 JlSeC pulse width and amaximum 140 kV pulse voltage was successfully completed. Test operations of a L-band klystron(1.296 GHz) planned to be used at the high-f3 structure of the linac are also accomplished, producinga 5MWpeak RF power at a 400 Jlsec pulse duration. Feasibility of the L-band RF source ofa 6 MWpeak power with a 600 JlSeC pulse duration will be discussed.IN1RODUCTIONIn the present design of the I-GeV proton linac for the JHP 1,2, 36 L-band klystrons are required as the highpower RF source for the high-~ accelerating structure. In order to achieve a stable accelerating field in thecavity for a 600 fJSec pulse duration, a feedback system is proposed3 to compensate beam loading; the outputpower of the klystron will be modulated by a drive power. In this scheme, the klystrons must be operated atunsaturated state. Another crucial point to consider is reliability of the operation for such a large number ofklystron system. Therefor, every components ofa prototype test station are designed to meet the requirementfor producing a 6MW peak power, even though a nominal peak power is 4MW at the proposed linac. Theline type modulator was constructed4 at KEK for the test station to test the RF power generation with suchthe long pulse duration. After the initial operation of the modulator with 200 J.1sec of pulse length andmaximum 140 kV of cathode voltage, the pulse width was doubled by extending a PFN section of themodulator. Fairlywell operationoftheklystron was achieved up toamaximum 5MWpeak powerwith almost400 JlSeC of pulse duration at a cathode voltage of 130 kV. An accumulation of the experiences of such thelong pulse operation will be very essential to reach our final goal.MODULATOR AND PULSE TRANSEFORMERThe expected 6MW outputpower klystron would need 140 kV and 105 A pulses where the 40% conversionefficiency and 2 x 1()-6 beam perveance areestimated from currentlyavailable pulsedklystrons. The maximumpulse voltage of the modulator is chosen as 20 kV for assurance of a stable operation of thyratrons which areused as a switching device of the modulator. Consequently, a 7:1 step up ratio is required for a pulsettansformer. The details of the transformer are discussed in the reference 5.As shown in Fig. 1, the modulator consists ofa high voltage DC power supply, a charging circuit, the[1493]/5152/[1494] M.ONOETAL.Rectifier l Filtert------------------- to@)D.C. Power SupplyPFN 26.70 34 cells : 400~s50 cells : 600~sL34 L2 L1PFN and Discharge CircuitI---- (jharglng (jlrcUltCoax. DespikerCable L0::::1:::JB--......a..a.;~-Q1--.......'"rT'4-...--..-~~ JIIII"""' .........I0.............<2)--_....Figure 1 Schematic circuit of the prototype modulator.(Dashed boxes show a possible future extension).PFN and thyratron switching devices. The present PFN consists of 34 cells which correspond to a 400 JlSeCpulse width. At the final objective, 50 cells will be installed to generate 600 Jlsec pulses. The crowbarthyratron is also installed not only for a protection of the klystron but also for the stable operation of the mainthyratron. In the high duty modulator as in our case, it is suspected that the main thyratron will not alwayscease it's fire at a pulse-end period for a wide range impedance of the klystron load. Because an amount ofcharging up voltage at the pulse-end timing is proportional to a square of the duty. (Notice; Once the holdoff diode becomes turned on, the charging up current flow into the PFN continuously, irrelevant to theconduction of the main thyratron.) And because , at low rating power, a positive mismatch of the klystronload may o~cur and a reflected positive voltage may be applied to the anode of the main thyratron for morethan twice the pulse width. The crowbar thyratron triggered at the pulse-end timing always shorten thepositive voltage reflection, then it cease the main thyratron conduction. This pulse-end triggered conductioncurrent of the crowbar is not so heavy (both peak current and pulse duration), therefor we expect much stableoperation of this tube compared to the main thyratron. The klystron protection is another function of thecrowbar. However, it is expected that a energy dissipation at the klystron is not so much even when an arcingis happened inside ofitbecause almostall