Experimental study of fast hydromagnetic waves in an argon plasma

Abstract

In these experiments fast hydromagnetic waves are excited by discharging a capacitor through a single turn coil surrounding a cylindrical column of magnetized argon plasma. The plasma column is 200 cm long and 22 cm in diameter, and the axial magnetic field strength is varied in the range from 1 to 6 kG. The wave amplitude is typically 10 G, and the frequency is varied between 1$cdot$2 and 6 times the ion cyclotron frequency. Measurement of the radial variation and the relative amplitudes of the three components of the wave magnetic field shows that the oscillation is the lowest axially-symmetric mode. As predicted by the theory, the wave is elliptically polarized in the r$\theta$ plane with the magnetic vector rotating in the same sense as the electron cyclotron rotation. The experimental results demonstrate the cut-off of this mode both as the frequency is decreased and as the axial magnetic field strength is increased. Measurements of the axial wave number and absorption coefficient are in good numerical agreement with theoretical dispersion curves computed from the measured plasma parameters. This work provides quantitative evidence to support the theories currently used in treating hydromagnetic oscillations, both stable and unstable, of magnetized plasmas.