Enhanced D Alpha H Mode Studies In The Alcator C Mod Tokamak
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| Author | : Earl S. Marmar |
| Publisher | : |
| Total Pages | : 6 |
| Release | : 2000 |
| Genre | : |
| ISBN | : |
A favorable regime of H-mode confinement, seen on the Alcator C-Mod tokamak is described. Following a brief period of ELM-free H-mode, the plasma evolves into the Enhanced D-Alpha (EDA) H-mode which is characterized by very good energy confinement, the complete absence of large, intermittent type I ELMs, finite impurity and majority species confinement, and low radiated power fraction. Accompanying the EDA H-mode, a quasi-coherent (QC) edge mode is observed, and found to be responsible for particle transport through the edge confinement barrier. The QC-mode is localized within the strong density gradient region, and has poloidal wavenumber and lab-frame frequency of 100 kHz. Parametric studies show that the conditions which promote EDA include moderate safety factor, high triangularity (d>0.35) and high target density (ne>1.2x20 m-3). EDA H-mode is readily obtained in purely ohmic and well as in ICRF auxiliary-heated discharges.
| Author | : Amanda E. Hubbard |
| Publisher | : |
| Total Pages | : 16 |
| Release | : 2005 |
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| ISBN | : |
Comparisons of H-mode regimes were carried out on the Alcator C-Mod and JFT-2M tokamaks. Shapes were matched apart from aspect ratio, which is lower on C-Mod. The High Recycling Steady (HRS) H-mode on JFT-2M and Enhanced D-alpha (EDA) regime on C-Mod, which both feature very small or no ELMs, are found to have similar access conditions in q95-nu* space, occurring for pedestal collisionality nu* greater than 1. Differences in edge fluctuations were found, with lower frequencies but higher mode numbers on C-Mod. In both tokamaks an attractive regime with small ELMs on top of an enhanced D-alpha baseline was obtained at moderate nu* and higher pressure. The JFT-2M shape favoured the appearance of ELMs on C-Mod, and also resulted in the appearance of a lower frequency component of the quasicoherent mode during EDA.
| Author | : Amanda E. Hubbard |
| Publisher | : |
| Total Pages | : 18 |
| Release | : 2000 |
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| ISBN | : |
High resolution measurements on the Alcator C-Mod tokamak [I.H. Hutchinson et al, Phys. Plasmas 1, 1551 (1994)] of the transport barrier in the "Enhanced Da" (EDA) regime, which has increased particle transport without large edge localized modes, show steep density and temperature gradients over a region of 2-5 mm, with peak pressure gradients up to 12 MPa/m. Evolution of the pedestal at the LH transition is consistent with a large, rapid drop in thermal conductivity across the barrier. A quasi-coherent fluctuation in density, potential and Bpol, with fo%7E50-150 kHz and kq%7E 4 cm-1, always appears in the barrier during EDA, and drives a large particle flux. Conditions to access the steady-state EDA regime in deuterium include d> 0.35, q95> 3.5 and L-mode target line average density> 1.2 x 1020 m-3. A reduced q95 limit is found for hydrogen discharges.
| Author | : Yunxing Ma |
| Publisher | : |
| Total Pages | : 210 |
| Release | : 2013 |
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| ISBN | : |
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.
| Author | : Alexander Mazurenko |
| Publisher | : |
| Total Pages | : 320 |
| Release | : 2001 |
| Genre | : |
| ISBN | : |
Phase Contrast Imaging (PCI) is a new diagnostic that was built for the Alcator C-Mod tokamak. It measures line-integrated (along 12 vertical chords) plasma density perturbations with good temporal (2-500 kHz) and wavenumber (0.5-12 /cm) resolution. The Quasi-Coherent (QC) fluctuation mode was studied using the PCI and other diagnostics. The mode was found to cause fluctuation of density, electric and magnetic field in the plasma edge with typical frequency of 100 kHz and typical poloidal wavenumber of about 5/cm. The mode was found to be responsible for confinement properties of the "Enhanced D-alpha H-mode" (a particularly favorable regime of tokamak operation). Through numerical modeling, the physical origin of the fluctuations was tentatively identified as "resistive X-point" mode (a kind of resistive ballooning mode strongly affected by the X-point configuration of magnetic field lines). The PCI system has been upgraded to detect waves in the ion cyclotron range of frequencies (ICRF, 40-80 MHz) by means of optical heterodyning - a technique based on modulation of the diagnostic laser beam near the wave frequency. The upgraded system was then used to study propagation of the Fast Magnetosonic Waves. These waves, which have never been measured in detail in past experiments, are being used to heat the tokamak plasma at the megawatt power level. The measured results were compared to the simple cold-plasma dispersion relation and to predictions of the full-wave 3D numerical modeling.
