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Fast wave heating and current drive in DIII-D discharges with negative central shear

Description: The noninductive current driven by fast Alfven waves (FWCD) has been applied to discharges in DIII-D with negative central shear. Driven currents as high as 275 kA have been achieved with up to 3 MW of fast wave power with the efficiency and profile as predicted by theory-based modeling. When counter-current FWCD was applied to discharges with negative central shear, the negative shear was strengthened and prolonged, showing that FWCD can help to control the current profile in advanced tokamak discharges. Under some conditions in negative central shear, the plasma spontaneously makes a transition into a regime of improved performance, with a reduction in both the ion and the electron heat diffusivities. Up to 3 MW of fast wave power has been successfully coupled into H-mode discharges with large edge localized modes through use of an innovative decoupler/hybrid power splitter combination.
Date: October 1, 1996
Creator: Prater, R.; Austin, M.E. & Baity, F.W.
Partner: UNT Libraries Government Documents Department

Electron cyclotron current drive and current profile control in the DIII-D tokamak

Description: Recent work in many tokamaks has indicated that optimization of the current profile is a key element needed to sustain modes of improved confinement and stability. Generation of localized current through application of electron cyclotron (EC) waves offers a means of accomplishing this. In addition to profile control, electron cyclotron current drive (ECCD) is useful for sustaining the bulk current in a steady state manner and for instability suppression. ECCD is particularly well suited for control of the current profile because the location of the driven current can be regulated by external means, through steering of the incident EC waves and setting the magnitude of the toroidal magnetic field. Under most conditions the location of the driven current is insensitive to the plasma parameters. Central ECCD has been studied in a number of tokamaks and found to have characteristics commensurate with theory as expressed through ray tracing and Fokker-Planck computer codes. The present experiments on DIII-D explore central current drive and are the first to test off-axis ECCD. These experiments are unique in using internal measurements of the magnetic field to determine the magnitude and profile of driven current.
Date: July 1998
Creator: Prater, R.; Luce, T. C. & Petty, C. C.
Partner: UNT Libraries Government Documents Department

Electron cyclotron heating experiments on the DIII-D tokamak

Description: Initial experiments on heating and current drive using second harmonic electron cyclotron heating (ECH) are being performed on the DIII-D tokamak using the new 110 GHz ECH system. Modulation of the ECH power in the frequency range 50 to 300 Hz and detection of the temperature perturbation by ECE diagnostics is used to validate the location of the heating. This technique also determines an upper bound on the width of the deposition profile. Analysis of electron cyclotron current drive indicates that up to 0.17 MA of central current is driven, resulting in a negative loop voltage near the axis.
Date: January 1998
Creator: Prater, R.; Austin, M. E. & Bernabei, S.
Partner: UNT Libraries Government Documents Department

ELECTRON CYCLOTRON CURRENT DRIVE EFFICIENCY IN GENERAL TOKAMAK GEOMETRY

Description: Green's-function techniques are used to calculate electron cyclotron current drive (ECCD) efficiency in general tokamak geometry in the low-collisionality regime. Fully relativistic electron dynamics is employed in the theoretical formulation. The high-velocity collision model is used to model Coulomb collisions and a simplified quasi-linear rf diffusion operator describes wave-particle interactions. The approximate analytic solutions which are benchmarked with a widely used ECCD model, facilitate time-dependent simulations of tokamak operational scenarios using the non-inductive current drive of electron cyclotron waves.
Date: March 1, 2003
Creator: LIN-LUI,Y.R; CHAN,V.S & PRATER,R
Partner: UNT Libraries Government Documents Department

First results on fast wave current drive in advanced tokamak discharges in DIII-D

Description: Initial experiments have been performed on the DIII-D tokamak on coupling, direct electron heating, and current drive by fast waves in advanced tokamak discharges. These experiments showed efficient central heating and current drive in agreement with theory in magnitude and profile. Extrapolating these results to temperature characteristic of a power plant (25 keV) gives current drive efficiency of about 0.3 MA/m{sup 2}.
Date: July 1, 1995
Creator: Prater, R.; Cary, W.P. & Baity, F.W.
Partner: UNT Libraries Government Documents Department

