Developments of a MEVVA-type ion source for ion implantation studies

An ion implantation facility incorporating a metal vapor vacuum arc (MEVVA) ion source has been in operation within ANSTO for the past 2 years. This pulsed source is powered by a 0.300 V pulse forming network (pfn) which produces maximum arc currents of 500 A and extracted ion beam currents to 100 mA. Operating at pulse repetition frequencies of 1--10 Hz, this ion implanter has now accumulated over 3 x 106 shots in routine use with a variety of cathode materials. To enhance performance and provide more diagnostic information, several additions and modifications have been made to the original electronic and electrical systems. In particular the trigger power supply has been redesigned to improve the arc initiation reliability for difficult-to-trigger materials such as tungsten and silicon. Some typical results obtained with this unit will be presented. Also, a digital target current integrator based upon a voltage-frequency converter has been installed on the system. This simple device gives a relative measure of the ion dose delivered on-target. Finally, the design and performance of a high-voltage (2,000 V), high-impedance (5Ω) pfn will be briefly described. This compact unit was developed to operate continuously at trigger rates in the range 0--50 Hz while generating arc currents similar to those reported above. The higher voltage was chosen in order to investigate the influence of pfn voltage on arc current chopping which has been observed with some cathode materials. The results of a preliminary study of this effect will also be presented