Barbiellini, Guido

Bozzo, M

Darriulat, Pierre

De Zorzi, G

Fainberg, A

Grosso-Pilcher, C

Holder, M

Macri, M

Maderni, G

McFarland, A

Orito, S

Pilcher, J E

Rubbia, Carlo

Santroni, A

Sette, G

Staude, A

Strolin, P

Tittel, K

CERN Document Server

OBSERVATION OF THE GEOMETRY OF COLLIDING PROTON-PROTON BEAMS BY MEANS OF BEAM-BEAM ELASTIC COLLISIONSG. Barbiellini, M. Bozzo, P. Darriulat, G. De Zorzi, A. Fainberg, C. Grosso-Pilcher, M. Holder,A. McFarland, M. Macri, G. Maderni, S. Orito , J. Pilcher, C. Rubbia, A. Santroni, G. Sette, A. Staude,P. Strolin and K. TittelIII. Physikalisches Institut der Tedmischen Hochschule, Aachen, Germany.CERN, Geneva, Switzerland.Department of Physics, Harvard University, Cambridge, Massachusetts 02138 (USA).Istituto di Fisica dell'Universita, INFN, Sezione di Genova, Genova, Italy.Istituto di Fisica dell'Universita, INFN, Sezione di Torino, Torino, Italy.(presented by P. Strolin)1. Introduction 2. The methodrecoil protonVERllCAL PLANE \ (beam -gos coil)-+-WR, \ W~ L-:_t--proton2-~--~ ----~---=:::r----T I+--=+----~proton 1 - 1---W,The principle of the method is shown in Fig. 1.The vertex of the interaction is determined by thetracks reconstructed in the wire chamber telescopesWI and W2 • The analysis of a sufficiently largesample of events provides a map of the beam-beam in-teraction diamond, such as the one shown in Fig. 2.A non-destructive observation of the geometryof coasting beams in the Proton Intersecting StorageRings (ISR) cannot)be easily performed with conven-tional techniques 1 since i) in the absence ofbunching the RF beam monitors are ineffective, andii) the ultra-vacuum makes it almost impossible touse beam profile indicators based on ionizationcollection2 ). The latter method has, in fact, ledto the proposal of rather complicated techniques inwhich the ionization is artificially increased byscanning the coasting beams with intense molecularor atomic beams 3) • These methods cannot be con-tinuously applied near to the interaction pointswhere the physics experiments are performed, sincelocal pressure increases of several orders of magni-tudes are clearly unacceptable. Furthermore, ifused continuously they would perturb the geometry ofthe beams appreciably.We have considered yet another possibility forcontinuous, non-destructive observation of the geo-metry of the beams, based on the reconstruction ofsome specific type of beam-beam collisions. In factin the almost perfect vacuum conditions of the ISR,by far the most apparent effect of the beams is thatof beam-beam interactions. Amongst the very manydifferent types of events, elastic scatterings areideally suited for our purposes since:i) they occur in great abundance, i.e. ? 15% of allinteractions"');Fig. 1HORI lCNTAL PLANESchematic view of the experimentalarrangement. The wire chamber telescopesWI and W2 detect the forward scatteredprotons. The telescope WRl detects therecoil protons scattered out in collisionswith the residual H2 in the vacuum chamber.ii) most events give scattered particles within anarrow forward angular cone; at 24 GeV, approximately70% of all elastic events are emitted between 6 and26 mrad, which is the angular range of the presentwork;iii) they can be promptly identified by a siwplecollinearity requirement in the c.m. system"').In order to exemplify the method, we have ana-lysed events collected during an experiment at theISR on small-angle elastic scattering"'). The speci-fic application had not been envisaged at the timeof the design of the apparatus. A specifically de-signed apparatus could be much simpler, more precise,and about one order of magnitude faster in the dataacquisition rate. A very modest on-line computercould reconstruct these events for immediate displayof the results.Histograms of the horizontal profiles of the singlebeams are sorted out by viewing the diamond along therespective beam directions. In a way, we are makinguse of one beam in order to take the horizontal scanof the other one. The resolution of each wire planeis 'U 0.35 nnn. The geometry of the set-up (distancebetween the chambers, distance from the beam-beamdiamond and from the vaCUlDll chamber window) determineshow this and some multiple Coulomb scattering on thevacuum chamber window reflect onto the resolution inthe reconstruction of the vertex. In practice, aspatial resolution of a fraction of a millimetre iseasily achieved, which is largely adequate when com-pared to the relatively large dimensions of the storedbeams.