The H- Accelerator consists of a 750 keV Cockcroft Walton preaccelerator and
an Alvarez type 50 MeV linac. The accelerator has been in operation since 1961.
Since 1981, it has been used as the injector for the Intense Pulsed Neutron
Source (IPNS), a national user facility for neutron scattering. The linac
delivers about 3.5x1012 H- ions per pulse, 30 times per second (30 Hz), for
multi-turn injection to a 450 MeV Rapid Cycling Synchrotron (RCS). IPNS
presently operates about 4,000 hours per year, and operating when scheduled is
critical to meeting the needs of the user community. For many years the IPNS
injector/RCS has achieved an average reliability of 95%, helped in large part
by the preaccelerator/linac which has averaged nearly 99%. To maintain and
improve system reliability, records need to show what each subsystem
contributes to the total down time. The history of source and linac subsystem
reliability, and improvements that have been made to improve reliability, will
be described. Plans to maintain or enhance this reliability for at least
another ten years of operation, will also be discussed.Comment: 3 pages, 1 figur

Brumwell, F. R.

Donley, L. I.

McMichael, G. E.

Stipp, V. F.

English

arXiv

UNT Digital Library

*,’ fw- /!!P/tLs/@- /0/ 7’5/RELIABILITY HISTORY AND IMPROVEMENTS TO THE ANL 50 MEV IZACCELERATOR*L.I. Donley, V.F. Stipp, F. R. Brumwell, G. E. McMichael, ANL, Argonne, IL 60439, USAAbstractThe H- Accelerator consists of a 750 keV CockcroftWalton preaccelerator and an Alvarez type 50 MeV linac.The accelerator has been in operation since 1961. Since1981, it has been used as the injector for the IntensePulsed Neutron Source (IPNS), a national user facility forneutron scattering. The linac delivers about 3.5x 10’2 Hions per pulse, 30 times per second (30 Hz), for multi-turninjection to a 450 MeV Rapid Cycling Synchrotron(RCS). IPNS presently operates about 4,000 hours peryear, and operating when scheduled is critical to meetingthe needs of the user community. For many years theIPNS injector/RCS has achieved an average reliability of95%, helped in large part by the preaccelerator/linacwhich has averaged nearly 99970. To maintain andimprove system reliability, records need to show whateach subsystem contributes to the total down time. Thehistory of source and linac subsystem reliability, andimprovements that have been made to improve reliability,will be described. Plans to maintain or enhance thisreliability for at least another ten years of operation, willalso be discussed.1 INTRODUCTIONAfter operating with 200 ps pulses at 0.25 Hz for 15years as an injector for the Zero Gradient Synchrotrons(ZGS), the linac repetition rate was increased to 30 Hzwith 70 w pulses in the mid- 1970’s when it also becamean injector for the Rapid Cycling Synchrotrons (RCS).Direct injection to the ZGS ended in 1979 and the RCS,which was originally built to be a booster for the ZGS,became the source of high-energy protons for the IntensePulsed Neutron Source (IPNS), a national user facility forneutron scattering. Since IPNS began operation in 1981,it has accumulated over 60,000 hours of 30 Hz beamoperations. The overall accelerator reliability (beamhours versus scheduled hours) exceeds 93% and for thepast 10 years, that number is greater than 95%.Thoughout, the preacceleratorllinac reliability has hoveredaround 99~0, and this has been achieved while operating atroughly 30 times its original duty factor.The evolution and performance of the preacceleratorand linac was described in a 35& anniversary paperpresented at the Linac96 Conference [1]. In this presentpaper, we examine overall system and subsystemreliability and the impact of changes in hardware andsoftware on reliability, and discuss improvements that willbe necessary to maintain current high performance for atleast another ten years.The linac rf system was the first linac amplifier builtusing the 7835 triode. The rf amplifier system wasdesigned and built by Continental Electronics Corporationin the late 1950’s and comprises a single 4-stage amplifiersystem with a rated output of 5 MW and a maximumpulse length of about 200 vs. With the exception of thepower supplies, most of which had to be upgraded duringthe late 1970’s when the duty factor was increased, theoriginal configuration and in many cases, originalequipment, is still functioning well. In the almost 40years since startup, seventeen new or rebuilt 7835 tubeshave accumulated a total of over 200,000 filament hoursin this single station, equivalent to over 23 years ofcontinuous operation.2 PREACCELERATORThe preaccelerator is a standard 4-stage 750 kVCockcroft-Walton power supply built by Haefely. Therehave been no failures of the Haefely high-voltagecomponents in the life of the facility. The original motorgenerator set supplying 400 Hz ac to the high-voltagerectifiers was replaced by one made in the USA, Theoriginal 400 Hz isolated generator used for supplyinginput power to the ion source enclosure was also replacedwith a brushless 60 Hz unit to reduce maintenance. It hasbeen trouble-free for the past 10 years.The first ion source was a proton