M. Kishinevsky

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.

Experimental simulation of a gaseous plasma collector

Hydrogen plasma with a density of 3×1014 cm−3 and electron temperature of 20 eV is injected parallel to a 0.1 T B field, into hydrogen (and helium) neutral gas at 0.1–2 Torr. This plasma is opaque to molecular hydrogen but transparent to Franck–Condon neutrals. The axial density and temperature scale length increased with decreasing gas pressure but were insensitive to plasma density and axial magnetic field strength. The plasma decay is explained by radial ion diffusion resulting from collisions with fast neutrals while the fast neutral density is determined by the radial pressure balance between fast neutrals and the cold background gas.

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.

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.

Experimental simulation of a gaseous divertor: Measurements of neutral density inside the plasma

Direct measurements of the neutral density in the core of hydrogen plasma with a density of 3–4×1014 cm−3 and electron temperature of 15–20 eV in a magnetic field of 0.2 T, injected into hydrogen neutral gas at a pressure of 0.1–2 Torr are performed with plasma emission spectroscopy. The data are in agreement with the results of measured plasma decay [Phys. Fluids B 2, 837 (1990)] and can be explained by radial pressure balance between fast neutrals inside the plasma and cold background gas.

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...

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.

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.

A tokamak with inclined toroidal field coils

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.