Particle-Transport Analysis in Reversed Field Pinch Helical States

This Letter reports the result of a numerical study of particle transport in self-organized single helicity (SH) and quasi-SH reversed field pinch plasmas. Our code, benchmarked against experimental data, predicts a large improvement in particle transport for SH compared to the standard multiple helicity states. The contribution of neoclassical effects is noted. An estimate of the ambipolar electric field in helical states and in fully stochastic magnetic fields is given.

Figure 1

RFX Shot #11183 @ $t=42\mathrm{m}\mathrm{s}$: $b_{\varphi }$ mode spectrum (a) with the dominant radial eigenfunction (b), and comparison between Poincaré map (c) and SXR emissivity pattern (d).

Figure 2

Topology-induced transport asymmetries: loss time versus poloidal angle for ions (a) and electrons (b), with loss border at $r_L=28.9\mathrm{c}\mathrm{m}$ (${\psi }_p=0.062$), in MH (c), deposition at $r_{\mathrm{d}\mathrm{e}\mathrm{p}}=18.3\mathrm{c}\mathrm{m}$ (dashed line), QSH (d), $r_{\mathrm{d}\mathrm{e}\mathrm{p}}=18.3\mathrm{c}\mathrm{m}$, and SH (e), $r_{\mathrm{d}\mathrm{e}\mathrm{p}}=18.8\mathrm{c}\mathrm{m}$.

Figure 3

$\theta =0$ section of collisionless ion orbits (a) and time evolution of the poloidal angle (b). ▲, ▽, and ● symbols indicate poloidally trapped (initial pitch ${\lambda }_0=0.1$), helically trapped (${\lambda }_0=0.4$) and passing (${\lambda }_0=0.9$) orbits.

Figure 4

Ambipolar fields. The radial electric potential in MH (a),(b) causes quasineutrality for $k\simeq 0.18\mathrm{k}\mathrm{e}\mathrm{V}$ (deposition at $r_{\mathrm{d}\mathrm{e}\mathrm{p}}=17.40\mathrm{c}\mathrm{m}$, loss border at $r_L=22.44\mathrm{c}\mathrm{m}$). In SH with a helical potential (c),(d) quasineutrality is obtained for $k_{\mathrm{S}\mathrm{E}\mathrm{P}}\sim 0.11\mathrm{k}\mathrm{e}\mathrm{V}$ ($r_{\mathrm{d}\mathrm{e}\mathrm{p}}=18.60\mathrm{c}\mathrm{m}$, $r_L=19.90\mathrm{c}\mathrm{m}$).