In fall 1994 the new heavy ion injector at CERN was brought into operation successfully and a lead beam of 2.9´107 ions per pulse was accelerated in the SPS up to an energy of 157 GeV/u. The ion source, which was supplied by GANIL (France) was in operation almost continuously over a period of about one year and proved to be very reliable. It pro-duces a current of more than 100 µA of Pb27+ (after the first spectrometer) during the afterglow of the pulsed discharge. The current stays within 5% of the maximum value for a time of about 1 ms, which is more than required by the accel-erators. Measurements of the charge state distribution, emittance and energy spread, which were made during this window, are presented together with other operating data

Bougarel, M P

Haseroth, H

Hill, C E

Langbein, K

Tanke, E

Bougarel, MP

Hill, CE

CERN Document Server

Performance of the ECR ion source of CERN’s heavy ion injector

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHORGANISATION EUROPEENNE POUR LA RECHERCHE NUCLEAIRECERN - PS DIVISIONPS/HI/Note 95-12Performance of the ECR Ion Source of CERN’s Heavy Ion InjectorM. P. Bougarel*, C. E. Hill, H. Haseroth, K. Langbein, E. TankePS-Division, CERN, Geneva, Switzerland*GANIL, B.P. 5027, F-14021 Caen Cedex, FranceAbstractIn fall 1994 the new heavy ion injector at CERN was brought into operation successfully and a lead beam of 2.9×107ions per pulse was accelerated in the SPS up to an energy of 157 GeV/u. The ion source, which was supplied byGANIL (France) was in operation almost continuously over a period of about one year and proved to be very reliable.It produces a current of more than 100 µA of Pb27+ (after the first spectrometer) during the afterglow of the pulseddischarge.  The current stays within 5% of the maximum value for a time of about 1 ms, which is more than requiredby the accelerators. Measurements of the charge state distribution, emittance and energy spread, which were madeduring this time window, are presented together with other operating data.Presented at the 12th International Workshop on ECR Ion Sources, April 25-27, 1995, RIKEN, JapanGeneva, SwitzerlandJuly 19952Performance of the ECR Ion Source of CERN’s Heavy Ion InjectorM. P. Bougarel*, C. E. Hill, H. Haseroth, K. Langbein, E. TankePS-Division, CERN, Geneva, Switzerland*GANIL, B.P. 5027, F-14021 Caen Cedex, FranceAbstractIn fall 1994 the new heavy ion injector at CERN was brought into operation successfully and a lead beam of 2.9×107ions per pulse was accelerated in the SPS up to an energy of 157 GeV/u. The ion source, which was supplied byGANIL (France) was in operation almost continuously over a period of about one year and proved to be very reliable.It produces a current of more than 100 µA of Pb27+ (after the first spectrometer) during the afterglow of the pulseddischarge.  The current stays within 5% of the maximum value for a time of about 1 ms, which is more than requiredby the accelerators. Measurements of the charge state distribution, emittance and energy spread, which were madeduring this time window, are presented together with other operating data.IntroductionAfter the successful completion of the last lightion run in 1992, CERN’s Linac I, which hadbeen in operation for more than thirty years,was dismantled and replaced by a new heavy ionlinac (Linac III) [1,2]. The various componentsof this new injector were supplied by five col-laborating institutes.The ECR ion source, was constructed andsupplied by GANIL, France. The low energybeam transport system (LEBT), was contributedby INFN, Legnaro, who also designed andsupplied the RFQ accelerator. The linac, whichis a so called Interdigital-H (IH) accelerator,was developed at GSI, Darmstadt, who alsosupplied most of the RF systems. The filter lineafter the linear accelerators  was contributed byINFN, Torino and the final debuncher is a con-struction of the IAP, University of Frankfurt [3].The pulse length required by the synchro-trons at CERN is only 400-500µs with a repe-tition rate of 1 (or alternatively 10) Hz.  