The 9th Symposium on Accelerator &ience and Technology, Tsukuba, Japan1993

RF Phase Studies of Internal and External Beams for JAERI A VF Cyclotron
Mitsuhiro Fukuda, Susumu Okumura, Kazuo Arakawa, Yoshiteru Nakamura,
Watalu Yokota, Takayuki Nara, Takashi Agematsu, limo Ishibori andTakashi Karasawa
Japan Atomic Energy Research InstituteWatanuki 1233, Takasaki, Gunma 370-12, Japan

Abstract
Beam phases have been corrected by using a phase probe
to accomplish isochronous conditions. Phase deviation has
been reduced within ±5' after a few times iterative corrections.
Phase history of the internal beam has been obtained frombeam current distributions for different frequency shifts. Phaseband width corresponds to around 57' RF for 10 MeV H+ and41' RF for 50 MeV 4He2+. Time spectrum of the externalbeam pulses has been measured with a plastic scintillator. Thepulse width for 10 MeV H+ corresponds to 7.3' and 18'
FWHM of RF phase with and without cuts made by the phaseslits, respectively.

I. INTRODUCTION
The JAERI AVF cyclotron[l] is a variable-energy, multiparticle cyclotron designed to accelerate ions in a wide range ofenergy: 5 to 90 MeV for protons and 2.5XM to 110XQ2/M
MeV for heavy ions, where M is a mass number and Q acharge state. Harmonic numbers of I, 2 and 3 are available forions with mass to charge ratio up to 6.5. The extensive beamdevelopment work is required for accelerating various kinds ofions in a wide energy range necessary for research[2] inmaterials science and biotechnology.
One of the important attributes of the internal beam is theRF phase of the beam in the acceleration process. The beamphase depends on the field deviation from isochronous
conditions. The RF phase of the beam is quite sensitive tofield pr.rtnrl-><Hion because the turn number amounts to 550 for
a harmonic number of 1, 265 for 2 and 210 for 3, which aredctnmined by a constant orbit method. The RF phase
acceptance is determined by two sets of phase defining slits
installe-d in the central region[3]. Extraction efficiency can beimproved by reduction in transmission of RF phase withinternal cuts. The width of the external beam pulse depends on
the phase width of the internal beam and energy spread. In thiswork the beam studies are focused on the beam phase behavior
in the cyclotron.

the signal cable length is within 4 mm, so that the error of thephase measurement due to the variation is negligible. The
pickup signal from the electrode detected by a digital storage
oscilloscope is shown in Fig. 1. The signal contains RF noisewith large amplitude. Relative beam phases are measured
exactly by detecting the point where the signal wave intersects
the noise wave.
Initial main coil and circular trim coil currents forgenerating the isochronous field are calculated by anoptimization code based on measured field maps. After fine.
adjustment of the base field and the central bump field, phasedeviations of the beam are converged within 30'. The measured
beam phases are used for trim-coil-current correction, made bya least-square fitting program, to minimize the field deviationfrom the isochronous field. The phase deviation can be fmallyreduced within ±5' after a few times iterative corrections.
The isochronism of the internal beam is cross-checked by
the phase drifts for positive and negative frequency shifts of.M/f=2.4Xl0-4 equivalent to the same amount of a base fieldchange 6B/B. Phase drifts for 50 MeV 4He2+ is shown in Fig.
2. The amount of the phase drifts for the positive frequency
shift is the same as that for the negative one. The measured
beam phases are consistent with the calculated ones. The fielddeviation from the isochronous condition is estimated to bewithin 1 gauss.

II. CORRECTION OF BEAM PHASE
The phase probe consists of ten pairs of rectangular
pickup electrodes placed in a radial direction. Each electrodewith the gap of 30 mm is radially 58 mm long, and Fig. 1 Pickup signal from the phase probe electrode for 10azimuthally 40 mm wide for the inner threes, 60 mm for the MeV H+ beam.
middle fours and 93 mm for the outer threes. The variation of-306-

0.5-er::: 0......
til-0.5-1~-L~~-L~~-L~~-L~~-L~

Fig. 2 Phase drifts for 50 MeV 4He2+ due to the positive andnegative frequency changes of M/f=±2.4xi0-4. Measured andcalculated phases are indicated by circles and solid lines,respectively.

0200

400

600

800

1000

8001000

Radius (mm)1350~-r~rT,-~-r~~-r~~~~~

0.5-e-.s0til-0.5-10400

600

Radius (mm)Fig. 4 Phases of the 10 MeV H+ and 50 MeV 4He2+ beams.
The circles and squares indicate the phase bands deduced
from the I vs. R curves for the positive and negative
frequency shifts, respectively.
where h is the harmonic number, r the radius, mc2 the massand ~ the peak energy gain per turn.
Beam current distributions of 10 MeV H+ for different
600
1000400800200
frequency shifts measured with a radial main probe are shown
Radius (mm)in Fig. 3. The frequency is shifted at an interval of M/f =Fig. 3 Beam current versus radius for 10 MeV H+ beam. The4xlo-4. The phase plots for 10 MeV H+ and 50 MeV 4He2+
upper and lower distributions are obtained for the positive andbeams are shown in Fig. 4. The phase slits are inserted into
negative frequency shifts at an interval of M/f=4xl0--4.
the beam of the first turn to cut the beam phase. The cut forthe 10 MeV H+ beam is different from that for the 50 MeV
beam. In case of 10 MeV H+ the phase band for a4He2+
III. PHASE DISTRIBUTION OF INTERNAL BEAM
series of negative frequency changes is shifted by sin$=0.2
The phases measured with the phase probe correspond to from that for a series of positive ones. The cause of theradially averaged ones because the pickup signals are produced difference is under investigation. The internal phase widthby the particles passing through the electrode covering the corresponds to around 57' for 10 MeV H+ and 41' for 50 MeV
4radial length of 58 mm. The phase behavior of the internal He2+. The deduced phase widths are wide in spite of thebeam is deduced from records of beam current versus radius (I phase cuts. The defining effect of the phase slits appears to bevs. R) at several radio frequencies above and below the incomplete. Figure 3 shows that the I vs. R curves for theoptimum frequency by using the method described by Smith positive frequency shifts have a long tail just before vanishing.
and Garren(4]. An additional amount in phase for the A small amount of beam still remains in the lead phase region.
frequency shift M/f with non-relativistic approximation is The phase history of the 50 MeV 4He2+ beam is shown inFig. 5. The change of phases obtained from the I vs. R curve
given byis consistent with that of the measured ones with the phase~sin<fii!F =2nh ~iRF ~me 2nfRF 'hefRF 2 ~Eprobe.

