Charge state distribution of light heavy ions accelerated in a single ended Van de Graaff accelerator

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Nuclear Instruments and Methods in Physics Research 220 (1984) 154-157 North-Holland, Amsterdam

C H A R G E S T A T E D I S T R I B U T I O N O F L I G H T H E A V Y I O N S A C C E L E R A T E D IN A SINGLE ENDED VAN DE GRAAFF ACCELERATOR I. H U N Y A D I , A.Z. KISS, I. KISS, E. K O L T A Y and Gy. S Z A B O Institute of Nuclear Research, Hungarian Academy of Sciences, Debrecen, Hungary

Ion-atom collision experiments require, in many cases, accelerated light heavy ions of various charge states in the energy region of a few MeV. These types of beams can be produced using Penning ion sources combined with a charge state selector in the terminal of single ended Van de Graaff accelerators. We propose, in addition, the use of stripper targets in different locations in the beam transport system. In the present paper the experimental results obtained with pre-analyser and post-analyser strippers are presented and the performances of such systems are discussed.

1. Introduction Experiments in ion-atom collisions require light heavy ion beams of various charge states with energy definition and intensity moderate compared to those required in nuclear measurements. The few MeV energy range is often too low for tandem generators except for the Accel-Decel system [1] where low energy ions can be obtained in very high charge states. Single-ended Van de Graaf accelerators with a Penning ion source and a charge state selector used in the terminal [2-4] produce a broad selection of light heavy ions in different charge states. In the present work we propose the application of a charge exchange target at properly chosen positions in the beam transport system. With a stripper at the entrance to the analysing magnet [5] highly charged heavy ions can easily be deflected. Moreover, a neutral component appears at the extension of the entrance direction. The energy definition of the beam will be poorer than in normal runs, because the energy stabilisation is driven by the weak component of the selected charge state. The neutral beam will contain neutral particles from the acceleration tube and from molecular components, too. The beam quality will be improved with the stripper behind the analyser magnet. In this case the energy stabilisation is driven by the deflected single charged component and the neutral component originates from the stripper only. However, due to the higher magnetic rigidity of the single charged ions the maximum energy is limited more by the mass-energy product than by the maximum voltage of the accelerator. The components from the stripper can be separated by the switching magnet nearby. Some features of the application of a single-ended Van de Graaff generator in this field have been investi0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

gated on the 5 MV accelerator of this Institute (for references to a technical report see ref. 6).

2. Ion source and strippers 2.1. Radio-frequency ion source An inductively coupled radio-frequency ion source [7] is used here for H, He, C, N and O ions. Components of higher charge states appear at non-usable levels. A careful search has been performed for doubly-charged helium ions normally available at low intensity from capacitively coupled sources. Special care has been taken in distinguishing between 4He2+ and 2H + components unresolved in magnetic deflection. The two components behave differently in elastic scattering, so the direct beam has been analysed in a scattering chamber. The spectrum shown in fig. 1 contains an insignificant 4He2+ peak. To make the spectrum less complex the scattering measurement was performed in the analysed beam, as well. From these measurements the intensity of the 4He2+ component was found to be less than 0.01% of the 2H + background. Direct visual evidence of the very low intensity ratio 4HeZ+/2H + in the beam is given in fig. 2. A solid state nuclear track detector type CR-39 [9] was exposed to a single shot of 2 bts duration behind the analysing magnet. Black spots are the tracks of 4He2+ ions while the dense structure is provided by the 2H + ions. The exposure was made one hour after the ion source had been switched from hydrogen to helium. 2.2. Penning ion source For use with a stripper target a Penning ion source and a magnetic charge state selector has been con-

I. Hunyadi et a L / Charge state distribution of light heavy ions

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structed and tested. The source is a simplified version of that published by Banmann and Bethge [2,3]. During the test runs we measured the discharge and extraction characteristics for H, He, C, N and O. A minor modification is underway and the source will be put in the generator in the near future.

