Optimization of the Safety Factor Profile for High Noninductive Current Fraction Discharges in DIII-D

Journal Article · Sat Jan 01 00:00:00 EST 2011 · Nuclear Fusion

DOI:https://doi.org/10.1088/0029-5515/51/6/063026· OSTI ID:1037087

Ferron, J R ^[1]; Holcomb, C T ^[2]; Luce, T C ^[1]; Politzer, P A ^[1]; Turco, F ^[3]; White, A E ^[4]; DeBoo, J C ^[5]; Doyle, E J ^[6]; Hyatt, A W ^[1]; La Haye, R ^[1]; Murakami, Masanori ^[7]; Petrie, T W ^[1]; Petty, C C ^[1]; Rhodes, T L ^[6]; Zeng, L ^[6]

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1. General Atomics, San Diego
2. Lawrence Livermore National Laboratory (LLNL)
3. Oak Ridge Associated Universities (ORAU)
4. Massachusetts Institute of Technology (MIT)
5. General Atomics
6. University of California, Los Angeles
7. ORNL

In order to assess the optimum q profile for discharges in DIII-D with 100% of the current driven noninductively (f(NI) = 1), the self-consistent response of the plasma profiles to changes in the q profile was studied in high f(NI), high beta(N) discharges through a scan of q(min) and q(95) at two values of beta(N). As expected, both the bootstrap current fraction, f(BS), and f(NI) increased with q(95). The temperature and density profiles were found to broaden as either q(min) or beta(N) is increased. A consequence is that f(BS) does not continue to increase at the highest values of q(min). A scaling function that depends on q(min), q(95), and the peaking factor for the thermal pressure was found to represent well the f(BS)/beta(N) inferred from the experimental profiles. The changes in the shapes of the density and temperature profiles as beta(N) is increased modify the bootstrap current density (J(BS)) profile from peaked close to the axis to relatively flat in the region between the axis and the H-mode pedestal. Therefore, significant externally driven current density in the region inside the H-mode pedestal is required in addition to J(BS) in order to match the profiles of the noninductive current density (J(NI)) to the desired total current density (J). In this experiment, the additional current density was provided mostly by neutral beam current drive with the neutral-beam-driven current fraction 40-90% of f(BS). The profiles of J(NI) and J were most similar at q(min) approximate to 1.35-1.65, q(95) approximate to 6.8, where f(BS) is also maximum, establishing this q profile as the optimal choice for f(NI) = 1 operation in DIII-D with the existing set of external current drive sources.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• DOE Contract Number: DE-AC05-00OR22725
• OSTI ID: 1037087
• Journal Information: Nuclear Fusion, Vol. 51, Issue 6
• Country of Publication: United States
• Language: English