MHD-particle simulations and collective alpha-particle transport: analysis of ITER scenarios and perspectives for integrated modelling

Transcription

1 MHD-particle simulations and collective alpha-particle transport: analysis of ITER scenarios and perspectives for integrated modelling G. Vlad, S. Briguglio, G. Fogaccia, F. Zonca Associazione Euratom-ENEA sulla Fusione, C.R. Frascati - C.P I Frascati, Rome, Italy and M. Schneider Ass. Euratom-CEA, CEA/Cadarache, Saint Paul-lez-Durance, France G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 1

2 Introduction Next generation Tokamaks (e.g., ITER), should approach the so called ignition condition: heating due to fusion α-particles (hot particles) able to sustain the burning plasma Good confinement of the α-particles is crucial in getting such condition Fusion α-particles are generated with v Η v A =B/(4πn i m i ) 1/2 and peaked density profile free-energy source for the resonant destabilization of shear Alfvén modes (TAE, EPM, ) Interaction of such modes with energetic-particle can induce outward α-particle transport G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 2

3 Introduction-cont. Transport codes aimed to define burning-plasma scenarios do not include the possibility of Alfvén mode - α-particle interactions Are the proposed scenarios consistent with the α-particle collective dynamics? Which are the effects on α-particle profiles and confinement? Aim of our investigation: to check the consistency of several ITER scenarios by means of particle simulation techniques If α-particle pressure profile in presence of fully saturated modes is: close to initial one different from initial one scenario is consistent scenario could be inconsistent G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 3

4 The Model Hybrid MHD-Gyrokinetic Code (HMGC): thermal plasma described by zero pressure reduced O(ε 3 ) Magnetohydrodynamic (MHD) equations (circular shifted magnetic flux surfaces); energetic particles described by nonlinear guiding-center Vlasov equation (k ρ Η <<1) solved by particle-in-cell (PIC) techniques; energetic particles are loaded according to an isotropic slowingdown distribution function, with birth energy E fus =3.52 MeV and critical energy E crit 33.0 T e (r) (Stix); assume n D =n T =n i /2 and n i =n e. energetic particles and thermal plasma are coupled through the α particle pressure tensor, which enters the MHD momentum equation; mode-mode coupling neglected (only particle nonlinearities retained). G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 4

5 ITER scenarios Three different ITER scenarios have been considered: a) the reference monotonic-q scenario ( scenario 2, SC2, from ITER Joint Work Site): inductive, 15 MA scenario, with 400 MW fusion power and fusion yield Q=10; b) the reversed shear scenario ( scenario 4, SC4, from ITER Joint Work Site): steady-state, 9 MA, weak-negative shear, with about 300 MW fusion power and Q=5; q min 2.4 and r qmin /a c) a recently proposed hybrid scenario ( scenario H, SCH, from CRONOS package simulations): 11.3 MA weak-positive shear, with about 400 MW fusion power and Q 5. Simulations are performed retaining on-axis equilibrium magnetic field, major and minor radii, the safety-factor q, the plasma density n e, the electron temperature T e and the α-particle density n H. G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 5

6 Equilibrium profiles and parameters SC2 SC4 SCH a(m) R 0 (m) q B T (T) n e0 (10 20 m -3 ) T e0 (kev) n H0 (10 18 m -3 ) β H0 (%) α H,max = max{-r 0 q 2 β H} r α H,max /a G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 6

7 Linear dynamics: SC2 Power spectra of scalar-potential fluctuations in the [r,ω] plane, during the linear phase (β H0 =β H0, SC2 ). Upper and lower Alfvén continuous spectra are also plotted (black). n=2 n=4 n=6 G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 7

8 Linear dynamics: SC2-cont. Toroidal mode number n=2 is found to be the most unstable. The mode is located around r α H,max /a in radius, and below the Alfvén continuum in frequency. The mode belongs to a MHD branch (it exists also at zero β H0 ). G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 8

9 Linear dynamics: SC4 Power spectra of scalar-potential fluctuations in the [r,ω] plane, during the linear phase (β H0 =β H0, SC4 ). n=2 n=4 n=6 G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 9

10 Linear dynamics: SC4-cont. Toroidal mode number n=2 is found to be the most unstable. The mode is located close to r qmin /a radius, and close to the tip of the lower Alfvén continuum in frequency. A second weaker mode appear in the toroidal gap around r/a , for n=2. The mode belongs to a MHD branch (it exists also at zero β H0 ). G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 10

