F.W. Baity

Coupling of fast waves in the ion cyclotron range of frequencies to H-mode plasmas in DIII-D

Measurements of low power ( 1 mW) antenna loading are used to study the coupling of a compact loop antenna structure to plasmas in the divertor configuration in DIII-D heated by neutral beam injection (NBI) or electron cyclotron heating (ECH). When a transition to the H-mode regime occurs during NBI, the antenna loading resistance drops by approximately a factor of two. This coupling decrease is due to a steepening of the edge density profile near the separatrix, accompanied by a reduction in edge density in the scrape-off layer. During edge localized modes, the opposite effects occur, and the antenna coupling increases transiently. The loading measurements are compared with theoretical calculations which take into account the measured density profiles as well as the conducting side-walls of the recessed antenna housing. Absolute agreement between the theoretical and the experimental results is obtained, including the correct dependence on the density, antenna position, RF frequency and antenna geometry. The theoretical interpretation of the results is discussed, together with the technological implications for future high power experiments.

Plasma properties in the ELMO bumpy torus

Experiments with 200 kW of applied electron cyclotron heating (ECH) power have demonstrated electron temperatures of about 1 keV in the ELMO Bumpy Torus-Scale (EBT-S) device. Electron densities are in the range of (0.5-1.5)*1018 m-3 and increase as the square root of the applied ECH power. A potential well is present, and its depth in V closely follows the electron temperature expressed in electron volts. Only low charge states of impurities are found, and Zeff approximately=1.0. Data from the electrons can be compared with simple scaling laws when scale lengths are held constant. These comparisons indicate that electron densities, temperatures, and confinement times scale according to neoclassical expectations.

Heating experiments in the ion cyclotron range of frequencies on EBT-S

Heating experiments in EBT-S have been conducted in steady-state plasmas at continuous wave power levels up to 20 kW in the ion cyclotron range of frequencies (15–30 MHz). Power coupling efficiencies and wave propagation measurements have been obtained at reduced power levels over the frequency range 15–50 MHz. Wave dispersion properties are in qualitative agreement with simplified theory. Substantial ion heating is obtained in deuterium plasmas above 25 MHz, the point at which wave propagation begins. At this frequency, only the 3rd-10th ion cyclotron harmonics of deuterium are present in the core plasma. Ion heating under these circumstances is considerably greater than theoretically predicted for EBT-S parameters. A residual proton component is also heated. However, collisional energy transfer from the protons to the deuterons is too small to account for the deuteron heating. The stored energy of the relativistic electron annuli increases substantially with the application of wave power. Near the antenna, the stored energy more than doubles at a power level of 15 kW. The mechanism producing this dramatic effect is not well understood at present. A small reduction in the ambipolar electric field is also observed during wave heating.

Fast wave current drive in H mode plasmas on the DIII-D tokamak

Current driven by fast Alfven waves is measured in H mode and VH mode plasmas on the DIII-D tokamak for the first time. Analysis of the poloidal flux evolution shows that the fast wave current drive profile is centrally peaked but sometimes broader than theoretically expected. Although the measured current drive efficiency is in agreement with theory for plasmas with infrequent ELMs, the current drive efficiency is an order of magnitude too low for plasmas with rapid ELMs. Power modulation experiments show that the reduction in current drive with increasing ELM frequency is due to a reduction in the fraction of centrally absorbed fast wave power. The absorption and current drive are weakest when the electron density outside the plasma separatrix is raised above the fast wave cut-off density by the ELMs, possibly allowing an edge loss mechanism to dissipate the fast wave power since the cut-off density is a barrier for fast waves leaving the plasma.

