JAERI-Conf 95-021

PB07
Non-destructive Beam Profde Monitor at HIMAC
S. Sato, N. Araki, M. Hosaka, Y. Sano*, H. Takagi*, M. Torikoshi,K. Noda, E. Takada, H. Ogawa, T. Honma** J. Matsui***, and T. Nagabuchi****National Institute ofRadiological Sciences, Chiba, 263, Japan*Accelerator Engineering Corporation, Chiba, 263, Japan**cyclotron and Radioisotope Center, Tohoku University, Sendai, 980, Japan***Mitubishi Electric Corporation, Wadasaki-cho, Kobe, 652, Japan****Toshiba Corporation, Yokohama, 230, Japan
Abstract

2. DESIGN CONSIDERATIONNon-destructive profile monitors (NDPM), based on
micro-channel plate (MCP), have been developed andinstalled in both the synchrotron ring and high-energy beamtransport (HEBT) line at HIMAC. Beam test using thesemonitors have been carried out since April of 1995 toinvestigate a change of vertical beam size in synchrotron anda possibility of observing beam with high energy by one
pass. In this paper the measurement system is mainly
reported, and the preliminary results are also briefly presented.

I. INTRODUCTION
NDPM based on MCP has been studied anddeveloped at many accelerator facilities [I ,..._, 3], because of apowerful method for non-destructive beam diagnostics. TheNDPM utilizes residual gas ionization. The ion-electron pairs
created by the impact of high energy charged particles on
residual gas are accelerated in a uniform electric field, andions or electrons bombard the surface of MCP. Electrons
emitted by the bombardment are multiplied in MCP when abias voltage is supplied between both ends of MCP. The
multiplied electrons in MCP are collected to multi-anodestrips or resistive anode using charge division method. Onedimensional beam profile is obtained as electric signal
through a read out circuit.
NDPMs at HIMAC have been studied and installed
for following purposes. (I) To investigate a change of verticalbeam size of circulation beam during acceleration andextraction at synchrotron, because the measured verticalemittance of the extracted beam is not consistent with thecalculated value based on the adiabatic dumping. (2) To
monitor the beam profile non-destructively at HEBT during
irradiation treatment of tumor. Therefore, two NDPMs havebeen designed, and installed to the synchrotron ring andHEBT, respectively.
The paper reports the design considerations of theNDPM and the preliminary testing results.

The NDPM consists of an accelerating electrode ofions or electrons created by a beam, a cascade-MCP with
32ch multi-anode strips and a read out circuit.
2.1 Estimation of output signal level
The expected signal amplitude obtained from amonitor was roughly estimated using the well-known BetheBloch formula. The number of ion-pairs, that can beproduced along the unit length of a 290Me V/n carbon beam,is first estimated. In this case, 5.7pairs/cm/Torr is obtained
for the beam revolution frequency of I. 5MHz in the averagevacuum pressure of lxl0-9Torr in the ring. Taking thesevalues into account, since the gain of a cascade-MCP and thebeam intensity are assumed to be 107 and l0 8pps,
respectively, the estimated output current per channel is44nA. In NDPM at HEBT, the output current is estimated at
2.9pA/ch under the conditions of lxl0- 7Torr of the vacuum
pressure and 10 7 ofMCP's gain.
The estimation suggests that measurement in HEBT
needs cascade-MCP to obtain large gain, and possibly worse
vacuum condition, in the ring , lower gain is enough forassumed intensity. Nevertheless, cascade-MCP was adopted
also for the ring to cover the request for weaker beam.
2.2 Structure ofNDPM
Fig. 1 shows a photograph of the NDPM at HIMAC.
The NDPM has 7 electrodes, which are arranged to realize
a uniform electric-field to accelerate ions or electrons createdby a beam. The field was calculated by using the code
Poisson as shown in fig. 2. The maximum voltage is ± 25k V.
The effective area that beam can pass through is 180x7 5mm2for measuring a vertical profile in synchrotron, and100xl00mm2 for a horizontal beam profile in HEBT. Theelectrode at the opposite end of the field cage to MCP is madeof mesh, in order to use UV-rays for gain calibration of eachanode. In both cases, the size ofMCP is 55x8mm2 (Hamamasu
Photonics F4772-0l), and the interval between neighboring
anode-electrodes is 1.7mm. The output of the MCP was-266-JAERI-Conf 95-021operated at O.SkV negative to the anode strips which are at
the grand level.
In the case ofring, the electric field of the NDPM may
distort a closed orbit by about 8mm during the injection. Inorder to correct this disturbance, an additional electrode isinstalled at just downstream of the NDPM. This correction
electrode has a similar structure to the monitor, and is fed
high voltage with the inverse polarity by same power supply
as the NDPM. In the case ofHEBT, however, the correction
electrode is not used because an orbit distortion is negligibledue to one pass high energy beam.
In order to keep high vacuum in the ring, the NDPM
was baked at 200"C for 24hr before installation to the ring,and the resistor of 20M Q to divide the high voltage to eachelectrode. are equipped outside of the vacuum chamber toremove out-gas sources.

