Bhandari, R K

Bose, D K

Taki, G S

English

IACS Institutional Repository

Indian J. Phys. 66B  (5 &  6). 629  -  637 (1992)Status of VEC ECR heavy ion sourceD K Bose, G S Taki and R K BhandariVariable Encigy CyclolKwi Ccnlrc, Department of Atomic Energy, 1/AF Bidhan Nagar, Calcuiia-700064, IndiaAbstract : An Electron Cyclotron Resonance (ECR) heavy ion source, basically a hot electron plasma machine having B^ in im um  field configuration is going to be operational soon at this centre. The source which is iiufigenously built will act as a heavy ion injector lor the Variable Energy Cyclotron (V EC ) replacing the existing penning (PIG) internal light ion source. This programme was undertaken to facilitate acceleration of muluply charged heavy ions (M CH I) in the cyclotron. The heavy ions, substantially energetic compared to the light ions will be utilised to initiate new fields of research with heavy ions The present ECR  source is expected to pr<xluce fully stnppcd light heavy ions and parually stripped heavy ions. It will be suiublc for metallic ions also. Radio frequency (if) ppwer is transferred to the ionising electrons in this source at a frequency in resonance with the electron cyclotron frequency (ECR). V EC  EC R  source is a two suge, room temperature heavy ion source, “Stage-1 acts as an injector for the stage-2**. the main stripping suge which has a B -minimum magnetic field configuration. Powerful samarium-cobalt sextupolar permanent magnet generates the radial field of the second stage. The resonance frequency of operation is same for both sUges and is 6.4 GHz. Two commercially available 6.4 GHz, 3 K W  microwave generators have been procured to supply the rf power. I'he source has been partly assembled in the EC IH S laboratory of VECC. A  vacuum of 1 x 10“"^  lorr in the source and 5 x  10~* lorr in the analysing line have been obtained. A  charge analysing system comprising two 45® normal entry and normal exit dipoles and four quadrupoles (QDQ Q D Q  system) has been buUt. ll ic  fabrication work of enUre injection line comprising optical elements, beam iransporl lines, radio frequency bunchcr, beam inflccior, etc. coming in the phasc-2 programme shall suit immediately after the source commissionmg.K e y w o rd sline.ECR, accclcrauon, multiply charged heavy ions, plasma, B-mmimiim, injectionPACS No. : 2S.25NI1. IntroductionThe Variable Energy Cyclotron (VEC) at Calcutta is being utilised efficiently since 1981 for research and applications. So far, light ions mostly alpha particles, have been accelerated using internal Penning (PIG) light ion source. The achievable energy from VEC is given byE = 130Q^/AMcV.where Q is the charge number and A is the mass number.Equauon (1) gives 130 MeV as the maximum energy for alphas which have Q = 2 and A = 4.© 1992 lACSH 5&6(23)6 3 0 D  K  Bose, G  S Taki and R K  BhandariFor fully stripped comparatively heavier ions, for example O** and A E(0**) = 520 Me V E(A'**)= 1053 MeV.It is clear that by accelerating highly stripped heavy ions in the cyclotron, there will be significant gain in energy per nucleon. These energetic ions in turn when used as projectiles can dump large amount of energy on target giving rise to many new phenomena. In fact heavy ion capability of cyclotron will open up new fields of research in many branches of nuclear physics. MCHI beams from ECR source can also be utilised for offline research in atomic physics.1.1. Consideration in favour of ECR source :The present internal PIG source is incapable of producing MCHI beams. Modified PIG heavy ion source [1 ] can produce low charge state heavy ions and has the major drawback of having only 2/3 hours cathode life time. In present days, electron beam ion source EBIS \2] and electron cyclotron resonance ion source ECRIS [3-6] are well known advanced types of heavy ion sources. EBIS has the disadvantage that it involves ultra high vacuum technology and it is basically a pulsed device. On the other hand. ECRIS requires only high vacuum technology and produces continuous beam of heavy ions. The latter is an electrodeless discharge which takes place by very efficient ECR mechanism with microwave power. Its other merits are capability of working stably over days together and having low phase space area (emittance) ion beam. Because of these advantages ECR heavy ion source is the best choice for cyclotron application.1.2. Physics of ECR heavy ion source :Multiply charged ions with reasonably good amount of yield are efficiently produced in ECR source by successive ionization of species by hot electrons. The equilibrium charge states distribution (CSD) is determined by the balance between ion production rate and loss rate. Two important quantities in regard to production of high charge states arc the product and electron temperature T,, in