M. Vukovic

Alfvén wave experiments in the Phaedrus‐T tokamak*

Heating in the Alfven resonant regime has been demonstrated in the Phaedrus‐T tokamak [Fusion Technol. 19, 1327 (1991)]. Electron heating during injection of radio‐frequency (rf) power is indicated by a 30%–40% drop in loop voltage and modifications in sawtooth activity. Heating was observed at a frequency ωrf≊0.7Ωi on axis, using a two‐strap fast wave antenna operated at 7 and 9.2 MHz with 180° phasing (N∥∼100). Numerical modeling with the fast wave code fastwa [Plasma Phys. Controlled Fusion 33, 417 (1991)] indicates that for Phaedrus‐T parameters the kinetic Alfven wave is excited via mode conversion from a surface fast wave at the Alfven resonance and is subsequently damped on electrons.

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.

Discrete spectrum of Alfvén ion–ion hybrid waves

In the Phaedrus‐T tokamak [R. Majeski et al., Phys Fluids B 5, 2506 (1993)], Alfven waves are indirectly driven by a fast wave antenna array. Small fractions of minority ions can couple Alfven and ion–ion hybrid waves and have a large effect on the wave numbers accessible for a given launched frequency. A discrete spectrum and toroidal damping for these modes has been identified by measuring dispersion properties at the edge. Landau damping is predicted to be large and spatially localized and to be responsible for the experimentally observed electron heating (T. Intrator et al., ‘‘Alfven ion–ion hybrid wave heating in the Phaedrus‐T tokamak,’’ to appear in Phys. Plasmas) and current drive near the core of the tokamak plasmas.

Measurements on rotating ion cyclotron range of frequencies induced particle fluxes in axisymmetric mirror plasmas

A comparison of phenomenological features of plasmas is made with a special emphasis on radio-frequency induced transport, which are maintained when a set of two closely spaced dual half-turn antennas in a central cell of the Phaedrus-B axisymmetric tandem mirror [J. J. Browning et al., Phys. Fluids B 1, 1692 (1989)] is phased to excite electromagnetic fields in the ion cyclotron range of frequencies (ICRF) with m=−1 (rotating with ions) and m=+1 (rotating with electrons) azimuthal modes. Positive and negative electric currents are measured to flow axially to the end walls in the cases of m=−1 and m=+1 excitations, respectively. These parallel nonambipolar ion and electron fluxes are observed to be accompanied by azimuthal ion flows in the same directions as the antenna-excitation modes m. The phenomena are argued in terms of radial particle fluxes due to a nonambipolar transport mechanism [Hojo and Hatori, J. Phys. Soc. Jpn. 60, 2510 (1991); Hatakeyama et al., J. Phys. Soc. Jpn. 60, 2815 (1991), and Phys...

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.

Current Drive Experiments in the Phaedrus‐T Tokamak

Experiments in progress on the Phaedrus‐T tokamak focus on effects associated with fast wave current drive at low harmonics of the cyclotron frequency, typically either 3ΩCD or 1.5ΩCH on axis. Areas of investigation include edge effects, directionality of wave launch, and comparison of wave absorption to numerical predictions. More general aspects of current drive, such as wave helicity effects which can be viewed as part of a complete picture of the nonlinear contributions to current drive,1 will be extensively studied. Early Thomson scattering data appears to indicate that rf power coupling to electrons is affected by antenna phasing. However, current drive has not yet been observed. Several innovations have also been implemented on the experiment, including insulating limiters on the Faraday shield to reduce rf ‐ edge plasma interactions, an antenna design which reduces inductive coupling between the straps for operation at arbitrary phase, modelling of the coupled straps to allow predictive retuning o...

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.

Modeling of Alfven wave heating and current drive in Phaedrus‐T

Theoretical analysis and numerical modeling of Alfven wave plasma heating and current drive experiments on the Phaedrus‐T tokamak is presented. The full‐wave hot‐plasma code, ALFA, is used in these calculations. The code features toroidal geometry and poloidal magnetic field effects. It is essentially a 2D full‐wave code, but can obtain a 3D picture of RF wave fields and absorbed power via Fourier composition of solutions for many toroidal modes. The stand‐alone current diffusion code, DIFF, is intergrated with ALFA to model the transient processes of current drive in the Phaedrus‐T tokamak. Comparison of numerical calculations to experimental data is given thus permitting a deeper understanding of AWCD processes.

Alfven Wave Current Drive experiments in Phaedrus‐T

Following experimental demonstration of Alfven Wave Current Drive (AWCD) on the Phaedrus‐T tokamak a redesigned high power antenna has been installed that couples 0.5 MW to the plasma. Evidence is shown for core electron heating coexisting with AWCD. There was no observable increase in the AWCD efficiency during these heating experiments, although the spread in kz launch made it difficult to determine if the ratio of wave phase speed to electron thermal speed was actually reduced and whether any decrease in efficiency due to changes in the electron trapping fraction occurred. Scans of toroidal magnetic field show systematic changes in the time dependence of the drop in loop voltage during the RF pulse. Reflectometer data indicates two radial locations for RF fluctuations.

Evidence of coupling to Global Alfvéne Eigenmodes during Alfvén wave current drive experiments on the Phaedrus‐T tokamak

A series of experiments designed to explore mechanisms of power deposition during Alfven wave current drive experiments on the Phaedrus‐T tokamak has shown evidence of power deposition via mode conversion of Global Alfven Eigenmodes at the Alfven resonance. Observation of radially localized RF induced density fluctuations in the plasma and their location vs. BT is in agreement with the predictions of behaviour of GAE damping on the AR by the toroidal code LION. Furthermore, the change in the time evolution of the loop voltage, is consistent with the change of effective power deposition radius, rPD, and is in agreement with the density fluctuations radius.