Statistical analysis of magnetic divertor configuration influence on H-mode transitions
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- Imperial College, London (United Kingdom); General Atomics
- Imperial College, London (United Kingdom); Univ. of Warwick, Coventry (United Kingdom)
- Imperial College, London (United Kingdom)
- Imperial College, London (United Kingdom); Univ. of Cambridge (United Kingdom). Cavendish Lab.
- Coventry University (United Kingdom). Fluid and Complex System Research Centre
- Univ. of California, Los Angeles, CA (United States)
- Univ. of Wisconsin, Madison, WI (United States)
DIII-D plasmas are compared for two upper divertor configurations: with the outer strike point on the small angle slot (SAS) divertor target and with the outer strike point on the horizontal divertor target (HT). Scanning the vertical distance between the magnetic null point and the divertor target over a range 0.10–0.16 m is shown to increase the threshold power, $$P_\mathrm{th}$$, and edge plasma power, $$P_\mathrm{Loss},$$ for the low-to-high confinement (L–H) and H–L transitions respectively, by up to a factor of 1.4. The X-point height scans were performed at three L-mode core plasma line average electron densities, $$\bar{n}_\mathrm{e} = $$ 1.2, 2.2 and 3.6 $$\times 10^{19}\,\mathrm{m}^{-3}$$, to investigate the density dependence of divertor magnetic configuration influence on $$P_\mathrm{th}$$. The X-point height, $$Z_\textrm{x-pt}$$, was further extended across the range 0.16–0.22 m with the more open HT divertor configuration, for which a clear decrease in $$P_\mathrm{th}$$ with increasing $$Z_\textrm{x-pt}$$ is observed. The dependence of $$P_\textrm{th}$$ on divertor magnetic geometry is further investigated using a time-dependent probability density function (PDF) model and information geometry to elucidate the roles played by pedestal plasma turbulence and perpendicular velocity flows. The degree of stochasticity of the plasma turbulence is observed to be sensitive to the plasma heating rate. The calculated square of the information rate shows changes in the relative density fluctuations and perpendicular velocity PDFs begin 2–5 ms prior to the L–H transition for three plasmas; providing a crucial measurement of the dynamic timescale of external transport barrier formation. Additionally, both information length and rate provide potential predictors of the L–H transition for these plasmas.
- Research Organization:
- General Atomics, San Diego, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Fusion Energy Sciences (FES)
- Grant/Contract Number:
- FC02-04ER54698; FG02-08ER54984; FG02-89ER53296; FG02-08ER54999; AC05-00OR22725; SC0020287
- OSTI ID:
- 2329314
- Journal Information:
- Plasma Physics and Controlled Fusion, Journal Name: Plasma Physics and Controlled Fusion Journal Issue: 5 Vol. 66; ISSN 0741-3335
- Publisher:
- IOP ScienceCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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