the plasma. Here n , is the ionising electron density Md \  is the ion confinement time in tfie plasma T, is to be 3 to 20 KeV or more and n, to be 10*-10*® for totally stripped light heavy ions. For fully shipped heavy ions of the high mass numbers, T, and n,Xi should be higher. Since critical electron density in a plasma is proportional to the square of the rf frequency and electron temperature is proportional to the power of rf, it is beneficial to go to higher frequencies and higher power to achieve high yield of high charge states.7.3. Mechanism of ECR source :ECR heavy ion source essentially produces a hot electron plasma in a B-minimum configuration inside a plasma chamber. Axial mirror B, field is produced by solenoids and radial stabilising B^  field by a set of sextupole. In this field configuration, there is a ellipsoidal surface (Figure 1) where magnetic field B satisfies the resonance condition, i.e.Status o f V E C  E C R  heavy ion source 631corresponding to this B, the electron cyclotron frequency equals the applied rf frequency. Plasma chamber dimension is so chosen (multimode cavity) that there always exists component of E field perpendicular to the B field. As a result, on the ellipsoidal resonance surface, efficient power transfer lakes place from E-M wave to the electrons. At a given pressure (« KH torr) plasma electrons oscillating back and forth between the mirror pointsare heated up stochastically to quite a few KeV by crossing the resonance surface large number of times. These hot electrons (3-20 KeV) are responsible for the production of high charge state ions by successive ionisation.F ig u re  1. Layout of the V L C  ECR source. RF I, RE2, microwave power leeds, H V l, HV2 high voltage isolatoi. W I . W'2 nucrowavc windows, S I stage I . S2 stage 2; C I -C4 solenoid coils, M  sextupole magnet. AP additional port; PI, P2 : pumping portsAchievement of high charge state is favoured by operation at low pressure which restricts charge-exchange loss with neutrals and the yield is substantially increased by addition of a first stage. That is why most ECR sources are a two stage device [6]. Mam purpose of first stage is to supply electrons together with low charge state ions into the mamstage.2. Design of VEC ECR sourceIn this regard the foremost work was to fix up the requirements. These lead us to airiye at a suitable design of the source 17,8] to be developed within the fixed time period, t so included selecUon of microwave frcquency. The ECR programme is a quite complex one and it involves many techniques and fabrication works like(1) Fabricauon of high power solenoids for generating the axial magneue fields.(2) Technique of launching high power microwave into the plasma.(3) Very high vacuum technology (1Q-’ torr in the source and a few umes 10  ^ torr in632 D  K  Bose, G  S  Taki and R K  Bhandari(4)(5)Fabrication of plasma chambers — stage 1 and main stripping cavity (stage 2). Handling of powerful and fragile rare earth permanent magnets (Bre = 9000 gauss) and fabrication of scxtupolcs with these magnets.Precautions against microwave and X~ray radiations.Isolation of pumps, wave guides, magnet coils, etc., from the source at high voltage.Fabrications of charge analysing system.Fabrication of an advanced external injection system [9] comprising optical elements, rf bunching system, diagnostics, beam intlector etc.Schematic diagram of VFC ECR heavy ion source is shown in Figure 1. Some of the technical details are given in Table 1.Table 1. Technical details of V EC  EC R  heavy ion source(6)(7)(8) (9)1 Length of the source 88 0 cmII. Outer diameter of the yoke cyclinder .S3 6 cmili Number ol .solcniod coils 4IV Frequency of operation-  Stage 1 6 4 GHz-  Stage 2 6 4 GHzV Pla.sma chamber diameici-  Stage 1 94.0 mm- Stage 2 108.0 mmvi. Scxtupole magnets“ Length 305.0 mm-  Field diameter 118.0 mmField on the pole Up 4 0 KGaussVII. Estimated soieniod power- For 6 4 GHz operation 25 0 K W-  For ro 0 G Hz operation .52 0 K W2. /. Plasma chamber and vacuum requirements :VEC ECR source is a room temperature compact two stage device. The entire source is enclosed in a 25 mm thick iron cylinder acting as the yoke for the return flux. Four sets of coils C l, C2, C3 and C4 held within the yoke cylinder generate the resonance and mirror fields in both the stages. Additional sextupol magnet is provided for producing radial field in the second stage.Stage I:Plasma chamber SI is made of 94 mm internal diameter (1 D) copper tube and has a co-axial 20 mm ID  quartz tube inside it. On the left side there is a slot cut on the copper end-flange forStatus o f  V E C  E C R  heavy ion source 633CiMinccting WR-137 wave guide and a vacuum window. It is also having connection for gas inletStage 2 :The second stage plasma chamber S2 is made out of a solid oxygen