- 432 -Fig. 3 Wire spark chamber telescope.llAM 2 lUi ... )UAM-IIAM,"'UACfION DIAMONDEvolution of the horizontal beam profilesand of the vertical profile of the diamondduring a high-current run.Evolution of the horizontal beam profilesand of the vertical profile of the diamondduring a low-current run (x positive to-wards the outside of the ISR, z positiveupwards).IUM I (I., A)The currents circulating in the two rings are11 and 12 , We indicate with (x - xo) the displace-ment of average beam position starting from the ini-tial positions Xo' The sign conventions are suchthat x - Xo < 0 means spiralization to a smallerradius. A first important observation is that aradial change, negligible for "low" beam currents,is clearly noticeable at "higher" currents (Fig. 6).We have divided our observations into two typi-cal cases of "low" beam currents ('\, 1. 5 A x 1. 5 A)and "high" beam currents ('\, 2.5 A x 2.5 A). It isalready known 1) that in the case of more intensebeams the beam lifetime is considerably shortenedand the beam-beam effective height, which is relatedto the luminos i ty, grows up much fas ter • Our obser-vations complete this picture.Figures 4 and 5 show the observations for two-hour runs for the "low" and "high" currents condi-tions respectively. The time dependence of severalparameters are shown in Fig. 6.12.' Ai II...M 2 12.5 AI~l .......,......IN",ACIION DIAMOND ~.~~~ !=!\~ ~~. ~~-" -20 .,.... -1.5 -15 2.S 1.5 -.. -,a . ,. ..x 1M"') Z !MM] x [MMIFig. 4Fig. 5The beam 2, which presents approximately a threetimes as fast rate of current loss exhibits also aradial shift rate that is about twice as fast. Weattribute these shifts to the presence of an addi-tional average energy dissipation of 10- 3 eV/turnand of 2 x 10- 3 eV/turn for beams 1 and 2, respec-Horizontal distribution of 65,000 recon-structed vertices and horizontal profilesof the single beams.Fig. 2Data acquisition and analysis is simple andfast. It has been done in the general context of theexperiment, which uses an IlW 1800 on-line computer.A few thousand events give a quick test of the beamquality. High statistic beam profiles may requireas many as 104 events.4. Experimental observationsWe present observations of typical beam evolu-tions during several hours of unperturbed operationof the ISR. Observations usually have extended overas many as 1010 revolutions. We would like to dis-cuss some of these findings, mainly in order to elu-cidate the potentialities of the method.The beam-beam elastic event rate has been typi-cally of the order of 300 per second at 2.5 x 2.5 A2circulating currents. Unfortunately, in order toprevent refiring, the present type of chambers couldnot be operated faster than 30 per second. Thislimi tation could be easily overcome by using multi-wire proportional chambers.3. The apparatusOne of the nine chamber telescopes is shown inFig. 3. It consists of six magnetostrictive chambersaccurately positioned on a rigid reference plane,which also supports the trigger counters, not shownin the figure. The chambers have been designed inorder to be fully sensitive 'V 7 llUII away from the va-cuum chamber, so as to permit detection of small-angle scattering events with the longest possiblelever arm between the chamber.planes.- 433 -15.015.34 t 15.34 GeVicVERTICAL BEAM (S) PROFILEI)' 1.81 AT2 ' 1.948 ABEAM- BEAM FROM ELASTIC SCATTERING-10,0 - 5.0 0.0 5,0 10.0MILLIMETERS FROM THE MEDIUM PLANE OF THE ISR1010010005.pro10,000We have demonstrated the feasibility of observ-ing the vertical beam profile of a single beam byreconstructing, also with spark chambers, the vertexof the forward proton with the recoil proton genera-ted by elastic scatterings of the protons of one ofthe beams with the residual hydrogen gas in thevacuum chamber. Because of the high quali ty of thevacuum which is normally provided (~ 10- 11 Torr)in order to have an acceptable beam-gas interactionrate of the order of 10 per second, w~ have produceda modest temporary increase of the gas pressure byheating the vacuum chamber walls locally. The pro-file of a single beam and of the beam-beam are shownin Fig. 7. They are well-consistent with each other,as well as with the effective beam-height measurementsusing the beam displacement method1 ,5).10 t hours)ZZ__22G..VHIGH INTENSITY (Z!iAl"'0"t--="=-_~__------------ ::rms(mm)rm.Z(mri,z,10 t(hours)Evolution of beam parameters during lowand high intensity runs.QD I I 11522--22G~VlOW INTENSITY (1.SA)2QS.11101+---==~-_[1'2rms(mm) , -1-rm~mm2) --tI - --~ IxrmsFig. 