A FAST POLARIMETER FOR e+e- STORAGE RINGS
K. NAKAJIMA, M. ARINAGA, T. KAWAMOTO, Y. MIZUMACHI, Y. MORI
A. OGATA and K. YOKOYANatinal Laboratory for High Energy Physics
1-1 Oho, Tsukuba-shi, lbaraki-ken, Japan
Abstract
A fast polarimeter for a precise measurement of transverse
polarization of electron (positron) beam is presented. This polarimeter measures an asymmetry in the Compton scattering ofCircularly polarized laser beam on polarized electrons. The parameters of the laser polarimeter are discussed for study of beampolarization at the KEK TRISTAN main ring (MR) in the energy
range of 25- 32 GeV. A measurement of t4e beam polarization
is expected to achieve a statistical accuracy of ±1 % in several
seconds.

diction of the polarization level can be calculated by a computercode SLIM 131 • Fig. 1 shows the expected polarization of the MR in
the energy range 24 to 32 GeV, calculated with simulated magnetic field imperfections. This calculation takes no account ofcontributions from three large solenoids for the colliding detector. The absolute polarization level is expected to be very low
around the depolarization resonance. It occurs when the spintune v = EI0.44065 satisfies v = n + n,v, + nyvy + n.v., wherev,, vy, v. are the horizontal, vertical and synchrotron tunes, respectively and n, n,, ny, n. are any integers. A polarization time
will be 42 sec for a 30% polarization level at 29 GeV in the MR.

Introduction
In e+e- and ep physics experiments the role of polarized electron and positron beams are increasing with energy. Electrons(positrons) are transversely polarized anti-parallel (parallel) tothe guiding magnetic field by the Sokolov - Ternov mechanism 1' 1•
A natural polarization of electron beam has been observed inmany storage rings 121 with energy less than 25 GeV. As energy increases it is expected that the degree of polarization becomes low
due to strong depolarization effects. These are crucial problems
to achieve high level polarized beams in high energy storage rings
exceeding the 30 GeV range. The TRISTAN covers the energy
range of 25 to 32 GeV, in which the study of beam polarization
provides useful information.
A fast precision polarimeter is an essential tool to study polarization phenomena in e+e- storage rings. We have developed
a fast polarization monitor to measure a transverse polarizationof electron beam at TRISTAN MR, based on the Compton scattering of laser light on the electron beam. A fast data taking andan excellent precision are crucial points to measure a low level
polarization with a short buildup time, expected in the MR. Features of our polarimeter design are to minimize a systematic error
as well as a statistical error and to allow for measurement of thelongitudinal polarization simultaneously.
Beam Polarization in TRISTAN. In the _presence _of depolarizing effects originating from magnetic fi:ld Imperfections and misalignments in a real storage ring,
the bmldup of .the beam polarization can be expressed in termsof the effective buildup time r asP.(t) = rfrpPo(1- exp( -tfr)),(1)where Po is the ideal asymptotic polarization level, Po= 8/(5../3)= 92.38%, and rP is the natural polarization buildup time. Theb~ildup time of the polarization for the beam energy E in GeV isgiven byTp = 98.66(lI E 5 )( Rl p) sec,

An obs~rva~le buildup time depends on the asymptotic degreeof the polanzatJon determined by depolarization effects. The pre--30 --g""

20100v.
2426~,,
26~30w32

E (CeV)

Fig. 1 The expected beam polarization of TRISTAN MR in theenergy range of 24 to 32 GeV.
Compton Polarimetry
The laser polarimeter is a desirable tool to make a fast polarization measurement. The principle is based on the asymmetry
in the Compton scattering of circularly polarized photons froma laser on an electron (positron) beam. A transversely polarized
beam gives rise to an up-down asymmetry in the backscattered 1
rate, depending on the left-right photon polarization. The differential Compton scattering cross section I•J of circularly polarized
photons on transversely polarized electrons or positrons is written
in the electron rest frame as

dajdf! = (r~l2)(qlq0 ) 2 ((1 + cos 2 0) + (q0 - q)(1- cosO)± P.P., cos tP( 1 - cos O)q sin OJ(3)

where qo and q are the incoming and outgoing photon energies and() and tP are the scattering angles. The kinematics and Lorentz
transformation between the electron rest frame and the laboratory
system are summarized using energies and momenta in units ofelectron rest mass:q = qol[1 + qo(l- cos())],(2)

where p is the bending radius in meters and R is the machinemean radius. In the MR the natural polarization buildup time
would be Tp = 2.88 x 109 I E 5 sec, typically 2 minutes for 30 GeV.

