An Alfvén wave maser in the laboratory

Abstract

A frequency selective Alfvén wave resonator results from the application of a locally nonuniform magnetic field to a plasma source region between the cathode and anode in a large laboratory device. When a threshold in the plasma discharge current is exceeded, selective amplification produces a highly coherent (δω∕ω<5×10−3), large amplitude shear Alfvén wave that propagates out of the resonator, through a semitransparent mesh anode, into the adjacent plasma column where the magnetic field is uniform. This phenomenon is similar to that encountered in the operation of masers∕lasers at microwave and optical frequencies. The current threshold for maser action is found to depend upon the confinement magnetic field strength B0. Its scaling is consistent with the condition for matching the drift speed of the bulk plasma electrons with the phase velocity of the mode in the resonator. The largest spontaneously amplified signals are obtained at low B0 and large plasma currents. The magnetic fluctuations δB associated with the Alfvén maser can be as large as δB∕B0≈1.5% and are observed to affect the plasma current. Steady-state behavior leading to coherent signals lasting until the discharge is terminated can be achieved when the growth conditions are well-above threshold. The maser is observed to evolve in time from an initial m=0 mode to an m=1 mode structure in the transition to the late steady state. The laboratory phenomenon reported is analogous to the Alfvén wave maser proposed to exist in naturally occurring, near-earth plasmas.