2.3. Charge exchange gas stripper In order to meet the experimental requirements a gas stripper was preferred to a foil stripper for the following reasons: neutral beams can be obtained with high intensities; the limited life of foil strippers can be avoided; and since residual gas in the experimental chamber will

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not affect the equilibrium charge state of the beam component separated by the magnet, provided the gas target is of sufficient thickness. The intensities of the different beam components were measured in the arrangement shown in fig. 3. In the experiments with pre-analyser stripper, a stripper tube of 20 cm length was arranged in a differential pumping block near to the entrance slits. Neutral beam was measured at the extension at 0 ° through the current of secondary electrons released from a target [10]. A positive grid and a negative suppressor ring were used for correct intensity measurements. Data for the postanalyser stripper were taken with a stripper tube of 10 cm length situated in a second differential pumping block behind the exit slit of the analysing magnet. In these measurements the neutral and charged components were measured at the 0 ° and 45 ° channels of the switching magnet, respectively.

3. Results The charge state distributions predicted by the semiempirical expression of Dmitriev and Nicolaev [11,12] for C, N, O and Ne beams of the energies 1 and 5 MeV are shown in fig. 4. Full and dotted lines correspond to stripper gases helium and nitrogen, respectively. The measured data given in fig. 5 both for preanalyser and post-analyser strippers with H, He and N ions clearly demonstrate the performance of the method. Nitrogen data for post-analyser stripper are in agreement with the calculated charge state distribution: the full triangles in fig. 4 stand for the measured values. No direct comparison can be made for the neutral component because of the different transport properties of the 0 ° and 45 ° channels and the lack of focusing for neutral particles. In the case of pre-analyser stripper the intensity of the neutral component is higher than expected. The increase can be ascribed to particles neutralised in the acceleration tube as well as to neutralised molecular component. In fig. 5 crosses indicate the nitrogen beam ratios available from the Penning source. As can be clearly seen the stripper method results in intensity ratios higher than those for the Penning source. On the other hand, the acceleration of separated multiply-charged ions will result in analysed beams of higher energy. Taking into account the additional requirement of energy homogeneity for the neutral component, the best results can be obtained with a combination of the Penning source and charge state selector with a post-analyser stripper.

Fig. 2. Low intensity He z+ component (black tracks) appearing in 2H + background, as shown by a solid state nuclear track detector of CR-39 type. IV. NEW ACCELERATORS

I. Hunyadi et al. / Charge state distribution of light heavy ions

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L Hunyadi et al. / Charge state distribution of light heavy ions

References [1] T.K. Saylor, J.E. Bayfield, L.D. Gardner, Y.Z. Gulkok and S.D. Sharma, IEEE Trans. NS-28 (1981) 1024. [2] H. Baumann and K. Bethge, Nucl. Instr. and Meth. 122 (1974) 517. [3] H. Baumann and K. Bethge, Nucl. Instr. and Meth. 189 (1981) 107. [4] B. Spellmeyer, 3rd Int. Conf. on Elstat. Accelerator Technology, Oak Ridge (1981) IEEE 81CH1639-4, p. 71. [51 K. Brand, presented at SNEAP'82. Seattle (October 6-8, 1982). [6] A. Kiss, E. Koltay, G. Nagy and E. Somorjai, ATOMKI K6zlem6nyek 20 (1978) 263.

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[7] I. Kiss, E. Koltay and P. Bornemisza-Pauspertl, Rev. de Phys. Appl. 12 (1977) 1481. [8] J. Freemann, B.W. Hooton and P. Humphries, Int. Conf. on the Technology of Elstat. Accelerators, Daresbury (1973) D N P L / N S F / R 5 , p. 283. [9] G. Somogyi and I, Hunyadi, in: Solid State Track Detectors, ed., H. Francois (Pergamon, Oxford-New York, 1983) p. 443. [10] D. l~dizs, E. Koltay and A. Szalay, Nucl. Instr. and Meth. 94 (1971) 537. [11] I.S. Dmitriev and V.S. Nikolaev, Sov. Phys. JETP 20 (1965) 409. [12] H.G. Price, Daresbury Report N D P L / N S F / R 2 (1972).

IV. NEW ACCELERATORS