During the SPS run in the year 2000 further test measurements were performed with the rest gas monitor. First, profiles of single circulating proton bunches were measured and the bunch charge progressively reduced, in order to determine the smallest bunch intensity which can be scanned under the present operating conditions. The image detector in this case was a CMOS camera. Using a multi-anode strip photo-multiplier with fast read-out electronics, the possibility to record profiles on a single beam passage and on consecutive turns was also investigated. This paper presents the results of these tests and discusses the expected improvements for the operation in 2001. Moreover, the issue of micro channel plate ageing effects was tackled and a calibration system based on electron emission from a heating wire is proposed. The gained experience will be used for the specification of a new monitor with optimised design, to be operated both in the SPS and in the LHC

Ferioli, G

Fischer, C

Koopman, J

Sillanoli, M

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN    SL DIVISIONGeneva, SwitzerlandMay 2001CERN-SL-2001-026  BISensitivity Studies with the SPS Rest Gas ProfileMonitorG. Ferioli; C. Fischer; J. Koopman; M. Sillanoli.CERN, Geneva – CHAbstractDuring the SPS run in the year 2000 further test measurements were performed with the rest gasmonitor.First, profiles of single circulating proton bunches were measured and the bunch chargeprogressively reduced, in order to determine the smallest bunch intensity which can be scannedunder the present operating conditions. The image detector in this case was a CMOS camera.Using a multi-anode strip photo-multiplier with fast read-out electronics, the possibility to recordprofiles on a single beam passage and on consecutive turns was also investigated. This paperpresents the results of these tests and discusses the expected improvements for the operation in2001.Moreover, the issue of micro channel plate ageing effects was tackled and a calibration systembased on electron emission from a heating wire is proposed. The gained experience will be used forthe specification of a new monitor with optimised design, to be operated both in the SPS and in theLHC.Presented at DIPAC 2001 – Grenoble– France13 – 15  May 2001SENSITIVITY STUDIES WITH THE SPS REST GAS PROFILE MONITORG. Ferioli, C. Fischer, J. Koopman, M. Sillanoli, CERN, Geneva, SwitzerlandAbstractDuring the SPS run in the year 2000 further testmeasurements were performed with the rest gas monitor.First, profiles of single circulating proton bunches weremeasured and the bunch charge progressively reduced, inorder to determine the smallest bunch intensity which canbe scanned under the present operating conditions. Theimage detector in this case was a CMOS camera.Using a multi-anode strip photo-multiplier with fastread-out electronics, the possibility to record profiles on asingle beam passage and on consecutive turns was alsoinvestigated. This paper presents the results of these testsand discusses the expected improvements for theoperation in 2001.Moreover, the issue of micro channel plate ageingeffects was tackled and a calibration system based onelectron emission from a heating wire is proposed. Thegained experience will be used for the specification of anew monitor with optimised design, to be operated bothin the SPS and in the LHC.1  INTRODUCTIONA residual gas ionisation beam profile monitor (IPM) isconsidered as one of the instruments for measuring thetransverse beam size of the proton beams in the SPS andin the LHC. A monitor from DESY has been modifiedand is under test in the SPS [1][2]. Previous measurementcampaigns have shown that adequate accuracy andresolution can be achieved. During 2000 the sensitivitylimit of the monitor was probed. It was operated in both ahigh spatial resolution read-out mode, using a standardCMOS camera, and a high speed read-out mode,employing a miniature photo-multiplier tube with 16anode strips.In the LHC the instrument will have to deal with beamintensities varying from one pilot bunch, (5u109 protons),up to 2808 bunches of 1.67u1011 protons each (ultimatecurrent): a dynamic range in the order of 105.The acquisition speed is another important issue, sincethe device may also be used to verify the quality of thebetatron matching at injection into the SPS and the LHC[3]. For that purpose a single  nominal