Max-Planck-Institut für Plasmaphysik ITPA T&C Group meeting, CCFE, He & Impurity transport modelling He & Impurity transport Introduction Remarks on modeling aspects C. Angioni J. Candy and R.E. Waltz are warmly acknowledged for providing GYRO, M. Kotschenreuther and W. Dorland for providing GS2 with special thanks to C. Bourdelle, E. Fable, T. Hein

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE, Motivation  Impurity transport produced by combination of neoclassical and turbulent effects  Practical operational interest, to learn how to avoid too large dilution and radiation losses in the core  Physical interest, impurity transport is the natural complement to electron transport in the validation of the entire theoretical paradigm of particle transport  Theory of turbulent transport asked to reliably predict both D and V separately (and not only V/D like in electron particle transport)  Size of D from turbulent transport is critical in determining the relative impact of the neoclassical pinch, and of the central source of He ash  Impurity charge (and mass) provides additional handle to characterize experimental observations in terms of theoretically predicted transport processes

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE, Turbulent transport, complex theoretical pattern of inward and outward contributions  Framework for theory validation: Do experiment exhibit (qualititatively, quantitatively) the same pattern in out in out in Thermodiffusion Pure Convection ITG TEM ITG TEM Role of Collisions ITG TEM in out in electrons impurities in resonanc e only  d slab resonance limit

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE, Impurity charge provides additional handle to identify different transport processes  Although electrostatic turbulent transport is produced by fluctuating ExB drift, dependences on Z and A arise from the resonances, provided by the perpendicular and parallel gyro- centre motions [Bourdelle PoP 07]  Perpendicular motion, curvature and grad B drift prop. to 1/Z  Parallel motion, electric force term proportional to Z/A, pressure term proportional to 1/A

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE, Relevant parameters for comparison between theory and experiment  Transient transport experiments by impurity laser ablation or gas puffs can determine both diffusion and convection separately  One goal is to identify and agree on a set of parameters suited to compare experimental results with theoretical predictions  Dimensionless forms have to be preferred, because not directly limited by the requirement of matching heat fluxes in simulations which have to predict absolute values (in m^2/s) of the diffusivity  Most natural choice (already adopted in several exp. papers) where and

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE, Application to He transport at typical H- mode parameters (ITER standard scenario)  Input parameter of linear and nonlinear simulations provided by a GLF23 simulation of the ITER standard scenario  The predicted value of D/  does not change significantly with increasing values of R/LT (blue curve 20% smaller)  D is an actual (incremental )diffusivity,  is a power balance conductivity  Predicted values of D/  rather constant along minor radius and around 2, most of experimental estimates indicate lower values ( around 1 or less )

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE,  predicted to decrease ratio D/   Requires quantitative comparisons  Theoretically predicted dependence to be validated against experimental results  Qualitatively in agreement with observations in DIII-D [Petty PoP 04]  Could be of some concern for very high beta scenarios in case the drop of diffusivity becomes too large [ Hein & Angioni PoP 10 ]  Too strong effect of central source of He ash on He peaking  Too weak reduction of impact of neo inward pinch of high Z impurities by turbulent D

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE, Turbulent convection of He at typical H- mode parameters (ITER standard scenario)  He found to be convected inward for typical H-mode parameters (outward thermodiffusion (ITG) does not compensate inward convection )  The same takes place for heavier impurities (B, C), and this appears to not account for observations of flat/ hollow density profiles of B and C in H-modes [ AUG McDermott yesterday, JET Weisen (NF 05) and Giroud today ] thermodiffusion pure convection [ GYRO linear and nonlinear]  On the other hand, this He transport provides a He profile which has the same shape as the predicted electron density profile, in agreement with some observations [ DIII-D, Wade PoP 95 ] [ Angioni NF 09]

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE,  has some (limited) effect also on V / D  Note opposite direction of thermodiffusion between He and T due to the different charge  Magnetic flutter practically negligible on diffusion & thermodiffusion, gives up to 10% correction for the pure convection piece [ Hein & Angioni PoP 10 ]

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE,  has some (limited) effect also on V / D  Summing all effects, beta is predicted to lead to weak accumulation of intermediately heavy impurities (typical H-mode parameters)  b.t.w, this goes in the wrong direction to get flat/hollow C profiles in H- modes  Effect on V/D of light impurities is weak [ Hein & Angioni PoP 10 ]

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE, Outward turbulent convection  The only mechanism identified so far which can produce a total outward turbulent convection of intermediate / heavy impurities is parallel compression of parallel velocity fluctuations  This requires usually R/LTe >> R/LTi, as in the case of the simulations at r/a = 0.2 in the presence of ECH ( AUG case, agrees with experimental measurements on Si )  Note, at r/a = 0.5 all Z go inward (in agreement with Si exp measurements, but also C is predicted inward … )  Still, one could speculate (= hope ) that by appropriate choice of parameters, for impurities like B and C, conditions where thermodiffusion (outward in ITG) is large enough to prevail over inward convection can be idenitified (… not yet though) [ Angioni PPCF 07]

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE, Outward turbulent convection in NL simulations  The mechanism of outward impurity convection in the presence of electron drift propagating turbulence has been confirmed in nonlinear gyrokinetic simulations with GYRO (case Qe ~ 2Qi )  For ion and electron heat fluxes which are of comparable size, the pure convection is directed inward  Observations of outward convection of impurities provide real challenges for theory / modelling and are effective for validation  In turbulence, outward convection obtained only when specific transport processes prevail over the inward ExB compression pinch  In addition, plasma conditions leading to outward (or weak inward) convection of impurities are also operationally attractive [ Angioni NF 09] GYRO

He and Impurity transport modellingC. Angioni, ITPA T&C, CCFE, Conclusions  The combination of intense current and past experimental studies on impurity transport (whose review with specific focus on He is the topic of the present session) should allow us to characterize experimental phenomenology in a more comprehensive way  This gives also the conditions for an unprecedented effort in validation of turbulent theory of impurity transport  Investigate of size and main parametric dependences of the ratio of the turbulent diffusivity to the effective heat conductivity  Proposed key objectives  Identify conditions leading to outward impurity convection, for more effective validation of theoretical predictions  The combination of these studies with those on other transport channels and/or with additional informations from fluctuation measurements makes the validation effort more complete and conclusive