RADIAL TRANSPORT EFFECTS ON ECCD IN THE TCV AND DIII-D TOKAMAKS AND ON OHMIC DISCHARGES IN THE MST RFP

Description: The comprehensive CQL3D Fokker-Planck/Quasilinear simulation code has been benchmarked against experiment over a wide range of electron cyclotron conditions in the DIII-D tokamak (C.C. Petty et al., 14th Topical Conf. on RF Power in Plasmas, 2002). The same code, in disagreement with experiment, gives 560 kA of ECCD for a well documented, completely ECCD-driven, 100 kA TCV shot [O. Sauter et al, PRL, 2000]. Recent work (R.W. Harvey et al, Phys. Rev. Lett., 2002) has resolved the differences as due to radial transport at a level closely consistent with ITER scaling. Transport does not substantially affect DIII-D ECCD, but at similar ECH power has an overwhelming effect on the much smaller TCV. The transport is consistent with electrostatic-type diffusion (D{sub {rho}{rho}} constant in velocity-space) and not with a magnetic-type diffusion (D{sub {rho}{rho}} {proportional_to} |v{parallel}|). Fokker-Planck simulation of Ohmic reversed field pinch (RFP) discharges in the MST device reveals transport velocity dependence stronger than |v{parallel}| will give agreement with current and soft X-ray spectra in standard discharges, but in the higher confinement, current profile controlled PPCD discharges, transport is again electrostatic-like. This is consistent with the object of PPCD, which is to replace magnetic turbulence driven current with auxiliary CD to improve transport. The tokamak and high-confinement RFP results mutually reinforce the constant-in-velocity-space ''electrostatic-type turbulence'' conclusion. The steady-state energy and toroidal current are governed by the same radial transport equation.
Date: July 1, 2002
Creator: HARVEY, R.W.; SAUTER, O.; PRATER, R.; NIKKOLA, P.; O'CONNELL, R. & FOREST, C.B.
Partner: UNT Libraries Government Documents Department

PHYSICS OF ELCTRON CYCLOTRON CURRENT DRIVE ON DIII-D

Description: OAK A271 PHYSICS OF ELCTRON CYCLOTRON CURRENT DRIVE ON DIII-D. Recent experiments on the DIII-D tokamak have focused on determining the effect of trapped particles on the electron cyclotron current drive (ECCD) efficiency. The measured ECCD efficiency increases as the deposition location is moved towards the inboard midplane or towards smaller minor radius for both co and counter injection. The measured ECCD efficiency also increases with increasing electron density and/or temperature. The experimental ECCD is compared to both the linear theory (Toray-GA) as well as a quasilinear Fokker-Planck model (CQL3D). The experimental ECCD is found to be in better agreement with the more complete Fokker-Planck calculation, especially for cases of high rf power density and/or loop voltage. The narrow width of the measured ECCD profile is consistent with only low levels of radial transport for the current carrying electrons.
Date: September 1, 2002
Creator: PETTY,CC; PRATER,R; LUCE,TC; ELLIS,RA; HARVEY,RW; KINSEY,JE et al.
Partner: UNT Libraries Government Documents Department

COMPLETE SUPPRESSION OF THE M/N = 2/1 NEOCLASSICAL TEARING MODE USING RADIALLY LOCALIZED ELECTRON CYCLOTRON CURRENT DRIVE ON DIII-D AND THE REQUIREMENTS FOR ITER

Description: A271 COMPLETE SUPPRESSION OF THE M/N = 2/1 NEOCLASSICAL TEARING MODE USING RADIALLY LOCALIZED ELECTRON CYCLOTRON CURRENT DRIVE ON DIII-D AND THE REQUIREMENTS FOR ITER. DIII-D experiments demonstrate the first real-time feedback control of the relative location of a narrow beam of microwaves to completely suppress and eliminate a growing tearing mode at the q = 2 surface. long wavelength tearing modes such as the m/n = 2/1 instability are particularly deleterious to tokamak operation. Confinement is seriously degraded by the island, plasma rotation can cease (mode-lock) and disruption can occur. The neoclassical tearing mode (NTM) becomes unstable due to the presence of a helically-perturbed bootstrap current and can be stabilized by replacing the missing bootstrap current in the island O-point by precisely located co-electron cyclotron current drive (ECCD). The optimum position is found when the DIII-D plasma control system (PCS) is put into a search and suppress mode that makes small radial shifts (in about 1 cm steps) in the ECCD location based on minimizing the Mirnov amplitude. Requirements for ITER are addressed.
Date: July 1, 2003
Creator: LAHAYE,RJ; LUCE,TC; PETTY,CC; HUMPHREYS,DA; HYATT,AW; PERKINS,FW et al.
Partner: UNT Libraries Government Documents Department

