Session YI02: Magnetic Confinement Fusion/Postdeadline

Achieving reduced turbulent transport in stellarators through enhanced nonlinear energy transfer

Advances in stellarator optimization have significantly enhanced the prospects of stellarators as fusion reactors.  Configurations with excellent neoclassical transport properties have been demonstrated experimentally, however recent results from the W7-X stellarator show significant ion thermal transport that effectively clamps peak ion temperatures as predicted by strong ion-temperature-gradient turbulence.  These results motivate efforts to find turbulence optimized stellarator configurations.  A scheme for using three-dimensional shaping to reduce ion-temperature-gradient-driven turbulent transport is formulated that emphasizes turbulent saturation physics.  Strong nonlinear energy transfer from unstable to damped eigenmodes results in lower nonlinear fluctuation levels and correspondingly reduced turbulent transport.   A fluid model for predicting nonlinear energy transfer is implemented in a new computational optimization framework.  Nonlinear energy transfer between modes is mediated by geometry-dependent resonant three-wave interaction lifetimes and coupling coefficients and can be effectively predicted from fast linear eigenvalue calculations, providing a natural optimization metric.  Optimization calculations on candidate quasihelically symmetric stellarator configurations show a strong correlation between increases in resonant three-wave coupling lifetimes between unstable and stable modes and reductions in transport as predicted by nonlinear gyrokinetic simulations with the GENE code.  Analysis of the dominant interactions in the fluid and gyrokinetic models indicate a key feature for improving nonlinear energy transfer is localization of the coupled eigenmodes to the same location along magnetic field lines.  New candidate configurations for turbulence-optimized stellarators obtained through these calculations will be presented.

    

Understanding and designing quasisymmetric stellarators: a topological approach

Designing viable stellarator configurations requires a search in a large space of parameters. Although optimization in such a space has proven useful and provided many designs, its complexity makes this approach seem like a `black box' that provides designs piecemeal. We present an alternative topology-mediated perspective on the space of configurations, focusing on quasisymmetric stellarators. The main change in perspective comes from considering configurations beginning from the axis and moving outward (the "inside-out" model) rather than the traditional approach that begins from the outermost flux surface and moves inward (the "outside-in" model). We characterize configurations in a highly reduced model, starting with the topological structure of a closed curve (the magnetic axis), along with a few additional shaping parameters. Doing so confers configuration space a topological structure (quasisymmetric phases and phase transitions) that organises it in a powerful way, in which the desirability of configurations as quasisymmetric designs can be simply assessed. This description can be used to understand the nature of typical quasisymmetric designs as well as explore the space of possibilities more exhaustively, leading to new design possibilities.   

Efficient Stellarator Optimization and Analysis with DESC

We present an overview of the new stellarator design and analysis capabilities made possible by the DESC code suite [1-4]. This software package couples equilibrium and optimization codes together to efficiently solve the numerical optimization problems required for next-generation stellarator designs. Unlike finite differences or adjoint methods, automatic differentiation provides access to exact derivatives of any objective function and allows the inclusion of more physics constraints, such as metrics for particle confinement and stability. Boundary conditions specifying a Poincaré section instead of the last closed flux surface help to understand the existence of solutions with nested flux surfaces and reveal the evolution of tokamaks into stellarators. This approach to stellarator optimization is valid for finite-β solutions throughout the full plasma volume and has been extended to free-boundary equilibria. DESC is the first stellarator optimization code to use only a single equilibrium solution at each iteration, which reduces the computation time by three orders of magnitude in tests compared to STELLOPT and enables exploration over a higher-dimensional parameter space. Examples of these computational advances are demonstrated along with a discussion of the novel physics insights they provide.   

Space Object Identification by Measurements of Orbit-Driven Waves (SOIMOW)

