Following experimental demonstration of Alfvén 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.

A small insertable oven for benchmarking the boronizing characteristics of solid compounds, such as decaborane and carborane, has been developed for the Phaedrus-T tokamak. Assembly and installation of the oven are relatively easy as the oven design utilizes a Langmuir probe drive assembly, which is standard equipment on most tokamaks and allows the oven to be inserted into the tokamak without requiring a vent. Films deposited by heating carborane into the vapor state with the oven are found to be spatially nonuniform in both thickness and in the ratio of boron to carbon as compared to films deposited with trimethylboron, a gaseous compound. Overall plasma performance is not found to be greatly affected by whether decaborane, carborane or trimethylboron is used for boronization in Phaedrus-T.

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

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

The first experimental evidence of Alfvén 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.

Summary form only given, as follows. Physics and engineering principles of a tokamak with vertically inclined toroidal field (TF) coils are considered. This device can be viewed as a hybrid between the tokamak and stellarator configurations. The stellarator-like properties include the existence of vacuum flux surfaces with finite rotational transform. The present report extends the previous more general studies towards recommendations for a specific device of this type with parameters close to that of the Phaedrus-T tokamak. Our analysis includes both the physics and engineering aspects. In the physics part, we analyze various types of TF coils for their effectiveness to generate the stellarator-like properties. The optimization of the configuration with respect to the angle of inclination and number of TF coils, and various poloidal field coil systems is given. The physics part is based on the UBFIELD and other codes. The engineering part includes the analysis of the magnetic forces on various parts of the coils and the possible engineering implementation of the above device. The numerical code EFFI is used for the engineering analysis. The physics and engineering optimization of the configuration is studied to ensure its feasibility for construction.

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.

Summary form only given. The first experimental evidence of low frequency current drive in a tokamak has been observed on the Phaedrus-T tokamak (R/sub major/=0.92 m, r/sub minor/=0.255 m, B/sub T//spl ap/0.6-1 T, I/sub p/<100 kA, n/sub e0/=0.2-1.5/spl times/10/sup 19/ m/sup -3/). Low frequency current drive utilizes waves with frequencies below the ion cyclotron frequency to inject momentum to electrons to drive a toroidal current, and is often referred to as Alfven wave current drive (AWCD). Like other noninductive current drive techniques, AWCD would allow fusion tokamak reactors to operate as steady state devices. AWCD would also allow tailoring of the energy and current density profiles. Properly modified profiles would make the plasma less susceptible to instabilities. The presence of noninductive current is inferred from the behavior of the plasma loop voltage measured at the edge of the plasma The loop voltage can be roughly related to the sum of the ohmic dissipation, product of plasma current and resistance, and the time rate of change in the stored magnetic energy of the plasma during a plasma discharge, the plasma current is kept constant through automatic feedback control and is produced by pulsed magnetic induction. Therefore, the loop voltage can decrease if there is a decrease in plasma resistance, a change in stored magnetic energy, or a noninductive current source is present.

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, as follows. Energy confinement has been studied on the Phaedrus-T Tokamak. The study was performed using (1) a diamagnetic loop to measure the total plasma energy, W, and global energy confinement times, /spl tau//sub E/; (2) Thompson scattering to measure the electron temperature, T/sub e/, and electron density, n/sub e/, profiles; (3) a visible bremsstrahlung diagnostic to determine bremsstrahlung emission profiles and thus the effective ion charge, Z/sub eff/, profiles; (4) pyrobolometers to measure the power leaving the plasma as radiation, P/sub rad/. The profile measurements allowed us to determine not only global confinement times but local electron energy confinement times. The study included data from different tokamak operating modes including ohmic, biased probe induced H-Mode and plasma with RF power at different phasings. How /spl tau//sub E/ scales with several plasma parameters, including line density, n/sub l/, plasma current, I/sub p/, toroidal magnetic field, B/sub T/, and RF power, P/sub rf/, has been studied. The determined plasma scalings are compared to several experimentally and theoretically derived scaling laws. The energy confinement time is also compared to the particle confinement time, /spl tau//sub P/.

In the course of our Alfven wave heating and current drive experiments several different two and four strap antennas have been installed in Phaedrus‐T. The motivation focusing the redesign of the antenna into a four strap design was to enable traveling wave phasing, and to reduce the k∥≊0 component of the wavenumber spectrum, and consequent edge power deposition. The latest modifications to the 4 strap antenna have dramatically improved its behavior, and enabled us to suppress its RF power induced impurity generation. The remaining gas reflux fueling is significant and is not local to the antenna.

