Max-Planck-Institut für Plasmaphysik ITPA T&C Group meeting, CCFE, 22-25.3.2010He & Impurity...

He & Impurity transport modelling

Max-Planck-Institut für Plasmaphysik

ITPA T&C Group meeting, CCFE, 22-25.3.2010

He & Impurity transportIntroduction

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 modelling C. Angioni, ITPA T&C, CCFE, 22-25.3 2010

2

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


3

Turbulent transport, complex theoretical pattern of inward and outward contributions

Framework for theory validation: Do experiment exhibit (qualititatively, quantitatively) the same pattern

in

inout

out

inout

out

in

in

ThermodiffusionPure Convection

ITG TEM ITG TEM

Role of Collisions

ITG TEM

inout

out in

electrons

electrons

impurities

impurities

inresonan

ceonly

d

slabresonance

limit


4

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


5

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)

whereand


6

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)

1

2

3

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 )


7

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 0.8

3.2

2.4

1.6

[ 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


8

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]


9

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 ]


10

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 ]


11

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]


12

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 validationIn 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


13

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

Max-Planck-Institut für Plasmaphysik ITPA T&C Group meeting, CCFE, 22-25.3.2010He & Impurity...

Documents

Transcript of Max-Planck-Institut für Plasmaphysik ITPA T&C Group meeting, CCFE, 22-25.3.2010He & Impurity...