Advancing the physics basis for prediction and control of spherical tokamaks via experimental investigation of energetic ion driven instabilities and validation of simulations

This proposal is for renewal of an existing grant. The goals are to experimentally investigate the effects ofenergetic ion-driven instabilities on the energetic ions themselves and on the bulk plasma in the Mega AmpSpherical 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 theobjectives of the MAST-U program, which seeks to break new ground in neutral beam heating and currentdrive 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 currentdrive. 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) andcompressional (CAE) Alfvén eigenmodes. The effects of these modes are potentially wide ranging. Forinstance, CAEs and GAEs correlate with anomalous core energy transport and electron temperaturebroadening [D Stutman, PRL 2009, NA Crocker, PPCF 2011, NN Gorelenkov, NF 2010, E Belova, PRL2015]. This profile broadening has potentially significant consequences for advanced tokamak scenariodevelopment and may be the cause of the favorable scaling of confinement of with collisionallity in STs [SKaye, NF 2013]. Fishbones and TAEs, on the other hand, have been shown to cause significant energeticion 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, ITChapman, NF 2001]. A physics-based understanding of transport by these modes is vital for predicting theireffects 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 burningplasmas, 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 vianon-invasive diagnostics that are robust in a burning plasma environment [KG McClements, NF 2015]. Theproposed research also has an educational objective. It will be performed by UCLA grad. students, whoseparticipation will fulfill the research requirements for a PhD Thesis, and a UCLA postdoc or young scientistworking together with the PI and Co-PIs. The objectives of the proposed research will be achieved througha combination of experimental techniques, including fluctuation, energetic ion population and plasmaprofile measurements, carefully designed transport experiments, and transport modeling with codes likeTRANSP. 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, oras 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 withsimulations in collaboration with MAST-U diagnostic experts. A powerful suite of diagnostics is availablefor these measurements, including soft x-ray detector arrays with collimated views, several multichannelDoppler Backscattering systems, which can be used for density perturbation measurements, and beamemission spectroscopy, which provides a 2D array of density perturbation measurements, including poloidalwavenumber measurements. An interferometer will be available, external magnetic sensors to measuretoroidal mode number and polarization, and a magnetic sensor for ion cyclotron emission. Energetic iontransport will be determined with a powerful suite of energetic ion population diagnostics including a totalneutron detector, a collimated array of neutron detectors, a fast-ion Dα spectrometer (FIDA), fast-ion lossand fusion proton detectors and a solid-state neutral particle analyzer.In addition to undertaking these experimental investigations, UCLA will also contribute to MAST-Uenergetic ion physics research in another way. Dr. Michael will continue to serve as the Responsible Officerfor the MAST-U FIDA diagnostic, and he will expand the FIDA diagnostic coverage of energetic ion phasespace by implementing a vertical view to complement the existing toroidal view.