free high conductivity copper billet This chamber has ID 108 mm, and length 345 mm and forms a multi mode cavity for rf. This chamber is water cooled and carries 305 mm long rare earth sextupole magnet bars externally. Three radial ports arc provided for vacuum gauge, diagnostics and oven connections.At 6.4 GHz microwave frequency ECR discharge t^cs place at a pressure of about 1 X 10"^  torr in the quartz tube of stage-1 plasma chamber. This stage injects electrons and low charge state ions into the main stripping stage by diffusion. In this stage, more powerful ECR discharge occurs at a pressure of about 1 x 10^ torr or lower. The plasma which is stabilised in B-min configuration, has hig^ i energy electrons which by stepwise ionisation, produce highly snipped heavy ions. Heavy ions are extracted through a 8 mm hole in the copper emission electrode with the help of a shaped stainless steel puller electrode. The puller is placed 25 mm away from the emission electrode and has a 10 mm aperture at the cenue. The source first stage and the extraction chamber are pumped by two 600 litre per sec diffstak pumps. Second stage is mainly pumped by the plasma itself. Additional pumping is provided through the extraction side. A pressure of 1 x 10~^  torr in stage 1 and extraction chamber is routinely obtained in absence of gas leak. The beam analysing part of the transport line is pumped by a 1500 litre per sec cryopump producing a pressure of 7 x 10~* torr. A pressure of about 1 X IQ-^  in the beam transport line is needed for transport of MCHI beam with negligible loss by charge exchange.2.2. Microwave:For the present application, 6.4 GHz has been chosen as the ECR frequency. It has been planned to use a single 3 KW microwave generator to feed both the stages by making internal or external power division. Microwave power will be injected in both the stages parallel to the axis. Teflon microwave windows have been used to isolate wave guide at atmospheric pressure from the part in high vacuum. Two 3 KW 6.4 GHz commercially available microwave generators have been procured and tested with dummy load.2.5. Magnets:As is common with conventional ECR source two types of magnets arc used. One type i? solenoid magnets for axial mirror fields and SmCo5 permanent magnets for sextupolar fieldsin the main stage.Four solenoid coils C1-C4 comprise 27 pancakes. Each pancake is made of 6.3 mm square copper conductor having 3 mm central hole for water cooling. The conductor is covered with thin insulating t^)e while winding. Wound pancakes are epoxy casted to make their outer diameter concentric with the inner diameter which help alignment inside the iron634 D K Bose, G S Taki and R K Bhandariyoke of 486 mm internal diameter. There are two types of pancakes as shown in Table 2. Table 2 . Types o f pancakes.Pancake I D  O D  Num bers o f ThicknessType (m m ) (m m ) tums/layers (m m )AB190240485485211714.814.8Figure 2, gives an example of solenoid fields obtained using Poisson code. The lower curve corresponds to 6.4 GHz operation and upper one for 10.0 GHz operation of bothF ig u re  2 . A x ia l m agnetic fie ld  due to the solenoidal coils. U pper curve corresponds to 10 G H z  and low er curve lo  6 .4  G H z  operation.F ig u re  3 . A x ia l magnetic fie ld  o f  the solenoidal coil w ith  fa lling  fie ld  configuration in stage 1 fo r  6 .4  G H z  operation.stages. It is also possible to generate working field configurations without exciting the first coil, that is no minor field in stage 1 as shown in Figure 3.Status o f  V E C  E C R  heavy ion source 635The second stage has a B-minimum field configuration. This is needed for plasma stabilisation against MHD instability. Plasma stabilisation is important for achieving high ion confinement time ("q). The radial stabilising field is produced by a sextupole of length 305 mm and field radius 59 mm and pole tip field 4 KGauss. Each pole is made up of five individual blocks of samarium cobalt (StpCos) rare earth magnets of size 61 mm by 38.1 mm by 50 mm and magnetised to a residual magnetism of 9 KGauss. Calculated field plots for the sextupole is shown in Figure 4.F ig u r e  4 . R ad ia l fie ld  due u> sextupole m agnet. C urve  1 ; along the radius going into  the pole  centre; C urve  2  : along ihe radius going between tw o  adjacent poles.2.4. Charge state analyser and external injection system [9] .