bunch, (1.1u1011protons), should be measured on a turn by turn basis(23.1 Ps in the SPS and 88.9 Ps in the LHC).One of the problems encountered, when exploitingIPM monitors, is the ageing of the micro channel plate(MCP). This ageing affects the area of the MCP wherethe beam is imaged. To track this effect and correct for it,a remote controlled built-in calibration system would bevery useful. A method is proposed using a heating wireacting as an electron source. The feasibility of such acorrection system has been checked in a dedicatedlaboratory set-up.2 SENSITIVITY LIMIT2.1 High spatial resolution read-out set-up.In the first half of the 2000 SPS run, a read-out systemintegrating a standard CMOS camera, with a 25 Hz framerate associated to CERN designed acquisition electronics,was installed. Beam profiles, integrated over 866 SPSturns, were provided every 40 ms. Figure 1 shows 108consecutive horizontal profiles acquired during the SPSacceleration cycle, starting at injection.These measurements are performed on a single bunchof 6u1010 protons, (half the nominal LHC intensity).Profiles are very well defined, as can be seen in Figure 2,and the shrinking of the r.m.s. beam size (from 1.2 to 0.7mm) during acceleration can be easily distinguished.The profile of a bunch of 6.109 protons, nearly an LHCpilot bunch, is displayed in Figure 3. This measurementwas performed with all gains set at maximum, whilemaintaining the nominal SPS rest gas pressure of 10-8hPa. The signal, although rather noisy, is still exploitable.Figure 1: 108 consecutive horizontal profiles of abunch of 6u1010 protons accelerated in the SPS.Figure 2: Six horizontal individual bunch profiles ofa bunch of 6u1010 protons accelerated in the SPS.This confirms that this set-up is suitable for transverseprofile measurements throughout the full intensity rangeof LHC beams. The large dynamic range can be handledwith by acting on several parameters: the MCP gain,  thephosphor screen gain, the camera lens diaphragmopening, and the video gain. A further possibility is tolocally increase the residual gas pressure by injection ofgas (N2). This will allow for precision measurements atlow beam intensities, (pilot bunches), in the LHC wherethe residual pressure will be lower than in the SPS by atleast one order of magnitude.2.2 High speed read-out set-up.In the second half of the SPS run a Photo MultiplierTube (PMT) with 16 anode strips and dedicated, CERNdesigned, high speed acquisition electronics [4], wereassociated to the IPM. The phosphor used was of the P46type, claimed to have a decay time of 0.3 Ps down to 10%and 90 Ps down to 1%. This set-up allowed for profilemeasurements at the SPS beam revolution frequency(43.3 kHz). Such profiles measured at injection on 6consecutive SPS turns, with good definition, arerepresented in Figure 4.Beam size and position oscillations following injectioncan be observed in Figure 5, (time axis from upper rightto down left corner). Figure 6 represents the associatedaverage position oscillation with a maximum excursion ofr3mm.The previous measurements were carried out with abeam of 1.5u1012 protons consisting of 40 bunches. Theywere repeated with a single bunch of 3.5u1010 protons.Results are displayed in Figure 7. The signal is somewhatnoisier, with a few random spikes, but it is stillexploitable.The evolution of the bunch size measured over 195SPS turns just after injection is represented in Figure 8. Ablow-up of about 5 mm, created deliberately, is clearlyobserved on the rms value.One drawback of this high speed read-out system wasits low spatial resolution. Both position oscillations andbeam-size variations had to be coped with. A range of50.5 mm at the beam level was covered with the 16channels of the PMT, resulting in a resolution of 3.16mm/strip. Reducing this range to 40 mm should beacceptable. Moreover, a new design is under way thatFigure 3: Horizontal profile of a bunch of 6u109 protons.Figure 4: Horizontal profiles of a beam of 1.5u1012protons (40 bunches), on six consecutive SPS turns.Figure 5: Profiles measured on 37 consecutive turns afterinjection of a beam of 1.5u1012 protons (40 bunches).Figure 6: Corresponding oscillations in averageposition (r3mm maximum amplitude).Figure 7: Horizontal profiles of a bunch of 3.5u1010protons, on six consecutive SPS turns.time3 mmuses a PMT with 32 channels. Hence, the optics of thesystem can be modified to reach a resolution of 1.25mm/strip. 