Ion temperatures have been measured in the Madison Symmetric Torus (MST) [Dexter et aL, Fusion Technol. 19, 131 ( 199 1 j] reversed-field pinch (RFP) with a five channel charge exchange analyzer. The characteristic anomalously high ion temperature of RFP discharges has been observed in the MST. The ion heating expected from ion-electron collisions is calculated and shown to be too small to explain the measured ion temperatures. The charge exchange determined ion temperature is also compared to measurements of the thermally broadened CV 227.1 nm line. The ion temperature, Ti~250 eV for 1=360 kA, increases by more than 100% during discrete dynamo bursts in MST discharges. Magnetic field fluctuations in the range OS-5 MHz were also measurd during the dynamo bursts. Structure in the fluctuation frequency spectrum at the ion cyclotron frequency suggests that the mechanism of ion heating involves the dissipation of dynamo fluctuations at ion cyclotron frequencies.

Most Madison Symmetric Torus ͑MST͒ ͓Fusion Technol. 19, 131 ͑1991͔͒ reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2-5 ms, corresponding to magnetohydrodynamic ͑MHD͒ instability and increased transport. However, in discharges where the plasma-facing wall has been boronized, the plasma resistivity is reduced, and sawteeth are often suppressed for periods up to 20 ms. The energy confinement time during these sawtooth-free periods is triple the normal value, corresponding to a higher plasma temperature and lower Ohmic input power. In addition, the steady growth of the dominant magnetic fluctuations normally observed between sawtooth crashes is absent. © 1996 American Institute of Physics. ͓S1070-664X͑96͒02703-5͔Sawtooth oscillations have been observed in several reversed-field pinch ͑RFP͒ experiments. 1-3 Similar to the case in tokamaks, the RFP sawtooth crash coincides with a sudden transfer of energy from the core plasma to the edge and is interpreted as magnetohydrodynamic ͑MHD͒ relaxation. During the slow sawtooth rise phase in the RFP, the profile, where (r)ϭ 0 J-B/B 2 , peaks as a result of resistive diffusion and the peaked parallel equilibrium inductive electric field. The amplitudes of the dominant poloidal mode number mϭ1 magnetic fluctuations ͑or modes͒, resonant in the plasma core, gradually increase during this phase. The faster sawtooth crash phase results in the redistribution of parallel current, flattening (r). The crash is a discrete dynamo event, during which edge current is driven, and toroidal flux is generated. Sawteeth degrade confinement in two ways. First, the growth of magnetic fluctuations increases transport. Second, plasma-wall interaction, most prominent in the crash phase, injects radiative impurities into the plasma. Hence, control of sawtooth oscillations and understanding of their origin is essential.In the Madison Symmetric Torus ͑MST͒ RFP, 4 sawteeth with periods of several ms have, until recently, accompanied nearly all discharges. 5 In this paper we report the production of discharges, following boronization 6 of the plasma-facing wall, in which sawteeth are absent for extended periods ͑up to 20 ms͒. We believe that these sawtooth-free periods ͑SFP͒ are due primarily to decreased current diffusion ͓and slowed peaking of (r)͔ arising from a reduced plasma resistivity. During SFP we observe that ͑1͒ the energy confinement time triples relative to normal sawtoothing discharges, and ͑2͒ the steady growth of the dominant mϭ1 modes normally observed between sawtooth crashes is absent. Also observed in SFP ͑and in some boronized discharges without SFP͒ are discrete bursts of mϭ0, toroidal mode number nϭ1 mode activity, each corresponding to a small increase in the toroidal flux.Shown in Fig. 1 are data from a typical MST discharge in which sawteeth occur throughout. Each sawtooth crash causes a rapid increase in the overall toroidal magnetic flux ͓Fig. 1͑a͔͒, while enhancing the reversal of the toroidal magnetic field at the edge ͓Fig. 1͑b͔͒. Increased ...

Current drive using the lower-hybrid slow wave is shown to be a promising canflidale for improving cGnfinement properties of a reversed field pinch (RFP). Ray-tracing calculations indicate that the wave will make a few poloidal turns while spiraling radially into a target zone inside the reversal layer. The poloidal antenna wavelength of the lower hybrid wave can be chosen so that efficient parallel current drive will occur mostly in the poloidal direction in this outer region. Three-dimensional resistive magnetohydrodynamic (MHD) computation demonstrates that an additive poloidal current in this region will reduce the magnetic fluctuations and magnetic stochasticity.

The total magnetic fluctuation-induced electron thermal flux has been determined in the Madison Symmetric Torus ͑MST͒ reversed-field pinch ͓Fusion Technol. 19, 131 ͑1991͔͒ from the measured correlation of the heat flux along perturbed fields with the radial component of the perturbed field. In the edge region the total flux is convective and intrinsically ambipolar constrained, as evidenced by the magnitude of the thermal diffusivity, which is well approximated by the product of ion thermal velocity and the magnetic diffusivity. A self-consistent theory is formulated and shown to reproduce the experimental results, provided nonlinear charge aggregation in streaming electrons is accounted for in the theory. For general toroidal configurations, it is shown that ambipolar constrained transport applies when remote magnetic fluctuations ͑i.e., global modes resonant at distant rational surfaces͒ dominate the flux. Near locations where the dominant modes are resonant, the transport is nonambipolar. This agrees with the radial variation of diffusivity in MST. Expectations for the tokamak are also discussed.

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...