Benedikt, Michael

Cyvoct, G

Hancock, S

Jansson, A

Lindroos, M

Métral, G

CERN Document Server

 87 MONITORING BEAM QUALITY IN THE PS COMPLEX DURING THE LHC ERA M. Benedikt, G. Cyvoct, S. Hancock, A. Jansson, M. Lindroos, G. Metral, PS operations team PS Division, CERN, Switzerland Abstract Continuous monitoring of beam intensity and beam losses is provided todayat the PS complex with beam current transformers and beam loss monitors.The beam intensity is continuously displayed over a video network, togetherwith relevant information concerning the machine status. In the future, duringthe LHC era, further important beam quality parameters will be brightness,bunch length and momentum spread.For maximum beam brightness, any increase of emittance must be avoided,and its value should be monitored continuously. Transverse coherent beamposition oscillations, due to injection steering errors, can be measured withconventional pickups. A new system, using two quadrupole pickups, willallow continuous monitoring of both position oscillations and beam sizeoscillations, the latter caused by injection matching errors. Beam size, aftersmear-out consequent to any injection errors, is measured with fast wire-scanners on a daily basis. The possibility of monitoring it non-invasively andcontinuously using the signals from the quadrupole pickups will be explored.A longitudinal phase space plot can be created on-line by applying phasespace tomography to bunch shape data from a longitudinal pick-up. Thisallows many important longitudinal parameters - among them emittance andmomentum distribution - to be deduced with unprecedented precision.We discuss the problems associated with i) automating all thesemeasurements, ii) the limits of the different systems and iii) how the resultscan be used for on-line beam quality monitoring.  1. INTRODUCTION The PS Division is responsible for the operation of seven accelerators which deliver beams of protons,antiprotons, lead ions and electrons either directly to the users, or to the SPS for further acceleration.Four areas are fed with beam directly: the East Hall where there is an experiment (DIRAC) andnumerous tests of experimental equipment, the ISOLDE facility, the new AD area and the LEA andSLF areas where tests of detector and vacuum components take place. During 2001, the Division tookover responsibility for operating the ISOLDE facility. The breadth of the CERN physics programmerequires that normally several of the different particle beams operate simultaneously, which requirespulse-to-pulse modulation of the machines and a complicated interweaving of different machine cyclesin a supercycle. The accelerators operate for close to 6500 hours per year, which requires the reliableoperation of all the component parts of all the machines for extended periods. In general the manydifferent users have high requirements on beam quality, but with different emphasis on particular beamcharacteristics. In this paper the monitoring of the future LHC beam will be discussed. In particular, theissue of providing on-line beam quality data for the subsequent machine in the injector chain will beaddressed.  88 2. ON-LINE BEAM MONITORING FOR THE PS USERS At present a display showing the actual magnetic cycle in the PS, the ejected beam intensity and thebeam destination is broadcast over a video network to experiments and subsequent machines (theUsers). Measurements of transverse and longitudinal beam characteristics are done on a daily basis atall machines by the PS operation crew, by some experiments and at injection into the next machine inthe chain. The PS operators correct for all irregularities and work on improving general aspects of thebeam quality. Such general aspects can be a poor beam profile or a undesired beam halo. Irregularitiesobserved by the Users which cannot be verified or understood by the operation crew are discussed inspecially arranged meetings. Weekly meetings are also held to inform the Users about the machineprogramme and status, and the work in progress.  3. THE LHC BEAM The emphasis at the PS division has historically been on high-intensity beams. Major investments havebeen made to increase the intensity over the years with impressive results. The LHC beam is of lowintensity but high brightness. This translates into a small emittance beam with a high longitudinal andtransverse particle density. To enable the control and observation of such a beam, a project waslaunched in 1993 to adapt the machine hardware, the longitudinal and transverse beam control, and theinstrumentation of the PS machines forming part of the LHC injector chain.  