Impurity Asymmetries in the Pedestal Region of the Alcator C-Mod Tokamak

Impurity Asymmetries in the Pedestal Region of the Alcator C-Mod Tokamak
Author: Randy Michael Churchill
Publisher:
Total Pages: 216
Release: 2014
Genre:
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In an effort to illuminate the effects of the strong plasma gradients in the pedestal region on impurity transport, research was conducted to measure complete sets of impurity density, poloidal and parallel velocity, and temperature at two separate poloidal locations in the pedestal region of the Alcator C-Mod tokamak. To this end, the diagnostic technique gas puff-CXRS was refined and expanded on, allowing for the first time in a tokamak complete measurements of impurities at the high-field side (HFS). Large in-out B5+ impurity density asymmetries were measured in H-mode plasmas with strong boundary electron density gradients, with a build-up of impurity density at the HFS. Impurity temperatures were also found to be asymmetric in the pedestal region, with larger temperatures at the low-field side (LFS). Such temperature asymmetries suggest a significant asymmetry in electron density near the separatrix. In contrast to these H-mode results, plasmas with low boundary electron density gradients, such as L-mode and I-mode, exhibit constant impurity density on a flux surface, even if strong electron temperature gradients are present. Mechanisms which could drive such poloidal asymmetries are explored. Experiments provide evidence against localized impurity sources and fluctuation-induced transport as primary causes. Particle transport timescales are compared, showing that the radial transport becomes comparable to or faster than the parallel transport in the pedestal region. Additionally, modelling of impurity transport using conventional, one-dimensional neoclassical physics fails to correctly reproduce the measured flux-surface averaged impurity density, suggesting along with the timescale estimates that a more complete two-dimensional treatment of impurity particle transport is required. The measured impurity velocities at the LFS and HFS are compared to the canonical form for particle flow velocity within the flux surface of a tokamak. Within the error bars of the measurement, agreement is found with the canonical form. The implications of exact matches to the canonical form are low radial transport, and the E x B drift dominating the perpendicular impurity flow. Further work is motivated into more precise velocity measurements to determine if the velocities exactly match this canonical form.

Edge Plasma Phenomena in the Alcator C-Mod Tokamak Measured by High Resolution X-ray Imaging Diagnostics

Edge Plasma Phenomena in the Alcator C-Mod Tokamak Measured by High Resolution X-ray Imaging Diagnostics
Author: Thomas Sunn Pedersen
Publisher:
Total Pages: 206
Release: 2000
Genre:
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(cont.) A second array views the top of the plasma. The x-ray emissivity measured with this array also shows a distinct and narrow pedestal in H-mode. However, it is located significantly closer to the separatrix and is often narrower. Both of these differences increase with the safety factor at the edge, q95. Thus, there is a significant poloidal asymmetry in the impurity density in the H-mode edge region, which increases with q95. Therefore, the impurity transport in the H-mode edge is highly two-dimensional. The strong poloidal asymmetries measured show some quantitative agreement with theories developed to explain poloidal impurity asymmetries. However, none of the theories are strictly applicable to the Alcator C-Mod edge, and they all significantly underestimate the actual asymmetries that we observe.

Impurity Transport Studies on Alcator C-Mod Tokamak Using Charge Exchange Recombination Spectroscopy

Impurity Transport Studies on Alcator C-Mod Tokamak Using Charge Exchange Recombination Spectroscopy
Author: Igor Olegovich Bespamyatnov
Publisher:
Total Pages: 322
Release: 2008
Genre: Charge exchange
ISBN:

