The present CERN proton linac (Linac2) was commissioned in 1978 and since that date has been the primary source of protons to the CERN accelerator complex. During the past 18 years, the machine has had a very good reliability record in spite of the demands made upon it. Modifications have been made with the view of maintaining this reliability with reduced resources and new requirements from the users. Further demands will be made in the future for LHC operation. In 1994, a new linac for heavy ion production was put into service replacing the original CERN proton linac. As this machine was built within an international collaboration, operation had to take into account the novelty of the techniques used and the variety of equipment supplied by outside collaborators. Even so, the new machine has also had very good reliability

Charmot, H

Dutriat, C

Hill, C E

Langbein, K

Lombardi, A M

O'Neill, M

Tanke, E

Vretenar, Maurizio

CERN Document Server

OPERATIONAL EXPERIENCE WITH THE CERN HADRON LINACS H. Charmot, C. Dutriat, C.E. Hill, K. Langbein, A.M. Lombardi, M. O'Neil, E. Tanke and M. VretenarPS Division, CERN, 1211 Geneva 23,SwitzerlandAbstractThe present CERN proton linac (Linac2) wascommissioned in 1978 and since that date has been theprimary source of protons to the CERN accelerator complex.During the past 18 years, the machine has had a very goodreliability record in spite of the demands made upon it.Modifications have been made with the view of maintainingthis reliability with reduced resources and new requirementsfrom the users.  Further demands will be made in the futurefor LHC operation.In 1994, a new linac for heavy ion production was putinto service replacing the original CERN proton linac. As thismachine was built within an international collaboration,operation had to take into account the novelty of thetechniques used and the variety of equipment supplied byoutside collaborators. Even so, the new machine has also hadvery good reliability.Linac1Proton OperationAlthough the original CERN proton linac (Linac1) nolonger exits, from the operational point of view it is interestingto review the changes made to the machine since itsconstruction [1,2]. Major performance improvements cameabout quite early with the change of the grid focusing in tank1 with pulsed quadrupoles. Later, an easy access to thesequadrupole power supplies was invaluable in tuning beamintensities for other particles. Further improvements inperformance came with the addition of transient beam loadingcompensation for the RF systems and pre-accelerator HTsystems, the installation of a high gradient HT column with aduoplasmatron ion source and improved matching in theLEBT.Linac1 was commissioned in 1958 and was built with thetechnology of that age. It was very much a hands-on,manpower intensive machine with little scope for moderncontrol technology. A considerable effort had to be expendedto maintain operational availability throughout its life. Thetechnology of the tanks, RF liner inside a vacuum envelope,made alignment unstable and the constructional methods werenot of good vacuum engineering standards. The unstabilisedRF structure also gave rise to instabilities that had to be dealtwith on an “ad hoc” basis. These are some of the reasons themachine became unsuitable for the performance demands ofthe new CERN accelerators, hence the construction of Linac2.Development EraFollowing the commissioning of Linac2, the old machinebecame available for development work. Linac 1 had alreadyaccelerated light ions in the 2EO mode and more extensivestudied were carried out on the production and acceleration ofalphas. The success of a gas target stripper in the LEBTenabled useful quantities of D and D particles to be acceleratedand stored in the ISR.The presence of a spare accelerator also gave rise to aninteresting operation where Linac1 acted as a cheap source ofparticles for cooling and set-up tests for LEAR. Opportunitywas also taken to replace the aging, and increasinglyunreliable, SAMES high voltage generators and the pre-injector by an RFQ [3]. This exercise was successful with animprovement in injection reliability,Light IonsThe positive experience with very light ions encouragedthe users to demand other ions. Experiments demonstrated thatit could be just possible to increase the accelerating fields inthe tanks by 33%. Achievement of these levels on an oldmachine required more access for maintenance and repairsthan was possible during normal PS operation. To overcomethese difficulties, the linac was moved to allow the installationof a shielding wall.  Surprisingly, the linac worked after thismove supplying LEAR again with protons.Installation of an O6+ injector [4] followed soonafterwards with the proton injection line entering at 30o  to theion line. The use of the linac with these two particles wasmutually exclusive but free time could be used to commissionions. This proved to be a difficult process due to sparking intank 1.  