Status of advanced tokamak scenario modelling with off-axis electron cyclotron current drive in DIII-D
M. MurakamiH.E. St. JohnT. A. CasperM. S. ChuJ. C. DeBooC. M. GreenfieldJ. E. KinseyL. L. LaoR.J. La HayeY. R. Lin‐LiuT. C. LucePeter PolitzerB. W. RiceG. M. StaeblerT. S. TaylorM. R. WadeDIII-D Research Team
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Abstract:
The status of modelling work focused on developing the advanced tokamak (AT) scenarios in DIII-D is discussed. The objective of the work is twofold: (a) to develop AT scenarios with ECCD using time dependent transport simulations, coupled with heating and current drive models, consistent with MHD equilibrium and stability; and (b) to use time dependent simulations to help plan experiments and to understand the key physics involved. Time dependent simulations based on transport coefficients derived from experimentally achieved target discharges are used to perform AT scenario modelling. The modelling indicates that off-axis ECCD with approximately 3 MW absorbed power can maintain high performance discharges with qmin > 1 for 5-10 s. The resultant equilibria are calculated to be stable to n = 1 pressure driven modes. The plasma is well into the second stability regime for high-n ballooning modes over a large part of the plasma volume. The role of continuous localized ECCD is studied for stabilizing m/n = 2/1 tearing modes. Progress towards validating current drive and transport models, consistent with experimental results, and developing self-consistent, integrated high performance AT scenarios is discussed.Keywords:
DIII-D
Bootstrap current
Ballooning
Tearing
The stability of ballooning modes is investigated in the neighborhood of the magnetic axis of an axisymmetric toroidal configuration of the Tokamak type. It is shown that the stability criterion for these modes is the same as that for localized modes.
Ballooning
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The DIII-D high and low beta stability results have established the basic feasibility of the divertor and H-mode configurations up to elongation 2.0 for next generation tokamaks. The 6.8% beta T achieved has already exceeded projected operating requirements of next generation devices. beta T > 6% has been sustained for 800 ms. Stability calculations and patterns of MHD mode behavior suggest a central expanding zone of ballooning instabilities leads ultimately to unstable m/n=2/1 modes which cause beta collapse or disruption. The pressure gradient at the plasma edge just reaches the first regime ballooning limit prior to ELMs. ECRH has proven effective for generating H-mode, sawtooth suppression, and ELM suppression.
DIII-D
Ballooning
Sawtooth wave
BETA (programming language)
Edge-localized mode
Pressure gradient
Limiter
Bootstrap current
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Progress is reported on simulations of electromagnetic drift-resistive ballooning turbulence in realistic single-null tokamak geometry using the BOUT three-dimensional fluid code [1] that solves Braginskii-based fluid equations [2]. The simulation domain models the actual magnetic geometry of the DIII-D tokamak. The simulations follow unstable drift resistive ballooning turbulence in the edge region to saturation. Fluctuation amplitudes, fluctuation spectra, and particle and thermal fluxes are compared to experimental probe and beam-emission-spectroscopy data for a well-characterized L-mode discharges in DIII-D. Post-processing of the simulation data using synthetic diagnostics facilitates the comparisons. The simulations are comprised of a suite of runs in which the physics model is extended to include more fluid fields and physics terms. The relative agreement of the simulation results with the experimental data improves as more physics is included in the model. The simulations yield results for fluctuation amplitudes, correlation lengths, particle and energy fluxes and diffusivities in reasonable agreement with measurements near the outer midplane of the discharge. The effects of sheared ExB poloidal rotation are included, and a density scan is presented.
DIII-D
Ballooning
Resistive touchscreen
Mode (computer interface)
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DIII-D
Ballooning
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Strongly ballooning modes have been found as precursors to high {beta} disruptions on TFTR. The modes are typically localized to a region spanning about 60{degree} in the toroidal direction. The toroidal localization is associated with lower frequency, global Magneto-Hydro-Dynamic (MHD) activity, typically an ideal n = 1 kink mode. They have moderate to high frequency (f = 10--20 f{sub rot}), implying toroidal mode numbers in the range n = 10--20. The growth rates for the modes are large, of order 10{sup 4}/sec.
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BETA (programming language)
Mode (computer interface)
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The stability of ideal ballooning modes in the presence of a rigid toroidal rotation is revisited. Two stabilising contributions arising due to rigid toroidal rotation are identified. These stabilising contributions can be particularly significant for present and future generations of low and ultra-low aspect ratio tokamaks.
Ballooning
Plasma stability
Gyrokinetics
Kink instability
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DIII-D
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The effect of nonaxisymmetry on Alfven modes in toroidal geometry is investigated numerically. A model equation is used which simplifies the analysis on the resonant surfaces. Alfven modes, characterized by their poloidal and toroidal node numbers (m,n), are found to exist in moderate nonaxisymmetry as well. For fixed (m,n), however, the mode collapses from a global feature on the resonant surface into an infinitely thin Alfven ballooning mode along a field line, if the nonaxisymmetry exceeds a critical threshold or, with given nonaxisymmetry, if the poloidal variation does so. Alfven balloonings are polarized within the magnetic surfaces and are stable.
Ballooning
Alfvén wave
Mode (computer interface)
Field line
Line (geometry)
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Ballooning
Accretion disc
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