Ezura, E

Fujieda, M

Kanazawa, M

Mori, Y

Muramatsu, R

Nakayama, H

Noda, K

Ohmori, C

Sato, Y

Takagi, A

Toda, M

Uesugi, T

Yamamoto, M

Yoshii, M

CERN Document Server

HIGH FIELD-GRADIENT CAVITIES LOADED WITH MAGNETICALLOYS FOR SYNCHROTRONSC. Ohmori, E. Ezura, M. Fujieda, Y. Mori, R. Muramatsu, H. Nakayama,Y. Sato, A. Takagi, M. Toda, T. Uesugi, M. Yamamoto, and M. Yoshii, KEKM. Kanazawa and K. Noda, NIRSAbstractVery high field-gradient has become available by a newMagnetic Alloy (MA)-loaded cavity developed for high in-tensity proton synchrotrons. The available RF voltage percore is ten times larger than that of the ordinary ferrite core.The maximum voltage of 20 kV has been achieved by theHigh-Field Gradient Cavity(HGC) of 40 cm in length[1].Because the intrinsic Q-value of the MA core is low, ac-celeration without any tuning system also becomes pos-sible. The first beam acceleration test using the HGChas been performed successfully at the HIMAC(Heavy IonMedical Accelerator in Chiba). Furthermore, the dual har-monic RF and barrier bucket experiments have been carriedout.Another advantage of the MA-loaded cavity is that it iseasy to compensate the beam loading. The feed forwardbeam compensation was applied for both HGC on the testbench using electron beam and MA-loaded cavity installedin the AGS for the barrier bucket experiment. A new devel-opment for high-Q HGC using cut core configuration willbe also reported.1 INTRODUCTIONThe field gradient of a RF cavity is one of the most im-portant parameters to design a high intensity machine. Inorder to minimize the circumference of the synchrotron andto increase the repetition rate of acceleration cycle for thehigh beam current, high field gradient is required. In caseof JHF(Japan Hadron Facility)[2], the total RF voltage of420 kV is necessary for the 3 GeV Booster synchrotron.Some future upgrades including the energy up to 5-6 GeVand/or 50 Hz operation require about 800 kV. So far, themaximum field gradient in the frequency region of 2-3.4MHz was about 15 kV/m for the ferrite-loaded cavity be-cause of the characteristics of the ferrite core. The shuntimpedance of ferrite core becomes very low at the mag-netic flux density of few 100 Gauss because the saturationflux density(Bs) is small as shown in Fig. 1. If ordinaryferrite-loaded cavities are used for the JHF, the total lengthfor them becomes about 60 m. It means that most of thelong straight sections are occupied for the RF stations.To achieve high field gradient, the characteristics of themagnetic core should be stable in high RF field. Recently,soft magnetic alloys (MA) with high permeability, such asFINEMET[3], Metglas[4] and other amorphous have beendeveloped. Some of the cores have very high saturationE-mail: chihiro@insuty.tanashi.kek.jpflux density of more than 10 kGauss, as shown in Fig. 1.On the other hand, the hysteresis loss in the ferrite core willbe increased at the large RF amplitude. However, the lossin the MA core does not depend on the RF amplitude. TheMA has conductivity and the RF power is lost mainly bythe eddy current in the material. To reduce the eddy loss,the material is composed of the tape of 16 m thicknesswhich is insulated by a thin Silica layer.Bs-MABs-ferriteBHferriteMagnetic AlloyFigure 1: The hysteresis curves of magnetic materials. Bsmeans the saturation magnetic flux density. The usableBRFis much smaller than Bs.The characteristics of the MA and ferrite cores havebeen measured by a test bench using a 1 kW transistoramplifier[3]. The results are shown in Fig. 2. To evaluatethe characteristics of magnetic materials, the product of rel-ative permeability(), quality factor of the material(Q) andfrequency(f), so called Qf -product has been used. TheQf -product is given by Rp = 0 t ln(a=b) (Qf),where Rp, t, a and b are shunt impedance, thickness,outer diameter and inner diameter, respectively. The Qf -product depends on the magnetic flux density in a core,BRF. It is given by BRF=V!S, where V and S are thevoltage and cross section of the core, respectively. BRFis the averaged value in the core and the flux density nearthe inside is larger than it. The MA core shows an excel-lent characteristics at very high magnetic flux density of2 kGauss. The characteristics