Three-dimensional Simulation of H-mode Plasmas with Localized Divertor Impurity Injection on Alcator C-Mod Using the Edge Transport Code EMC3-EIRENE. PDF Download

Author:
Publisher:
ISBN:
Category :
Languages : en
Pages :

View

Book Description
We study experiments in Alcator C-Mod to assess the level of toroidal asymmetry in divertor conditions resulting from poloidally and toroidally localized extrinsic impurity gas seeding show a weak toroidal peaking (~1.1) in divertor electron temperatures for high-power enhanced D-alpha H-modeplasmas. This is in contrast to similar experiments in Ohmically heated L-modeplasmas, which showed a clear toroidal modulation in the divertor electron temperature. Modeling of these experiments using the 3D edge transport code EMC3-EIRENE [Y. Feng et al., J. Nucl. Mater. 241, 930 (1997)] qualitatively reproduces these trends, and indicates that the different response in the simulations is due to the ionization location of the injected nitrogen. Low electron temperatures in the private flux region (PFR) in L-mode result in a PFR plasma that is nearly transparent to neutral nitrogen, while in H-mode the impurities are ionized in close proximity to the injection location, with this latter case yielding a largely axisymmetric radiation pattern in the scrape-off-layer. In conclusion, the consequences for the ITER gas injection system are discussed. Quantitative agreement with the experiment is lacking in some areas, suggesting potential areas for improving the physics model in EMC3-EIRENE.

Author: John E. Rice
Publisher:
ISBN:
Category :
Languages : en
Pages : 86

View

Book Description
(cont.) These edge values of the transport coefficients during EDA H-mode are qualitatively similar to the neoclassical values. In ELM-free H-mode discharges, impurity accumulation occurs, dominated by large inward impurity convection in the pedestal region. A scaling of the impurity confinement time with H-factor reveals a very strong exponential dependence. In ITB discharges, there is significant impurity accumulation inside of the barrier foot, typically at r/a = 0.5. Steady state impurity density profiles in L-mode plasmas have a large up-down asymmetry near the last closed flux surface. The impurity density enhancement, in the direction opposite to the ion BxB drift, is consistent with modeling of neo-classical parallel impurity transport.

Author: Dmitri A. Mossessian
Publisher:
ISBN:
Category :
Languages : en
Pages : 34

View

Book Description
For steady state H-mode operation, a relaxation mechanism is required to limit build-up of the edge gradient and impurity content. C-Mod sees two such mechanisms - EDA and grassy ELMs, but not large type I ELMs. In EDA the edge relaxation is provided by an edge localized quasicoherent electromagnetic mode that exists at moderate pedestal temperature T400 eV, high pedestal density and high edge safety factor ...3.5, and does not limit the build up of the edge pressure gradient. The q boundary of the operational space of the mode depends on plasma shape, with the q95 limit moving down with increasing plasma triangularity. At high edge pressure gradients and temperatures the mode is replaced by broadband fluctuations (f

Author: John E. Rice
Publisher:
ISBN:
Category :
Languages : en
Pages : 25

View

Book Description

Author: Dmitri A. Mossessian
Publisher:
ISBN:
Category :
Languages : en
Pages : 32

View

Book Description
Under most operational conditions of Alcator C-Mod the dominant type of H-mode is the steady state enhanced Da mode (EDA), characterized by good energy confinement, continuously degraded impurity confinement and absence of regular ELMs. In this regime a quasicoherent (QC) electromagnetic mode ... is observed, localized in the region of the density pedestal. Experimental evidence suggests that the mode is responsible for enhancement of particle transport. It is shown experimentally that the QC mode can exist in a well defined region in edge temperature-safety factor space, favoring high edge q values ... and requiring moderate pedestal temperatureTeped ... As edge temperature and pressure gradient increase, the quasicoherent mode is replaced by broadband low frequency fluctuations (f

