The source SERSE is operational at LNS since June 1998 and many improvements have been carried out in this period. The frequency has been increased from 14.5 GHz to 18 GHz and the use of two frequency heating has given positive results. Metallic ion production has been tested by means of a high temperature oven and the preliminary results are described. Tests of magnetic field scaling and frequency scaling have confirmed the results of previous tests with SC-ECRIS at lower frequency and seems to suggest that the upgrading of the source to higher frequency may be considered

Briand, P

Castro, M

Celona, L

Chines, F

Ciavola, G

Gammino, S

Girard, A

Guillaume, D

Ludwig, P

Marletta, S

Melin, G

Seyfert, P

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PRODUCTION OF INTENSE HIGHLY CHARGED ION BEAMS WITHSERSES. Gammino, L. Celona,      G. Ciavola   , M. Castro, F. Chines, S. MarlettaINFN-Laboratori Nazionali del Sud, Via S. Sofia 44, 95123 Catania, ItalyG. Melin, P. Briand, A. Girard, P. Ludwig, P. Seyfert, D. GuillaumeCEA/DRFMC, 17 Rue des Martyrs, 38041 Grenoble, FranceAbstractThe source SERSE is operational at LNS since June 1998and many improvements have been carried out since then.The frequency has been increased from 14.5 GHz to 18 GHzand the use of two frequency heating has given positiveresults. The chamber insulation was improved, so that thesource can be operated now up to 30 kV. The hightemperature oven for metallic ion production has beenmounted and it will be tested through next weeks. Tests of magnetic field scaling at different frequency havebeen carried out and they have confirmed the results ofprevious tests with SC-ECRIS at lower frequency,suggesting that the source upgrading to higher frequenciesmay be considered.1 THE INSTALLATION AT LNSThe design of the superconducting Electron CyclotronResonance (ECR) ion source SERSE (fig. 1), described inmany previous papers [1,2,3,4,5], was based on the conceptof High B mode [1,6]. The main features of the source aredescribed in tab. 1.Frequency 18 GHz + 14.5 GHzType of launching WR62, off-axisAxial maxima distance 490 mmBmax (injection side) 2.7 TBmin 0.3 to 0.6 TBmax (extraction side) 1.6 TResonance zone length < 100 mmHexapole length 700 mmBrad (at chamber wall) 1.55 T maximumBiased disk 300 to 600 V, 1 to 2 A mAf  plasma electrode 8 mmf  puller 12 mmExtraction voltage 30 kV  maxTab. 1: The main features of SERSEIn fig. 1 it is shown the plasma chamber (f= 130 mm.)surrounded by the hexapole, which is enclosed into astructure on which the solenoids are placed. Both thehexapole and the solenoids are made by superconductingwires. On the right there are the microwave and gasinjection, and on the left the three-electrode movableextraction system. A biased disk is placed on-axis at theinjection side and can be moved along the same axis.Pumping units at the injection and extraction side provide anoperational vacuum of a few 10-8 mbar, without plasma(typical values with gas and plasma are 1 to 4× 10-7 mbar).The source construction started in fall 1993 and in 1995 allthe components were operational, except the hexapole,which was able to attain only 70% of the nominal field, avalue which did not meet our request to operate the source inHigh B mode (i.e. with a magnetic field exceeding the valueof 2·BECR, where BECR is the resonance magnetic field,corresponding to 0.52 T for the frequency of 14.5 GHz and0.64 T for the frequency of 18 GHz). Therefore we could notaccept he magnets and we ordered new superconductingmagnets, which delayed the project for two years.Fig. 1 - The SERSE source.In spring 1997 the new magnets were ready and operatingabove the specification [2] and in summer 1997 the firstbeams of highly charged ions were obtained, with alreadyexcellent performance. The tests on the bench site ofGrenoble [4] were completed in March 1998 and the sourcewas moved to LNS, where it was operated at 14.5 GHz sinceJune, for a few months.When the source was installed at LNS, two kinds ofproblems limited the reliability. The first problem was apoor HV insulation, both on the chamber and on themicrowave DC break, because the source was originallydesigned for lower voltages. The dc break was changed witha more robust mechanical design and a 1.0 mmpolypropylene sheet (a simple mechanical polishing of theexternal surface of the chamber was also done, because a toorough surface triggered the discharge) replaced the chamberinsulation sheet. Since then 20 kV operations have becomereliable and we demonstrated also the reliability ofoperations up to 30 kV through few days, as required by theK-800 Superconducting Cyclotron axial injection [7].The second problem was about cryogenics: there was a largerhelium consumption of the cryostat, which was settled bystep-by-step optimization of the LHe dewar pressure, inorder to limit the losses in the refilling, and there was a badefficiency of LHe transfer from the main reservoir to theSERSE dewar, which was settled only recently by replacinga part of