The DIII-D 3 MW, 110 GHz ECH System

Description: Three 110 GHz gyrotrons with nominal output power of 1 MW each have been installed and are operational on the DIII-D tokamak. One gyrotron is built by Gycom and has a nominal rating of 1 MW and a 2 s pulse length, with the pulse length being determined by the maximum temperature allowed on the edge cooled Boron Nitride window. The second and third gyrotrons were built by Communications and Power Industries (CPI). The first CPI gyrotron uses a double disc FC-75 cooled sapphire window which has a pulse length rating of 0.8 s at 1 MW, 2s at 0.5 MW and 10s at 0.35 MW. The second CPI gyrotron, utilizes a single disc chemical-vapor-deposition diamond window, that employs water cooling around the edge of the disc. Calculation predict that the diamond window should be capable of full 1 MW cw operation. All gyrotrons are connected to the tokamak by a low-loss-windowless evacuated transmission line using circular corrugated waveguide for propagation in the HEl 1 mode. Each waveguide system incorporates a two mirror launcher which can steer the rf beam poloidally from the center to the outer edge of the plasma. Central current drive experiments with the two gyrotrons with 1.5 MW of injected power drove about 0.17 MA. Results from using the three gyrotron systems will be reported as well as the plans to upgrade the system to 6 MW.
Date: July 1, 1999
Creator: Callis, R.W.; Lohr, J.; Ponce, D.; O'Neill, R.C.; Prater, R. & Luce, T.C.
Partner: UNT Libraries Government Documents Department

Polarization Measurements During Electron Cyclotron Heating Experiments in the DIII-D Tokamak

Description: The polarization of the launched electron cyclotron wave has been optimized for coupling to the X-mode by adjusting the inclination of grooved mirrors located in two consecutive mitre bends of the waveguide. The unwanted O-mode component of the launched beam can be positively identified by the difference in the power deposition profiles between X-mode and O-mode. The optimal polarization for X-mode launch is in good agreement with theoretical expectations.
Date: July 1, 1999
Creator: Petty, C.C.; Luce, T.C.; Austin, M.E.; Ikezi, H.; Lohr, J. & Prater, R.
Partner: UNT Libraries Government Documents Department

Multi-megawatt 110 GHz ECH system for the DIII-D tokamak

Description: Two 110 GHz gyrotrons with nominal output power of 1 MW each have been installed on the DIII-D tokamak. The first 110 GHz gyrotron built by Gycom has a nominal rating of 1 MW and a 2s pulse length, with the pulse length being determined by the maximum temperature allowed on the edge cooled boron nitride window. This gyrotron was first operated into the DIII-D tokamak in late 1996. The second gyrotron was built by Communications and Power Industries (CPI) was commissioned during the spring of 1997. The CPI gyrotron uses a double disc FC-75 cooled sapphire window which has a pulse length rating of 0.8s at 1 MW, 2s at 0.5 MW and 10s at 0.2 MW. Both gyrotrons are connected to the tokamak by a low-loss-windowless evacuated transmission line using circular corrugated waveguide for propagation in the HE(11) mode. Using short pulse lengths to avoid breakdown inside the air filled waveguide, the microwave beam has been measured inside the DIII-D vacuum vessel using a paper target and an IR camera. The resultant microwave beam was found to be well focused with a spot size of approximately 8 cm. The beam can be steered poloidially from the center to the outer edge of the plasma. The initial operation of the Gycom gyrotron with about 0.5 MW delivered to a low density plasma for 0.5 s showed good central electron heating, with peak temperature in excess of 10 keV. A third gyrotron, being built by CPI, will be installed later this year. Progress with the first CPI tube will also be discussed and future plans for the ECH installation and physics experiments will be presented.
Date: November 1, 1997
Creator: Callis, R.W.; Lohr, J.; O`Neill, R.C.; Ponce, D. & Prater, R.
Partner: UNT Libraries Government Documents Department