ofthe suppliedpulsedenergy (stored energy in the PFN) is reflectedat the shortenedloadand is finally dissipatedat the modulator. Theestimatedenergy dissipationat the klystronis about 10 joules for 600 Jlsec duration if the arcing voltage across the klystron gun is the order of 100 Vandthe current is around 180 A. (Owing to the characteristic impedance of the PFN, the current is restricted lessL-BAND HIGH-POWER RF SOURCE [1495]/53than the twice the nominal current, even if the load is shortened). This means thatnot so much a fast triggeringsystem « IJlsec) is required for the klystron protection. The basic parameters of the modulator aresummarized in Table I. More details are discussed in the reference 4.TABLE I Basic Parameters.(* indicate at 600 JlSec pulse width).(I) Modulator OutputPeak PowerPulse WidthPulse Rise TimePulse Repetition RateAverage Power(2) Pulse TransformerStep-up Ratio(3) PFN parameterCell NumberImpedanceCharging VoltageFlat top rippleInductance/cellCapacitance/cellKLYSTRON AND DUMMY LOAD15 MW ; 20 kV x 750 A400 JlSec , (* 600 Jlsec)< 25 JlSeC50pps300 kW , (* 450 kW)7: 134, (*50)26.7040kV<0.5%160 flH0.22 JJ.FA high power pulsed klystron (Thomson, TH2104A) which can produce a peak power ofmore than 5MW ata600Jlsec pulse duration was installedat the test station todo high power test ofthe modulator, thepulse trans-former, the klystron itself and other components. The TH2104A tube, according to the factory data, canproducea 7.3MWpeakpowerfor areduced pulseduration (270Jlsec) when the 140kVand 108 Abeam poweris supplied. The produced RF power is directory led to a dummy load (Varian, L650BA4) guided by a WR-650 waveguide system which is filled with a SF6 gas at 1.5 bar absolute pressure for preventing breakdownof RF windows of the klystron and dummy load. The klystron was operated at 1298 MHz and 400 JlSec ofpulse duration. Typical operational parameters of the klystron are described in the next section. Thewaveguide type water dummy load with a beryllia ceramic window has capability of a 5MW peak and 300kW average power. The limitation to the energy perpulse is also exist; the maker guarantee up to 2000 joules/pulse which correspond to a 5MW peak and 400 J.1sec duration. A factory data sheet shows a small VSWR« 1.1) for a water temperature range between 15°C and 30°C. More powerful dummy load system, for ex-ample parallel use of it, is needed at the 6MW peak and 600 flSec duration power test.mGH POWER lESTThe output power of the klystron is calibrated by a calorimetric measurement of the dissipated power at thedummy load. The power measured by a directional coupler mounted on the waveguide is consistent with thecalorimetric measurementwithin 5% discrepancy. Neither adissipation power at the waveguide nora escapeof the heat to atmosphere are taking into account in the calorimetric measurement, because these correctionsare negligible small compared to the uncertainty of the temperature difference and water flow rate54/[1496] M. ONOET AL.measurements. A calculated dissipation power at the waveguide is around 144W/m for 5MW peak and 3%duty operation for an aluminum waveguide and a temperature rise at the surface ofthe waveguide is estimatedaround 30°C at 30°C atmosphere. The measured temperature rise of the surface is quite consistent with thecalculated one, even though this comparison has been done at low duty (0.3%) operation with 5MW peakpower. In any case, a lowering the temperature rise orcooling of the waveguide may be required at the higherduty operation. In the calorimetric measurement, the pulse width of the drive power is reduced to 300 J.1secto produce an almost rectangular shape output power as shown in Fig. 2. Other quantities such as cathodevoltage, beam current, reflection power and phase fluctuation are also shown as indicated in the Fig. 2.I50 lJSec/div I Output Power = 5.1 MWCathode Vol. =130 kVFIGURE 2 Typical pulse shape data at 5MW output.The rise time ofthe cathode voltage is around 28 J.1sec at this moment A small dip near the middle of the pulsetop can be seen in the phase measurement even though no clear dip can be observed in the cathode voltageat this measuring scale. We don't yet finish the fmal adjustment of the inductance of the PFN for flatteningthe pulse