M. Doczy

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.

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.

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.

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...

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.

Plasma fueling by sputtering and gas desorption during high power RF operation on the Phaedrus-T tokamak

Summary form only given. The particle confinement time in tokamaks, /spl tau//sub p/, is at least an order of magnitude less than the length of the discharge. In order to maintain a constant density during the discharge, it is necessary to provide an external hydrogen fueling source. On Phaedrus-T, approximately 4/spl times/10/sup 17/ hydrogen atoms per millisecond must be puffed into the vessel to maintain a constant density. Ion impact onto materials within the vacuum vessel causes sputtering and gas desorption, and these processes provide an additional uncontrolled fueling source. On the Phaedrus-T tokamak, high power RF (100-400 kW, 4-7 MHz) operation increases the sputtering and gas desorption rates. This increased gas influx, called RF fueling, can lead to disruptions or create ambiguity in interpreting the nature of the RF physics. Antenna designs have played a critical role in determining the amount of RF fueling that occurs. On Phaedrus-T, insulating boron nitride (BN) protection limiters intercept the plasma flow to the antenna straps. This is different from conventional tokamak antenna designs which surround their antenna with a conducting shield.

Experimental evidence of low frequency current drive in the Phaedrus-T tokamak

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.

Edge measurements during Alfven wave heating and current drive on the Phaedrus-T tokamak

Summary form only given, as follows. The properties of tokamak boundary plasmas play an important role in the success of using waves in the ion cyclotron range of frequency for heating and current drive. Edge measurements have been conducted on the Phaedrus-T tokamak under a variety of operating conditions to study the relevant processes in coupling Alfven wave power to plasma and the effects of Alfven waves on the plasma boundary. With the adjustment of gas puffing, the line density during rf can be successfully controlled and kept at the same constant level as ohmic on the Phaedrus-T tokamak. The edge density is then found to have no significant change with the application of Alfven waves. Edge electron temperature, however, is found to increase with increasing rf power and the rise time is much shorter than the energy confinement time, which indicates direct power deposition in the boundary of the plasma. Some data suggest less edge heating corresponds to better rf power coupling to the bulk plasma. Edge electrostatic fluctuation measurements show increases in the fluctuation induced particle flux and heat flux during rf, indicating a degradation of particle and energy confinement time by the Alfven waves. The effects of rf phasing will also be discussed.