3 research outputs found
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
Magnetic Fields with General Omnigenity
Omnigenity is a desirable property of toroidal magnetic fields that enables
the confinement of trapped particles. Equilibrium solutions that approximate
omnigenity have previously been discovered for the special cases of
quasi-symmetry and quasi-isodynamicity, but general omnigenity is a much larger
design space than these subsets. A new model is presented and employed in the
DESC stellarator optimization suite to represent the full omnigenity parameter
space. Examples far from quasi-symmetry with poloidally, helically, and
toroidally closed contours of magnetic field strength are shown to have low
neoclassical collisional transport throughout a volume.Comment: 7 pages, 5 figure