There-fore CERN’s accelerator complex is ideallysuited for the application of the afterglow of theECR discharge, which can produce ion pulseswith durations ranging from a few hundred mi-croseconds to several ms. Furthermore the af-terglow is particularly well suited for the pro-duction of highly charged heavy ions (the spe-cies presently required at CERN is Pb27+). Thevery successful application of the afterglowduring the former operation with sulphur ions[4,5] and the good results obtained for lead ionsat GANIL [6,7], gave great confidence, that theadvantages of the afterglow could be utilized forCERN’s heavy ion project.Layout and SpecificationsFig.1 gives an overview of the preinjector.  Theion source, which is of the ECR4 type [8], isoperated at a frequency of 14.5GHz; the maxi-mum power available is 2kW. The source hasc.w. capabilities, but was optimized for after-glow operation. The discharge is pulsed with apulse length of 50ms and a repetition rate of10Hz.The RFQ requires an injection energy of2.5keV/u, which leads to a preacceleration volt-age around 20kV, depending on the chargestate of lead. All components have been de-signed to accelerate Pb25+. Presently chargestate 27+ is preferred as this requires a lowerRF level in the accelerators.Fig.1. Low energy part of heavy ion injector3Oxygen is used as a carrier gas and iso-topically pure lead 208 is evaporated into theplasma using a micro oven [7], which is situatedinside the coaxial RF feed line.The Pb-ion current originally specified inthe design study [2], a figure of merit, on whichall further (very optimistic) estimates in the ac-celerator chain were based, was 80µA of Pb27+with a pulse duration of 600µs. In the designphase of the injector this current could bereached, however the duration was a critical pa-rameter, as the position of the cutoff edge of theafterglow is usually not very stable.The ion beam is extracted from an aper-ture of  13mm diam. with a single extractionelectrode at ground potential. The extractiongap is  43 mm. After a drift of  approx. 1m thebeam is focused onto the entrance slit of thespectrometer using a solenoid. The  verticalmatching to the spectrometer can be optimizedby a single quadrupole. The spectrometer is asplit pole dipole with a deflection angle of 135°.Higher order corrections, which are obtained bya special curvature of the inner edges of thepoles, result in the very high resolution of∆p/p = 0.003 . Its design is similar to that firstinstalled at GSI’s HLI [9].After the spectrometer, the beam passesthrough an exit slit, which can be closed to±3 mm without losses of the desired chargestate losses. A Faraday cup with electrostatic-and additional magnetic secondary electronsuppression is used to measure the ion currentafter the spectrometer. The radial symmetry ofthe beam is restored by a set of three independ-ent quadrupoles and the final focusing into theRFQ is done by a second solenoid. The RFQ,which accelerates the beam from 2.5 to250keV/u uses the four-rod principle, originallydeveloped by A. Schempp [10]. The Legnarodesign [11] used here employs four modulatedvanes, which act as “rods” of the resonator.The IH structure, designed byU. Ratzinger, GSI [12], accelerates the beam to4.2MeV/u. At this energy the ion beam isstripped in a carbon foil to a distribution ofcharge states ranging from 49 to 57. A secondspectrometer is then used to select charge state53+ for the further acceleration in the synchro-trons.ResultsThe originally specified 80µA of Pb27+ werereached soon after the installation of the sourcein Jan. 1993. After an extensive study of the pa-rameter field, the performance could be furtherimproved  and currents of up to 140µA Pb27+could be extracted during the afterglow undercertain conditions. To classify the infinite rangeof possible operating conditions  four operatingregimes, mainly characterized by the magneticfield distribution were defined.a)  The “GANIL” or “standard” modeIn this case the currents in the two coils of thesource lie around 1080/1020 A. The afterglowpulse (see Fig.2a) obtains a relatively flat top ofa duration of up to 600µs, which is followed bya sharp cutoff edge. The maximum current ob-tained in this mode is 100µA. The beam currentmay fluctuate by a few percent from pulse topulse and the position of the cutoff edge variesby a few tens of microseconds.b) The “maximum” modeThe highest currents (up to 140µA, see Fig.2b)can be obtained at high magnetic fields (e.g., at1100/1020A). The duration of the