Laser-Induced Fluorescence Measurement of the Ion-Energy-Distribution Function in a Collisionless Reconnection Experiment

Observations in space and laboratory plasmas suggest magnetic reconnection as a mechanism for ion heating and formation of non-Maxwellian ion velocity distribution functions (IVDF). Laser-induced fluorescence measurements of the IVDF parallel to the $X$ line of a periodically driven reconnection experiment are presented. A time-resolved analysis yields the evolution of the IVDF within a reconnection cycle. It is shown that reconnection causes a strong increase of the ion temperature, where the strongest increase is found at the maximum reconnection rate. Monte Carlo simulations demonstrate that ion heating is a consequence of the in-plane electric field that forms around the $X$ line in response to reconnection.

Figure 1

Chopper signal (top trace), PMT signal fluctuations (middle trace), and loop voltage $U_{\mathrm{loop}}$ and total plasma current $I_{\mathrm{plasma}}$ (bottom traces).

Figure 2

A typical VTF shot with approximately 750 reconnection cycles. (a) Time evolution of the toroidal plasma current amplitude ${\widehat{i}}_{\mathrm{plasma}}(t)$. (b) PMT fluctuation signal rms value with and without the reconnection drive (black and gray line, respectively).

Figure 3

(a) LIF measurements of the IVDF and the ion temperature. Error bars are obtained from the 95% confidence interval of the periodogram estimation. Gaussians are fitted to the measured data: without reconnection (gray diamonds) and with reconnection (black squares). (b) Residua (in %) show the goodness of the two least-square fits.

Figure 4

(a) Time evolution of the ion temperature $T_i$ during a VTF shot (cf. Fig. 2): without reconnection (gray diamonds) and with reconnection (black squares). Spline fits (dashed lines) are included to guide the eye. (b) Gray scale plot of the time evolution of the IVDF during a VTF shot. Contours of Gaussians fitted to the thermal component are indicated as solid lines.

Figure 5

(a) Phase resolved measurement of the ion temperature $T_i$ (black squares with spline fits). (b) Evolution of the IVDF during a reconnection cycle: Shown are the raw data (gray shaded plot) and contours of thermal component fitted by Gaussians (solid lines). The hatched area indicates the interval where no LIF signal is obtained.

Figure 6

Probe measurements of the perpendicular electric potential at two separate time instants, $t=12$ (a) and $t=48\mu \mathrm{s}$ (b). Pairs of magnetic field lines (dashed lines) are plotted bold to indicate the poloidal motion of the magnetic flux. (c) Calculated orbit of a ion (going left to right) in the in-plane fields at the $X$ line. (d) Histogram of the energy distribution of ions with initial energy $f(\mathcal{E})=0.3\mathrm{eV}$ (solid line) after passing the $X$ line once.