The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Non-Destructive Velocity Monitor for the HIMAC Injector
Tsuyoshi KATOH. Takeshi MURAKAMI*, Satoru YAMADA*, Jun YOSIDZAW A*,Chihiro KOBAYASHI, Yasuo HONDA and Syohbun HARA * *Accelerator Engineering Corporation
2-13-1 Konakadai, Inage-ku, Chiba 263 JAPAN*National Institute of Radiological Sciences
4-9-1 Anagawa, Inage-ku, Chiba 263 JAPAN* * Sumitomo Heavy Industries, Ltd.
5-2 Sobiraki, Niihama, Ehime 792 JAPANAbstract
A non-destructive monitor for measuring the velocities and intensities of beams from the HIMAC injector wasdeveloped. The beam velocity is estimated from the phasedifference between the signals from the two electric pickups.

1 Introduction
HIMAC (Heavy Ion Medical Accelerator in Chiba)
began its operation in 1993, and is supplying beams not
only for clinical trials, but also for basic research during
the night time and on the weekends [1]. In the basic research, ion species from He to Ar are being used, while theclinical trials employ C beams.
The injector supplies beams of 6MeV/u to threecourses (two synchrotron rings and a direct use of injectorbeams) "simultaneously'', changing the beam direction.
Furthermore, an upgrade of the time-sharing acceleration(TSA) is in progress. In the TSA mode, the injector supplies different ion species to three courses, changing ion
sources on a pulse-to-pulse basis. After the upgrading,therefore, it is necessary to condition the injector system fora given beam without interfering with other beams. In thepresent system, the beams must be stopped during beamtuning, because the energy is estimated using a bending
magnet, and the intensity is measured by a Faraday cup.
For using in the TSA mode, a non-destructive monitor wasdeveloped and tested. The new monitor must be capable ofmeasuring the beam velocity with an accuracy of 111000 tocarry out tuning for synchrotron injection. It must also copewith a short duration of the beam pulses because of a duty
factor of 1/1000 of the injector.

Fig. 1. Schematic diagram of the non-destructive monitor
system. ( BPF: band pass filter, ATI: attenuator)
diameter of 44 mm and a length of 60 mm, were installedin the beam line about 11.5 m downstream of the injector
linac. The RF probes, having a band width of DC to 2 GHz,
were attached to the pickups as near as possible (40 rom).
Amplifiers with a gain of 10 were placed inside of shieldboxes next to the probes to maximize an SIN ratio, whileother modules were set up in a different room, 10m apart.
Read-out data of the phase meter are sent to a personal
computer in the control room through the GPIB.

3 Results of beam tests

3.1 Beam intensities and output signal levels
Figure 2 shows typical output signals of amplifiers,.
mduced by C e+ beams with an intensity of 240 J.LA.

2 System composition
A schematic diagram of the system is shown in Fig.1.
[2]. Signals induced in two electric pickups by the periodic
beam bunches are picked up by RF probes, and supplied toamplifiers, band-pass filters (BPF), and frequency converters. The Phase difference t/1 between the two signals is related to the beam velocity 1J as follows :(1 )IJ 1=360 L I t/1 ,acceleration
theofwavelength
thedenoteLandwhere 1frequency and the distance between the two pickups, respectively.
In the HIMAC injector, the RF frequency is 100MHz
and the beam velocity is around 6MeV/u (IJ=0.11295).
Since an accuracy of 1 deg is obtainable for a phasedifference measurement, the distance between two pickups
was chosen to be 1017 mm, giving a phase difference of 61r: rad. Electric pickups, made of SUS304 and having aFig. 2. Signals from the amplifiers. ( 5ns/div. and
500mV/div.)-249-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

The peak-to-peak voltage is around 700 mY, while thenoise amplitude without the beam is around 10 mY. The
structure of the signals may reflect the particle distribution
inside the bunches.
The relation between the beam intensities and theoutput voltage showed good linearlity, as displayed in Fig.3.
An intensity of 240 JlA is enough to be used by the synchrotron being operated at full intensity.
800--·
700i~::;l600~ 500
ts 400 - - -·t;
~0300
200
100-/vy /" I7/

3.3 Velocity measurements/The phase difference was measured· by changing the-----/--~00

10050

The intensity dependence of the phase-meter output
was also examined. The beam intensities were varied by anattenuator, which comprised metal sheets with slits ofvariable sizes and was installed between the ion sources
and the injector, so that the beam sizes and duration werenot altered. A slight dependence was observed: averaged
outputs with an intensity of 70 JlA were different by 1.3 deg(26mY) from those of 200 JlA. When the beam intensity
was less than 20 f.lA, the phase-meter outputs fluctuatedsignificantly, and did not give definite results. The outputof the amplifier was too small to be fed to the phase meter.

