Gurleen Bal (UCLA)

Genevieve DeGrandchamp (UCI)

Jonah Duran (U Tenn Knoxville)

Kenneth Gage (UCI)

Alvin Garcia (UCI)

Daniel Lin (UCI)

Gabriel Player (UCI)

Quinn Pratt (UCLA)

Aaron Rosenthal (MIT)

Alex Saperstein (Columbia)

Kamil Sklodowski (UCLA)

Elizabeth Tolman (MIT)

Jeff Lestz (Princeton/UC Irvine)

Philip Bonofiglo (PPPL)

Noah Hurst (Wisconsin)

Alex Tinguely (MIT)

More details on this ITER International School are available or forthcoming at https://iis2020.sciencesconf.org/. We are hoping to host the next IIS in the US, but now this will have to be in 2022 at the earliest.

 

Understanding transport and stability mechanisms that enable high bootstrap fraction scenarios in tokamaks is a key goal for achieving steady-state burning plasmas as one potential path to economical fusion energy. In this Research Highlight, Xiang Jian and colleagues at UC-San Diego, General Atomics and ORAU analyze and simulate the mechanisms predicted to control transport in the internal transport barrier (ITB) present in DIII-D high poloidal beta scenarios that lead to relatively large bootstrap fraction.

Role of High-Field-Side MicroTearing Turbulence on DIII-D High Core Plasmas*

X. Jian¹, C.Holland¹, J.Candy², S. Ding³ , E. Belli², V.Chan², G.M.Staebler², A.M.Garofalo², J. Mcclenaghan² , and P. Snyder²

¹ University of California, San Diego

² General Atomics

³ Oak Ridge Associated Universities

Author e-mail: xijian@ucsd.edu

Developing economically attractive steady-state tokamak reactor scenarios requires simultaneous achievement of high plasma confinement and high bootstrap current fraction, in order to generate the desired amount of fusion power without requiring a significant fraction of that power to “recirculate” into external current drive actuators. Because the bootstrap current fraction (where with and the plasma pressure and poloidal magnetic field), high scenarios have been extensively pursued as a possible steady-state scenario^1-3. In these scenarios, high plasma confinement is achieved through the formation of a strong large radius internal transport barrier (ITB). These ITBs are enabled by the use of stabilization, which is an inherent stabilization effect coming from the toroidal geometry of tokamaks. The parameter is proportional to in the limit of infinite aspect ratio shifted-circle geometry, and effectively quantifies the Shafranov shift of a given flux surface. Large values of can sufficiently modify the effective magnetic drift frequency to stabilize both microscopic and macroscopic instabilities. Through this mechanism, potentially attractive operating scenarios with self-sustaining high confinement and self-generated bootstrap current (also driven by density and temperature gradients) can be developed.

In order to fully develop these scenarios and confidently extend them to future reactor parameters, it is essential to have a comprehensive understanding of the underlying transport physics, particularly of the crucial ITB region which is essential for achieving the needed pressure and confinement levels. Recent work has shown that in the ITB region of DIII-D high discharges, the stabilization process suppresses conventional ballooning drift-wave turbulence. Transport modeling of these discharges indicates that the ion thermal transport is neoclassical across almost the entire plasma volume, while the electron transport remains anomalous^4,5, and not well-described by the TGLF quasilinear transport model⁶. Motivated by this result, extensive linear and nonlinear gyrokinetic simulations of the ITB region in a typical high discharge were performed. These simulations have revealed that a novel slab-like microtearing mode (MTM) localized to the high-field side of the plasma controls the electron thermal transport in the ITB region.

To investigate the local gyrokinetic stability of the ITB region, we employ the CGYRO code⁷ and run it with a very high numerical resolution. By a systematic scaling of in the electrostatic limit, it is shown that all drift waves, from long-wavelength trapped electron modes (TEMs) to short-wavelength electron temperature gradient (ETG) modes ( ranges from 0.1 to 50) are fully stabilized at ~70% of the experimental value (Fig. 1(a)). In these plasmas, the ion temperature gradient (ITG) mode is stable in the ITB region even when a = 0. More realistic electromagnetic calculations which include both perpendicular and parallel magnetic field fluctuations show that the kinetic ballooning mode (KBM), instead of the TEM, is in fact the dominant instability in the low-k region. Similar to the electrostatic case, the KBM can be fully suppressed by stabilization. We note that the analysis presented here is a local gyrokinetic one, and nonlocal effects which might destabilize a more ‘global’ KBM are not included. While all modes remain stable at the experimental value of in the electrostatic case, in the electromagnetic case a new mode in the low-k region is destabilized, as shown in Fig. 1(b). In order to clearly see how the dominant mode evolves with an increase of the frequency and growth rate for = 0.2 as a function of are shown in Fig. 1(c) and (d). The = 0.2 KBM is fully suppressed by ~50% of the experimental value, leaving a stable gap at larger values. However, above 75% of the experimental value, a new mode with phase velocity in the electron diamagnetic direction is destabilized by further increases in (Fig. 1d), which has been identified as microtearing mode based on its mode parity, parametric dependencies, and transport characteristics (both linear and nonlinear)⁸. Beyond these linear characteristics, nonlinear gyrokinetic simulations predict that this MTM branch can drive electron energy fluxes consistent with the experimental levels but drives no ion thermal transport. When combined with the complete stabilization of all other turbulent modes, these findings demonstrate that the MTM instability uniquely regulates the electron energy flow through the ITB region of these scenarios and helps prevent the plasma pressure from growing to the global MHD limit.

