Core Transport Modeling and Characterization for Compact Tokamak Reactor Scenarios

Motivated by the current interest in compact, high-field approaches to fusion power plants, the OMFIT STEP integrated modeling workflow has been used to generate self-consistent core plasma transport solutions representative of potential compact tokamak reactor operating scenarios. In this study, solutions for an idealized Rmaj = 4 m, B0 = 8 T tokamak “use case reactor” (UCR) were developed, with the intention of providing starting parameters for more comprehensive future transport studies in the spirit of the CYCLONE base case. Both inductive pulsed (UCR-P) and steady-state (UCR-SS) solutions potentially capable of producing 1 GW of fusion power and 200 MW or more net electric power have been identified. A common feature of both scenarios is that the core confinement time is long enough for the plasmas to be well- coupled, even though core collisionality is low. This situation leads to significant core ion thermal transport, despite the heating being predominantly to the electrons, and a corresponding dominance of long-wavelength ion temperature gradient modes. A similar situation is found to hold for ITER and SPARC plasma scenarios, and is argued to be an inherent property of power plant-relevant burning plasmas. For both UCR scenarios, the EPED code predicts peeling-limited pedestals with extremely weak sensitivity to core pressure values, enabling use of a fixed boundary condition in core transport modeling. With this constraint, another key finding of this study is the extreme sensitivity of the results to the quantitative stiffness level of the transport model used as well as the predicted critical gradients, with outcomes ranging from runaway ignition to radiative collapse possible depending upon the choice of TGLF saturation rule. Given this uncertainty, new analysis presented in the paper details initial benchmarking of TGLF against linear and nonlinear gyrokinetic simulations. The gyrokinetic results are broadly consistent with the relevant TGLF predictions, but highlight the need to improve the accuracy of transport stiffness and particle flux predictions, especially at larger radii.