150200250

BEAM INTENSITY[.UA]

Fig. 3. Relation between the beam intensities and output
voltages.

3.2 Phase meter output
The phase difference between the two pickups wasobserved using carbon beams with a pulse duration of 450JlS. Ordinarily, the input of the phase meter is supposed tobe signals with DC or long duration. The stability of thephase-meter output. therefore, is of concern,· because thebeam duration from the IUMAC injector is 700 JlS in a
typical case. Figure 4 shows the relation between thepickup output corresponding to the beam pulse, and a gate
pulse given to the phase meter. A gate pulse, during which
the signal was fed to the phase meter, was delayed for 150
JlS to avoid any large oscillation at the beginning of thesignal. The sample-hold signal, and therefore the read-out
timing, of the phase meter output was varied 50 f.lS to 400
JlS from the beginning of the gate pulse. The phase-meter
outputs were found to be stable within 0.2 deg, including
the intensity fluctuation from pulse to pulse.

beam velocities. The beam velocity was varied by shifting

the acceleration RF phase of the No.3 DTL tank. The beamenergies were monitored by a couple of magnets: a 70 deg
bending magnet and a 20 deg pulse-operated switchingmagnet. installed about 14.5 m downstream the injector
linac. Two slits with a width of 1 mm, upstream anddownstream of the magnets, define the beam orbit. Using afield strength measured by a NMR, one can estimate thebeam momentum with an accuracy of 1/1000.
Figure 5 shows the relation between the NMR outputof the bending magnet and the output of the phase meter.
The fluctuation of the individual data was within the error
bars. Two outputs are consistent, and an accuracy of 1/1000
seems obtainable.

400390>..§. 380

I~ 370

t-!

0 360

350
704.5

'I

1

•
705

-+I~

ij4~--t--!

705.5+-111005

I..

Ii

I

706
706.5
NMR[mT]

707

707.5

Fig. 5. Relation between the magnetic-field strength of thebending magnet and outputs of the phase meter.

Fig. 4. Relation between the amplifier outputs (top) and agate pulse (middle). The bottom signal is a timing
pulse of a sample hold, when the output of the phasemeter was read out (100 f.lS/div. ).

The beam velocities were calculated based on thephase difference. The phase difference caused by cables andcircuits was estimated by supplying the same test pulses
into the two RF probes. The results were compared with thebeam energies calculated from the magneic-field strengthand the bending radius. The two results agree reasonably
with each other, as shown in Fig. 6 by the solid triangles.
Since our purpose is to monitor the variation of the beamenergies, not to calculate the absolute energies, the result isquite satisfactory.

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The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

6. 010 ,.........,........,..,.......,.......,..,........,..,.......,...,...,.'T"T"'..,.......,..,...,...,........,""T"T",....,.....,,.................,

~0

t:.-.
z..eo>::EII)~ 6 6.000

g~~ :::> 5.990
tl:)tl:!

~~~5.980 ......................""-'-...........""--!-'u...J...............................................................................................................
6.010
6.000
5.990
5.980

NMR MEASUREMENT [MeV/u]
Fig. 6. Comparison between the beam energies measured
by the velocity monitor and those evaluated by thebending magnet field. The solid triangles are thecalculated results by two methods, while the solid
circles include a correction for an effect of theswitching magnet.
As noted before, the 20 deg pulse-operated switchingmagnet was involved in the energy measurements. Since it
is not practical to carry out fine-tuning of the pulseoperated magnet, the field of the switching magnet waskept constant during the energy measurements. The data
corrected for the effect of the switching magnet wereplotted with solid circles in Fig. 6, giving a slightly better
agreement.

4 Conclusion
The non-destructive monitor tested showedsatisfactory results. A modification of the control system inorder to install the monitor is in progress. The monitor
should be improved so as to reduce the intensity
dependence of the· phase difference, and to be capable ofmeasuring the phase difference at very low intensities.

References[1] T. Murakami, et al., "Status of the lllMAC Injector ",Proc. XVIII Int. Lin. Conf., (1996) 830.
T. Fujimoto, et al., " Time-Sharing Acceleration in theIDMAC Injector ", In this proceeding.
[2] Z. Igarashi, et al., " Beam velocit~ monitor for KEK 40
MeV proton Linac ", Proc. 17 Lin. Accel. Conf.,
Sendai (1992) 198.

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