Spontaneous Formation of Closed-Field Torus Equilibrium via Current Jump Observed in an Electron-Cyclotron-Heated Plasma

Spontaneous current jump resulting in the formation of closed field equilibrium has been observed in electron-cyclotron-heated toroidal plasmas under steady external fields composed of a toroidal field and a relatively weak vertical field in the low aspect ratio torus experiment device. This bridges the gap between the open field equilibrium maintained by a pressure-driven current in the external field and the closed field equilibrium at a larger current. Experimental results and theoretical analyses suggest a current jump model that is based on the asymmetric electron confinement along the field line appearing upon simultaneous transitions of field topology and equilibrium.

Figure 1

Time evolution of typical discharge. (a) Plasma current, (b) line densities, (c) hard x-ray intensity, (d)–(k) contour plots of current distribution and poloidal flux (the center of the plasma current is denoted by +), (l)–(r) visible light plasma images. Three-dimensional field lines for the cases of (d) and (j) are shown in (s) and (t), respectively.

Figure 2

Various characteristic currents for current jump versus $B_{\mathrm{v}}$.

Figure 3

Equilibrium characteristics for various aspect ratios.

Figure 4

(a) Temporal evolutions of plasma current ($I_{\mathrm{p}}$), electron pressure ($p_{\mathrm{e}}$), (b) electron temperature ($T_{\mathrm{e}}$), and (c) density ($n_{\mathrm{e}}$). (d) Relationship of $I_{\mathrm{p}}{R}_{\mathrm{p}}/p_{\mathrm{e}}$ versus $B_{\mathrm{v}}$ at the initial discharge stage. $R_{\mathrm{p}}$ is the plasma major radius.

Figure 5

Poloidal flux surfaces and confined areas in the velocity space for various levels of the self field.