Download or read book Lower Hybrid Current Drive Experiments in Alcator C Mod written by and published by . This book was released on 2007 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: A Lower Hybrid Current Drive (LHCD) system has been installed on the Alcator C-MOD tokamak at MIT. Twelve klystrons at 4.6 GHz feed a 4x22 waveguide array. This system was designed for maximum flexibility in the launched parallel wave-number spectrum. This flexibility allows tailoring of the lower hybrid deposition under a variety of plasma conditions. Power levels up to 900 kW have been injected into the tokomak. The parallel wave number has been varied over a wide range, n.

Download or read book Lower Hybrid Experiments at the 1 MW Level on Alcator C written by Miklos Porkolab and published by . This book was released on 1984 with total page 24 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Lower Hybrid Current Drive and Heating Experiments Up to the 1 MW Level in Alcator C written by Miklos Porkolab and published by . This book was released on 1983 with total page 16 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Lower Hybrid Heating and Current Drive on the Alcator C Mod Tokamak written by and published by . This book was released on 2009 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: On the Alcator C-Mod tokamak, lower hybrid current drive (LHCD) is being used to modify the current profile with the aim of obtaining advanced tokamak (AT) performance in plasmas with parameters similar to those that would be required on ITER. To date, power levels in excess of 1 MW at a frequency of 4.6 GHz have been coupled into a variety of plasmas. Experiments have established that LHCD on C-Mod behaves globally as predicted by theory. Bulk current drive efficiencies, n20IlhR/Plh ~ 0.25, inferred from magnetics and MSE are in line with theory. Quantitative comparisons between local measurements, MSE, ECE and hard x-ray bremsstrahlung, and theory/simulation using the GENRAY, TORIC-LH CQL3D and TSC-LSC codes have been performed. These comparisons have demonstrated the off-axis localization of the current drive, its magnitude and location dependence on the launched n.

Download or read book Overview of the Alcator C MOD Research Program written by and published by . This book was released on 2007 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Recent research on the high-field, high-density diverted Alcator C-MOD tokamak has focussed on the plasma physics and plasma engineering required for ITER and for attractive fusion reactors. Experimental campaigns over the past two years have focused on understanding the physical mechanisms that affect the plasma performance realized with all-molybdenum walls versus walls with low-Z coatings. RF sheath rectification along flux tubes that intersect the RF antenna is found to be a major cause of localized boron erosion and impurity generation. Initial lower-hybrid current drive (LHCD) experiments (PLH

Download or read book Lower Hybrid Modeling and Expiments sic on Alcator C Mod written by John E. Liptac and published by . This book was released on 2006 with total page 256 pages. Available in PDF, EPUB and Kindle. Book excerpt: (cont.) Clear evidence of current drive was seen as the loop voltage decreased by about 50%. Studies were performed by varying the phase, density, and magnetic field as well as changing the direction that the waves were launched. Analysis using CQL3D indicates that a current drive efficiency of 0.15 (1020 m-2A/W) was obtained by driving 167 kA of LH current with 410 kW at a line-average density of 5.5 x 1019 m-3, without an electric field. This efficiency exceeds what was observed on Alcator C, but is within the! range observed on FTU for similar densities. Including the residual electric field increased the LH driven current to 308 kA, corresponding to an increased effective efficiency of 0.28. In addition to experiments, extensive modeling of current profile control through phase variation was studied, including compound spectra, using CQL3D. It was found that over 200 kA of off-axis LH current can be generated in a variety of profile shapes in an H-mode target plasma. Finally, time dependent modeling of an integrated scenario was performed using the transport code TRANSP to explore what performance can be ultimately achieved on C-Mod. Results indicate that fully non-inductive, quasi-steady-state plasmas are possible with bootstrap fractions as high as 75%.

