Intermittent electron density and temperature fluctuations and associated fluxes in the Alcator C-Mod scrape-off layer

The Alcator C-Mod mirror Langmuir probe system has been used to sample data time series of fluctuating plasma parameters in the outboard mid-plane far scrape-off layer. We present a statistical analysis of one second long time series of electron density, temperature, radial electric drift velocity and the corresponding particle and electron heat fluxes. These are sampled during stationary plasma conditions in an ohmically heated, lower single null diverted discharge. The electron density and temperature are strongly correlated and feature fluctuation statistics similar to the ion saturation current. Both electron density and temperature time series are dominated by intermittent, large-amplitude burst with an exponential distribution of both burst amplitudes and waiting times between them. The characteristic time scale of the large-amplitude bursts is approximately 15{\mu}s. Large-amplitude velocity fluctuations feature a slightly faster characteristic time scale and appear at a faster rate than electron density and temperature fluctuations. Describing these time series as a superposition of uncorrelated exponential pulses, we find that probability distribution functions, power spectral densities as well as auto-correlation functions of the data time series agree well with predictions from the stochastic model. The electron particle and heat fluxes present large-amplitude fluctuations. For this low-density plasma, the radial electron heat flux is dominated by convection, that is, correlations of fluctuations in the electron density and radial velocity. Hot and dense blobs contribute approximately 6% of the total fluctuation driven heat flux

High-field side scrape-off layer investigation: Plasma profiles and impurity screening behavior in near-double-null configurations

New experiments on Alcator C-Mod reveal that the favorable impurity screening characteristics of the high-field side (HFS) scrape-off layer (SOL), previously reported for single null geometries, is retained in double null configurations, despite the formation of an extremely thin SOL. In balanced double-null, nitrogen injected locally into the HFS SOL is better screened by a factor of 2.5 compared to the same injection into the low field side (LFS) SOL. This result is insensitive to plasma current and Greenwald fraction. Nitrogen injected into the HFS SOL is not as well screened (only a factor of 1.5 improvement over LFS) in unbalanced double-null discharges, when the primary divertor is in the direction of B×∇B. In this configuration, impurity ‘plume’ emission patterns indicate that an opposing E × B drift competes with the parallel impurity flow to the divertor. In balanced double-null plasmas, the dispersal pattern exhibits a dominant E × B motion. Unbalanced discharges with the primary divertor opposite the direction of B×∇B exhibit excellent HFS screening characteristics – a factor of 5 enhancement compared to LFS. These data support the idea that future tokamaks should locate all RF actuators and close-fitting wall structures on the HFS and employ near-double-null magnetic topologies, both to precisely control plasma conditions at the antenna/plasma interface and to maximally mitigate the impact of local impurity sources arising from plasma-material interactions. Keywords: Alcator C-Mod; Impurity screening; Double null; High field side scrape-off layerUnited States. Department of Energy (Contract DE-FC02-99ER54512

Measurements of divertor target plate conditions and their relationship to scrape-off layer transport

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: Ph. D. in Applied Plasma Physics, Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2019Cataloged from student-submitted PDF of thesis.Includes bibliographical references.Cross-field filamentary transport in the scrape-off layer (SOL) is important for controlling SOL profiles, main-chamber recycling fluxes, and divertor operation. However, questions remain about the extent to which divertor target conditions play a role in setting transport levels. The Alcator C-Mod SOL is well diagnosed and extensively characterized, making it an ideal platform to assess the impact of divertor target conditions on SOL filamentary transport and the resultant upstream profiles, in particular, density shoulder formation. To facilitate the investigation, a set of high heat flux handling, flush-mounted rail Langmuir probes were designed for the Alcator C-Mod divertor. They were validated and proved to be robust, reliable diagnostics. Main chamber SOL fluctuations and density profiles were observed and found to be strongly correlated with divertor target conditions when the core plasma Greenwald fraction was increased.However, no trend was observed when local changes to near SOL divertor conditions were made using N₂ impurity seeding. To understand these results, a numerical model for filament transport was constructed that includes realistic magnetic geometry effects (e.g. magnetic shear) and collisionality profiles, both of which have been identified by theory to be important parameters. In validating the numerical model, a discrepancy was highlighted: experimental observations find fluctuation timescales in the SOL to be independent of location, whereas theories assume that timescales are set by local parameters--not accounting for the nonlocal effect of filaments being formed in the near SOL and propagating outwards.The numerical model reveals that strong distortions to the magnetic geometry in the near SOL, due to proximity to the X-point, electrically disconnect the main chamber SOL from divertor target conditions, offering an explanation for the experimental observations, and further suggesting that divertor heat flux mitigation may be optimized without direct impact on main chamber plasma profiles. When the divertor is electrically connected to the main chamber SOL, simulations indicate that increasing divertor collisionality causes a decrease to filament velocity, contrary to published literature. In summary, the combined impact of SOL collisionality and magnetic geometry effects were found to be strong controlling parameters on cross-field filamentary transport consistent with theoretical expectations, providing strong motivation for including these effects in SOL transport simulations and in interpreting experimental results.by Adam QingYang Kuang.Ph. D. in Applied Plasma PhysicsPh.D.inAppliedPlasmaPhysics Massachusetts Institute of Technology, Department of Nuclear Science and Engineerin

Lower hybrid wave edge power loss quantification on the Alcator C-Mod tokamak

For the first time, the power deposition of lower hybrid RF waves into the edge plasma of a diverted tokamak has been systematically quantified. Edge deposition represents a parasitic loss of power that can greatly impact the use and efficiency of Lower Hybrid Current Drive (LHCD) at reactor-relevant densities. Through the use of a unique set of fast time resolution edge diagnostics, including innovative fast-thermocouples, an extensive set of Langmuir probes, and a Lyα ionization camera, the toroidal, poloidal, and radial structure of the power deposition has been simultaneously determined. Power modulation was used to directly isolate the RF effects due to the prompt (t1.0×10[superscript 20] (m[superscript −3])). Results will be shown addressing the distribution of power within the SOL, including the toroidal symmetry and radial distribution. These characteristics are important for deducing the cause of the reduced LHCD efficiency at high density and motivate the tailoring of wave propagation to minimize SOL interaction, for example, through the use of high-field-side launch.United States. Department of Energy. Office of Fusion Energy Sciences (Award No. DE-FC02-99ER54512-CMOD

Overview of the SPARC tokamak

© 2020 The Author(s). The SPARC tokamak is a critical next step towards commercial fusion energy. SPARC is designed as a high-field (T), compact (m, m), superconducting, D-T tokamak with the goal of producing fusion gain 2$]]> from a magnetically confined fusion plasma for the first time. Currently under design, SPARC will continue the high-field path of the Alcator series of tokamaks, utilizing new magnets based on rare earth barium copper oxide high-temperature superconductors to achieve high performance in a compact device. The goal of 2$]]> is achievable with conservative physics assumptions () and, with the nominal assumption of, SPARC is projected to attain and MW. SPARC will therefore constitute a unique platform for burning plasma physics research with high density (), high temperature (keV) and high power density () relevant to fusion power plants. SPARC's place in the path to commercial fusion energy, its parameters and the current status of SPARC design work are presented. This work also describes the basis for global performance projections and summarizes some of the physics analysis that is presented in greater detail in the companion articles of this collection