of a ferrite core are also0-7803-5573-3/99/$10.00@1999 IEEE. 413Proceedings of the 1999 Particle Accelerator Conference, New York, 1999shown in the figure. Below 100 Gauss, ferrite core haslarger Qf -product than the MA core. However, the MAhas larger value at the large RF amplitude. Another mea-surement using 30 kW tetrode amplifier also shows a largeMA core of 550 mm O.D. was stable at the magnetic fluxdensity of 1.3 kGauss and 5.5 kV was obtained by the corewhich had the thickness of 2.5 cm.0.00E+001.00E+102.00E+101 10 100 1000 10000Brf (Gauss)upQfSY-2FT3-smallFT3-largeFigure 2: Magnetic flux density dependence of magneticmaterials. SY2 is a typical ferrite core and the magneticflux density dependence was observed. FT3-small and -large are MA cores of 570mm O. D. and 70 mm O.D., re-spectively. The MA cores have a constant shunt impedanceup to 2 kGauss.The characteristics of MA core are summarized as fol-lowings;(1) Qf -product of the MA core does not depend onthe flux density. One of MA cores, FINEMET, has largersaturation flux density than other MA and can be used atthe very high RF flux density of 2 kGauss.(2) The intrinsic Q-value of the MA core is 0.6-1. It issuitable for the wide frequency sweep without tuning sys-tem, barrier bucket, and multi-harmonic RF acceleration.(3) The permeability of the MA core is about 10 timeshigher than that of ferrite. Although the Q-value is low, theQf -product is not small to use for RF cavity.(4) The MA core has very high Curie temperature of570oC. And the characteristics of the core remained con-stant at high temperature of 170oC.(5)The Q-value of the core can be changed up to morethan 10 by using the cut core technique(see section 5). TheR/Q-value of the cavity is a variable without changing theshunt impedance.(6) The core consists of thin MA tape which is coated bysilica insulator. It is possible to make a large core of about100 cm diameter.2 HIGH-FIELD GRADIENTCAVITY(HGC)2.1 High Field GradientA prototype of HGC has been developed as shown in Fig.3. The cavity has one acceleration gap and consists of twosets of 14 cell. Each cell composed of 3 MA cores and theyare installed in a water tank to cool by water directly. Thesize of MA cores is 305mm I.D., 670mm O.D. and 26mmthickness. The impedance of the cavity is shown in Fig.4. The impedance is very broad because of the character-istics of core. The cavity has been driven by a push-pullamplifier using two sets of 150 kW tetrode, 4CW150,000.The maximum voltage of 20 kV has been achieved for thepulse operation. The field gradient per cavity length was50 kV/m. For the high duty operation, 15 kV has been ob-tained. The voltage and duty are limited by the capacitiesof the plate power supply and cooling water.Figure 3: High field gradient cavity. The length of the cav-ity is 40 cm.2.2 Fast Frequency SweepBecause of the broadband impedance of the MA core, anydelay effcts on the magnetic material, such as a magnetic-after effect of ferrite core, has not been observed. Very fastrepetition of about 1 kHz for the frequency sweep has beenachieved. Figure 5 shows the envelope of the gap voltagewhen the 1-5 MHz frequency sweep was performed in 1.1ms.3 BEAM TEST AT HIMACThe HGC has been installed in the HIMAC for the beamtest. The HIMAC is a heavy ion synchrotron for the cancer414Proceedings of the 1999 Particle Accelerator Conference, New York, 1999-150-100-500501001502002503003500 5 10 15 20 25 30Frequency (MHz)Z(Ohm) , Phase(deg) PhaseRe(Z)Im(Z)Figure 4: Very broad impedance curve of the HGC. Theresonance frequency was shifted when the high power am-plifier was connected.Figure 5: The envelope of the gap voltage while the fastfrequency sweep was performed. Because of the character-istics of the drive amplifier, the RF voltages was low at 1MHz.therapy. The main parameters of the HIMAC are listed inTable 1.Table 1: Main Parameters of HIMACCircumference 129.6 mInjection Energy 6 MeV/uExtraction Energy 800 MeV/uRF frequency 1-8 MHz for Si1-5 MHz for HeHarmonic Number 4Repetition 0.3 HzRamping Time 1 sBeam Intensity 1:4 1011 at InjectionExtraction Slow Extraction3.1 Beam Acceleration without Any FeedbackLoopBeam acceleration test has been performed for several ionbeams, He, C, Ne and Si[5]. Because the HGC has a verybroad