THE 110 GHz MICROWAVE HEATING SYSTEM ON THE DIII-D TOKAMAK

Description: OAK-B135 Six 110 GHz gyrotrons in the 1 MW class are operational on DIII-D. Source power is > 4.0 MW for pulse lengths {le} 2.1 s and {approx} 2.8 MW for 5.0 s. The rf beams can be steered poloidally across the tokamak upper half plane at off-perpendicular injection angles in the toroidal direction up to {+-} 20{sup o}. measured transmission line loss is about -1 dB for the longest line, which is 92 m long with 11 miter bends. Coupling efficiency into the waveguide is {approx} 93% for the Gaussian rf beams. The transmission lines are evacuated and windowless except for the gyrotron output window and include flexible control of the elliptical polarization of the injected rf beam with remote controlled grooved mirrors in two of the miter bends on each line. The injected power can be modulated according to a predetermined program or controlled by the DIII-D plasma control system using real time feedback based on diagnostic signals obtained during the plasma pulse. Three gyrotrons have operated at 1.0 MW output power for 5.0 s. Peak central temperatures of the artificially grown diamond gyrotron output windows are < 180 C at equilibrium.
Date: July 2003
Creator: Lohr, J.; Callis, R. W.; Doane, J. L.; Ellis, R. A.; Gorelov, Ya; Kajiwara, K. et al.
Partner: UNT Libraries Government Documents Department

COMPLETE SUPPRESSION OF THE M=2/N-1 NEOCLASSICAL TEARING MODE USING ELECTRON CYCLOTRON CURRENT DRIVE ON DIII-D

Description: A271 COMPLETE SUPPRESSION OF THE M=2/N-1 NEOCLASSICAL TEARING MODE USING ELECTRON CYCLOTRON CURRENT DRIVE ON DIII-D. The first suppression of the important and deleterious m=2/n=1 neoclassical tearing mode (NTM) is reported using electron cyclotron current drive (ECCD) to replace the ''missing'' bootstrap current in the island O-point. Experiments on the DIII-D tokamak verify the maximum shrinkage of the m=2/n=1 island occurs when the ECCD location coincides with the q = 2 surface. The DIII-D plasma control system is put into search and suppress mode to make small changes in the toroidal field to find and lock onto the optimum position, based on real time measurements of dB{sub {theta}}/dt, for complete m=2/n=1 NTM suppression by ECCD. The requirements on the ECCD for complete island suppression are well modeled by the modified Rutherford equation for the DIII-D plasma conditions.
Date: March 1, 2003
Creator: PETTY,CC; LAHAYE,LA; LUCE,TC; HUMPHREYS,DA; HYATT,AW; PRATER,R et al.
Partner: UNT Libraries Government Documents Department

PHYSICS OF OFF-AXIS ELECTRON CYCLOTRON CURRENT DRIVE

Description: Electron cyclotron current drive is a key option for driving current off-axis in a tokamak, as needed for example for current profile control or for suppression of neoclassical tearing modes. Experiments in DIII-D at low beta have shown that the partial cancellation of the Fisch-Boozer co-current by the Ohkawa counter-current can cause strong deterioration of the current drive efficiency at larger minor radius. However, more recent experiments at higher power have shown that the loss in efficiency can be mostly recovered if the target plasma has higher electron beta, {beta}{sub e}. The improvement in efficiency with beta can be understood from a theoretical viewpoint by applying the Fokker-Planck code CQL3D, which shows excellent agreement with experiment over a wide range of parameters, thereby validating the code as an effective means of predicting the ECCD.
Date: July 1, 2002
Creator: PRATER, R.; PETTY, C.C.; HARVEY, R.; LIN-LIU, Y.R.; LOHR, J.M. & LUCE, T.C.
Partner: UNT Libraries Government Documents Department

