Kamino, Y

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10 MeV 25KW INDUSTRIAL ELECTRON LINACY. KaminoMitsubishi Heavy Industries, Ltd. Nagoya Aerospace Systems10 Oye-Cho Minato-ku Nagoya,455 JapanAbstractA 10 MeV 25 KW class electron LINAC was developed forsterilization of medical devices. The LINAC composed of astanding wave type single cavity prebuncher and a 2 m electro-plated traveling wave guide uses a 5 MW 2856 MHz pulseklystron as an RF source and provides 25 KW beam power atthe Ti alloy beam window stably after the energy analyzingmagnet with plus-minus 1 MeV energy slit. The practicalmaximum beam power reached 29 KW and this demonstratedthe LINAC as one of the most powerful S-band electronLINACs in the world. The control of the LINAC is fullyautomated and the "One-Button Operation" is realized,which isvaluable for easy operation as a plant system. 2 systems havebeen delivered and are being operated stably.IntroductionA 20 KW (beam power) class LINAC has the greatestmarket in the electron beam sterilization market. Several typesof 20 KW class S-Band (2856 MHz or 2998 Mhz) LINACshave been developed and marketed but most of them areunstable at the 20 KW operation and the practical beam poweris limited to under 20 KW range. Under this situation, a small,stable and efficient S-Band sterilization LINAC with thecapacity of 25 KW plus class (marginal beam power) is lookedforward to for an expanding sterilization needs. We havedeveloped a 25 KW plus class sterilization LINAC systemsuccessfully and two systems are operated at the customer'splant stably. In this paper, the detail of this new age 25 KWplus class S-Band electron LINAC is presented.Design ConceptThe main cause of the instability of most of the S-BandLINACs is the thermal instability of the accelerator guideunder an intense heat load. In some cases, 20 to 30 KW RFpower is dissipated in a 1 to 2 m accelerator guide with lessthan 10 cm diameter cavities. Beam loss in the acceleratorguide gives an additional lumped heat load to some part of theaccelerator guide where the beam is lo t. The intense anduneven heat load distorts the cavities of the accelerator guide todestruct a correct acceleration phase relationship and cause thebeam instability.Our design concept is to reduce the RF loss and beam lossto the theoretical limit.System OutlineThe system construction is shown in Fig. 1. The systemblock diagram is shown in Fig. 2. The operational parameterand the performance is as follows.Beam Energy :10 MeV variable from 9 to 11 MeVEnergy Spread :1 MeV (90% beam current)Beam Power at :25 KW (guaranty)  PPS = 550 Hzbeam window 29 KW (actual)      PPS = 630 HzPPS : 700 Hz (Max)Beam pulse width :13.5 micro sec (nominal)Beam current :340 mA peak (nominal)Frequency :2856 MHzKlystron power : 5 MW peakIrradiation Surface :60 cm from the beam windowBeam spot size :16 cm diameteron the irradiation surfaceBeam Scan Width :30 cm to 80 cm variableScan Uniformity :within plus minus 5% (guaranty)ELECTRONGUNPREBUNCHERKLYSTRONTANKACCELERATORGUIDESCAN MAGNETSCAN HORNTi ALLOY BEAMWINDOWENERGYSLITGUN HVDECK KLYSTRONINPUT WAVE GUIDERF WINDOW270deg BENDING MAGNETOUTPUT WAVE GUIDEDUMMY LOADSOLENOIDFig. 1:  System construction (accelerator side view).Accelerator GuideA traveling wave CG type guide is selected because acirculator and dummy load system for the input wave guidewhich constitutes a major loss can be deleted with thetraveling wave guide and the klystron output port can beconnected directly to the input port of the accelerator guide.The guide parameter is selected as 2 m long (60 cavities) and0.38 Neper (attenuation factor) CG type to get the 10 MeVbeam energy at the nearly heaviest loading condition withavailable klystron power of 4.8 MW at the guide input by theload line analysis. The beam current is 360 mA. The initial 8cavities of the accelerator guide is the tapered buncher sectionand the disk spacing is determined by the electron phaseanalysis to get the best bunch suppression ratio for a tightenergy spectrum and the phase limit of - 90deg (just on thecrest of the accelerating field) for the best energy conversionefficiency. The energy conversion efficiency of the acceleratorguide with the injection in the next paragraph is as high asabout 70% (measured value)and nearly the theoretical limit.This good energy conversion efficiency reduces the heat load ofthe accelerator guide to as low as 10 KW (average) under the25 KW beam power operation.LINAC 96836The new cooling scheme of the acc lerator guide -ADAPTIVE COOLING (PAT. Pending) is introduced tocontrol the phase relationship under the intense and unev nheat load.InjectorThe electron gun is a triode gun. A rather high gun voltage(140 KV) is selected for a better capture of injected electronbeam at the buncher section. The gun voltage is supplied fromthe klystron modulator as a pulse voltage from the pulsetransformer for the klystron. The electrode geometry of theelectron gun is designed with simulation code E-GUN to getthe best optics at the designed beam current of 400 mA. Thegun cathode assembly is CPI EIMAC Y-845. The electron gunhas a grazer type magnetic lens to compensate the variation ofthe optics due to the mounting tolerance of the cathode. Thebeam current is controlled by the grid voltage and stabilizedwith a feedback control. The current stability is less than plusminus 1% for a long period of operation.The prebuncher cavity is a nose reentrant type cavity tosuppress a multipactor discharge. The cavity is made of OFHCand the RF power is supplied with a over-coupled condition(coupling coefficient beta = 30). The effective