impedance, it can be used for 1-8 MHz without thetuning system. The block diagram of the HGC accelerat-ing system is shown in Fig. 6. The preprogrammed volt-age control system was employed to obtain the constant RFvoltage during acceleration. Without preprogramming, theRF voltage became rather low at the particular frequenciesbecause of the gain characteristics of the driver amplifier.The RF voltage was gradually changed during the adiabaticcapture process and beginning of the acceleration. The pre-programming controlled the gap voltage accurate enoughto accelerate the beam without beam loss. The AGC(AutoGain Control) loop was not necessary to control the gapvoltage. The RF frequency pattern for the beam acceler-ation using another ferrite-loaded cavity is stored in thememory of a DDS system. The same RF frequency pat-tern was used for the beam acceleration using the HGC.PINDiodeAttenuatorDDSArbitoraryFunctionGeneratorHP33120AThamway1 kWTransistorAmplifierPowerAmplifier4CW30,000X2HighFieldGradientCavityStartDDS Output Modulated RF Signal Gap VoltageModulation PatternfgaiCharacteristics of  1kW AMPBeamFigure 6: The Block Diagram of the HGC accelerating sys-tem.3.2 Multi-harmonic ExperimentsThe HGC RF system is broadband and can generate themulti-harmonic frequencies on the gap simultaneously[6].The dual harmonic RF was applied successfully. The fun-damental RF frequency is divided by a splitter and doubledby a frequency doubler circuit. Both harmonic frequencieswere combined after adjusting the phase by a phase shiftercircuit. The amplitudes of both frequencies were controlledby PIN diode attenuators. The bunching factor of the beamwas increased from 0.28 to 0.40 by the dual harmonic RFas shown in Fig. 7. The beam loss was reduced. The DDSsystem for the multi-harmonic RF is under development forthe dual harmonic acceleration. Another multi-harmonicexperiment to produce the saw-tooth RF on the gap hasbeen performed[6].415Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Bunch Shape-0.0100.010.020.030.040.050.060.07300 400 500 600 700 800 900 1000 1100 1200Single RFDual HarmonicsTime (nsec.)Bunch Shape(arb.)Figure 7: The difference of the bunch shapes by applyingthe dual harmonics.3.3 Bunch Compression By Barrier VoltagesThe barrier RF experiment has been carried out for thebunch compression experiment[6]. The wideband RF sys-tem is suitable for the barrier bucket experiment[7, 8]. Twobarrier RF voltages were excited by the single HGC. Theshape of the barrier is under development. Figure 8 showsthe gap voltage after the improvement. The overshoot hasbeen disappeared.Figure 8: The barrier RF voltage. The barrier voltage is 1.1kV and the repetition of the barrier RF is 261 kHz.4 BEAM LOADING4.1 Beam Loading Compensation Test usingElectron BeamThe beam loading is one of the most severe problems on theRF system for the high intensity accelerator. The HGC haslow impedance and this is suitable for the stational beamloading because the relative loading parameter becomessmall. However, R/Q-value is not ideal because of lowQ-value. The beam loading effects on the HGC has beentested by the electron beam[10]. The energy of the electronbeam is 180 keV and it is modulated with the frequency of3 MHz to simulate the circulating beam. The length of thepulse train is 5 sec. The electron beam current was mea-sured by a fast current transformer and faraday cup. Thebeam transmission rate was almost 90 %. The wake volt-age was about 1.2 kV when the electron beam peak currentwas 8.6 A.The wake voltage was compensated by the feed forwardscheme as shown in Fig. 9. The electron beam currentpicked up by the CT was divided into 3 harmonics(3, 6, 9MHz) by the filters, the gain and delay were adjusted, in-dividually, and all harmonics were combined together. Thecombined signal was returned to the driver amplifier. These3 harmonics were compensated, remarkably[11].60kW AMP1kW AMPCT ElectronBeam175 keVOscilloscope1:1000H.V. Probe~9AHigh-Field Gradient CavityFilteringAttenuationDelayFigure 9: Set up of the compensation test system.4.2 Feed Forward Beam Compensation at AGSA MA-loaded cavity has been installed in the AGS forthe barrier bucket experiments[8]. The feed forward tech-nique was applied to compensate it and the