Self-organization, anomalous resistance and anomalous heating in magnetized plasmas

Abstract

The nonlinear dynamics and structure of plasmas with tightly twisted magnetic field lines have been studied using a toroidal plasma device. Stepwise magnetohydrodynamic (MHD) relaxation occurs, resulting in a discontinuous change in the pitch of magnetic field lines. This discrete nature of the pitch stems from the instability of kink (torsional) modes. The MHD relaxation stabilizes kink modes by selecting (self-organizing) appropriate pitches. The self-organized state displays the characteristic of a ‘dissipative structure’ in that it is accompanied by enhanced energy dissipation; the global resistance of the plasma current is substantially enhanced. The magnetic energy, which is generated by the internal plasma current, first changes into fluctuation energy through the kink instability, and then it goes mainly to ion thermal energy through viscous dissipation of the fluctuating flow. The viscosity dissipates the fluctuation energy with conservation of helicity. The self-organization of the stabilized magnetic field is characterized by the preferential conservation of the helicity.

Tokamak, reversed field pinch and intermediate structures as minimum-dissipative relaxed states

“…Such observations of low q-equilibria have also been reported earlier in Repute-1 17 as well as in Toriut-6. 20 An effort to understand the equilibrium of a cylindrical plasma in the ULQ regime was earlier made by Yoshida et al, 21 by solving the force-free equation by assuming a nonconstant Lagrange multiplier . The present work is an attempt to understand the relaxation mechanisms that are operative in these low-q type of toroidal discharges as well as to present the different configurations within the framework of a single relaxation model as is often indicated by the experimental results.…”

3D nonlinear MHD simulations of ultra-low q plasmas

“…Also of interest is the efficiency of plasma heating by ohmic dissipation of the plasma current (note that the efficiency is particularly high since similar ion and electron temperatures, T i ∼ T e , are generally observed in those configurations [4][5][6][7]). These pros have historically motivated research in low q configurations such as the reversed field pinch (RFP), where q wall 0, and, somewhat discontinuously, on the intermediate configuration, the so-called 'ultra-low q' configuration, ULq, where 0 < q wall < 1 (see reviews on ultra-low q research in [4,8]).…”

“…In contrast, ultralow q operation is characterized by the poor tendency of nonlinear coupling among the modes. In fact, the sequence of emergence, saturation and healing of different instabilities occurs in well definite discrete steps which accompany macroscopic adjustments of the plasma current and edge value of the safety factor, q edge (see [4,8] and references therein). Therefore, in view of current mode control capabilities and of the interest in the study of transport properties of helical regimes in current carrying 'axisymmetric' devices, RFX-mod offers the opportunity of re-assessing ultra-low q operation and its possible advantages.…”

Charge-to-mass-ratio-dependent ion heating during magnetic reconnection in the MST RFP

“…Several theoretical models have been put forward to explain the ion heating mechanism. There are three models which are extensively discussed in relation to the observations in the reversed field pinch: viscous damping of the flows generated at reconnection, [15][16][17] stochastic heating, 14,18 and cyclotron damping of magnetic fluctuations 19,20 Viscous damping of flows requires high flow velocity (on the order of ion thermal velocity) and strong velocity gradient (on the order of ion gyro radius) to result in any significant ion TABLE I. Impurity ion species, atomic transition wavelengths and their charge-to-mass ratio (Z=l, where Z is the ionic charge and l is the ratio of the ion mass to the proton mass.). Impurity ion species Wavelength (Angstrom) Charge-to-mass ratio (Z/l) heating, which have not yet been observed experimentally.…”