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High power helicon waves (whistler or very high harmonic fast lower hybrid waves) at a frequency of 476 MHz are being tested for efficient off-axis current drive on DIII-D with the goal of demonstrating profile control in AT plasmas [1-4]. In agreement with earlier theoretical predictions, strong Parametric Decay Instability (PDI) has been observed at injected RF power levels in the range of 0.05-0.5 MW with corresponding electric fields of 10-30 kV/m [5,6]. The dominant driver of the PDI is the E×B and the polarization drift velocity which can drive ion cyclotron quasi-modes and lower hybrid (or IBW) sideband waves unstable [5,6]. Initial experimental results have been obtained with powers up to 0.3 MW showing evidence of strong PDI measured with high-frequency one-turn magnetic probes located at both the outboard and the inboard wall at frequencies set by the usual selection rules [7,8]. Here we review the appropriate analytic formulation to predict such instabilities and present numerical evaluation of frequencies and growth rates relevant to DIII-D plasma parameters. We also assess the convective thresholds for the PDIs, and compared them with experimental observations. [ABSTRACT FROM AUTHOR]

Experiments in DIII-D have measured the width of the power deposition and current drive profiles for electron cyclotron (EC) waves by modulating the gyrotrons and fitting the periodic response of the electron temperature profile or motional Stark effect signals to a linearized diffusion equation. The measured EC power deposition widths are scattered around the predicted values, including both narrower and wider cases. The average width for single gyrotron modulation is ≈88% of the theoretical value from ray tracing, while the average width for multiple gyrotron modulation is ≈127% of theory; aiming misalignment between the different EC launchers may explain why multi-gyrotron measurements are wider than for single gyrotrons. The measured EC current drive widths have nearly the same amount of profile broadening (≈131% of theory, on average) as does multi-gyrotron power deposition. Finally, while the measured locations of EC current drive are well centered on the expected values, the measured heating locations are skewed to larger radius. This difference may be due to rapid heat pulse propagation causing an inherent outward bias to the deduced power deposition profile, whereas magnetic flux diffusion has less effect on current drive measurements. [ABSTRACT FROM AUTHOR]

"Steady-state" hybrid plasmas in DIII-D with zero surface loop voltage have been maintained for up to two current relaxation times using 3.4 MW of central electron cyclotron current drive (ECCD). In addition to driving ≈0.2 MA of plasma current, central ECCD leads to significant changes in Alfvén eigenmode (AE) activity and thermal transport. For neutral-beam-only heating, strong AE activity is observed that causes a ~35% degradation in the neutron rate. With central ECCD this AE activity is suppressed, replaced by a bursty energetic particle mode that appears more benign as the neutron rate is closer to the classical value. The electron thermal diffusivity increases by ≈50% for 2.4 MW of ECCD compared to neutral-beam-only cases. Fortunately, the global thermal confinement factor remains the same (H98y2=1.4) as the higher thermal transport for PEC=2.4 MW hybrids is offset by the decreased fast ion transport resulting from AE suppression. [ABSTRACT FROM AUTHOR]

Theoretical work, computation, and results from TCV [J. Decker "Effect of density fluctuations on ECCD in ITER and TCV," EPJ Web of Conf. 32, 01016 (2012)] suggest that density fluctuations in the edge region of a tokamak plasma can cause broadening of the ECH deposition profile. In this paper, a GUI tool is presented which is used for analysis of ECH deposition as a first step towards looking for this broadening, which could explain effects seen in previous DIII-D ECH transport studies [K.W. Gentle "Electron energy transport inferences from modulated electron cyclotron heating in DIII-D," Phys. Plasmas 13, 012311 (2006)]. By applying an FFT to the Te measurements from the University of Texas's 40- channel ECE Radiometer, and using a simplified thermal transport equation, the flux surface extent of ECH deposition is determined. The Fourier method analysis is compared with a Break-In-Slope (BIS) analysis and predictions from the raytracing code TORAY. Examination of multiple Fourier harmonics and BIS fitting methods allow an estimation of modulated transport coefficients and thereby the true ECH deposition profile. Correlations between edge fluctuations and ECH deposition in legacy data are also explored as a step towards establishing a link between fluctuations and deposition broadening in DIII-D. [ABSTRACT FROM AUTHOR]

We compare, qualitatively and with a simple semi-quantitative model, the difficulty of coupling to fast waves at 0.5 GHz ('whistlers' or 'helicons') and to slow waves ('lower hybrid waves') at 5 GHz and n‖=3 under conditions relevant to the DIII-D tokamak. Though the density at which the slow wave begins to propagate is much lower than the fast wave, the stronger evanescence in vacuum for the higher frequency wave makes excitation of the SWs more difficult than for the FW case. Enforcing a minimum density at the coupler face, as might be produced by local gas puffing, is shown to likely be more effective for the SW case. The fact that the surface admittance for the slow wave at 0.5 GHz is much larger than for the FW at the same frequency has possibly significant implications for attempts to excite the FW. [ABSTRACT FROM AUTHOR]

