Download or read book Electron Cyclotron Current Drive and Current Profile Control in the DIII D Tokamak written by and published by . This book was released on 1998 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Current Profile Modification with Electron Cyclotron Current Drive in the DIII D Tokamak written by and published by . This book was released on 1998 with total page 8 pages. Available in PDF, EPUB and Kindle. Book excerpt: Proof-of-principle experiments on the suitability of electron cyclotron current drive (ECCD) for active current profile control are reported. Experiments with second harmonic extraordinary mode absorption at power levels near 1 MW have demonstrated ability to modify the current profile. This modification is manifested in changes in the internal inductance and the time at which sawteeth appear. Measurements of the local current density and internal loop voltage using high resolution motional Stark effect spectroscopy to half of the minor radius in discharges with localized deposition clearly demonstrate localized off-axis ECCD at the predicted location. Comparison with theory indicates the detrimental effect of trapped electrons on the current drive efficiency is less than predicted. Modification of the theory for finite collisionality is the leading candidate to explain the observations.

Download or read book ELECTRON CYCLOTRON CURRENT DRIVE IN DIII D written by and published by . This book was released on 2003 with total page 11 pages. Available in PDF, EPUB and Kindle. Book excerpt: A271 ELECTRON CYCLOTRON CURRENT DRIVE IN DIII-D: EXPERIMENT AND THEORY. Experiments on the DIII-D tokamak in which the measured off-axis electron cyclotron current drive has been compared systematically to theory over a broad range of parameters have shown that the Fokker-Planck code CQL3D provides an excellent model of the relevant current drive physics. This physics understanding has been critical in optimizing the application of ECCD to high performance discharges, supporting such applications as suppression of neoclassical tearing modes and control and sustainment of the current profile.

Download or read book Modeling of Electron Cyclotron Current Drive Experiments on DIII D written by and published by . This book was released on 1999 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: Electron Cyclotron Current Drive (ECCD) is considered a leading candidate for current profile control in Advanced Tokamak (AT) operation. Localized ECCD has been clearly demonstrated in recent proof-of-principle experiments on DIII-D. The measured ECCD efficiency near the magnetic axis agrees well with standard theoretical predictions. However, for off-axis current drive the normalized experimental efficiency does not decrease with minor radius as expected from the standard theory; the observed reduction of ECCD efficiency due to trapped electron effects in the off-axis cases is smaller than theoretical predictions. The standard approach of modeling ECCD in tokamaks has been based on the bounce-average calculations, which assume the bounce frequency is much larger than the effective collision frequency for trapped electrons at all energies. The assumption is clearly invalid at low energies. Finite collisionality will effectively reduce the trapped electron fraction, hence, increase current drive efficiency. Here, a velocity-space connection formula is proposed to estimate the collisionality effect on electron cyclotron current drive efficiency. The collisionality correction gives modest improvement in agreement between theoretical and recent DIII-D experimental results.

Download or read book Electron Cyclotron Current Drive in DIII D written by and published by . This book was released on 1999 with total page 9 pages. Available in PDF, EPUB and Kindle. Book excerpt: Clear measurements of the localized current density driven by electron cyclotron waves have been made on the DIII-D tokamak. Direct evidence of the current drive is seen on the internal magnetic field measurements by motional Stark effect spectroscopy. Comparison with theoretical calculations in the collisionless limit shows the experimental current drive exceeds the predictions by a substantial amount for currents driven near the half radius. In all cases the experimental current density profile is broader than the predicted one.

Download or read book PHYSICS OF OFF AXIS ELECTRON CYCLOTRON CURRENT DRIVE written by and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Welcome and Guide to Dunkeld Cathedral written by and published by . This book was released on 1976 with total page 4 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book ADVANCED TOKAMAK PROFILE EVOLUTION IN DIII D written by and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Using off-axis electron cyclotron current drive (ECCD), self-consistent integrated advanced tokamak operation has been demonstrated on DIII-D, combining high[beta] (>3%) at high q(q[sub min]> 2.0) with good energy confinement (H[sub 89][approx] 2.5) and high noninductive current fraction (f[sub BS][approx] 55%, f[sub NI][approx] 90%). Modification of the current profile by ECCD led to internal transport barrier formation even in the presence of type I edge localized modes. Improvements were observed in all transport channels, and increased peaking of profiles led to higher bootstrap current in the core. Separate experiments have shown the ability to maintain a nearly steady-state current profile for up to 1 s with q[sub min]> 1.5. Modeling indicates that this favorable current profile can be maintained indefinitely at a higher[beta][sub N] using tools available to the near-term DIII-D program. Modeling and simulation have become essential tools for the experimental program in interpreting the data and developing detail plans for new experiments.

