Emissive probes for technical applications

The physics of plasmas has an enormous importance for modern technology, in particular for surface treatment and nanotechnology, intensive light sources, ion thrusters, welding and cutting techniques, waste treatment and nuclear fusion technology. However, frequently plasmas show strongly unstable behaviours, which lead to their deterioration. Therefore the parameters of a plasma should be monitored constantly in order to counteract detrimental effects in time if possible. Consequently, the diagnostics of plasma parameters is one of the most important branches of plasma physics. One of the most important parameters is the plasma potential. Due to nonlinear phenomena the plasma potential can either strongly oscillate or fluctuate with high amplitudes or can show stationary deviations from a flat profile. There are very few methods to measure the plasma potential, and these are either complicated and costly or they can disturb the plasma. One of the methods to determine the plasma potential directly with acceptable temporal and spatial resolution is a probe that emits an electron current into the plasma. This is usually realized by a small loop of a refractory metal wire (W or Re) that is heated electrically. Then the floating potential of the emissive probe corresponds quite exactly to the plasma potential, since the plasma electron current is compensated by the emission current,. Although emissive probe are know since many years, they are not completely understood. But we have achieved very good results with emissive probes even in hot and dense plasmas of various fusion experiments and possess now a good expertise of them. Based on that we plan, on one side, to study effects of emissive probes which are not yet completely understood in order to improve their reliability and applicability, and, on the other hand, to develop them further in order to make them a reliable and well-understood standard tool for the diagnosis of the plasma potential in as many plasma types as possible, especially in industrial plasmas and fusion plasmas. To this end we plan to pursue two directions: (i) to perform specific experimental investigations of emissive probes in various plasmas, aided by computer simulations, (ii) to develop new, highly reliable and robust emissive probes for technological and fusion plasma applications. The most promising new type would be a laser-heated emissive probe, in which the electron emitting probe head is heated by a strong infrared beam from an IR diode laser with a power up to 50 W. These objectives should partly be pursued in collaboration with two other European plasma physics laboratories