Download or read book Effects of RF Power on SOL Density Profiles and RF Coupling on the Alcator C Mod Tokamak written by Cornwall HongMan Lau and published by . This book was released on 2013 with total page 203 pages. Available in PDF, EPUB and Kindle. Book excerpt: A 100-146GHz swept-frequency X-mode reflectometer constructed and installed at three poloidal locations adjacent to the lower hybrid (LH) launcher and one location adjacent to the new field aligned ion cyclotron range of frequencies (ICRF) antenna has been used to measure the scrape-off-layer (SOL) density profiles under a wide range of plasma parameters in order to study plasma-antenna coupling and non-linear RF-SOL interactions on Alcator C-Mod. After validating the reflectometer density profiles with other density profile diagnostics in easily diagnosed and well understood plasma conditions, detailed reflectometer density profile measurements in plasmas with application of ICRF, LH and ICRF+LH power will be shown in order to understand the physical mechanisms for RF-induced density profile modifications. Results indicate that both ICRF and LH power create significant poloidal density profile asymmetries that are correlated with video camera emissivity measurements. These results will be shown to depend on various plasma parameters such as launched nll of the LH waves, ICRF antenna location, and toroidal magnetic field direction. Both LH and ICRF power have been experimentally observed to modify the poloidal flow in the SOL. These flows are reminiscent of LH and ICRF induced convective cells that have been discussed in the literature. A 2-D diffusive-convective model has been applied to quantify the effects of these RF-induced ExB drifts on the SOL density profiles. For the LH case, the simulation reproduces the experimental trends at all three reflectometer poloidal locations adjacent to the LH launcher and indicates that LH-induced ExB drifts is the dominant physical mechanism producing the experimentally measured density profile modifications. Understanding this RF-induced transport helps elucidate these nonlinear RF-SOL mechanisms. One consequence of these density profile modifications is a change in LH coupling efficiency. A 2-D slab LH coupling model has also been used to quantify the effects of the density profile modifications on LH coupling. It will be shown that LH coupling is extremely sensitive to the edge density profile and to poloidal asymmetries in the density profiles. The inclusion of LH and ICRF-induced ExB drift effects on the observed density profile asymmetries is necessary to understand the experimentally measured LH coupling results during LH only and LH+ICRF operations.

Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1994 with total page 1028 pages. Available in PDF, EPUB and Kindle. Book excerpt: Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.

Download or read book Radio Frequency Edge Physics on the Alcator C Mod Tokamak written by James Christian Reardon and published by . This book was released on 1999 with total page 152 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Rf Modeling and Design of a Folded Waveguide Launcher for the Alcator C Mod Tokamak written by and published by . This book was released on 1993 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: The folded waveguide (FWG) launcher is being investigated as an improved antenna configuration for plasma heating in the ion cyclotron range of frequencies (ICRF). A development FWG launcher was successfully tested at Oak Ridge National Laboratory (ORNL) with a low-density plasma load and found to have significantly greater power density capability than current strap-type antennas operating in similar plasmas. To further test the concept on a high density tokamak plasma, a collaboration has been set up between ORNL and Massachusetts Institute of Technology (MIT) to develop and test an 80-MHz, 2-MW FWG on the Alcator C-Mod tokamak at MIT. The radio frequency (rf) electromagnetic modeling techniques and laboratory measurements used in the design of this antenna are described in this paper. A companion paper describes the mechanical design of the FWG.

