Laboratory Study of Magnetic Reconnection with a Density Asymmetry across the Current Sheet

The effects of a density asymmetry across the current sheet on anti-parallel magnetic reconnection are studied systematically in a laboratory plasma. Despite a significant density ratio of up to 10, the in-plane magnetic field profile is not significantly changed. On the other hand, the out-of-plane Hall magnetic field profile is considerably modified; it is almost bipolar in structure with the density asymmetry, as compared to quadrupolar in structure with the symmetric configuration. Moreover, the ion stagnation point is shifted to the low-density side, and the electrostatic potential profile also becomes asymmetric with a deeper potential well on the low-density side. Nonclassical bulk electron heating together with electromagnetic fluctuations in the lower hybrid frequency range is observed near the low-density-side separatrix. The dependence of the ion outflow and reconnection electric field on the density asymmetry is measured and compared with theoretical expectations. The measured ion outflow speeds are about 40% of the theoretical values.

Figure 1

(a) A cross section of MRX. The flux core contains both the PF coil for driving magnetic reconnection and the TF coil for creating the plasma. Magnetic probes are inserted to monitor the evolution of the 2D magnetic geometry. (b) Schematic view of the ion dynamics during the plasma formation period. The blue arrows along the flux cores indicate the direction of the TF coil current. The red arrows between the flux cores stand for the ion flow vectors. (c) Radial electron density profiles at $Z=0$ for both asymmetric and symmetric cases early in the quasisteady period. The ion skin depth of the asymmetric case based on the low-density side is about 13 cm, while that of the symmetric case is 5 cm. (d) Radial profiles of the reconnecting magnetic field component ($B_Z$) at $Z=0$ early in the quasisteady period. For the asymmetric case, the ion gyroradius is 5 cm, and the plasma $\beta$ is about 0.14 on the low-density side. These quantities are not available for the symmetric case due to the lack of an ion temperature measurement in deuterium plasmas.

Figure 2

2D profiles of the out-of-plane magnetic field ($B_Y$) with contours of the poloidal flux for asymmetric (a) and symmetric (b) cases. Compared to the symmetric case, the Hall magnetic field component is enhanced on the high-density side ($R>37.5\mathrm{cm}$) and suppressed on the low-density side ($R<37.5\mathrm{cm}$). Black lines indicate contours of the poloidal magnetic flux which represent magnetic field lines. In-plane ion flow vector profiles for asymmetric (c) and symmetric (d) cases. Red crosses indicate the radial location of $V_R=0$ at $Z=0$. For the asymmetric case, the ion inflow stagnation point is shifted to the low-density side. The density ratio ($n_1/n_2$) for the asymmetric case is about 6, while it is about 1.2 for the symmetric case.

Figure 3

Radial plasma potential profiles at $Z=0$ for asymmetric (blue) and symmetric (red) cases. With the density asymmetry, the potential profile becomes asymmetric with a larger well depth on the low-density side.

Figure 4

(a) 2D electron temperature profile for the asymmetric case. Strong bulk electron heating exists near the low-density-side separatrix. The magenta asterisk indicates the measurement location of high-frequency fluctuations. (b) Time trace of typical fluctuations in $E_Y$ (Discharge 138 796). (c) Fourier spectrum of $\delta E_Y$ averaged over 12 discharges.

Figure 5

(a) Measured ion outflow velocity compared to the hybrid Alfvén velocity. The measured values are about 40% of theoretically predicted values. The black dashed line indicates a linear fit. (b) The measured reconnection electric field versus $E_{\text{th}}$ in Eq. (3). Values of $E_{\text{ms}}$ agree with those of $E_{\text{th}}$ when the measured outflow density, $n_{\text{out},\mathrm{ms}}$ (blue asterisks) is used as $n_{\text{out}}$ instead of the theoretical estimate, $n_{\text{out},\mathrm{th}}$ (red circles).