The SOIMOW project measures in situ electromagnetic waves excited by satellites and space debris moving through the earth’s plasma in low earth orbit by exploiting satellite conjunctions.  Detection of spacecraft and debris is part of national effort to identify satellites below the normal detection thresholds of existing space-track sensors.  This is traditionally accomplished with satellite sensors employing optics and ranging sensors.  SOIMOW uses in situ plasma receivers to detect space objects during orbital conjunctions that excite electromagnetic and electrostatic plasma waves.  SOIMOW measures low frequency plasma modes with electric and magnetic field receivers on host satellites to characterize passing space objects.  Satellites moving through the near-earth ionosphere between 200 and 1000 km altitude become electrically charged by both electron collection and photo emission in sunlight.  These hypersonic charged objects pass through the ionosphere setting up electric currents and electric potentials that can produce a wide range of plasma waves.  The SOIMOW technique may detect electromagnetic plasma waves launched by satellite motion out to ranges of tens of kilometers.  Measurements of the time signature of the waves can provide information on space objects’ presence, shape, orbit, and propulsion system.  SOIMOW is not hampered by terrestrial cloud cover, insensitivity due to large stand-off distances, and the need to radiate signals for target illumination. The SOIMOW concept has been demonstrated using the Radio Receiver Instrument (RRI) on the SWARM-E satellite with detection of micro-satellites and space debris in low earth orbit (LEO).  The goal of future research is to extend current detection capabilities to objects less than 10 cm in size using state-of-the art electric-field receiver technology.  Both machine learning and plasma wave analysis will be applied to the SOIMOW observations to determine the impact of object physical characteristics and background plasma environment on system performance. 

    

High temperature plasmas in ST40

ST40 is a high field Spherical Tokamak, built and operated by Tokamak Energy Limited, with R_geo≈0.4-0.5m, A≈1.6-1.9, κ<1.7, I_P≈0.4-0.8MA and B_T≈0.8-2.2T. Two co-injected hydrogen neutral beams deliver 0.8MW at 50kV and 0.7MW at 24kV. With deuterium the corresponding figures are 1.0MW/55kV and 0.8MW/24kV, respectively. The vessel walls were conditioned with helium GDC and boronization. Merging-compression start-up was used. The plasmas were mainly limited on the high field side, although some diverted plasmas were obtained. The primary goal for the 2021-2 operations was the attainment of the business objective, T_i(0) > 100MK (8.6keV). A central deuterium ion temperature of 9.6keV was obtained together with ??_i(0).??_??(0).??_E ≈ 6 ± 2 × 10¹⁸ m⁻³.keV.s. More detailed analysis of the high T_i data will be presented at this meeting by M. Sertoli and M. Romanelli. Such high ion temperatures have only previously been achieved in significantly larger devices and never reached in a ST. However, in order to access the high T_i regime, the electron temperature must be high enough that the collisional power transfer from ions to electrons is significantly smaller than the input power density. In the best ST40 shots, T_e(0) > 3keV was observed, which is in contrast to the T_e(0) <2.4keV in neutral beam heated STs of twice the major radius but with B_T <0.8T. The results from ST40 are consistent with the NSTX observation of an approximately linear B_T dependence of energy confinement time. A limited TF scan and comparisons between high T_i data at 1.6 and 1.92T, on axis, demonstrate the beneficial effect of high B_T. The dominant loss channel is via electrons and so the observation of high temperatures lends support to the high field ST route to economical, compact fusion reactors.

    

Long Pulse High Performance H-mode Plasmas Achieved on EAST

A world-record duration of 310s H-mode plasma (H_98y2>1.3, n_e/n_GW>0.6, f_BS>50%) has been recently achieved on EAST, exploiting the device's improved long pulse capabilities. The experiment demonstrates good control of impurities, core/edge MHD stability, and heat exhaust with an ITER-like tungsten divertor and zero injected torque, establishing a milestone on the path to steady-state long-pulse high performance scenarios in ITER and CFETR. Important synergistic effects are leveraged towards this result, which relies purely on RF power for heating and current drive. On-axis Electron Cyclotron Heating enhances the heating and current drive from Lower Hybrid Wave injection, increasing confinement quality and enabling fully non-inductive operation at high density (n_e/n_GW ~60%) and high poloidal beta (β_P ~2.5). A small ELM regime facilitates the RF power coupling to the H-mode edge and reduces divertor sputtering/erosion. The very high energy confinement quality (H_98y2>1.3) is achieved with a slightly reversed magnetic shear and an internal transport barrier (ITB) in the T_e profile, at rho~0.3. Transport analysis suggests that TEMs dominate in the core region, and reveals that these EAST plasmas are limited by ITG turbulence in the outer region (rho>0.7). The detailed physics processes (RF synergy, core-edge integration, confinement properties, etc.) of the steady-state operation will be illustrated. The latest experiments will also be discussed, where a broader current profile with q_min>2 and li~0.7 is obtained by early heating using off-axis ECH, and is sustained with combined LHW, ICRF and NBI powers. Based on this high q_min profile, future experiments will pursue the formation and sustainment of an ITB at larger radius, in order to further extend the fusion performance of long pulse plasmas on EAST.