Experimental Demonstration of Hydromagnetic Waves in an Ionized Gas

Abstract

A regular hydromagnetic oscillation was excited in a linear discharge tube with metal walls. An externally applied axial magnetic field and discharge currents less than 104 amperes were essential to the production of the oscillation. Complete spatial current distributions derived from measurements of the radial and time dependence of the three components of magnetic field showed a helical notch (screw thread) of reduced current density which rotated with uniform angular velocity. The pitch and apparent rotational frequency of the oscillation depended on the gas density, discharge current, and applied axial magnetic field; the right- or left-handedness and rotational sense depended on the relative orientation of the applied axial magnetic field and the self field of the discharge current. The helical regularity could be described as a superposition of hydromagnetic waves traveling in the axial and circumferential directions.

The many faces of shear Alfvén waves

“…In yet another early experiment by Sawyer et al, 23 hydrodynamic waves were observed in a linear discharge of 60 cm long and 15 cm in diameter. There was an axial magnetic field of 500 G. The plasma was produced with a 200 kJ capacitor bank in relatively high (0.1-10 mm) pressure deuterium.…”

Laboratory experiments on shear Alfvén waves and their relationship to space plasmas

“…A standing wave of the appropriate frequency was observed by Bostick and Levine [1952] in a pulsed toroidal device. Subsequent experiments in toroidal [Jephcott, 1959] and linear devices [Sawyer et al, 1959;Wilcox et al, 1960] measured the wave phase velocity from the phase shift of detected wave magnetic fields and verified its dependence on magnetic field strength and plasma density. The dispersion of the shear wave was measured by Jephcott and Stocker [1962] in a cylindrical column.…”

Experimental observations of shear Alfvén waves generated by narrow current channels

“…The properties of Alfvén waves have been the subject of several laboratory investigations (Bostick and Levine 1952, Jephcott 1959, Sawyer et al 1959, Wilcox et al 1960, Jephcott and Stocker 1962, Cross and Lehane 1967a, b, Lehane and Paoloni 1971, Muller 1973, Cross 1988). The first experiments on Alfvén waves were performed in pinches and arcs which produce high-density plasmas, but suffer from high noise levels, large shot-to-shot variability and high collision rates.…”