Laboratory Realization of an Alfvén Wave Maser

It is demonstrated that a frequency selective Alfvén-wave resonator can be realized by applying a nonuniform magnetic field to a plasma region bounded between a cathode and a semitransparent mesh anode. When a current threshold is exceeded, selective amplification results in a highly coherent, large amplitude wave that propagates into an adjacent plasma column.

Figure 1

The broken curve is the ambient noise frequency spectrum at the plasma center at $z=4.9\mathrm{m}$, $B_0=1000\mathrm{G}$. The peak near $0.6f_{\mathrm{c}\mathrm{i}}$ corresponds to the first resonant Alfvén mode within the cathode-anode cavity. The quenching at higher frequencies results from the nonuniform magnetic field in the cavity. The solid curve is the prediction of a model consisting of partial wave reflections between cathode and anode.

Figure 2

Middle panel: single-shot temporal evolution of the horizontal component of the magnetic field. Center of the plasma, $z=8.75\mathrm{m}$, $B_0=700\mathrm{G}$. Bottom panel: time dependence of current injected by cathode into plasma column. Top panel: expanded temporal behavior showing remarkable coherency. Frequency corresponds to that of the mode with a spectral peak around $0.6f_{\mathrm{c}\mathrm{i}}$ in Fig. 1.

Figure 3

Spatial properties of flaring mode. The lower curve is the dependence of spectral amplitude on the horizontal position across the plasma column (the plasma center is at 32 cm). The hodograms show transverse polarization at the various horizontal positions.

Figure 4

A slow temporal decrease in the frequency of flaring mode is shown. The time traces of the $Bx$ component are compared starting 0.2 ms before the peak of the flare (solid curve), and starting 0.7 ms after the peak (dotted curve).

Figure 5

Flare model in which instability due to electron drift drives a shear Alfvén wave that experiences partial reflection. The solid curve is the resonant phase difference of the mode. Broken curves are proportional to the negative of the slope of the electron distribution function evaluated at the phase velocity for a given frequency. The numbers correspond to the current injected at the cathode.