| Author | : Liang Lin (Ph. D.) |
| Publisher | : |
| Total Pages | : 189 |
| Release | : 2009 |
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| ISBN | : |
(cont.) Our study shows that although the short wavelength turbulence in the ETG range is unstable in the linear ohmic regime, the nonlinear simulation with k[theta][rho]s up to 4 does not raise the electron thermal diffusivity to the experimental level, where k[theta] is the poloidal wavenumber and [rho]s is the ion-sound Larmor radius. The H-Mode studies focus on plasmas before and during internal transport barrier formation in an enhanced D[alpha], H-Mode plasma. The simulated fluctuations from GYRO agree with experimental measurements in the ITG regime. GYRO also shows good agreement in transport predictions with experimental measurements after reducing the ion temperature gradient (~15%) and adding ExB shear suppression, all within the experimental uncertainty.
| Author | : Jerry Wayne Hughes |
| Publisher | : |
| Total Pages | : 452 |
| Release | : 2005 |
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Edge transport barriers (ETBs) in tokamak plasmas accompany transitions from low confinement (L-mode) to high confinement (H-mode) and exhibit large density and temperature gradients in a narrow pedestal region near the last closed flux surface (LCFS). Because tokamak energy confinement depends strongly on the boundary condition imposed by the edge plasma pressure, one desires a predictive capability for the pedestal on a future tokamak. On Alcator C-Mod, significant contributions to ETB studies were made possible with edge Thomson scattering (ETS), which measures profiles of electron temperature (20 [leq] Te[eV] [leq] 800) and density (0.3 [leq] ne[10^20m^-3] [leq] 5) with 1.3-mm spatial resolution near the LCFS. Profiles of Te, ne, and pe = neTe are fitted with a parameterized function, revealing typical pedestal widths [delta] of 2-6mm, with [delta]Te [geq] [delta]ne , on average. Pedestals are examined to determine existence criteria for the enhanced D[alpha] (EDA) H-mode. A feature that distinguishes this regime is a quasi-coherent mode (QCM) near the LCFS. The presence or absence of the QCM is related to edge conditions, in particular density, temperature and safety factor q. Results are consistent with higher values of both q and collisionality [nu]* giving the EDA regime. Further evidence suggests that increased abs([nabla]pe) may favor the QCM; thus EDA may have relevance to low-[nu]* reactor regimes, should sufficient edge pressure gradient exist.
| Author | : Yuichi Takase |
| Publisher | : |
| Total Pages | : 24 |
| Release | : 1996 |
| Genre | : |
| ISBN | : |
| Author | : Alexander Mazurenko |
| Publisher | : |
| Total Pages | : 38 |
| Release | : 2001 |
| Genre | : |
| ISBN | : |
The Quasi-Coherent (QC) mode, observed at high densities during enhanced D-alpha (EDA) H-Mode in the Alcator C-Mod tokamak, has been studied with a phase contrast imaging (PCI) diagnostic. The QC mode is believed to be responsible for the reduced particle (and impurity) confinement during H-mode operation, so as to allow quasi-steady state operation of the tokamak. It is proposed that the QC mode is a form of resistive ballooning mode known as the resistive X-point mode. The measured dispersion and mode stability are found to be in good agreement with the resistive X-point mode predicted by the BOUT (Boundary Plasma Turbulence) code.
| Author | : John Wesson |
| Publisher | : Oxford University Press |
| Total Pages | : 828 |
| Release | : 2011-10-13 |
| Genre | : Science |
| ISBN | : 0199592233 |
The tokamak is the principal tool in controlled fusion research. This book acts as an introduction to the subject and a basic reference for theory, definitions, equations, and experimental results. The fourth edition has been completely revised, describing their development of tokamaks to the point of producing significant fusion power.
