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L–H power threshold studies in JET with Be/W and C wall.
- Source :
- Nuclear Fusion; 2014, Vol. 54 Issue 2, p023007-023021, 15p
- Publication Year :
- 2014
-
Abstract
- A comparison of the L–H power threshold (P<subscript>thr</subscript>) in JET with all carbon, JET-C, and beryllium/tungsten wall (the ITER-like choice), JET-ILW, has been carried out in experiments with slow input power ramps and matched plasma shapes, divertor configuration and I<subscript>P</subscript>/B<subscript>T</subscript> pairs. The low density dependence of the L–H power threshold, namely an increase below a minimum density n<subscript>e,min</subscript>, which was first observed in JET with the MkII-GB divertor and C wall and subsequently not observed with the current MkII-HD geometry, is observed again with JET-ILW. At plasma densities above n<subscript>e,min</subscript>, P<subscript>thr</subscript> is reduced by ∼30%, and by ∼40% when the radiation from the bulk plasma is subtracted (P<subscript>sep</subscript>), with JET-ILW compared to JET-C. At the L–H transition the electron temperature at the edge, where the pedestal later develops, is also lower with JET-ILW, for a given edge density. With JET-ILW the minimum density is found to increase roughly linearly with magnetic field, , while the power threshold at the minimum density scales as . The H-mode power threshold in JET-ILW is found to be sensitive both to variations in main plasma shape (P<subscript>sep</subscript> decreases with increasing lower triangularity and increases with upper triangularity) and in divertor configuration. When the data are recast in terms of P<subscript>sep</subscript> and Z<subscript>eff</subscript> or subdivertor neutral pressure a linear correlation is found, pointing to a possible role of Z<subscript>eff</subscript> and/or subdivertor neutral pressure in the L–H transition physics. Depending on the chosen divertor configuration, P<subscript>thr</subscript> can be up to a factor of two lower than the ITPA scaling law for densities above n<subscript>e,min</subscript>. A shallow edge radial electric field well is observed at the L–H transition. The edge impurity ion poloidal velocity remains low, close to its L-mode values, ⩽5 km s<superscript>−1</superscript> ± 2–3 km s<superscript>−1</superscript>, at the L–H transition and throughout the H-mode phase, with no measureable increase within the experimental uncertainties. The edge toroidal rotation profile does not contribute to the depth of the negative E<subscript>r</subscript> well and thus may not be correlated with the formation of the edge transport barrier in JET. [ABSTRACT FROM AUTHOR]
- Subjects :
- CARBON
BERYLLIUM
NUCLEAR energy
PLASMA gases
LINEAR statistical models
Subjects
Details
- Language :
- English
- ISSN :
- 00295515
- Volume :
- 54
- Issue :
- 2
- Database :
- Complementary Index
- Journal :
- Nuclear Fusion
- Publication Type :
- Academic Journal
- Accession number :
- 94383146
- Full Text :
- https://doi.org/10.1088/0029-5515/54/2/023007