Recent Progress in Stellarator Optimization D. A. Gates1, A. H. Boozer2, T. Brown1, J. Breslau1, D. Curreli3, M. Landreman4, S. A. Lazerson1, J. Lore5, H. Mynick1, G.H. Neilson1, N. Pomphrey1, P. Xanthopoulos6, A. Zolfaghari1 1Princeton Plasma Physics Laboratory, Princeton, NJ 08543, U.S.A. 2Columbia University, New York, NY 10027 3U. Illinois Champaign-Urbana, Champaign, IL 61820 4University of Maryland, College Park, 20742 5Oak Ridge National Laboratory, Oak Ridge, TN 6Max Planck Insitut-fur-Plasmaphysik, Greifswald, Germany EX/P3-39 EX/P3-39 Coil Simplification A four part program for improved stellarators A design activity based on the four new optimization capabilities would lead to an improved stellarator reactor concept Coil optimization with spatial constraints Fast Particle Confinement Optimization Limited access leads to low availability Basic code changes include: Modular coil winding topology used on NCSX and ARIES-CS designs leads to small component, port based maintenance approach Details develop that allow straightened MC back legs and the code revised to receive input of engineering supplied MC surface geometry to locate the MC winding centers, The MC winding geometry now is developed with spline representations enabling spatial constraints to be placed on the coil locations, Coding effort was also made to smooth the shaping of the modular coil winding, add torsion constraints, freeze coil geometries and much more. Fast particle confinement in modern optimized designs such as W7-X and NCSX is much improved compared to conventional stellarators, but remains one of the main challenges for the concept G. Grieger, et al., Phys. Fluids B 4, 2081 (1992) STELLOPT has recently been coupled to the gyro-center following parts of the BEAMS3D code allowing massively parallel computations. M. McMillan and S. A. Lazerson, Plasma Phys. Control. Fusion 56, 095019 (2014) NCSX 4.4 AR 1.4-M Raxis ARIES-CS 4.5 AR 7.75-M Raxis ARIES-CS port based maintenance approach A European tokamak study looking at an equivalent ITER configured DEMO design found that an operating availability barely above 50% could be achieved. COILOPT++ was run with constraints applied using aspect ratio 6 case from Ku and Boozer The results of this work is a newly developed stellarator configuration with much improved maintenance features Primary minimization cost function was combination of rms and max dB/B over the boundary COILOPT++ with splines was substantially easier to run with the splines than COILOPT with Fourier representation The use of saddle coils was deemed unnecessary Magnetic field error after one pass was rms2.27e-2 and Max = 5.28e-2 Spline coil centers from COILOPT++ with applied constraints Achieved equilibrium Target equilibrium A fast particle loss prediction verification exercise will be undertaken on W7-X using the nearly radially injected neutral beams Fast lost ion detectors will be installed to verify predicted losses Future optimization strategies Plasma boundary well matched after single iteration Turbulent Transport Optimization Divertor Heat Flux Optimization Mynick et al., recently demonstrated the reduction of turbulent heat flux by employing a novel “proxy function” within the STELLOPT code. H. Mynick, et al., Phys. Rev. Letters 105, 095004 (2010) The proxy function is developed to rapidly estimate the transport level of a given configuration Nonlinear GENE run to corroborate that the evolved configuration in fact has reduced transport. Most work to date has focused on optimizing for ion temperature gradient (ITG) turbulence. Without scraper Overloaded elements (>10MW/m2, rated for 5) With scraper MW/m2 Load on low-rated tiles reduced below 1MW/m2 A method has been developed to rapidly determine the heat flux footprint on a stellarator facing surface using field line tracing with diffusion J. Lore, et al., IEEE Transactions on Plasma Science 42 539 (2014) This method can be included in STELLOPT with the parallelized FIELDLINES code The result can then be compared to a more accurate model, such as EMC3- EIRENE Automated divertor plate design can make machine design much more rapid Can also verify robustness of the design to equilibrium variations Poloidal cross-sections and (b)surface-averaged heat flux Q versus time from GENE simulations for NCSX and 2 turbulence-optimized configurations QA_40n (red) and QA_35q (green), showing the reduction in Q from NCSX by factors of 2 to 2.5. Xanthopoulos has since generated a W7-X-like configuration with turbulent flux minimized across the entire cross-section. P. Xanthopoulos, et al., Phys. Rev. Letters 113, 155001 (2014). The MPX configuration Comparison between field line tracing and EMC3-EIRENE Parallelized field line tracing using the FIELDLINES code Peak flux: ~16MW/m2 Peak flux: ~12MW/m2 DIV3D EMC3 Comparison of the normalized heat flux between MPX and W7-X