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A major challenge for spherical tokamak development is to start-up and ramp-up the plasma current (Ip) without using a central solenoid. Experiments in the National Spherical Torus eXperiment (NSTX) demonstrated that 1.4 MW of 30 MHz high-harmonic fast wave (HHFW) power could generate an Ip = 300 kA H-mode discharge with a non-inductive Ip fraction, fNI ~ 0.7. The discharge had an axial toroidal magnetic field (BT(0)) of 0.55 T, the maximum BT(0) available on NSTX. NSTX has undergone a major upgrade (NSTX-U), that will eventually allow the generation of BT(0) ≤ 1 T and Ip ≤ 2 MA plasmas. Full wave simulations of 30 MHz HHFW and medium harmonic fast wave (MHFW) heating in NSTX-U predict significantly reduced FW power loss in the plasma edge at the higher BT(0) achievable in NSTX-U. HHFW experiments will aim to generate stable, fNI ~ 1, Ip = 300 kA H-mode plasmas and to ramp Ip from 250 to 400 kA with FW power. Time-dependent TRANSP simulations are used to develop non-inductive Ip ramp-up and sustainment using 30 MHz FW power. This paper presents results from these RF simulations and plans for developing non-inductive plasmas heated by FW power. [ABSTRACT FROM AUTHOR]

Several experiments on different machines and in different fast wave (FW) heating regimes, such as hydrogen minority heating and high harmonic fast waves, have found strong interactions between radio-frequency (RF) waves and the scrape-off layer (SOL) region. This paper examines the propagation and the power loss in the SOL by using the full wave code AORSA, in which the edge plasma beyond the last closed flux surface (LCFS) is included in the solution domain and a collisional damping parameter is used as a proxy to represent the real, and most likely nonlinear, damping processes. 3D AORSA results for the National Spherical Torus eXperiment (NSTX), where a full antenna spectrum is reconstructed, are shown, confirming the same behavior found for a single toroidal mode results in Bertelli et al, Nucl. Fusion, 54 083004, 2014, namely, a strong transition to higher SOL power losses (driven by the RF field) when the FW cut-off is moved away from in front of the antenna by increasing the edge density. Additionally, full wave simulations have been extended to "conventional" tokamaks with higher aspect ratios, such as the DIII-D, Alcator C-Mod, and EAST devices. DIII-D results show similar behavior found in NSTX and NSTX-U, consistent with previous DIII-D experimental observations. In contrast, a different behavior has been found for Alcator C-Mod and EAST, which operate in the minority heating regime unlike NSTX/NSTX-U and DIII-D, which operate in the mid/high harmonic regime. A substantial discussion of some of the main aspects, such as (i) the pitch angle of the magnetic field; (ii) minority heating vs. mid/high harmonic regimes is presented showing the different behavior of the RF field in the SOL region for NSTX-U scenarios with different plasma current. Finally, the preliminary results of the impact of the SOL region on the evaluation of the helicon current drive efficiency in DIII-D is presented for the first time and briefly compared with the different regimes mentioned above. [ABSTRACT FROM AUTHOR]

Field-aligned losses of HHFW power in the SOL of NSTX have been studied with IR cameras and probes, but the interpretation of the data depends somewhat on the magnetic equilibrium reconstruction. Both EFIT02 and LRDFIT04 magnetic equilibria have been used with the SPIRAL code to provide field mappings in the scrape off layer (SOL) on NSTX from the midplane SOL in front of the HHFW antenna to the divertor regions, where the heat deposition spirals are measured. The field-line mapping spiral produced at the divertor plate with LRDFIT04 matches the HHFWproduced heat deposition best, in general. An independent method for comparing the field-line strike patterns on the outer divertor for the two equilibria is provided by measuring Langmuir probe characteristics in the vicinity of the outer vessel strike radius (OVSR) and observing the effect on floating potential, saturation current, and zero-probe-voltage current (IV=0) with the crossing of the OVSR over the probe. Interestingly, these comparisons also reveal that LRDFIT04 gives the more accurate location of the predicted OVSR, and confirm that the RF power flow in the SOL is essentially along the magnetic field lines. Also, the probe characteristics and IV=0 data indicate that current flows under the OVSR in the divertor tiles in most cases studied. [ABSTRACT FROM AUTHOR]

