FY2023

R(23-1):  Transport characteristics of high performance, high temperature ST40 discharges

Description: ST40 is a small, high toroidal field spherical tokamak with 2 MW of neutral beam heating owned and operated by Tokamak Energy, UK. Collaborative work through a PPPL/ORNL CRADA with Tokamak Energy has helped ST40 researchers achieve their main business milestone of producing plasmas with 100M degree ion temperatures at toroidal fields of over 2 T, more than a factor of two greater than toroidal fields in present STs. The ST40 physics program planned for FY23 will focus on characterizing these high performance plasmas with full kinetic profile measurements, and also develop divertor operation for accessing robust H-modes. The program is also planning to perform parametric scans in toroidal field and additional scans in plasma current and density to characterize the confinement and transport underlying performance and achievable temperature across these ranges. The TRANSP code will be used to determine the transport characteristics of these plasmas, and in particular whether the strong toroidal field dependence of confinement seen in a number of STs holds at these high toroidal fields. TRANSP will be used also to determine the dependence of the transport and confinement on collisionality to determine whether this dependence remains strong as well. The TRANSP calculations will be used as a basis for subsequent gyrokinetic studies, to understand the nature of the underlying transport, and to produce energetic particle distribution functions for subsequently assessing energetic particle confinement and energetic particle-driven modes.

R(23-2):  MHD equilibrium and stability studies on MAST-U

Description: The MAST-U spherical tokamak is a new device, having just completed its first physics campaign in 2021. As such, the operational space of the device is still being expanded in various plasma parameters, and stability boundaries are being approached. Methods for modeling and control of key parameters, such as q0 and βN, based on experimental measurements, will be explored. Characterization of disruption indicators in MAST-U will be assessed and the operational space and stability boundaries will be monitored. This collaborative research also supports the NSTX-U five-year plan objective to develop low-disruptivity operation of steady-state compact fusion devices. These activities require high quality equilibrium reconstructions, which will be generated for this purpose. Active monitoring of the approach to the n = 1 no-wall ideal stability limit, can be assessed by experimentally determining the plasma response to applied magnetic perturbations. This resonant field amplification technique will be established for MAST-U. The multimodal nature of the plasma response was recently measured for the first time in the DIII-D, allowing for ELM suppression and heat flux spreading, by varying the applied poloidal spectrum. Modeling of plasma response to applied external fields in MAST and NSTX-U, done with the resistive MHD code MARS, predicted the presence of a detectable multimodal plasma response also in spherical tokamaks. Data from MAST-U will be used to explore the possibility to measure multimodal plasma response for the first time in a spherical tokamak, and, if measurable, to study its dependence on equilibrium quantities like βN.