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Title: Control of the Tokamak q Profile via MPC to Facilitate Reproducibility of High-qmin Steady-State Scenarios at DIII-D

Authors:
; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1337004
Report Number(s):
LLNL-CONF-696907
DOE Contract Number:
AC52-07NA27344
Resource Type:
Conference
Resource Relation:
Conference: Presented at: IEEE Multi-Conference on Systems and Control 2016, Buenos Aires, Argentina, Sep 19 - Sep 22, 2016
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION

Citation Formats

Wehner, W, Lauret, M, Schuster, E, Ferron, J, Holcomb, C, Luce, T, Humphreys, D, Walker, M, Penaflor, B, and Johnson, R. Control of the Tokamak q Profile via MPC to Facilitate Reproducibility of High-qmin Steady-State Scenarios at DIII-D. United States: N. p., 2016. Web.
Wehner, W, Lauret, M, Schuster, E, Ferron, J, Holcomb, C, Luce, T, Humphreys, D, Walker, M, Penaflor, B, & Johnson, R. Control of the Tokamak q Profile via MPC to Facilitate Reproducibility of High-qmin Steady-State Scenarios at DIII-D. United States.
Wehner, W, Lauret, M, Schuster, E, Ferron, J, Holcomb, C, Luce, T, Humphreys, D, Walker, M, Penaflor, B, and Johnson, R. 2016. "Control of the Tokamak q Profile via MPC to Facilitate Reproducibility of High-qmin Steady-State Scenarios at DIII-D". United States. doi:. https://www.osti.gov/servlets/purl/1337004.
@article{osti_1337004,
title = {Control of the Tokamak q Profile via MPC to Facilitate Reproducibility of High-qmin Steady-State Scenarios at DIII-D},
author = {Wehner, W and Lauret, M and Schuster, E and Ferron, J and Holcomb, C and Luce, T and Humphreys, D and Walker, M and Penaflor, B and Johnson, R},
abstractNote = {},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 7
}

Conference:
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  • Cited by 9
  • The results from experiments on DIII-D [J. L. Luxon, Fusion Sci. Technol. 48, 828 (2005)] aimed at developing high β steady-state operating scenarios with high-qminqmin confirm that fast-ion transport is a critical issue for advanced tokamak development using neutral beam injection current drive. In DIII-D, greater than 11 MW of neutral beam heating power is applied with the intent of maximizing β N and the noninductive current drive. However, in scenarios with q min>2 that target the typical range of q 95= 5–7 used in next-step steady-state reactor models, Alfvén eigenmodes cause greater fast-ion transport than classical models predict. Thismore » enhanced transport reduces the absorbed neutral beam heating power and current drive and limits the achievable β N. Conversely similar plasmas except with q min just above 1 have approximately classical fast-ion transport. Experiments that take q min>3 plasmas to higher β P with q 95= 11–12 for testing long pulse operation exhibit regimes of better than expected thermal confinement. Compared to the standard high-q min scenario, the high β P cases have shorter slowing-down time and lower ∇β fast, and this reduces the drive for Alfvénic modes, yielding nearly classical fast-ion transport, high values of normalized confinement, β N, and noninductive current fraction. These results suggest DIII-D might obtain better performance in lower-q 95, high-q min plasmas using broader neutral beam heating profiles and increased direct electron heating power to lower the drive for Alfvén eigenmodes.« less
  • The results from experiments on DIII-D [J. L. Luxon, Fusion Sci. Technol. 48, 828 (2005)] aimed at developing high β steady-state operating scenarios with high-qminqmin confirm that fast-ion transport is a critical issue for advanced tokamak development using neutral beam injection current drive. In DIII-D, greater than 11 MW of neutral beam heating power is applied with the intent of maximizing β N and the noninductive current drive. However, in scenarios with q min>2 that target the typical range of q 95= 5–7 used in next-step steady-state reactor models, Alfvén eigenmodes cause greater fast-ion transport than classical models predict. Thismore » enhanced transport reduces the absorbed neutral beam heating power and current drive and limits the achievable β N. Conversely similar plasmas except with q min just above 1 have approximately classical fast-ion transport. Experiments that take q min>3 plasmas to higher β P with q 95= 11–12 for testing long pulse operation exhibit regimes of better than expected thermal confinement. Compared to the standard high-q min scenario, the high β P cases have shorter slowing-down time and lower ∇β fast, and this reduces the drive for Alfvénic modes, yielding nearly classical fast-ion transport, high values of normalized confinement, β N, and noninductive current fraction. These results suggest DIII-D might obtain better performance in lower-q 95, high-q min plasmas using broader neutral beam heating profiles and increased direct electron heating power to lower the drive for Alfvén eigenmodes.« less
  • Ideally, tokamak power plants will operate in steady-state at high fusion gain. Recent work at DIII-D on the development of suitable high beta discharges with 100% of the plasma current generated noninductively (f{sub NI} = 1) is described. In a discharge with 1.5 < q{sub min} <2, a scan of the discharge shape squareness was used to find the value that maximizes confinement and achievable {beta}{sub N}. A small bias of the up/down balance of the double-null divertor shape away from the ion B x {del}B drift direction optimizes pumping for minimum density. Electron cyclotron current drive with a broadmore » deposition profile was found to be effective at avoidance of a 2/1 NTM allowing long duration at {beta}{sub N} = 3.7. With these improvements, surface voltage {approx} 0-10 mV, indicating f{sub NI} {approx} 1, was obtained for 0.7 {tau}{sub R} (resistive time). Stationary discharges with {beta}{sub N} = 3.4 and f{sub NI} {approx} 0.9 that project to Q = 5 in ITER have been demonstrated for {tau}{sub R}. For use in development of model based controllers for the q profile, transport code models of the current profile evolution during discharge formation have been validated against the experiment. Tests of available actuators confirm that electron heating during the plasma current ramp up to modify the conductivity is by far the most effective. The empirically designed controller has been improved by use of proportional/integral gain and built-in limits to {beta}{sub N} to avoid instabilities. Two alternate steady-state compatible scenarios predicted to be capable of reaching {beta}{sub N} = 5 have been tested experimentally, motivated by future machines that require high power density and neutron fluence. In a wall stabilized scenario with q{sub min} > 2, {beta}{sub N} = 4 has been achieved for 2 s {approx} {tau}{sub R}. In a high internal inductance scenario, which maximizes the ideal no-wall stability limit, {beta}{sub N} {approx} 4.8 has been reached with f{sub NI} > 1.« less