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FY2019 NSTX-U Research Milestones

R(19-1):  Expanded disruption prediction and avoidance for NSTX-U and tokamaks

Description: Predicting and avoiding damaging plasma disruptions in fusion-producing tokamaks is the present “grand challenge” in tokamak stability research. Meeting this significant goal will require multiple approaches, data from a variety of tokamaks, and use of various physical models and control methods. The present milestone will focus on the results of automated disruption event characterization and forecasting (DECAF) that identifies chains of events that lead to eventual disruptions. Once these chains are determined, methods of breaking the chains using all available forms of actuators can be defined and tested. Initial results using the developing DECAF code have successfully demonstrated predictive models for resistive wall mode onset and automatic determination of rotating tearing mode activity, rotation bifurcation events, and mode locking precursors to disruptions. These successes will be expanded to satisfy several milestone tasks including further development of physics modules that address the dominant causes of disruptions, application to a greatly expanded NSTX/NSTX-U plasma database (including plasmas not suffering disruptions before the intended shot duration), and resultant statistics determining the success and false positive rates of DECAF analysis. Building on the results of the FY-2017 milestone on error field correction and FY-2018 milestone on tearing mode physics, an array of physical topics causing disruptions will be addressed including density limits, MHD mode locking, and RWM destabilization, as well as more technical considerations. The analysis will also identify how the automated algorithm can be improved through greater physics understanding of the disruption chain events, and will demonstrate the effectiveness of altering the predictive physics models used in the code. Through arranged collaborations, the analysis will expand to include input from both national and international tokamaks (DIII-D, JET, and KSTAR), which is critical for a generalized DECAF product applicable to ITER and future devices. The general DECAF algorithm will be applied to the existing NSTX-U database to identify how the device actuators can be best used to avoid disruptions in the continued operation of NSTX-U, and also to improve the prediction of disruptions in real-time to inform and aid the further development of the automated NSTX-U plasma shutdown handler.

R(19-2): Assess the effects of neutral beam injection parameters on the fast ion distribution function and neutral beam driven current profile

Description:  Accurate knowledge of neutral beam (NB) ion properties is of paramount importance for many areas of tokamak physics. NB ions modify the power balance, provide torque to drive plasma rotation and affect the behavior of MHD instabilities. Moreover, they determine the non-inductive NB driven current, which is crucial for future devices such as ITER, FNSF and STs with small or no central solenoid. With the additional more tangentially-aimed NB sources, NSTX-U is uniquely equipped to characterize a broad parameter space of fast ion distribution (Fnb) and NB-driven current properties, with significant overlap with conventional aspect ratio tokamaks.  The two main goals of this milestone are (i) to characterize the NB ion behavior and compare it with classical predictions, and (ii) to document the operating space of NB-driven current profile. Fnb will be characterized through the upgraded set of NSTX-U fast ion diagnostics (e.g. fast-ion D-alpha: FIDA, solid-state neutral particle analyzer: ssNPA, scintillator-based fast-lost-ion probe: sFLIP, neutron counters, and possibly a fusion products diagnostic) as a function of NB injection parameters (tangency radius, beam voltage) and magnetic field. Building on the initial results obtained in the NSTX-U FY-2016 run campaign, well controlled, single-source scenarios at low NB power will be used to compare fast ion behavior with classical models (e.g. the NUBEAM module of TRANSP) in the absence of fast ion driven instabilities. Diagnostics data will be interpreted through the “beam blip” analysis technique and other dedicated codes such as FIDASIM. Then, the NB-driven current profile will be documented for the attainable NB parameter space by comparing NUBEAM/TRANSP predictions to measurements from Motional Stark Effect, complemented by the vertical/tangential FIDA systems and ssNPA to assess modifications of the classically expected Fnb.  Particular emphasis will be placed on documenting driven current profile variations as a function of injecting beam tangency radius.  During FY2019, significant effort will be put toward fast-ion diagnostic re-commissioning and MSE commissioning for these experiments on NSTX-U.  If NSTX-U cannot support plasma operations during FY2019, additional emphasis will be placed on collaboration on MAST-U and possibly DIII-D to support the beam characterization and driven current profile research goals of this milestone.  

 R(19-3): Assess H-mode energy confinement, pedestal, and scrape off layer characteristics with higher BT, IP 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, pedestal and scrape off layer trends over an expanded range of engineering parameters.  H-mode studies in NSTX have shown that the global energy confinement exhibits a more favorable scaling with collisionality (BtE ~ 1/n*e) than that from ITER98y,2.  This strong n*e scaling unifies disparate engineering scalings with boronization (tE~Ip0.4BT1.0) and lithiumization (tE ~ IP0.8BT-0.15).  In addition, the H-mode pedestal pressure increases with ~IP2, while the divertor heat flux footprint width decreases faster than linearly with IP.  With double BT, double IP and double NBI power with beams at different tangency radii, NSTX-U provides 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 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.  The scaling of the divertor heat flux profile with higher IP and PNBI will also be measured to characterize the peak heat fluxes and scrape off layer widths, and this will provide the basis for eventual testing of heat flux mitigation techniques. Scrape-off layer density and temperature profile data will also be obtained for several divertor configurations, flux expansion values, and strike-point locations to validate the assumptions used in the physics design of the cryo-pump to inform the cryo-pump engineering design. During FY2019, 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 FY2019, emphasis will be placed on collaboration on MAST-U to support the core transport and pedestal structure research goals of this milestone.  

R(19-4):  Commission physics and operational tools for obtaining high-performance discharges in NSTX-U 

Description:  Steady-state, high-beta conditions are required in future ST devices, such as a FNSF or Pilot Plant.  NSTX-U is designed to provide the physics knowledge for the achievement of such conditions by demonstrating stationary, long pulse, high non-inductive fraction operation. A major research goal during the first year of operation of NSTX-U is to develop high-performance H-mode scenarios accessing toroidal fields up to at least ~0.8 T and plasma currents up to ~1.6 MA. Several plasma facing component (PFC) conditioning methods including boronization and lithium coatings will be assessed to determine which are most favorable for longer pulse scenarios. Impurity control techniques, an example of which is ELM pacing, will be developed for the reduction of impurity accumulation in otherwise ELM-free lithium-conditioned H-modes. Utilizing the scenario modeling framework developed in FY2017 and FY2018, high elongation (2.5 < k < 2.8) plasma shapes anticipated to result in high non-inductive fraction in NSTX-U will be developed, and the vertical stability of these targets will be assessed, with mitigating actions taken if vertical instability problems arise. Building upon the results of the FY2016 NSTX-U run campaign and the FY2017-18 milestones on error field correction and tearing mode physics, 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. 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 FY2019, significant effort will be put toward commissioning real-time diagnostics and scenario-control-relevant actuators for these experiments on NSTX-U. If NSTX-U cannot support plasma operations during FY2019, 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.