Download or read book The LHCD Launcher for Alcator C Mod Design Construction Calibration and Testing written by J. Hosea and published by . This book was released on 2005 with total page 18 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Measurements and Modeling of Lower Hybrid Driven Fast Electrons on Alcator C Mod written by Andréa Elizabeth Wilhelm Schmidt and published by . This book was released on 2011 with total page 176 pages. Available in PDF, EPUB and Kindle. Book excerpt: A Lower Hybrid Current Drive (LHCD) system has been implemented on Alcator C-Mod with successful coupling to the plasma of up to 1 MW of power. Nearly fully non-inductive current drive has been achieved for several current relaxation times at low but ITER-relevant densities. One major advantageous feature of the C-Mod LH system is its phasing flexibility, allowing it to produce spectra with a wide range of peak parallel refractive index (nI). Theory predicts that LH power deposition location is strongly dependent on ng, as well as on other parameters such as electron temperature, electron density, and plasma current. Several diagnostics exist on Alcator C-Mod which can measure the effects of LHCD power on the plasma. The primary diagnostic for measuring LH-driven fast electrons is a hard x-ray (HXR) camera that measures fast electron Bremsstrahlung emission. This is a horizontallyviewing diagnostic with 32 spatial chords that span the plasma cross-section. Each chord ends at a cadmium zinc telluride (CZT) detector that detects individual x-ray photons as current pulses. The output from these detectors is shaped and digitized. Post-processing of the raw pulse train allows for flexible time and energy binning. Another diagnostic for detecting fast electrons is the Electron Cyclotron Emission (ECE) diagnostic. This diagnostic is designed to measure electron temperature in a Maxwellian plasma that is optically thick in the second harmonic. However, the LHdriven fast electrons have relativistically downshifted electron cyclotron frequencies and contribute to additional emission at frequencies just below the second harmonic, where the plasma is optically thin. A third useful diagnostic for LHCD operation is the Motional Stark Effect (MSE) diagnostic, which measures the magnetic field pitch angle profile and therefore can be used to infer the current profile inside the plasma. This current profile is the sum of ohmic current, LH current, and bootstrap current and has been observed to change when LH power is applied to an ohmic plasma. GENRAY/CQL3D is a ray-tracing/Fokker-Plank code package that solves iteratively for a self-consistent electron distribution function in the presence of LH waves, given a plasma scenario and LH wave spectrum. This code includes synthetic diagnostics that can be compared to experimental HXR and ECE measurements. Although an MSE synthetic diagnostic does not currently exist in CQL3D, the inferred current profile from MSE can be compared with the current profile output by CQL3D. Modeling has been carried out for multiple plasma scenarios to benchmark the code package and to further our understanding of how to interpret the experimental results. An experiment in which LH power is square-wave modulated on a time scale much faster than the current relaxation time does not significantly alter the poloidal magnetic field inside the plasma and thus allows for realistic modeling and consistent plasma conditions for different nII spectra. Inverted hard x-ray profiles show clear changes in LH-driven fast electron location with differing nil. Boxcar binning of hard x-rays during LH power modulation allows for 1 ms time resolution, which is sufficient to resolve the build-up, steady-state, and slowing-down of fast electrons. Ray-tracing/Fokker-Planck modeling in combination with a synthetic hard x-ray diagnostic show quantitative agreement with the x-ray data for high ni cases. The time histories of hollow x-ray profiles have been used to measure off-axis fast electron transport in the outer half of the plasma, which is found to be small on a slowing down time scale. This work is supported by the US DOE awards DE-FC02-99ER54512 and DE-AC02-76CH03073.

