Anisotropic Electron Tail Generation during Tearing Mode Magnetic Reconnection

The first experimental evidence of anisotropic electron energization during magnetic reconnection that favors a direction perpendicular to the guide magnetic field in a toroidal, magnetically confined plasma is reported in this Letter. Magnetic reconnection plays an important role in particle heating, energization, and transport in space and laboratory plasmas. In toroidal devices like the Madison Symmetric Torus, discrete magnetic reconnection events release large amounts of energy from the equilibrium magnetic field. Fast x-ray measurements imply a non-Maxwellian, anisotropic energetic electron tail is formed at the time of reconnection. The tail is well described by a power-law energy dependence. The expected bremsstrahlung from an electron distribution with an anisotropic energetic tail ($v_{\perp }>v_{\parallel }$) spatially localized in the core region is consistent with x-ray emission measurements. A turbulent process related to tearing fluctuations is the most likely cause for the energetic electron tail formation.

Figure 1

(a) Evolution of $U_{\text{mag}}$ for two magnetic relaxation cycles in a 500 kA standard plasma in MST. (b) Evolution of tearing mode amplitudes for the edge-resonant $m=0$, $n=1$ (red, solid) mode and the (innermost) core-resonant $m=1$, $n=6$ (blue, dashed) mode. The insets show the magnetic energy (top) and tearing mode amplitudes (bottom) averaged over 485 events (shaded region represents the standard error of the mean), with time relative to MR.

Figure 2

A schematic of the FXR detector views.

Figure 3

The evolution of x-ray energy relative to MR (0 ms), with colors indicating x-ray flux. Black indicates no flux and dark red indicates high flux.

Figure 4

X-ray spectra measured in the radial view for $20\mu \mathrm{s}$ windows 0.5 ms before (black), during (red) and 0.5 ms after (blue) MR. Each spectrum is fit with a power law (solid lines), from which ${\gamma }_{\perp }$ is calculated. The inset shows ${\gamma }_{\perp }$ as a function of time relative to MR.

Figure 5

(a) EDF used in cql3d to model parallel anisotropy for a power-law tail distribution in $v_{\parallel }$. (b) Modeled bremsstrahlung x-ray emission for toroidal parallel (black), toroidal antiparallel (red), and radial (blue) pencil-like lines of sight. (c) EDF used in cql3d to model perpendicular anisotropy for a power-law tail distribution localized in $v_{\perp }$. (d) Modeled bremsstrahlung x-ray emission for the same lines of sight as in (b). For both cases, the tail is Gaussian and $v_{\parallel }$ and $v_{\perp }$ are normalized to $v_{\text{norm}}=1.9\times {10}^8\mathrm{m}/\mathrm{s}$. The modeled emission is normalized for volume of viewing cones through the plasma core.

Figure 6

X-ray spectra measured for parallel (black), antiparallel (red) and radial (blue) views during a $20\mu \mathrm{s}$ window during MR. The inset shows the evolution of $\gamma$ for the parallel (black) and radial (blue) views.