Angular Resolved Measurements of Particle and Energy Fluxes to Surfaces in Magnetized Plasmas

In fusion experiments, the energy flux to the target plates is an important issue. In order to spread the heat load, surfaces are usually designed to intersect magnetic field lines at very shallow angles. In the course of this work, a sensitive probe allowing simultaneous measurements of energy flux and current density as functions of a bias voltage was developed. Extensive experimental data on the particle and energy flux densities as functions of the angle between a surface and the confining magnetic field are provided. An analytical model is developed in order to reveal the physics involved; it is in good qualitative agreement with the experimental results. The experiments were conducted at the PSI-2 facility, a linear divertor simulator with moderate magnetic field strength (B ≈ 0.1 T, ne ≈ 10 − 10 m−3, Te ≈ 1− 15 eV, Ti ≈ 2/3Te). The probe was rotated in a spatially homogeneous plasma. The active area, a tungsten covered Peltier module, was immersed in a ceramic surface, closely resembling the geometry of a flush mounted probe. Its dimensions were comparable to the ion gyro radius ri. While the electrons were strongly magnetized (Hall parameter he ≈ 10), the ion conditions varied between unmagnetized, hi < 1, and hi ≈ 10 depending on the ion species. Sheath parameters (ion current density ji, floating potential Uf , energy flux density q, ion energy reflection coefficient RE and sheath energy transmission coefficient γ) were determined as functions of the angle α between the probe surface normal and the magnetic field. An apparent asymmetry in the angular dependence of the particle and energy flux densities was found experimentally; they could be explained qualitatively by basic geometric considerations. For |α| exceeding about 80◦ some interesting deviations from the normal incidence conditions (α = 0◦) case were found: a pronounced reduction of the floating potential is observed. This is also recovered in the angular dependence of the sheath energy transmission coefficient γ. Additionally, an increase of the ion energy reflection coefficient RE depending on the ion gyro radius is found. With respect to an application in fusion research, the combination of both, the reduction of the floating potential and the increase of the ion energy reflection coefficient at shallow angles of incidence, should result in reduced heat loads and, possibly, lower sputtering yields beyond the mere reduction of the ion and electron flux densities.