Author: Amanda E. Hubbard
Publisher:
ISBN:
Category :
Languages : en
Pages : 6

View

Book Description

Author: Sanjay Gangadhara
Publisher:
ISBN:
Category :
Languages : en
Pages : 10

View

Book Description

Author: Nils Plesner Basse
Publisher:
ISBN:
Category :
Languages : en
Pages : 96

View

Book Description
The recently upgraded phase-contrast imaging (PCI) diagnostic is used to characterize the transition from the low (L) to the enhanced D[alpha] (EDA) high (H) confinement mode in Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plasmas, 1511 (1994)] plasmas. PCI yields information on line integrated density fluctuations along vertical chords. The number of channels has been increased from 12 to 32, the sampling rate from 1 MHz to 10 MHz. This expansion of diagnostic capabilities is used to study broadband turbulence in L- and EDA H-mode and to analyze the quasi-coherent (QC) mode associated with EDA H-mode. Changes in broadband turbulence at the transition from L- to EDA H-mode can be interpreted as an effect of the Doppler rotation of the bulk plasma. Additional fluctuation measurements of D[alpha]-light and the poloidal magnetic field show features correlated with PCI in two different frequency ranges at the transition. The back transition from EDA H- to L-mode, the so-called enhanced neutron (EN) mode, is investigated by new high frequency (132 and 140 GHz) reflectometer channels operating in the ordinary (O) mode. This additional hardware has been installed in an effort to study localized turbulence associated with internal transport barriers (ITBs). The EN-mode is a suitable candidate for this study, since an ITB exists transiently as the outer density decreases much faster than the core density in this mode. The fact that the density decays from the outside inward allows us to study fluctuations progressing towards the plasma core. Our results mark the first localized observation of the QC-mode at medium density: 2.2 x 1020 m-3 (132 GHz). Correlating the reflectometry measurements with other fluctuating quantities provides some insight regarding the causality of the EN-mode development.

Author: Florian Effenberg
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

View

Book Description
A first-time numerical exploration of radiative power exhaust in limiter startup and island divertor scenarios has been performed at the new optimized quasi-isodynamic stellarator Wendelstein 7-X (W7-X). The experiments conducted in this study confirm the main transport features predicted by simulations with the 3D plasma fluid and kinetic neutral edge transport Monte Carlo Code EMC3-EIRENE and the feasibility of radiative power dissipation with impurity seeding in the W7-X island divertor for the first time. The heat and particle transport in the limiter scenarios is found to be governed by the 3D helical magnetic geometry and well characterized by the simple scrape-off layer (SOL) model. The correlation between heat fluxes and connection lengths predicted by 3D modeling has been confirmed with experimental IR camera measurements. Reduction of the limiter heat fluxes was predicted to be very effective with Neon (Ne) seeding because it enhances the radiation ($P_{rad}$) at the upstream location while Nitrogen (N$_2$) radiates deeper in the SOL. Experiments confirmed this, but the limiter plasmas are shown to be more frequently terminated by radiative instabilities in case of Ne seeding. The internal shear of the magnetic islands creates much longer and more complex heat and particle transport channels in the parallel and perpendicular direction in the island divertor scenarios. Ne and N$_2$ seeding in the island divertor demonstrates stable power exhaust substantially reducing $T_e$, and $q_{\parallel, div}$. Ne seeding generally features $P_{rad}$ enhancement with slow decay over several seconds after termination of the puff suggesting high recycling. N$_2$ seeding shows fast recovery of $P_{rad}$, $T_e$ and $q_{\parallel, div}$ after injection termination indicating low recycling. The feasibility of impurity exhaust control has been demonstrated by manipulation of the island geometry with control coils. 3D modeling provides evidence for reduced overall Ne impurity dwell times with increased islands and reduced connection lengths. The potential impact of equilibrium effects on the edge island geometry and plasma transport has been anticipated based on a high $\beta$ scenario calculated with the 3D MHD code HINT.

Author: John Reel Walk (Jr.)
Publisher:
ISBN:
Category :
Languages : en
Pages : 64

View

Book Description
Using a spatially-resolving x-ray spectrometer system, toroidal impurity rotation in Alcator C-Mod plasmas was measured. The propagation of the rotational velocity from the edge to the core of the plasma column was measured during the L- to H-mode transition. Momentum transport was measured in both Ohmic and ICRF-heated discharges, which produced EDA and ELM-free H-modes. The momentum transport was modeled by a simplified diffusion model, in which momentum diffusivity was substantially higher than neoclassically predicted values.