Download or read book Modification of the Current Profile in DIII D by Off Axis Electron Cyclotron Current Drive written by and published by . This book was released on 1999 with total page 10 pages. Available in PDF, EPUB and Kindle. Book excerpt: Localized non-inductive currents due to electron cyclotron wave absorption have been measured on the DIII-D tokamak. Clear evidence of the non-inductive currents is seen on the internal magnetic field measurements by motional Stark effect spectroscopy. The magnitude and location of the non-inductive current is evaluated by comparing the total and Ohmic current profiles of discharges with and without electron cyclotron wave power. The measured current agrees with Fokker-Planck calculations near the magnetic axis, but exceeds the predicted value as the location of the current drive is moved to the half radius.

Download or read book RECENT DEVELOPMENTS ON THE 110 GHZ ELECTRON CYCLOTRON INSTATLLATION ON THE DIII D TOKAMAK written by and published by . This book was released on 2002 with total page 12 pages. Available in PDF, EPUB and Kindle. Book excerpt: OAK A271 RECENT DEVELOPMENTS ON THE 110 GHZ ELECTRON CYCLOTRON INSTALLATION ON THE DIII-D TOKAMAK. Significant improvements are being implement4ed to the capability of the 110 GHz electron cyclotron system on the DIII-D tokamak. Chief among these is the addition of the fifth and sixth 1 MW class gyrotrons, increasing the power available for auxiliary heating and current drive by nearly 60%. These tubes use artificially grown diamond rf output windows to obtain high power with long pulse capability. The beams from these tubes are nearly Gaussian, facilitating coupling to the waveguide. A new fully articulating dual launcher capable of high speed spatial scanning has been designed and tested. The launcher has two axis independent steering for each waveguide. the mirrors can be rotated at up to 100{sup o}/s. A new feedback system linking the DIII-D Plasma Control System (PCS) with the gyrotron beam voltage waveform generators permits real-time feedback control of some plasma properties such as electron temperature. The PCS can use a variety of plasma monitors to generate its control signal, including electron cyclotron emission and Mirnov probes. Electron cyclotron heating and electron cyclotron current drive (ECH and ECCD) were used during this year's DIII-D experimental campaign to control electron temperature, density, and q profiles, induce an ELM-free H-mode, and suppress the m=2/n=1 neoclassical tearing mode. The new capabilities have expanded the role of EC systems in tokamak plasma control.

Download or read book Electron Cyclotron Current Drive Experiments on DIII D written by and published by . This book was released on 1990 with total page 7 pages. Available in PDF, EPUB and Kindle. Book excerpt: Electron Cyclotron Current Drive (ECCD) experiments on the DIII-D tokamak have been performed using 60 GHz waves launched from the high field side of the torus. Preliminary analysis indicates rf driven currents between 50 and 100 kA in discharges with total plasma currents between 200 and 500 kA. These are the first ECCD experiments with strong first pass absorption, localized deposition of the rf power, and [tau]{sub E} much longer than the slowing-down time of the rf generated current carriers. The experimentally measured profiles for T{sub e}, [eta]{sub e} and Z{sub eff} are used as input for a 1D transport code and a multiply-ray, 3D ray tracing code. Comparisons with theory and assessment of the influence of the residual electric field, using a Fokker-Planck code, are in progress. The ECH power levels were between 1 and 1.5 MW with pulse lengths of about 500 msec. ECCD experiments worldwide are motivated by issues relating to the physics and technical advantages of the use of high frequency rf waves to drive localized currents. ECCD is accomplished by preferentially heating electrons moving in one toroidal direction, reducing their collisionality and thereby producing a non-inductively driven toroidal current. 6 refs., 4 figs.