3 MCP CALIBRATION SYSTEM.One recurrent problem with instruments employingmicro channel plates is ageing. MCP’s lose gain afterhaving delivered a certain amount of charge, resulting inerroneous measurements. To correct for this phenomena abuilt-in remote controlled electron source could beemployed. This source must deliver a  uniform and, evenmore important, stable distribution of electrons onto theMCP input face. One of the most simple electron sourcesis a glowing wire. Applying an electrical extraction fieldwith sufficient strength will induce enough energy to theliberated electrons to excite the MCP.A laboratory set-up was built to test the principle.Inside a windowed vacuum tank, a wire made of an alloyof Tungsten (75%) and Rhenium (25%), with a diameterof 50 Pm, was stretched in a 80 mm wide supporting fork.This fork was placed on a carriage allowing the distancebetween MCP-input and wire to be varied from 5 mm to60 mm. A DC voltage was applied to the wire endsinducing a current of 0.5 A, causing the wire glow red.Behind the wire and around the input face of the MCP,two large parallel plates were mounted to ensure theuniformity of the extraction field. A voltage of a few tensof Volts was applied on the wire with respect to the platebehind it in order to reject the emitted electrons towardsthe MCP. An extraction field of several hundreds of Voltswas applied between the wire and the MCP input, thusgiving enough energy to the electrons to excite the MCP.The phosphor behind the MCP was of the P46 type andcould be observed through the window.The first results are encouraging. Figure 9(a) shows animage of the light density distribution from the phosphorobtained with the glowing wire at a distance of 60 mmfrom the MCP input face. The distribution looks fairlyhomogeneous. The exercise has been repeated a fewweeks later under the same conditions. No alteration ofthe pattern was observed, indicating that the distributionmay be reproducible with time. Ageing of the wire shouldnot be an issue, since it is operated only for very shortperiods of time.One of the problems encountered in this set-up was anerratic emission pattern along the wire in the xx’direction, Figure 9(b), clearly observable when it wasplaced at 5mm from the MCP input face. Neitherpolishing nor cleaning the wire in a solvent cleared theproblem. Heating the wire for some minutes at a veryhigh temperature, the wire was then lighting up white,did, however, improve the emission pattern: Figure 9(c).The obtained light density distribution is nothomogeneous enough yet to be used as an absolutecalibration system. A peak to peak modulation of about25% can be measured. It should be sufficient, however, totrack the ageing of the MCP. Using two or more wires inparallel at some distance may improve the uniformity ofthe light distribution. The principle of this system will beintegrated into a new IPM design, under preparation, tobe installed next year in the SPS. 4 CONCLUSION.The tests carried out in the SPS during the year 2000with the rest gas monitor, show that it will probably bepossible to acquire profiles down to the LHC pilot bunchlevel (5u109 p). In the turn by turn mode, an improvementof the resolution by a factor of 2.5 is expected, in order toalso use the device for injection matching studies.A future rest gas monitor design will incorporate twoparallel heating wires, emitting electrons, to track andcorrect for the ageing of the micro channel plate.REFERENCES[1] Fischer, C., Koopman, J., Measurements made in theSPS with a Rest Gas Profile Monitor by collectingElectrons, BIW 2000, Boston, USA, May 2000.[2] K. Wittenburg, Emittance measurement in the ProtonAccelerator in DESY, Emittance in Circ. Accel., KEK 95-7, Sept. 1995.[3] Bovet, C., Jung, R., A new Diagnostic for BetatronPhase Space matching at Injection into a circularAccelerator, EPAC ‘96 Sitges, Spain, Jun 1996.[4] Ferioli, G. et Al., Beam profile measurements at 40MHz in the PS to SPS transfer channel, DIPAC’99.Chester, UK, May 1999.Figure 8: Evolution of the horizontal rms value of abunch of 3.5u1010 protons over 195 turns.        (a)                              (b)                           (c)Figure 9: Light density distributions from the phosphor.x’x

Sensitivity Studies with the SPS Rest Gas Profile Monitor

Abstract

Similar works

Full text

Available Versions

CERN Document Server