The dynamics of arc cathode spots in vacuum. Part III: measurements with improved resolution and UV radiation

Are cathode spots in vacuum with Cu cathodes have been registered by combining framing and streak channels of a fast image converter camera equipped with high-magnification optics. In a special version UV radiation could be used for imaging. In this way the internal fragment structure of the spots could be resolved down to values < 5 mu m with exposure times of 10 ns. With light from the spectral range 200-800 nm the spot intensity is about ten times higher than when in the visible range 350-800 nm. The smallest observable fragment size is less than or equal to 5 mu m. It is demonstrated that the spot fragments can change their position within 10-50 ns, the corresponding velocities reaching 1000 m s(-1). These positional changes are associated with splitting and merging of the fragments. Prior to splitting and after merging the fragments become very bright. Characteristic brightness peaks have been found in the nanosecond as well in the microsecond range. The average intervals between fluctuations or groups of fluctuations in each range amount to about 19 ns, 180 ns, 2 mu s and 16 mu s respectively. The fragment dynamics lead to random displacement of the spot as a whole. The streak pictures show this spot movement for times down to 100 ns. Within 100 ns, the spot velocity can reach about 100 m s(-1). Hence, spots and fragments are nanosecond phenomena. Apparent time constants >> 1 mu s of the fragments are explained by the nonlinear increase of the spot luminosity with increasing sojourn probability of the fragments in a given area. The effect leads to overestimating the phases of small displacements and of fragment merging when analysing time-integrated pictures.