Transition from electrostatic to electromagnetic modes of low-frequency fluctuations in RT-1

Abstract We report the electrostatic and electromagnetic behaviors of low-frequency fluctuations and their spatial structures observed in the RT-1 (Ring Trap 1) levitated dipole experiment. By using movable Langmuir probes capable of operating under the high-heat flux conditions, we investigated the spatial structures of electrostatic fluctuations in the plasma and compared them with magnetic fluctuation properties. Low-frequency electrostatic fluctuations in low-beta plasma transact into electromagnetic modes in high-beta operation, the latter of which has been found with edge magnetic probes in previous studies. Multi-point measurements with the Langmuir probes revealed that, in low-beta plasma, the fluctuations propagate in the electron diamagnetic direction and exhibit a toroidal mode number of 3 or 4 in a broad region across different magnetic surfaces. In the high-beta plasma, the phase velocity of the fluctuations has a clear dependence on the magnetic surfaces and reverses its toroidal propagation direction according to plasma conditions. These observations are consistent with the interpretation that density fluctuations transported by the drift motion of plasma generate magnetic fluctuations in high-beta conditions, suggesting a similarity with the so-called entropy mode.