afterglowpeak decreases to about 200µs and the stabilityis poor. This mode of operation does howevergive an indication of the potential for furtherimprovements.c) The “stable” modeA third very distinct operating regime wasfound at very low magnetic fields (860/900A).In this mode of operation the afterglow has acompletely smooth shape (see Fig.2c) as earlierpredicted in ref. [5]. The current maximum isreached after a rise time of about 500 µs and isfollowed by a long tail with a decay time of sev-eral ms. The sharp cutoff edge, which is encoun-tered in the other operating modes is probablydue to breakdowns in the extraction systemcaused by the impinging beam, which in turnlead to instabilities in the plasma and disturb thefurther extraction process. Thus, if those break-downs are reduced, which is evidently the case4in this mode of operation, the pulse to pulsestability of the afterglow becomes extremelygood: After some optimization it was possibleto obtain a stability which was so good, that itwas hardly possible to distinguish the continu-ously pulsing afterglow from a stored trace on adigital storage oscilloscope. This means that thepulse to pulse fluctuations are well below 1% ofthe total amplitude. Also the long term stabilityis greatly improved in this mode of operation.The ion current obtained in this mode are gen-erally lower than in the the other cases, although80 µA of Pb27+ can also be obtained.d) “Operational” modeDuring most of the running in phase of the ac-celerators and finally during the production run,the source was operated in a regime close to thestable mode but tending towards higher mag-netic fields (900/940A) and higher ion currents.In this mode the beginning of the afterglow issimilar to that of the stable mode, but the sharpcutoff appears again after about 1ms (seeFig.2d). The maximum current obtained for thiskind of operation was 100µA while the standardcurrent in operation was 80-90µA.Charge State DistributionA special technique (decribed ref. [15]) was de-veloped to measure the charge state distribution(CSD) during a given time window, maintainingthe same beam otics for all charge states.Fig.3 gives two examples of such a measure-ment, a) the CSD during the afterglow and b) -on a different scale - the CSD during the maindischarge. The CSD during the afterglow peaksaround charge state 26, which is overlapped bythe intense O2+ peak. The distribution during themain pulse differs from this and a characteristicdip for charge state 25 is often observed.Energy Spread and Voltage StabilizationDue to the high resolution of the spectrometerand the limited acceptance of the RFQ in theradial plane, the energy stability during the af-terglow is of great importance: A change in en-ergy with time results in a displacement of thebeam after the spectrometer and thereby causesa virtual growth of the emittance.Apart from the energy distribution of theions in the plasma, which is generally consideredto be low for ECR ion sources, an energy varia-tion can have two causes: Firstly the currentdrained from the HT supply varies greatly inpulsed operation, especially during the after-glow, where the total current leaving the sourcemay increase by a factor of two. This thenmodulates the source potential. Secondly, if oneassumes, that the afterglow is produced by ionstrapped in a negative plasma potential as sug-gested by Shirkov [13], this potential willchange during the course of the afterglow andthereby lead to a temporal energy variation.    Fig.2. Comparison of different afterglowmodes: a) “standard” mode, b) “max.” modec) ”stable” mode and  d) ”operational”mode.5The voltage variation was found to beabout 300V at the beginning of the main pulseand an additional 50V negative with respect tothe originally applied voltage during the after-glow. The time varying energy change could bemeasured using the high resolution spectrome-ter. Fig.4 shows a plot of momentum/timephase space, which was constructed from 100stored oscilloscope traces. On fig.4a one sees aconsiderable distortion of this figure, which in-dicates a large change of momentum at the be-ginning of the pulse. After the installation of anelectronic voltage stabilization, the change ofenergy with time disappeared almost completelyas shown in Fig.4b, allowing an