THE PLA POLARIZED PROTON SOURCE

J. M. Dickson Rutherford High Energy Laboratory

The design work on the polar~zed source was started in 1 9~\7. It was developed into a functioning source by 1959 and was first operated on the PLA in February 1961. The f:ime during which a polarized beam has been available for nuclear physics experiments now totals 4670 hours, comp!'ising 500 hours in 1961, 1500 in 1962., 1.725 if:. 1963 and 945 in the first six months of 1964. The operatior~al reliability of the source has L'een such that the percentage fault time of 1,he Pol ... A. has been only marginally lower with a polarized beam compared with ihatof the normal unpolartz:ed beam.

Engineering development of the source has been a continuing process durir:g its life and has been aimed at eliminating weaknesses in the original design, notably, lack of sufficient cooling, unreliability of control mecha­nisms bRtween ground and the 500 k'! platform, and alignment difficulties. The original ionizer was ineffici(:~rit, gave an. annoying inequality of II spin-

I' . d" . d ! i 1 .'. " -' . " t' d d d b 'Jp at: spm,- own po .an,zanOLS anu Inter:.Slles, an, pr'o ~;ce ' a eam which was not parallel to the a:xjs of the injector. Two successive re·· designs of the .lon.izer eliminated tbe polarlzaEon inequality and provided electrical stee:cing of the beam. The iOEize'c' EOW- used is show'n in Fig. 1. It has an ionization efficiency of about 10- 3, a-,,:: used, but by modifying tbe extraction electrode and the iOE mirror' a factor 5 improvement might be achieved. The ion,izer is followed by two pairs of deflector electrodes similar to the X, Y plates of an oscilloscope.

Surrounding the ionizer' ther;:; are three pairs of COilb arranged in the general form of Helmholtz coils. These are used to produce a low field (3 to 10 Oe) in the ior1.ization region to orient the polarized atomic beam in the desired direction. This system is v(:';ry convenier-.t to use and very flexible, in that any desired direction of protoY:. polarization can be produced aD.d the direction can be precisely r'eversed by reversing the current in one or two of these pairs of coils. For horizontal transve~~"se polarization the Helmholtz coils need not be used since tbe sex:tupole magr:ei: of the atomic beam system was designed to give a stray dipol.e field at the ionizer and the sextupole polarity can be reversed.

Proceedings of the 1964 Linear Accelerator Conference, Madison, Wisconsin, USA

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The atomic beam intensity has been increased by a factor of L 5 by a new collimator in the dissociator, This col1im:s..tor consists of a large number ('" 1000) fine bore pyrex tubes of uniform cross section (length '" 1 mm, bore '" O. 1 mm), It is similar in concept to the original collimator but the individual tubes are more uniform in size and the assembly technique produced much better alignment of the tubes.

Polarization and Intensitv 0<

The accelerated beam of protons, measured at the output end of +oank 3

8 has normally a polarization of 0.40 and a ~ean intens~ty of 0

2 x 10 protons / sec (at 10/0 duty cycle). The polanzatloF and mtenslty cannot be measured at any other point alor>g the accelerator. This leads to some slight difficulty during operation of the accelerator and in local>o lng some faults. The total beam current from the injector 18 about 20 i,~mes t,he proton current and the polarization cannot be measured at low er.ergy. However, the rf and other parameters of the PLA a!"e now weE enough monitored to overcome nearly all these difficulties.

There is now rather good i.:->.strumer:tation for measr,ring the p01 ar­iza'ior and intensity oi' the outpt.1 bearn (the mean er~ergy can also be measvred to good accuracy). The pol arizahon 1S mf."asured by degradu:,g the beam energy to 15 MeV (mearJ and 3catt .e::"ir_g the protons at 450 by a 2 Me':;/ rhick carbon target into four scintil1 atlO~. counters pl aced up, dowp, left and right of the proton beam. The degrader and target are mourt.ed or: a whee~. rotaTing at: 2,5 revs. j 5r?-~~. Variatior of the number of degrader~ target pairs from 2 up to 10 allows samplir:g of from one protor, pulse ir teD up to alten:ate pulses. in gC-Jeraltwo 50 MeV and two 30 MeY de­grader-target pairs have been monr'.ed, 50 that on changing machire energy from 50 MeV to 30 MeVtbe only cha~ige required is an ''3.d;ll].sr· ment to the gating circuits to er;.st;re iJ-';i~t counts fromi.he approp'~late 1:arget are registered. A sigro.} :'rom the rotating wheel is vsed to tr 19" ger the accelerator timing system a.!:;.d er,.Sl.~."es synchroniza t ~(lr:: of tht" t.arget wheel and the proton puLses. Elastic;;..lly scatter'O.d protons from ihe carbon target are selected by discnminators ir, the courHng sysTE'm. More deta:as of this apparaL~s are giver: k the 1962 and 1963 PLA Prog· ress Reports.