Heating in the Alfvén 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 Alfvén wave is excited via mode conversion from a surface fast wave at the Alfvén resonance and is subsequently damped on electrons.

Evidence is given that the presence of an open ion cyclotron resonance layer in a tandem mirror can result in enhanced radial ion transport.

Plasma density turbulence has been measured with the beam emission spectroscopy (BES) diagnostic system, using a low-power neutral beam with He0 and H0 as beam species. In general, He0 (588 nm) provided the best signal-to-noise ratio due to its lower edge plasma background interference. Simultaneous measurements of edge density fluctuations have been made with BES and Langmuir probes; the spectra are seen to be essentially identical, and the fluctuation amplitudes from both diagnostics are in close agreement. A poloidal coherence length of about 2–4 cm was observed. Radial propagation of modes was not seen, but a lab-frame poloidal phase velocity at r/a=0.77 of about 7×105 cm/s in the electron diamagnetic direction was observed, corresponding to m=8–75 kHz.

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 on phase changes, and a two‐channel transformer coupled transmitter.

Phaedrus-T is a moderate sized tokamak with low toroidal field and substantial radio frequency power capabilities. This machine has only recently become operational and full RF power is expected in the next year. The initial studies include edge modifications due to variable phased antenna structures and RF helicity current drive experiments. Spheromak particle refueling tests will be started in the Fall of 1991. The tokamak is also being used as a test site for diagnostic development.

A flexible beam emission spectroscopy diagnostic system is being installed on the Phaedrus-T tokamak. It consists of a low-power diagnostic neutral beam (H0 or He0) coupled with visible and vacuum UV collection optics to study low-amplitude, high-frequency fluctuations in local plasma density in ohmically and rf-heated plasmas. Neutral beam geometry and optical sightlines are chosen to optimize localization and radial resolution (<1 cm) over the whole plasma region. The ability to inject either a H or He neutral beam and observe in both the vacuum UV and the visible spectral ranges allows a wide choice of atomic transitions (e.g., Hα, Lα, He singlet or triplet lines, etc.) to be compared and optimized for a given experimental condition.

Electrostatic end plugging is observed in a completely axisymmetric, three cell tandem mirror under conditions where the central-cell plasma density is always larger than the end-cell density. A factor of 4 increase in the central-cell density, to a maximum of 1.2×1013 cm−3 with simultaneous plasma beta of 13%, occurs upon application of the end plugging potential. Ion confining potentials of 25 V and 80 V at the two ends of the device, respectively, result in a factor of 2.5 increase in the axial confinement time for Tic =40 eV in agreement with the collisional flow model for ion confinement. The non-Boltzmann ion confining potential is caused by electron heating in the end cells by rf near the ion-cyclotron frequency. The initial central-cell density rise is caused by an increase in the ionization rate that occurs because of an increase in the electron temperature. The density remains high throughout the end-cell heating pulse as a result of increased particle confinement time. There is no nonambipolar radial ion loss in the core plasma (r≤16 cm) but inward radial transport of ions is observed at a rate consistent with ion–neutral collisions and a radial electric field in the negative radial direction. Steady-state thermal-barrier-like potential dips that are in agreement with the Boltzmann model for potentials are observed in the transitions between the central cell and the end cells.

Cold neutral gas density has important consequences for the energy and particle balances of the hot ion plasma in the end cells of tandem mirrors. Charge exchange loss from cold gas penetration into this plasma detracts from the pressure in the end cells needed for magnetohydrodynamic stability of the mirror. Unfortunately, cold gas can be a by-product of neutral beam injection and of fueling. This report describes measurements of neutral pressure in the rf heated tandem mirror Phaedrus. The emphasis is on the dynamics of gas flow into the end cells, the effects of cold gas on the plasma, and on wall reflux for different wall conditions. The effects of neutral beams and neutral beam generated wall reflux are also studied. A numerical gas flow code is used to model the effects of plasma pumping, charge exchange, and wall reflux on end cell plasma parameters. The titanium sticking coefficient for hydrogen gas was determined to be 0.08±0.08. Wall reflux coefficients in the end cell were determined to be 1.4±0.2 without titanium gettering and 0.75±0.25 with gettering.