Status report of a 1.7 MV tandem accelerator for solid state research

48

Nuclear Instruments and Methods in Physics Research A244 (1986) 48-49 North-Holland, Amsterdam

STATUS

REPORT

S. B E R G E R

O F A 1.7 M V T A N D E M

ACCELERATOR

FOR SOLID STATE RESEARCH

a n d F. D W O R S C H A K

IFF, Kernforschungsanlage Jhlich, FRG

An accelerator system is described which is in use for radiation damage studies, analytical (RBS) and structural (channeling) measurements, and high energy ion implantation. An important feature of the system is its high current capability for radiation damage. The injector is equipped with three ion sources and is operated at 80 kV to permit high intensity ion beam currents to be accelerated. A parallel fed voltage multiplier located within SF6 gas of 8.3 bar pressure produces stable voltages in the range of 0.400 to 1.700 MV. The beam is transported by a switching magnet to four target stations.

A 1.7 MV tandem accelerator has been installed in the I F F of K F A Jiilich and became operational in January 1984. It is used both for radiation damage and the analysis of surface structure (RBS and channeling experiments) as well as ion implantation. The terminal energy was chosen to be a compromise for the aspects of the different users and also a financial one. The accelerator is a standard 1.7 MV Tandetron from G I C equipped with three ion sources each with an 80 keV injector. They are for hydrogen, helium and heavy ions, respectively. An important feature of the system is its high current capability for radiation damage experiments. Fig. 1 shows a layout of the accelerator system and fig. 2 is a view at the three injector legs. The center, or zero-degree, injector is designed to produce negative hydrogen ions from a duoplasmatron. These ions are focused by an einzel lens and accelerated

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by a preacceleration tube, they then pass through a gridded lens and a cross-field analyzer before entering the injection magnet chamber. The cross-field analyzer rejects deuterium ions and hydrogen molecules from the beam. The helium ion injector has a positive-ion duoplasmatron, followed by an einzel lens and a lithium vapor charge-exchange canal. The negative helium ions are then accelerated through the preacceleration tube, pass a vertical steerer and a gridded lens before entering the injector magnet at the - 3 0 ° port. The heavy-ion source is a sputter source which uses cesium to sputter ions from a variety of target materials. The ions are focused by an einzel lens and accelerated by the preaccelerator tube. They pass a vertical steerer and a gridded lens and enter the injector magnet at the + 30 ° port.

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Fig. 1. Layout of the accelerator system showing the three injector legs, the injector magnet, the accelerator tank and the high energy switching magnet. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

S. Berger, F. Dworschak / Tandem accelerator for solid state research

49

Fig. 2. View of the accelerator system from the low energy side showing the three injector legs. The specifications of the different ion sources as measured at the LE Faraday cup are: H - : 120 #A; H e - : 1 #A; B-: 10 #A; C - : 100 p.A; Si-: 230 #A; N i - : 35 #A; A u - : < 100 #A. The injector magnet has an air gap of 51 mm and a maximum field of 10 kG. The radius of curvature is 70.5 cm and the mass-energy product is 24 amu MeV at 30 °. The tandem accelerator consists of: 1) A tee-shaped tank structure for the high voltage insulation with SF6 (max. pressure 9 bar). 2) The acceleration tubes which are constructed by sandwiching titanium electrodes between glass rings and which are equipped with permanent magnets to suppress electron loading and hence to reduce X-ray production by secondary electrons. The terminal voltage is divided by a resistor string connecting each electrode. 3) A gas stripper at the terminal, the gas of which is pumped through the HE accelerator tube. 4) A Dynamitron-type high voltage solid state power supply [1]. The circuit is excited by a frequency of 39 kHz. 5) A generating voltmeter to measure and regulate the terminal voltage. The beam transport at the high energy end consists

of an electrostatic quadrupole and a switching magnet (four beam ports, with a radius of curvature of 140 cm and a mass-energy product of 132 amu MeV at 15 °) after which the beam lines for the different experiments are installed. The vacuum pressure produced by four 1000 l / s turbopumps is between 10 -5 and 10 -7 mbar depending upon location and type of operation. The optimum transmission for protons through the tandem was 80% with the terminal at 1.7 MV. This transmission can only be achieved with a rather high gas pressure in the charge-exchange canal at the terminal leading to a heavy gas load in the HE accelerator tube. Due to this gas load the current through the resistive grading of the HE accelerator tube decreases drastically because the stripper gas becomes ionized. A solution to this problem is pumping at the terminal and the installation of a pump is being worked out with GIC.

Reference [1] M.R. Cleland and P. Farrell, IEEE Trans. Nucl. Sci. NS-|2 (1965) 227. I. EXISTING FACILITIES