•Heavy ion beam extracted by a potential difference of 10 KV is composed of ionic species of various charge states. This beam with the help of two glasers (magnetic lenses) LM-1 and LM-2 [10] is injected into the 90® charge analysing system (Figure 5). It consists of two unit cells each cell comprising two quadrupole (QT) magnets and a 45° dipole magnet (MT) [11].The magnetic rigidity Bp (B = field in pole gap, p = bending radius) is about 20 KGauss cm for 10 KV dculeron beam. And the resolution R = Q!M — 1/20 for full (20 mm) object slit opening. This can be improved further by reducing the slit width.The beam after charge state selection is to be injected into the centre of the cyclotron for acceleration which is the end use of heavy ion beam produced by the ECR source. The scheme of the injection system is shown in Figure 5. The beam after the analyser is transported to the centre of the cyclotron telescopically with the help of four magnetic lenses LI-1 to LI-4 and finally by a glaser LI-5 on to the 45® electrostatic mirror inflector. Inflector inflects the beam into the cyclotron for subsequent acceleration by the rf fields. The inflector is a copper electrode placed at an angle of 45® to the vertical and is applied with an ion- repelling voltage of about 10 KV. The inflector electrode is enclosed in a grounded copper tube having a slit for beam exit. There is a wire grid placed in front of the infiector for636 D  K  Bose, G  S  la id  and R K  Bhandaricreating desired electrical field in between. Apart from the lenses mentioned above there is a beam rotator (BRI)—a solenoid magnet which can impart a rotation to the beam. This rotation may be needed for maximum transfer of beam from the inflector to the cyclotron. There is also a rf buncher system in the vertical line. The continuous low intensity MCHI beam passing throuth the buncher is bunched in time for increasing the intensity. This is done by accelerating the slower ions and slowing down the faster ones in the rf cycle of the buncher. Since cyclotron accepts ion beam only over a small time interval of the rf cycle, bunching helps to increasing the intensity of the heavy ion beam number of times compared to the unbunched beam.F ig u r e  5 . Schem e o f external in jection  system . L M l ,  L M 2  : g laser lenses; 0 ' I '1 - Q T 4  : quadnipole magnets; U 1 -L 1 4  ; solenoid lenses; B R I : beam rotator.3. Present statusAs on November 1990, the source component have been assembled in the laboratory with part of the charge analysing vacuum line. All the magnet coils have been installed inside the iron yoke cylinder, together with vacuum chambers, plasma chambers. High vacuum pressure 1 x 10"'^  tonr in the ion source and 5 x 1 0 “* torr in the part of the transport line have been obtained. Main stage OFHC copper chamber will be taken out to mount the samarium cobalt magnets for the sextupole. All the power supplies for the solenoid magnets have been commissioned. Arrangements are being made to shift the source to high bay area on the top of the cyclotron which is the location decided for the source assembly. The source is expected to go for trial riin within a few weeks.AcknowledgmentIt is acknowledge with gratefulness the helpful discussions we had with T Anuiya of Michigan State University, who also arranged a 250 mm diameter OFHC copper billet as aStatus o f V E C  E C R  heavy ion source 637gift. We are also thankful to C M Lyncis and D Clark of Lawrence Berkeley Laboratory, University of California, for many helpful discussions. Wc acknowledge with thanks the efforts of J Reich which enabled us to receive as gift a set of samarium cobalt permanent magnet bars and OHFC copper rods from Internationals Bureau, Kern Forschungs AnUutge julich. We are grateful to our Director Dr Bikash Sinha for his constant encouragement, support and advice. We also thank our colleagues N K Mukhopadhyay, C Mallik, P R vSarma, M H Rashid for their interest and support.R e fe re n c e sft] D  J Clark 1981 Proc 9th hiti Conf on C\\ lotnms and itheir AppluatKms Caen Sept 7 231[2] M P Slockli 1987 hitl Ccnif on ECR Ion Sonne and Their Applu alums N S C L, East ReportM S l lC P -4 7  219131 R Gellcr, F  Bourg. P Bnand, J Dcbcmardi, M  Delaunay. B Jacqiiot. P Ludwig, R Paiilhenei, M  Pontonnicr and PSoitais 1987 ibid 1|4] C  M  Lyncis 1987 Proc Nth Inti Conf on Cytlotion and Their Applumions, Tokyo I9S6 707[5] Y  Jongen, C  Pirart and G Ryekcwacil 1982 Pnn 4ih lull Worshop on ECR Ion Son n e and Related Topics. Grenoble 3 I(6J T  A Antaya and Z  Q  Xic 1986 Pnn 7th bill. Wotkshop on ECR Ion Scnirres, Jiilich 72[7] D  K  Bose and T Antaya 1987 Inti Conf on ECR Ion Sources and Their Applications. N.SCE. East Uuisiny.Report M S U ( 'P -4 7  ^71(81 D  K Bose, G .S Taki, J Chaudlnm and R K Bhandan 1989 !2th Inti Conf on ( \c  lotions and I heir Applications, lieilin IVO[9] R K  Bhandan 1988 /boc Svmp on Nm lent Phvsic \ {DAE PARC, Ifonibav) M{li) 92 [ lof MM Rasind, P R .Surma and R K Bhanduri 1991 Indian I Phws 65A 402 1111 P R  Sarma 1988 (unpublished)B 5&6(24)

Status of VEC ECR Heavy Ion Source

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Status of VEC ECR Heavy Ion Source

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