Transport reduction by current profile control in the reversed-field pinch

Abstract

An auxiliary poloidal inductive electric field applied to a reversed-field pinch (RFP) plasma reduces the current density gradient, slows the growth of m = 1 tearing fluctuations, suppresses their associated sawteeth, and doubles the energy confinement time. This experiment attacks the dominant RFP plasma loss mechanism of parallel streaming in a stochastic magnetic field. The auxiliary electric field flattens the current profile and reduces the magnetic fluctuation level. Since a toroidal flux change linking the plasma is required to generate the inductive poloidal electric field, the current drive is transient to avoid excessive perturbation of the equilibrium. To sustain and enhance the improved state, noninductive current drivers are being developed. A novel electrostatic current drive scheme uses a plasma source for electron injection, and the lower-hybrid wave is a good candidate for radio-frequency current drive. 0 1995 American Institute of Physics. INTRODUCTlONIn the reversed-field pinch (REP), the loss of plasma results primarily from particle convection along stochastic magnetic field lines generated by large-amplitude magnetohydrodynamic (MHD) fluctuations. Measurements',2 of the magnetic-fluctuation-induced electron particle and heat losses in the Madison Symmetric Torus3 (MST) directly identify large transport associated with the magnetic fluctuation, while in other RPP experiments,4 the estimated magnetic-fluctuation-induced energy loss can account for the observed global energy flux. In MST, the measured fluxes agree with expectations for convective stochastic magnetic field diffusion,' but the electron loss occurs at the ion rate as a result of an ambipolarity constraint on the particle flux, i.e., an outward pointing electric slows the electron loss.More than 90% of the RPP magnetic fluctuation l? results from several poloidal mode number m = 1, toroidal mode number n -2Rla core-resonant tearing (or resistive kink) instabilities. The amplitudes of these fluctuations are typically -1% of the mean field,6 and the close spatial proximity of their resonant magnetic surfaces encourages magnetic island overlap and stochasticity. Since the dominant plasma loss results from this stochasticity, researchers proposed methods for reducing the fluctuation with hope of improving RFP confinement. Tearing fluctuation stems from the current density gradient, so the proposals employ auxiliary electrostatic7 or radio-frequency (RF)8V9 poloidal current drive in the outer region of the plasma, eliminating the need for fluctuation-dynamo sustainment of the RFP These theoretical and computational studies demonstrate reduction of the tearing fluctuations and the restoration of closed magnetic surfaces in the core of the plasma.In this paper the first observation of reduced transport resulting from current profile control in a RFP is presented. The experimental technique employs auxiliary inductive poloidal current drive. Unlike electrostatic or RF current drive, poloidal inductive current drive is inherently transient...

E×B flow shear and enhanced confinement in the Madison Symmetric Torus reversed-field pinch

“…Note that these events occur during PPCD discharges as well ͑as will be shown below͒. 28 To further illustrate the reduction of electrostatic fluctuations in EC discharges, we plot standard and EC electrostatic fluctuation power spectra in Fig. 5.…”

High confinement plasmas in the Madison Symmetric Torus reversed-field pinch

“…With the goal of modifying this gradient in Madison Symmetric Torus ͑MST͒ 2 reversedfield-pinch ͑RFP͒ plasmas, inductive auxiliary parallel current drive was applied at the plasma boundary. [3][4][5][6] Comprised of a poloidal electric field induced by a transient change in the toroidal flux in the plasma, this technique resulted in a reduction of the mϭ1 fluctuations and an increase in the global energy confinement time to 5 ms from the MST standard 1 ms.…”

“…First, we now know that the previously achieved mϭ1 fluctuation reduction and confinement improvement were limited by bursts of edge-resonant mϭ0 magnetic fluctuations that were observed in conjunction with the auxiliary current drive. [3][4][5][6][7][8] The mϭ1 fluctuations account for the bulk of energy transport in standard MST plasmas. 9 Thus, the mϭ0 fluctuations have not previously been considered to play a significant role in transport ͑and their exact role is still not established͒.…”

Equilibrium evolution in oscillating-field current-drive experiments

“…Similar m =0 activity can occur in nonoptimized inductive current-profile control experiments when the applied E ʈ ͑a͒ is made larger than needed to suppress fluctuations. 31 Perhaps this intersawtooth activity corresponds to linear instability of edge-resonant m = 0 modes, unlike RFP sawtooth activity, where the m = 0 are nonlinearly driven by coreresonant m = 1 modes. 30 The m = 0 are typically linearly stable in a RFP with a sufficiently close-fitting conducting shell like MST's.…”