estimate of theenergy spread obtained in the plasma: Using thefull width of the peak with voltage stabilzationone arrives at a momentum spread of ∆p/p of0.005, from which spread of 50 eV per chargecan be calculated.Emittance MeasurementsA further uncertainty was the emittance of thebeam during the afterglow. Earlier measure-ments had only been performed in c.w. opera-tion. At CERN it was possible to measure theemittance during a time window of only 200µsusing several hundred source pulses. The emit-tance was measured at the location of the RFQentrance in both planes, using a slit and profileharp arrangement. It was found, that the emit-tance during the afterglow lies inside the re-quired value of 200 pi mm mrad (see fig.5a).However, also some strong distortions of theemittance figure were found at certain LEBTsettings (see fig. 5b). These distortions may be aresult of an inhomogeneous plasma density atthe outlet aperture, caused by the sextupolemagnets. Furthermore spherical aberrations maybe induced by the two solenoids. Nevertheless,the transmission through the RFQ was >90%,indicating, that the emittance lies well inside  theacceptance of the RFQ. Detailed results of  theLEBT investigations are found in [14].Long Term OperationDuring the installation and setting up period ofthe injector, the source was operated almostcontinuously (24h per day) for almost one year,with short stops only for servicing. After thefirst injection into the booster synchrotron inJune 1994 until the end of the year, there wasvirtually no deterioration of source performanceand only minor readjustments were required perday. Thus the source proved to be extremely re-liable also in long term operation.Fig. 3. Comparison of charge state distribu-tions a) during afterglow and b) during mainpulse (discharge optimized for afterglow).Fig. 4. Momentum/ time phase diagram of thePb 27+ beam during the afterglow6Performance of the Lead InjectorThe results of the 1994 ion run are summarizedon fig.6: Although a maximum current of140µ A of Pb27+ can be reached, the stabilitywas considered more important and a current of80-90µ A was adjusted during the productionrun. The highest current obtained at the exit ofthe RFQ was 80µ A, the operational value being70µA. The transmission through the IH linacwas also very good and the current at its exitwas 60µ A. After stripping and filtering thereremained a current of 25µ A of Pb53+ for furtheracceleration.The beam was transferred to the boostersynchrotron (PSB) with almost no losses. Thecircular machines at CERN accelerate a total offour consecutive pulses from the linac duringthe PS supercycle of 16 seconds. Within thosefour pulses, the booster was able to accelerate1.2×109 ions to 94Mev/u, the PS accelerated7.5×108 ions to an energy of 4.2GeV/u and fi-nally the SPS accelerated 2.9×108 fully strippedlead ions to an energy of 157GeV/u for fixedtarget experients.References[1] “Concept for a Lead Ion Accelerating facility at CERN”, H. Haseroth (ed.), CERN 90-01 (1990).[2] “CERN Heavy-Ion Facility Design Report”,D. Warner (ed.), CERN 93-1 (1993).[3] A. Firjahn-Andersch, “Untersuchungen von Buncherresonatoren hoher Effekivitaet”, Diplomar-beit, University Frankfurt a.M., 1993[4] P. Briand, R. Geller, H. Haseroth, C. Hill,K. Langbein, Proc. 11th Int. Workshop on ECRIS, KVI-Report 996, p.135 (1993)[5] K. Langbein in “Notes of the Workshop on ECRSource Problems” (1992), PS / HI / Note 92-05.[6] P. Sortais,  Rev. Sci. Instrum., Vol. 63, No. 4 (1992)[7] P. Sortais, ibid. ref. [5][8] P. Bricault et al., ref. [6], p. 2494[9] N. Angert et al., GSI Sci. Rep. 1986, GSI-87-1, 317[10] A. Schempp et al., NIM B10/11 (185) 831[11] A. Bezzon et al., Proc. LINAC 94, Tsukuba, 1994[12] N. Angert et al., Proc. LINAC 94, Tsukuba, 1994[13] G. Shirkov, ibid. ref. 4[14] A. Lombardi, A. Pisent, M. Weiss, K. LangbeinA.M. Lombardi, E. Tanke, H. H. Umstätter, Proc. LINAC 94, Tsukuba,1994[15] K. Langbein, this conferenceFig. 5. Emittance measurements  of the Pb27+beam at the entrance of the RFQ.Fig.6. The heavy ion injector at CERN: Results ofthe 1994 lead ion run.7

Performance of the ECR ion source of CERN's heavy ion injector

Performance of the ECR ion source of CERN's heavy ion injector

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