4. MONITORING TRANSVERSE LHC BEAM CHARACTERISTICS In order to verify that the beam emittance is not blown up by bad matching between machines, a newmeasurement system has been developed to monitor PS injection (since the Booster uses multiturninjection, matching is not as much of an issue there). This system is currently being installed andconsists of two quadrupole pick-ups positioned in consecutive straight sections of the machine. Aquadrupole pick-up is a non-intercepting device that measures, apart from the beam position, thequadrupole moment of the beam. The PS pick-ups [1] have been designed [1] so that  k  can be measuredon a turn-by-turn basis for each bunch separately (see Fig. 1).(1)Any oscillatory part of  k  as a function of machine turn is due to mismatch. If the latticedispersion is non-zero (as in the PS case), beam size oscillations due to injection errors in  b x  anddevelop differently as a function of turn. Consequently, it is possible to separate these two effects in thehorizontal plane. The vertical dispersion mismatch cannot, however, be easily distinguished from avertical betatron mismatch, but it is expected to be very small. Since the ratio of  b  values (horizontal to vertical) are different at the the two pick-up locations,the system of equations given by Eq. (1) for the measured  k  values can be solved for the emittances.This requires a knowledge of the  b ’s, which may be oscillating due to mismatch. However, by using thevalues of  b  and  k  averaged over many turns, the filamented emittance can be calculated. This istheoretically very straightforward, but has yet to be demonstrated experimentally. Since  s  is the rmsbeam size, the result is the true rms emittance, independent of distribution. The momentum spread, which is needed to quantify the dispersion part of the signals, iscalculated from the cavity voltage and the bunch length. The latter can also be determined from thepick-up data. Thus, the measurement system is self-contained in the sense that it will not require anyadditional input of measured beam parameters by the user. Therefore, it can employ a very simple userinterface, or even run continuously in the background as an emittance watchdog, alerting the operatorwhen correction is needed. The application program for this system will be integrated with the ABS(Automatic Beam Steering and shaping) one developed for the PS Complex [2], allowing the simplecorrection of detected injection errors. k s s e b e b s =  x2- = - +y x x y y p xD2 2 2Dx2 89 Since the quadrupole pick-up system was developed primarily with injection studies in mind,some of its components (hybrids and amplifiers) are bandwidth-limited to about 30 MHz. This meansthat it is not possible with the present configuration to make single-bunch measurements in the very lastpart of the cycle, when multiple splitting has significantly shortened the bunch length to fit the SPSrequirements. An increase of the bandwidth is possible if required, but the emittance at PS ejection willanyway be monitored using the Optical Transition Radiation (OTR) screens [3] in the transfer linetowards the SPS.  Fig. 1:  Quadrupole moment of a proton bunch in the PS measured with a quadrupolepick-up over several machine turns. Apart from the on-line monitoring of the injection using the quadrupole pick-ups, it is foreseenthat the operators measure the emittances at different times in the cycle on a daily basis using the wirescanners [4]. This will provide a valuable cross-check of the on-line results, verify the beam profile thatcannot be studied with the pick-ups, and provide day-by-day emittance statistics.  5. MONTORING OF LONGITUDINAL LHC BEAM CHARACTERISTICS The main longitudinal beam characteristics can be determined from a measurement of the longitudinalbunch shape. The longitudinal pick-ups in the PS and the PS Booster can be read with a digitizingoscilloscope and the resulting image can be displayed for one entire turn. It is also possible to acquireturn-by-turn data with an oscilloscope triggered by a revolution train synchronous with the bunch, orbunches, in the accelerator. The software for the first system permits the operator to save referenceimages, which can be compared to the present image yielding information on the phase and bunchshape. This is particularly important for the LHC beam where there is a second bunch-to-buckettransfer which must be synchronised at the energy of the first batch waiting in the