source. This waschanged to a modified duoplasmatron-type 11 source in1976, and then to the present magnetron-type H- source in1983. The present source uses cesium to enhance the H-current and produces 45-50 mA, 70 microsecond beampulses at the required 30 Hz rate. It has been very reliableand typically is dismantled and cleaned every six months,with new electrodes installed about every year. Closecontrol of the cesium flow from the ion source isnecessary to keep the spark rate of the 750 kV columnlow. However, proof of success is the fact that thecolumn has not been dismantled in over 14 years.Detailed operating logs have been kept since thebeginning of operation and for the last five years, thetrouble log has been computerized. Accelerator downtimeis tracked, rounded to the closest 5 minutes. Over the lastfive years (20,000 hours of beam operation) there were143 interruptions of 5 minutes or more charged to thepreaccelerator and 120 to the linac. Breakdown bysubsystem is given in Tables 1 and 2.*This work is supported by the US DOE under contract no. W-31- 109-ENG-38.DISCLAIMERThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.Table 1: Lost beam time in last 20,000 hours of operationdue to preaccelerator faults (number of beaminterruptions, lost beam time, mean-time-between-failureand mean-time-to-repair).IChopper I 15 4.0 7.9 0.3 JVacuumfWater 22 7.3 I 5.4 I 0.3Beam Transport 9 7.3 13.2 0.8 ~3 LINACThe 50 MeV Iinac is an Alvarez structure, constructedas eleven sections that are bolted together to form a singlerf cavity that is 0.94 m in diameter and 33.5 m long.There are dc quadruple magnets in each of the 124 drifttubes powered by twelve mag-amp type dc supplies.These units are original to the linac and are nearly 40years old. The change from tube to semiconductorregulators in 1988 cleared up most of the maintenance andfailure problems with these systems. The tank, drift tubesand quadruples have been very reliable. With theexception of a failed interlock and loss of water flow inthe first eight quadruple magnets in 1988 (in whichexternal soft-solder joints melted and separated but nonoticeable internal damage to the magnet coils occurred),these systems have caused no significant downtime.Occasional rf arcing in the linac tank after vacuum workhas generally been cured by mild reconditioning, andthere is sufficient redundancy of vacuum pumps thatfailures can be left to the next scheduled maintenanceperiod for repair.Over the life of the machine, there have been significantchanges in the vacuum pumps used on both thepreaccelerator and linac. Some of the original 2000 l/sion pumps are still in use (after many rebuilds to replacethe titanium elements and bead-blast internal surfaces).Others have been replaced with cryopumps orturbomolecular pumps to increase pumping speed andallow operation to continue despite small leaks. Vacuumpump upgrades are expected to continue over the life ofthe facility because improvements in vacuum havepositive effects on many of the subsystems.As is evident from Table 2, the rf transmitter accountsfor most of the downtime, and over the past five yearsalmost half the downtime for the preaccelerator/Iinaccombined is caused by two components, the dc blockingcapacitors in the triode cavity. These capacitors, whichare custom made by Continental Electronics and unique toour transmitter, only became a problem after the changeto 30 Hz operation. However, in the past twenty years,the upper capacitor has failed 16 times and the lower 10times. Although average lifetimes are approximately 25and 40 operating weeks for the upper and lower capacitorsrespectively, the range is broad. There was a period fi-om1987 through 1995 (170 weeks) with no failures, whereasothers have failed after only a few weeks. Frequency offailures have been higher over the past few years, possiblydue to higher-order modes in the cavity and to difficultiesthe manufacturer is encountering in maintaining a rebuildcapability for such limited-demand items.Table 2: As Table 1 but for linac faults.I I #of I Down I IISubsystem Faults Time MTBF MTTR(hours) (weeks) (hours)DT power supplies 14 9.6 8.5 0.7RF (capacitors) 15 109.6 7.9 7.3RF (triode) 1 4.5 I 118.7 4.5RF (other) 64 34.4 I 1.9 0.5Vacuum/Water 19 11.3 6.2 I 0.6Controls I 7 3.1 I 17.0 I 0.4The 7835 triode amplifier tubes used in our linac and atseveral other accelerator facilities (e.g., LANL, FNAL)were originally developed and manufactured by RCACorporation. For many years, the only supplier for thesetubes has been Burle Industries, a small company set upby some former employees of RCA. Lifetime of the tubesis a significant operational consideration because of thehigh item cost (of order $150,000 for a new tube and halfthat if the failed tube can be repaired) and delivery timesare 6 to 12 months. Since startup in 1961, we haveaccumulated over 200,000 filament hours on 17 new orrebuilt tubes. In the early years of the facility, most of thetubes failed during operation because of grid-cathodeshorts. Lifetimes varied from 3,000 to 17,000 hours(10,000 hours average) for the eight tubes that developedshorts, and each resulted