Role of energetic particles in heat and particle exhaust in a poloidal divertor RFP

In a closed poloidal divertor experiment in the STE-2 reversed field pinch (RFP), heat and particle exhaust into the RFP divertor region has been demonstrated. The exhaust efficiency, however, was lower than expected from equilibrium flux surface geometry near the divertor throat. In addition, indications of enhanced plasma-wall interaction (PWI) have also been observed. Particle orbit calculations have been carried out in the poloidal divertor configuration, showing that the complicated orbits depend strongly upon the particle energy, pitch angle and initial position. It has been concluded that energetic particles such as edge superthermal electrons or anomalously heated ions, both of which are common to the RFP, possibly enhance PWI near the divertor throat due to the effect of large Larmor radii, and reduce the exhaust efficiency also. It has also been shown that the orbits are quite sensitive to the initial positions near the separatrix, indicating the importance of edge magnetic fluctuations to the divertor action. Issues for future RFP divertor studies are also discussed. (orig.)