UW Madison Edge Meeting

This is a weekly cross-campus meeting to discuss plasma edge physics and plasma material interaction from a variety of perspectives engaging views on the field from a broad variety of devices on campus.

ROOM CHANGE!! The meeting takes place every Friday at noon in room 2421 in Engineering Hall.

—- Next talks with title and abstract:

– March, 13 2015: Three-dimensional edge plasma and neutral gas modeling with the EMC3-EIRENE code on the example of RMP application in tokamaks – status and development plans”

Heinke Frerichs

The development of a reliable computational model for the plasma edge in non-axisymmetric configurations is essential for both the interpretation of present day magnetic confined fusion experiments and for guiding the design activities for future next step devices such as ITER and Wendelstein 7-X. Three-dimensional models are naturally more computationally challenging than their two-dimensional counterparts, but they are required for a detailed analysis of the impact of resonant magnetic perturbations (RMPs) in tokamaks and for intrinsically non-axisymmetric stellarator configurations. One such tool is the EMC3-EIRENE code, a coupled transport solver for the fluid edge plasma in self-consistent interaction with neutral gas. It is a flexible tool regarding the magnetic field configuration (it has been applied to single null, disconnected double null and snow-flake divertor configurations at tokamaks, as well as to classical, quasi-helical and quasi-isodynamic stellarator configurations), but it still lacks the physical sophistication of present two-dimensional transport codes.
We will present an overview on results from application of the model to scenarios with RMP fields at DIII-D and highlight the shortcomings of the model compared to the experiment. This assessment focuses in particular on the power balance and parallel heat transport as well as the particle balance in high density regimes. We show first steps on advancing the model regarding both aspects and concentrate on recent activities facing the challenge to access detached divertor configurations with this numerical tool. A generalization of the particle balance was implemented allowing for localized gas sources as well as volume recombination. Recent exploratory simulations exhibit numerically unstable behavior which may be attributed to the iterative simulation procedure itself. We present an analysis of this numerically unstable state and propose an adaptive relaxation method for its stabilization.

– March, 20 2015: Investigation of limiter heat loads and impurity transport in Wendelstein 7-X startup plasmas with EMC3-Eirene

Florian Effenberg

Wendelstein 7-X is a new optimized stellarator in Greifswald, Germany, aiming to start plasma operation in 2015. The first plasmas will be in a limiter configuration. In this field configuration the plasma boundary does not include magnetic islands and the scrape-off layer (SOL) is defined by five poloidal graphite limiters located at the bean shaped symmetry planes. The limiters define the position of the last closed flux surface and are positioned such that they can prevent high heat fluxes onto the unprotected main chamber wall and metallic frame structure of the later installed graphite divertor targets. They are optimized for a preferably even distributed heatload deposition. Yet, in the environment of a 3D helical magnetic edge topology peaks in the deposition patterns are unavoidable. Considering startup plasmas with heating power up to 4MW and densities up to 9×10^{19} m^{?3} heat loads to plasma facing components (PFCs) and the generation of impurities due to plasma surface interaction become a concern. A topology analysis and plasma transport simulations are performed with the 3D fluid plasma edge and kinetic neutral transport Monte Carlo code EMC3-Eirene. Results are discussed for a systematic scan of plasma scenarios assessing the production and transport of impurities in the 3D SOL. In particular, radiation cooling by intrinsic carbon and active injected nitrogen is studied as a means to reduce damaging heat loads on PFCs.

—- Previous talks with title and abstract:

– February, 27 2015: Ryan Norval

“Edge Physics in the MST: Past, Present, and Future Experiments”

While the understanding of RFP core physics has seen significant advancement over the years, the RFP edge has only recently started to receive attention. MST is beginning a boundary physics program with the goals of understanding and controlling the RFP edge. The RFP in normal operation has a stochastic core with high transport, but paths to improved confinement include edge current drive, or driving the core to a 3D self organized state. The effects of these enhanced confinement discharges on PWI will be presented. Past and present works on boronization and a new boundary viewing camera will be presented. A brief look at future plans will also be presented.


– February, 20 2015: Recent advances in plasma microturbulence: turbulent saturation, the MST edge, and parity oddities in the pedestal

M. J. Pueschel

Abstract: Selected results from current electromagnetic gyrokinetic turbulence research are presented, with special emphasis on their relevance for edge physics. While many of the topics covered — zonal flows, subdominant microtearing, ELMs, the non-zonal transition, magnetic fluctuations in MST, and instability parity reversal in high-gradient regions — may appear to be rather theoretical in nature, this presentation is aimed at experimentalists and other interested parties, emphasizing both practical implications for confinement and challenges for future modeling.

– February, 13 2015: Overview of HSX Edge Program and Experiments

A. Akerson and L. Stephey

Abstract: The symmetric design of the HSX stellarator has been shown to improve particle confinement, reduce neoclassical transport, and reduce drift orbit excursions. However, while the core physics of HSX is now relatively well-understood, many aspects of edge physics remain largely unexplored.  A new thrust of edge studies is underway at HSX to better understand edge physics through both experiment and modeling using EMC3-EIRENE. These topics include the contribution of plasma potential, parallel and perpendicular transport, islands and flux bundles, particle transport, neutrals and plasma sourcing, wall recycling, and limiter structures. Two-dimensional Langmuir probe measurements have been made and are compared to EMC3-EIRENE modeling. Future work will include particle transport, particle balance, and neutral source measurements. A brief summary and overview of future work will be presented.