The CERN heavy ion community, and some other high energy physics experiments, are starting to demand other ions, both heavy and light, in addition to the traditional lead ions. Studies of the behaviour of the afterglow for different operation modes of the ECR4 at CERN have been continued to try to understand the differences between pulsed afterglow and continuous operation, and their effect on ion yield and beam reproducibility. The progress in adapting the source and ion beam characteristics to meet the new demands will be presented, as will new information on voltage holding problems in the extraction

Hill, C E

Küchler, D

Scrivens, R

Wenander, F

Hill, CE

CERN Document Server

Studies on ECR4 for the CERN ion programme

1STUDIES ON ECR4 FOR THE CERN ION PROGRAMMEC. E. Hill, D. Küchler, R. Scrivens, and F. WenanderPS Division, CERN, 1211 Geneva 23, SwitzerlandAbstractThe CERN heavy ion community, and some other highenergy physics experiments, are starting to demand other ions,both heavy and light, in addition to the traditional lead ions.Studies of the behaviour of the afterglow for differentoperation modes of the ECR4 at CERN have been continuedto try to understand the differences between pulsed afterglowand continuous operation, and their effect on ion yield andbeam reproducibility. The progress in adapting the source andion beam characteristics to meet the new demands will bepresented, as will new information on voltage holdingproblems in the extraction.I IntroductionBesides the normal lead operation scheduled for 2002, itis necessary to prepare several new types of beam for physicsexperiments in the CERN Super Proton Synchrotron (SPS)and to anticipate requests that could arise in the Large HadronCollider (LHC) era. The preparation of a He1+ beam and thefirst trials of an indium beam are given here. Although the leadbeam has been in demand for Ion Induced Desorption Studies[1] in test vacuum chambers, free time has been used to testnew ideas and modifications. Some experiments on theafterglow and a study of the extraction system are alsoreported.II. He1+ operationA muon production experiment requested an a-particlebeam. Because of the configuration of the CERN PS Booster(PSB) this requires a He1+ beam, (similar accelerationparameters to the Pb53+ beam) from the linac which would bestripped after the PSB. Because of the strictly fixed injectionenergy of 2.5 keV/u into the Radio Frequency Quadrupole(RFQ) the extraction voltage of the source should have been10 kV for He1+. Experimentally, a slightly higher voltage (10.8kV) gave more intensity at the end of the linac.For 500 emA at the entrance of the RFQ, there was270 emA at exit of the RFQ and 160 emA at the end of thelinac at 4.2 MeV/u. The high losses can be attributed to thelimits of the accelerating structure and emittance problems. Toobtain the above results, the same extraction gap as forstandard lead operation was used, but when this gap wasreduced by a factor 2 to give an extraction field strengthcomparable to nominal lead operation, the current was ~40%lower and unstable, with a sawtooth structure. III. Indium operationIn 2003 a SPS fixed target physics run with indium isscheduled. Although the initial request was for silver (mostcommon isotope 107Ag, 53%) the strict energy acceptance ofthe PSB would have required the use of pure silver isotopes.Indium, being virtually mono-isotopic, readily available andhaving a vapour pressure curve between silver and leadseemed more suitable. A test was initiated in the linac usingmetallic indium with the standard micro-oven (Figure 1)evaporating the metal vapour into the ion source oxygenplasma. Sample Tube Filament inside insulator Heat shield and outer conductor FIG. 1 The oven used for lead or indium.If it is assumed that the vapor pressure of indium inthe plasma chamber should be comparable to that of lead, thena temperature of around 650 °C (melting point 156.6 °C) isneeded. At this temperature the indium is very liquid, easilyflowing out of the sample tube. As liquid indium wets quartzand ceramics, short-circuits occurred in the heater filament.Closing the sample tube with a leaky plug of molybdenumalleviated the problem. However, if the oven power wasmaintained constant (instead of the voltage as is usual), theoven could maintain temperature, even with partial short-circuits. The necessary modifications to heat the oven underpower control should be available for the next test.With up to 80 emA of In21+ out of the RFQ andaccelerated through the linac to 4.2 MeV/u, approximately25 emA of In 37+ was obtained after the stripper foil withoutoptimising the linac parameters (over and above a simplescaling). It was of interest to note that the optimum chargestate  for linac operation (In 15+  which is very similar to Pb 27+)proved difficult to produce in any quantity as is shown in atypical charge state distribution (Figure 2) whereas 21+ couldbe readily maximised. This could possibly be due to a too highplasma temperature and a poor yield for this charge state inafterglow mode. No attempt was made in the time available toexperiment with the "normal" pulsed mode of operation.2FIG. 2 Indium charge state distribution (2.5 keV/u).Overall, the long-term stability would not besatisfactory for operation. The source has to be reoptimisedseveral times per day, as compared to one or two times perweek for lead operation. Considerable care was needed tomaintain the oven power constant. Improvement of thestability will be the main objective in the next series of tests.Two possible solutions will be tried to improve stability:firstly to modify the present, simple, voltage control of theoven heating to a constant power controller; and secondly toinvestigate the use of indium oxide instead of metallic indium,possibly with a support gas other than oxygen.Additionally, for operation it would probably takearound 24 hours to change the oven and to come back to astable and intense beam (~4 hours in the case of lead) mainlydue to the care needed to avoid overheating the sample tube.This could become an important consideration since theconsumption of indium (15-20 mg/day) was somewhat higherthan that of lead (4-8 mg/day) during these tests.IV. New investigations on the afterglow effectThe flexibility of the source timing system enabledsome experiments to be carried out with lead beams inafterglow mode to investigate the effects of plasma heatingtimes. As these tests required the source timing to beasynchronous with respect to the linac, beam intensities couldonly be measured at a Faraday cup (FC2) in the dc injectionline [3]TABLE 1. Effect of heating time (Pb 27+)RF pulse length ms. ion current in FC2 emA.                   50 (standard) 5640 5230 3220 12Initially, for a given source setting the length of themicrowave pulse (i.e. plasma heating) was varied whilstmaintaining  the standard source pulse repetition rate of 10 Hz.Table 1 shows the variation of current with heating time whichis a demonstration of the breeding time of the chosen chargestate. Figure 3 shows the Faraday cup signals at 10 and 40Hz.FIG 3. Effect of heating time and repetition rate for Pb 27+.Upper trace, beam current, lower trace RF power. Repetitionrate is indicated by Ch2 Frequency.However, if the repetition rate for the 20 ms heatingwas increased,  the peak current at FC2 rose until at 40 Hz itreached  48 emA. There are two possible explanations. Firstly,at the instant the RF is switched on, electrons that remaintrapped in the magnetic field in the plasma chamber create aninitial electron component in the plasma which leads to ashorter breeding time for the desired charge state [3].Alternatively, together with the electrons, there are still ions inthe plasma chamber (the afterglow plasma has not completelydecayed) that help create a new plasma with the nextmicrowave pulse and thus reduce the breeding time. VUV orX-ray measurements of the characteristic lines of ions betweenthe heating pulses could indicate which explanation is correct.Changing the RF repetition rate on a O2+ beam showsa smooth transition between real pulsed operation and quasiconstant beam as can be seen in figure 4 where the repetitionrate varies from about 3 Hz at the top to 10 Hz at the bottom.The afterglow is most intense at an intermediate frequency.3FIG 4. Repetition rate effects for O 2+. The beam current zerois marked by the black boxed numbers.V. Solution of the voltage holding problemThe ECR4 uses a simple two electrode extractionsystem and an axially adjustable extraction mechanism wasdesigned. With the desire to simplify the manufacture of thegrounded electrode  a smooth puller was produced [4].Unfortunately this gave extreme voltage holding problemswith high leakage currents (several mA) with magnetic field inthe extraction gap and independent of the presence of  plasmabut unrelated to the distance between puller and plasmaelectrode. The discharge is believed to originate in low fieldareas between the cylindrical part of the puller and the mainceramic insulator. This problem  was finally solved  by addinga small "geometrical perturbation" (a lip), as in the originaldesign, to the smooth puller about 1 cm. from the groundplane.VI. OutlookApart from the continuing effort to improve the yield oflead ions from the ECRIS, it is evident that an effort will beneeded to understand the limitations of the source inproducing high intensities of  light and medium mass ions [5].This will become important in the LHC era in defining thenumber of ion sources and injectors that will be needed tosatisfy the experimental physics requirements.References[1] M. Chanel, J. Hansen, J-M. Laurent, N. Madsen, and E.Mahner, Particle Accelerator Conference, Chicago, (2001). Tobe Published.[2] C. E. Hill and K.Langbein, Rev. Sci. Instrum. 67, 1328(1996).[3] A. Heinen, M. Ruether, H. Ortjohann, Ch. Vitt, and H.J.Andrae, Proc. 14th International Workshop on ECR SourcesECRIS 99, Geneva,  (1999). CERN/PS 99-052 (HP), Geneva(1999), p 224[4] D. Kuchler, C.E. Hill, and F. Wenander, Production ofIntense Beams of Highly Charged Ions, Catania , 33, (2000),edited by S. Gammino and G. Ciavola, Italian PhysicalSociety, Bologna, (2001).[5] M. Chanel, Proc. Workshop on Beam Cooling and RelatedTopics, Bad Honnef. (2001), to be published

Studies on ECR4 for the CERN ion programme

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