The RFP dynamo: MHD to kinetic regimes

The hallmark of magnetic relaxation in an RFP plasma is profile flattening of ${\bf J_0\cdot B_0}/B^2$ effected by a dynamo-like emf in Ohm's law. This is well-studied in single-fluid MHD, but recent MST results and extended MHD modeling show that both $<\bf{V_1\times B}_1>$ and the Hall emf, $-<{\bf J_1\times B_1}>/en_e$, are important, revealing decoupled electron and ion motion. Since dynamo is current-related, the electron fluid emf, $<{\bf V_{e,1}\times B_1}>$, captures both effects. In MST, the electron flow is dominantly ${\bf V_{e,1}\approx E_1\times B_0}/B^2$, implying $<{\bf V_{e,1}\times B_1}>\approx<{\bf E_1\cdot B_1}>/B$. This and the Hall emf are measured in MST for comparison in Ohm's law. A finite-pressure response is also possible, e.g., ``diamagnetic dynamo", $\nabla\cdot< p_{e,1} {\bf B_1}>/en_e$, associated with diamagnetic drift, and ``kinetic dynamo" associated with collisionless streaming of electrons in a stochastic magnetic field. Correlation measurements $$ and $$ using FIR interferometry and Thomson scattering reveal these as small but finite in MST. A kinetic emf might be expected for any high-beta plasma with inhomogeneous pressure. Support by DOE/NSF