Observation of Ion Cyclotron Waves in a Hot Plasma

Abstract

According to the theory of the propagation and absorption of ion cyclotron waves in a hot magnetized plasma, the wavelength approaches a minimum, nonzero value as the wave propagates into a magnetic beach where the wave frequency approaches the local ion cyclotron frequency. Furthermore, the damping of the waves is a sharply resonant function of the magnetic field strength, the width of the resonance being proportional to the square root of the ion temperature for motion parallel to the static magnetic field. These effects are observed in a plasma confined in a linear magnetic mirror geometry for perpendicular ion temperatures in the range 10–110 eV. The wave fields are measured with two magnetic probes, the plasma density is measured with two microwave interferometers, and the perpendicular ion temperature is measured with a diamagnetic probe. The theory is developed for the transport of spatially damped waves in a hot, finite plasma immersed in a slightly inhomogeneous magnetic field. Quantitative agreement between theory and experiment is obtained. These measurements represent the first experimental observations of thermally broadened collisionless cyclotron absorption in a laboratory plasma. Observations of collisional damping of ion cyclotron waves are also presented.