High-power electron cyclotron (EC) waves are used to increase performance in several Advanced Tokamak (AT) regimes on DIII-D where there is a simultaneous need for high noninductive current and high beta. In the Quiescent High-confinement mode (QH-mode), a direct measurement of the electron cyclotron current drive (ECCD) profile is made using modulation techniques, and a trapped electron mode (TEM) dominated regime with core Te>Ti is created. In the 'highqmin' AT scenario, ECCD provides part of the off-axis noninductive current and helps to produce a tearing stable equilibrium. In the hybrid regime, strong central current drive from EC waves and other sources increases the noninductive current fraction to ≈100%. Surprisingly, the core safety factor remains above unity, meaning good alignment between the current drive profile and the desired plasma current profile is not necessary in this scenario. [ABSTRACT FROM AUTHOR]

Experiments on DIII-D have measured the electron cyclotron current drive (ECCD) efficiency for co- and counter-injection in low density plasmas with radiation temperatures from electron cyclotron emission (ECE) above 20 keV. The radiation temperature is generally higher than the Thomson scattering temperature, indicating that there is a significant population of non-thermal electrons. The experimental ECCD profile measured with motional Stark effect (MSE) polarimetry is found to agree with quasi-linear theory except for the highest power density cases (QEC/ne2>1). Radial transport of the energetic electrons with diffusion coefficients of ∼0.4 m2/s is needed to model the broadened ECCD profile at high power density. [ABSTRACT FROM AUTHOR]

This paper evaluates a new type of ambipolar plasma thruster that uses multiply-charged ions as propellant. Beginning with an electron cyclotron heated (ECH) mirror plasma with energetic electrons, the ion charge state distribution and confining electrostatic potentials are self-consistently modeled for different fill gases using the particle, charge, and energy conservation equations and Pastukhov-flow confinement. The specific impulse is found to be high (∼6000 s) and easily varied. Although the thrust efficiency is low, 24% for double-ended operation and 45% for single-ended operation, this ambipolar thruster is capable of producing high thrust in a compact source because ECH mirror plasmas can operate at high density. [ABSTRACT FROM AUTHOR]

In strongly driven ECCD experiments, consideration of radial transport can be crucial for accurate modeling of otherwise localized electron cyclotron current drive. The DIII-D experiment is in an intermediate driven regime with ttransport∼tslowing for the EC driven electrons. We report computational results from the CQL3D Fokker-Planck simulation code showing radial spreading of driven ECCD in DIII-D. Progress on implementation of a new iterative sparse matrix fully-implicit solve for the full 3D electron distribution, f(u,θu,ρ,t) and toroidal electric potential, Vloop(ρ,t) is described. We give a new algorithm for implicit determination of the self-consistent solution of the Ampere-Faraday equation for the time-dependent toroidal electric field. [ABSTRACT FROM AUTHOR]

A new measurement approach is presented which directly determines the noninductive current profile from the periodic response of the motional Stark effect (MSE) signals to the slow modulation of the external current drive source. A Fourier transform of the poloidal magnetic flux diffusion equation is used to analyze the MSE data. An example of this measurement technique is shown using modulated electron cyclotron current drive (ECCD) discharges from the DIII-D tokamak. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The absorption of fast Alfvén waves (FW) by ion cyclotron harmonic damping in the range of harmonics from fourth to eighth is studied theoretically and with experiments in the DIII-D tokamak. A formula for linear ion cyclotron absorption on Maxwellian ion species is used to estimate the single-pass damping for various cases of experimental interest. It is found that damping on fast ions from neutral beam injection can be significant even at the eighth harmonic if the fast ion beta and the background plasma density are both high enough. The predictions are tested in several L-mode experiments in DIII-D with FW power at 60 MHz and at 116 MHz. It is found that 4th and 5th harmonic absorption of the 60 MHz power on the beam ions can be quite strong, but 8th harmonic absorption of the 116 MHz power appears to be weaker than expected. Possible explanations of the discrepancy are discussed. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

