Turbulence and transport science on MAST-U: Magnetic and density turbulence, turbulence flow, GAMs, and zonal flows

UCLA will continue its successful scientific collaboration on the MAST-U spherical tokamak located in Culham, UK. During FY21 UCLA along with colleagues at the Culham Centre for Fusion Energy successfully installed and operated the UCLA Q-Band (33-50 GHz) Doppler backscattering system. Doppler backscattering (DBS) is a technique that measures local low to intermediate wavenumber density fluctuations, fluctuation flow, GAMs, and zonal flows. This technique, developed by worldwide researchers over the last ~20 years, has added significant insight into turbulence and flow behavior in fusion plasma research devices. The UCLA diagnostic installation on MAST-U also includes a cross-polarization scattering system (CPS) for the measurement of internal magnetic fluctuations. Measurements of internal magnetic fluctuations are believed critical to better understand the physics of transport and confinement in the high-beta MAST-U regimes. UCLA's scientific goals include leading turbulence and transport experiments as well as providing data analysis, interpretation, etc. in support of the US fusion energy research and the overall MAST-U Research Program. Already in FY22 UCLA has on the schedule three sets of experiments on MAST-U: initial testing of the cross-polarization system, final testing of the DBS system, and a dimensionless scaling experiment focused on the local turbulence, transport, and overall confinement behavior as collisionality is varied (the goal is a variation in ?* of a factor of 3 or more). For clarity, a listing of the overall goals/objectives of this proposal is shown below. (1) The main focus of UCLA's effort will be the advancement of scientific knowledge and understanding of spherical tokamak science relevant to the US fusion energy goals and to MAST-U's goals. The testing and validation of linear and non-linear gyrokinetic turbulence simulations are key experimental components of this effort. UCLA has the ability to perform linear gyrofluid and gyrokinetic simulations (eg. TGLF , CGYRO, etc.) but will depend on collaborations to perform the more complicated nonlinear turbulence simulations (eg CYRO, GENE, etc). (2) The science of a newly discovered Ohmic H-mode ELMing like regime will be pursued. Density turbulence from the new UCLA DBS diagnostic showed changes in intensity and Doppler shifts similar to NBI H-modes. Ohmic H-mode have been observed for many years on tokamaks however we are not aware of any results showing density control along with ELM-like activity in Ohmic H-mode. (3) UCLA will operate the newly installed DBS CPS scattering in support of MAST-U experiments. These systems will measure low to high-k density and magnetic turbulence and turbulence flows relevant to ITG, TEM, micro-tearing, and KBM type instabilities. In addition to supporting its own scientific pursuits, UCLA will provide experimental design expertise, data analysis and interpretation, contributions to talks and papers, etc. to the MAST-U team and collaborators. (4) UCLA will design, fabricate, install a higher frequency, tunable DBS/CPS system to access higher plasma density and magnetic fields on MAST-U. This will take advantage of the already installed remotely operable quasi-optical system. This is a low dollar cost investment that will capitalize on DOE-FES investment and provide significant returns on the science and understanding of spherical tokamaks. (5) In collaboration with Dr. Valerian Hall Chen (via a consultation agreement) UCLA will work on developing a quantitative ability to predict and interpret Doppler backscattering data. The method uses the 3D code SCOTTY, developed by Dr Hall-Chen, whose inputs are the probe beam parameters, density profiles, and magnetic equilibria. Note that this is not a fitting code. This is the next step to developing a quantitative measure of ñ with DBS. (6) Education is an extremely important part of UCLA's goals that will be addressed through the participation of an on-site post-doctoral scholar and graduate student.