Ducted Alfv\'{e}n Waves in Helicon Plasmas

Recent data from \textit{Hinode} spacecraft, the new high resolution solar imaging mission [\textit{Science}, Dec. 2007], has provided strong evidence for the presence of Alfv\'{e}n waves in Sun's corona and coronal loops. Damping of Alfv\'{e}n waves is a strong candidate for explaining the million degree corona. The high density and steep density profile of a typical helicon plasma source makes helicon plasmas a good analog to coronal loop plasma conditions (for example $V_{A-HELIX} /V_{A-Corona} \approx 0.1-1$, $\beta _{HELIX} /\beta _{Corona} \approx 0.1-10)$. Here we present observations of Alfv\'{e}n waves launched via amplitude modulation of the helicon RF antenna in HELIX (Hot hELIcon eXperiment). Plasma parameters include an ion gyro radius to system length ratio of $\rho _i /L\approx 0.01-0.1$, and electron and ion skin depth to system size ratios of $\delta _e /L\approx 0.04-0.4$ and $\delta _i /L\approx 2-100$ (corresponding coronal values for the same parameters are 0.05-0.2, 0.01-0.1 and 1-100, respectively). The waves are excited at sub-cyclotronic frequencies in argon and helium plasmas. Phase velocity and amplitude measurements in the high density region are reported and compared to an Alfv\'{e}n wave model that includes the effect of a strong density gradient.