Download or read book Waveguide Coupling During Lower Hybrid Current Drive in a Tokamak written by Kunizo Ohkubo and published by . This book was released on 1986 with total page 34 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Lower hybrid heating Experiments on the Alcator C and the Versator II Tokamaks written by and published by . This book was released on 1982 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Initial results are reported from lower hybrid wave heating experiments carried out on the MIT Alcator C and Versator II tokamaks. In the Alcator C experiments a 4 waveguide array, with internally brazed ceramic windows has been used to inject 160 kW of microwave power at 4.6 GHz into the plasma with n0 less than or equal to 1 x 1015 cm−3, and B0 less than or equal to 12 T. An RF power density of 8 kW/cm2 has been transmitted into the plasma without RF breakdown. RF coupling studies show optimal coupling (R less than or equal to 10%) when the local density at the waveguide mouth is 25 to 50 times overdense. Initial heating experiments show an ion tail formation in hydrogen discharge peaking at a density of anti n approx. = 2.7 x 1014 cm−3 at B = 8.9 T, and bulk ion heating at a density of anti n approx. = 1.5 x 1014 c−3 at B approx. = 11 T. Evidence of RF current enhancement has been observed at a density of n approx. = 3 x 1013 cm−3. In the Versator II tokamak initial ion heating studies have been carried out using an 800 MHz, 140 kW klystron. With 50 kW of net RF power injected through a 4 waveguide grill at B = 1.3 T and anti n = 2.5 x 1013 cm−3, Doppler broadening of the OVII and NVI lines shows a .delta.T/sub i/ = 50 eV rise in the bulk ion temperature. A significant RF produced ion tail is also observed by charge exchange analysis. We have succeeded in combining a toroidal ray-tracing code and a 1-D transport code to study the heating density bands and heating efficiencies.

Download or read book Study of High Performance Mode Access Conditions on the Alcator C Mod Tokamak written by Yunxing Ma and published by . This book was released on 2013 with total page 210 pages. Available in PDF, EPUB and Kindle. Book excerpt: Usually when sufficient heating power is injected, tokamak plasma will make an abrupt transition into a state with improved confinement, known as the high-confinement mode, or H-mode. Given the greatly enhanced fusion yield, H-mode is foreseen as the baseline scenario for the future plasma operation of the International Thermonuclear Experimental Reactor (ITER). Many research efforts have been given to understand the criteria for H-mode access. To further contribute to this research, a primary focus of this thesis is characterizing the H-mode access conditions in the Alcator C-Mod tokamak, across a broad range of plasma density, magnetic field, and plasma current. In addition, dedicated experiments were designed and executed on C-Mod, to explore the effects of divertor geometry, ICRF resonance location, and main ion species on H-mode access conditions. Results from these experiments will be included in this thesis. The underlying physics of H-mode access is very complex, and the critical mechanisms remain largely unresolved. To promote our understanding, some models proposed for the H-mode transition are tested, using well documented local plasma conditions, obtained in C-Mod experiments. In particular, this thesis pioneers the test of a recently developed model for H-mode threshold power predictions.

