Cleaning of inner vacuum surfaces in the Uragan-3M facility by radio-frequency discharges

A method for cleaning vacuum surfaces by a low-temperature (T _e∼ 10 eV) relatively dense (n _e≈ 10¹²cm^-3) plasma of an RF discharge was developed and successfully applied at the Uragan-3M torsatron. The convenience of the method is that it can be implemented with the same antenna system and RF generators that are used to produce and heat the plasma in the operating mode and does not require retuning the frequencies of the antennas and RF generators. The RF discharge has a high efficiency from the standpoint of cleaning vacuum surfaces. After performing a series of cleanings by the low-temperature RF discharge plasma (about 20000 pulses), (i) the intensity of the CIII impurity line was substantially reduced, (ii) a quasi-steady operating mode with a duration of up to 50 ms, a plasma density of n _e≈ 10¹²cm^-3, and an electron temperature of up to T _e∼ 1 keV was achieved, and (iii) mass spectrometric analysis of the residual gas in the chamber indicated a significant reduction in the impurity content.