The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan 1993

Development of a Beam Phase Monitor with a Micro-Channel Platefor the RIKEN Ring Cyclotron
M. Kase, T. Kawama*, T. Nakagawa, N. Inabe, I. Yokoyama,
A. Goto, andY. Y ano
The Institute of Physical and Chemical Research (RIKEN)
Wak:o-shi, Saitama 351-01, Japan

Abstract
A beam monitor with a micro-channel plate (MCP) has
been developed for measurement of time structure of various
kinds of beams at the RIIffiN Ring Cyclotron (RRC). Itworks not only as a phase monitor used for stabilizing
magnetic fields inside cyclotrons, but also as a monitor
giving information about single turn extraction and turnnumber inside cyclotrons. It has a good sensitivity with alarge dynamic range of beam intensities, compared with theold-type phase monitor with capacitive pick-up probes.

I. INTRODUCTION
The RIKEN Ring Cyclotron (RRql) is a post-stage
accelerator, coupled with an A VF cyclotron (AVF) or a heavyion linac (RILAC). To get a high-quality beam stably for along duration, it is important to keep the beam phase constanton each accelerator. For this purpose, a beam phasemeasurement system with capacitive pick-up probes has been
installed, and beam phases are continuously monitored in thecyclotrons and in the beam lines. For low-intensity beams,
which is required frequently by experimenters, however, thesignal becomes so small that the system does not work fully.
Thus a phase probe with a wide dynamic range of beam
intensity is necessary for this case. The new device is also
required to allow us a continuous measurement for the wholebeam time. Accordingly it must be of a non-beam-destructive
type.
Recently, we developed a very sensitive monitor by use ofa micro channel plate (MCP), which is widely used in a fieldof experimental nuclear physics2).

acceleration. The wire is biased up to -3 kV against theentrance surface of the MCP, which is biased independently at-2.4 kV to its anode at the ground potential. The whole
electrodes including the target and the MCP can be moved
remotely in perpendicular to the beam axis, keeping a constantdistance (50 mm) between the wire and the MCP. In this waythe position of wire with respect to the beam is selectable inorder to adjust a signal rate (0.1 - lOkcps) from the MCP.
Fast and large signals with a rise time less than 1 ns and witha gain larger than 107 are obtained from the tapered coaxialshaped anode of the MCP.
Timings of these signals with respect to the time origin
determined by an RF master oscillator are measured with atime-to-analog converter (TA C) and analyzed by a multichannel analyzer.
The device is installed inside a beam-diagnostic chamber
located in the extracted beam line of the RRC andpreliminary tests with various kinds of beams were done.

050IOOmm

IT. DEVICEThe structure of the new phase probe is shown in Fig. 1.
A micro-channel plate (MCP, F4655, Hamamatsu photonics
Ltd.) with a sensitive region of 14 mm<l>, is set parallel to thebeam axis. It has a small target of 0.3 mml!> tungsten wire
strung in parallel with the MCP surface. The target intersects
only a small fraction of beam (a few percents at maximum).
Electrons and/or photons produced on the target by beams areintroduced to the MCP through double slits. There are four
sets of electrodes for producing an electric field for the electron* Sumijyu Accelerator Service, Ltd.

-474-Fig. 1. A structure of a new type of beam phase
monitor with an MCP. All electrodes are made ofstainless steel and supported by ceramic insulatorrods. Each electrode is biased so that a uniform
field is produced from the wire to the MCP.

B. Monitoring single-turn extraction of the RRC

ill. RESULTS
A typical example of time spectrum obtained forlOOMeV/n 18o beam is shown in Fig. 2. In this case, theRRC is operated with the AVF. Beam bunches appear withtime intervals of an rf period of the AVF, T inj• which isequal to 2 x TRRe. an rf period of RRC. The time spread ofbeam bunch was measured to be 940ps in FWHM,corresponding to an rf phase spread of 10 degrees.
Here, signals from the MCP are due to photons (considered
to be mainly x-rays). Yields of photons and electrons dependon kinds of beams. For heavier ions with low energy, forexample 7Me V/n 40 Ar, main signals are due to electrons
emitted from the target. For protons and deuterons withenergies greater than 100 MeV, the rate of signal originatingfrom the wire is very low, and in these cases, a thicker target
is required. For light ions, such as carbon and oxygen, withenergies higher than 100 MeV/n, the yields of photons andelectrons are comparable.
A. Stabilizing a magnetic field of the RRC
The position of peak in Fig. 2 is always relating to abeam phase to an rf phase. This allows the device to work asa phase probe. The shift of the peak caused by the change ofmagnetic field in the RRC is shown in Fig. 3. An operator
can stabilize the magnetic field by adjusting a magnet current
so as to keep this peak at the same position.

