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FY2021 Research Milestones

R(21-1):  Assess H-mode energy confinement and pedestal characteristics with higher field, plasma current, and NBI heating power

Description:  Future ST devices such as ST-FNSF will operate at higher toroidal field, plasma current and heating power than NSTX.  To establish the physics basis for future STs, which are generally expected to operate in lower collisionality regimes, it is important to characterize confinement and pedestal structure over an expanded range of engineering parameters.  H-mode studies in NSTX and MAST have shown that the global energy confinement exhibits a more favorable scaling with collisionality (Bt~ 1/n*e) than that from ITER98y,2.  In addition, the H-mode pedestal pressure increases with ~IP2.  With higher BT, IP, and NBI power with beams at different tangency radii, NSTX-U and MAST-U provide an excellent opportunity to assess the core and boundary characteristics in regimes more relevant to future STs and to explore the accessibility to lower collisionality.  Specifically, the relation between H-mode energy confinement and pedestal structure with increasing IP, BT and PNBI will be determined and compared with previous NSTX and MAST results, including emphasis on the collisionality dependence of confinement and beta dependence of pedestal width. Coupled with low-k turbulence diagnostics and gyrokinetic simulations, the experiments will provide further evidence for the mechanisms underlying the observed confinement scaling and pedestal structure.  During FY2020, significant effort will be put toward profile and turbulence diagnostic commissioning for these experiments on NSTX-U, and if NSTX-U cannot support plasma operations during FY2020, emphasis will be placed on collaboration on MAST-U to support the core transport and pedestal structure research goals of this milestone.

R(21-2):  Commission operational tools that enable high-performance discharges in NSTX-U

Description:  NSTX-U is designed to develop the physics and technical basis required for stationary, long pulse, high non-inductive fraction operation in a low-aspect-ratio tokamak. A major research goal during the FY21commissioning campaign on NSTX-U is to develop high-performance H-mode scenarios that simultaneously exceed the current (Ip > 1.4 MA) and magnetic field strength (BT > 0.55) achieved on NSTX. A critical component of these scenarios is maximizing the achievable elongation at low internal inductance (li). This effort will build on the ramp-up simulation development that identifies ramp-up scenarios that optimize the achievable elongation. Dedicated experiments will quantify the vertical and MHD stability limits in the ramp-up phase in order to compare to the simulation results and identify avenues for potential expansion of these limits through new scenario or control tools. Building upon the results of the FY16 NSTX-U run campaign and the FY17-18 milestones on error field identification and correction, a re-assessment of low-n error fields, mode-locking, and optimal error field correction will be made. Further, RWM control and dynamic error field correction algorithms using both proportional and state-space n ≥ 1 feedback schemes will be implemented taking advantage of the spectrum flexibility provided by the 2nd SPA power supply. This effort will enable access to large bN/li, which is critical for high-current H-mode scenarios. Resonant field amplification measurements, ideal MHD stability codes, and kinetic stability analysis will be used to evaluate the no-wall and disruptive stability limits. These physics and operational tools will be combined to enable new plasma operating scenarios and to make an initial assessment of the non-inductive current drive fraction across a range of toroidal field, plasma density, boundary shaping, and neutral beam parameters. During FY21, significant effort will be put toward commissioning real-time diagnostics and scenario-control-relevant actuators for these experiments on NSTX-U. This may include the integration of ELM-pacing actuators in order to control the accumulation of impurities. If NSTX-U cannot support plasma operations during FY21, additional emphasis will be placed on collaboration on MAST-U to support the high-current access, shape control, error-field correction, and stability analysis research goals of this milestone.

R(21-3): Optimization of NBI mix for AE-mitigated scenarios

Desscription: A broad deposition profile from neutral beam injection, e.g. by NBI aiming tangentially on the outboard midplane, is usually assumed to reduce the drive for Alfvénic instabilities (AEs) by reducing the radial gradient of the fast ion density profile. However, NSTX-U results from the FY16 campaign show evidence that tangential NB injection, including off-axis injection near the plasma mid-radius, can also lead to undesired effects such as the destabilization of AEs, thus potentially leading to enhanced fast ion redistribution. Based on previous work, dedicated experiments will be performed to assess the optimal mix of on- and off-axis NBI that simultaneously maximizes NB current drive with mitigated or suppressed AE activity. Predictive time-dependent analysis with the TRANSP code will inform on the expected fast ion distribution function resulting from different NBI configurations. TRANSP results will then be used as input for AE stability analysis, which in turn provides input for updated TRANSP simulations including enhanced fast ion transport by AEs through the “kick” and RBQ fast ion transport models interfaced with TRANSP. Predictions from the TRANSP/AE stability/transport loop will be tested against available data from NSTX-U, and against data from the MAST-U and DIII-D devices.

R(21-4):  Multi-mode correction of error fields in NSTX-U and tokamaks

Description: A small non-axisymmetric error field (EF) can cause a disruptive field penetration or mode locking instability and therefore must be properly controlled to achieve high performance in tokamaks. Although the empirical scaling based on a single dominant n=1 mode approximation has been partially successful, multi-mode EF, including n > 1 modes, can also degrade plasma confinement depending on the operating regime, as has recently been demonstrated in various tokamaks. EF identification and correction in the 2016 NSTX-U campaign showed that the plasma response to the inner-TF and PF5 n = 1 misalignment can change significantly as the current profile evolves, and therefore these EFs are not easily correctable by standard control algorithms.  This sensitivity of the plasma response is due to the coupling to the m = 1 component, the generically complex phase variation of the high-field-side (HFS) EFs, and also possibly the shift of the magnetic axis. These NSTX-U EF data will be further investigated and compared with other new experiments from COMPASS, DIII-D, MAST-U, KSTAR, and EAST, in order to develop single-mode and multi-mode EF threshold scaling and correction strategies for future tokamak devices. The regime dependence of EF thresholds for error field penetration and mode locking will be studied by applying leading theories relevant for each linear or non-linear MHD, and also by applying hybrid and extended MHD codes to study dynamics and bifurcations of islands due to EFs predicted in theory. Improved EF physics understanding and correction criteria will be used for ITER, MSTX, NSTX-U and also for ST40, to identify error field effects and develop mitigation/correction strategies. Coil metrology, EF characterization, and non-axisymmetric response experiments will be revised for newly assembled NSTX-U, and multi-n correction strategies against penetration and TM/NTM mode locking will be developed and tested.