Download or read book Energy Research Abstracts written by and published by . This book was released on 1988 with total page 872 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Quantification of Lower Hybrid Wave Absorption in the Edge of the Alcator C Mod Tokamak written by Ian Charles Faust and published by . This book was released on 2016 with total page 209 pages. Available in PDF, EPUB and Kindle. Book excerpt: A 1 MW Lower Hybrid Current drive (LHCD) radiofrequency system is used to replace inductive drive on the Alcator C-Mod tokamak. It was designed to test Advanced Tokamak (AT) scenarios for future steady-state diverted, high field tokamaks. However, at reactor-relevant densities (n̄e > 1 . 10 20 m-3), an anomalous current drive loss is observed. This loss, known as the LHCD density limit, occurs in diverted plasmas and is correlated with the plasma current and plasma density. Several mechanisms have been implicated in the loss of current drive, with both experimental and theoretical results suggesting edge power loss. Power modulation is a standard technique used for characterizing power sources and plasma power balance. In this case, the Lower Hybrid radiofrequency (LHRF) power is modulated in time in a set of plasmas across the density range from efficient to negligible current drive. This data is used to characterize the absorption of LHRF power through the calculation of the LHRF power balance within 15%, typical of power balance studies. This power balance is used to derive characteristics of the cause behind the LHCD density limit. The immediate nature of LHRF-induced conducted and radiated power losses confirm that LHRF power is absorbed in the edge plasma, even at the lowest densities. The edge losses increase to balance the reduced current drive, indicating that the observed power in the scrape-off-layer (SOL) limits the available power for current drive and the edge losses represent a parasitic mechanism. Unlike edge losses of other radiofrequency systems, this absorption occurs with a high degree of toroidal symmetry near the plasma separatrix. This indicates absorption occurs just inside the separatrix, or just outside the separatrix over multiple SOL traversals. Measurements of the poloidal distribution of ionization and recombination in the edge were made using a specially designed Ly[alpha] pinhole camera. It utilizes a MgF2 filter and AXUV diode array to measure Ly[alpha] emission from the lower to upper divertor. Edge deposited LHRF power was found to promptly ionize the active divertor plasma in all diverted topologies. This result highlights the power flow and importance of the divertor plasma in the LHCD density limit. Three independent characteristics indicate the thermal absorption of LHRF power. First, in- /out balance of radiated and conducted LHRF power change with the reversal of the tokamak magnetic fields. Second, comparisons of the conducted heat via Langmuir probes and IR thermography are similar with and without LHRF power. Lastly, the Langmuir probe ratio of Vf l/Te does not significantly modulate with modulated LHRF. A second experiment utilized a high strike-point diverted discharge to determine the edge loss of fast electrons. The high strike point could be observed using the hard X-ray camera, which can compare core and edge X-ray emission. The measured count rates from thick-target bremsstrahlung were interpreted into fast electron fluxes using the Win X-ray code. Theoretical treatments of the fast-electron confinement time were also calculated for Alcator C-Mod. In all cases the fast-electron edge losses are minimal and will be unimportant for future tokamaks due to the small fast electron diffusivity and their large size. The loss of current drive in high density diverted plasmas correlates with high edge plasma collisionality. The newly derived characteristics set stringent requirements in nk for electron Landau damping to cause the edge absorption of LHRF power. Several observed attributes, namely high frequency modulation and low density absorption do not correlate with Landau damping characteristics. However, parasitic collisional absorption in the divertor plasma yields the necessary plasma current, topology, symmetry, thermal, and ionization characteristics. High divertor plasma collisionality is expected if not required for future tokamaks. LHRF systems of future tokamaks must must avoid propagation through collisional regions, even on the first traversal through the SOL.

Download or read book Fusion Energy Update written by and published by . This book was released on 1986 with total page 100 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Mode Conversion Current Drive Experiments on Alcator C Mod written by Alexandre Parisot and published by . This book was released on 2007 with total page 197 pages. Available in PDF, EPUB and Kindle. Book excerpt: (cont.) Based on simulations of the saw-tooth cycle with the Porcelli trigger model, localized electron heating, which could also explain the experimental results, was found not to be dominant compared to the current drive effect. The experimental results demonstrate that, while not optimal, MCCD can be used for sawtooth control.

