The DESC Stellarator Code Suite Part III: Quasi-symmetry optimization

The DESC stellarator optimization code takes advantage of advanced numerical methods to search the full parameter space much faster than conventional tools. Only a single equilibrium solution is needed at each optimization step thanks to automatic differentiation, which efficiently provides exact derivative information. A Gauss-Newton trust-region optimization method uses second-order derivative information to take large steps in parameter space and converges rapidly. With just-in-time compilation and GPU portability, high-dimensional stellarator optimization runs take orders of magnitude less computation time with DESC compared to other approaches. This paper presents the theory of the DESC fixed-boundary local optimization algorithm along with demonstrations of how to easily implement it in the code. Example quasi-symmetry optimizations are shown and compared to results from conventional tools. Three different forms of quasi-symmetry objectives are available in DESC, and their relative advantages are discussed in detail. In the examples presented, the triple product formulation yields the best optimization results in terms of minimized computation time and particle transport. This paper concludes with an explanation of how the modular code suite can be extended to accommodate other types of optimization problems.Comment: 18 pages, 5 figures, 2 tables, 2 listing

The DESC Stellarator Code Suite Part I: Quick and accurate equilibria computations

3D equilibrium codes are vital for stellarator design and operation, and high-accuracy equilibria are also necessary for stability studies. This paper details comparisons of two 3D equilibrium codes, VMEC, which uses a steepest-descent algorithm to reach a minimum-energy plasma state, and DESC, which minimizes the MHD force error in real space directly. Accuracy as measured by final plasma energy and satisfaction of MHD force balance, as well as other metrics, will be presented for each code, along with the computation time. It is shown that DESC is able to achieve more accurate solutions, especially near-axis. DESC's global Fourier-Zernike basis also yields the solution everywhere in the plasma volume, not just on discrete flux surfaces. Further, DESC can compute the same accuracy solution as VMEC in an order of magnitude less time

Optimization of Nonlinear Turbulence in Stellarators

We present new stellarator equilibria that have been optimized for reduced turbulent transport using nonlinear gyrokinetic simulations within the optimization loop. The optimization routine involves coupling the pseudo-spectral GPU-native gyrokinetic code GX with the stellarator equilibrium and optimization code DESC. Since using GX allows for fast nonlinear simulations, we directly optimize for reduced nonlinear heat fluxes. To handle the noisy heat flux traces returned by these simulations, we employ the simultaneous perturbation stochastic approximation (SPSA) method that only uses two objective function evaluations for a simple estimate of the gradient. We show several examples that optimize for both reduced heat fluxes and good quasisymmetry as a proxy for low neoclassical transport. Finally, we run full transport simulations using T3D to evaluate the changes in the macroscopic profiles

A robust solution for the resistive MHD toroidal Δ′ matrix in near real-time

We introduce a new near real-time solution for the tokamak resistive MHD Δ′ matrix. By extending state transition matrix methods introduced in [Glasser et al., Phys. Plasmas 25(3), 032507 (2017)] and leveraging the asymptotic methods of [A. H. Glasser, Phys. Plasmas 23, 072505 (2016)], we have developed STRIDE—State Transition Rapid Integration with DCON (Asymptotic) Expansions—a code that solves for Δ′ in <500 ms. The resistive MHD stability remains a foremost challenge in successful tokamak operation, and its numerically demanding analysis has received attention for many years. Our code substantially improves upon the speed and robustness of earlier Δ′ calculation methods, affording solutions for previously intractable equilibria and helping enable the real-time control of ideal and resistive MHD tokamak stability. In this paper, we pedagogically review tearing stability analysis and motivate and define Δ′ in the slab, cylindrical, and toroidal geometries. We also benchmark STRIDE against the calculations of [Nishimura et al., Phys. Plasmas 5, 4292–4299 (1998)] and Furth et al. [Phys. Fluids 16, 1054 (1973)] for Δ′ in a cylindrical geometry, and the Δ′ matrix calculations of [A. H. Glasser, Phys. Plasmas 23, 112506 (2016)] in the full toroidal geometry

Optimization of the snowflake divertor for power and particle exhaust on NSTX–U

In this paper, simple analytical modeling and numerical simulations performed with the multi-fluid edge transport code UEDGE are used to identify optimal snowflake divertor (SFD) configurations for heat flux mitigation and sufficient cryopumping performance on the National Spherical Torus eXperiment Upgrade (NSTX–U). A model is presented that describes the partitioning of sheath-limited SOL power and particle exhaust in the SFD as a result of diffusive transport to multiple activated strike points. The model is validated against UEDGE predictions and used to analyze a database of 70 SFD-minus equilibria. The optimal location for the entrance to a divertor cryopumping system on NSTX–U is computed for enabling sufficient pumping performance with acceptable power loading in a variety of SFD-minus configurations. UEDGE simulations of one promising equilibrium from the database indicate that a significant redistribution of power to the divertor legs occurs as a result of neutral particle removal near one of the SFD-minus strike points in the outboard scrape-off layer. It is concluded that pump placement at the optimal location is advantageous as the large number of compatible equilibria reduces the precision required of real-time SFD configuration control systems and enables acceptable divertor solutions even if UEDGE-predicted power redistribution slightly reduces the achievable pumping performance

Hamilton-Jacobi Modelling of Stellar Dynamics

Abstract One of the physical settings emerging in the galaxy and stellar dynamics is motion of a single star and a stellar cluster about a galaxy center. The potential availability of analytical treatment of this problem stems from the smallness of mass of the star and cluster relative to the galactic mass, giving rise to Hill&apos;s restricted three-body problem in the galaxy-cluster-star context. Based on this observation, this paper presents a Hamiltonian approach to modelling stellar motion by the derivation of canonical coordinates for the dynamics of a star relative to a star cluster. First, the Hamiltonian is partitioned into a linear term and a high-order term. The HamiltonJacobi equations are solved for the linear part by separation, and new constants for the relative motions are obtained, called epicyclic orbital elements. The effect of an arbitrary cluster potential is incorporated into the analysis by a variation of parameters procedure. A numerical optimization technique is developed based on the new orbital elements, and quasiperiodic stellar orbits are found