In order to facilitate the planning and analysis of experiments in which fast wave power is applied to discharges in DIII-D, a code has been developed to run conveniently a ray tracing code (GENRAY) and/or a full wave code (TORIC) for experimental or planned conditions. The code gathers information about the kinetic profiles (Te, Ti, ne, Zeff) and the neutral beam injection and calls the ONETWO transport code to obtain the density and equivalent Maxwellian temperature profiles of the energetic beam ions. Using all these data and the wave frequency and toroidal and poloidal spectra characteristic of any DIII-D antenna, GENRAY or TORIC can be run. For ray tracing some self-consistency checks on the absorption and current drive physics may be made using the output files, and comparisons with analytic expressions have been made. A goal of this program is the development and validation against experiment of computational models for FWCD which will be suitable for use on ITER. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Fast wave current drive has been performed in JET plasmas with internal transport barriers, ITBs, and strongly reversed magnetic shear. Although the current drive efficiency of the power absorbed on the electrons is fairly high, only small effects are seen in the central current density. The main reasons are the parasitic absorption of RF power, the strongly inductive nature of the plasma and the interplay between the fast wave driven current and bootstrap current. The direct electron heating in the FWCD experiments is found to be strongly degraded compared to that with the dipole phasing. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Electron cyclotron current drive (ECCD) experiments on the DIII-D tokamak are solidifying the physics basis for localized, off-axis current drive in both standard and advanced operating modes, the goal being to validate a predictive model for ECCD. The measured ECCD efficiency is found to be in agreement with theoretical calculations using the CQL3D FokkerPlanck code over a range of toroidal injection angles, absorption radii, and electron beta. The latter dependence is especially important since the ECCD efficiency is shown to increase with local electron beta, which is favorable for current profile control using off-axis ECCD in highbeta advanced tokamaks. [ABSTRACT FROM AUTHOR]

A detailed quantitative study of the power absorption and predicted current drive, currently using Ehst-Karney parameterisation, will be presented on three tokamaks; ASDEX, DIII-D, and JET. The 2D full wave toroidal code TORIC is used to calculate the FW current drive efficiency weighted over the complete launched spectrum from the antenna. The code can simulate the complex physics of the efficiency and power balance of all three damping mechanisms as a function of plasma parameters and of n[sub ]. The code takes into account electron Landau damping of both the Fast Wave (FW) and Ion Bernstein waves and cyclotron damping at the fundamental and first harmonic. Results have shown that the current drive efficiency (normalized to the density) is machine dependent. The results have also demonstrated the importance of taking into account the full spectrum. [ABSTRACT FROM AUTHOR]

RF electron heating and current drive in DIII-D are observed to typically reduce the core toroidal rotation velocity and core ion temperature when added to target discharges with rotation established by neutral beam heating. Two cases are noted here in which electron cyclotron heating and current drive are observed to increase co-toroidal rotation in different discharge regimes. In the first case electron cyclotron current drive (ECCD) is used to stabilize a 3/2 neoclassical tearing mode (NTM) and the stabilization is accompanied by an increase in rotation, ion temperature and plasma beta. In the second case electron cyclotron heating (ECH) added to a nominally Ohmic target discharge results in an increase in the co-toroidal rotation. [ABSTRACT FROM AUTHOR]

Tearing modes have been shown to limit β and confinement in conventional ELMing H-mode tokamak regimes. The tearing modes grow from a "seed" island due to the destabilizing effect of pressure flattening in the island leading to a reduction in the local bootstrap current. Recent experiments on the DIII-D tokamak have demonstrated stabilization of m = 3/n = 2 tearing modes in the presence of sawteeth through localized electron cyclotron current drive (ECCD). Variation of the deposition location indicates the ECCD remains localized despite the beam traversing an ELMing edge. The effect of the ECCD on the mode is consistent with predictions that the ECCD must be within the island for stabilization. The calculated EC current density (J[sub EC]) is greater than the calculated local bootstrap current density (J[sub BS]) also in accord with predictions. A closed-loop feedback scheme has been successfully operated for the first time using position control and magnetic signals as the actuator and sensor, respectively. [ABSTRACT FROM AUTHOR]

Increased efficiency of off-axis electron cyclotron current drive (ECCD) has been obtained in discharges in DIII-D with higher electron beta, and these results have been understood by studying computationally the particle fluxes in velocity space driven by the EC waves. The relativistic cyclotron resonance tends to shift away in velocity space from the trappedpassing boundary for low-field-side launch as the electron temperature and/or density rise, increasing the net efficiency toward that of the trapping-free Fisch-Boozer level. Calculations with the CQL3D Fokker-Planck code illustrate clearly that the flux in velocity space due to rfinduced diffusion moves away from the trapping boundary for values of β[sub e] which have been realized in the experiments. [ABSTRACT FROM AUTHOR]

A local representation is used to determine localized features in the current profiles due to ECCD in the DIII-D tokamak discharges from equilibrium reconstruction based on spectroscopic and external magnetic measurements. Initial results indicate that reconstruction using a local basis function can resolve very peaked ECCD profiles. The reconstructed narrow ECCD profiles are consistent with the quasi-linear Fokker-Planck results from the CQL3D code. [ABSTRACT FROM AUTHOR]

© 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Direct observation of the generation of sheared ExB flows via Ion Bernstein Waves (IBW) at the fourth ion cyclotron harmonic is reported in the toroidal magnetized plasma Thorello. The measured ExB sheafing rate from IBW induced Reynolds stress increases with increasing RF power. The experimental results show a decrease of the radial correlation length in the region of the shear layer. [ABSTRACT FROM AUTHOR]