The authors present an analysis of the influence of various physics phenomena (which are not necessarily independent) on current drive performance in tokamaks. Such phenomena include diffraction and other non-geometrical optics processes, k|| modification, and multiple-pass absorption as well as antenna characteristics such as recessed cavity and septa geometry, poloidal extent and poloidal location of the current straps. The two-and-one-half-dimensional (21/2-D) full-wave code PICES is used for modelling ion cyclotron resonance heating and current drive. PICES is based on poloidal mode and reduced-order expansions. By 21/2-D, we mean that 3-D wave fields are calculated in axisymmetric geometry (2-D solution domain - ρ,θ), while the correct toroidal dependence of the antenna source currents is obtained from a 2-D (ρ ,ϕ) recessed antenna code. The calculation includes the poloidal and toroidal structure of the antenna, modification of the k|| spectrum due to the poloidal magnetic field and a complete solution for E||. A semi-analytic model for current drive, including trapped electron effects, is employed. These calculations are used extensively to model fast wave current drive in DIII-D, ITER and other tokamaks

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

The Alfvén resonances may occur on the low- and high-field sides for a low-frequency fast wave current drive scenario proposed for the International Thermonuclear Experimental Reactor (ITER) [Nucl. Fusion 31, 1135 (1991)]. At the resonance on the low-field side, the fast wave may be mode converted into a short-wavelength slow wave, which can be absorbed by electrons at the plasma edge, before the fast wave propagates into the core area of the plasma. Such absorption may cause a significant parasitic power loss.

Electromagneticfield calc_ationsforradio frequency(rf)antennas in two dimensions(2-D) neglectfinite antenna lengtheffects as wellas thefeedersleadingto the main current .strap.Comparisons with experimentsindicate that these 2-D calculations can overestimate the loadingof the antenna and fail to givethe correctreactive behavior.The 2-D calculationsalsopredictthat the returncurrentsin the sidewalls of the antenna structuredepend stronglyon plasma parameters,but thisprediction isalsosuspectbecause of experimental evidence.To study the validity of the 2-D approximation,the MultipleAntenna Implementation System (MAntIS) has been used to perform 3-D modeling of the power spectrum, plasma loading,and inductancefora relevant loop antenna design.Effects on antenna performance caused by feedersto the main currentstrap,conductingsidewalls, and finite phase velocity are considered.The plasma impedance matrix for the loadingcalculation is generatedby use of the ORION-ID code. The 3-D model isbenchmarked with the 2-D model in the 2-D limit.For finite-length antennas,inductance calculations are found to be in much more reasonable agreement with experiments for 3-D modeling than for the 2-D estimates. The modeling shows that the feeders affect the launchedpower spectrum in an indirect way by forcingthe drivenrf currentto returnin the antenna sidewalls ratherthan in the plasma as in the 2-D model. Thus, the feedershave much more influence than the plasma on the currentsthatreturnin the sidewall. It has alsobeen found thatpoloidaldependenciesin the : plasma impedance matrix can reducethe loadingfrom that predictedin the 2-D model. For some plasma parameters,the combined 3-D effects can lead to a reductionin the predicted loading by as much as a factorof 2 from that given by the 2-D model.

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