ii: fields. ::;jF;te!:j of. "'1 cctror: collectior., thi: scmner gives the projected proton density dis ... face,. The electric equipotential surfaces are arranged to be parallel.
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THE CPS GAS-IONIZATIO:~
BEAM SCANNER
C.D. Johnson and L. Thorndahl CERN, Geneva, Switzerland Crossed A non-destructive beam scanner was irstalled in the CPS ring early in 1 962. 3erivirg a sIgr1a1 from the clectronc liberated .oy the protori bemr fro]! the rcir:i.dual. gas in the vaouum chamber lor!cies up to 20 kHs are achieved using a Fave-tek waveform generator arid a high-voltage 15C VJ amplifier. Fast sweeps a~e made using Triggered L-C ringing circni:s of fixed frequecties. Signals are amplified by a transistor impedance, mounted pre-amplifier of low output directly onto the detector and a main amplifier located nearby in a reg$on of low radiation. The overall voltage gain is 250 and the bandwidth dc to 10 MHz. Experimental
Transition
Results RF radial shift
The prototype IDS has been in straight section 12 of the CPS since Easter 1968. The fast scan ?a:ility was developed very recently and is still bcillg perfected. The radial position indication given by the horizontal IRS has been compared with that obtained from induction electrodes and internal targets. A shift in radial position can be measured to wlthin 1 mm. Absolu'te position measurements, after correction for the space charge poterrtial effect, are in error by an a-:lovlilt correspording to fhc difference in ir;pedante between the plates e, and e2 (Fig.1) and ear%h. The .~se of precision components in the potential divider chains should reduce this error
Fast ejection (2 bunches)
Debunching
.r 0 '1 TJ .
Spatial resolution has 'oeen checked in the laboratory using thin tungsten filaments and a narrow electron beam ;o simulate the protonbeam. The spa;ial resolution is fcund to depend as expected OIL the voltage applied across the detector plates and the highes: resolution is such that true Seam profiles are made wider instrumehtallv /_ byI less than 1 mm. A good check of this claim was made recently in collaboration w-?tn E. Brouzet. A 24 GeV/c bunched proton bear was set up on the 'flat top' of the CPS magnet cycle. -A fork target having 10 mm internal separation ‘between the vertical prongs of the fork was centered with respect to the bea: (by minimizing target interactions). The IBS indicated a horizontal beam full width of IO to 11 mm with the fork in position.
Target
Target
RF radial
shift
control
1 S
voltage
A radial display of the full cycle of Fig. 5. operatiol? of the CPS. The trace running along the edge of the photograph is the RF radial shift control voltage and the corresponding Xote the change in beam movement can be seen. sense of these shifts after transition, and the the debunching,which * double beam, formed during . is present during target 1 and 8 operation. 1 large division = 50 ns. The vertical scale is: 911
Fig. 6. A radial display of a machine operation ending with slog ejection fro:!. straight section 62. h$o+ze the asymmetrical profile during the slow ejection spill-out and the large radial position shift thei marks the beginning of :he ejection. In this photograph one large horicontal division = 15 mm within tie time scale: CPS. Vertical 1 iarge division = 100 ms Radial profiles during slow ejection Fig. 10. spill-out tests at 24 GeV / c. These photographs SLOW the vaiety Of profiles which may be Observed dJring the setting-u;) of a slow ejection operat i on. The normal szoo t!l asynii,ietric profile of slow ejection shown in Fig. 5 ha,;, in -the::e examples, become broker; up into two, three or even four separated beams, Vertical time scale: 1 large division = 25 KS.
Fig. 7. Beam gynnasties at high enerpJ. Target 6 operation during accelerationat 21 GeV/c, fast ejection from ss 58 on a 'flat top' at 24 GeV/c, followed by slow ejection fron ss 62 on asecond magnetic 'flat top' at 19.2 GeV/c, were responsible for the many radial position shifts in The trace on the side of the this photograph. photo is the magnetic 'flat top' monitor, Vertical time scale: 1 large division = 100 ms.