Fast-wave heating will be a major heating scheme on ITER, as it can heat ions directly and is relatively unaffected by the large machine size unlike neutral beams. However, fast-wave interactions with the plasma edge can lead to deleterious effects such as, in the case of the high-harmonic fast-wave (HHFW) system on NSTX, large losses of fast-wave power in the scrape off layer (SOL) under certain conditions. In such scenarios, a large fraction of the lost HHFW power is deposited on the upper and lower divertors in bright spiral shapes. The responsible mechanism(s) has not yet been identified but may include fast-wave propagation in the scrape off layer, parametric decay instability, and RF currents driven by the antenna reactive fields. Understanding and mitigating these losses is important not only for improving the heating and current-drive on NSTX-Upgrade but also for understanding fast-wave propagation across the SOL in any fast-wave system. This talk summarizes experimental results demonstrating that the flow of lost HHFW power to the divertor regions largely follows the open SOL magnetic field lines. This lost power flux is relatively large close to both the antenna and the last closed flux surface with a reduced level in between, so the loss mechanism cannot be localized to the antenna. At the same time, significant losses also occur along field lines connected to the inboard edge of the bottom antenna plate. The power lost within the spirals is roughly estimated, showing that these field-aligned losses to the divertor are significant but may not account for the total HHFW loss. To elucidate the role of the onset layer for perpendicular fast-wave propagation with regards to fast-wave propagation in the SOL, a cylindrical cold-plasma model is being developed. This model, in addition to advanced RF codes such as TORIC and AORSA, is aimed at identifying the underlying mechanism(s) behind these SOL losses, to minimize their effects in NSTX-U, and to predict their importance in ITER. [ABSTRACT FROM AUTHOR]

The Moon is used as a target volume for ultra-high energy neutrino searches with terrestrial radio telescopes. The LUNASKA project has conducted observations with the Parkes and ATCA telescopes; and, most recently, with both of them in combination. We present an analysis of the data obtained from these searches, including validation and calibration results for the Parkes-ATCA experiment, as well as a summary of prospects for future observations. [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

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]

ELMs reduce the stored energy achieved with HHFW heating compared with the ELM free case as also occurs for NBI heating alone. This reduction can be attributed both to direct ELM ejection of stored energy and to an increase in edge density with ELMs that exceeds the onset density for perpendicular wave propagation near the antenna [1,2], and leads to significantly more edge RF power deposition. This latter effect causes a more intense RF 'hot' zone in the lower divertor scrape off region due to an increase in edge RF power propagating to the divertor from the antenna region along the magnetic field lines. Fast IR measurements of the direct ELM heat deposition at the lower divertor shows it to be peaked in the vicinity of the outer strike radius and to fall off strongly as the 'hot' zone is approached, indicating little direct ELM effect on the 'hot' zone heating. Physics studies of the 'hot' zone have begun with sweeping the RF 'hot' zone spiral over Rogowski instrumented divertor region tiles to show that the tile currents track the location of the zone. The relation of the results here to the AORSA RF code simulations including the SOL region is pointed out. [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 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]

The implementation in TRANSP of a recent version of TORIC capable of calculating power deposition for HHFW conditions is used to analyze NSTX plasma under different operating conditions. The power deposition profile into the electrons is obtained for high-Te conditions —Te<=5 keV— obtained in He and D plasmas with ITB. HHFW heating of NBI-induced H-mode plasmas is discussed. At the RF onset the RF power is divided evenly between the electrons and the fast particles, but as the latter thermalize and the electron density increases, the HHFW power repartition shifts progressively toward the electrons. Power deposition profiles for the electrons and for the fast particles are shown. [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]

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 is the electron thermal speed. Consequently these waves are well-suited to driving current in the plasma periphery where the electron temperature is lower, making LH current drive (LHCD) a promising technique for off-axis (r/a>=0.60) current profile control in reactor grade plasmas. Established techniques for computing wave propagation and absorption use WKB expansions with non-Maxwellian self-consistent distributions. In typical plasma conditions with electron densities of several 1019 m-3 and toroidal magnetic fields strengths of 4 Telsa, the perpendicular wavelength is of the order of 1 mm and the parallel wavelength is of the order of 1 cm. Even in a relatively small device such as Alcator C-Mod with a minor radius of 22 cm, the number of wavelengths that must be resolved requires large amounts of computational resources for the full wave treatment. These requirements are met with a massively parallel version of the TORIC full wave code that has been adapted specifically for the simulation of LH waves [J. C. Wright, et al., Commun. Comput. Phys., 4, 545 (2008), J. C. Wright, et al., Phys. Plasmas 16 July (2009)]. This model accurately represents the effects of focusing and diffraction that occur in LH propagation. It is also coupled with a Fokker-Planck solver, CQL3D, to provide self-consistent distribution functions for the plasma dielectric as well as a synthetic hard X-ray (HXR) diagnostic for direct comparisons with experimental measurements of LH waves. The wave solutions from the TORIC-LH zero FLR model will be compared to the results from ray tracing from the GENRAY/CQL3D code via the synthetic HXR diagnostic and power deposition. [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]

