Efficient generation of variable, high k⊥, kinetic Alfvén waves

In laboratory studies of shear Alfvén waves, control of the perpendicular wavenumber is an important tool for testing dispersion relations, particle scattering, plasma heating, etc. Efficient generation (wave amplitude per unit driver current) is also desirable for the lab and spacecraft. Generally, time harmonic induced electric fields aligned with the background magnetic field will excite shear waves. These can be produced by biasing electrodes or through the use of rotating magnetic field (RMF) antennas[1]. The field generated by RMF antennas is proportional to the driving current. In this work, field enhancement was accomplished when a ferrite core was inserted in the loop. A shear wave antenna (μ= 80, 32 turns) launched a shear wave (f/fci = 0.65) with 80 times the amplitude than an identical antenna without a ferrite core. Two or more ferrite based antennas have launched shear waves with kperp/kparallel = 98 in the kinetic regime (v_A>v_Te). This work reports the use of multiple loops driven with phase-shifted currents in the Large Plasma Device at UCLA. Each loop produces an Alfvén wave cone enhanced in amplitude by the ferrite. Importantly, the orientation of the various loops can be easily varied in order to control the spectrum of launched perpendicular wavelengths. The resulting interference patterns, wave amplitudes, and wavenumber spectra are presented.

These experiments are conducted in the Large Plasma Device at UCLA's Basic Plasma Science Facility and is jointly funded by the U.S. DoE and NSF. Additional support provided by an AFSOR MURI award.

[1] A. Gigliotti et al, Phys. Plasmas, 16 (2009)