Arai, E

Funaki, H

Ishiguro, Y

Katayama, M

Oguri, Y

Shimizu, K

Shimizu, M

CERN Document Server

SECTION IIIon Sources and InjectorsParticle Accelerators, 1992, Vols. 37-38, pp. 39-46Reprints available directly from the publisherPhotocopying permitted by license only© 1992 Gordon and Breach Science Publishers, S.A.Printed in the United KingdomCLUSTER BEAM ACCELERATION USING ATANDEM VAN DE GRAAFFE. ARAI, H. FUNAKI, Y. ISHIGURO, M. KATAYAMA, Y. OGURI,K. SHIMIZU and M. SHIMIZUTokyo Institute of Technology, Ohokayama 2-12, J-152 Tokyo-Meguroku, Japan.(Received 3 December 1990)A beam of accelerated clusters is one of the new tools for fundamental study of fusion reactions. We haveinvestigated the possibility of cluster beam generation by using a tandem Van de Graaff. The key issueis the efficiency of transmission through the charge exchanger located in the high-voltage terminal. Clusterbeams such as C3 and Si2 have been accelerated using a 1.7-MV NEC 5SDH-2 tandem Van de Graaffwhich is provided with a SNICS II sputter type negative-ion source. We have determined parameters thatmaximize the transmission of injected cluster beams. The charge exchanger is a canal filled with nitrogengas. The nitrogen-gas feeding rate, the accelerating voltage, and the pumping speed of nitrogen from thecharge exchanger are the factors which have been investigated in this work. We have observed transmissioncoefficients of 12 and 1.5% for C3 and Si 2 , respectively.INTRODUCTIONThe last fifteen years have seen a veritable explosion of cluster research broughtabout by new experimental advances in physics and chemistry 1,2. Especially, newphysics using atomic clusters with kinetic energy in the keY and even MeV regionopens new perspectives for attractive application, because clusters are quasineutral,yet they can be accelerated, steered and focused by electromagnetic devices. Forexample, Schweikert et al. 3 suggested that the secondary ion emission can beenhanced by a large factor by means of cluster bombardment. A major concernis to maintain the integrity of the MeV-energy cluster during acceleration and beamtransport. In the present work we have investigated how to generate carbon andsilicon cluster beams with energies in the MeV range by using a tandem Van deGraaff. A sputter type source produces negative clusters such as C;;, F ~- and Si;;where n = 1, 2, 3, ... As is well known, a tandem has a big advantage: we can accessthe source very easily because it is located on an open platform with an electricpotential as low as some 10 kV.The problem with a tandem is the transmission of clusters through the strippersection in the tandem terminal and through the accelerating tubes with vacuum aslow as 10- 7 to 10- 6 torr. In the following sections we describe the experimental setupand results of measurements performed to determine optimum parameters anddiscuss the result.39402 EXPERIMENTALE. ARAI et al.Figure 1 illustrates the experimental arrangement. Cluster ion beams of carbon andsilicon are generated by the SNICS II negative ion source. The cathode and extractionvoltage are typically 7 kV and 15 kV, respectively. The 22-keV negative beams wereanalyzed by the switching magnet SW1 to define the mass of clusters. The 5SDH-2type tandem is provided with a 500 lis turbo pump at each end of the acceleratingtubes.The inset illustrates schematically the structure of the stripping system, whichconsists of a stripper canal with 6.35 mm ID and 635 mm length. Nitrogen gas is fedthrough a metering valve to the canal from the center. A baffle with a 6.35-mmaperture is mounted at the low energy end of the stripper to prevent the strippinggas from entering the low-energy accelerating tube. The stripper housing is providedwith a