The width of the adjustable polishing head gap was changed from 2 mm to 10 mm in 2 mm steps, the applied polishing potential difference U from 260 V to 320 V in 20 V steps and the velocity of the polishing head from 0.3 mm s−1, 1 mm s−1 and 5 mm s−1. A 30 mm wide area in the tube was plasma polished. The polishing head went through the tube 10 times with a polishing head gap of 1 mm. The current I was measured by the machine control over a current-to-applied potential difference converter.