On the possiblity of direct electrical power extraction from scrape-off layer currents in tokamaks

Currents flowing in the scrape-off layer (SOL) are routinely observed in a number of tokamaks. They are usually closed through the divertor plates, either between the inner and outer target or between different poloidal sections of each target. The most common driving mechanisms of SOL currents are the thermoelectric effect (due to electron temperature difference between the two targets) and Pfirsch-Schlüter flows caused by radial pressure gradient. The SOL currents are commonly used for diagnostic purposes-e.g. as ELM monitors or for real-time estimation the divertor electron temperature. In this paper, we report on attempts to extract the SOL currents and related electrical power from the tokamak COMPASS using a dedicated insulated divertor tile and electronics setup, which allows for fast switching between grounded and floating state. It is observed that plasma acts as an ideal power source and that the magnitude of SOL currents scales with upstream separatrix temperature T _e,sep.