Behavior of magnetic field fluctuations during dynamo activity and its effect on energy confinement in a reversed-field pinch

Abstract

Fluctuations of magnetic fields and related magnetohydrodynamic (MHD) phenomena are investigated in the TPE-1RM15 reversed-field pinch experiment [Plasma Physics and Controlled Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. 2, p. 453]. Mode analysis of fluctuations measured by multichannel coils reveals that nonlinear interactions between m=1 and m=0 modes, such as nonlinear coupling and phase locking, play significant roles during a dynamo event (i.e., the flux genertion process in the sustainment phase), resulting in transition from an unstable state to a stable state. Behaviors of these fluctuations are found to be toroidally asymmetrical due to strong nonlinearity. Study of the current ramping experiment shows that the inverse of global energy confinement time depends on the squared fluctuation level offset linearly, which is consistent with the prediction of the transport model based on the diffusion of stochastic field lines. By examining the dependence of the resistive part of the loop voltage on the fluctuation level, the input power to the electrons and ions are estimated to be about 70% and 30% of the total input power, respectively.

Phase locking of multi-helicity neoclassical tearing modes in tokamak plasmas

“…This behavior is reminiscent of the socalled "slinky mode" generated by the nonlinear phase locking of tearing modes in reversed-field pinches. [39][40][41][42] V. TOROIDAL COUPLING OF TWO NEOCLASSICAL TEARING MODES…”

Scaling of Local Core Transport with Lundquist Number in the Reversed Field Pinch

“…Besides the simplest theoretical prediction based on the dimensionless Ohm's law,b B~S 21͞2 , quite a wide effort has been dedicated to investigate the behavior of magnetic fluctuations in the RFP [4][5][6][7][8][9][10][11][12]: exponents a for theb B~S 2a scaling law have been found ranging from a ഠ 0.07 to a ഠ 0.5. In particular, in a recent paper Stoneking et al [11] have extended the range of S over which the scaling is experimentally studied and they have proposed a narrow range of measured exponents a, i.e., 0.07 # a # 0.18, depending on the density regime.…”

Model for relaxation in reversed-field pinch plasmas

“…29 The observations of discrete relaxation in the RFP plasmas showed that the relaxation may go on in an oscillatory manner. The loop voltage, reversal parameter, and electron current all have sawtooth character [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] during the relaxation. The loop voltage and fluctuation amplitudes peak at the sawtooth crash.…”

“…63. Since the MHD dynamo action was introduced into the RFP plasma, 55 the MHD dynamo theory has enjoyed considerable success in the physics of the laboratory plasma and is well supported by the experimental phenomenology [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] and the numerical simulations. 66 -77 It is of great importance to the physics of the RFP plasma that the MHD dynamo electric field and mean-field Ohm's law have been verified in the recent experiment.…”

“…30 The cycle goes on and forms the discrete relaxation process which is probably responsible for the ''discrete dynamo events'' observed in the experiments. 15,[30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] Now we discuss the fluid velocity U. The kinetic energy is dissipated by the viscosity, especially, the turbulent viscosity.…”