the cryogenic transfer pipe. Now the LHeconsumption is below 4 l/h and the Dewar must be refilledseven times per month, according to the design.On the other way, the operations with the superconductingmagnets were fully reliable and no more quenches happenedsince the end of training, in July 1998, except for a re-training quench without consequences, due to a partial warm-up of the coils (magnets temperature attained 160 K duringthe summer stop). After this incident, we have not permittedanymore to the magnet temperature to rise above 77 K.As for the currents of highly charged ions measured at 14.5GHz at LNS, they were slightly higher than the onesmeasured at CEA, Grenoble, because of a higher pumpingspeed in the extraction region (charge exchange processaffected less the high charge states buildup). The most oftime was spent for Krypton an Xenon whichwere nevertested there, and that featured current values for the highestcharge states largely exceeding (tab. 2) the best resultsobtained by the other ECR ion sources.2 THE FREQUENCY UPGRADINGIn a recent paper [8] we estimated that the source SERSE,operated with an 18 GHz generator, should increase itsplasma density of about 60%, as compared to 14.5 GHzoperations and then an increase of current of the same orderof magnitude was expected. The condition for an effectiveupgrading is that the magnetic field is also scaled with thefrequency (BECR is proportional to frequency).In fig. 2 the axial magnetic field is shown; the profile wasmore or less the same for the two frequencies, if anormalization factor f2/f1 is used; the radial confining fieldwas lso increased from 1.1 T to 1.45 T.Except for a few cases, a significant increase of currents wasobserved, as described in tab. 2. It needs to be pointed outthat the test with increased frequency were carried out with alower available power (our 18 GHz generator is able tosupply only 1100 W, in spite of nominal 1500 W). Afurther increase of currents will be obtained with higher RFpower, once that the 18 GHz generator will be repaired.14.5 GHz 18 GHz 14+18 A.I.E. optimumO7+ 200 208O8+ 40 55Ar14+ 80 84Ar16+ 17 21Ar17+ 1 2.6Ar18+ 0.05 0.4Kr22+ 46 66Kr25+ 20 35Kr27+ 4.5 7.8Xe30+ 12 17 21.2 27 38.5Xe31+ 7.6 10.3 14.7 18.3 23.5Xe33+ 1.5 3.3 6 7.2 9.1Xe34+ 1 1.6 3 4.1 5.2Xe36+ 0.4 0.9 1.3 2Tab. 2: Typical currents (in eµA) produced by SERSE (allthe currents are measured at 15 kV except for last column,which refers to 25 kV operations with improved vacuum).00,511,522,530 20 40 60 80 100 120 14014.5 GHz18 GHzAxial field (T)z (cm)Fig. 2 – Axial magnetic field of SERSE.3 TWO FREQUENCY HEATINGRecently we have began to operate the source with the “twofrequency heating” and the results have not been clear,although the currents were higher. The two frequencyheating was more effective in the case that the volume of thesecond resonance was narrow and the amount of power waspoor (50 to 90 W). In this case an average increase of 20 ofthe xenon currents was measured for the highest chargestates, because of the higher plasma density and of thepresence of two resonance surfaces. In fig. 3 a typical chargestate distribution for Xe is shown (power was 1040 W at 18GHz and 60 W at 14.5 GHz). Charge states up to 38+wereclearly observed just by decreasing the gas input. Thereproducibility of the values in tab. 2 was excellent (withina few %) and the stability over a few hours was acceptable.01020304050603,5 4 4,5 5 5,5 6Current (eµA)Q/M33+28+26+31+30+27+34+25+Fig. 3 – Charge state distribution for Xe, optimized for 30+.4 ANOMALOUS ISOTOPE EFFECT ANDOTHER FEATURESThese impressive results were not obtained only with thehigher frequency but even other factors played a role, as theimprovement of chamber conditioning and the use of 18O asmixer, according to the experience gained at KVI with the“anomalous isotope effect” [9]. Following the suggestion ofA.G. Drentje, we changed the mixer, by using 18O2 in placeof 16O2, used as standard mixer. The result was a remarkableincrease of currents (>20%) for highly charged Xenon ions.We repeated the same experience for Krypton, but in thiscase the current increase was moderate with respect to theone of Xenon (about 10%). We also used different othermixers, as 15N, 20Ne, 22Ne, and it came out that they areworse than 16O2, whereas 18O2 is the best mixer for thehighest charge states according to [9,10]  (fig. 4).Another relevant improvement consisted of the decrease ofthe base pressure in the extraction region, obtained byinserting a 1200 l/s getter pump after the 600 l/sturbomolecular pump following the solenoid (fig. 5 showsthe analysis section of the beamline to the cyclotron). Thebase pressure decreased from 3 to 1*10-8 mbar, increasing thecurrents of highly charged ions (last column in tab. 2) andimproving the stability of the extracted beam.Recently we replaced the two pumps with a 1000 l/s TMP.Last upgrading of the source consisted of the mounting of anoven [11] that can be used for many types of metallic ions,the other being obtained by means of the sputtering method[12] or by laser ablation [12]. The oven has been mountedand tests will begin in the next weeks.24681030507022 23 24 25 26 27 28 2916 O18 O20 Ne22 Ne15 NCurrent (eµA)Charge stateFig. 