A novel aspect of this new MTM branch is the intimate connection between its destabilization by large and its slab-like mode-structure and localization to the high-field-side of the plasma. Visualizations of the electrostatic and magnetic potential components of the linear eigenmode in the (R,Z) plane are shown in Fig. 2a and 2b. Both fluctuation fields’ amplitudes peak at the inboard midplane, and exhibit radial structures tightly localized around individual rational surfaces. These properties persist in the nonlinear simulation. Snapshots of turbulent fluctuations from a nonlinear simulation are shown in Fig. 2c and 2d. As can be seen, the eddies in both and are always radially narrow and poloidally extended. This structure is particularly different from either the streamer-like linear eigenmodes of typical ballooning instabilities or the much more isotropic eddies typically observed in nonlinear simulations of those instabilities, e.g. in simulations of ITG turbulence. The narrow radial extent of these MTM fluctuations likely makes them more resistant to the flow shearing that can suppress ballooning-type modes through radial decorrelation of the fluctuations. In fact, no impact on the predicted transport levels is found by including equilibrium ExB shear in the simulations, despite the fact that the shearing rate is approximately four times larger than the maximum MTM linear growth rate.

Figure 1. The (where is the linear growth rate) of the electrostatic instabilities (a) and of the electromagnetic run (b) for different scaling factors of experimental (legend is ). The frequency (c) and growth rate (d) of dominant electromagnetic instability of
= 0.2 versus . ⁸

 

Figure 2. Linear eigenfunction structure of (a) and (b) of the MTM mode. Nonlinear fluctuation contour plot of the MTM are shown in (c) and (d) with only finite-k_y components included.

In conclusion, a detailed transport analysis has convincingly identified a novel high-field-side MTM as the unique cause for the thermal transport in the ITB region of the high scenario with the conventional micro-instabilities candidates, like ITG, TEM, ETG, and local KBM fully suppressed by the strong stabilization. More broadly, this finding generalizes the conclusions of other recent transport studies⁹, which identify MTMs as a “mode of last resort” in controlling the structure of transport barriers from edge to core, and therefore, to the whole plasma, when conventional instabilities are suppressed by mechanisms such as stabilization or shear. For more details, please see Ref 8.

Acknowledgments: This work was supported by the U.S. Department of Energy under awards DE-SC0018287, DE-SC0017992, DE-FG02-95ER54309, DE-SC0017992 and DE-FC02-04ER54698. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DEAC02-05CH11231

Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

 

References:

1 Gormezano, C. et al. Steady state operation [Progress in the ITER Physics Basis (PIPB)]. 47, S285-S336 (2007).

2 Garofalo, A. M. et al. Compatibility of internal transport barrier with steady-state operation in the high bootstrap fraction regime on DIII-D. 55, 123025 (2015).

3 Fujita, T. et al. Quasisteady high-confinement reversed shear plasma with large bootstrap current fraction under full noninductive current drive condition in JT-60U. 87, 085001 (2001).

4 Ding, S. et al. Confinement improvement in the high poloidal beta regime on DIII-D and application to steady-state H-mode on EAST. 24, 056114 (2017).

5 McClenaghan, J. et al. Transport modeling of the DIII-D high scenario and extrapolations to ITER steady-state operation. 57, 116019 (2017).

6 Staebler, G., Kinsey, J. & Waltz, R. J. P. o. P. A theory-based transport model with comprehensive physics. 14, 055909 (2007).

7 Candy, J., Belli, E. A. & Bravenec, R. J. J. o. C. P. A high-accuracy Eulerian gyrokinetic solver for collisional plasmas. 324, 73-93 (2016).

8 Jian, X. et al. Role of Microtearing Turbulence in DIII-D High Bootstrap Current Fraction Plasmas. 123, 225002 (2019).

9 Kotschenreuther, M. et al. Gyrokinetic analysis and simulation of pedestals to identify the culprits for energy losses using ‘fingerprints’. 59, 096001 (2019).

* This work was supported by the US DoE under Grants DE-FG02-99ER54531 and DE-FC02-04ER54698.

 

Calendar of Burning Plasma Events

2020

JET DT-campaign (/resources/ref/Web_Seminars/Litaudon-JET-%202019-05-02-vf.pdf)
JT-60SA First Plasma (http://www.jt60sa.org/)
Oct 12-16, 2020	Theory of Fusion Plasmas Joint Varenna- Lausanne International Workshop	Varenna or Lausanne
Oct 12-15, 2020	38th ITPA Diagnostics Topical Group Meeting	Remote only
Oct 12-22, 2020	29th Scrape-Off Layer & Divertor ITPA Topical Group Meeting	Remote only
Nov 9-13, 2020	62nd Annual Meeting of the APS Division of Plasma Physics	Remote only
Nov 15-19, 2020	Technology of Fusion Energy (TOFE) 2020	Chicago, IL
Nov 16-20, 2020	ITER International School	Marseille, France
Dec 6-10, 2020	47th International Conference on Plasma Science (ICOPS)	Singapore Virtual
Dec 13-17, 2020	High Temperature Plasma Diagnostics Conference	Santa Fe, NM Virtual
Dec 16-17, 2020	Fusion Power Associates 41st Annual Meeting and Symposium	Washington, D.C. Virtual

 

2021

Jan 24-29, 2021	International Conference on Plasma Surface Interactions	Jeju, South Korea
May 10-15, 2021	28th IAEA Fusion Energy Conference (FEC2020)	Nice, France