11 Linear dynamics: SCH Power spectra of scalar-potential fluctuations in the [r,ω] plane, during the linear phase (β H0 =β H0, SCH ). n=2 n=4 n=8 G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 11

12 Linear dynamics: SCH-cont. This scenario is weekly unstable at nominal β H0 =β H0, SCH value: the mode belongs to a MHD branch (a similar mode exists also at zero β H0 ); it is localized close to the magnetic axis. Above a certain threshold in β H0 a stronger unstable mode appears: it belongs to a EPM branch; it is localized at r α H,max /a, where the local drive is maximum. G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 12

13 Linear Dynamics: EPMs localization Radius: the mode is localized where the local drive α H has its maximum. Frequency: expected wave-particle resonances (ε<<1, small orbit width, well circulating/deeply trapped particles): ω = ωt l = 1 transit frequency: precession frequency: precession-bounce frequency: ω = ω d precession-bounce and precession resonances seem to dominate. E fus 2m H 1 q(r)r 0 E fus m H R 0 ω ch nq(r) r ω = ω d + lω B,l = ±1 ω = ω d +ω B ω B 1 R 0 q(r) ω = ω d ω B E fus m H ( r ) 1/2 R 0 β H0 =3.3 β H0, SCH. Mode disappears if mirroring term is switched off! SCH, n=4 SCH, n=6 SCH, n=8 G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 13

14 Nonlinear dynamics: phenomenology Maximum gradient of rβ H shifts outward: first steepening (convective phase, avalanche), then relaxing (diffusive phase: saturated e.m. fields scatter α-particles) rβ Η r max Linear phase r/a rβ H Convective phase r max r/a rβ H Diffusive phase r max r/a G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 14

15 Nonlinear dynamics: cont. SC2 (n=2): scarcely affected by nonlinear saturation (mode localization close to linear-phase one); continuum damping barrier prevents the mode to displace outwards. SCH: similar to SC2 at nominal β H0 value. SC4 (n=2): the mode is located around q min ; α particles have larger orbit width (higher q): larger convection and diffusion. at saturation G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 15

16 Nonlinear dynamics: cont.-2 Define r y (t) as the radial position of the surface containing a fraction y of the α- particle energy: y = r y rβ H (r;t)dr rβ H,init (r)dr SC2, n=2 SC4, n=2 0.8 r 95% /a β H0 tω A0 Time behavior of central α-particle beta, β H0 : tω A0 G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 16

17 Nonlinear dynamics: cont-3. Increase artificially β H0 to investigate the dependence of transport on the intensity of energetic particle drive. Convection phase: SC2, n=2 SC4, n=2 SCH, n=8 No α-particle dynamics, relaxed value! No α-particle dynamics, relaxed value! Also diffusion, characterized by: τ diff,95% r 95% [ r 95% / t] -1, τ diff,βh0 β H0 [ β H0 / t] -1, increases with β H0. G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 17

18 Conclusions Standard ITER monotonic-q scenario (SC2): unstable w.r.t. α-particle driven Alfvén modes; nonlinear saturation produces negligible modifications to the α-particle radial profile: consistent scenario; central β H0 very sensitive to increasing drive, r 95% not sensitive. Reversed shear scenario (SC4): unstable; appreciable changes to the α-particle radial profile (r 95% ): possibly inconsistent scenario; both β H0 and r 95% sensitive to increasing drive. Hybrid scenario (SCH): Very weakly unstable (MHD branch mode); Above 1.6 β H0, SCH EPM branch is driven strongly unstable. Caveat: multiple toroidal-mode-number dynamics could enforce α transport; dynamic approach to the reference α-particle pressure could regulate the stronger energetic particle mode dynamics via nonlinear effects of weaker modes. G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 18

19 Next Multiple toroidal-mode-number simulations: mode-mode coupling could enforce α transport. Could the dynamic approach to the reference α-particle pressure regulate the stronger energetic particle mode dynamics via nonlinear effects of weaker modes? (Counter example: ALE?). New (upgraded) code under development: full MHD, general curvilinear geometry (start with a linear module, MARS), energetic particles described by nonlinear gyro-kinetic Vlasov equation (finite k ρ Η ) solved by particle-in-cell (PIC) techniques. Needs for detailed inputs (α H, n beam, n ICRH detailed distribution functions, radial profiles, ). How to integrate the results of α-particle collective dynamics with transport code? G. Vlad et al. 12th EFPW, 6th-8th December 2004, Witney, UK 19