PHYSICS OF ELECTRON CYCLOTRON CURRENT DRIVE ON DIII-D

Description: OAK A271 PHYSICS OF ELECTRON CYCLOTRON CURRENT DRIVE ON DIII-D. Recent experiments on the DIII-D tokamak have focused on determining the effect of trapped particles on the electron cyclotron current drive (ECCD) efficiency. The measured ECCD efficiency increases as the deposition location is moved towards the inboard midplane or towards smaller minor radius for both co and counter injection. The measured ECCD efficiency also increases with increasing electron density and/or temperature. The experimental ECCD is compared to both the linear theory (Toray-GA) as well as a quasilinear Fokker-Planck model (CQL3D). The experimental ECCD is found to be in better agreement with the more complete Fokker-Planck calculation, especially for cases of high rf power density and/or loop voltage.
Date: November 1, 2002
Creator: PETTY,CC; PRATER,R; LUCE,TC; ELLIS,RA; HARVEY,RW; KINSEY,JE et al.
Partner: UNT Libraries Government Documents Department

INCREASED STABLE BETA IN DIII-D BY SUPPRESSION OF A NEOCLASSICAL TEARING MODE USING ELECTRON CYCLOTRON CURRENT DRIVE AND ACTIVE FEEDBACK

Description: OAK A271 INCREASED STABLE BETA IN DIII-D BY SUPPRESSION OF A NEOCLASSICAL TEARING MODE USING ELECTRON CYCLOTRON CURRENT DRIVE AND ACTIVE FEEDBACK. In DIII-D, the first real-time active control of the electron cyclotron current drive stabilization of a neoclassical tearing mode (here m/n=3/2) is demonstrated. The plasma control system is put into a search and suppress mode to align the ECCD with the island by making either small rigid radial position shifts (of order 1 cm) of the entire plasma (and thus the island) or small changes in toroidal field (of order 0.5%) which radially moves the second harmonic resonance location (and thus the rf current drive). The optimum position minimizes the real-time mode amplitude signal and stabilization occurs despite changes in island location from discharge-to-discharge or from time-to-time. When the neutral beam heating power is programmed to rise after mode suppression by the ECCD, the plasma pressure increases above the peak at the onset of the neoclassical tearing mode until the magnetic island reappears due to the ECCD no longer being on the optimal position. Real-time tracking of the change in location of q=3/2 due to the Shafranov shift with increasing beta is necessary to position the ECCD in the absence of a mode so that higher stable beta can be sustained. The control techniques developed for the m/n=3/2 NTM are also being applied to the more deleterious m/n-2/1 NTM. For the first time in any tokamak, an m/n=2/1 mode has been completely suppressed using radially localized off-axis ECCD.
Date: September 1, 2002
Creator: LAHAYE,RJ; HUMPHREYS,DA; LOHR,J; LUCE,TC; PERKINS,FW; PETTY,CC et al.
Partner: UNT Libraries Government Documents Department

Modeling of 110 GHz electron cyclotron wave propagation and absorption on DIII-D

Description: In modeling electron cyclotron (EC) waves for heating (ECH) and current drive (ECCD) applications, the standard approach describes wave propagation using geometric optic ray tracing with cold plasma dispersion and wave absorption using a relativistic warm plasma expression. However, recent vertical O-mode transmission measurements on Tore Supra indicate that wave trajectories near the fundamental resonance frequency can significantly deviate from predictions of cold plasma analysis. The experimental results were attributed to warm plasma refraction effects caused by the anomalous dispersion associated with wave-particle resonance. Here, warm plasma refraction effects on wave propagation and absorption are examined in the context of a slab model in the parameter regime of interest to the upcoming 110 GHz electron cyclotron heating and current drive experiments on DIII-D.
Date: April 1, 1997
Creator: Lin-Liu, Y.R.; Luce, T.C.; Chiu, S.C.; Miller, R.L. & Prater, R.
Partner: UNT Libraries Government Documents Department

Initial results from the multi-megawatt 110 GHz ECH system for the DIII-D tokamak