PS. The on-linesoftware for the turn-by-turn system includes a tomographic reconstruction algorithm [5] whichpermits the measurement of longitudinal phase space density (see Fig. 2). Additional input of somemachine parameters, e.g. the RF voltage and the magnetic field, is necessary for the reconstruction. Thelongitudinal rms emittance together with the momentum distribution can easily be deduced from theresulting tomograms. Our tomography algorithm has proved extremely robust for errors in the inputdata [6]. An example of the on-line display is shown in Fig. 3 where the Booster beam was captured atthe wrong frequency. –20–15–10–505100 5 10 15 20 25 30 35 40k (mm2)machine revolutions 90 Fig. 2:  A tomographic reconstruction of an unusual bunch distribution. Withouttomography it would not be trivial to visualize the longitudinal phase spacedistribution from the turn-by-turn data. Fig. 3:  A small error in the RF capture frequency at injection caused this phase spacedistribution. From the profile data alone it would not be possible to diagnose thisfault.-200 -100 0 100 200-6-4-20246[ns][MeV]-200 -100 0 100 200[ns]–600 –400 –200 0 200 400 600[ns]–1–0.500.51[MeV]00.10.20.30.40.5[A]2x1061.5x1061x1065x105 0[e/eV]0 10 20 30 40Iterations03.3x1012 [e/eVs]e <rms=0.703 eVs BF=0.369 2s     /p=5.37x10–3 prms Ne=1.85x1012 91 6. CONCLUSIONS The LHC beam is more sensitive to a deterioration of the beam quality than the present beams. The newinstrumentation developed for this beam will, as we have shown, yield reliable on-line data for thebeam characteristics. This can be used for on-line alerts (and correction) but it can also be directlybroadcast to the subsequent machine. This would enable the crew at that machine to monitor and evenstore the initial beam conditions and relate that to observed abnormalities in their accelerator. Thiswould require a different form of broadcast compared to today, which will need a common controlstructure (under study). It would also differ from the present situation due to the type of data that isbeing monitored and broadcast e.g. longitudinal and transverse beam emittance, beam shape andlongitudinal phase shift (compared to a given reference). Maybe the most important difference is that,with this new type of monitoring, the operator of the subsequent machine would not have to rely on amanual intervention to have important beam characteristic data at hand. Longitudinal tomography not only yields emittance data, but it could also serve as a powerfulvisual aid to see small longitudinal beam perturbations. The present style of video screens with asummary of the most important machine data will probably continue to be broadcast over the existing,or an improved, video network. The tomograms are visually attractive and would form a natural focuson such a screen. The present progress in computing power should permit us to compute on-linetomograms for every machine cycle at the LHC start-up in 2006.  References [1] A New Optimised Quadrupole Pick-Up Design using Magnetic Coupling, A. Jansson,D.J. Williams, submitted to Nuclear Instruments and Methods in Physics research A, vol. 459, 16(2001) [2] Generic Automated Beam Steering and Beam Shaping Programs with an Object OrientedApproach, M. Arruat, A. Jansson, F. Di Maio, G-H. Hemelsoet, M. Lindroos, O. Tungesvik,International Conference on Accelerator and Large Experimental Physics Control Systems,Beijing, P. R. China, 3 - 5, Nov 1997 [3] New Methods to Derive the Optical and Beam Parameters in Transport Channels, G. Arduini,M. Giovannozzi, K. Hanke, D. Manglunki, M. Martini, Accepted for publication in NuclearInstruments and Methods in Physics research A [4] S. Hancock, M. Martini, M. van Rooij and Ch. Steinbach, Experience with a fast wire scannerfor beam profile measurements at the CERN PS, in: Proceedings of the Workshop on AdvancedBeam Instrumentation, KEK, Tsukuba, Japan, 1991. [5] Longitudinal phase space tomography with space charge, S. Hancock, S. Koscielniak,M. Lindroos, International Computational Accelerator Physics Conference, Darmstadt,Germany, 10 - 14 Sep 2000. Publ. in: Physical Review Special Topics - Accelerators and Beams,vol. 3, 124202 (2000), (CERN-PS-2000-068-OP) [6] Longitudinal phase space tomography, S. Hancock, Presentation at the 9th ICFA Mini-workshopon High-energy, High-brightness Hadron Beams, CERN, 7 - 10 Mar 2000, http://psdata.web.cern.ch/psdata/www/icfa9/slides/Icfa00_sbh.htm 

Monitoring Beam Quality in the PS Complex during the LHC Era

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