Speaker
Dr
Brian LaBombard
(MIT Plasma Science and Fusion Center)
Description
The improved impurity screening characteristics of the high-field side scrape-off layer to local impurity sources, previously reported for single null geometries, is found to be retained in double null configurations - strengthening the argument for locating current drive and heating actuators on the high-field side.
The high-field-side (HFS) scrape-off layer (SOL) is known to exhibit extremely low levels of cross-field transport [1] and excellent impurity screening characteristics [2] in single-null magnetic configurations. It has been proposed that future tokamaks should exploit these remarkable HFS characteristics to solve critical plasma-material interaction (PMI) and sustainment challenges – relocate all RF actuators and close-fitting wall structures to the HFS and employ near-double-null magnetic topologies, to precisely control plasma conditions at the antenna/plasma interface and mitigate the impact of PMI [3]. Dedicated experiments were performed on Alcator C-Mod during the 2015 experimental campaign to quantify impurity screening characteristics and scrape-off layer profiles in near-double-null configurations. Nitrogen screening by the HFS SOL is found to be a factor of 2.5 better than LFS in balanced double-null discharges, despite an extremely thin scrape-off layer. Impurity screening is found to be insensitive to current and Greenwald fraction. HFS impurity screening is least effective (only a factor of 1.5 improvement) in unbalanced double-null discharges that favor the active divertor in the direction of B×∇B. Unbalanced discharges that favor the most active divertor opposite the direction of B×∇B have excellent HFS screening characteristics, a factor of 5 better than LFS. The latter situation is particularly promising for the use of HFS RF actuators in I-mode plasmas – a high confinement, steady state, ELM-free regime that is accessible at high magnetic field to a large range of input power for this magnetic topology [4].
[1] N. Smick, et al., Nucl. Fusion 53 (2013) 02300; [2] G. McCracken, et al., Phys. Plasmas 4 (1997) 1681; [3] B. LaBombard, et al., Nucl. Fusion 55 (2015) 053020; [4] A. Hubbard, et al., IAEA FEC2014, paper EX/P6-22.
*This material is based on work supported by U.S. Department of Energy, Office of Fusion Energy Sciences under Award Number DE-FC02-99ER54512 on Alcator C-Mod, a DoE Office of Science User Facility.
| Country or International Organization | USA |
|---|---|
| Paper Number | EX/P3-6 |
Primary author
Dr
Brian LaBombard
(MIT Plasma Science and Fusion Center)
Co-authors
Mr
Adam Kuang
(MIT Plasma Science and Fusion Center)
Dr
Bob Mumgaard
(MIT Plasma Science and Fusion Center)
Dr
Daniel Brunner
(MIT Plasma Science and Fusion Center)
Prof.
Dennis Whyte
(MIT Plasma Science and Fusion Center)
Dr
Earl Marmar
(MIT Plasma Science and Fusion Center)
Dr
Gregory Wallace
(MIT Plasma Science and Fusion Center)
Dr
James Terry
(MIT Plasma Science and Fusion Center)
Jerry Hughes
(MIT Plasma Science and Fusion Center)
Dr
John Walk
(MIT Plasma Science and Fusion Center)
Dr
Mark Chilenski
(MIT Plasma Science and Fusion Center)
Dr
Matthew Reinke
(Oak Ridge National Laboratory)
Stephen Wukitch
(MIT Plasma Science and Fusion Center)
Dr
Steve Wolfe
(MIT Plasma Science and Fusion Center)
Dr
Yijun Lin
(MIT Plasma Science and Fusion Center)
