Unipolar arcs

The most essential plasma device characteristics that are needed in order to obtain impurity release rates are the fluxes of photons and of charged and neutral particles to the wall. It will be necessary to have detailed information on the energy spectra and fluxes to walls, limiters and beam dumps of thermal electrons and ions, photons, a-particles, runaway electrons, charge exchange neutrals, neutral beam and impurity neutrals and ions. The effects of sheath potentials, secondary electron emission and unipolar arcing need to be included in these calculations. 

Although observed quite early in the development of plasma devices, unipolar arcing has only recently gained widespread attention as both a major source of plasma impurities and as a potentially severe erosive effect for first walls, limiters and other material surfaces in close proximity to Tokamak plasmas. 

The principal method of introducing metal impurities into early pinch discharges was considered to be arcing. The externally applied voltages, although low, still permit the occurrence of unipolar arcing between the plasma and the wall when driven by the sheath potential. Local electron emission from a cathode spot is balanced by a uniform flow back to the surface of energetic electrons in the tail of the Maxwellian distribution. 

There is still the possibility that the metals may be sputtered by multiply charged low-Z impurities such as oxygen and carbon. Self sputtering is also a possibility but at present the question is still to be resolved. Evaporation as a possible source of metal impurities has also been dismissed. Unipolar arcing appeared to be the most likely mechanism leading to the observed impurity level81. ... 

Dc arc spectrography is still a most powerful method for trace determinations in solids even with difficult matrices such as U3Og. Here the detection limits for many elements are down in the sub-pg/g range [358]. It is still in use for survey analysis, especially in the case of ores, minerals and geological samples. In work with unipolar arcs, re-ignition of the arc is often facilitated by providing an hf discharge over the dc arc, by which the arc channel is kept electrically conductive. 

The bipolar membranes are used in a more or less conventional ED stack together with conventional unipolar membranes. Such a stack has many acid-alkali producing membranes between a single pair ol end electrodes. The advantages uf the process compared tu direct electrolysis seem iu be that because only end electrodes arc required, the cost of the electrodes used in direct electrolysis is avoided, and the energy consumption at such electrodes is also avoided,... 

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