Destruction of a Magnetic Mirror-Trapped Hot Electron Ring by an Alfv\'{e}n Wave

Highly energetic electrons produced naturally or artificially can be trapped in the earth's radiation belts for months, posing a danger to valuable space satellites. Concepts that can lead to mitigation of the radiation belts have drawn a great deal of interest. In this work, we demonstrated that a shear Alfv\'{e}n wave can effectively de-trap energetic electrons confined by a magnetic mirror field.\footnote{Y. Wang, W. Gekelman, P. Pribyl, and K. Papadopoulos, Phys. Rev. Lett. 108, 105002 (2012).} The experiment is performed in a quiescent afterglow plasma in the Large Plasma Device (LaPD) at UCLA ($n_e =0.1-1\times 10^{12}/cm^3$, $T_e =0.5eV$, $B_0 =400-1600G$, $L=18m$, and $diameter=0.6m)$. A hot electron ring (along with hard x-rays of energies up to 3 MeV) is generated by 2nd harmonic ECRH ($P=25kW$, $\tau _{pulse} =20-50ms$, $f=2.45GHz)$ and is trapped in a magnetic mirror field ($L=3m$, $R_{mirror} =1.1-4)$. A shear Alfv\'{e}n wave ($f\approx 0.5f_{ci} $, $B_{wave} /B_0 \approx 0.1\% )$ is launched with a rotating magnetic field antenna with arbitrary polarity. The circularly polarized Alfv\'{e}n wave is observed to dramatically scatter the trapped fast electrons out of the mirror within as little as 10 wave cycles. A collimated detector outside the vacuum vessel detects X-rays of E$>$100 keV. X-ray images are reconstructed from more than 1000 chord projections at each axial location by computed tomography. The de-trapped electrons are observed outside the mirror field region.