Studies of Lower Hybrid Range of Frequencies Actuators in the ARC Device

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1 Studies of Lower Hybrid Range of Frequencies Actuators in the ARC Device P. T. Bonoli, Y. Lin. S. Shiraiwa, G. M. Wallace, J. C. Wright, and S. J. Wukitch MIT PSFC, Cambridge, MA th Annual Meeting of the APS Division of Plasma Physics Milwaukee, WI October 23-27, 2017 Work supported by the U.S. DoE, Office of Science, Office of Fusion Energy Sciences, User Facility Alcator C-Mod under DE-FC02-99ER54512 and a PSFC Theory Grant under DE-FG02-91-ER54109 Abstract CO

2 RF actuators are critical for fusion energy development Radio-frequency (RF) sources are used in reactor designs such as the ARC Device: Ion cyclotron radio-frequency (ICRF) fast wave power for core heating and current drive Lower hybrid radio-frequency (LHRF) power for off axis-current drive that augments the bootstrap current. High field side (HFS) placement of RF actuators has a number of distinct advantages: Reduced heat flux, quiescent nature (turbulent reduction), and screening of impurities Improvement in core wave physics Refined lower hybrid current drive (LHCD) predictions for ARC using a hierarchy of LHCD models result in higher current drive efficiency with improved profiles for current profile control: ACCOME: Adjoint code + 1D (v ) power absorption + ray tracing GENRAY + CQL3D: 3D (r, v, v ) + ray tracing Abstract CO

3 Plasma Sustainment Challenge Efficient, robust, steady state current drive is required to make the tokamak a viable concept for fusion electricity. Power required for current sustainment is a major constraint upon plant efficiency. RF actuators have been long recognized as essential tools for steady state tokamak. Maintaining a large radius of shear reversal helps with improved confinement and MHD stability Maximizes bootstrap current Ideal-wall β N limit rises as current profile is broadened External off axis current drive supplements bootstrap current profile at 0.6 < ρ < This FNSF Design was done with the LH launcher on the LFS FNSF Design Target Adapted from C. Kessel et al, Fusion Science & Tech. (2015). Abstract CO

4 Quiescent high field side SOL is ideal for RF antennas Transport in tokamak sends heat and particles to low field side scrape off layer (SOL) Expect coupling to remain quiescent. Expect reduced scattering from turbulent density perturbations. ELMs are attenuated in single null and do not reach HFS in double null. N. Smick et al, Nucl. Fusion 53 (2013) Abstract CO

5 High field side SOL plasma profiles allow for optimal RF antenna coupling Transport in tokamak sends heat and particles to low field side scrape off layer (SOL) Expect coupling to remain quiescent. Expect reduced scattering from turbulent density perturbations. ELMs are attenuated in single null and do not reach HFS in double null. Steep HFS SOL Density Profile Allows for Coupling Optimization Lower density measured in HFS Double Null (DN) plasmas: Potential to optimize coupling through magnetic balance. Density control at launcher by adjusting inner gap Magnetic Balance (USN, LSN, DN) allows control of SOL flows and impurity screening. N. Smick, B. LaBombard, C.S. Pitcher, Journal of Nuclear Materials, 2005 See Poster JP by S. J. Wukitch The High Field Path to Practical Fusion Energy Abstract CO

6 Conventional Reduced heat flux on high field side (HFS) favors HFS placement of RF actuators Tokamak power exhaust strongly favors HFS launch. Innovation RF CD launcher Injecting power from HFS removes the launcher from high heat flux region. Conventional approach has launchers facing into high heat exhaust and turbulent plasma. In reactor, ~0.5 m of actively cooled shield and blanket region. Innovative RF launchers can be accommodated. > 0.5 m shield & blanket Abstract CO