6tively. Ionization losses in the residual gas in thevacuum chamber are insufficient to account for theeffect at least at the quoted average ring pressures.Another clear effect is apparent in the incre-ments of the squared mean deviations from the cen-tral orbit in the vertical component of the beam-beam interaction volume. Since the vertical shapeof the single beams is approximately Gaussian (seeSection 5), the various contributions to ztms addlinearly. By describing z~(t) in the formz:ms(t) = z~s(O) + At ,1) J. Borer, S. Hansen, E. Jones, M. de Jonge,B.W. Montague, D. Neet, B. de Raad, W. Schnell,L. Thorndahl, K. Unser, S. Van der Meer,H. Verelst, B. Vosicki and K. Zankel, Instrumen-tation and beam diagnostics in the ISR, paperpresented at this conference.2) C.D. Johnson and L. Thomdahl, The CPS gasionization beam scanner, Proc. of the NationalAce. Conference, Washington (1969).3) B. Vosicki, Beam profile indicator, CERN intemalnote ISR (1968), unpublished.AcknowledgementsWe would like to express our gratitude to theISR staff, and in particular to Drs. K. Johnsen,F. Bonaudi, E. Jones and E. Fischer for cons tantinterest, support, and encouragement.Referenceswe find constant values for A, A = 10- 2 hour- 1 rmn2for the "low" current condition and A = 10- 1 hour- 1mm2 for the "high" current case. The calculatedmultiple Coulomb scattering on the residual averagegas pressures of 5 x 10- 11 Torr Nz equivalent inthe two rings, although in reasonable agreement withthe first case, is insufficient to explain the secondanomalously large value of A. Of course one cannotexclude the possibility that indeed somewhere in themachine the residual gas pressure could have beenconsiderably increased by the beam itself, exceedingthe average values provided by the gauges.Furthermore, we observe a substantial incrementin the r.m.s. horizontal beam width xrms ' again muchfaster in the high current conditions. Moreover, ifone transposes the observed vertical blow-up to thehorizontal plane, one calculates values for theblow-up that are much smaller than the observations.Finally, the aperture limitations are at alltimes far beyond the observed limits of the profilesof Figs. 4 and 5. Hence a simple multiple scatteringcausing a gradual beam blow-up and final beam lossesat the limi ts of the acceptance cannot be the maincause of the increased current losses at highercurrents.Fi g. 7 Vertical profile of a single beam and ofthe diamond from beam-gas and beam-beamscattering data.- 434 -E. Koudmirenko, P. Strolin, B. Vosicki andK. Zankel, CERN internal report NP 70-28 (1970).4) M. Holder, E. Rademacher, A. Staude,G. Barbiellini, P. Darriulat, M. Hansroul,S. Orito, P. Palazzi, A. Santroni, P. Strolin,K. Tittel, J. Pilcher, C. Rubbia, G. De Zorzi,M. Macri, G. Sette, C. Grosso-Pilcher,A. Fainberg and G. Maderni, Phys. Letters 3S B,3S5 (1971) and Phys. Letters 3S B, 361 (1961).M. Holder, E. Radennacher, A. Staude,G. Barbiellini, P. Darriulat, P. Palazzi,A. Santroni, P. Strolin, K. Tittel, J. Pilcher,C. Rubbia, M. Bozzo, G. De Zorzi, M. Macri,S. Orito, G. Sette, C. Grosso-Pilcher,A. Fainberg and G. Maderni, submitted toPhysics Letters (1971).5) S. Van der Meer, CERN internal reportISR-PO/68-3l (1968).DISCUSSIONM. Q. BARTON: Would someone from the ISH staffcomment on the slow change of radius?W. SCHNELL: We have been aware of this effect andat present have no explanation. When we have time,we shall investigate it.

Observation of the geometry of colliding proton-proton beams by means of beam-beam elastic collisions

Observation of the geometry of colliding proton-proton beams by means of beam-beam elastic collisions

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