Studies on Suitable Materials for a Laser-heated Electron-emissive Plasma Probe

The direct measurement of the plasma potential poses a major challenge to plasma diagnostic, and the sole tools for localized measurement of this parameter are certain forms of electric probes. For many decades emissive probes have been used in laboratory plasmas for a direct determination of the plasma potential Φpl. Only during the last years emissive probes have been applied for the first time also in the edge plasma region of fusion experiments for measuring Φpl and related parameters such as the electric field and fluctuations of the potential and electric field [1, 2]. Usually the probes are made of tungsten and/or tantalum wire heated to emission. Unfortunately, this type of probes has some disadvantage. For example, if a wire probe is not heated very cautiously, it often melts and needs replacement, in particular when high emission currents are needed. On the other hand, when an emissive wire probe is used, the entire battery or power supply has to float electrically with the probe. Due to the inevitably large capacity of such instruments, together with the also inevitable cable capacities, the upper cut-off frequency is reduced. In order to overcome all