18In Fig. 2, small peaks are observed at the middle points
between large main-beam peaks. These small peaks are due tomulti-turn extraction of the RRC. In case of the AVF
injection, a harmonic number of the RRC is an odd number offive, and T inj = 2 x T RRe (the AVF bunches exist every
other acceleration phase of the RRC). These conditions resultin the extracted bunches as illustrated in Fig. 5 (A), when themulti-turn extraction occurs in the RRC. The small peaks
correspond to the fragments of the bunches in the last internal
orbit. They are peeled off by the deflector septum before
entering the extracted orbit. By observing the time spectrum
such as Fig. 2, it is quite easy to know to what degree thesingle tum extraction is achieved.
In case of the RILAC injection, however, this method is
not available, because the fragments overlap the main beambunches (Tinj is equal to TRee). In order to observe thefragments clearly, we cut the RILAC-injected beam bunches
in certain duration by using the electric chopper nstalled inthe injection line of the RILAC3). Figure 5 shows thespectrum obtained, when a beam is chopped for the durationof l.2Jls, for the case of the RRC harmonic number of 11.
The beam loss was in the order of 3% under the following
conditions: the voltage was 3kV; the rise time shorter than 50ns; and the repetition cycle 25kHz. Figure 5 (B) illustrates theprinciple of this method. After the chopper is turned on, 11bunches (the number corresponds to the RRC acceleration
harmonics) without fragments accompanied are observed,
because the fore-going fragments disappear. After turned off,
11 fragments are obsearved first. By use of this method, we
can observe to what degree the single-tum extraction isachieved for the RILAC-injected beams.

f=29.0MHz

0 lOOMeV/n

1000

900 f8001-:>

700 f--§600"E::s soo

0400

1-f------- T;.; ~'--T-l-u 300 1200 f100

-I 00

200

300

400

Kf13+ E = 10.6McV/n fRee= 20MHz h = 10I
II.::II

SOO,u:SOO
600

Channel No.
Fig. 284IA

A.

.1.tFWHM=940psSSO

6!50600---TIME

Example of time spectrum of signals fromthe MCP for a lOOMeV/n 18o beam. Thesesignals are considered to originate from photons
produced at the wire by the beam, because noaccelration voltage is applied of the electrodeduring this measurement. A start signal for theT AC is given by the MCP signal and a stop
signal is an rf reference. Shadowed peaks are
due to a main beam, and unshadowed peaks are
due to the effect of multi-tum extraction.

-475-1ero•••••••••••••••••••••<II••••••••••••••••••••••~0:II

I---~----~~~ ~----~------~--0.01
0.00
-0.01

0.1.B/B (%)

Fig.3 The peak shift (that is, beam phase) vs thechange of magnetic field in the RRC.

'-"'~

Ar 7MeV/n

f=18MHz (TitRc=56ns) h=ll

600.sooFig. 4.
A time spectrum ofbeam bunches when the beam iscut partially (1.2J.Ls) by thebeam chopper installed in theRILAC injection line. The startof the T AC is done by thetrigger signal for the chopper,
which is sychronized to the rfphase. The stop for the T AC isthe MCP signal.

:X:.g400§0 300u 200
100100200300

Channel No.

TIME----.-

IV. CONCLUSIONREFERENCES

The beam monitor with the MCP is very useful not onlyfor the measurement of beam phase but also for the judgementof single-tum extraction. For other applications, themeasurement of turn number inside the cyclotron is available.
These measurements can be done with a slight beam loss.
.
{almost non beam destructive).
Now we are preparing to install these devtces m theinjection line to the RRC as well as in the extraction beamline. These monitors will help us to maintain the beamstably and also to tune the beam quickly.

Deflector'-"'!"-

1) Y. Yano et al.: this proceedings.
2) T. Mizota, K. Yuasa-Nakagawa, S.M. Lee, and T.
Nakagawa: Noel. Intsr. and Meth., A305(1991)125-128.
3) M. Hemmi, H.Kumagai, andY. Miyazawa: "A Beam
Chopper for RILAC using High Speed Mo~-FF:TModules", Proc. the 17th Linear Accelerator Meeting mJapan, 1992 Sendai, p309.

Extracted beam(A).::::.

.::::.

14TMc

1:1t

Fragment

4

Tinj

Extracted beam _ _ _ _....,.

(B).::::.

~...... .::::.
11fragments

No beam

11bunches

O!opper ON l.2J.1S-476-~
TRRc

Fig. 5
Conceptional
illustration of multi-turn
extraction of the RRC. (A): in case of the A VF
injection, harmonic number
being 5. (B) : in case of theRILAC injection, harmonic
number being 11, bychopping beam bunches
partially.