This system enables one to measure the transverse (vertical ar d horizontal) componen1:S of the be",.m polarization contirmo'~s1y. During a typical experiment, running for several hundred hours, the meaSI. .. rEc:1 beam polarization varied by only ± 0.01 (standard deviation) per comiir>.g cycle; each of which had a statistical countiEg error of: 0.002. At the same time the horizontal component of the polarizatLor,,;aried bei.wtCen ... 0.008 and + 0.003 + 0.002 and averaged 0.002.

Proceedings of the 1964 Linear Accelerator Conference, Madison, Wisconsin, USA

466

L , ..... l ~ SI.PP<a1'

J rn..uo_ .. I0I>l'

5 rIUJlOlT StPf'()I:T"S

6 ..,...,. ILEftCI)O

~ JOM fIII1ICR.

8 llNZ.n. LD&5

9 FIL. 3UPPL! PUT'll:

10 IICStJl.A.l'I».

Fig . 1. Ionizer

U NFUARI SED E

R 9)

-~}--- - -----T,

(al

POLARISED

(bl

Fig. 2. Scattering Geometry for the Measurement of the Carbon Polarimeter Analysing Power.

Proceedings of the 1964 Linear Accelerator Conference, Madison, Wisconsin, USA

467

The beam intensity is mea$ured in an ion chamber filled with 1 atmosphere argon + 100/0 methane mixture. Mean proton current s down to 104 protons/ sec can be measured using a dc amplifier an d peak pulse currents of 10 7 protons/sec can be detected in a pulse am­plifier.

Absolute Calibration of Polarization

The polarimeter described above could be calibrated from data available in the literature for (p~C) polarization at 'about 15 MeV but it was thought desirable that an absolute calibrat ion measurement should be performed. The apparatus for this calibration is shown schematically in Fig. 2. From the three asymmetries measured with a polarized and an unpolarized beam the polarization of the beam is found from t he rela­tion

From the asymmetry a' measured by the continuous polarimeter its analyzing power pi = al/p can be found, pi = O. :562 ±. O. 006 at 50 MeV. The ana.).yzing power, however, is a function of the mean proton energy incident on tll:;; polarim eter t arget so a correction factor had to be calcu­lated from the results of a subsidiary experiment to enable one to m eas­ure the beam polarization for energies other than that used during t he calibration. The absolute v alue of the beam polarization can be meas­ured to an accuracy of ±. 0.005 (standard deviation).

To obtain a beam with zero polariz ation it has been found neces ­sary not just to switch off the sextupole magnet of the PPS, but to de ­gauss the magnet by reversing the current several times while decreas­ing it. Otherwise residual polarizations of 0.01 may be observ ed.

Future ModifiCations

2Since the stati stical accuracy of polarization measurement s depen ds on P • it is always desirable to increase the beam polarization. The present source has a theoretical maxirnum polarization of O. 50 so not much improvement can be expected. However, by using t he adiabatic passage "spin-Dip" techniql;le first described by Abragam and Winter and so successfully exploited by the Saclay cyclotron group a factor of two improvement Can be achieved. The necessary modifications to the source are now being proposed for installation later this year.

If the atomic beam, after selection of the J = + t states in the sextupole magnet, is passed through an rf field in a low magnet ic fie l d,

Proceedings of the 1964 Linear Accelerator Conference, Madison, Wisconsin, USA

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which varies slowly t h r ough resonance, t he I = + i ' s t ate can b e completely converted into a I = - t state. If the beam is theA ionized in a s t rong

magnE;ti c fie ld (-1000 Oe ), a theoretical prot on polar ization of 1. 0 can be achi eved. Since ionizati on in a str ong field fs more efficient, the proton current will also be increased. It i s expected that a polar izat ion of 0.80 at an intens ity of about 10 9 prot ons! se c will b e obtained~

One disadv antage of the modified source will be t hat the present Hel mholtz coil system cannot be used. The polarization direction .will be v ertical initially, but it can be rotated in the transv erse plane by a sole ­noid, located in .the 500 kV term inal. To obt ain lon gitudin al polariz ation a crossed electric and magneti c field spin rotator will hav e t o be installed.

Scheduling of experi mental time on t he PLA is handicapped by the. fact that the unpolarized beam intens ity obt a inable from t he PPS is lower than most expe rimenters require, an d changin g over to the normal source r equires at least a 24 - hour shut down. It is desir able, t herefo re, to com ­bine in the same vacuum system the PPS and a proton sourc e of a few mA peak cu rrent.

Acknowledgement s

D. A . G. Broad has been responsible for the recent development work on the polarized source . He h as b een a ss i sted by R. C. Carte r and A. G. D. Payne in the labor at ory and the engineering h a s been the r esponsi­bility of K. McAinsh and J. A. T aylor.

MARTIN; J . H.: What was t he pulsing r ate of the mac hine?

DICKSON: It is 50 pulses p er s e c on d and t he p ul se length is 200 fL s.

FEATHERSTONE : Are you using the h i gh fre quency rf tr ansition at · about 1400 Mc or a re you using the l ow frequency ?

DI CKSON: We are using the low frequency transition at about 10 G and 8 Mcl s whi ch gives a t ransition from s t at e 1 to state 3 by the adiab atic passage method.

Proceedings of the 1964 Linear Accelerator Conference, Madison, Wisconsin, USA

469