Eventually, with a computer controlled formationprogram reliable operation of the tanks was achieved and O6+accelerated and passed to physics. Operationally, the linacproved to be much more reliable than had been hoped for.Even so much effort had to be expended in keeping allcomponent performance at its peak but the failure rate couldbe held below 10% for these short experimental periods.After beams of milliamps, beams of microamps provedmuch more difficult to measure and diagnostics remained amajor problem. This was not helped by an inherent instabilityin the ion source. With the demand for S12+ with higher massbut lower intensity, the problem was aggravated. Linac1finished its active life in 1992 after 33 years of service. It wasa labour intensive machine which required considerableattention but which was sufficiently flexible so as  to  allowconsiderable abuse of its original components. Many moreproblems were experienced with modern ones.LINAC 96360Linac2Reasons for a New LinacThe demands made on the old linac following thecommissioning of the PSB and ISR machines proved difficultto satisfy reliably and efficiently. The input matching, lowinjection energy, the FFDD focusing structure and instabilitiesin the RF at high currents made the old machine much moredifficult to handle. Thus a new machine capable ofaccelerating 200 Ps pulses of up to 150 mA of protons wasdesigned. Improvements built into this machine were a higherinjection energy (750 instead of 500 kV), a post coupledstabilised RF structure, feedback stabilised RF amplifiers andbetter diagnostics.  Advantage was taken of the technology ofthe era to simplify wherever possible and to add remotecomputer control [5].Initial ExperienceThe linac was commissioned in stages with extensivetesting between them. When in 1978, the linac was deemedready it was used for one physics period for injection into thePSB. At this time Linac 1 was held in reserve in case ofproblems but not used. The improved intensity, energystability and reproducibility gave immediate benefits to theother users. From 1979 it became the proton workhorse of theCERN accelerator complex. Reliability was high and over thenext ten years a 5% downtime would give rise to concern.ImprovementsSome problems were experienced with the high voltageholding of the original 750 kV high voltage column but after arigorous program of cleaning and electrode polishing,acceptable to good sparking rates were achieved. In the period1983 to 1992 the sparking rate varied from 1 to 3.5 sparks perday.  However, there were intensity limitations with the highgradient column and although the requirements for LHC couldbe just met, it was felt that more margin was needed. Thesuccess of RFQ1 on Linac1 and the promise of better injectioninto Linac2 from a RFQ led to the decision to replace theCockroft-Walton system by a RFQ [6] with an injectionenergy of 90 kV. This was installed in 1993. Problems relatedto damage to vacuum pumps during HT flashovers did causesome vacuum pollution in the RFQ and a general increase inits sparking. Long term conditioning has rectified thesituation.PerformanceTable 1 shows typical performance figures for 1994 bothfor standard operation and for high performance, LHC type,beams. For the moment, the high performance beam is usedonly for test purposes as some upgrading of the RF would beneeded for longer beams required by some users.For comparison, in 1995 the linac ran for 6630 hours with98.5% availability, all sources of beam loss included.Operation(Typical)LHC(50% duty)Current source 250 350 mAInto Linac 155 220 mALinac out 140 195 mAInto PSB 130 > 170 mAPulse Length 20 - 120 30 - 60 PsRise Time 20 30 PsH Emittance 1.7 1.8 Pm (1V norm)V Emittance 1.2 1.0 Pm (1V norm)Energy Spread r170 r200 keV (V)Operation 6250 > 250 hoursAvailability 97.6 |99% %Table 1.  Performance figures for 1994.Light IonsAlthough Linac2 was designed entirely for protons, it didprove possible, with little effort to accelerate deuterons in themachine. About 20 ePA of deuterons were sent to the PSB andproved invaluable in the commissioning of the early light ionsin the accelerator complex in 1985. With the RFQ this facilityhas now been lostLinac3HistoryThe Light Ion programme was initiated as a collaborationbetween CERN, GSI and LBNL. Following the success of thisexperiment and the impossibility of further upgrading thevenerable Linac1 for heavier ions, a study was launched intothe possibility of building a new linac dedicated to heavy ions.A cost / energy analysis based on possible source technology,restrictions from other machines and interest from physicsindicated that a linac capable of accelerating Pb25+ to 4.2MeV/u with stripping to Pb53+ was feasible in the existing hall.The project [7] came to fruition as an internationalcollaboration between CERN (infrastructure and 200 MHzRF), GANIL Caen (source), INFN Legnaro (LEBT, RFQ andMEBT), GSI (100 MHz RF and IH linac), INFN Torino(HEBT including ion filter), IAP Frankfurt (debuncher).Additional assistance was furnished by the Czech Republic,India, Sweden and Switzerland. In spite of the variety ofequipment supplied by the collaboration, the new linac wasready on time to supply beam to the next accelerators insummer 1994. Beam was supplied to physics for nine weeksof operation later that year [8].First OperationThere  had  