Interaction between EGAMs and turbulence in full-f gyrokinetic simulations

Transcription

1 Interaction between EGAMs and turbulence in full-f gyrokinetic simulations David Zarzoso 1 X Garbet 1, Y Sarazin 1, V Grandgirard 1, J Abiteboul 1, A Strugarek 1,2, G Dif-Pradalier 1, R Dumont 1, G Latu 1 and Ph Ghendrih 1 1 CEA, IRFM, F Saint-Paul-lez-Durance, France. 2 Laboratoire AIM Paris-Saclay, CEA/Irfu Université Paris-Diderot CNRS/INSU, Gif-sur-Yvette, France 1 / 15

2 Towards the control of turbulence? Efficient mechanism of turbulence reduction Control of E r Poloidal rotation E r shearing Biglar-1990 GAMs (due to geodesic curvature) Landau damped γ L e q2. How to excite GAMs in steady state? Interaction with turbulence? 2 / 15

3 Towards the control of turbulence? Efficient mechanism of turbulence reduction Control of E r Poloidal rotation E r shearing Biglar-1990 GAMs (due to geodesic curvature) Landau damped γ L e q2. How to excite GAMs in steady state? Interaction with turbulence? 3 / 15

4 Fast particles excite GAMs EGAMs GAMs unstable due to fast particles (e.g. bump-on-tail): E F eq v =v res > 0 EGAMs Theoretically Fu-2008, Berk-2010, Qiu-2010 Experimentally Nazikian-2008 Numerically Zarzoso-2011 to be submitted and Poster on Friday, P2.16 EGAM: new mode ω EGAM + radial structure determined by GAM continuum ( T ) and FLR effects Fu-2008 Radial structure of the fast particles source (Sfp ) Qiu-2010,2011 Simulations with Gysela 4 / 15

5 Fast particles excite GAMs EGAMs GAMs unstable due to fast particles (e.g. bump-on-tail): E F eq v =v res > 0 EGAMs Theoretically Fu-2008, Berk-2010, Qiu-2010 Experimentally Nazikian-2008 Numerically Zarzoso-2011 to be submitted and Poster on Friday, P2.16 EGAM: new mode ω EGAM + radial structure determined by GAM continuum ( T ) and FLR effects Fu-2008 Radial structure of the fast particles source (Sfp ) Qiu-2010,2011 Simulations with Gysela 5 / 15

6 EGAMs simulations with Gysela code Gysela code: global full-f 5D gyrokinetic code to model electrostatic turbulence Grandgirard-2008, Sarazin-2010, Abiteboul et al. this afternoon (A4.3) GK equation + quasineutrality Brizard-2007 ( ) ( ) B tf + B ẋg F + vg, B v G, F = C(F ) + S bulk + S fp e (φ φ ) 1 ( mi n ) eq. T e,eq n eq eb 2 φ = n G n eq n eq Adiabatic electrons C (F ) collisions operator Dif-Pradalier-2011 S bulk bulk heating (flux-driven simulations) Sarazin-2011 S fp fast particles energy source 6 / 15

7 Fast particles source implementation EGAM excitation E F eq v =v res > 0 Resonance in v. Need to invert the slope E F eq v =v res < 0 v 0 0 E F eq v =v res > 0 S bulk and S fp do not inject particles. 7 / 15

8 Comparing simulations with/without EGAMs Two flux driven simulations S = S bulk + S fp. Only difference: S fp with v 0 = 0 and v 0 = 2. Same heating power. Total heating such that T ITG turbulence. S Ed 3 v χ neo < T crit No expected ρ = 1/64 (ρ ITER = ), ν = 0.1 (banana regime) N r = 128, N θ = 128, N ϕ = 64, N v = 128, N µ = 16 N proc = / 15

9 New source successful at exciting EGAMs When v 0 = 0: E F eq < 0 Landau damped GAMs. When v 0 = 2: E F eq > 0 EGAMs excited at ω EGAM ω GAM /2. 9 / 15

10 EGAMs lead to improved confinement! EGAMs Temperature gradient locally increased Core temperature increases with EGAMs Improved confinement 10 / 15

11 Improved confinement ITG turbulence Improved confinement T > T crit ITG turbulence Destabilization of resonant modes k = m + nq = 0. Broad turbulent spectrum 11 / 15

12 EGAMs modify neoclassical transport EGAMs Positive shearing rate, ω E B φ > 0 (ω E B < 0 without fast part.) Present understanding: Berk-1967, Hazeltine-1989, Shaing-1992, Kagan-2010 Modification of neoclassical transport through orbit squeezing factor S orb = 1 + q2 φ χ ɛ 2 neo = χ0 neo Sorb α φ > 0 S orb > 1 χ neo < χ 0 neo φ < 0 S orb < 1 χ neo > χ 0 neo Our simulations are in qualitative agreement with this explanation With EGAMs Sorb 1.5 Without EGAMs S orb / 15

13 Saturation of EGAMs Without turbulence Wave-particle trapping as a mechanism for nonlinear saturation of bump-on-tail instability O Neil-1965, Berk EGAMs (recently invoked in Qiu-2011, Zarzoso-2011) 2 nd harmonic. Wave-particle trapping E F 0 With turbulence E F starts decreasing only when turbulence saturates. Turbulence contributes to EGAM saturation. Wave-particle trapping is not excluded. 13 / 15

14 Saturation of EGAMs Without turbulence Wave-particle trapping as a mechanism for nonlinear saturation of bump-on-tail instability O Neil-1965, Berk EGAMs (recently invoked in Qiu-2011, Zarzoso-2011) 2 nd harmonic.. Wave-particle trapping E F 0 With turbulence E F starts decreasing only when turbulence saturates. Turbulence contributes to EGAM saturation. Wave-particle trapping is not excluded. 14 / 15

15 Conclusion and perspectives EGAMs efficiently excited in full-f GK simulations by means of a convenient external source. A mechanism of energy transfer from energetic particles to turbulence has been identified (1) EGAMs ω E B. (2) ωe B decreased neoclassical transport and increased temperature gradient Improved confinement (3) Improved confinement ITG turbulence. (4) EGAM saturation occurs only when turbulence saturates. Analysis to understand the role of turbulence in progress. 15 / 15