The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan1998

Preliminary Results of a Test of a Longitudinal Phase-Space Monitor
Eiji Kikutani, Yoshihiro Funakoshi, Takashi Kawamoto and Toshihiro Mimashi
KEK, National Laboratory for High Energy Physics
1-1 Oho, Tsukuba-shi, lbaraki-ken, 305 Japan

Abstract

the betatron oscillation and (t:.pfp)7J is the displacement
due to the energy deviation. 7J is, as usual notation, theA prototype of a longitudinal phase-space monitor has been dispersion at the pickup. Thus non-DC component is thedeveloped in TRISTAN Main lling at KEK. The principle superposition of the synchrotron oscillation and the betaof the monitor and its basic components are explained. tron oscillation. If we measure the position of a bunch
Also a result of a preliminary beam test is given.
successively for many turns, the time series of the data
can be Fourier-analyzed. Then we can pick up only the. synchrotron oscillation component by preparing a window
I. Introduction
favorable to.the synchrotron oscillation frequency. By thistechnique we can extract the energy deviation from theThe phase-space information of a bunch which is circulat- data of the transverse position.
ing in an accelerator is very helpful to us for understanding the dynamics of the bunch motion. In TRISTAN Main
m. The measuring system
Ring, the transverse phase-space monitor has already been
installed and it has been successfully operated[1]. Thenthe next step will be to install a longitudinal phase-space
A. Pickups
monitor in the ring. _The longitudinal phase-space monitor which we are de- The longitudinal position and the transverse position areveloping. is the semi-real time system, that is, the oscilla- measured by independent pickups, which are installed intion will be visualized within a few seconds after the actual the separated positions in the ring. Both pickups are usual
button electrodes. The. transverse pickups are installed atmotion .. Also data can be stored in disks if necessary.
In this article, we discuss the possibility of the longitu- the southernmost point of the ring and the longitudinal
dinal phase-space monitor and give preliminary results of pickup is installed at the position whose distance from thetransverse pickup is about 1/8 of the circumference. Natthe beam-test of the monitor in TRISTAN Main Ring.
urally, it is desirable that both the data are taken in one
point. But it is not a serious problem because the phase of
the longitudinal osciUation rotates only ...., 271" /90 throughH. Principle
this distance.
To make a phase-space plot, two kinds of signals, aposition~like variable and a momentum-like variable, must
be observed. In the case of the longitudinal motion ofparticles, the position-like variable is the time at which
the bunch center passed by a pickup (relative to the synchronous particle). This relative time is measured directly
by comparing the bunch timing with the reference RF signal.
On the other hand, the momentum-like variable is thedeviation of the average energy of.the particles in the bunchfrom the nominal energy. This signal is obtained by measuring the transverse position of a bunch at a point where
the dispersion is not 0. In general, the transverse positionof a bunch, x; is expressed asX= XDC + X/J + (;) 7},

TRISTAN Main Ring

longitudinal
position pickuptransvers position pickup( 1) Figure 1: Layout of the pickups and the control room.

where x0 c is the static deviation of the bunch position fromthe nominal center (COD), x, is the displacement due toB. Electronics-388-

The front-end electronics of the transverse monitor has already been described in ref. [1]. Here we give a brief explanation on the front-end circuit of the longitudina l position detection system[2]. A block diagram of the circuit isshown in Fig.2. The main componen t is a double balanced
mixer ( dbm) which detects the phase difference between
two signals. One of these two signals is the reference signalthat is made by multiplyin g (6-time) the RF signal which
has the common source with the signal for the acceleration. The other signal is obtained by band-pass filtering
the output of the pickup electrode. The center frequencyof the band-pass filter is 3050 MHz, which is again the 6times of the acceleratio n frequency. The DC component of
the output of the dbm is proportion al to the longitudina lposition. In this method, the adjustmen t of the relative
phase of two signals is very important.