Q factor is 300.This coupling scheme realizes a broad frequency characteristicsto allow the frequency control of the accelerator guide (forreactive beam distortion compensation and adaptive cooling)and to reduce the phase and amplitude perturbation due to thebeam loading.Beam DynamicsThe beam dynamics is evaluated with PARMELA. Thecalculated and measured transmission efficiency are 90.2% and90% respectively. Both values coincides fairly well. Thecalculated and measured beam profiles coincide well both in thespatial distribution and in energy distribution. The system isdesigned as a practical production machine and can notaccommodate a farady cup and a slit for quantitative energyanalysis. The energy spectrum is measured with a fluorescentplate (Desmarquest AF995R) at the slit position. The beamprofile (Fig. 3) shows that the almost all current is included inthe range of 1 MeV. The FWHM is about 0.4 MeV. There isno low en rgy tail which is usually observed for the industrialand medical LINAC systems which becomes a large amount ofcu rent integrated over a wide energy range and constitutes alarge amount of beam loss at the energy slit. There was somehigh energy component observed but the componentdis p eared after the gun triggering timing alignment to theRF pulse to compensate an initial beam loading effect. Thebeam loss in the accelerator guide is 10% and the beam loss ofthe energy slit is 5%. The total beam loss between the electrongun and the beam window is only 15%.The heavy beam loading causes a reactive phase distortion.This is compensated by adjusting the RF frequency upward byabout 200 KHz to get the heaviest beam loading [1, 2].Beam Break Up ConsiderationThe beam break up (BBU) is evaluated thoroughly, becausea longer pulse width (13.5 micro sec) compared with otherexisting LINAC and a rather large beam current (400 mA) willlead to the BBU which results in a fatal failure in satisfyingthe performance specification.Fig. 2:  System block diagram.LINAC 968371.0 MeV0.5 MeV0 (10 MeV)–0.5 MeV–1.0 MeVFig. 3: Energy spectrum easured at the energy slit. Energyanalyzed beam profile at the energy slit position onDesmarquest AF995R plate.A Regenerative BBU (R-BBU) is the BBU expected for thisLINAC. The starting current of R-BBU is inverselyproportional to the electron beam energy and occurs at theinitial part of the accelerator guide where the electron beamenergy is low. In CG type accelerator guide, HEM11 modesexcited in the upper stream region can not go through thedown stream region, because the HEM11 modes excited in theupper stream region fall in the stop band in the down streamregion and are trapped and become standing waves. The 15cavities (tapered buncher section-8 cavities and initial part ofCG section-7 cavities, 15 cavities in total) are modeled and alldipole modes are sorted out with MAFIA. There are 10 dipolemodes. In these dipole modes, pi-like modes are selected andthree modes with the highest shunt impedance are evaluated asfollows. The starting current estimation is based on Wilson'smethod [3, 4] including the effect of the magnetic focusing andthe pulsed nature. The following value is a rough estimation,but a great margin is secured for 400 mA operation. There isno indication of BBU observed in the real operation and thebeam pulse shape is quite stable.     Mode     Dipole Mode Freq.     Starting Current  No. 3 4213.2 MHz 3.25 ANo. 4 4226.8 MHz 2.10 ANo. 5 4234.8 MHz 3.09 AKlystron ModulatorThe klystron modulator is a line type pulser with a highaverage current thyratron (EEV CX-1720MN) as a mainswitching device. The command charging is introduced tosecure the recovery time of the thyratron and a long life ofPFN capacitors by minimizing the HV time of the capacitors.The klystron is Thomson TH-2154.System ControlThe whole accelerator system is controlled by aProgrammable Logic Controller. A whole monitor and controlfunction is integrated on a LCD touch sensing panel to realize'ONE BUTTON CONTROL'. The communication with othercomputer system is provided to realize a fully remote controland one-man operation of a whole sterilization plan . Theaccelerator system reaches the predetermined operationalcondition from the stand-by mode in 1 minutes after the startup command full automatically.ConclusionA 10 MeV 25 KW plus class electron LINAC forsterilization of medical devices is successfully developed withthe state of the art LINAC technology. Two systems are beingoperated stably at the customer's sterilization plant.AcknowledgementThe author is grateful to Dr. P.B. Wilson of SLAC for hisassistance in study of beam break up theory andDr. J.W. Wang, also of SLAC for his theoretical assistanceand prebuncher development. The author is grateful toengineers at Nihon Koushuha Co. Ltd. for their contributionin developing a Klystron modulator and RF components (RFwindow, etc).The author is grateful to HOGY Medical Co. Ltd. forgiving him the chance to develop the significant system andvarious valuable suggestions as a pioneer in the field ofcommercial high energy electron beam sterilization in Japan.References[1]S. Arai et al., "Detuning effect in a traveling-wave acceleratorsturucture due to beam loading", Particle Accelerators, 1984,vol. 15, pp. 99–114.[2]S. Arai et al., "Detuning effect in a traveling-wave electronLINAC", Particle Accelerators, 1984, vol. 15, pp.115–136.[3]P.B. Wilson, HEPL Report 297, June, 1963.[4]P.M. Lapostolle and A.L. Septier, "Linear Accelerators",North-Holland Publishing Company, 1970, pp. 190–192.LINAC 96838

10 MeV 25 KW Industrial Electron Linac

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