40 -qo ~ 21ko,

1tan(JL = sin(JI(l- cos()),

k ~ 1q(cos() -1),

(4)

where 1 is Lorentz factor of the incident electron beam Elm c2
ko and k momenta of the incoming and outgoing photons in ~h~laboratory system, and ()L the scattered photon polar angle withrespect to the incoming electron beam direction.

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15.0

In the laboratory system the maximum asymmetry of thebackscattered 1 rate occurs in the polar angles of OL ~ 1h,typically 20 Jl rad in the TRISTAN energy range. The angulardivergence of the beam and the angular resolution of the detector
must be small compared to this angle so that the asymmetry of
the 1 ray angular distribution cannot be lost. The net angularresolution of the polarimeter with a rms position resolution 6 ofthe detector and a beam emittance t is given by

12.5

10.0"<7.5:0!J0~5.0:;

2.5

if0.0~~;~

where L is the distance of the detector from the laser interaction
point and a, (3 are the beam Twiss parameters at the interaction point. The parameters are chosen so as to minimize theangular resolution. This is optimized by choosing an interaction
point where the (3- function is its maximum value. Usually in theFig. 4 The dependence of a 0 , aVao and A= a2/ao on the vertical
position used in the measurement.

e+e- storage rings, the horizontal emittance is large enough tosmear out an asymmetry of the projected profile on the detector.
Therefore only vertical distribution of the 1 rays is measured.
The vertical (3 and a functions at the interaction point are 23.7
m and ~ 0 m, respectively, and L is set to be 38 m because ofrestriction on the installation space. A typical vertical emittanceis 2.5 nm for 25 GeV, assuming a 2 %of the horizontal one. The
projected beam spot size on the detector is 0.46 mm. If we use
the detector with the position resolution 126 Jlm, the net angularresolution is 12.5 Jlrad, sufficiently smaller than 1h.

Two methods have been proposed to measure the verticalasymmetry of 1 rays. In the single photon counting method 121
the electron beam is illuminated by a low power, high repetition
rate laser pulse. Each backscattered photon is measured by theposition sensitive detector and the calorimeter. The multi-photon
technique 151 uses a high peak power laser to produce numerous
1 rays per interaction. The number of photon is recorded asa deposited energy in the calorimeter. We adopted the single
photon counting method to avoid systematic errors resulting froma drift of the closed orbit and a photon counting detector.

The degree of polarization Pe can be determined by the vertical asymmetry of the measured 1 rate and the analyzing power
A for 100 % photon polarization:

Pe = (1/A)(N+- N_)f(N+ + N_)(6)

where N+ and N_ are the numbers of 1 events counted at a vertical position for the two photon helicities. The vertical distributionof 1 rays is calculated by integrating over the horizontal distribution and the 1 energy distribution. Taking into account theangular divergence of the incident beam with a Gaussian distribution, an expected vertical profile at the detector is obtained asshown in Fig. 2. Fig. 3 shows the vertical asymmetry for two beamemittances. The analyzing power is defined by the ratio of thespin-dependent term a 2 of cross section to the non spin-dependent
term a 0 . It depends on bins used in the asymmetry measurement
and the cutoff 1 energy. Since the accuracy of the polarization
measurement is given by li.Pe ~ 1/(AVF/) with N = N+ + N_,
these values should be chosen so as to optimize the quantity ofA 2 N ex aV a0 • The dependence of relevant quantities on the vertical bins used in the measurement is shown in Fig. 4.

Fig. 2 The vertical distribution of 1 rays at the detector.

Y(mm)Laser Polarimeter

Laser
We use a cavity-dumped Arion laser producing the 514.5 nm(2.41 eV) line. The cavity-dumped pulses are synchronized withthe beam revolution frequency to illuminate the electron beam ateach crossing. The laser pulse has a width of 15 ns (FWHM) anda peak power of 50W. The linearly laser light is converted to either
right or left circularly polarized light with a KD*P Pockels cell.
The laser beam is guided through the transport optics consistingof 5 mirrors and a final focusing lens. The alignment of the laser
beam is performed with the optical target remotely inserted at
the interaction point. The laser beam is focused at this pointwith a rms spot size approximately 1 mm.
Experimental layoutThe experimental layout for polarimetry is shown in Fig. 5.
The laser beam is introduced from the laser building into the MR
beam pipe and then deflected towards the interaction point where
the laser light crosses the electron beam at an angle of 8 mrad in
the horizontal plane. The backscatterted 1 rays travel along with

Fig. 3 The asymmetries at the detector for two beam emittances.
The K denotes a ratio ty/tx·-220-

the electron beam, then leave the beam pipe at the end of the firstmain dipole and finally reach the 1 detector located about 38 mdownstream from the interaction point. Some modifications of
the magnet and vacuum chamber have been worked out to make

1-Rate and Background
The luminosity of the electron beam- laser interaction with ahorizontal crossing angle a is given by

the path of the 1 rays.