Description: The first of three MW-level 110 GHz gyrotrons was operated into the DIII-D tokamak in late 1996. Two additional units will be commissioned during 1997. Each gyrotron is connected to the tokamak by a low loss, windowless, evacuated transmission line using circular corrugated waveguide carrying the HE{sub 11} mode. The microwave beam spot is well focused with a spot size of approximately 6 cm and can be steered poloidally from the center to the outer edge of the plasma. The initial operation with about 0.5 MW delivered to a low density plasma for 0.5 s showed good central electron heating, with peak temperature in excess of 10 keV. The injection was 19{degree} off perpendicular for current drive.
Date: April 1, 1997
Creator: Callis, R.W.; Lohr, J.; O`Neill, R.C.; Ponce, D.; Luce, T.C.; Prater, R. et al.
Partner: UNT Libraries Government Documents Department

Applications of fast wave in spherical tokamaks

Description: In spherical tokamaks (ST), the magnetic field strength varies over a wide range across the plasma, and at high betas it deviates significantly from the 1/R dependence of conventional tokamaks. This, together with the high density expected in ST, poses challenging problems for RF heating and current drive. In this paper, the authors investigate the various possible applications of fast waves (FW) in ST. The adjoint technique of calculating current drive is implemented in the raytracing code CURRAY. The applicability of high harmonic and subharmonic FW to steady state ST is considered. They find that high harmonic FW tends to be totally absorbed before reaching the core and may be considered a candidate for off axis current drive while the subharmonic FW tends to be absorbed mainly in the core region and may be considered for central current drive. A difficult problem is the maintenance of current at the startup stage. In the bootstrap ramp-up scenario, the current ramp-up is mainly provided by the bootstrap current. Under this condition, the role of rf becomes mainly the sustainment of plasma through electron heating. Using a slab full-wave code SEMAL, the authors find that the ion-ion-hybrid mode conversion scheme is a promising candidate. The effect of possible existence of edge Alfven resonance and high harmonic cyclotron resonance is investigated and regimes of minimization of edge heating identified.
Date: April 1, 1997
Creator: Chiu, S.C.; Chan, V.S.; Lin-Liu, Y.R.; Miller, R.L.; Prater, R. & Politzer, P.
Partner: UNT Libraries Government Documents Department

Polarization, propagation, and deposition measurements during ECCD experiments on the DIII-D tokamak

Description: The power deposition profiles for different poloidal and toroidal launch angles have been determined by modulating the ECH power and measuring the electron temperature response. The peak of the measured power density follows the poloidal steering of the ECH launcher, and perpendicular launch gives a narrower deposition profile than does oblique (current drive) launch. The difference in wave refraction between X-mode and O-mode allows positive identification of an unwanted O-mode component of the launched beam.
Date: March 1, 1999
Creator: Petty, C.C.; Luce, T.C.; Lin-Liu, Y.R.; Lohr, J.; Prater, R. & Austin, M.E.
Partner: UNT Libraries Government Documents Department

A design study for the ECH launcher for ITER

Description: The Design Description Document for ITER calls for 50 MW of electron cyclotron power at a frequency of 170 GHz, upgradeable to 100 MW. This power is intended to heat the plasma from Ohmic temperatures to ignition, in concert with power from some combination of neutral injection and/or ICRF heating. The second major application of ECH power is current drive. In the advanced steady-state scenarios, the total current is 12 to 16 MA, of which 75% is driven by bootstrap effects. The current drive requirement is 2 to 3 MA at a relative minor radius of 0.7, plus a small current near the center of the discharge. ECH power is also used for plasma initiation and startup, using a separate ECH system of two fixed frequencies between 90 to 140 GHz and total power to 6 MW. Suppression or control of MHD instabilities like neoclassical tearing modes, sawteeth, ELMs, and locked modes are also important objectives for the ECH systems. However, the launching and power characteristics of the ECH for these applications is highly specialized. The ability to modulate at high frequency (at least several tens of kHz), the ability to redirect the beams with precision at relatively high speed, and the requirement that the stabilization be carried out at the same time as the bulk heating and current drive imply that separate and specialized ECH systems are needed for the stabilization activities. For example, for stabilization of neoclassical tearing modes current must be driven inside the islands near the q = 2 surface. If this is done near the outboard mid plane, a system with optimized frequency might be much more effective than what could be done with the main 170 GHz system. This paper does not treat the launchers for the stabilization systems.
Date: April 1, 1997
Creator: Prater, R.; Grunloh, H.J.; Moeller, C.P.; Doane, J.L.; Olstad, R.A.; Makowski, M.A. et al.
Partner: UNT Libraries Government Documents Department

