P.H. Probert

The Phaedrus-T antenna system

Abstract A two strap fast wave antenna has been developed which is capable of operating at arbitrary phasing for any level of plasma loading resistance. Recent advances in the understanding of the rf-edge plasma interaction have been incorporated in the design as well. The result is an antenna which operates without Faraday shielding while greatly reducing rf-induced scrape-off layer perturbations and impurity influx.

Biased H mode experiments in Phaedrus-T

Inserting a positively biased electrode to just inside the Phaedrus-T tokamak limiter results in typical H mode behaviour (i.e. Hα or Dα drop, density rise, increase in stored energy, profile steepening, and reduction of edge turbulence and radial transport) in deuterium, hydrogen and helium discharges. Hα or Dα 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

Poloidally asymmetric potential increases in tokamak scrape-off layer plasmas by radiofrequency power

Langmuir probe data are presented which show poloidally asymmetric increases in floating potential, electron temperature and, hence, plasma potential on magnetic field lines which map to the Faraday shield of an ICRF antenna in a medium size tokamak, Phaedrus-T, during radiofrequency power injection. These data are consistent with and suggestive of the existence of radiofrequency generated sheath voltages on those field lines

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.

The magnetic field structure in a modular stellarator

The numerically predicted magnetic surface structure within the separatrix of the interchangeable module stellarator (IMS) [IEEE Trans. Plasma Sci. PS‐9, 212 (1981)] has been measured experimentally with the use of an electron beam. The results show nested, well‐formed surfaces with a rotational transform profile corresponding to the predicted profile. In the area outside the separatrix, regions of localized magnetic flux emergence from the coil volume (modular divertors) have been seen using an electron beam as well as during plasma operation, and correspond to regions of localized particle flux emergence from between the modular coils.

Rf Edge Interactions and Insulating Limiters in Phaedrus‐T

Inductively generated rf sheaths and self‐bias effects at antenna Faraday screens are at least partly responsible for the impurity influx and edge modifications often seen in rf heating experiments1,2. Analysis of JET results has shown that the effectiveness of solutions utilized in that experiment to reduce rf impurity influxes can be explained in terms of rf sheath effects.1 On both JET and TFTR magnetic field‐aligned Faraday shield elements, low‐Z coatings, and 180° phasing of adjacent antenna straps have been effective at reducing impurities. However, alignment of Faraday shield elements is difficult, low‐Z coatings do not remove the underlying causes of sputtering, and 180° phasing reduces loading resistance and is incompatible with certain rf goals, such as fast wave current drive. Here we show that rf generated impurity influxes in the Phaedrus‐T tokamak are due to self‐bias effects in the edge plasma generated by rf sheaths at the Faraday shield, and that these effects can be largely eliminated fo...

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.

Convective transport due to poloidal electric fields during electron cyclotron heating in IMS

Steady state hollow density profiles, observed during electron cyclotron resonant heating in the Interchangeable Module Stellarator (IMS), are shown to be consistent with a transport model that includes convection in the particle balance equation. The factor of two difference in the confinement time between a hollow profile and a fairly flat profile is due to particle convection and is in good agreement with that calculated from the equilibrium profiles. It is observed that the poloidal electric fields are greater for the hollow profile than for the less hollow case, indicating that they are most likely the cause of the convection.

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...

Comparison of the folded stripline and stacked stripline concepts to the folded waveguide launcher

Two new concepts are being developed as possible upgrades to the folded waveguide launcher. The folded stripline is a folded waveguide with an additional conductor positioned inside. The term stripline refers to the resemblance of the design to microwave microstrip line. The conductor provides support for TEM mode propagation, which eliminates cutoff and the nonlinear frequency dependence of the waveguide impedance and phase velocity. A natural extension to the folded stripline is the stacked stripline, which comprises several stacked, independent TEM waveguides. Initial measurements indicate that both concepts have better magnetic flux coupling than the folded waveguide.