Download or read book Optimized Profiles for Improved Confinement and Stability in the DIII D Tokamak written by and published by . This book was released on 2001 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Simultaneous achievement of high energy confinement, [tau][sub E], and high plasma beta, [beta], leads to an economically attractive compact tokamak fusion reactor. High confinement enhancement, H=[tau][sub E]/[tau][sub E-ITER89P]= 4, and high normalized beta[beta][sub N]=[beta]/(I/aB)= 6%-m-T/MA, have been obtained in DIII-D experimental discharges. These improved confinement and/or improved stability limits are observed in several DIII-D high performance operational regimes: VH-mode, high[ell][sub i] H-mode, second stable core, and high beta poloidal. The authors have identified several important features of the improved performance in these discharges: details of the plasma shape, toroidal rotation or ExB flow profile, q profile and current density profile, and pressure profile. From the improved physics understanding of these enhanced performance regimes, they have developed operational scenarios which maintain the essential features of the improved confinement and which increase the stability limits using localized current profile control. The stability limit is increased by modifying the interior safety factor profile to be nonmonotonic with high central q, while maintaining the edge current density consistent with the improved transport regimes and the high edge bootstrap current. They have calculated high beta equilibria with[beta][sub N]= 6.5, stable to ideal n= 1 kinks and stable to ideal ballooning modes. The safety factor at the 95% flux surface is 6, the central q value is 3.9 and the minimum in q is 2.6. The current density profile is maintained by the natural profile of the bootstrap current, and a modest amount of electron cyclotron current drive.

Download or read book Modeling of Trapped Electron Effects on Electron Cyclotron Current Drive for Recent DIII D Experiments written by and published by . This book was released on 1999 with total page 4 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Current Profile Modeling to Extend the Duration of High Performance Advanced Tokamak Modes in DIII D written by and published by . This book was released on 1998 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: In DIII-D, as in a number of tokamaks, high performance is obtained with various optimized magnetic shear configurations that exhibit internal transport barriers. Negative central shear (NCS) discharges are created transiently during the current ramp-up by auxiliary heating and current drive from neutral beam injection. Both q{sub min} and the radius at which it occurs, [rho]{sub qmin}, decrease with time as the Ohmic current diffuses inward. The q-profiles calculated using EFIT with external magnetic and Motional Stark Effect (MSE) measurements as constraints are comparable to those calculated with the Corsica code, a time-dependent, 2D equilibrium and 1D transport modeling code. Corsica is used to predict the temporal evolution of the current density from a combination of measured profiles, transport models and neoclassical resistivity. Using these predictive capabilities, the authors are exploring methods for increasing the duration and [rho]{sub qmin} of the NCS configuration by local control of the current density profile with simulations of the possible control available from the electron cyclotron heating and current drive system currently being upgraded on DIII-D. Their intention is not to do a detailed investigation of transport models but rather to provide a reasonable model of heat conductivity to be able to simulate effects of electron cyclotron heating (ECH) and current drive (ECCD) on confinement in NCS configurations. The authors adjust free parameters (c, c1 and c2) in the model to obtain a reasonable representation of the temporal evolution of electron and ion temperature profiles consistent with those measured in selected DIII-D shots. In all cases, they use the measured density profiles rather than self-consistently solve for particle sources and particle transport at this time.

Download or read book FWCD and ECCD Experiments on DIII D written by and published by . This book was released on 1994 with total page 9 pages. Available in PDF, EPUB and Kindle. Book excerpt: Fast wave current drive and electron cyclotron current drive experiments have been performed on the DIII-D tokamak as part of the advanced tokamak program. The goal of this program is to develop techniques for controlling the profile of the current density in order to access regimes of improved confinement and stability. The experiments on fast wave current drive used a four strap antenna with 900 phasing between straps. A decoupler was used to help maintain the phasing, and feedback control of the plasma position was used to keep the resistive loading constant. RF pickup loops demonstrate that the directivity of the antenna is as expected. Plasma currents up to 0.18 MA were driven by 1.5 MW of fast wave power. Electron cyclotron current drive experiments at 60 GHz have shown 0.1 MA of plasma current driven by 1 MW of power. New fast wave and electron cyclotron heating systems are in development for DIII-D, so that the goals of the advanced tokamak program can be carried out.

Download or read book PHYSICS OF ELCTRON CYCLOTRON CURRENT DRIVE ON DIII D written by and published by . This book was released on 2002 with total page 19 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Electron Cyclotron Heating Experiments on the DIII D Tokamak written by and published by . This book was released on 1998 with total page 7 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.