Modeling of Trapped Electron Effects on Electron Cyclotron Current Drive for Recent DIII-D Experiments

Description: Owing to its potential capability of generating localized non-inductive current, especially off-axis, Electron Cyclotron Current Drive (ECCD) is considered a leading candidate for current profile control in achieving Advanced Tokamak (AT) operation. In recent DIII-D proof-of-principle experiments [1], localized off-axis ECCD has been clearly demonstrated for first time. The measured current drive efficiency near the magnetic axis agrees well with predictions of the bounce-averaged Fokker-Planck theory [2,3]. However, the off-axis current drive efficiency was observed to exceed the theoretical results, which predict significant degradation of the current drive efficiency due to trapped electron effects. The theoretical calculations have been based on an assumption that the effective collision frequency is much smaller than the bounce frequency such that the trapped electrons are allowed to complete the banana orbit at all energies. The assumption might be justified in reactor-grade tokamak plasmas, in which the electron temperature is sufficiently high or the velocity of resonant electrons is much larger than the thermal velocity, so that the influence of collisionality on current drive efficiency can be neglected. For off-axis deposition in the present-day experiments, the effect of high density and low temperature is to reduce the current drive efficiency, but the increasing collisionality reduces the trapping of current-carrying electrons, leading to compensating increases in the current drive efficiency. In this work, we use the adjoint function formulation [4] to examine collisionality effects on the current drive efficiency.
Date: August 1999
Creator: Lin-Liu, Y. R.; Sauter, O.; Harvey, R. W.; Chan, V. S.; Luce, T. C. & Prater, R.
Partner: UNT Libraries Government Documents Department

COMPLETE SUPPRESSION OF THE m/n=2/1 NEOCLASSICAL TEARING MODE USING RADIALLY LOCALIZED ELECTRON CYCLOTRON CURRENT DRIVE ON DIII-D AND THE REQUIREMENTS FOR ITER

Description: OAK-B135 DIII-D experiments demonstrate the first real-time feedback control of the relative location of a narrow beam of microwaves to completely suppress and eliminate a growing tearing mode at the q=2 surface. long wavelength tearing modes such as the m/n = 2/1 instability are particularly deleterious to tokamak operation. Confinement is seriously degraded by the island, plasma rotation can cease (mode-lock) and disruption can occur. The neoclassical tearing mode (NTM) becomes unstable due to the presence of a helically-perturbed bootstrap current and can be stabilized by replacing the missing bootstrap current in the island O-point by precisely located co-electron cyclotron current drive (ECCD). The geometry for the ECCD launch, the second harmonic resonance 2f{sub ce} and the q=2 surface are shown. The optimum position is found when the DIII-D plasma control system (PCS) is put into a search and suppress mode that makes small radial shifts (in about 1 cm steps) in the ECCD location based on minimizing the Mirnov amplitude.
Date: June 1, 2003
Creator: LA HAYE,RJ; LUCE,TC; PETTY,CC; HUMPHREYS,DA; HYATT,AW; PERKINS,FW et al.
Partner: UNT Libraries Government Documents Department

HIGH POWER LONG PULSE PERFORMANCE OF THE DIII-D GYROTRON INSTALLATION

Description: At DIII-D, five 110 GHz gyrotrons are operating routinely for 2.0 s pulses at generated power levels {ge}750 kW per gyrotron. A sixth gyrotron is being installed, which should bring the generated power level to >4 MW and the injected power to about 3.0 MW. The output power now can be modulated by the plasma control system to fix T{sub e} at a desired value. The system is being used as a tool for control of current diffusion, for current profile control and other experiments leading to advanced tokamak operation.
Date: May 1, 2002
Creator: LOHR, J.; GORELOV, Y.A.; CALLIS, R.W.; GRUNLOH, H.J.; PEAVY, J.J.; PINSKER, R.I. et al.
Partner: UNT Libraries Government Documents Department