2L(kHz/mb) = 257.1IeWL{ a;Y + aly} - 1/X { cos 2 (a/2)(a;x+ aix) + sin2 (a/2)(a;. + aL)} - 1/ 2(7)where Ie is the electron beam current in A, WL the laser energy perpulse in ji.J, O"ex. O"ey' O"ez and O"Lx, O"Ly, O"Lz are the rms dimensionsof the electron bunch and of the laser beam in mm. If a 1.88 ji.Jlaser pulse with a round profile O"Lx = O"Ly = 1 mm collides theelectron beam at an angle 8 mrad, the expected luminosity is 62.2
Hz/mb for a 1 rnA TRISTAN beam. The total cross section ofthe Compton scattering is 385 mb for 2.41 eV photons on a 25
GeV electron beam. Thus an estimate of the backscattered 1 rate

0.4mL022 4m

is 24 kHz/rnA.
Fig. 5 The TRISTAN polarimeter layout.
1 detector

A schematic diagram of the 1 detector is shown in Fig. 6. The
detector has two functions for measurement of the position andenergy distribution of 1 rays. The photon trigger is provided by 3
scintillation counters S1, S2, S3 and a lead glass calorimeter. The
counter Sl vetoes charged particles. A 2 mm thick (~ 0.6 r.l.)tungsten converter is followed by the trigger counters S2, S3 andthe lead glass counter with ~ 18 r.l. for the energy measurementof a e- - e+ pair created in the converter. The vertical positionof a converted photon is determined from hit channels of the silicon microstrip detector (SSD) with a 50 mmx50 mm sensitivearea. The SSD is composed of P-N junctions formed on a silicon
substrate with 42 Jl.ill pitch. A capacitive charge devided pulse
is read out every three strips. It is possible to detect a charged
particle with position resolution of 42 jl.m. We use 160 readout
channels over a 20 mm vertical width.
Pulse height signals generated in the SSD and the lead glass
calorimeter are digitized by fast ADC CAMAC modules (LeCroyFERA) in coincidence with the trigger gate. A FERA ECL bus
transfers encoded data to either of two 16K words memory module. Then data are read out through CAMAC bus by the computer each time a memory module is filled with data. This systemallows for fast data acquisition at the maximum rate of 80 kHz.

~ ., " .

-L-~ +H+=t4=F1==1====·-.••
VeloI

lead-gloss
colorimeler

The background for the Compton 1 events is high energy 1
rays produced from the gas bremsstrahlung. The total cross section of the bremsstrahlu ng in a gas with average atomic number
Z=5 is given by O"brem(> kjE) = 57.3{6.37[(k/E) -ln(k/E)] 2.34(k/ E) 2 - 4.03} mb, where k is the emitted photon energy andE is the electron beam energy. The 1 rate from a 1 rnA electron
beam on the residual gas of a pressure p x 10- 9 Torr in a 200 m
MR straight section leads to l.lp kHz/rnA for 0.19 < kj E < 0.48.
In a real measurement the bremsstrahlu ng background may
become comparable to the Compton events. Its background canbe subtracted from the collected data if we take only the backgrounds in the laser-off run besides the up -down asymmetry run.
The polarization can be corrected with the background events Nsas

Pe = (1/A)(N+- N_)j(N+ + N_- 2Ns)(8)

where N+ and N_ are the up and down asymmetry events including the background.
Conclusion
We have constructed a fast precision polarimeter for the e+eTRISTAN main ring. The first polarization measurement in the30 GeV range will be carried out with this polarimeter soon.
References
1. A.A. Sokolov and I.M. Ternov, Sov. Phys. Dokl. 8 (1964)
1203.
2. D.B. Gustavson et a!., Nucl. lnstr.Meth. 165 (1975) 177.
D.P. Barber eta!., Physics Letters 135B (1984) 498.
W.W. MacKay et al., Physics Review D29 (1984) 2485.
3. A. Chao, PEP note 263, 1978. SLAC-PUB-2564, 1980.
4. F.W. Lipps and H.A. Toelhoek, Physica XX (1954) 85, 395.
5. M. Placidi and R. Rossmanith, CERN/LEP- BI/86-25, 1988.

counler Wconverler

Fig. 6 The schematic diagram of the 1 detector.

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