a faster computer, we can observe the plots in the longitudinal phase space in semi-real time manner. In this case atrigger of order of 10Hz will be given to the ADC moduleand the computer.
IV. Prelimina ry data
To check the basic performance ofthe phase-space monitor,
we tried to observe the synchrotro n oscillation in TRISTAN Main Ring. The bunch current was 0.8 rnA and thenumber of bunch was only 1. The oscillation was artificially excited by shaking the phase of one of the klystrons
for the acceleration. The amplitude of the oscillation wasroughly measured by an oscilloscope and it was about 80
pico seconds in full width.
The transverse oscillation observed and its frequency
spectrum are shown in Figs. 3 and 4. The transverse position, which is the vertical axis of the graph in Fig. 3 israw data of the output of ADC. The fast Fourier transform was done using the data for 1024 turns. The datain the time domain roughly reproduces the sine-like oscillation. The frequency domain data shows that the main
peak is at 0.11, which is just the synchrotro n tune of theaccelerator.
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Figure 2: Block diagram of longitudin al position de-c:tector.

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C. Data acquisition

Both the transverse and longitudin al position signals are
input to two input connectors of a flash-ADC module[3].
This module consists of 4 ADC chips (8 bit, 100 Mega
samplesfs at maximum) , a clock circuit, a trigger circuit("conversion-stop") and memories. The revolution clockof the accelerator ('"" 100 kHz) is fed to the ADC chips
as the external clock for the conversion. By a CAMAC
command, the ADC chips start conversions synchronizing
with the revolution signal of the accelerator, namely theADC's catch the positions of a bunch turn by turn. Whenthe "conversion stop" signal is accepted, the ADC's stop
the conversion and the module becomes ready to be read.
Then the data aquisition computer reads the data stored
in the module. The memory size is 1 kilo-words/channel,
then we are able to catch the succesive position data for1024 turns at maximum.
For the preliminar y test described below, we used therelatively slow computer for data acuisition. But if we use

Figure 3: A typical data of transverse position.
A typical data for the longitudina l oscillation is shownin Fig. 5. Again, the vertical value is raw data of ADC
output. The oscillation found in Fig. 5 is not symmetricwith respect to its average level. Considering the fact thatthe oscillation is excited by a sine-curved signal, this asymmetry might come from the wrong adjustmen t of the phasedifference which we mensiond previously.
The result of the frequency analysis in the longitudinal
direction is shown in Fig. 6. There are three sharp peaks.
The peak at the lowest frequency corresponds to the synchrotron tune. The other two peaks are the 2-nd and 3-rd
harmonics of the synchrotro n tune. The origin of theseharmonics should be the asymmetry of the time-domain
data explained above.

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Figure 6: Longitudinal frequency spectrum.
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Figure 5: A typical data of longitudinal position.

Figure 7: Longitudinal "phase space".

At last, Fig. 7 is the "phase space" plot obtained inthis experiment. From the Fourier analysis of the transverse oscillation, the oscillation comes mainly from the energy oscillation not from the betatron oscillation. Thus weuse raw data for the transverse oscillation for plotting thegraph. The gain of the amplifier for the longitudinal direction signal is not sufficient. Thus the resolution is poorin the longitudinal direction. However, we can catch an
ellipse-like pattern as shown in the figure.

VI. Summary

We have constructed a prototype of the longitudinal phasespace monitor. The preliminary experiments show us that
this system can be a useful tool in the accelerator machine
study.

V. Plan to the next step[1] T. Shintake, "Phase-Space Monitor System", Particle
Accelerators, Vol. 30, pp.27-32, 1990.

Looking at t.he figures above, we find that the longitudinal
data is less reliable. At first we must establish the technique to adjust the phase difference of two signals. Additionally, the gain of the amplifier for the longitudinal signal
is rather low with the bunch current of"" 8mA. Thus wemust prepare a higher-gain amplifier and some adjsutableattenuator to fit the signal level to the input range of theADC.

References(2) The basic idea of the longitudinal position detection
was given by the SLAC B-Factory Accelerator DesignGroup. See SLAC-372, P. 359.
[3) This module is a product of REPIC Coporation,
Japan. The module consists of 4 ADC chips, Analog
Devices Company, AD9002, and 4x 1-kw memory.

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