4 – Krypton current with different oxygen isotope.Fig. 5 - SERSE with the analysis section of the beamline.5 TESTS OF SCALINGSystematic tests about the magnetic field scaling for theoperations at 14.5 GHz and 18 GHz were carried out withthe same procedure already used for SC-ECRIS at MichiganState University [13]. The trend that was observed forSERSE was quite similar to the one reported in [13]. Thecurrents increase up to value around Bradial= 2· ECR then theyremain stable or decrease. In fig. 6 the currents are plottedversus the radial magnetic field for 14.5 and 18 GHz,featuring a similar trend but a higher current for the higherfrequency. For axial field scaling (fig. 7) a value above4·BECR on the injection side and 2·BECR on the extraction sideimproves the plasma stability, as already observed for SC-ECRIS. In fig. 8 the comparison between the results ofSERSE and the ones of SC-ECRIS is presented for O7+. Itcan be seen that, except for the absolute values of thecurrent, the shape of the curves is quite similar for the twosources. The maximum current is not scaled with the squareof frequency, as it is foreseen by the Geller’s scaling laws,because space charge effects at the ex raction play a role(total current is above 4 mA for SERSE, well above theSC-ECRIS values). It can be concluded that the frequencyscaling is effective, provided that the magnetic field is highenough, as we underlined some years ago [6].1101001,2 1,6 2 2,4 2,8Current (eµA)B/BE C R14.5 GHz18 GHzFig. 6 – Xe27+ currents vs. Bradial/BECR.6 PERSPECTIVESIn conclusion, the 18 GHz upgrading, along with the twofrequency heating and anomalous isotope effect, havepermitted to increase enormously the SERSE currents forhighly charged ions. The results that we have obtained withSERSE have confirmed the validity of our choice to build anECR ion source with superconducting magnets, even if thetime for the construction has been long and the investmentshave been relatively high. In fact, the operations of theSuperconducting Cyclotron will benefit of the performanceof SERSE, which allows to boost the currents extractedfrom the cyclotron and also to rise the energy, especially forthe heaviest ions. The former result is particularlyimportant, because the EXCYT project [14] needs highcurrents of fully stripped light ions (up to 7 pµA) thatcannot be provided with the radial injection by means of theTandem followed by a stripper.More developments are scheduled for the next future,especially the study of different methods of metallic ionproduction and the study of two frequency heating. We havealso bought recently an 8-18 GHz generator, which will beuseful to study systematically the effects of frequency on theion production. This study will be crucial for the futureupgrading to 28 GHz, to be carried out in fall 1999, in theframework of collaboration with CEA and ISN of Grenoble,GSI and CERN [15].0,11101001 2 3 4 5 6B1 (18 GHz)B2 (18 GHz)B1 (14.5 GHz)B2 (14.5 GHz)Current (eµA)B/BECRFig. 7 – Xe27+ currents vs. Baxial/BECR on injection (1) andextraction side (2).0,11101000,5 1 1,5 2 2,5 3Oxygen current (eµA)B/BECRSERSE (18 GHz)SC-ECRIS (6.4 GHz)Fig. 8 – O7+ current  vs. Bradial/BECR.7 ACKNOWLEDGEMENTSThe authors acknowledge the expertise of E. Messina in thesolution of mechanical problems and thanks M. Cafici, A.Pagano and F. Speziale for the solution of cryogenicproblems. We thanks S. Pulvirenti and S. Cavallaro fortheir support for the contribution to computer controls.8 REFERENCES[1] G. Ciavola, S. Gammino, Rev. Sci. Instr. 63(4), (1992) 2881[2] P. Ludwig et al., Rev. Sci. Instr. 69(2), (1998) 653 and refs. therein[3] P. Ludwig et al.,, Proc. 13th Workshop on ECRIS, College Station(1997), 119[4] P. Ludwig et al., Rev. Sci. Instr. 69(12), (1998) 3312[5] S. Gammino et al., “18 GHz upgrading of the superconducting ECRion source SERSE”, submitted to Rev. Sci. Instr.[6] S. Gammino, G. Ciavola, Plasma Sources Science & Technology 5,(1996), 19[7] D. Rifuggiato et al, “First years of operation with the SuperconductingCyclotron”, Proc. of the 15th Conf. on Cyclotrons and Appl., Caen, June1998, in press[8] S. Gammino, G. Ciavola, Rev. Sci. Instr. 69(8), (1998), 3081[9] A. G. Drentje, Proc. 13th Workshop on ECRIS, Groningen (1993), 155[10] G. Melin, A.G. Drentje, A. Girard, D. Ditz, to be published on Phys.of Plasmas[11] G. Di Bartolo et al., Rev. Sci. Instr. 69(2), 1998, 725[12] R. Harkewicz et al., Proc. 12th Workshop on ECRIS, RIKEN (1995),126[13] S. Gammino et al., Rev. Sci. Instr. 67 (12), (1996) 4109[14] G. Ciavola et al., Proc. of 1999 Part. Acc. Conf., New York, in press[15] N. Angert et al., these proceedings

Production of intense highly charged ion beams with SERSE

Production of intense highly charged ion beams with SERSE

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