9. A radial file of a well m hefined doublebeam sii produced by misadj-Asting the de2 bunching operation @ at the star-t of the magnetic 'flat g g top' at 19.2 GeV/c. A negative slop" g on the flat top sends the double 5 beam across to the outside of the vacuum chamber where it is evenagainst the chamber wall. Vertical 10 ms between scans. Fig.
tually lost time scale:
is again narrcw Wl.ilC a few il.-: lntor ,,-till th: proffles 5 and i have become wicc, as they rcmain for the whole of the slow ejection (cf.FS;;s 6 and 8) . The :;~cond phctui;ro.ph si7owl; urofile:; j and 4 on an expanded time scale. These benm size observations can be 'explzined in terms of the theory of the debuncoing oper-tion. The two photographs which follow are the which have been made first ones to be presented using the IBS with an electron multiplier as detector. The bandwidth of the system extends up to 10 KHz and so as we now can see the RF structure of the beam as an intensity modulation on the bean profiles, some explanation of the information contained in these profiles is reIn the CPS the RF is the 20th harmonic quired. of the revolution frequency. The latter is about 150 BHz at injection and 500 IcHe at high Consider a profile obtained by scanning energy. through the CPS beam in a tine of 15 pS just after injection when the bunches are forming. The result will be as shown in Fig. 12.
rare example of a double Another rather Fig. 9. beam on the 'flat top' at high energy with RF The exact machine conditions (no debunching). which cause this splitting are not known. H.G. Hereward (private communication) 'has suggested that one beam is the normal bunchedbe=m different while the second beam, with slightly energy, and therefore radius, has escaped from Vertical the bunches and is coasting alongside. time scale: IO ns between scans.
912
Tl Fig. jest
Fig. 15 shows horizontal profiles just after injection without acceleration (no bunches). Note the intensity modulation of profile No. 9 due to the tail of the beam. The beam spirals to tke inside of the vacuum chamber as the magnetic field rises.
T2
12. .F&mple of a wide-band after icjectlon.
IBS profile
is the proIn Fig. 12 the upper fzrvelope which we norrlally a ssociate with the IBS. Thi :: -1rofile is noddated -oy the bunches. Also, d1i: to irl~:~mllt3tr fill.i!lg, 5ltere is a lOO$ modu1 stion at the re~~olluxio!l freq-Len-y (p0.i.r.t:: T, , we see the tail of the bear. T2), i.e. Elsewhere the modulatj.on i.r: less thax 10% due to tne bunches being incoil.plote. The lower envelope therefore , gives the dc profile, or the profile of tl:e u*lhu:chrd portiof the beaq. This nay n?t correspond ex,ictly to that of the bunched p3:tiOll. A:: aczelcration proceed:; and the bLulche:; are com~lctcd, the 3 MHz modulation become:; 104!,. During; debunching the reverse SCCI:X and. zgzin the beam profiles CK be separated i r-t:) ? ufl::hc6 an.l unbunchod portion::. file
Fig. 16 was taken u~31:r tke same conditions but with xcceleration. Time between scans is 15 pS, the time for the first scan through the beam is 1 @. This increases as the amplitude of the scanning voltage decreases (ringing circ.;lit). Horizont:tl scale : 1 l:zrge division = 4 cm in the CPS. Conclusions Xith the 13 st two photo~;riph:: wt :~~y+:ir to have renched the Ill timnte s'::in%l ng rprr-4 of ilLi> Ins. . There is s3me interest tc e;n fa.:ter fol, obscrvnti.on of the unbunched bean after jnj,:;'tion, Sut for a bunchrd bear]. {F;rtiater i;peed ipoirltlo:::;, since tlic> intt>n:.:i-ty mr,LIulz.ti.or~ du< I,0 5112 l)ur:che~ cpoils the pcofilc-::. 0111~woul d t1::vc to ::t-nn iii :, itirt. ::11ort. !-'nmp:..r\x 7,' t,lli, tc, l!i,‘.ill !,Yl1,C i!l::li-:l1 -It>!! kh, C.f'. 1 ni:, 'r-1 l,l~.!.BP?~ prof‘i.l.e:: of ::.in~rl.c~ bun&r>::. l'l..ir: i:; k?ibyon;i I !I(' ~::L~~:i?lilitii::: :' th!, IX:.
t7 ri:;ht).
turci
iii: &~::critci? cc: t:.i:-1 y I‘t?f:1rL 3 ij
~~>o‘JL',
the
ilU~lCl1
With ;11; usi' of 1:; feasible IO- a Tom, collection
i:trnc-
i s ti!t: lOr$ ir!o,l1&,ti roii duo to the ' !;nil' cf tlic bear?. Injectiotl i:; c~~veral 11; 31!1'(?Tf! t11:: f'i r:;: :‘c:i,!' and tti; IJ-~ilch~ :I are dlre;idy furini~,~ as tiri:: ccar. i:: made. 300 !I:; 1 ztcr 'i:!c lrur-lctc::: xi! ful.ly for-r&. 11ote t11at tlw urlbwtc:t!u~:l p,lr ti1.i:: of the: boxn dr i.f 1:; toinitrd:, the inside cf the 17P:; rice (the left-hand ::ide prO~t!c3::. of tnl~:: pl~ot,u,~r:t~:~:) a:3 nc celcration Xote 31: 0 tlie beam blow-~=, which oc:urs b
