A three- to five-fold enhancement of the energy confinement time in a reversed-field pinch (RFP) has been achieved in the Madison Symmetric Torus (MST) by reducing the amplitude of tearing mode fluctuations responsible for anomalous transport in the core of the RFP. By applying a transient poloidal inductive electric field to flatten the current density profile, the fluctuation amplitude (b) over bar/B decreases from 1.5% to 0.8%, the electron temperature T-e0 increases from 250 to 370 eV, the ohmic input power decreases from 4.5 to approximately 1.5 MW, the poloidal beta beta(theta) increases from 6% to 9%, and the energy confinement time tau(E) increases from 1 to similar to 5 ms in I-phi = 340 kA plasmas with density (n) over bar = 1 x 10(19) m(-3). Current profile control methods are being developed for the RFP in a program to eliminate transport associated with these current-gradient-driven fluctuations. In addition to cent rot ling the amplitude of the tearing modes, an understanding of the physics of these fluctuations is being pursued. In particular, plasma flow, both equilibrium and fluctuating, plays a critical role in a diversity of physical phenomena in MST. The key results are: (1) Edge probe measurements show that the MHD dynamo is active in low collisionality plasmas, while at high collisionality a new mechanism, the "electron diamagnetic dynamo", is observed. (2) Core spectroscopic measurements show that the toroidal velocity fluctuations of the plasma are coherent with the large-scale magnetic tearing modes; the scalar product of these two fluctuating quantities is similar to that expected for the MHD dynamo electromotive force. (3) Toroidal plasma flow in MST exhibits large radial shear and can be actively controlled, including unlocking locked discharges, by modifying E-r with a robust biased probe.

The local electron energy flux produced by magnetic fluctuations has been measured directly in the edge plasma (r/a > 0.75) of the Madison symmetric torus (MST) reversed field pinch (RFP), continuous current tokamak (CCT), and the scrape-off layer of the TEXT-U tokamak. The Bur produced by electrons travelling parallel to a fluctuating magnetic field is obtained from correlation between the fluctuations in the parallel heat flux and the radial magnetic field. The fluctuations in the parallel heat flux were measured with a fast insertable pyrobolometer. The measurements reveal fundamental differences in the nature of electron energy transport in the RFP and the tokamak. In the RFP the fluctuation-induced energy flux is large (approximate to 100 kW m(-2), comparable to the total ohmic heating power) inside the reversal surface where the magnetic field is expected to be stochastic, and small in the edge. The magnetic fluctuation induced radial energy flux Q and radial particle flux Gamma (measured independently) are related by a 'convective' formula Q approximate to 3/2T Gamma. The electron heat transport is significantly lower than the value predicted by the Rechester-Rosenbluth transport model. This feature of the electron energy transport can be explained using self-consistent calculations that account for clumping of electrons streaming along the magnetic field. In the tokamak the magnetic fluctuations do not contribute to the total energy transport except in the vicinity of the q = 2 magnetic surface, where the transport is associated with large amplitude Mirnov oscillations.

The reversed field pinch (RFP) is a toroidal, high beta plasma confinement configuration with great potential as an attractive, compact, high power density fusion reactor core. Relatively poor confinement has been a main obstacle in establishing the viability of the RFP. Recent progress in understanding magnetic-fluctuation-induced transport in the RFP has spawned the idea of current profile control to reduce fluctuations and transport. With the addition of inductive poloidal current drive in the Madison Symmetric Torus (MST) device, the energy confinement time is increased five-fold from 1.2 ms to 6 ms. The lowest magnetic fluctuation level and highest electron temperature observed in MST are also obtained with inductive current profile control. These results strongly encourage the development of improved and steady-state current profile control.

Most Madison Symmetric Torus (MST) [Fusion Technol. 19, 131 (1991)] reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2–5 ms, corresponding to magnetohydrodynamic (MHD) 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.

The total magnetic fluctuation-induced electron thermal flux has been determined in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] 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 surfaces) 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.

Data are presented showing the first demonstration that H mode plasmas can be produced with a biased electrode in the presence of substantial auxiliary power (the Alfven wave power was roughly twice the corresponding ohmic power). The data also show the first demonstration of Alfven wave heating of electrons during an H mode. Although the H mode has been produced in some tokamaks (without the use of a biased electrode) by auxiliary heating greater than a threshold value dependent on plasma conditions and the auxiliary heating method, for the first time, measurements presented here show that subthreshold Alfven wave power increases the edge particle transport (in both ohmic and H mode plasmas) and that this auxiliary power degrades rather than enhances the biased electrode H mode. In addition, for the first time, it is shown that Alfven waves generated by + pi /2 (as compared with a) phasing result in greater RF fuelling in H mode plasmas

Two important effects of MHD fluctuations in the RFP and tokamak are current generation (the dynamo effect) and mode locking. In the T1 and MST RFP experiments new results reveal the mode dynamics underlying these phenomena. In T1 the effect of specific magnetic Fourier modes on the current density profile is evident. In MST, the MHD dynamo term ( delta v* delta B) is measured in the plasma edge, and found to account for the time dependence of the edge current throughout a sawtooth cycle. As edge resistivity is increased in T1 the fluctuation amplitude increases to maintain the dynamo-driven current, as expected from MHD computation. The modes responsible for the dynamo often lock to the local magnetic field error at the vertical cut in MST. The plasma rotation velocity has been measured with a fast Doppler spectrometer to a time resolution of 1 mu s. The plasma rotation and mode phase velocity are remarkably well-correlated, with both slowing, in the presence of an impulsive field error, in a 100 mu s timescale.

Summary form only given. The energy loss in the reversed field pinch (RFP) predominantly results from parallel streaming in a stochastic magnetic field. This stochasticity results from B/spl tilde//B/spl sim/1% magnetic fluctuations which accompany m=1, n/spl sim/2R/a tearing (or resistive kink) instabilities in the plasma core. Major research goals in the MST are to understand fluctuation induced transport and to improve plasma confinement using this understanding. Magnetic fluctuation induced transport in the plasma core is studied during a period of high magnetic activity preceding sawteeth events. The flow dynamics of bulk plasma rotation is examined by measuring Doppler shifts of impurity ions spectral lines. Both, spontaneous and actively driven confinement improvement regimes have been observed. After machine conditioning with solid-target boronization, a high confinement regime, characterized by the absence of sawteeth, spontaneously appears during low-density discharges. Similar improvements result by actively applying a transient auxiliary inductive electric field to the MST plasma. The current density gradient is reduced, the growth of the m=1 tearing fluctuations slows, and the energy confinement time doubles. To sustain and enhance the improved plasma, electrostatic and Rf current drivers are being developed.

Summary form only given, as follows. Recent theoretical studies (Uchimoto et al., 1994) strongly motivated the development of a radio-frequency current drive scheme for current density gradient reduction in the outer region of a reversed field pinch. The preliminary experiments (Sarf et al., 1994) using inductive current drive, indicate that such current density profile modification reduces the magnetic fluctuation amplitude and related energy and particle losses. To test the theoretical predictions and to further improve confinement in the MST, we are planning a series of lower-hybrid wave experiments. The initial phase is the design and optimization of a low-power antenna to study slow wave propagation in a frequency range 2-3 f/sub LH/ (200-300 MHz) with parallel index of refraction n/sub /spl par///spl sim/10. Ray-tracing calculations, for typical MST plasma parameters, indicate that such a wave will spiral radially into a target zone inside the reversal layer. The antenna consists of an array of tunable loops arranged in the poloidal direction. The design is compatible with the existing box-port openings in the MST conductive shell to prevent additional magnetic field errors associated with large portholes. Antenna vacuum characteristics are studied on a test-stand designed to approximate the geometry of the MST shell. For the initial measurements of plasma response and antenna loading, we designed a reduced, easily insertable, vacuum antenna structure. The results of plasma impedance measurements will be compared with the numerical modeling results and incorporated in the optimized design of the antenna for wave propagation experiments.

Inserting a positively biased electrode to just inside the Phaedrus-T tokamak limiter results in typical H mode behaviour (i.e. H-alpha or D-alpha drop, density rise, increase in stored energy, profile steepening, and reduction of edge turbulence and radial transport) in deuterium, hydrogen and helium discharges. H-alpha or D-alpha emission suggests that the improvement in particle confinement with H mode is poloidally asymmetric, with the greatest improvement occurring on the low field side. The radial conductivity is examined and measured values are compared with theory.

Summary form only given. The dominant energy loss mechanism in the reversed field pinch (RFP) results from parallel streaming in a stochastic magnetic field. This stochasticity results from B~/B/spl sim/1% magnetic fluctuations which accompany m=1, n/spl sim/2R/a tearing (or resistive kink) instabilities in the plasma core. Inductive, electrostatic, and RF current drive are being explored as techniques to reduce the tearing fluctuation and the associated energy loss. For example, by applying an auxiliary poloidal inductive electric field to the MST RFP plasma, the current density gradient is reduced, the growth of the m=1 tearing fluctuations slows, and the energy confinement time doubles. Sawteeth associated with the m=1 instabilities are also suppressed. Since a toroidal flux change linking the plasma is required, inductive current drive must be transient to avoid excessive perturbation of the equilibrium, To sustain and enhance the improved plasma, electrostatic and RF current drivers are being developed. The novel electrostatic current drive scheme utilizes a plasma source for electron injection.

Summary form only given. The Madison Symmetric Torus (MST) reversed-field pinch is a toroidal, ohmically-heated, magnetic fusion research device capable of plasma currents of I 700 kA and plasma temperatures of several hundred eV. Recent MST discharges have exhibited energy confinement times, /spl tau//sub E/, that are 2-3 times greater than normal. An important benchmark of the MST's performance, /spl tau//sub E/=(total plasma energy)/(input ohmic heating power) and is about 1 ms in normal discharges. /spl tau//sub E/ is limited by transport of energy and particles out of the plasma core due primarily to fluctuations in the structure and magnitude of the internal magnetic field. The dominant magnetic fluctuations are sawtooth events, so-named for the shape of their waveform in many plasma parameters. Sawteeth arise due to overpeaking of the current-density profile on the magnetic axis. During each sawtooth, the total plasma energy decreases, and the ohmic heating power increases, resulting in a sharp drop in /spl tau//sub E/. Recently, discharges with sawtooth-free periods have been achieved, resulting in enhanced confinement. One method of preventing sawteeth is to actively drive current in the edge plasma. This flattens the normally-peaked current-density profile. The best results with this method have been achieved with careful conditioning by solid-target boronization of the plasma-facing inner wall of the vacuum vessel.

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.

This Thomson-scattering diagnostic is used to measure the electron temperature and density of the plasma in the Madison Symmetric Torus reversed-field pinch, a magnetic confinement fusion research device. This diagnostic system is unique for its type in that it combines high performance with simple design and low-cost components. In the design of this instrument, careful attention was given to suppression of stray laser line light with simple and effective beam dumps, viewing dumps, apertures and a holographic edge filter. This allows use of a single-grating monochromator for dispersion of the Thomson scattered spectrum onto the microchannel plate detector. Alignment and calibration procedures for the laser beam delivery system, the scattered light collection system, and the spectrometer and detector are described. A sample Thomson-scattered spectrum illustrates typical data.

Current drive using the lower-hybrid slow wave is shown to be a promising candidate for improving confinement properties of a reversed field pinch. 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 computation demonstrates that an additive poloidal current in this region will reduce the magnetic fluctuations and magnetic stochasticity.

A solid rod of hot-pressed boron carbide is being used as the source of boron during boronization of MST. The most striking result of this procedure is the reduction in oxygen contamination of the plasma (OIII radiation, characteristic of oxygen at the edge, falls by about a factor of 3 after boronization.). The radiated power fraction drops to about half its initial value. Particle reflux from the wall is also lowered, making density control simpler. The rod (12.7 mm diameter) is inserted into the edge plasma of normal high-power RFP discharges. B4C is ablated from the surface of the rod and deposited in a thin film (a-B/C:H) on the walls and limiters. The energy flux carried by “superthermal” (not “runaway”) electrons at the edge of MST appears to enhance the efficient, nondestructive ablation of the boron carbide rod.