Effect of bias on trapping probes and bolometers for Tokamak edge diagnosis

Analysis is presented for the particle and energy flow to an electrically biasable probe immersed in the magnetic field of the boundary plasma of a Tokamak or other magnetic confinement device. The analysis is appropriate to the operation of the probe in the voltage-current (Langmuir) probe mode, the thermal power (bolometer) mode, hydrogenic ion trapping mode or impurity deposition mode. The formulation of the analysis permits the use of any cross-field diffusion model, i.e. classical, semi-classical, empirical. The principal conclusion of the analysis is that a probe should be operated simultaneously in at least two modes, e.g. Langmuir and bolometer modes. In this way it is possible to measure plasma density and electron and ion temperatures separately with several consistency checks also available. Under some circumstances it is also possible to measure the hydrogenic cross-field diffusion coefficient and impurity ion density, temperature and charge state. It is necessary to operate a bolometer in a biased mode in order to infer heat loads to non-floating surfaces, e.g. limiters and divertor targets, since heat flux is a strong function of the potential difference between the plasma and surface.