An idea for directly heating the thermal ion population with radio frequency waves at frequencies below the ion cyclotron frequency is proposed. This method would increase fusion efficiency by avoiding the lossy electron channel [ABSTRACT FROM AUTHOR]

Alcator C-Mod is similar in density, field, and plasma shape to ITER and consequently, the lower hybrid (LH) wave dispersion is very similar. The differences in temperature between the two devices do affect the relation between n∥ and the location at which damping occurs. Even with a parallel code, LH on ITER is a petascale problem requiring on the order of 100 000 processor cores and 10 000 poloidal modes to complete in one hour. Alcator C-Mod is 1/10th the scale of ITER requiring 1000 times less computation and simulations of LH in this machine have required on the order of 1000 cpu-hours. Therefore, we focus on analysis of full-wave physics effects in LH propagation in Alcator C-Mod using the LH version of the TORIC code [J. C. Wright et al., Phys. Plasmas 11, 2473 (2004)] and contrast those results with ray tracing calculations. Non-Maxwellian effects through development of the quasilinear plateau also play a role and both codes have a generalized dielectric using numerically calculated distributions [Valeo in this proceedings] to incorporate this effect. We will discuss issues of resolution requirements, algorithm improvements, and convergence as well, and speculate on further changes to the algorithms that may enable simulations of ITER with less than petascale requirements. [ABSTRACT FROM AUTHOR]

Self-consistent full-wave and Fokker-Planck simulations of wave heating in tokamaks have been generalized to include multiple energetic ion populations. To simplify the problem, it is assumed that, because of their low densities, the individual ion tails do not interact with each other in the Fokker-Planck solution. They do, however, interact self-consistently in the wave solution. Preliminary results are presented for DIII-D at 60 MHz and 116 MHz. [ABSTRACT FROM AUTHOR]

This paper describes physics research on wave-plasma interactions in the ion cyclotron range of frequencies (ICRF) and the lower hybrid range of frequencies (LHRF) that takes advantage of advances in large scale computation as well as applied mathematics expertise. A combined Fokker-Planck and full-wave solver is used to simulate the interaction of ICRF waves with nonthermal ions generated by the wave itself, with fast ions from neutral beam injection (NBI), and with alpha particles from fusion processes. Finite ion drift orbit effects are included in this analysis through the use of a Monte Carlo approach. Models for simulating the linear and nonlinear interaction of an ICRF antenna with the tenuous edge plasma are also described. Finally we describe advances that have been made to perform a combined full-wave and Fokker Planck analysis of nonthermal electrons during LH current drive. [ABSTRACT FROM AUTHOR]

HHFW RF power delivered to the core plasma of NSTX is strongly reduced as the launched wavelength is increased—for B[uppercase_phi_synonym] = 4.5 kG, heating is ∼1/2 as effective at k[lowercase_phi_synonym] = -7 m-1 as at 14 m-1 and ∼1/10 as effective at -3 m-1. Measured edge ion heating, attributable to parametric decay (PDI), increases with wavelength but not fast enough to account for the observed power loss. Surface fast waves (FW) may enhance both PDI and also losses in sheaths and structures around the machine—FW fields propagate closer to the wall with decreasing B[uppercase_phi_synonym] and k∥ (onset ne∝B[uppercase_phi_synonym]×k∥2). A dramatic increase in core heating efficiency is observed at -7 m-1 when B[uppercase_phi_synonym] is increased to 5.5 kG—central Te near 4 keV at PRF = 2 MW. Also, the PDI losses are a weak function of B[uppercase_phi_synonym] and k∥, whereas the far-field RF poloidal magnetic field (at 5.5 kG) increases a factor of ∼3 when k∥ is reduced from 14 m-1 to -3 m-1, suggesting a large increase in wall/sheath power loss and a major effect of surface fast waves on edge losses. [ABSTRACT FROM AUTHOR]

