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A remarkable characteristic of the cathode spot of a low-pressure arc is that it can remove an oxide layer preferentially. Recently, the cathode spot of a low-pressure arc has been used for cleaning metal oxide surfaces before thermal spraying or surface modification. However, few reports have described the cathode spot movement or the oxide removal process. This experiment was carried out using the SS400 cathode workpiece and a cylindrical copper anode. The cathode spot movement was recorded using a high-speed video camera. The images were later analyzed using plasma image processing. The workpiece surface was covered with a thick oxide of 9.67 [micro]m. The workpiece surface was analyzed using a laser microscope after processing. The cathode spot movement shows differences as the processing time increases and as the oxide layer roughness changes. Index Terms--Cathode spot movement, low-pressure arc, moved distance, oxide layer, processing time, surface roughness, tracks of cathode spots, vacuum arc.

Low-pressure arc cleaning is a process for removing an oxide layer. Currently, chemical and mechanical means are typically used to remove such layers. However, both methods present difficulties such as a liquid waste, dust, and noise. Regarding the low-pressure arc cleaning, waste comes from one source: the oxide layer. In addition, the cathode spot has very high temperatures that are sufficient to remove the oxide layer. This paper describes the removal of the nanometer-thick oxide layer from a thin metal plate. An oxide layer of 27-157 nm was removed, thereby, obtaining a smooth surface whose respective arithmetical mean height ([R.sub.a]) and average length of an outline curve element ([R.sub.sm]) are 0.04 and 6.4 [micro]m. In that case, [R.sub.a] and [R.sub.sm] increased with an increasing oxide-layer thickness at 397-1680 nm. Those results depend on the oxide-layer thickness. Therefore, although the surface is cratered and rough after a cathode-spot treatment on the chemical oxide layer (6.7 nm), a smooth surface is obtainable after the cathode-spot treatment on the thermal oxide layer (27, 66, and 157 nm). Surface roughness depends on the processing time to produce one crater, which depends on the oxide-layer thickness. Index Terms--Cathode spots, low-pressure arc, oxide layer, processing time, smooth surface, vacuum arc.

When using the boundary biased-probe method, the nonlinear curve attained in experiments is contradictory to the linearity expected when Tassicker's formula is applied to this method. In this paper, we analyze the probe model theoretically and obtain a nonlinear relation. Our analysis shows that the space charge near by the biased probe is the main cause of this nonlinearity. This space-charge effect was ignored in Tassicker's formula. Our theoretical analysis and experimental results show that Tassicker's formula can be improved, in particular, when the electric field region is rich in space charge. In this paper, we propose a way to improve Tassicker's formula. Index Terms - Biased-probe method, coaxial three-cylinder system, discharge, ion current, Poisson's equation, space charge.

Decreased electrode erosion is important to maintain an arc and reduce costs. Normally, a vacuum or rare gases such as argon and helium are used to prevent electrode erosion. However, associated equipment and its arrangement are expensive. If the anode attachment of the arc is moving, electrode erosion can be decreased even in an air atmosphere. This paper specifically addresses arc movement, especially the restrike phenomenon, in a rail-gun. The are length was measured using a plasma image processing on pictures taken with a high-speed video camera. Thereby, the lengths and positions of the arc are determined over time. The average velocity of the anode is as fast as that of the cathode, even in restrike mode. Index Terms--Anode attachment, arc mode, plasma image processing, rail gun.