The torsion vacuum microbalance technique useful for the determination of the sputtering yield of wall materials for fusion reactors was developed. In order to measure the rate of weight change of the samples during sputtering continuously, it is very important to keep the null point drift of the microbalance as small as possible. The deflection angle of the balance beam was detected and amplified by optical lever, and was transformed to electrical output by linear photopotentiometer. The null point drift was mainly caused by temperature change in the laboratory, which induced relative displacement by thermal expansion between the balance system and the optical measuring system. The drift was reduced less than the equivalent weight of 4 × 10 ^-7 g/hr when the balance system wosset in the thermostat kept at 20 ± 0.5 °C. The sensitivity of the balance was 8.3 × 10 ^-7 g/mV.
The microbalance technique developed was applied to the measurement of sputtering yield of Cu by Ar⁺. The measurement was carried out with Ar⁺ ion energy of 0.22 keV, beam current of 0.51.5 μA, and sputtering time of 2 hrs. The initial weight of Cu sample was 0.4g (15 mm × 15 mm sheet) and the sample temperature during sputtering was about 120 °C. The sputtering yields obtained show good agreement with the results reported by Wehner et al. [8] and Weijsenfeld [9].

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A new acceleration method in hot hollow cathode discharge (HHCD) plasma beam is proposed.
A neutral gas (He, Ar) in a central region along the discharge is pumped out locally and kept in an optimum pressure where electrons are collisionless and ions are collisional. Then, a voltage to accelerate electrons is applied for the central region while ion motions are restricted by the ion-neutral collisions to reduce a space charge of accelerated electrons. Electrons in a Ta hollow cathode discharge are accelerated up to 1.5 keV along a uniform magnetic field of 700 gauss. As a result, a plasma electron beam concentrated in a diameter of 8 mm is produced with a current which is controllable up to 80% of the discharge current (50 A max.) and independent of its accelerating voltage. It is remarkable also that the cathode is protected from bombardments with back stream ions and and that the starting of discharge is easy.
Thus, we expect that a power efficiency entering into a sample anode will be much improved in comparison with an ordinary low voltage hollow cathode discharge.

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