2 2()2-307-attenuator, and that accidental noise events are negligible
compared with the incident ions, full widths of the time
distribution with and without phase cuts are around 5.6 ns and.g10
8.8 ns corresponding to 30' and 47' of RF phase, respectively.
'-"
FWHM is 7.3' for the cut beam and 18' for the uncut beam.
~
The measured width of the uncut beam is narrower than that of
jg 00.
the internal beam with cuts obtained from the phase plots.
~Assumingthat the uncut distribution consists of three
u-10>
components, time widths of the components are much".:::2~
narrower. The beam phase is assumed to be reduced by a certain~ -20200~L..-JL..-JL-J'--1'--1--'-600-L--'---"---'--8..1.oo-..1.-..1.-LI--'ooo amount in the extraction
region of the cyclotron because theRadius (mm)efficiency of extraction from the deflector to the first Faraday
cup is around 60 %. The pulse width of the external beam
Fig. 5 Phase histories of the 50 MeV 4He2+beam. Phases
depends upon the phase width of the internal beam, the spatial
measured with the phase probe are indicated by circles. Thelag boundary of the phase band deduced from the I vs. R
beam cut in the extraction region and the energy spread of thecurves are indicated by triangles.
beam particles. Flight path length is around 41 m from theentrance of the deflector to the plastic scintillator. If the 1060() ~TnMTMnTMMT~Tn~nTTnTMnTMM~
MeV H+ beam has an energy spread of ~EIE=lx10-3, the500
width of the time distribution additionally extends to 0.5 ns
equivalent to a RF phase of 2.6'. The effect of the energy"' 400~ 300
spread cannot be evaluated in the present spectrum because theenergy resolution of the plastic scintillator is not good. Precise~200
analysis for the time structure of the external beam pulses is~ 100
required.
'On' 20~cr::::1£)N

00 a-t+H+HI"f'f-H+H++H++II-H-H++ +++H=l"t-11-H-82500

V. CONCLUSION~2000§ 1500Isochronism of the internal beam has been checked byphasemeasurement with little variation of RF frequency. It is81000
estimated that the isochronous field is generated within 1500
gauss. The phase distribution of the internal beam has been
investigated by analyzing the beam current versus radius
measurement. The phase band width is around 57" RF for 10
MeV
H+ and 41' RF for 50 MeV 4He2+. The time structure
Fig. 6 Time structure of the external I 0 MeV H+ beam pulse.
oftheexternal beam pulses has been obtained by measuring
The upper and lower spectrum show the time structure without
and with cuts made by the phase slits, respectively. Phases are beam particles with a plastic scintillator. The pulse width for
10 MeV H+ corresponds to 7.3' and 18' FWHM of RF phaserelative, and the zeros are not absolute.
with and without cuts made by the phase slits, respectively.

IV. 'DME STRUCTURE OF EXTERNAL BEAM

VI. REFERENCES

The time structure of the external beam pulse has beenmeasured with a plastic scintillator at the end of the beam line.
The plastic scintillator with a diameter of 5 em is 10 em long,
and around 41 m distant from the deflector. The beam particles
passing through a vacuum partition foil of 50 11m thicktitanium are detected directly by the plastic scintillator in theair. The beam intensities can be reduced down to a factor of at
10 with three sets of beam attenuators[5] installed inleast
the injection line. The incidence time of the beam particles
relative to a stop signal generated by the RF pickup has beenmeasured with a good resolution below 0.5 ns FWHM. Thetime spectrum has been obtained for the events selected by agate on pulse height.
The time structure of an extracted 10 MeV H+ beam bypreliminary analysis is shown in Fig. 6. Assuming that thetime structure is independent on the reduction rate of the beamw-

[1] K. Arakawa, et al., "Construction and first year's operationof the JAERI AVF cyclotron," in Proceedings of thethirteenth Int. Conf. on Cyclotrons and their Applications,
Vancouver, Canada, July 6-10, 1992, p.ll9.
[2] R. Tanaka, et al., "JAERI A VF cyclotron for research ofadvanced radiation technology," in Proceedings of thetwelfth Int. Conf. on Cyclotrons and their Applications,
Berlin, Germany, May 8-12, 1989, p. 566.
[3] M. Fukuda, et al., "Beam studies of injection to extraction
system for JAERI AVF cyclotron," in Proceedings of thethirteenth Int. Conf. on Cyclotrons and their Applications,
Vancouver, Canada, July 6-10, 1992, p.423.
[4] A.A. Garren and L. Smith, CERN 63-19, 18(1963)[5] K. Arakawa, et al., "Status report on the JAERI AVF
cyclotron," presented at this symposium.

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