A Charge-Exchange Recombination Spectroscopy (CXRS) diagnostic has been installed on Alcator C-Mod to study the transport of light impurities in plasma. The system provides spatially (1 cm) and temporally (12.5 msec) resolved measurements of the impurity density, temperature and flow velocities of the particular impurity. Two optical arrays: poloidal (19 channels) and toroidal (10 channels), collect the light emitted from excited impurity ion populated by charge exchange process from the Diagnostic Neutral Beam (DNB) particle. The attention of this dissertation is focused on the B4 (n = 7 [-->] 6) spectral line emitted by B4 ion formed in the following charge exchange reaction (H0 + B5+ [-->] H+ + B4+*). A complex spectral model was developed to simulate emission. The high magnetic fields of C-Mod result in broad Zeeman patterns which must be taken into account for the interpretation of the line shift and broadening in terms of impurity ion velocity and temperature. After the spectral line fitting and careful identification of the charge exchange component, the calculated Doppler broadening and shifts of the spectral line profile yield information on the ion temperature and rotation. Together with the calculation of the beam density, the absolute calibration of the CXRS optical system provides us with B5+ density measurement capabilities. One of the main objectives of this work was to use the acquired impurity density, temperature and flow velocity profiles to investigate plasma transport behavior and infer the radial electric field E[subscript R] from plasma force balance equation. The focus here was placed on the region of the Internal Transport Barrier (ITB) formation 0.35

Measurement of Impurity Ion Densities and Energies in the Divertor and Edge Regions of Alcator C-Mod Tokamak. [Lab. for Plasma Research, Univ. of Maryland, College Park].

Measurement of Impurity Ion Densities and Energies in the Divertor and Edge Regions of Alcator C-Mod Tokamak. [Lab. for Plasma Research, Univ. of Maryland, College Park].
Author:
Publisher:
Total Pages: 6
Release: 1992
Genre:
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A study to investigate impurity production and transport in the divertor and edge regions of the Alcator C-Mod tokamak through spectroscopic techniques is described. A 0.75-meter Czerny-Turner spectrometer with a 1200-g/mm grating and a 35-meter quartz optic bundle transmission line were tested. A high-resolution 2-meter spectrometer will be ordered. Data acquisition considerations are being addressed. (RWR).

Measurement of Impurity Ion Densities and Energies in the Divertor and Edge Regions of Alcator C-Mod Tokamak. Final Progress Report, March 1994--January 1995

Measurement of Impurity Ion Densities and Energies in the Divertor and Edge Regions of Alcator C-Mod Tokamak. Final Progress Report, March 1994--January 1995
Author:
Publisher:
Total Pages: 5
Release: 1995
Genre:
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The authors are investigating impurity production and transport in the divertor and edge regions of the Alcator C-Mod tokamak through spectroscopic techniques. The emphasis is on the low ionization states found in the edge and divertor regions which are indicative of the physical processes related to impurity generation and particle and energy transport in this region. The authors are using a high-resolution visible/ultraviolet spectrograph capable of measuring the Doppler shifts associated with neutral and ion flows and the Doppler broadening associated with neutral and ion temperatures.

Physics and Application of Impurity Plume Dispersal as an Edge Plasma Flow Diagnostic on the Alcator C-Mod Tokamak

Physics and Application of Impurity Plume Dispersal as an Edge Plasma Flow Diagnostic on the Alcator C-Mod Tokamak
Author: Sanjay Gangadhara
Publisher:
Total Pages: 273
Release: 2003
Genre:
ISBN:

(Cont.) A Monte Carlo impurity transport code (LIM) was used to simulate the plumes. Results indicate that contributions to the emission from sputtering explain the cross-field plume width, and that the parallel extent of emission generated in the far SOL is well-described using a sputter launch-energy distribution for the impurities. In the near SOL, the presence of a localized parallel electric field arising from background ion recycling off the probe surface is necessary to explain the parallel extent of emission generated in this region. This electric field accelerates impurity ions formed near the probe tip away from the probe, causing jet-like behavior. LIM was also used to investigate causes for the vertical elongation of the impurity emission. Results suggest the existence of a probe-induced E x B drift, of order ca. 1000 m/s in the near SOL. This drift may be responsible for the transport of both impurity and bulk plasma ions down the probe axis. Values for vII in the far SOL and Er in the near SOL have been extracted from the plume structure. A comparison between plume and probe results for Er suggests that calculations which employ a probe-sheath model may be in error, and that measurement of the poloidal propagation velocity of edge plasma fluctuations may be a more reliable means of inferring Er from probe data. Comparisons between plume- and probe-inferred values for the parallel Mach number suggest that the probe over-estimates parallel flow to the divertor in the far SOL, where effects of short field line connection to the divertor are important ...