Design of Probe to Investigate Energetic Electrons in Lower Hybrid Wave Plasmas in the TST-2 Spherical Tokamak

The plasmas whose current is driven by the lower-hybrid wave (LHW) alone have been investigated on the TST-2 spherical tokamak. We are developing a probe to obtain the information about energetic electrons generated by the LHW. We investigated a detector, motivated by the lost fast-ion probe (LIP) or fast ion loss detector (FILD), which can identify the energy and pitch angle of captured ions. However, the pitch angle of electrons of interest is expected to be so small that it was found that a conventional configuration with the normal vector of an entrance-orifice surface almost perpendicular to the magnetic field line was not applicable. Thus, we accessed the feasibility of the configuration with two tiny orifices by which orbits, or the energy and pitch angle, of detected electrons can be well specified. We carried out numerical calculations by using calculation conditions and geometries which are manufacturable. The results suggest that it is difficult to identify the energy of detected electrons, but that it is possible to identify their pitch angle well. Based on the results, we designed the diagnostic system with 3 channels which will detect energetic electrons with the pitch angle of 3 -5, 5 -9, and 9.5-12.5 degrees, respectively. (c) 2023 The Japan Society of Plasma Science and Nuclear Fusion Research