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Lower hybrid waves are very important for heating and current drive in contemporary tokamaks. Phased arrays of rectangular waveguides are frequently used as launcher. While the simple theory of parallel plate waveguide coupling to the plasma slab with the linear density profile was successful [1], it neglected the coupling of the fast to slow wave in the inaccessibility region and could not handle the effect of the poloidal field. These drawbacks were overcome by 2D grill theory assuming rectangular waveguides and the full wave solution in the plasma [2]. However, this method is rarely used for large structures as it requires the evaluation of a very large number of 2D infinite integrals in k-space [3]. Here, we overcome this drawback by using high order Gaussian quadratures and 2D B-splines of the integrands in Npol, Ntor-space. We adopt efficient and accurate methods of handling the eigenmodes excited by the LH grill in the inaccessible Ntor -region and clarify the role of collisions. [ABSTRACT FROM AUTHOR]

Fast Waves (FWs) at 60 MHz and 90 MHz are used in DIII-D for central electron heating and current drive. Coupling of FWs to high-performance discharges is limited by low antenna loading in these regimes. To extend the application of high-power FWs to such regimes, methods of increasing the antenna loading in these regimes are needed. A systematic study of loading enhancement techniques has been carried out in DIII-D, including reduction of the antenna/plasma distance, gas puffing into the far scrape-off layer (SOL), and control of other parameters that affect the particle balance in the far SOL. Quantitative understanding of the physics of the loading resistance and its dependence on edge density profiles is demonstrated. The core FW heating efficiency appeared to be ∼100% in the Advanced Inductive regime, consistent with the high first-pass direct electron absorption of ∼75% that is predicted by the ray-tracing code GENRAY in this high electron beta regime. [ABSTRACT FROM AUTHOR]

30 MHz high-harmonic fast wave (HHFW) heating and current drive are being developed to assist fully non-inductive plasma current (Ip) ramp-up in NSTX. The initial approach to achieving this goal has been to heat Ip = 300 kA inductive plasmas with current drive antenna phasing in order to generate an HHFW H-mode with significant bootstrap and RF-driven current. Recent experiments, using only 1.4 MW of RF power (PRF), achieved a non-inductive current fraction, fNI∼0.65. Improved antenna conditioning resulted in the generation of Ip = 650 kA HHFW H-mode plasmas, with fNI∼0.35, when PRF≥2.5 MW. These plasmas have little or no edge localized mode (ELM) activity during HHFW heating, a substantial increase in stored energy and a sustained central electron temperature of 5-6 keV. Another focus of NSTX HHFW research is to heat an H-mode generated by 90 keV neutral beam injection (NBI). Improved HHFW coupling to NBI-generated H-modes has resulted in a broad increase in electron temperature profile when HHFW heating is applied. Analysis of a closely matched pair of NBI and HHFW+NBI H-mode plasmas revealed that about half of the antenna power is deposited inside the last closed flux surface (LCFS). Of the power damped inside the LCFS about two-thirds is absorbed directly by electrons and one-third accelerates fast-ions that are mostly promptly lost from the plasma. At longer toroidal launch wavelengths, HHFW+NBI H-mode plasmas can have an RF power flow to the divertor outside the LCFS that significantly reduces RF power deposition to the core. ELMs can also reduce RF power deposition to the core and increase power deposition to the edge. Recent full wave modeling of NSTX HHFW+NBI H-mode plasmas, with the model extended to the vessel wall, predicts a coaxial standing mode between the LCFS and the wall that can have large amplitudes at longer launch wavelengths. These simulation results qualitatively agree with HHFW+NBI H-mode data that show decreasing core RF heating efficiency and increasing RF power flow to the lower divertor at longer launch wavelengths. [ABSTRACT FROM AUTHOR]