Download or read book Physics Briefs written by and published by . This book was released on 1993 with total page 1334 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Particle Transport on the Alcator C Mod Tokamak written by Cindy R. Christensen and published by . This book was released on 1999 with total page 184 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 Impact of Wave filament Scattering in the Scrape off Layer During Lower Hybrid Current Drive written by Bodhisatwa Biswas and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Lower Hybrid Current Drive (LHCD) is an efficient means to control the current profile in a tokamak. The performance of LHCD is sensitive to the phase-space trajectory of the wave as it propagates and damps in the plasma. These trajectories are scattered by turbulent density fluctuations in the scrape-off layer (SOL), which is hypothesized to be the reason for poor agreement between LHCD experiments and simulations (which do not account for scattering). This thesis investigates the impact of scattering processes on LHCD, and attempts to bridge this discrepancy. Two novel scattering models are developed. The filament-refraction model generalizes beyond the weak turbulence assumption made in previous works. The full-wave/statistical model further generalizes beyond the Wentzel-Kramer-Brillouin (WKB) limit. This latter approach results in excellent agreement with experimental observations. The filament-refraction model couples realistic SOL turbulence profiles to a ray-tracing code. Synthetic, 3-D turbulence profiles that mimic SOL turbulence are generated and coupled to the ray-tracing/Fokker-Planck codes GENRAY/CQL3D. In contrast with previous scattering models that employ the weak-turbulence approximation, this approach accounts for the spatial coherency of filamentary turbulence. As a result, the extent of scattering is shown to be greater in filamentary turbulence, leading to a significant modification of the current profile in the Alcator C-Mod tokamak. The full-wave/statistical approach employs a Mie-scattering technique to treat a single wave-filament interaction in full-wave formalism. The radiative transfer approximation is employed to treat scattering from a statistical ensemble of filaments in a turbulent layer in slab geometry. This approach extends beyond ray-tracing, and retains all single-filament full-wave effects while remaining computationally inexpensive. Notably, it is found that LH waves can asymmetrically scatter in wave-number phase-space due to full-wave interactions with spatially coherent density fluctuations. Coupling to GENRAY/CQL3D reveals current profiles that are robustly monotonic and peaked on-axis, in much better agreement with experiment. In contrast, simulations without scattering result in current profiles that are non-monotonic and peaked off-axis. The full-wave/statistical model is self-consistently coupled to GENRAY, allowing for LHCD simulations in the multi-pass regime. This multi-scale model couples local full-wave scattering physics to a global ray-tracing solver. Across multiple C-Mod discharges, simulations show that scattering plays a significant role in determining the current profile. Phase-space broadening due to scattering significantly broadens the power deposition profile, allowing for increased on-axis current and the mitigation of current valleys and off-axis peaks. These effects saturate for sufficiently intense SOL turbulence, and parametric scans suggest LHCD in C-Mod exists in this saturated regime. Furthermore, the asymmetric scattering effect is shown to significantly affect the current profile. At low and moderate densities, good agreement is found with experimental Motional Stark effect and Hard X-ray measurements. At high densities, the same general trends are found. However, uncertainties relating to Ohmic current, SOL collisionality, and parametric decay make comparisons with experiments challenging.

Download or read book Fast Ion Tails During Radio Frequency Heating on the Alcator C Mod Tokamak written by Jon Christian Rost and published by . This book was released on 1998 with total page 154 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Radio frequency Heating and Propagation in Tokamak Plasmas in the Ion Cyclotron Range of Frequencies written by Tak Kuen Mau and published by . This book was released on 1977 with total page 318 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Experimental Investigation of Toroidal Rotation Profiles in the Alcator C Mod Tokamak written by William Davis Lee and published by . This book was released on 2003 with total page 205 pages. Available in PDF, EPUB and Kindle. Book excerpt: (Cont.) Data taken during ITB discharges have shown rotation profiles that evolve from flat co-current to hollow, with rotation slightly counter-current inside the barrier foot. Radial electric field profiles have been infered using neoclassical theory; these profiles are found to be highly sheared around the barrier foot. A lower limit to the gradient of the radial electric field ([partial derivatice]E[sub]r/[partial derivative]r) approximately] 183 kV/m2 has been set, and a lower limit on the shearing frequency has been approximated as [omega] [sub]E [approximately] krad/s.

Download or read book Design Constraints for Rf driven Steady state Tokamak Reactors written by and published by . This book was released on 1979 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Plasma current density profiles are computed due to electron Landau damping of lower hybrid waves launched into model tokamak density and temperature profiles. The total current and current profile shape are chosen consistent with magnetohydrodynamic equilibrium for a variety of temperature and density distributions and plasma beta values. Surface current equilibria appear attractive and are accessible to waves with n/sub z/ as low as 1.2. By suitably choosing the spectrum location and width it is possible to drive the 9.8 MA current of a 7.0-m reactor with as little as 2.8% of the fusion power recirculated as rf input from the waveguides.

Download or read book Nuclear Fusion written by and published by . This book was released on 2006 with total page 1102 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book ICRF heated Enhanced Confinement Modes in the Alcator C mod Tokamak written by Yuichi Takase and published by . This book was released on 1996 with total page 24 pages. Available in PDF, EPUB and Kindle. Book excerpt: