C. Litwin

Lower‐hybrid poloidal current drive for fluctuation reduction in a reversed field pinch

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.

Dynamo effect and current drive by low‐frequency waves

The dynamo effect due to low‐frequency oscillations and its application to the nonresonant current drive are examined. The relevant single particle forces are analyzed and the possibility of the dynamo effect enhancement by thermal effects is investigated. In the collisional two‐fluid theory with isotropic pressure the flux‐surface averaged parallel dynamo field is found to be solely due to frictional forces. In the collisionless, double‐adiabatic approximation the previously reported steady‐state collisionless α‐effect is shown to be absent. The origin of this discrepancy is discussed.

Alfvén wave current drive in the Phaedrus‐T tokamak

The first experimental evidence of Alfven Wave Current Drive (AWCD) in a tokamak is shown. In a low‐density experiment, an estimated 20–35 kA out of 65 kA total current, or 30%–55% of the total current has been driven. The estimated efficiency for current driven per unit RF input power is approximately ICD/PRF≊0.2 A/W, which is near the predicted efficiency, and corresponds to the commonly used figure of merit, neR0ICD/PRF≊0.4×1018 A m−2 W−1, where ne is plasma density and R0 is the major radius. The significant 30%–40% drop in loop voltage observed cannot be explained by any plausible increase in electron temperature Te, or decrease in inductive plasma energy, or changes in plasma resistivity. Independently measured loop voltage, Te, effective ionic charge Zeff, and plasma inductance and resistance are all consistent with this conclusion.

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

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.

Current diffusion and toroidal electric field response to a non‐Ohmic current drive

Resistive current diffusion and inductive electric field evolution in response to an auxiliary current drive are analyzed for conditions typical for tokamak operation. Special emphasis is placed on a localized current drive. Relaxation time scales for a broad range of conductivity profiles are calculated, using the Wentzel–Kramer–Brillouin (WKB) approximation, and compared with numerical solutions. Validity of the effective circuit model is discussed.

Alfvén ion–ion hybrid wave heating in the Phaedrus‐T tokamak

In the Phaedrus‐T tokamak [R. A. Breun et al., Fusion Technol. 19, 1327 (1991)], Alfven waves are indirectly driven by a fast wave antenna array. Small fractions of minority ions are shown to have a large effect on the Alfven spectrum, as measured at the edge. An ion–ion hybrid Alfven mode has been identified by measuring dispersion properties. Landau damping is predicted to be large and spatially localized. These Alfvenic waves are experimentally shown to generate correlated electron heating and changes in density near the core of the tokamak plasma. Fast wave antenna fields can mode convert at a hybrid Alfven resonance and provide a promising route to spatially localized tokamak heating and current drive, even for low effective ionic charge Zeff≊1.3–2.

Alpha effect of Alfvén waves and current drive in reversed-field pinches

Circularly polarized Alfven waves give rise to an α-dynamo effect that can be exploited to drive parallel current. In a “laminar” magnetic the effect is weak and does not give rise to significant currents for realistic parameters (e.g., in tokamaks). However, in reversed-field pinches (RFPs) in which magnetic field in the plasma core is stochastic, a significant enhancement of the α effect occurs. Estimates of this effect show that it may be a realistic method of current generation in the present-day RFP experiments and possibly also in future RFP-based fusion reactors.

Confinement improvement in the MST reversed field pinch

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.

Analysis of loop voltage evolution in current drive experiments in the Phaedrus-T tokamak

The loop voltage response in the low‐frequency current drive experiments is analyzed in order to extract information about the current drive profile and efficiency.

On the loop voltage response to an rf current drive in a tokamak

Inductive electric field evolution in response to an rf current drive is analyzed for conditions typical for tokamak operation, with emphasis on a localized current drive. Relaxation time scales for a broad class of conductivity profiles are calculated, using the WKB approximation, and compared with numerical solutions. Loop voltage response in recent experiments in the Phaedrus‐T tokamak is exploited to infer the current drive.