title: Tokamak operation with safety factor q95 < 2 via control of MHD stability.
abstract: Magnetic feedback control of the resistive-wall mode has enabled the DIII-D tokamak to access stable operation at safety factor q(95) = 1.9 in divertor plasmas for 150 instability growth times. Magnetohydrodynamic stability sets a hard, disruptive limit on the minimum edge safety factor achievable in a tokamak, or on the maximum plasma current at a given toroidal magnetic field. In tokamaks with a divertor, the limit occurs at q(95) = 2, as confirmed in DIII-D. Since the energy confinement time scales linearly with current, this also bounds the performance of a fusion reactor. DIII-D has overcome this limit, opening a whole new high-current regime not accessible before. This result brings significant possible benefits in terms of fusion performance, but it also extends resistive-wall mode physics and its control to conditions never explored before. In present experiments, the q(95) < 2 operation is eventually halted by voltage limits reached in the feedback power supplies, not by intrinsic physics issues. Improvements to power supplies and to control algorithms have the potential to further extend this regime.
authors.affiliation: Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy.
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authors.lastname: Piovesan
authors.affiliation: Columbia University, New York, New York 10027, USA.
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authors.initials: JM
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authors.affiliation: Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy.
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authors.lastname: Martin
authors.affiliation: Columbia University, New York, New York 10027, USA.
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authors.affiliation: Columbia University, New York, New York 10027, USA.
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authors.affiliation: Columbia University, New York, New York 10027, USA.
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authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
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authors.initials: NM
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authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
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authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
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authors.initials: MJ
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authors.affiliation: Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543-0451, USA.
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authors.affiliation: Oak Ridge Institute for Science and Education, Oak Ridge, Tennessee 37831, USA.
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authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
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authors.initials: EJ
authors.lastname: Strait
authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
authors.forename: A D
authors.initials: AD
authors.lastname: Turnbull
authors.affiliation: Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy.
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authors.lastname: Zanca
authors.affiliation: Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy.
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authors.lastname: Baruzzo
authors.affiliation: Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy.
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authors.lastname: Bolzonella
authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
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authors.initials: AW
authors.lastname: Hyatt
authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
authors.forename: G L
authors.initials: GL
authors.lastname: Jackson
authors.affiliation: Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy.
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authors.initials: L
authors.lastname: Marrelli
authors.affiliation: Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Piron
authors.affiliation: Columbia University, New York, New York 10027, USA.
authors.forename: D
authors.initials: D
authors.lastname: Shiraki