THE SYSTEM OF NANOSECOND 280-keV-He PULSED BEAM

At Fast Neutron Research Facility, the 150 kV-pulses neutron generator is being upgraded to a 280-kV-pulsedHe beam for time-of-flight Rutherford backscattering spectrometry. It involves replacing the existing beam line elements by a multicusp ion source, a 400-kV accelerating tube, 45-double focusing dipole magnet and quadrupole lens. The multicusp ion source is a compact filament-driven of 2.6 cm in diameter and 8 cm in length. The current extracted is 20.4 μA with 13 kV of extraction voltage and 8.8 kV of Einzel lens voltage. The beam emittance has found to vary between 6-12 mm mrad. The beam transport system has to be redesigned based on the new elements. The important part of a good pulsed beam depends on the pulsing system. The two main parts are the chopper and buncher. An optimized geometry for the 280 keV pulsed helium ion beam will be presented and discussed. The PARMELA code has been used to optimize the space charge effect, resulting in pulse width of less than 2 ns at a target. The calculated distance from a buncher to the target is 4.6 m. Effects of energy spread and phase angle between chopper and buncher have been included in the optimization of the bunch length. INTRODUCTION At FNRF in Chiang Mai University, used to do the research about time-of–flight Rutherford backscattering spectrometry technique (TOF-RBS) for analyzing the material by using the accelerator which is produced pulsed D ion at 140 keV [1]. The experiment has done for analyzing the Cu-Au/Si sample. The experimental result however does not show separation of Cu and Au element due to low resolution of the system [2]. To achieve higher resolution, higher mass or higher energy ion should be used. With a constraint of the existing components which limit of the width of the x-y deflector in the chopper and the dimension of the buncher, He ion at 280 keV has been considered. To upgrading the accelerator to have more efficient some component will be change such as ion source, accelerating tube, 45 of double focusing magnet and also quadrupole magnet. In this paper describe in the detail of each component in the beam line. ION SOURCE SYSTEM The rf ion source will be replaced by a filament–driven multicusp ion source (MIS). Furthermore, we will install a 400-kV power supply that will supply terminal voltage to the helium ion. The MIS has been reported to have a stable discharge, small beam emittance, low axial energy spread and high gas efficiency for single charge ion [3]. It plasma chamber has a diameter of 2.6 cm, is 8 cm long and is made from copper free oxygen. The two rows of 16 samarium-cobalt magnets which are located in the wall of the ion source will form a longitudinal line-cusp field configuration in the chamber. The hair pin filament is made from tungsten with a diameter 0.5 mm. The extraction hole has a diameter of 1.0 mm. The electrode static lens, which is of the decelerate-accelerate type, has three electrodes and is exclusively used for focusing. The focusing power of the lens is realized by applying the voltage to the second electrode. The first and the third electrode of the einzel lens are kept at ground potential. Since the ion source housing is kept at a higher voltage than the ground potential, the additional voltage that exists between the source housing and the first electrode is being used to extract ion from the source. Figure 1 shows a schematic drawing of the ion source system. Ion source adapter Tip Ion source holder Screw holder Extraction & Einzel lens holder Einzel lens