Barzangy, F.

Drentje, A.G.

Kremers, H.R.

Sijbring, J.

English

Dissertations of the University of Groningen

   University of GroningenCan a Hexapole Magnet of an ECR Ion be too Strong?Drentje, A.G.; Kremers, H.R.; Sijbring, J.; Barzangy, F.Published in:Proceedings of the 14th International Workshop on ECR sources ECRIS99IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionFinal author's version (accepted by publisher, after peer review)Publication date:1999Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Drentje, A. G., Kremers, H. R., Sijbring, J., & Barzangy, F. (1999). Can a Hexapole Magnet of an ECR Ionbe too Strong? In B. W. Allardyce, H. Haseroth, C. Hill, & K. Langbein (editors), Proceedings of the 14thInternational Workshop on ECR sources ECRIS99 (Vol. PS-99-052-HP, blz. 94 - 97). [EXT-2000-093]CERN. https://cds.cern.ch/record/427298?ln=enCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 11-10-2022   University of GroningenCan a Hexapole magnet of an ECR Ion Source be too strong?Drentje, A. G.; Barzangy, F.; Kremers, H. R.; Meyer, D.; Mulder, J.; Sijbring, J.Published in:Default journalIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1999Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Drentje, A. G., Barzangy, F., Kremers, H. R., Meyer, D., Mulder, J., & Sijbring, J. (1999). Can a Hexapolemagnet of an ECR Ion Source be too strong? Default journal.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 30-03-20201Can a Hexapole magnet of an ECR Ion Source be too strong?A. G. Drentje, F. Barzangy, H. R. Kremers, D. Meyer, J. Mulder, J. SijbringK.V.I.  University GroningenZernikelaan 25,  9747AA Groningen, The NetherlandsIntroduction.Experience of many ECRIS designers and users during more than a decade has given afew experimental rules or “scaling laws”. Many of these have been discussed at theECRIS workshops. After the 1993 workshop it was concluded [1] that the properties ofthe magnetic trap, in particular the strength of the radial component, determine to a greatdeal the confinement characteristics.For that reason it was decided at the KVI to choose a strong magnet for the new 14 GHzECRIS4 to be used in the Atomic Physics experiments. The hexapole magnet designed bythe Giessen group [2] is a good example. There the higher field strengths were obtainedin a so-called Halbach configuration with 24 wedge shaped pieces of two special kinds ofpermanent magnetic material (Vacodym), where the inside diameter was reduced to 65mm as compared to the usual 70 – 75 mm). The field, measured 2.5 mm inside the poletips (i.e. at the wall of the plasma chamber) is more than 1.2 T.Whether or not the choice of this particular magnet was a good choice will be discussedin this contribution.Operation of ECRIS3 with moderate hexapole magnet.For ECRIS3 - in operation since 1994 - the ion beam currents for a few oxygen and argoncharge states are given in the table. These values were reported earlier [3]; they wereobtained after an extensive series of optimization experiments for oxygen, and a few forargon. For the oxygen beams, helium has been used as a mixing gas; for the argon beams,oxygen was used. The best results for the Ar14, 16+ beams were obtained using 18O as amixing gas [4], [5]. The hexapole magnet properties are shown in the figure, marked“ECRIS 3”. The quality of the magnetic trap can be given by the  “radial mirror ratio”,which is usually defined as R =  Bmax/ Breson, with Breson equals 0.5 T for a 14 GHzECRIS. In the present source is R = 1.86. The measured radial field strength of thishexapole magnet obeys closely the expression Br = 0.00080 r2 T (with r in mm).2Fig. 1 Properties of the hexapole magnets of the two KVI ECR ion sources.Operation of ECRIS4 with strong hexapole magnet.The strong hexapole magnet was installed in ECRIS4, which is in operation since 1995.ECRIS4 is to a large extent a copy of ECRIS3 at KVI; the main difference is thehexapole magnet.The measured radial field component Br obeys closely the expression Br= 0.00136 r2(with B in T, r in mm); see the curve marked “ECRIS 4” in the figure. For the stronghexapole magnet in ECRIS4 the radial mirror ratio R= 2.4.For ECRIS4, the optimization procedure was essentially similar as was applied for othersources, but it took much longer because the source was used intensively before beingcommissioned. In order to improve the beam transmission to the experimental stations, itwas tried to reduce the beam emittance by reducing the size of the hole in the plasmaelectrode from ϕ =5.7 to 3.7 mm. The argument was that aberrations could easily bepicked up during beam formation in the fringing field of the narrow and strong hexapolemagnet (See the discussion given in a separate contribution [6] to this workshop). ForECRIS4, four sets of currents are given, marked 1995, 1997 (with two values of ϕ) and1998. Without giving details it is remarked here that the corresponding configurations ofthe coils, with respect to each other and with respect to the hexapole magnet weresubstantially different.3ECRIS 3 . ECRIS4 ECRIS4 ECRIS4 ECRIS4 ECRIS4 ECRIS4=Edownstairs =Edownstairshexapole magnet Moderate Strong Strong Strong Strong Strong Strongaxial configuration "3" "5" "5" "8"coil distance 65 58 58 52 52 52intermediate coil No No No No No Yes Yesextr hole diam. (mm) 7 5.7 5.7 3.7 3.7 3.7 6.11995-98 1995 1997 1998 1998 Jan-99 Feb-99O6+ µΑ 220 110 90 93O7+ µΑ 55 13 13 13 12 16Ar8+ µΑ 70 55 55 60 64Ar9+ µΑ 74 25 25 41Ar11+ µΑ 52 8 12 26Ar13+ µΑ 15 1.7 2.5 8Ar14+ µΑ 4.7 0.6 0.7 3.9 4.1Ar16+ µΑ 0.27 0.003 0.014 0.22 0.21      Table 1. Best results obtained (in µA) with ECRIS3, and at various stages with ECRIS4.In reality even more changes than the ones mentioned above were made. One conclusioncould be that some basic parameter was limiting the output to a large extent, as the O7+beam current was at best 12 – 13 µA in all these cases. A second conclusion is that thesame O7+ beam current is obtained with different sizes of the extraction hole ϕ. Thiscould be consistent with the general observation that the emittance becomes smaller athigh charge states.A considerable step forward was made when a “intermediate” coil with reversed currentwas installed between the main axial coils. In that situation, in particular the argon chargestate distribution improved substantially. The Ar14+ and Ar16+ currents reached a value ofabout 80% of the ECRIS3 records, see column marked “Jan- 99” in the table. After that,the exit hole was increased from ϕ=3.7 to 6.1 mm. As can be seen in column “Feb-99” ofthe table, the effect on the high charge state was zero, but the effect on the Ar8+ currentwas surprisingly small too. Surprising, because similar experiments on other sources hadgiven for these beams an intensity increase with increasing hole size.Recently, further improvements have been made; this is reported in a separatecontribution to this workshop [6].The order of magnitude of the beam emittance was estimated from the transmission to agiven set of diaphragms. This gives (at 12 kV) an emittance of the order of 40πmm.mrad. This is an advantage in cases that the beam acceptance of the user’s set – up isalso small, which is the case in the particular application of ECRIS4.4DiscussionThe observations described above may give rise to the following questions:• Is the stronger hexapole magnet in this particular application limiting the performanceof the source?• Is the apparently smaller emittance related to the stronger hexapole magnet?From the properties of hexapole magnets it obviously follows that the radial size of theECR volume is smaller for a stronger magnet. For the given situation, the difference inradial size is 20%. Therefore, the radial size of the high-density core of the plasma issubstantially smaller as well. By making this “plasma size” smaller the charge statedistribution inside will certainly shift to higher charge states, but at the same time theextracted fluxes (i.e. the losses) will decrease.This might be true in the present situation, which then leads to the observed “limitation”.The effect on the beam formation could be as follows. The high-density core of theplasma maps onto a small part of the extraction area where the axial field lines are highlyconcentrated. That part will become even smaller for the reduced “plasma size” due tothe stronger hexapole magnet. If that part of the extraction area coincides with the actualextraction hole, a further increase of the size of the extraction hole will not lead to highercurrents. It might be that due to this effect even the beam emittance becomes smaller. Theobservations mentioned above are consistent with this model, but are certainly notaccurate enough to prove it.Conclusion.• When the output of ECRIS4 is compared to that of ECRIS 3, it is clear that the sourcewith the stronger magnet (i.e. ECRIS4) produces lower currents of highly chargedions.• By changing various parameters, including major alterations of the (axial) magnetconfiguration it seems that the stronger hexapole magnet is limiting (in terms of O7+and Ar14+ currents) the output in this particular source.• The limits could be upgraded by further changing the axial field profile andincreasing the axial mirror ratios to values substantially higher than those of ECRIS3.• The beam emittance of the source with strong hexapole magnet is smaller.• A higher RF frequency, e.g. 18 GHz might result in a more effective use of thisstrong hexapole magnet.1. A.G. Drentje Rev. Sci. Instrum. 65(1994) 1045   (Review ECRIS workshop 1993)2. M. Schlapp, R.Trassl, M.Liehr, E. Salzborn, Report ECRIS workshop Groningen (1993), p 2263. A.G. Drentje, H.R. Kremers, J. Sijbring, Report 12th ECRIS workshop Tokyo (1995), p 148.4. A. G. Drentje, H. R. Kremers, J. Mulder, J. Sijbring, Rev. Sci. Instrum. 69(1998)7285. A. G. Drentje, A. Girard, D. Hitz, G. Melin, contributed paper to this workshop.6. A.G. Drentje, F. Barzangy, H.R. Kremers, D. Meyer, J. Mulder, J. Sijbring, “Status report ofthe ECR ion sources at the KVI”, contributed paper to this workshop.                                                                                                                     agd:CoE99b.doc; 2804995

Can a Hexapole Magnet of an ECR Ion be too Strong?

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Can a Hexapole Magnet of an ECR Ion be too Strong?

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