High Beta Observations of the Hot Electron Interchange Instability

High frequency ($f > 1$ MHz) electrostatic fluctuations have been observed in high-beta plasma created in the Levitated Dipole Experiment (LDX). We have previously identified these fluctuations as the Hot Electron Interchange (HEI) instability.\footnote{E. Ortiz, \textit{et al.}, to appear in \textit{J. Fus. Energy}, (2006).} New observations have been made in the presence of the magnetic levitation fields. We find the HEI mode is characterized by frequency sweeping at the drift-resonance of trapped energetic electrons. The fluctuations often appear with coherent structures that have been detected on fast high-impedance electrostatic probes and edge Mirnov sensors. We observe phase shifts using multiple probes that will enable us to determine the toroidal mode number (m) and a higher sampling rate reveals frequency sweeping as high as 40 MHz. Measurements that characterize these modes now incorporate fast magnetic measurements in an attempt to put together a coherent picture of plasma behavior during these modes, including the consequences of these instabilities on plasma formation and pressure limits.