J. H. Rogers

Analysis of RF sheath interactions in TFTR

New theoretical and experimental tools are applied to the analysis of ICRF antenna-edge plasma interactions in the TFTR tokamak. A new numerical method for computing the three dimensional (3-D) rf sheath voltage distribution is used, and the quantitative predictions of rf sheath theory are compared with measurements of the edge density profile obtained by microwave reflectometry and with titanium impurity concentration data. It is shown that the local density depletion at the antenna is consistent with density pump-out by strong E × B convection into the Faraday screen (FS). Modelling of the FS impurity influx shows that the calculated titanium impurity concentration based on this direct influx agrees with the measured concentration for π phasing. It is also shown that screening of impurity neutrals by ionization in the SOL is a large effect and increases with rf power. At high power over many shots, a fraction of the metal impurities migrates around the machine and is deposited on the limiters, providing a secondary source of titanium. The data show that the central titanium concentration is strongly dependent on antenna phasing. Possible explanations for this phasing dependence are discussed.

Ion cyclotron range of frequency experiments in the Tokamak Fusion Test Reactor with fast waves and mode converted ion Bernstein waves

Recent experiments in the ion cyclotron range of frequencies (ICRF) in the Tokamak Fusion Test Reactor [Fusion Technol. 21, 13 (1992)] are discussed. These experiments include mode conversion heating and current drive, fast wave current drive, and heating of low (L)‐ mode deuterium–tritium (D–T) plasmas in both the hydrogen minority and second harmonic tritium regimes. In mode conversion heating, a central electron temperature of 10 keV was attained with 3.3 MW of radio‐frequency power. In mode conversion current drive experiments, up to 130 kA of current was noninductively driven, on and off axis, and the current profiles were modified. Fast wave current drive experiments have produced 70–80 kA of noninductively driven current. Heating of L‐mode deuterium and D–T plasmas by hydrogen minority ICRF has been compared. Finally, heating of L‐mode D–T plasmas at the second harmonic of the tritium cyclotron frequency has been demonstrated.

ICRF heating and profile control techniques in TFTR

In fast wave to ion Bernstein wave mode conversion experiments in DT supershot plasmas, localized efficient ion heating rather than electron heating was observed, which was due to Doppler broadened tritium cyclotron resonance overlap into the mode conversion region. The ion temperature heat pulse associated with RF power modulation in this regime could provide a diagnostic tool for measuring the local ion thermal conductivity in various confinement regimes. In direct launch ion Bernstein wave heating experiments, core power coupling was limited by the excitation of parasitic edge modes. However, a sheared poloidal flow was observed that is consistent in both magnitude and direction with theoretical models based on RF driven Reynolds stress. With the modest power coupled to the core (~360 kW), the magnitude of the shear in the observed flow was estimated to be a factor of 3-4 too low to trigger transport barrier formation through localized shear suppression of turbulence.

Role of Alfvén instabilities in energetic ion transport

Experiments with plasma heating by waves at the ion cyclotron resonance of a minority species have shown that the heating efficiency degrades above a certain power threshold. It is found that this threshold is due to the destabilization of a branch of shear Alfven waves, the Energetic Particle Modes, which causes a diffusive loss of fast ions. These modes not only play a fundamental role in the transport of the fast ions, but appear closely related to the formation of giant sawteeth.

Alpha-particle physics in the tokamak fusion test reactor DT experiment

A summary is presented of recent alpha-particle experiments on the tokamak fusion test reactor. Alpha particles are generally well confined in MHD-quiescent discharges, and alpha heating of electrons has been observed. The theoretically predicted toroidicity-induced Alfv?n eigenmode has been seen in discharges of of alpha power, but only in plasmas with weak magnetic shear.

Ion cyclotron range of frequency heating on the Tokamak Fusion Test Reactor

The complete ion cyclotron range of frequency (ICRF) heating system for the Tokamak Fusion Test Reactor (TFTR) [Fusion Tech. 21, 1324 (1992)], consisting of four antennas and six generators designed to deliver 12.5 MW to the TFTR plasma, has now been installed. Recently a series of experiments has been conducted to explore the effect of ICRF heating on the performance of low recycling, supershot plasmas in minority and nonresonant electron heating regimes. The addition of up to 7.4 MW of ICRF power to full size (R∼2.6 m, a∼0.95 m), helium‐3 minority, deuterium supershots heated with up to 30 MW of deuterium neutral‐beam injection has resulted in a significant increase in core electron temperature (ΔTe=3–4 keV). Simulations of equivalent deuterium–tritium (D–T) supershots predict that such ICRF heating should result in an increase in βα(0)∼30%. Direct electron heating has been observed and has been found to be in agreement with theory. The ICRF heating has also been coupled to neutral‐beam heated plasmas f...

Application of microwave reflectometry to the measurement of fast magnetosonic waves in the Tokamak Fusion Test Reactor

The application of microwave reflectometry to the study of fast magnetosonic waves in the Tokamak Fusion Test Reactor (TFTR) is investigated. Assuming the validity of geometric optics for reflectometer measurements, it is shown that linearity to the fast wave amplitude is restricted to very small density fluctuation levels (n/n<10−3). Beyond this level, both phase and amplitude ambiguities occur that makes interpretation difficult. Measurements of 30 MHz fast magnetosonic waves in the core of TFTR plasmas are presented.

Fast Wave Current Drive on TFTR

For recent Fast Wave Current Drive (FWCD) experiments on TFTR two strap ICRF antennas with ±90 degree phasing between the straps have been used. In one set of experiments an RF frequency of 63.6 MHz and toroidal magnetic field of 2.7 T were selected, which placed the H fundamental resonance on the high field side of the plasma the second harmonic H resonance out of the plasma on the low field side. H‐minority heating (43 MHz) was used simultaneously to raise Te. The difference in loop voltage observed is consistent with ∼70 kA of driven current with 2 MW of RF power. In a second experiment an RF frequency of 43 MHz and toroidal magnetic field of 4.3 T was selected, which placed the deuterium fundamental resonance on the high field side of the plasma and the hydrogen fundamental resonance out of the plasma on the low field side (TPX scenario). With 1.4 MW of RF power, the signal to noise ratio in the loop voltage measurement was too low to clearly resolve the effect from current drive.

ICRF Heating during DT Experiments on TFTR — System Improvements and Results*

In order to carry out a program of ICRF experiments in deuterium-tritium plasmas on the TFTR device a series of technical improvements have been made to the ICRF system. These improvements allow more flexible and reliable operation of the system which is crucial for the limited number of tritium discharges available. During the last year circuitry has been installed to feedback the plasma position on antenna loading, to lock the phase between antenna elements, and to detect arcs from the second harmonic content of antenna signals.

Recent radio frequency experiments in TFTR

Recent experiments and theory have fueled renewed interest in Ion Bernstein Waves (IBW) in tokamaks. A direct launch IBW antenna was used for the first time on the Tokamak Fusion Test Reactor (TFTR), and Deuterium-Tritium Mode Conversion of fast waves to IBW has been demonstrated for the first time on TFTR. An estimated 20−40% of the launched RF power was coupled to the core of the plasma during the direct launch IBW coupling experiments. The initial demonstration of D-T MCCD in 1996 suffered from significant damping from Li7 minority heating. In 1997, Li6 was used to condition the walls of TFTR to avoid the presence of Li7, and efficient heating was observed. In addition, fast wave experiments were performed to test the ITER start up plasma scenario, and direct electron heating was tested for application in reversed shear plasmas.