M. J. Mayberry

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.

Observation of parametric decay correlated with edge heating using an ion Bernstein wave antenna on DIII-D

Significant levels of parametric decay activity and correlated edge ion heating were observed during injection of high power ion Bernstein waves (IBWs) in DIII-D. Both minority hydrogen ions and majority deuterium ions showed the formation of a high energy perpendicular tail; no parallel heating was observed. The edge ion heating and the parametric decay activity were both strongest when an ion cyclotron harmonic was present at the plasma edge. Ion tail formation had a power threshold of several hundred kilowatts, above which the tail size increased with antenna power; a comparable power threshold for parametric decay instability (PDI) was observed. Both the PDI and the associated edge deuterium heating were found to be sensitive to the hydrogen-to-deuterium ratio

Ion Bernstein wave antenna loading measurements on the DIII-D tokamak

Antenna loading measurements carried out during high power ion Bernstein wave (IBW) heating experiments on the DIII-D tokamak indicate that efficient, direct coupling to the IBW at ω 2ωci as predicted by linear coupling theory did not occur. Whereas strong peaking in the loading resistance near cyclotron harmonics is predicted for high edge densities (ω < ωLH|edge), the observed loading resistance is nearly independent of the toroidal magnetic field. The loading anomaly can be explained by invoking the ponderomotive force which decreases the edge density immediately in front of the antenna, thus allowing coupling to the cold plasma lower hybrid wave (LHW). A linear LHW coupling code including the modified density profile due to the ponderomotive force reproduces the measured dependence of antenna loading on toroidal magnetic field, edge density, antenna frequency and antenna phasing. Further evidence for the ponderomotive force is obtained from reactive loading measurements which indicate that the plasma is pushed away from the antenna as the radiofrequency power level is increased. The results indicate that the lack of central ion heating observed during DIII-D IBW experiments may be due to a lack of efficient mode transformation from the coupled LHW to a centrally propagating IBW, possibly as a result of nonlinear mechanism(s)

Heating and confinement in H-mode and L-mode plasmas in DIII-D using outside launch electron cyclotron heating

Using the outside launch with vertical polarization, electron cyclotron heating (ECH) experiments in DIII-D were carried out at 60 GHz at fundamental and second harmonic frequencies. The results are consistent with wave absorption theory for accessibility. For fundamental heating with a cut-off layer between the launch location and the resonant surface, good absorption efficiency was obtained. A process for mode conversion at the wall from the extraordinary mode to the ordinary mode is postulated to explain this result. For most efficient heating it was important to avoid the presence of a locked mode. For low density plasmas at the fundamental, the global energy confinement exceeds Kaye-Goldston scaling by about a factor of 1.4. However, heating at the second harmonic agrees numerically with the Kaye-Goldston scaling formula, for ECH alone or for ECH combined with neutral beam injection (NBI), and shows a mass dependence consistent with TE ~ √Ai. The global energy increase was insensitive to the resonance location for qres 1, global energy confinement scales as TE ~ Bt−0.3. Since plasma current and plasma geometry were not varied for these discharges, a scaling with safety factor q is also consistent. Using ECH alone, H-mode confinement was achieved for second harmonic heating in the central core of the plasma at a power threshold of ~0.75 MW for plasma parameters Ip = 0.5 MA, and e = 1.1 × 1019 m−3. The energy confinement times during the H-mode are similar to Ohmic values with a normalized confinement time τE/Ip ~ 200 ms/MA. For ECH and NBI H-mode plasmas, the transition from the L-mode to the H-mode was correlated with reduced magnetic fluctuations in the divertor region. For ECH H-mode plasmas, transport analysis shows that (1) the H-mode obtained with ECH is accompanied by an improvement of the electron thermal diffusivity χe relative to the L-mode over at least part of the plasma, (2) the ion thermal diffusivity χi is larger than predicted by neoclassical theory for the L- and H-modes, and (3) χe is approximately equal to χi for the ohmically heated discharge and for the L-mode and H-mode discharges heated by ECH discussed in the paper.

Theoretical comparison of coupling of a recessed cavity antenna and a conventional loop antenna for fast waves and ion Bernstein waves

A numerical code has been developed to calculate the loading of a cavity loop antenna in threedimensional geometry. The loading of the cavity is calculated for fast wave and ion Bernstein wave coupling and compared with the loading of a conventional loop antenna. For fast waves, the cavity loading increases with increasing edge density, while the conventional loop loading is less sensitive to the edge density but shows a slight decrease of loading because of a steeper density gradient. For ion Bernstein waves, the two types of antenna behave similarly; however, in contrast to the loading for fast waves, the loading for ion Bernstein waves increases with decreased edge density and a steeper density gradient.

Electron absorption of fast magnetosonic waves by transit time magnetic pumping in JET

Direct electron damping of low frequency fast magnetosonic waves has been observed in the centre of high beta hydrogenic JET plasmas where transit time magnetic pumping is a significant component in the electron-wave interaction mechanism. The electron heating power profile was peaked on axis, extended across almost half the minor radius and accounted for 22 ± 5% of the total radiofrequency power coupled to the plasma. After passing through the plasma core, the fast wave was absorbed by hydrogen ions at the second harmonic cyclotron resonance which was placed inboard of the magnetic axis and intersected the equatorial plane at one third of the minor radius.

High power ion Berstein wave experiments on DIII‐D

Previous tokamak experiments with Ion Bernstein wave heating have exhibited efficient central ion heating and associated improvement in particle confinements. In experiments reported here central ion heating at 3/2ΩH with a small He4 minority was observed. the transmitter frequency was 38 MHz and the nominal central toroidal field was BT=1.8 T, which places the 3/2 ΩH layer a few centimeters outboard of the magnetic axis, and the 2ΩH layer just behind the radiating element in the antenna. (AIP)

Fast Wave Current Drive Antenna Performance on DIII‐D

Fast wave current drive (FWCD) experiments at 60 MHz are being performed on the DIII‐D tokamak for the first time in high electron temperature, high β target plasmas. A four‐element phased‐array antenna is used to launch a directional wave spectrum with the peak n∥ value (≂7) optimized for strong single‐pass electron absorption due to electron Landau damping. For this experiment, high power FW injection (2 MW) must be accomplished without voltage breakdown in the transmission lines or antenna, and without significant impurity influx. In addition, there is the technological challenge of impedance matching a four‐element antenna while maintaining equal currents and the correct phasing (90°) in each of the straps for a directional spectrum. In this paper we describe the performance of the DIII‐D FWCD antenna during initial FW electron heating and current drive experiments in terms of these requirements.

Phased operation of the DIII-D FWCD antenna array with a single power source☆

Abstract A phasing and matching system has been designed and implemented for the four-element fast wave current drive (FWCD) antenna array on DIII-D, This system permits phased operation using a single transmitter. Coupled power levels of 1.1 Mw have been reached with relative phasing of ±π/2 and equal magnitudes of current and voltage on all four lines. Use of a single power source requires the achievement of amplitude and phase control at high power, without feedback control of these quantities. The system uses only standard components consisting of transmission lines, unmatched tees, and manually controlled phase shifters and stubs. Phasing, matching, and amplitude control for all four current straps are done using a total of five tuning elements. This simplification is achieved through the use of two resonant loops, each connecting a pair of straps. A tuning algorithm developed for the system produces accurate matching, phasing, and amplitude balance within a small number of shots (≤ 5) in cases where the loading is sufficiently high, that is, when the resonant series load resistance (RSLR) > 2 ω, at values of kQ approaching 1. A coupled transmission line model of the antenna array and resonant loops has been created and used to determine changes in resistive and reactive loading, as well as changes in coupling between array elements during plasma shots. The design and modeling of this system and the operating experience are reviewed.

Modeling of Fast Wave Current Drive Experiments on DIII‐D

Modeling of fast wave current drive experiments for DIII‐D has been improved to include calculation of target temperature profiles consistent with the DIII‐D database and more accurate modeling of the launched spectrum. The calculations indicate that a measurable current will be driven by fast waves in the near‐term (30–200 kA). Modeling of the long‐range goal of 2 MA non‐inductive at high β indicates the proposed 18 MW of rf power will be adequate. The optimum frequency for the intermediate scenario is 120 MHz; this frequency selection is also adequate for the long‐term goals.