been  some  fears  that  the  diversity  of  theequipment would lead to reliability problems as CERN stafflearn to use it. Experience with ions in Linac1 had shown theimportance of beam diagnostics for such low intensity beamsLINAC 96361(80 ePA from the source, 20 ePA after stripping). Faradaycups, SEMgrids, beam transformers and phase probes provedto be invaluable during the commissioning. Also temporary100 MHz amplifiers had to be adapted to CERN controls fortwo operational periods.During the physics period, the fault rate was maintainedat under 2% influenced mainly by mains instabilities.However, at the end of this period when the machine wassupposed to deliver ions to LEAR for cooling experimentsmajor problems, which are still not yet fully resolved,  withthe microwave generator for the source caused a considerabledelay.ConsolidationAs the next physics period was not scheduled until theend of 1995, time was spent in consolidating  the machine foroperation.  Tests were carried out on the source improving theoutput current to 120 ePA. However, new problems with themicrowave generator arose requiring major repairs. The firstof the 100 MHz amplifiers from industry was delivered andcommissioned for the 1995 run but although it worked withoutfault during this period, the reliability has not been as good asexpected. The second amplifier was delivered in 1996 toreplace the equipment borrowed from GSI which had workedwell, in spite of its age, until the end of 1995. The new 100MHz amplifier is still undergoing debugging by themanufacturer.Between the IH structures, quadrupole triplets areinstalled which in their initial configuration made alignment ofthe linac very difficult. Modifications were made by thecollaborator but problems still exist in this area.Another cause for concern, the life of the 100 Pg/cm2stripper foils turned out to be unfounded. Although statisticsare not readily available, it has been found that a foil whichresists the > 100 ePA 4.2 MeV/u beam for more than a fewhours, will resist for months.The FutureLinac2 has been the supplier of protons to CERN for 18years and during that period has showed a remarkablereliability. However, it has recently started to show signs offatigue especially at the level of the vacuum seals. Majorworks are envisaged to change and improve vacuum sealing inthe next years. Also during its life no re-alignment has beencarried out  and there are indications that due to settlement ofits building this would be desirable. The demands of the LHCera will require more performance from the linac and thussome refurbishment of the machine will be needed to maintainreliability.Ion beams will continue to be in demand. In the shortterm, the reliability can be maintained with manpower effortbut some improvements are still needed. The use of LEAR asan intermediate ion store for LHC ion beams will requireoperation  at 10 Hz. Most of the components are alreadysuitable for this but the impact on reliability has yet to beassessed. Already Linac3 has shown itself to be a flexiblemachine capable of changing ion species at fairly short notice.When its teething problems have been overcome, it will be avery useful tool.AcknowledgmentsIn the history of the CERN hadron linacs the people whobuilt these machines should not be forgotten. Service groupsprovide us with the equipment and the repair service thatkeeps the machines running with high reliability. The authorswould like to thank them all.This paper is also dedicated to Pierre Tetu, who left uswith linacs that work and who’s knowledge of them is sorelymissedReferences[1] E. Regenstreif, The CERN Proton Synchrotron (2nd Part)”,CERN 60-26, 1960[2] H. Haseroth, C.E. Hill, K. Langbein, E. Tanke, C. Taylor,P. Tetu, D. Warner, M. Weiss, “History, Developments andRecent Performance of the CERN Linac 1”, 1992  Int. Conf.on Linear Accelerators, Ottawa,  AECL-10728, 58, 1992[3] E. Boltezar, H. Haseroth, Ch. Hill, F. James, W. Pirkl,   G.Rossat, P. Tetu, M. Weiss, Performance of the CERN RFQ(RFQ1 Project), Proc. 1984 Linear Accelerator Conference,Seeheim, 1984, GSI-84-11, 56, 1984[4] H. Haseroth, C. Hill, P. Tetu, M. Weiss, B.H. Wolf,        K.Lieble, P. Spaetke, J. Klabunde, B. Langenbeck,              N.Angert, R.A. Gough, J. Staples, R. Caylor, D. Howard,    R.McGill, J. Tanabe, “Ion Acceleration in the CERN Linac1”,Proc. 1986 Linear Accelerator Conference, Stanford, SLAC-303, 355, 1986[5] E. Boltezar, H. Haseroth, W. Pirkl, G. Plass, T. Sherwood,U. Tallgren, P. Tetu, D. Warner, M. Weiss, “The New CERN50 MeV Linac”, Proc. 1979 Linear Accelerator Conference,Montauk, BNL 51134, 66, 1979[6] C.E. Hill, A.M. Lombardi, W. Pirkl, E. Tanke,              M.Vretenar, “Performance of the CERN Linac2 with a HighIntensity Proton RFQ”, Proc. 1994 International. LinearAccelerator Conference, Tsukuba, 175, 1994[7] D. Warner (Editor), “CERN Heavy Ion Facility DesignReport”, CERN 93-01, 1993[8] H.D.  Haseroth, “The CERN Heavy Ion AcceleratingFacility“, Proc 1995 Particle Accelerator Conference,  Dallas,IEEE, V1, 411 (1996)LINAC 96362

Operational experience with the CERN hadron linacs

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