Stabilization of an electrostatic accelerator.

This thesis describes the research work performed on the positional stabilization of the mass-analysed ion beam, which is produced by the 500 keV, multi-ion, open terminal., electrostatic accelerator at the University of Surrey. This work was essentially an engineering project and was based on the available equipment and limited financial resources of a university department. The position of a mass-analysed ion beam, at either target of the accelerator, depends on the beam energy, and this, in turn, is main.ly a function of the accelerating voltage . At the start of the project, accelerating voltage instabilities in the range of 2.5 X 10-2 to 7.1 X 10-2, and with associated frequencies lying between 0.1 and 2 Hz and at close to 50 Hz, were established; these caused beam movements of between 48 mm and 136 mm at the target located at 5 m from the 45° exit of the mass-analysing magnet. The range of frequencies, over which the accelerator voltage could be corrected, was initially limited to about 5 Hz by the electrostatic voltage supply. By modulating the lower end of a stack of high voltage capacitors, placed between the ion-source terminal and ground, this range was extended to at least 1 kHz. Both the generator and the capacitors were supplied with out-of-balance beam positional information, obtained from two vertical jaws intercepting the sides of the mass-analysed ion beam. The stabilising system, constructed by the author, gave accelerating voltage stabilities between 4 and 75 x 10-5 over a frequency range of d.c. to at least 1 kHz, and so reduced the beam movements at the target of the 45° line to between about 0.076 mm and l.M-3 mm. The beam positional stability was thus improved by a factor of between 33.3 and 1775. The performance of the system was investigated with the aid of mathematical models describi.ng both the frequency response of the system and the beam deflection geometry associated with the analysing magnet. The engineering aspects of the system are also detailed