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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]

Theoretical considerations and high spatial resolution numerical simulations of radio frequency (rf) wave heating in tokamaks and in spherical toruses (ST) indicate that fast waves launched into tokamaks in the ion cyclotron range of frequencies (ICRF) or into spherical toruses in the high harmonic fast wave (HHFW) regime may excite a short wavelength slow mode inside of the plasma discharge due to the presence of hot electrons that satisfy the condition ω∥vte, where ω is the launched wave frequency, k∥ is the local parallel component of the wave vector, and vte is the local electron thermal speed. This excited slow wave may be related to the electrostatic ion cyclotron wave that propagates for frequencies above the fundamental ion cyclotron frequency [1] in warm plasmas or to a high frequency version of a kinetic Alfvén wave [2]. This slow wave, if physically real, would provide another path for rf power absorption in tokamaks and ST devices. [ABSTRACT FROM AUTHOR]

An analytic model for single particle motion in the presence of a wave field and multiple cyclotron harmonics is developed and investigated. The model suggests that even in the absence of Doppler broadening, cyclotron harmonic layers have finite spatial extent. This allows for particles to interact with more than one harmonic layer simultaneously, provided the layers are tightly packed. The latter phenomenon is investigated in the context of the model using symplectic mapping techniques. Then the model behavior is compared with numerical simulations of neutral beam particle trajectories in NSTX using the full-orbit code SPIRAL. [ABSTRACT FROM AUTHOR]

Lower hybrid (LH) waves have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons at (2.5-3)×vte, where vte = (2Te/me)1/2. The velocity at which damping occurs depends on the non-linear balance between quasilinear diffusion and collisions. For high efficiency current drive, a low parallel index of refraction, n∥, corresponding to a high phase velocity, is chosen. Depending on the plasma electron temperature this may put the wave propagation in a multi-pass regime. In cases of low parallel refractive index, ray tracing with no SOL has been shown to have differences with experiment [1] and collision effects in the scrape off layer may be important [2]. Using a coupled model of the full wave code, TORLH[3], and the Fokker-Planck code, CQL3D[4], the importance of full wave effects in weak and strong absorption regimes are studied. [ABSTRACT FROM AUTHOR]

The characterization of fluctuation amplitudes, spatial correlation lengths, and wave vectors through measurement of the correlation properties of reflected microwave diagnostic signals depends on a quantitaive knowledge of propagation in toroidal, magnetized plasma. The disparity between the radiation wavelength (mm) and the plasma size makes full wave computations challenging. We extend a two dimensional model [1] which computes propagation in a poloidal plane to include toroidal variation. The model reduces the computational burden compared to that of solving the full-wave equation everywhere-but retains both diffraction and refraction-by merging a description appropriate to the under dense plasma (paraxial) with the required full-wave description near the reflection layer. Initial results for ITER-like profiles demonstrate the utility of the tool as an aid in specifying antenna positioning and setting sensitivity requirements. [ABSTRACT FROM AUTHOR]

The goal of the high harmonic fast wave (HHFW) research on NSTX is to maximize the coupling of RF power to the core of the plasma by minimizing the coupling of RF power to edge loss processes. HHFW core plasma heating efficiency in helium and deuterium L-mode discharges is found to improve markedly on NSTX when the density 2 cm in front of the antenna is reduced below that for the onset of perpendicular wave propagation (nonset∝B*k|2/ω). In NSTX, the observed RF power losses in the plasma edge are driven in the vicinity of the antenna as opposed to resulting from multi-pass edge damping. PDI surface losses through ion-electron collisions are estimated to be significant. Recent spectroscopic measurements suggest that additional PDI losses could be caused by the loss of energetic edge ions on direct loss orbits and perhaps result in the observed clamping of the edge rotation. Initial deuterium H-mode heating studies reveal that core heating is degraded at lower k[lowercase_phi_synonym](-8 m-1 relative to 13 m-1) as for the L-mode case at elevated edge density. Fast visible camera images clearly indicate that a major edge loss process is occurring from the plasma scrape off layer (SOL) in the vicinity of the antenna and along the magnetic field lines to the lower outer divertor plate. Large type I ELMs, which are observed at both k[lowercase_phi_synonym] values, appear after antenna arcs caused by precursor blobs, low level ELMs, or dust. For large ELMs without arcs, the source reflection coefficients rise on a 0.1 ms time scale, which indicates that the time derivative of the reflection coefficient can be used to discriminate between arcs and ELMs. [ABSTRACT FROM AUTHOR]

Recent improvements in high-harmonic fast wave (HHFW) core heating in NSTX are attributed to using lithium conditioning, and other wall conditioning techniques, to move the onset density for perpendicular fast wave propagation further from the antenna. This has resulted in the first observation of HHFW core electron heating in deuterium plasma at a launched toroidal wavenumber, k[lowercase_phi_synonym] = -3 m-1, NSTX record core electron temperatures of 5 keV in helium and deuterium discharges and, for the first time, significant HHFW core electron heating of deuterium neutral-beam-fuelled H-mode plasmas. Also, k[lowercase_phi_synonym] = -8 m-1 heating of the plasma startup and plasma current ramp-up has resulted in significant core electron heating, even at central electron densities as low as ∼4×1018 m-3. [ABSTRACT FROM AUTHOR]

A full-wave electromagnetic field solver valid the lower hybrid range of frequencies (LHRF) has been developed that utilizes a semi-spectral representation for the RF electric field. Spurious numerical behavior of the field solver that was found to be related to the inclusion of finite electron Larmor radius terms in the wave equation is discussed. The removal of these terms is shown to eliminate all spurious mode generation, leading to well-behaved electric field solutions for parameters typical of LH current drive experiments in present day sized tokamaks. [ABSTRACT FROM AUTHOR]

The generation of energetic tails in the electron distribution function is intrinsic to lower-hybrid (LH) heating and current drive in weakly collisional magnetically confined plasma. The effects of these deformations on the RF deposition profile have previously been examined within the ray approximation [1, 2]. Recently, the calculation of full-wave propagation of LH waves in a thermal plasma [3] has been accomplished using an adaptation of the TORIC [4] code. Here, initial results are presented from TORIC simulations of LH propagation in a toroidal plasma with non-thermal electrons. The required efficient computation of the hot plasma dielectric tensor is accomplished using a technique previously demonstrated in full-wave simulations of ICRF propagation in plasma with non-thermal ions [5]. [ABSTRACT FROM AUTHOR]

The TORIC 2D full wave simulation code has been used to study the dynamics of waves with ω<ωci everywhere for all ions in the Alcator C-Mod tokamak. This potential heating regime can be accessed uniquely on C-Mod, because of its high magnetic field capability, BT<=8 T, and variable ICRF source frequency, 40–80 MHz. The simulations indicate that the launched fast waves can mode convert to a short wavelength slow wave on the high field side of the discharge that damps primarily on electrons. The degree to which the mode converted wave penetrates into the core of the plasma is found to depend on the equilibrium density profile. [ABSTRACT FROM AUTHOR]