Local Gas Injection as a Scrape-off Layer Diagnostic on the Alcator C-Mod Tokamak

Local Gas Injection as a Scrape-off Layer Diagnostic on the Alcator C-Mod Tokamak
Author:
Publisher:
Total Pages: 190
Release: 1996
Genre:
ISBN:

A capillary puffing array has been installed on Alcator C-Mod which allows localized introduction of gaseous species in the scrape-off layer. This system has been utilized in experiments to elucidate both global and local properties of edge transport. Deuterium fueling and recycling impurity screening are observed to be characterized by non-dimensional screening efficiencies which are independent of the location of introduction. In contrast, the behavior of non-recycling impurities is seen to be characterized by a screening time which is dependent on puff location. The work of this thesis has focused on the use of the capillary array with a camera system which can view impurity line emission plumes formed in the region of an injection location. The ionic plumes observed extend along the magnetic field line with a comet-like asymmetry, indicative of background plasma ion flow. The flow is observed to be towards the nearest strike-point, independent of x-point location, magnetic field direction, and other plasma parameters. While the axes of the plumes are generally along the field line, deviations are seen which indicate cross-field ion drifts. A quasi-two dimensional fluid model has been constructed to use the plume shapes of the first charge state impurity ions to extract information about the local background plasma, specifically the temperature, parallel flow velocity, and radial electric field. Through comparisons of model results with those of a three dimensional Monte Carlo code, and comparisons of plume extracted parameters with scanning probe measurements, the efficacy of the model is demonstrated. Plume analysis not only leads to understandings of local edge impurity transport, but also presents a novel diagnostic technique.

Impurity Plume Experiments in the Edge Plasma of the Alcator C-Mod Tokamak

Impurity Plume Experiments in the Edge Plasma of the Alcator C-Mod Tokamak
Author: Sanjay Gangadhara
Publisher:
Total Pages: 86
Release: 2003
Genre:
ISBN:

The physics of impurity transport in response to a local gas injection in the scrape-off-layer (SOL) of Alcator C-Mod is investigated. Carbon "plumes'' are formed at variable locations in the SOL -- up to the separatrix -- by puffing deuterated ethylene gas (C2D4) through the end of a reciprocating fast-scanning probe. CCD cameras are used to simultaneously record C+1 and C+2 emission patterns from two near-perpendicular views. The plume dispersal patterns are found to yield direct qualitative information about plasma flow, including the direction of VExB near the separatrix. Impurity transport and plasma-surface interaction physics implicit in the 3-D plume structure is explored in detail using a Monte Carlo impurity transport code, with the aim of extracting background plasma-flow quantities. A number of important local effects involving plasma-probe interaction have been identified: a vertical ExB drift near the probe surface, a parallel electric field above the probe tip arising from plasma recycling off the probe surface, and sputtering of a carbon layer that dynamically forms on the probe surface. The emission patterns are also found to yield important information on flows in the SOL: radial electric field (Er) in the near SOL and volume-averaged values of the parallel Mach number in the far SOL. Er values obtained from plume data compare favorably with estimates of Er based on the poloidal propagation velocity of edge plasma fluctuations measured by the scanning probe. Comparisons between parallel Mach numbers obtained from the plume data and probe measurements indicate that the probe over-estimates the parallel flow towards the divertor in the far SOL. This result supports the picture of particle balance in the SOL of Alcator C-Mod being dominated by main-chamber recycling, with weak plasma flow into the divertor.

Impurity Transport Studies in Tokamak Edge Plasmas Using Visibe Imaging

Impurity Transport Studies in Tokamak Edge Plasmas Using Visibe Imaging
Author: Sanjay Gangadhara
Publisher:
Total Pages: 6
Release: 2001
Genre:
ISBN:

Understanding impurity transport in the scrape-off layer (SOL) of tokamak plasmas is a necessary piece of developing the physics basis for designing next-generation reactors. A system for inferring impurity transport parallel and perpendicular to local magnetic field lines has been developed on Alcator C-Mod using gas-injection "plumes". In this system, impurity gas is injected at a fixed position in the SOL via a reciprocating fast-scanning probe, and the resulting emission is imaged. In this paper visible light emission patterns from C+1 and C+2 ions are presented.

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 ...

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:
ISBN:

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.