titanium gettering system to pump the nitrogen gas. The sublimator containsheater coils which are powered by the generator. The heater current is adjustable byvarying the rotating speed on the driving shaft of the generator. According to thedata provided by the manufacturer4 , the gas pressure in the stripper canal changeslinearly as a function of the housing vacuum. For accelerating the lighter heavy ionssuch as oxygen it amounts typically to 5 x 10- 3 torr at a housing vacuum of1 x 10- 5 torr and 1 x 10- 6 torr above the high energy turbo pump.Backscattering experiments have been done to verify the purity of the cluster beams.A 60-nm thick gold target is located in the center of the scattering chamber. A 50-cm2surface barrier detector (SBD) counts scattered particles at 1650 with respect to thebeam direction. Particles detected by the SBD must have an energy E calculated asfollows:E == eU(llm + lin), (1)where U denotes the terminal voltage, and m and n represent the number of atomsin a cluster as injected and as analyzed by SW2, respectively.During the experiment the vacuum is kept better than 5 x 10- 8 torr in the sectionbetween the ion source and the low-energy accelerating tube. A higher residual gaspressure of 3 x 10- 7 torr was observed in the section after the high-energy accelerat-ing tube due to the stripper gas bleeding.A graphite cathode was used to generate carbon clusters, and a polycrystallinesilicon for Si clusters. A series of clusters C;;; or Si;;; (m == 1, 2, 3, ...) was observed inthe mass spectrum obtained by SWl. The total beam current was measured to betypically 10 flA with the Faraday cup FCS located at the entrance focus plane ofSW1. We have chosen C3 and Si2 cluster beams for the present work. The clustermass was restricted by the small intensity of carbon clusters heavier than C 3 and bythe magnetic field of QD1 available for focusing silicon clusters. The electric currentof C3 and Si2 amounted to 160 nA and 3 flA at the entrance of the tandem,respectively.The electric current of accelerated clusters of C n+ (n == 1, 2, 3) and Sin+ (n == 1, 2)was nleasured at terminal voltages of 1.2 MV and 1.6 MV as a function of the pressureincrease above the high energy turbo pump. The sublimator heater current was 30 A.(j t"""f C CJ') ~ tT.l ~ l:t' tT.l :> ~ :> (j (j tT.l t"""f tT.l ~ :> ~ o ztoLINACRBSERDAcourseScatteringchamberNuclearreactionanalysiscourseNitrogengasRotatingshaft500I/sturbopump1Q011.7MVTandemStrippercanalf···\\···...··r·····----····;···:·<UtiBaffleBeam-----FaradaycupFC1Home-madePIGIEinzellensSW1~I/st~rbopump"t::rI"lNJ~~I0=1/5SDH-2SNICSIIsputtersourceSublimatorMeteringvalveGasstrippersystemintheterminalFIGURE1Layoutoftheexperimentalsetup.Theinsetillustratesschematicallythestrippingsysteminthetandemterminal.+:-.-42 E. ARAI et al.3 RESULTSFigure 2 shows pulse-height spectra of particles scattered from the gold target, wherea beam of C3 has been injected into the tandem, and C: clusters (n = 1, 2, 3) havebeen analyzed by SW2. The accelerating voltage was 1.6 MV. The peak positionsagree with the values predicted by Eq. (1) within the uncertainty caused by thethickness of the gold target and the energy spread of the SBD for low-energy carbonions. At low channel numbers a small amount of particles is observed. They havethe same magnetic rigidity with analyzed clusters and have passed through QDl andSW2. However, the disturbance from this impurity is negligible in this measurement.Figure 3a represents the transmission coefficient of each carbon cluster beam C n(n = 1, 2, 3) measured at a terminal voltage of 1.6 MV as a function of the increase ingas pressure observed on the high-energy-side turbo pump. The background vacuumis subtracted from the vacuum reading. The