ICRF antenna and feedthrough development at the Oak Ridge National Laboratory

Antennas, vacuum feedthroughs, Faraday shields, and antenna materials for application in the ICR frequencies have been analysed and tested. The emphasis on the RDL (resonant double loop) antenna.(AIP)

Absorption of fast waves at moderate to high ion cyclotron harmonics on DIII-D

The absorption of fast Alfven waves (FW) by ion cyclotron harmonic damping in the range of harmonics from 4th to 8th is studied theoretically and with experiments in the DIII-D tokamak. A formula for linear ion cyclotron absorption on ions with an arbitrary distribution function which is symmetric about the magnetic field is used to estimate the single-pass damping for various cases of experimental interest. It is found that damping on fast ions from neutral beam injection can be significant even at the 8th harmonic if the fast ion beta, the beam injection energy and the background plasma density are high enough and the beam injection geometry is appropriate. The predictions are tested in several L-mode experiments in DIII-D with FW power at 60 MHz and at 116 MHz. It is found that 4th and 5th harmonic absorption of the 60 MHz power on the beam ions can be quite strong, but 8th harmonic absorption of the 116 MHz power appears to be weaker than expected. The linear modelling predicts a strong dependence of the 8th harmonic absorption on the initial pitch-angle of the injected beam, which is not observed in the experiment. Possible explanations of the discrepancy are discussed.

Methods of calculating selected geometrical effects in the design of ICRH antennas

Abstract This paper discusses the effects of realistic antenna geometry on the design and performance of ion cyclotron resonance heating (ICRH) antennas and presents methods to calculate these effects and to modify performance predictions that are based on idealized antenna geometry. It emphasizes the engineering aspects of antenna design, such as the electrical characterization of the antenna for inclusion into the power distribution network, as well as detailed analysis of the Faraday shield structure. The analysis of the Faraday shield includes the calculation of the shields power transmission and dissipation properties (rf load), its effect on the strap phase velocity and characteristic impedance, and the rf heat distribution for the purpose of thermal analysis. The finite antenna length and its interaction with the recessed cavity and reduced phase velocity are presented in terms of an effective antenna length. Calculation of interstrap coupling with slotted septa separating the straps is presented, including a discussion of the basic trade-offs between directionality, loading, and circuit stability in the case of directional phased arrays for fast wave current drive at ICRH frequencies.

Fast wave current drive in neutral beam heated plasmas on DIII-D

The physics of non-inductive current drive and current profile control using the fast magnetosonic wave has been demonstrated on the DIII-D tokamak. In non-sawtoothing discharges formed by neutral beam injection (NBI), the radial profile of the fast wave current drive (FWCD) was determined by the response of the loop voltage profile to co, counter, and symmetric antenna phasings, and was found to be in good agreement with theoretical models. The application of counter FWCD increased the magnetic shear reversal of the plasma and delayed the onset of sawteeth, compared to co FWCD. The partial absorption of fast waves by energetic beam ions at high harmonics of the ion cyclotron frequency was also evident from a build up of fast particle pressure near the magnetic axis and a correlated increase in the neutron rate. The anomalous fast particle pressure and neutron rate increased with increasing NBI power and peaked when a harmonic of the deuterium cyclotron frequency passed through the center of the plasma. The experimental FWCD efficiency was highest at 2 T where the interaction between the fast waves and the beam ions was weakest; as the magnetic field strength was lowered, the FWCD efficiency decreased to approximately half of the maximum theoretical value.

The folded waveguide: a high frequency rf launcher☆

Abstract The folded waveguide, an alternative to loop antennas for launching power in the ion cyclotron range of frequencies (ICRF) into plasma devices, operates as a cavity with apertures for coupling RF power to the plasma. The RF field pattern is similar to that of a loop antenna, but with a lower ratio of electric to magnetic field. Power enters from a coaxial line via a sliding contact, whose position matches impedances between the coaxial line and the folded waveguide. The folded waveguide has operated at 1 MW and promises high power density. Calculations indicate a factor of 4 increase in power handling capability over a comparable loop antenna. The possible use of the folded waveguide on several tokamaks is discussed.

Ion heating in the range of high ion cyclotron harmonics on EBT

Wave heating of ions is obtained in Elmo Bumpy Torus (EBT) experiments under conditions where 18 ω/Ωi 3. Absorption of wave energy in the plasma centre is demonstrated. The fast magnetosonic wave is observed on electrostatically shielded loop probes placed at the edge of the plasma, but it is shown that the fast wave cannot directly heat the ions in EBT. The experiments suggest, however, that the waves that do produce the ion heating are coupled to the fast wave. The possibility that the heating is due to excitation of electrostatic waves is investigated theoretically.