top and shortening the rise time. Figure 3 shows the phase variation as a function of the cathodevoltage (Vk). Where the phase difference is measured between the drive and output powers. Using the slopeof Act> = 8.4°/kV and the phase dip of Fig. 2, the cathode voltage variation is estimated to be the order of 0.2%. The output versus drive powers at several operation points are shown in Fig. 4. Unfortunately, owing toapower limitationofthepresentdriveramplifier, the saturation is notclearlyobservedeven though the factorydata show the saturation at 120W drive power for 5.4MW peak power generation. Typical operationparameters at 130 kV are shown in Table 2. The measured gain and the efficiency are also a little less thanthe factory values as shown in Table 2. We don't yet resolve these discrepancy.~V,/'/ //'/',.Slope 8.4d eg.1 kV/~,/',1801601400 120(1)~c: 100.2ca·Cm 80>(1)C/') 60m.r:.a..4020o110L-BAND HIGH-POWER RF SOURCE114 118 122 126Cathode Voltage (kV)FIGURE 3 Phase variation vs. cathode voltage.TABLE II Typical operation parameters.(3rd column show the factory data).130[1497]/55Peak PowerPulse DurationCathode VoltageBeam CurrentJ,LPerveanceEfficiencyGainDrive PowerDISCUSSION5.2MW300 fJSeC130kV92.4 A1.9743.3 %44.2 dB200W5.4MW600 J,JSeC125kV91 A2.0647.5%46.5 dB120WThe prototype L-band test station can now generate a 5MWpeak power for around 400 Jlsec ofpulse duration.The extension ofthe pulse width ofthe modulator to 600Jlsec is feasible as we did at the pulse width wideningfrom 200 fJSeC to 400 fJSeC. However, we expect severe operational situation of the main thyratron at thehigher duty operation even though the thyratron is replace<f4 to a currently most powerful thyratron (ITT F-259; 50 kV, 10 kA peak and 25 A average) from two ITT F-175 thyratrons connected in parallel. Actually,even after the replacement, we observed a less stable operation of the modulator when the repetition rate isincreased at the present400Jlsecpulse width. In that case, a delicate adjustment ofthe operational parametersof the thyratron is required; for example, fine tuning of the reservoir voltage. The parallel use of the two F-259 thyratrons as the main switching device as shown in Fig. 1 will be one solution to withstand the higherduty operation. At the initial phase of this modulator, where the ITT F-175 thyratrons were used4, we havealready tried parallel operation of the thyratron and we feel confident of such parallel operation. For thecrowbar thyratron, the ITT F-175 is still used, because of a low duty operation for positive reflection short56/[1498] M.ONOETAL.and very low frequency of the true crowbar work. We are also seeking another possibility to use solid statedevices such as a SCR and GTO instead of tubes. Needless to say, the feasibility of the high duty and highpower klystron is the most essential point to attain our fmal objective. As stated before, the TH2104A tubealready produce 7MW for 270 Jlsec pulse duration. We should check the actual state of the art for presentavailable klystrons; how far is it from our goal, or is it already reached there ?25050 100 150 200Drive Power (W)FIGURE 4 Output power vs. Drive power.~-----..........--~ ,.-- ~" ."Hj A...-A~~/."~~~ :r- 8 .1 A~~/r.-.-.~)[r- ~ ............-.. I~ .ts A..........4cV,.......~ r 6~ A.....J,c,......... tsearr liurrerto 0-4i(j)"~ 3a..S.e-::)0256REFERENCES1. Y. Yamazaki et al., Proceeding of the Advanced Hadron Facility Accelerator DesignWorkshop, LA-11432-C, 80 (1988).2. Y. Yamazaki et al., "The 1 GeV Proton Linac for the Japanese Hadron Facility",Proc. 1988 Linear Accelerator Conf., Williamsburg, VA, USA.3. H. Hanaki et al., "Control of an RF Amplifier for Japanese Hadron Facility",Proc. 1988 Linear Accelerator Conf.,Williamsburg, VA, USA.4. S. ADami et al., "Design and Construction of a Long-Pulse Modulator",Proc. 1989 Particle Acceleration Conf., Chicago, IL, USA.5. Ta. Kubo et al.,"Design and Construction of a Pulse Transformer for a Long Pulseklystron", Contribution paper to this Conf.

Development Of L-band High Power Rf Source For The Japanese hadron Project proton Linear Accelerator

Development Of L-band High Power Rf Source For The Japanese hadron Project proton Linear Accelerator

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