Traditional particle-in-cell / time-domain methods excel in situations of complex geometry, but are difficult to use in fusion plasma type problems, primarily because of explicit time-step algorithms that become numerically unstable when confronted with temporally or spatially unresolved physics, at high frequency and short scale-length, such as plasma oscillations and sheaths. We discuss an approach for a progressive treatment of these difficulties with carefully planned intermediate steps, each fully functional and useful in its own right. In our first year we plan to implement, in an implicit algorithm, the standard linear magnetized cold-plasma dielectric tensor in time-domain to treat the RF propagation. Particles will be pushed, but initially only for comparison purposes with the currents of the linear dielectric medium. This will permit cataloging of kinetic and non-linear departures from the linear model, while at the same time assuring numerical stability. The linear equations are identical to the fluid equations, and so subsequent upgrades involve relaxation of fluid linearity, and eventual feedback from particle information into the fluid quantities, and incorporation of new sheath boundary physics models. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Self-consistent solutions for the wave electric field and particle distribution function are calculated for ion cyclotron heating in non-Maxwellian plasmas. The all-orders wave solver AORSA is generalized to treat non-thermal velocity distributions arising from fusion reactions, neutral beam injection, and wave driven diffusion in velocity space. Quasi-linear diffusion coefficients are derived directly from the wave electric fields and used to calculate velocity distribution functions with the CQL3D Fokker-Planck code. Self-consistent results are obtained by iterating the full-wave and Fokker-Planck solutions. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Thomson scattering laser pulses are synchronized relative to modulated HHFW power to permit evaluation of the electron energy confinement time during and following HHFW pulses for both heating and current drive antenna phasing. Profile changes resulting from instabilities require that the total electron stored energy, evaluated by integrating the midplane electron pressure Pe(R) over the magnetic surfaces prescribed by EFIT analysis, be used to derive the electron energy confinement time. Core confinement is reduced during a sawtooth instability but, although the electron energy is distributed outward by the sawtooth, the bulk electron energy confinement time is essentially unaffected. The radial deposition of energy into the electrons is noticeably more peaked for current drive phasing (longer wavelength excitation) relative to that for heating phasing (shorter wavelength excitation) as is expected theoretically. However, the power delivered to the core plasma is reduced considerably for the current drive phasing, indicating that surface/peripheral damping processes play a more important role for this case. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

High Harmonic Fast Wave (HHFW) heating experiments on NSTX have been observed to be accompanied by significant edge ion heating (Ti >> Te). This heating is found to be anisotropic with Tperp > Tpar. Simultaneously, coherent oscillations have been detected with an edge Langmuir probe. The oscillations are consistent with parametric decay of the incident fast wave (ω > 13ωc) into ion Bernstein waves and an unobserved ion-cyclotron quasi-mode. The observation of anisotropic heating is consistent with Bernstein wave damping and the Bernstein waves should completely damp in the plasma periphery as they propagate toward a cyclotron harmonic resonance. The number of daughter waves is found to increase with rf power and to increase as the incident wave’s toroidal wavelength increases. The frequencies of the daughter wave are separated by the edge ion cyclotron frequency. Theoretical calculations of the threshold for this decay in uniform plasma indicate an extremely small value of incident power should be required to drive the instability. While such decays are commonly observed at lower harmonics in conventional ICRF heating scenarios they usually do not involve the loss of significant wave power from the pump wave. On NSTX an estimate of the power loss can be found by calculating the minimum power required to support the edge ion heating (presumed to come from the decay Bernstein wave). This calculation indicates at least 20–30% of the incident rf power ends up as decay waves. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The most common method for treating wave propagation in tokamaks in the lower hybrid range of frequencies (LHRF) has been toroidal ray tracing, owing to the short wavelengths (relative to the system size) found in this regime. Although this technique provides an accurate description of 2D and 3D plasma inhomogeneity effects on wave propagation, the approach neglects important effects related to focusing, diffraction, and finite extent of the RF launcher. Also, the method breaks down at plasma cutoffs and caustics. Recent adaptation of full-wave electromagnetic field solvers to massively parallel computers has made it possible to accurately resolve wave phenomena in the LHRF. One such solver, the TORIC code, has been modified to simulate LH waves by implementing boundary conditions appropriate for coupling the fast electromagnetic and the slow electrostatic waves in the LHRF. In this frequency regime the plasma conductivity operator can be formulated in the limits of unmagnetized ions and strongly magnetized electrons, resulting in a relatively simple and explicit form. Simulations have been done for parameters typical of the planned LHRF experiments on Alcator C-Mod, demonstrating fully resolved fast and slow LH wave fields using a Maxwellian non-relativistic plasma dielectric. Significant spectral broadening of the injected wave spectrum and focusing of the wave fields have been found, especially at caustic surfaces. Comparisons with toroidal ray tracing have also been done and differences between the approaches have been found, especially for cases where wave caustics form. The possible role of this diffraction-induced spectral broadening in filling the spectral gap in LH heating and current drive will be discussed. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Equilibrium considerations infer that large localized radial electric fields are associated with internal transport barrier structures in tokamaks and other toroidal magnetic confinement configurations. In this paper, the effects of an equilibrium electric field on fast magnetosonic wave propagation are considered in the context of a cold plasma model. [ABSTRACT FROM AUTHOR]

