Plasma cloud detection and diagnostics from arcing by conditional averaging

Abstract

Plasma cloud formation from arcing is experimentally studied. The arcs are formed by a high voltage set-up in the space simulation chamber at UiT. The plasma clouds are observed as large structures in the time series recorded by Langmuir probe and reference probe. By using the method of conditional averaging, the structures are accentuated. The electron cloud formed by arcing was found to be characterized by a steep drop from zero current to a minimum, then relaxation towards zero current. Additionally, we found the magnitude of the amplitude to the electron cloud to be decreasing when the distance between the arc and the movable probe is increased. The average electron cloud velocity was found to be ve = 1:32 106 ms�����1. The ion cloud formed by arcing was found to be characterized by a current above the steady state, which then increases logarithmically to a maximum, and then relaxes back to the steady state. However, the ion cloud measurement revealed a negative spike before the ion cloud, which indicates the electrons from an arc are energetic enough to be recorded by a negatively biased probe. The average ion cloud velocity was found to be vi = 119:2 ms�����1, however the uncertainty was large due to high scattering. During the experiment, we discovered that breakdown of gas causes an arc-like be- haviour of the electrons. By applying the method of conditional average, a distortion was revealed. This result led us to a new condition that revealed that a high voltage breakdown gives rise to an electron cloud with the velocity vH = 7694ms�����1, while the low voltage breakdown give rise an electron cloud with the velocity vL = 5441ms�����1. Additionally, the amplitude of the electron cloud from the high voltage breakdown were signi cantly larger than he lower breakdown voltage. It is apparent the higher break- down voltage creates a more energetic electron cloud than a lower breakdown voltage.