in downtime to install andcondition the new tube. A manufacturing change by RCAabout 1970 cured the grid-cathode short problem, and nowthe tubes last until the cathode degrades. Cathodesdegrade gradually, giving several months during whichthe heater current can be periodically increased tomaintain sufficient emission and providing time toschedule the replacement during a maintenance period.Nine tubes have been replaced because of low emissionwith lifetimes ranging from 9,000 to 20,000 hours (15,000hours average). The only downtime charged to the tubesin the last five years was a 4.5 hour interruption in 1997, afew days after a blocking-capacitor failure, when the tubedeveloped an external arc (carbon track on the ceramic).The tube was removed. cleaned and returned to serviceand proceeded to run trouble-free for another 14,000hours.4 DISCUSSIONIPNS operation increased about five years ago to itspresent level of about 25 weeks per year. Duringoperating periods, typically two to three weeks on withone to two weeks off between runs, the facility isexpected to run 24 hours a day, seven days a week.External users come for periods of a couple days to aweek or more to make measurements on samplesrequiring individual exposure times that vary from lessthan an hour to several days. In general, beaminterruptions of half an hour or less are relativelyinsignificant to users unless they are frequent. Even then,the main effect is a lack of throughput proportional to thelost beam (equivalent to operating continuously at reducedproton current on target). Long interruptions, particularlythose exceeding eight hours, may mean that the user’ssamples don’t all get exposed or statistics are poor and theuser may get ambiguous or uninterpretable data. Therehave been only five beam interruptions frompreaccelerator/linac faults over the last five years thatexceeded eight hours and the longest was 20 hours. Allwere associated with blocking-capacitor failures. Theaverage preaccelerator/linac reliability (percent ofscheduled time when protons were delivered to target)was 98.8% and half of the total downtime was associatedwith blocking-capacitor failures. Downtime of all IPNSaccelerator systems over the past 12 years (including thepreaccelerator/linac) is shown in Figure 1. Overall, thepreacceleratorllinac accounted for about one quarter of thetotal accelerator downtime.IPNS Accel. Equipment Trouble[1]Figure 1. IPNS Accelerator Equipment Trouble ChartThe IPNS accelerators, for over 10 years, havedelivered high-energy protons to the spallation target withan average reliability of 95Y0. Experience has shown thatthis is a level at which the neutron-scattering users areseldom disappointed. However, on the occasional two-to-three week runs when reliability slips below 90Y0, thechances of some users not completing their experimentsare much increased. Also, the short nature of individualexperiments means that one cannot compensate for a lostday of beam by operating at 10% higher current for thenext 10 days. Thus for IPNS, mean-time-to-repair is atleast as important as total downtime.The challenge posed five years ago, of increasingoperation from the previous level of 15-17 weeks per yearto 25 or greater without impacting reliability, has beenmet. Present plans call for a further increase to about 30weeks per year. Additional pressures on reliability arethat several systems, specifically the high-voltageregulator and pulsed bouncer for the preaccelerator, andthe driver stage for the 7835 power triode in the linac rfsystem, will have to be replaced because the tubes used inthese units are no longer available. As it is probable thatthe new systems will result in an initial increase indowntime, it will be important to aggressively attack theIinac rf capacitor problem which now accounts for abouthalf of our total downtime. The pressurized cavities usedfor the 7835 tubes at FNAL and LANL are a possiblesolution, but could not be installed in the 8-10 weeksummer shutdown of our present operating schedule.Therefore we are hoping that our present work with themanufacturer to develop a more robust capacitor, coupledwith our work to decrease higher-order modes in thecavity, will provide relief.ACKNOWLEDGEMENTSThe authors strongly believe that the reliability of theIPNS accelerator systems is primarily attributable to thedesigners, subsystem managers, engineers, techniciansand operators who have systematically eliminated theweak components and have developed maintenance andoperating techniques that provide rapid detection andrepair when failures occur. Their response to calls, at anyhour of the day or night, is what keeps our mean-time-to-repair manageable.REFERENCESV. Stipp, F. Brumwell, and Gerald McMichael, “TheANL 50 MeV H- Injector - 35 Year Anniversary,1996 Linear Accelerator Conference, Geneva,Switzerland, Aug 26-30, 1996, CERN Report 96-07,NOV. 1996, p74.

Reliability history and improvements to the ANL 50 MEV H accelerator.

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Reliability History and Improvements to the ANL 50 MEV H- Accelerator

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