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.

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...

An overview of MST results is presented. Substantial advances have been achieved in RFP confinement, in the development of auxiliary sources for heating and current drive, and in studies of fluctuations, transport, and magnetic self-organization physics. The density in improved confinement plasmas has been increased above the empirical limit, n/n G =1.5, using pellet injection. A record density of 0.7×10 20 m -3 has been achieved in high current (0.5 MA) plasmas. Maximum energy confinement is attained at 0.5 MA and lower n/n G =0.13; this establishes confinement comparable to a same-size, same-current tokamak over MST's full range of plasma current. Experiments using a new 1 MW, 25 keV neutral beam injector are underway, to explore beta limits, energetic particle confinement, momentum transport, and current profile control. Analysis of the d-d neutron production evidences good fast ion confinement. Results for oscillating field current drive are in good agreement with nonlinear resistive MHD computation, bolstering the OFCD physics basis. Energy confinement with OFCD is measured comparable to that for standard induction. Non-collisional ion heating generates transient temperatures T i =2-3 keV. The heating efficiency is measured to scale with a fractional power of the ion mass, and to be anisotropic depending on the plasma density. Broadband magnetic turbulence exhibits a dissipative nonlinear cascade that could be connected to the ion heating mechanism. A new high rep-rate Thomson scattering diagnostic measures electron temperature fluctuations associated with residual magnetic islands and helical mode structure. The equilibrium and fluctuations of quasi-single-helicity plasmas are investigated with this and MST's other advanced diagnostics. Lower hybrid and electron Bernstein wave injection (~100 kW absorbed power for each) are in development for current drive and heating.

A Kelvin-Helmholtz instability has been identified numerically on an azimuthally symmetric Alfvdn resonant layer in an axially bounded, straight cylindrical coronal loop. The physical model employed is an incompressible, reduced magnetohydrodynamic (MHD) model including resistivity, viscosity, and density variation. The set of equations is solved numerically as an initial value problem. The linear growth rate of this instability is shown to be approximately proportional to the Alfv6n driving amplitude and inversely proportional to the width of the Alfv6n resonant layer. It is also shown that the linear growth rate increases linearly with m -1 up to a certain m, reaches its maximum value for the mode whose half wavelength is comparable to the Alfv6n resonant layer width, and decreases at higher m's. (m is the azimuthal mode number.)