Improved Particle Confinement in Transition from Multiple-Helicity to Quasi-Single-Helicity Regimes of a Reversed-Field Pinch

The quasi-single-helicity (QSH) state of a reversed-field pinch (RFP) plasma is a regime in which the RFP configuration can be sustained by a dynamo produced mainly by a single tearing mode and in which a helical structure with well-defined magnetic flux surfaces arises. In this Letter, we show that spontaneous transitions to the QSH regime enhance the particle confinement. This improvement is originated by the simultaneous and cooperative action of the increase of the magnetic island and the reduction of the magnetic stochasticity.

Figure 1

Example of intermittent QSH: time evolutions of (a) plasma current, (b) dominant mode, and root mean square of secondary modes, (c) $N_s$ and (d) $I_{\mathrm{SXR}}/n_e^2$, and total radiation $P_{\mathrm{rad}}$ measured by the central chord of the bolometer. Frame (e) shows the radial profile of $I_{\mathrm{SXR}}/n_e^2$ at $t_1$ during MH and at $t_2$ during the QSH regime. The vertical dashed line represents the radial position of the island.

Figure 2

Tomographic SXR images at (a) $t_1$ during the MH state and (b) $t_2$ during the QSH state.

Figure 3

Time evolutions of (a) core electron density $n_e$ and peaking factor $P$, (b) integrated $D_{\alpha }$ radiation, (c) particle confinement time ${\tau }_p$. Gray areas highlight the QSH states.

Figure 4

Poincaré sections of the magnetic fields (black points) in the MH (a),(c) and QSH (b),(d) cases. Overplotted, as a colored contour, the values of the loss time ${\tau }_L$ for ions (a),(b) and electrons (c),(d).

Figure 5

Correlations of (a) ${\tau }_p$ and (b) $T_e$ versus $N_s$. Correlation of ${\tau }_p$ versus (c) the dominant mode and (d) the rms of the secondary modes for MH (○) and QSH (●).