7 HFS antenna location also improves core performance of LHCD by allowing use of a lower parallel refractive index n = k c / ω LH wave accessibility [1] and the condition for electron Landau damping of the LH wave [2] (v / v te 2.5-3) determine an access window for wave penetration and absorption: n n n acc ELD, ω ω ω ω ( n ) n acc > , n ELD 30 / Te ( kev ) ω ω ω ω 2 2 1/2 pi pe pe pe e 2 2 ce ce ce( B) Higher magnetic field improves wave accessibility by lowering n acc thus allowing access to a higher T e with faster phase velocity LH waves: Can be done by raising B 0 through HFS launch. [1] M. Brambilla Nuc. Fusion 19 (1979) [2] M. Brambilla Physics of Plasma Close to Thermonuclear Conditions (Brussels, 1980) 291. Abstract CO

8 In the ARC device high magnetic field is combined with HFS launch to yield excellent CD access HFS concept forms the basis for the LHCD system in ARC [1]: n = , f 0 = 8 GHz B 0 = 9.25 T, I p = 8 MA a = 1.1m R 0 = 3.3 m n e (0) = m -3 T e (0) ~ T i (0) = 26 kev HFS placement of LHRF actuator improves wave penetration at conventional values of B 5T but enables significant penetration when combined with high B 10T. [1] B. N. Sorbom et al, Fusion Eng. and Design (2015). Abstract CO

9 Original ARC LHCD simulations (ACCOME code) solved for the LH current density (J rf ) via a response function approach Define and solve an Adjoint Problem for the Spitzer Harm function (χ): [Karney, NF 1985] J rf = 3 χ d p Γrf p Γ rf = D QL f e p The response function χ contains all the physics effects already in the numerical 2D and 3D FP solvers such as particle trapping, DC electric field effect, and momentum conserving corrections in C(f e ) Computation of J rf requires separate knowledge of Γ rf and f e. Γ rf and f e are evaluated from a 1-D (p ) solution of the Fokker Planck Equation which only captures parallel damping effects related to T >> T e Abstract CO

10 New LHCD simulations have been performed for ARC using a combined 3-D (p, p, r) Fokker Planck ray tracing model (GENRAY CQL3D) CQL3D computes the time dependent solution of the Fokker Planck equation, with and without the radial diffusion operator: p D rf ( p f ) p + Γ δ ( p s e + C( fe, p, p ) + 1 ) + r Ray tracing and FP solver iterate until a self-consistent D rf and f e are obtained. Approach captures important 2D (v, v ) velocity space effects on LH wave damping due to pitch angle scattering of electrons from the parallel to perpendicular directions resulting in T >> T e This effect was ignored in the original simulations which assumed T T e Abstract CO r rχ F fe r ee = f p f t e e

11 Coupled ray tracing / 3D Fokker Planck simulations yield a LH power deposition profile that is more peaked and slightly farther out in radius as compared to the adjoint + ray tracing More accurate LH power deposition profile places LHCD at location needed for FNSF J LH profile in CQL3D becomes more peaked and moves slightly outward with I LH increasing from 1.77 MA to 1.95 MA Abstract CO

12 Physical ray trajectories are identical in ACCOME and GENRAY Differences in wave absorption due to 2-D (v, v ) velocity space effects Single pass damping achieved with deep ray penetration Abstract CO

13 High field side scrape-off layer offers a number of important advantages for placement of RF sources Reduced heat flux, quiescent nature (turbulent reduction), and screening of impurities High magnetic field combined with HFS launch in ARC makes it possible to couple LH waves at densities where both the current drive efficiency is high and the bootstrap current fraction is high: f BS ~ 0.65 and f NI ~ 0.35 Comparison of lower hybrid current drive (LHCD) predictions for ARC using more advanced ray tracing / Fokker Planck models indicates: Two-dimensional velocity space effects increase the level of LHRF current generation and cause the spatial profile of J LH to be peaked slightly farther out and be narrower, which is better for control of the shear reversal point ACCOME: Adjoint code + 1D (v ) power absorption + ray tracing η LH ~ 0.31 (10 20 A/W/m 2 ) GENRAY + CQL3D: 3D (r, v, v ) + ray tracing η LH ~ 0.35 (10 20 A/W/m 2 ) These LHCD efficiencies are consistent with FNSF requirements No other CD technique can give this CD efficiency at this location. Abstract CO