Abstract
Experiments on the DIII-D tokamak have identified how multiple simultaneous Alfvén eigenmodes (AEs) lead to overlapping wave-particle resonances and stochastic fast-ion transport in fusion grade plasmas [C. S. Collins et al., Phys. Rev. Lett. 116, 095001 (2016)]. The behavior results in a sudden increase in fast-ion transport at a threshold that is well above the linear stability threshold for Alfvén instability. A novel beam modulation technique [W. W. Heidbrink et al., Nucl. Fusion 56, 112011 (2016)], in conjunction with an array of fast-ion diagnostics, probes the transport by measuring the fast-ion flux in different phase-space volumes. Well above the threshold, simulations that utilize the measured mode amplitudes and structures predict a hollow fast-ion profile that resembles the profile measured by fast-ion Dα spectroscopy; the modelling also successfully reproduces the temporal response of neutral-particle signals to beam modulation. The use of different modulated sources probes the details of phase-space transport by populating different regions in phase space and by altering the amplitude of the AEs. Both effects modulate the phase-space flows.
Highlights
Adequate confinement of fast ions is essential for a practical magnetic fusion reactor
The analysis showed that, since the mode amplitudes are small, the island widths associated with individual resonances are quite modest, DW=Wn $ 2%.11 (Here, W is the poloidal flux, and Wn is the difference between the flux at the magnetic axis and the last closed flux surface (LCFS).) because there are hundreds of resonances, and the modes have different toroidal mode numbers n, the small islands overlap and can cause orbit stochasticity
A comparison of a reduced critical gradient model with DIII-D data by Waltz et al found agreement with the fastion pressure but not the fast-ion D-alpha (FIDA) data, causing the authors to conclude that a proper treatment of different regions of phase space is essential to reproduce the data.[21]
Summary
Adequate confinement of fast ions is essential for a practical magnetic fusion reactor. The threshold occurs when orbits become stochastic in the portion of phase space interrogated by the fast-ion diagnostic. Details of the comparison between the measured threshold and stochasticity theory are the focus of a forthcoming paper.[7] Because different modes interact with fast ions in different parts of phase space, the measured threshold depends sensitively on both the diagnostic and on the fast-ion population that drives the modes.[7]. A comparison of a reduced critical gradient model with DIII-D data by Waltz et al found agreement with the fastion pressure but not the fast-ion D-alpha (FIDA) data, causing the authors to conclude that a proper treatment of different regions of phase space is essential to reproduce the data.[21]. The use of a modulated source provides new information about the fast-ion transport by AEs above the stochastic threshold.
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call