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Oct 13 – 18, 2014
Hotel Park Inn Pribaltiyskaya
Europe/Moscow timezone

Alfven Eigenmodes Can Limit Access to High Fusion Gain, Steady-State Tokamak Operation

Oct 17, 2014, 4:40 PM
Blue 1-5 (Hotel Park Inn Pribaltiyskaya)

Blue 1-5

Hotel Park Inn Pribaltiyskaya

Saint Petersburg, Russian Federation


Mr William W. Heidbrink (USA)


Experiments on the DIII-D tokamak show that Alfven eigenmode (AE) activity degrades fast-ion confinement in many high β_N, high q_{min}, steady-state scenario discharges. (β_N is the normalized plasma pressure and q_{min} is the minimum value of the safety factor.) An extensive set of diagnostics measure degraded fast-ion confinement: neutron detectors, fast-ion D_α (FIDA) spectrometers, neutral-particle analyzers, and fast-ion pressure and current profiles inferred from the equilibrium. All fast-ion diagnostics that are sensitive to the co-passing population exhibit reductions relative to classical predictions. The increased fast-ion transport in discharges with strong AE activity accounts quantitatively for the previously observed [1] reduction in global confinement with increasing q_{min}; however, not all high q_{min} discharges show appreciable degradation. In current ramp plasmas, stochastic transport by multiple resonances with many small-amplitude AEs causes “stiff” fast-ion transport; as a result, the achieved fast-ion profile is insensitive to the beam-deposition profile [2]. We postulate that a similar process often occurs in steady-state scenario plasmas. Initial linear stability calculations predict unstable toroidal AEs for these conditions; comparisons with critical-gradient models are underway. If AE degradation of fast-ion confinement can be avoided, modeling indicates that a discharge scenario with q_{min}>2 can provide the MHD stability and bootstrap fraction required for high fusion gain, steady-state operation. The broad current and pressure profiles consistent with elevated qmin enable stable operation at reactor-relevant β_N {\sim -} 5. One-dimensional modeling shows that these conditions are attainable in DIII-D using practical neutral-beam and electron-cyclotron current drive sources and that a self-consistent fully noninductive scenario exists. The challenge in future work is to incorporate calculations of AE-induced transport into the analysis. This work was supported by the US Department of Energy under SC-G903402 and DE-FC02-04ER54698. [1] J.R. Ferron, et al., Phys. Plasmas 20, 092504 (2013) [2] W.W. Heidbrink, et al., Nucl. Fusion 53, 093006 (2013)
Paper Number EX/10-1
Country or International Organisation USA

Primary author

Mr William W. Heidbrink (USA)


Dr Christopher T. Holcomb (Lawrence Livermore National Laboratory) Dr John Ferron (General Atomics) Dr Mario Podesta (Princeton Plasma Physics Laboratory) Dr Michael Van Zeeland (General Atomics)

Presentation materials