2.3 Read-out circuit and control system
Fig. 3 shows a schematic block diagram for the readout circuit and control system. In the case of ring, the charge
from the anode strip is converted to a voltage with theconversion ratio of 1V/nA, then it is digitized after a 32ch
analog-multiplexer. The digitized signal is thirdly stored in amemory. The signal acquisition is repeated in this way for1.4s at the minimum interval of 1Oms. The stored data istransferred to the control computer in the central control
room, and fmally displayed on a CRT. The local control
system is connected to the control computer through GPIB.
The operation of the NDPM is normally carried out at thecentral control room. In the case of HEBT, generated current
signal in the NDPM is integrated in the existing read out
circuit with exchanging a capacitor from 1OOOOpF to 500pF,
which is used for multi-wire proportional counter type
monitor[4]Fig. 1 Photo of the NDPM at lllMAC synchrotron.
Fig. 3 Block diagram of read-out circuit and control system.

3. PRELIMINARY TEST
3.1 Calibration

Fig.2 Equipotential line calculated by Poisson code. At the left
is the mesh electrode and MCP position is at the right end.

UV -rays were irradiated to investigate uniformity inthe gain characteristics ofMCP. Results of both before beam
test and after 300hr beam exposition are shown in fig. 4. Ascan be seen in fig. 4(a), the uniformity of gain was within± 5% at NDPM in the ring before beam test. In the case ofring, however, since the created electron intensity issomewhat high due to high revolution frequency, the gain
seems to become small gradually . It is thus necessary tocompensate the gain deterioration. At present, the fluctuationof the gain in each channel is measured by using UV -rays,
and it is compensated in the level of software, resulting in aflat response on a CRT. In HEBT, since the created electron
intensity is usually small due to one pass beam, such gain
deterioration has not been observed yet.

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JAERI-Conf 95-021
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j· : :. : : : : : : : : : : : : : : : : : : : : : : : : ~: : :~ ~: ~ : : : : : :2···················································································
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1020chaaael

30

2 ......................................
1 ...................................................................................

011020chaaael

30(a)
(b)
Fig. 4 Gain characteristics of anode channels ofNDPM in thering. (a)before beam test. (b )after 300hr use.

3.2 Profile measurement
As shown in fig. 5, the beam profiles can be observedin both cases of ring and HEBT under the conditions
S1.ll1ll11llrized on Table 1. In the case of ring, signal levels weredecreased as increasing a beam energy, because of reductionof energy loss.
.
When a correction electrode was not used in the ring,a decrease by 20'"'-30% in the beam intensity was seen due
to the kick by the accelerating electric field of the NDPM.
Such a decrease was compensated by using the correction
field. InHEBT, when the supplied voltage to the accelerating
electrode is higher than 20kV, an increase in the noise can befound As long as the vacuum pressure is better than 10"7Torr,this noise remain very small, however the beam profile was
not satisfactorily observed.
Table 1. Testing conditions.
synchrotronc6+
beam
lxl0-9Torr
vacuumenergy6'"'-350MeV/u
intensity
5xl08pps
MCP gain
5xl04,.,_ 17kV
field voltage(a)4i! oro o;ooo ! 00 ! !
0~~~~~~~~~~~
0510
15
20
25
30
35

HEBT
Ne 10+
3xl0-7Torr
400MeV/ulxl08pps

Channel(b)Fig. 5 Measured beam profile. (a)synchrotron.(from injection
to the end of extraction, the interval of 30ms), (b )HEBT

107'"'-20kV

4. SUMMARY

6. REFERENCESThe NDPMs at IDMAC have been designed andtested. As a preliminary result, they measured the expectedbeam profile in both the cases of ring and HEBT. Both in thering and HEBT, the measured output was consistent with theestimation. In HEBT, however, the beam profile was notsatisfactorily observed when the vacuum is better than 1o·7Torr and the beam intensity lower than 107pps.

[1] B.Hochadel, MPI H-1990-V-19.
[2] T.Kawakubo, et al., Nucl. Instr. and Meth.
A302(1991)397.
[3] T.Honma, et al., INS-T-52l,p.43.
T.Tanabe, ibid., p.49.
[4] M.Torikoshi, et al., AlP Conf Proc. 333, eds. G.H.Mackenzie, et al., p.412

5. ACKNOWLEDGEMENTS
The authors would like to express their thanks to the crewof Accelerator Engineering Corporation for skilful operationof the IDMAC, and to Dr. K. Kawachi, Dr. S. Yamada andthe other members of the Division of Ace. Phys. and Eng. atNIRS for their warm supports.

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