On the use of biased movable B4C-limiter for pumping O-2 and H2O during discharge cleaning

The movable limiter, the head plate of which is formed with a boron carbide plate hot-pressed in vacuum, is used for studying plasma impurity removal and the possibility of boronization during discharge cleaning in Uragan-3M torsatron. Measurements of current-voltage characteristics, boron and oxygen spectroscopies, residual-gas mass-spectrometry were carried out for different biased and unbiased limiter positions. It is shown that at a negative limiter bias during pulsed discharges cleaning the boron spectral signal from the plasma increases due to arcing. At the same time the oxygen spectral line intensity decreases and it probably means desorbed oxygen and water effective trapping by boronized surfaces and the limiter B4C-plate, since the main plasma parameters do not change at this moment, The estimations of the limiter application for impurity pumping and solid target boronization shows that the negatively biased movable B,C-limiter can be used to improve pulsed discharge cleaning in Uragan-3M torsatron.