voltage inducedby the 8  1012 proton per bunch was reduced up to 1/9successfully[7].5 CORE IMPROVEMENTS5.1 Cut CoreThe Q-value of the MA core can be increased by use of aradial gap in the cut core configuration without having largedeterioration of the Qf -product. Figure 10 shows the Q-value of the MA core for the various radial gap height. Bythis improvement, R/Q-value of the cavity will be reducedremarkably. In the JHF-50 GeV main ring, the peak beam416Proceedings of the 1999 Particle Accelerator Conference, New York, 1999current reaches more than 100 A because the bunching fac-tor will be 0.05 at the top energy. The very low R/Q-valueless than 100  is suitable to handle the bucket distortionby such a high peak beam current. However, we shouldnote that the beam loading for low-Q system can be easilycompensated. Furthermore, it is pointed out that Q-valueshould avoid around H/2 where H is the harmonic numberof the ring to reduce the transient beam loading effects[9].There are two choices for the JHF RF, that is, high-Q HGCwhich is very strong for the beam loading and low-Q HGCwhich is suitable for beam gymnastics and for beam load-ing compensation.0246810121416180 2 4 6 8f (MHz)Q-valueQ(0+2mm)(10+2mm)Q(15+2mm)Figure 10: The Q-value of the cut core vs. gap height.5.2 Variable-Q HGCIn order to investigate the optimum Q-value of the HGC, avariable-Q HGC is now under development. For the cavity,large cut cores of 950 cm O.D. will be used. The cavityhas two gaps and will be driven by a new high power am-plifier which use two set of 600 kW tetrode, TH558. Thebeam loading effects including the transient process will beexamined by the electron beam test bench. For the exper-iments, a new electron gun will be used to obtain the peakcurrent of 30 A.6 CONCLUSIONHigh-Field Gradient Cavity has been developed using anewly developed Magnetic Alloy and the field gradient of50 kV/m becomes possible. In the core, the field gradi-ent of 220 kV/m has been achieved. Q-value of the cavitycan be changed from 0.6(intrinsic Q-value) to more than 10by the cut core configuration. A low-Q HGC is suitable forthe barrier bucket scheme, multi-harmonic acceleration andwide frequency sweep. It is also easy to compensate thebeam loading on the cavity using the feed forward tech-nique. The scheme was applied for the HGC at the elec-tron beam test bench and for a MA-loaded cavity installedin the AGS for barrier bucket experiment. The voltage bythe intense AGS beam becomes 1/9 by this method. High-Q HGC is now under development and it will be suitablefor high intensity accelerators because of its low R/Q-valueand small shunt impedance.7 ACKNOWLEDGEMENTWe would like to thank operating crew of Accelerator En-gineering Corporation for operation of HIMAC facility. Weare grateful to Drs. Y. Tanabe, K. Saito, T. Yan, K. Koba,A. Noda, Y. Iwashita, K. Sato, F. Pedersen, R. Garoby, M.Meth, T. Roser and M. Brennan for their advices and com-ments for the HGC and MA.8 REFERENCES[1] Y. Mori et al., ”A New Type of RF Cavity for High Inten-sity Proton Synchrotron Using High Permeability MagneticAlloy”, Proc. of EPAC98, p299-301.[2] Y. Yamazaki et al., in this conference.[3] T. Uesugi et al., ”New magnetic material for proton syn-chrotron rf cavity”, Proc. of Symp. on Acc. Sci. and Tech.,Nishiharima, Japan 1997.[4] I. Bolotin et al., ”Ferromagnetic Cores Made from Amor-phous Material for Broad-band accelerating System” Proc. ofPAC95, p1833-1835.[5] R. Muramatsu et al., in this conference.[6] M. Yamamoto et al., in this conference.[7] M. Fujieda et al., in this conference.[8] M. Fujieda et al., ”MA-Loaded Cavity for Barrier Bucket Ex-periment”, Proc. of EPAC98, p1796-1798.[9] M. Yamamoto et al., ”Beam Loading Effects in JHF Syn-chrotrons”, Proc. of APAC98, p399-402.[10] Y. Hashimoto et al., ”Beam Loading Experiment with ShortBunched Electron Beam for New Type of Accelerating RFSystem of High Intensity Proton Synchrotron”, Proc. ofEPAC98, p1770-1772.[11] M. Yamamoto et al., in this conference.417Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

High Field-gradient Cavities Loaded With Magnetic Alloys For Synchrotrons

High Field-gradient Cavities Loaded With Magnetic Alloys For Synchrotrons

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