upper coordinate indicates nitrogen gasbled, which has been calculated from the pressure increase and the pumping speedof the turbo pump.The transmission coefficient for C 3 amounts to only 4.4% at 0.6 x 10- 7 torr. Itshows, however, a maximum value of 12.5% at 1.9 x 10- 7 torr. With increase of thestripper gas bleeding the transmission decreases until 1% at 6 x 10- 7 torr. The peakis centered at 2 x 10- 7 torr. This stripper gas bleeding is lower than that foraccelerating light heavy ions such as 0 2 ,3,4+ and F 2 ,3,4+ which require a pressureincrease of about 1 x 10- 6 torr on the turbo pump.The yield of C;-, which are products of the breakup reaction C3 -+ C2 + C t , showsa maximum at 2.5 x 10- 7 torr and decreases to 4% at 6 x 10- 7 torr.tn...c:so(.)c; clusters injected atAccelerating yoltage:1.6 MV300 SW2 magnetic fieldwas set for: Cr c;___ 400..·c::I>-..a'J......:a..a'J""-"Pulse height of scattered carbon atomsFIGURE 2 Pulse height spectra of carbon atoms produced from the backscattering of carbon clustersCn (n = 1,2, 3). C~ clusters are injected into the tandem with an accelerating voltage of 1.6 MV.CLUSTER BEAM ACCELERATION 43Leak speed of stripper gas N2 (SCC/s)10-5 2 4 10-4 2 4 10-1 2 415 a)Accelerating voltage: 1.6MVC3- injectiono A .A A0 C3 transmission 0 0 -AA ?P-00. C6 C2 transmission 0"- aa aD 200·:~ 10 A 0 aa-.... 0 C1 yield o 6"ec a0 0 tni co A «JE .....tn ~ a + ..C atV a (.).... 0 aa... 100+M 5 0 a0 0 o A Ib+~ .0aoaoL..-...--"----------...--------'""'--s:--------10-510.8 10- 7 10·Pressure Increase at TMP(Torr)15 b)Leak sDeed ·of stri·pper gas N 2 (SCC/s)1~-5 2 4 1p-4 2 4 10-3 2oo 0A.10~C-oiifI)EfI)CtVb 5.... Mo... ~oAccelerating voltage: 1.2MVC3 Injectiono C3 transmission6. C2 transmissionAo 0AlA 6o 06 0 AAoA. AoAAooO-------01--------...&.----- -4- ~10-6 10-7 10-6Pressure increase at TMP (Torr)FIGURE 3 Transmission coefficients of en clusters (n = 1,2,3) as a function of the pressure increaseabove the turbo pump (stripper gas pressure in the canal). a) measured at 1.6 MY and b) at 1.2 MY.P.A.·-e44 E. ARAI et al.a)1.6 Accelerating voltage: 1.6MV AA 15A51; injection AAAl:i. 5i2 transmissionpl0O.A o 00o Ii. Q)0 5i 1 yield 0 °0......1.0 A?P- o 0 ~ 10~ A A tN Ii.en G.....0 A en0c 0 .....2 0en 0 • 'tJen• 'iE 0.5 5 ):en A ~C 0co.. At- A0 •A0A0 I , !10-8 10-7 10-6Pressure increase at TMP(Torr)Leak speed of stripper gas N2 (Sec/s)10.5 2 4 10.4 2 4 10.3 2 43.0 b) 30Accelerating voltage: 1.2MV5i2 injectionl:i. Si2 transmission~ AA0 5i1 yield ~AN 2.0- ¥ 20tc;; ~ A...en0 AC A0 A ....-;; A 0 0000 0rn rPo 000 00'tJE o 0 "i0 A ):tn A 0 0c 1.0 10«I A.. 00 0t- o 0 000 A0f o A SACDAA0 I10·& 10-7 10-6Pressure increase at TMP(Torr)FIGURE 4 Transmission coefficients of Sin clusters (n = 1,2) as a function of the pressure increase abovethe turbo pump (stripper gas pressure in the canal). a) measured at 1.6 MV and b) at 1.2 MV.CLUSTER BEAM ACCELERATION 45The yield of C l , products of the break up reactions C 3 ~ 3Cl and C 3 ~ C2 + C l ,increases from 10% at 0.6 x 10- 7 torr to a maximum value of 200% of the injectedC; beam current at 6 x 10- 7 torr. This high C l yield value is kept constant until1 x 10- 6 torr and decreases slowly with increase of stripper gas bled.Figure 3b represents transmission coefficients of C: (n = 2, 3) measured at anaccelerating voltage of 1.2 MV. Except for the accelerating voltage, all other param-eters have been kept constant. The values of transmission coefficients for Ci andCj have been reproduced by this experiment. This result indicates that the clusterenergy has no large effect on the rate of charge exchange and on the breakup reactionsof C 3 clusters in this energy region. We have omitted the yield curve of C l becausethere is no essential change