Fujieda, M

Kanazawa, M

Mori, Y

Muramatsu, R

Noda, K

Ohmori, C

Sato, Y

Takagi, A

Uesugi, T

Yamamoto, M

Yoshii, M

CERN Document Server

MULTI-HARMONIC ACCELERATION WITH HIGH GRADIENT MACAVITY AT HIMACM. Yamamoto, M. Fujieda, Y. Mori, R. Muramatsu, C. Ohmori, Y. Sato, A. Takagi,T. Uesugi and M. Yoshii KEK-Tanashi, Japan; M. Kanazawa and K. Noda NIRS, Chiba, JapanAbstractA high field gradient cavity(HGC) which has been devel-oped for high intensity proton synchrotrons is suitable togenerate multi-harmonic voltage on the accelerating gapbecause of the broad band impedance of the cavity. Inthis paper, the experimental results on the dual harmonicrf, saw-tooth rf, and bunch compression by barrier bucketare described.1 INTRODUCTIONWe have developed a prototype HGC loaded with Mag-netic Alloy(MA) for synchrotrons. We have succeeded toachieve the field gradient of 50 kV/m [1], because of highstability of the MA core under the large rf magnetic field.Another advantage of the HGC is that it is a broad-bandcavity. High accelerating voltage over the wide frequencyrange can be obtained without the tuning system. In such awide band system, some different frequencies can be gen-erated simultaneously on the gap. The bunch shape can bemanipulated by the higher harmonics, it leads to suppress-ing the space charge effects. The HGC has been installed inHeavy Ion Medical Accelerator in Chiba(HIMAC) for thebeam experiments.2 EXPERIMENTParameters of the cavity are listed in Table 1. Using thecavity, the beam acceleration has been successfully carriedout [2].Freq. Range 1  8 MHzGap Voltage Max. 4 kVShunt Impedance 400 Ω @ 2 MHz(4 cores loaded)Q value 0.6Amplifier push-pull, 60 kW,4CW30,000A 2Length 40 cmTable 1: Parameters of the HGC installed in HIMAC.The experiment with the higher harmonics were demon-strated on the flat base(no acceleration). The control sys-tem of the higher harmonics for the acceleration is underdesigning.E-mail: masanobu@tanashi.kek.jpSome beam parameters are shown in Table 2.Particle 4He2+Number of Particle at Injection 1:4 1014 ppp(Typical)Injection Energy 6 MeV/uMomentum Spread at Injection 0.1 %Revolution Frequency 261.3 kHzTable 2: Parameters of the beam.2.1 Dual Harmonics MixingThe beam capture experiment by dual harmonic rf has beencarried out. Usually, the experiments with dual harmonic rfneed another cavity for the higher hamonics [3, 4]. In thisexperiment, only HGC has been used. The second higherharmonic was mixed with fundamental rf and the dual har-monic voltage on the gap was obtained. The gap voltageVgap and the potential U are written as;Vgap() = V1 sin + V2 sin 2(− s) (1)U() =Z s(Vgap()− V1 sins)d= −V1 f(cos− coss) + (− s) sinsg+12V2 [cosf2(− s)− 1g] ; (2)where V1 and V2 are the amplitude of the voltage for thefundamental rf and the 2nd higher harmonic one, respec-tively and s is a synchronous phase and s = 0 for the flatbase. The voltage and the potential well in case of V1 = 1kV and V2 = 500 V are shown in Fig. 1.Gap Voltage and Potential Well-1-0.500.51-400 -200 0 200 400(a) seconddualfundamentalTime(nsec)Voltage (kV)0246810-400 -200 0 200 400(b)singledualTime(nsec)Potential Well(keV)Figure 1: The gap voltage and the potential well for thedual harmonics.0-7803-5573-3/99/$10.00@1999 IEEE. 863Proceedings of the 1999 Particle Accelerator Conference, New York, 1999The bottom of the potential well in case of the dual har-monic rf(solid line) becomes more flat than that of the sin-gle rf (dotted line) as shown in Fig. 1-(b). Since a bunchshape is related to the shape of the potential well, it is ex-pected that the bunch is flattened and lengthened in case ofthe dual harmonic rf. It leads to alleviating the space chargeeffects.Bunch Shape300 400 500 600 700 800 900 1000 1100 1200single(a)Time (nsec.)Bunch Shape(arb.)300 400 500 600 700 800 900 1000 1100 1200dual(b)Time (nsec.)Bunch Shape(arb.)Figure 2: Experimental results of bunch lengthening bymixing the 2nd higher hamonic rf.The bunch shape measured by an electro-static beammonitor is shown in Fig. 2. Figure 2-(a) and -(b) are theshapes for the single and dual rf, respectively. As seenclearly, the bunch shape became flat by the dual harmonicrf, then the bunching factor of 0.28 for the single rf wasincreased to 0.4 by the 2nd harmonic. From Eq. (2), thebunching factor for the beam with momentum spread of0.1 % can be calculated numerically, and they are 0.27for the single rf and 0.4 for the dual harmonic rf. It hasgood agreement with the experimental results.2.2 Saw-tooth RFIn the case of single rf frequency, non-linearity appears forthe particles which have large amplitude in the longitudi-nal phase space. On the other hand, if complete saw-toothvoltage like Vgap = V0  (− <  < ) (solid line in Fig.3) is applied to the rf acceleration, then linear rf bucket willbe obtained, and it is useful to investigate non-linear effectsby comparing with the sinusoidal one.The saw-tooth voltage is expressed as the Fourier series;Vgap =2V01Xn=11nsinn ; (3)where V0 is the amplitude of the saw-tooth voltage. Wehave produced pseudo saw-tooth voltage by combining upto the 3rd higher harmonics in Eq. (3).The measured gap voltages in case of the pseudo saw-tooth rf(bold line) and the sinusoidal rf(dotted line) areshown in Fig. 3-(a). The amplitude of the sinusoidal rfis adjusted as the gradient at  = 0 is equal to the gradientof pseudo saw-tooth one. Because the saw-tooth voltagemakes a potential well to be a complete parabolic shape,the bunch shape in the the saw-tooth rf will become moreparabolic than that of the sinusoidal one. Figure 3-(b)shows that the bunch shapes for the saw-tooth rf(solid line)and the sinusoidal rf(dotted line).Saw vs Sinusoidal-3-2-101230 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2(a)Time (µsec)Gap Voltage(kV)-0.05-0.04-0.03-0.02-0.0100.010.020.030.040.050 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2(b)Time (µsec)Bunch ShapeFigure 3: The measured gap voltage and the bunch shapein saw-tooth rf.We assume the amplitude of the synchrotron motion issmall to obtain the equation of the synchrotron frequencyfor the sinusoidal voltage;!s =sheV !rev2jj cos s22E0; (4)where h is harmonic number, e is unit electric charge, !revis revolution frequency,  is slippage factor,  = v=c, E0is total energy of the particle. However, if the beam emit-tance occupies the large area of the rf bucket, then the mea-sured synchrotron frequency tends to deviate from Eq. (4)and become smaller value [5]. On the other hand, the syn-chrotron frequency at large amplitude of the synchrotronmotion is as same that at small one for the saw-tooth rf. Themeasured synchrotron frequency should be consistent withEq. (4). Actually, the measured synchrotron frequency of1.03 kHz had good agreement with calculated one of 1.08kHz for the saw-tooth rf, althogh the measured one was0.75 kHz for the sinusoidal rf.2.3 Barrier BucketBarrier bucket is a scheme to manipulate the bunch lengthby moving a single sinusoidal voltage [6]. It is mostly usedto store more beam in the ring [7]. Conversely, it is possibleto make coasting beam into one high density bunch by thebarrier backet, then it is profitable to produce high peakcurrent for the study of the wake field.In this experiment, the barrier voltage was 2 kV, whichcorresponded to the bucket height of 0.4 % with respectto the momentum spread. The bunch of 3.8 sec length was864Proceedings of the 1999 Particle Accelerator Conference, New York, 1999compressed into 1.4 sec by the barrier bucket gymnasticsspending 100 msec to conserve the longitudinal beam emit-tance.Mountain View of Gap Voltage and Bunch Shape(Barrier 2kV)024681012142 3 4 5 6 7Time (µsec)Gap Voltage in Time Evolution0.40.60.811.21.41.60 1 2 3 4 5Time (µsec.)Bunch Shape in Time EvolutionFigure 4: Bunch compression by the barrier backet opera-tion.Figure 4 shows the experimental result, the upper sideshows the barrier voltage gymnastics and the lower sideshows the bunch shape in the mountain range view. The in-terval of the trace is 4 ms. The compression was performedfor the beam of 7 1010 ppp. Since a half of the beam waslost, the peak current was not so high that the beam inducedvoltage has not been observed.3 SUMMARYThe experiments of the multi-harmonic rf have been suc-ceeded by a HGC due to its broad band impedance. Al-though the beam was lost, the bunch shape could be ma-nipurated.4 ACKNOWLEDGEMENTWe would like to thank AEC staffs for their careful opera-tion of HIMAC. We would like to thank to K. Sato and S.Ninomiya for their valuable suggestions and comments.5 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-p301, 1998[2] R. Muramatsu et al., ’The First Beam Acceleration Test usingHigh Gradient Cavity at HIMAC, in this conf., 1999[3] P. Bramham et al., IEEE Trans. Nucl. Sci. NS-24, No.3,p.1490(1977)[4] J.M. Baillod et al., IEEE Trans. Nucl. Sci. NS-30, No.4,p.3499(1983)[5] K. Koba, Longitudinal Impedance Tuner using High Perme-ability Material, in this conf., 1999[6] J.E.Griffin et al., IEEE Trans. Nucl. Sci. NS-30, No.4,p.3502(1983)[7] M. Fujieda et al., ’Magnetic Alloy Loaded RF Cavity for Bar-rier Bucket Experiment at AGS’, in this conf., 1999865Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Multi-harmonic Acceleration With High Gradient Ma Cavity At Himac

Multi-harmonic Acceleration With High Gradient Ma Cavity At Himac

Abstract

Similar works

Full text

Available Versions

CERN Document Server