Download or read book Energy Research Abstracts written by and published by . This book was released on 1995 with total page 886 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Polarimetry Measurements of Current Density Profile and Fluctuation Changes During Lower Hybrid Experiments on Alcator C Mod written by Peng Xu (Ph. D.) and published by . This book was released on 2009 with total page 186 pages. Available in PDF, EPUB and Kindle. Book excerpt: As a possible method to achieve steady-state plasma operation in a tokamak fusion reactor, Lower Hybrid Current Drive (LHCD) has been extensively studied on Alcator C-Mod. The measurement of current density profiles (or poloidal magnetic field or safety factor) is essential to the understanding of the LH physics. A polarimeter measures the line-integral of the electron density times the magnetic field parallel to a chord through the plasma, and is capable of deriving the current density profile with multi-chord measurements. A three-chord poloidally viewing far-infrared (FIR) polarimeter has therefore been developed for C-Mod. FIR optics, FIR detectors, electronics and mechanical components were custom designed and fabricated. Various sources of vibrations and systematic errors have been carefully dealt with. A synthetic polarimeter signal has been derived from density profiles from Thomson scattering and magnetic field profiles from EFIT. The polarimeter results confirm that the magnetic field profiles from Kinetic EFIT (with Motional Stark Effect and plasma pressure constraints) is much more accurate than those from normal EFIT (no internal constraints, central safety factor qo~0.9 5) during the LHCD. The polarimeter data were used to constrain qO, and the result shows that the current density profile is flattened, and qo increases to ~2 with strong LHCD. Experiments during LHCD with a plasma density scan, a current scan, and LH power and phase scans have been carried out. The Polarimetry observations confirm the off-axis current drive, and strongly decreasing driven current when the density exceeds a limit of about 1.Ox 102 0/m3 . They also indicate that the driven current is closer to the plasma edge for Ip ~820 kA than for I, ~580 kA, and that lower LH phasing is preferred for higher current drive efficiency. The polarimeter is the first diagnostic to measure core magnetic fluctuations in Alcator C-Mod. These include broadband magnetic fluctuations during EDA H-modes that are suppressed with the application of LH power.

Download or read book Full wave Modeling of Lower Hybrid Waves on Alcator C Mod written by Orso-Maria Cornelio Meneghini and published by . This book was released on 2012 with total page 241 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis focuses on several aspects of the Lower Hybrid (LH) wave physics, the common theme being the development of full-wave simulation codes based on Finite Element Methods (FEM) used in support of experiments carried out on the Alcator C-Mod tokamak. In particular, two non-linear problems have been adressed: high power antenna-plasma coupling and current drive (CD). In both cases, direct solution of the wave equation allowed testing the validity of approximations which were historically done and consider full-wave effects and realistic geometries. The first code, named POND, takes into account the interaction of high power LH waves and the plasma edge based on the non-linear ponderomotive force theory. Simulations found the effect of ponderomotive forces to be compatible with the density depletion which is measured in front of the antenna in presence of high power LH waves. The second code, named LHEAF, solves the problem of LH wave propagation in a hot non- Maxwellian plasma. The electron Landau damping (ELD) effect was expressed as a convolution integral along the magnetic field lines and the resultant integro-differential Helmholtz equation was solved iteratively. A 3D Fokker-Planck code and a synthetic Hard X-Ray (HXR) diagnostic modules are used to calculate the self-consistent electron distribution function and evaluate the resulting CD and bremsstrahlung radiation. LHEAF has been used to investigate the anomalous degradation of LHCD efficiency at high density. Results show that while a small fraction of the launched power can be absorbed in the SOL by collisions, it is a strong upshift in the nii spectrum that makes the overall LHCD efficiency low by allowing the waves to Landau damp near the edge. Wavelet analysis of the full-wave fields identified spectral broadening to occur after the waves reflect and propagate in the SOL. This work explains why on Alcator C-Mod the eikonal approximation is valid only in the low to moderate density regime, and why parasitic phenomena introduced in previous work can reproduce phenomenologically well the experimental results.

Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1995 with total page 994 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Fusion Energy Program written by United States. Congress. House. Committee on Science, Space, and Technology. Subcommittee on Investigations and Oversight and published by . This book was released on 1990 with total page 820 pages. Available in PDF, EPUB and Kindle. Book excerpt: