Articles you may be interested in Discharge regime of non-ambipolarity with a self-induced steady-state magnetic field in plasma sources with localized radio-frequency power deposition Phys.
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.
The non-inductive current drive from directional fast Alfven and electron cyclotron waves was measured in the DIII-D tokamak in order to demonstrate these forms of radiofrequency (RF) current drive and to compare the measured efficiencies with theoretical expectations. The fast wave frequency was 8 times the deuterium cyclotron frequency at the plasma centre, while the electron cyclotron wave was at twice the electron cyclotron frequency. Complete non-inductive current drive was achieved using a combination of fast wave current drive (FWCD) and electron cyclotron current drive (ECCD) in discharges for which the total plasma current was inductively ramped down from 400 to 170 kA. For steady current discharges, an analysis of the loop voltage revealed up to 195 kA of non-inductive current (out of 310 kA) during combined electron cyclotron and fast wave injection, with a maximum of 110 kA of FWCD and 80 kA of ECCD achieved (not simultaneously). The peakedness of the current profile increased with RF current drive, indicating that the driven current was centrally localized. The FWCD efficiency increased linearly with the central electron temperature as expected; however, the FWCD was severely degraded in low current discharges owing to incomplete fast wave absorption. The measured FWCD agreed with the predictions of a ray tracing code only when a parasitic loss of 4% per pass was included in the modelling along with multiple pass absorption. Enhancement of the second harmonic ECCD efficiency by the toroidal electric field was observed experimentally. The measured ECCD was in good agreement with Fokker-Planck code predictions
A dual frequency rf sheath is analyzed using a simple rf sheath model to study the interaction between the two driving rf currents and their effect on sheath parameters. A symmetric rf discharge with defined electron density and dc sheath potential is modeled using a sharp boundary sheath approximation. Three results of this study are reported: (1) reproduction of trends in ion energy distribution functions predicted and measured in previous studies, (2) a frequency-mixing-dependent relationship between the dc sheath potential and applied rf potential, and (3) an additional asymmetry in the ion energy distribution function generated by the intermodulation components resulting from the nonlinear sheath.
The three dimensional (3-D) nature of antennas for fusion applications in the ion cyclotron range of frequencies (ICRF) requires accurate modelling to design and analyse new antennas. In this article, analysis and design tools for radiofrequency (RF) antennas are successfully benchmarked with experiment, and the 3-D physics of the launched waves is explored. The systematic analysis combines measured density profiles from a reflectometer system, transmission line circuit modelling, detailed 3-D magnetostatics modelling and a new 3-D electromagnetic antenna model including plasma. This analysis gives very good agreement with measured loading data from the Tokamak Fusion Test Reactor (TFTR) Bay-M antenna, thus demonstrating the validity of the analysis for the design of new RF antennas. The 3-D modelling is contrasted with 2-D models, and significant deficiencies are found in the latter. The 2-D models are in error by as much as a factor of 2 in real and reactive loading, even after they are corrected for the most obvious 3-D effects. Three dimensional effects play the most significant role at low parallel wavenumbers, where the launched power spectrum can be quite different from the predictions of 2-D models. Three dimensional effects should not be ignored for many RF designs, especially those intended for fast wave current drive
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