High flux expansion divertor studies in NSTX
Projections for high-performance H-mode scenarios in spherical torus (ST)-based devices assume low electron collisionality for increased efficiency of the neutral beam current drive. At lower collisionality (lower density), the mitigation techniques based on induced divertor volumetric power and momentum losses may not be capable of reducing heat and material erosion to acceptable levels in a compact ST divertor. Divertor geometry can also be used to reduce high peak heat and particle fluxes by flaring a scrape-off layer (SOL) flux tube at the divertor plate, and by optimizing the angle at which the flux tube intersects the divertor plate, or reduce heat flow to the divertor by increasing the length of the flux tube. The recently proposed advanced divertor concepts [1, 2] take advantage of these geometry effects. In a high triangularity ST plasma configuration, the magnetic flux expansion at the divertor strike point (SP) is inherently high, leading to a reduction of heat and particle fluxes and a facilitated access to the outer SP detachment, as has been demonstrated recently in NSTX [3]. The natural synergy of the highly-shaped high-performance ST plasmas with beneficial divertor properties motivated a further systematic study of the high flux expansion divertor. The National Spherical Torus Experiment (NSTX) is a mid-sized device with the aspect ratio A = 1.3-1.5 [4]. In NSTX, the graphite tile divertor has an open horizontal plate geometry. The divertor magnetic configuration geometry was systematically changed in an experiment by either (1) changing the distance between the lower divertor X-point and the divertor plate (X-point height h{sub X}), or by (2) keeping the X-point height constant and increasing the outer SP radius. An initial analysis of the former experiment is presented below. Since in the divertor the poloidal field B{sub {theta}} strength is proportional to h{sub X}, the X-point height variation changed the divertor plasma wetted area due to variations in a SOL flux expansion, or more generally, area expansion. The flux expansion factor is defined as f{sub exp} = {lambda}{sub q}{sup SP}/{lambda}{sub q}{sup MP} {approx_equal} (B{sub {theta}}/B{sub tot}){sup MP}/(B{sub {theta}}/B{sub tot}){sup SP}, where B{sub tot} is the total magnetic field at the strike point R{sub SP} and midplane R{sub MP} major radii, and {lambda}{sub q} is the SOL parallel heat flux width. The SOL area expansion factor is f{sub exp} R{sub MP} = R{sub SP}.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 963522
- Report Number(s):
- LLNL-PROC-414328; TRN: US0903368
- Resource Relation:
- Conference: Presented at: 36th EPS Conference on Plasma Physics, Sofia, Bulgaria, Jun 29 - Jul 03, 2009
- Country of Publication:
- United States
- Language:
- English
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