Advancing the physics basis for prediction and control of spherical tokamaks via experimental
investigation of energetic ion driven instabilities and validation of simulations
Dr. Neal A. Crocker II, Dept. of Physics and Astronomy, UCLA, Los Angeles
Dr. Clive A. Michael, Dept. of Physics and Astronomy, UCLA, Los Angeles
Prof. Troy A. Carter, Dept. of Physics and Astronomy, UCLA, Los Angeles
This proposal is for renewal of an existing grant. The goals are to experimentally investigate the effects of
energetic ion-driven instabilities on the energetic ions themselves and on the bulk plasma in the Mega Amp
Spherical Tokamak Upgrade (MAST-U), and to validate the physics models of leading codes (e.g. HAGIS,
HALO, HYM, GTC) and analytic theory for the processes involved. The proposed research addresses the
objectives of the MAST-U program, which seeks to break new ground in neutral beam heating and current
drive in the spherical tokamak (ST) configuration. Energetic ions from neutral beams excite instabilities,
or modes, across a broad range of frequencies, and the resulting transport can affect both heating and current
drive. The investigation will extend from low frequency modes, including fishbones, reverse shear (RSAE)
and toroidicity-induced Alfvén eigenmodes (TAE), to high frequency modes, including global (GAE) and
compressional (CAE) Alfvén eigenmodes. The effects of these modes are potentially wide ranging. For
instance, CAEs and GAEs correlate with anomalous core energy transport and electron temperature
broadening [D Stutman, PRL 2009, NA Crocker, PPCF 2011, NN Gorelenkov, NF 2010, E Belova, PRL
2015]. This profile broadening has potentially significant consequences for advanced tokamak scenario
development and may be the cause of the favorable scaling of confinement of with collisionallity in STs [S
Kaye, NF 2013]. Fishbones and TAEs, on the other hand, have been shown to cause significant energetic
ion transport, particularly in reversed shear, qmin > 1 discharges [ED Fredrickson, NF 2013, M Cecconello,
PPCF 2014], which are a potential candidate for advance tokamak scenarios [M Turnyanskiy, NF 2009, IT
Chapman, NF 2001]. A physics-based understanding of transport by these modes is vital for predicting their
effects on heating and current drive, as well as developing strategies to mitigate or exploit those effects.
The proposed research also contributes to the physics basis for diagnosis of energetic ions in burning
plasmas, since energetic ion modes, including GAEs, CAEs and coherent ion cyclotron emission,
potentially provide useful information about the energetic ion distribution and are frequently detectable via
non-invasive diagnostics that are robust in a burning plasma environment [KG McClements, NF 2015]. The
proposed research also has an educational objective. It will be performed by UCLA grad. students, whose
participation will fulfill the research requirements for a PhD Thesis, and a UCLA postdoc or young scientist
working together with the PI and Co-PIs. The objectives of the proposed research will be achieved through
a combination of experimental techniques, including fluctuation, energetic ion population and plasma
profile measurements, carefully designed transport experiments, and transport modeling with codes like
TRANSP. Mode amplitude and structure will be obtained via internal and external fluctuation diagnostics.
These measurements are also of use for validating codes that predict mode structure, like HYM or GTC, or
as input for codes like HALO that simulate mode impact on energetic ion orbits and the resulting transport.
Synthetic diagnostics will be developed for interpretation of the measurements and comparison with
simulations in collaboration with MAST-U diagnostic experts. A powerful suite of diagnostics is available
for these measurements, including soft x-ray detector arrays with collimated views, several multichannel
Doppler Backscattering systems, which can be used for density perturbation measurements, and beam
emission spectroscopy, which provides a 2D array of density perturbation measurements, including poloidal
wavenumber measurements. An interferometer will be available, external magnetic sensors to measure
toroidal mode number and polarization, and a magnetic sensor for ion cyclotron emission. Energetic ion
transport will be determined with a powerful suite of energetic ion population diagnostics including a total
neutron detector, a collimated array of neutron detectors, a fast-ion Dα spectrometer (FIDA), fast-ion loss
and fusion proton detectors and a solid-state neutral particle analyzer.
In addition to undertaking these experimental investigations, UCLA will also contribute to MAST-U
energetic ion physics research in another way. Dr. Michael will continue to serve as the Responsible Officer
for the MAST-U FIDA diagnostic, and he will expand the FIDA diagnostic coverage of energetic ion phase
space by implementing a vertical view to complement the existing toroidal view.