Electrostatic ion cyclotron instability near threshold

By drawing an electron current through a plasma to a variably biased conducting button, the electrostatic ion cyclotron instability is excited, consistent with predictions of Drummond and Rosenbluth [Phys. Fluids 5, 1507 (1962)]. Recent results show that a nonuniform steady-state electric field reaches axially into the plasma. Our results indicate that electrons are accelerated along magnetic field lines and decelerated by collisions. The point in space where the electron drift speed exceeds a critical speed (vc∼0.2vte) determines the instability amplitude and frequency. This initiation point moved further into the plasma as the externally controlled electric field was raised. This experiment demonstrates the importance of applying biases so that eφ/T≲1, near threshold, to observe the linear electrostatic ion cyclotron instability