Rf power at 30 MHz has been applied in a variety of situations to NSTX plasmas. The response of the plasma is observed in order to study both the physics of High Harmonic Fast Wave (HHFW) heating and as a tool to extend the performance of NSTX plasmas. In this paper we will discuss the progress made to date towards these goals. [ABSTRACT FROM AUTHOR]

Effect of scattering of high harmonic fast magnetosonic waves (HHFW) by low frequency plasma turbulence is investigated. Due to the similarity of the wavelength of HHFW to that of the expected low frequency turbulence in the plasma edge region, the scattering of HHFW can become significant under some conditions. The scattering probability increases with the launched wave parallel-phase-velocity as the location of the wave cut-off layer shifts toward lower density edge. The scattering probability can be reduced significantly with higher edge plasma temperature, steeper edge density gradient, and magnetic field. The theoretical model could explain some of the HHFW heating observations on NSTX. [ABSTRACT FROM AUTHOR]

Modeling of HHFW heating in recent NSTX discharges using the CURRAY ray tracing code is reported. Experimental results have been obtained for both D and He discharges, and for two antenna spectra, k[sub ]≈14 m[sup -1] (slow) and k[sub ]∼7 m[sup -1] (fast), with launched power of up to ∼ 3 MW. Significant electron heating was observed in He discharges for the slow spectrum, where ΔT[sub e0] ∼ 0.5 keV with ∼2 MW launched power. Weaker electron heating occurs in D plasmas with the same wave spectrum, and also for the fast spectrum in either D or He discharges. CURRAY analysis confirms strong single-pass electron absorption in the best heating cases in He, with the absorption peaked off the magnetic axis. The power flow path off the mid-plane agrees with that from recent full-wave analysis. It has also been shown that absorption by minority H in D plasmas may contribute to the low level of electron heating at k[sub ]∼7 m[sup -1]. Calculations indicate that as the hydrogen tail energy and density increase, H can compete with electrons in wave absorption. Wave scattering due to edge density fluctuations may be a second contributing factor, and absorption of the scattered wave has been investigated. [ABSTRACT FROM AUTHOR]

Mode-converted electron Bernstein waves (EBWs) potentially allow the measurement of local electron temperature (T) and the implementation of local heating and current drive in spherical torus (ST) devices, which are not directly accessible to low harmonic electron cyclotron waves. This paper reports on the measurement of X-mode radiation modeconverted from EBWs observed normal to the magnetic field on the midplane of the CDX-U and NSTX ST plasmas. The radiation temperature of the EBW emission was compared to T measured by Thomson scattering and Langmuir probes. EBW mode conversion efficiencies of over 20% were measured on both CDX-U and NSTX. Sudden increases of mode conversion efficiency, of over a factor of three, were observed at H-mode transitions on NSTX, when the measured edge density profile steepened. The EBW mode conversion efficiency was found to depend on the density gradient at the mode conversion layer in the plasma scrape off, consistent with theoretical predictions. The EBW emission source was determined by a perturbation technique to be localized at the electron cyclotron resonance layer and was successfully used for radial transport studies. Recently, a new in-vessel antenna and Langmuir probe array were installed on CDX-U to better characterize and enhance the EBW mode conversion process. The probe incorporates a local adjustable limiter to control and maximize the mode conversion efficiency in front of the antenna by modifying the density profile in the plasma scrape off where fundamental EBW mode conversion occurs. Initial results show that the mode conversion efficiency can be increased to ∼ 100% when the local limiter is inserted near the mode conversion layer. Plans for future EBW research, including EBW heating and current drive studies, are discussed. [ABSTRACT FROM AUTHOR]