in the 1.2-MV data in comparison with the 1.6-MV results.Figure 4a represents the 1.6-MeV Si2 measurement. The transmission of Si2(triangles) starts to increase at 7 x 10- 8 torr and reaches a maximum value of 1.5%at 1.8 x 10- 7 torr. After passing the peak it decreases rapidly to 0.1 % at 7 x 10- 7torr. Products of the breakup reaction Sil (circles) show a broad peak centered around3.5 x 10- 7 torr and a long tail that reaches the vacuum region of 10- 6 torr.In Figure 4b triangles represent the transmission of 1.2-MeV Si2 clusters, whichshow a maximum value of 2.3% around 1.8 x 10- 7 torr.The yield of charge exchange drops rather rapidly after passing the peak andamounts to 0.1 % at 7 x 10- 7 torr. The yield of Sit circles shows also a very longtail as observed in the 1.6-MV measurement. The peak value amounts to 15%.The transmission coefficients of C 3 and Si 2 were measured as a function of thesublimator heater current. As a result of the measurements, the transmission for eachcluster species has not been improved by increasing the pumping speed of thesublimator. The detailed explanation of the results is too lengthy to include here.4 DISCUSSION AND CONCLUSIONThe yield of charge exchange shows a maximum value at a pressure increase of1.8 x 10- 7 torr for both cluster species studied in the present work. This vacuumvalue corresponds to a nitrogen bleeding of 1.2 x 10- 4 SCCjs (standard cubiccentimeters per second) and to an estimated nitrogen pressure of 9 x 10- 4 torr inthe stripper canal. However, each cluster species shows a quite different yield value:the peak transmission coefficient is as high as 12% for C 3 clusters with energiesbetween 1.2 and 1.6 MeV, whereas the peak value for Si 2 clusters is as low as 2.20/0at 1.2 MeV and 1.50/0 at 1.6 MeV. It is also interesting to note that the Si2transmission increases with decrease of accelerating voltage.Products of the breakup reaction Ci show a yield curve which is shifted a littleto the high gas bleeding side; however, the values are not different from those of Cj.The yield curve of ct has a maximum value of 2000/0 at 5 x 10- 7 torr, whereas thatfor Sit has a peak value of only 12-15%. In the stripper canal 12% of injected C;clusters are charge exchanged and an additional 12% are broken up to C l + C 2 .The rest of C3 clusters are broken up to produce ct. The charge exchange starts toproduce multiply charged ions of carbon, and the Ctyield begins to decrease with46 E. ARAI et aleincrease of the vacuum reading over 1 x 10- 6 torr on the turbo pump. These figuressuggest that a tandem is one of the practical accelerators for generation of clusterbeams with energy in the MeV regime. However, Si2 clusters are more unstable thanC 3 clusters and are broken up heavily in the stripper section, and only 1.5-2.2% ofthe SiI is available for cluster experiments. The lower value of Si + yield can beexplained by the preferential production of multiply charged silicon ions at the gaspressure in the stripper canal, which is as low as 1 x 10- 3 torr.ACKNOWLEDGMENTSThe authors wish to acknowledge the encouraging discussion with Prof. Lee Sangmu of Tsukuba University.REFERENCES1. M. L. Mandich, W. D. Reents, Jr., and V. E. Bondybey, in Atomic and Molecular Clusters (E. R.Bernstein ed., Elsevier, Amsterdam 1990), Chap. 2, pp. 69-357.2. Y. Hatano, in Electronic and Atomic Collision (D. C. Lorents, W. E. Meyerhof, and 1. R. Peterson, eds.,Elsevier Science Publishers B.V., Amsterdam, 1986), pp. 153-173.3. E. A. Schweikert, M. G. Blain, M. A. Park, and E. F. Da Silveira, Nucl. Instrum. Meth. B 50307 (1990).4. James A. Ferry, private communication.

Cluster beam acceleration using a tandem van de graaff

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