The full-wave ICRF code TORIC has been modified to use numerical MHD equilibrium solutions obtained from EFIT. The EFIT equilibrium provide a more accurate geometry than the moments representation based analytic flux surface models currently used in TORIC. Several additional enhancements were made, allowing TORIC to fully use asymmetric EFIT equilibrium. A comparison of EFIT/TORIC and stand-alone TORIC is presented here for both CMOD and NSTX. [ABSTRACT FROM AUTHOR]

Internal transport barrier (ITB) modes have been observed with intense, off-axis ICRF heating in Alcator C-Mod. This barrier mode is routinely produced in enhanced D[sub α] H-mode, 4.5 T, 0.8 MA discharges (H cyclotron position is r/a ∼ 0.5 to the high field side). From TRANSP analysis, the χ[sub eff] is significantly reduced inside r/a∼0.4 where the density peaking is observed. With the barrier formation, the toroidal plasma rotation near the axis reversed from co-current (+50 km/sec) to counter current direction (-20 km/sec). In an effort to maintain the barrier, experiments utilizing a third antenna, operated at 70 MHz, allowed central ICRF heating of the ITB mode. Initial experiments with up to 1.2 MW confirmed the energy transport barrier and allowed significant increase in the ion temperatures. With the central heating, the ITB was sustained and the density and impurity peaking were arrested. Furthermore, the central rotation was observed to rotate in the co-current direction with the central heating. We have also had success in detecting electron density fluctuations at the RF frequency with a heterodyned phase contrast imaging diagnostic. The fast wave was observed with peaked density profiles and the measured wave number was in reasonable agreement with the cold plasma dispersion relationship. The mode-converted ion Bernstein wave (IBW) has also been observed in H (³He, D) plasmas. The measured wave number is in good agreement with the hot plasma dispersion relationship and indicates the wave is a backward wave as expected from the dispersion relationship. Simulations utilizing the full wave code TORIC have been critical to this analysis. TORIC has recently been upgraded to allow for higher poloidal mode number resolution. The predicted electron absorbed power more accurately reflects the measured values. Recent success in commissioning a 4-strap (J-port) antenna has allowed the maximum injected power to reach 2.5 MW from this... [ABSTRACT FROM AUTHOR]

High-Harmonic Fast-Wave (HHFW), a radio-frequency technique scenario applicable to high-beta plasmas, has been selected as one of the main auxiliary heating systems on NSTX. The HHFW antenna assembly comprises 12 toroidally adjacent current elements, extending poloidally and centered on the equatorial plane. This paper reviews experimental results obtained with a symmetrical (vacuum) launching spectrum with k[sub //] = 14 m[sup -1] at a frequency of 30 MHz. We describe results obtained when HHFW power is applied to helium and deuterium plasmas, during the plasma-current flattop period of the discharge. Application of 1.8-MW HHFW pulse to MHD quiescent plasmas resulted in strong electron heating, during which the central electron temperature, T[sub eo] more than doubled from ≈0.5 keV to 1.15 keV. In deuterium plasmas, HHFW heating was found less efficient, with a T[sub e0] increase of the order of 40% during a 1.8-MW HHFW pulse, from ≈400 eV to ≈550 eV. (At HHFW power of 2.4 MW, T[sub eo] increased by 60%, reaching 0.625 keV.) HHFW heating in presence of MHD activity is also discussed. A short neutral beam pulse was applied to permit charge-exchange recombination spectroscopy (CHERS) measurement of the impurity ion temperature T[sub i]. Preliminary CHERS analysis show that T[sub i] ≈ T[sub e] during HHFW heating. Of special interest are deuterium discharges, where the application of HHFW power was done during the current ramp-up. We observe the creation of large density gradients in the edge region. In the latter case, the density rose spontaneously to n[sub eo] ≤ 8 × 10[sup 13] cm[sup -3]. [ABSTRACT FROM AUTHOR]