Transcript C Angioni

Max-Planck-Institut
für Plasmaphysik
He & Impurity transport
Introduction
Remarks on modeling aspects
C. Angioni
with special thanks to C. Bourdelle, E. Fable, T. Hein
J. Candy and R.E. Waltz are warmly acknowledged for providing
GYRO, M. Kotschenreuther and W. Dorland for providing GS2
He & Impurity transport modelling
ITPA T&C Group meeting, CCFE, 22-25.3.2010
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 modelling
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
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Turbulent transport, complex theoretical
pattern of inward and outward contributions
Thermodiffusion Pure Convection Role of Collisions
wd
electrons
resonanc
impurities
e
only
ITG
TEM
in
out
ITG
TEM
ITG
TEM
out
in
out
in
in
out
in
electrons
slab
resonance
impurities
limit
in
out
in
in
out
 Framework for theory validation: Do experiment exhibit
(qualititatively, quantitatively) the same pattern
He and Impurity transport modelling
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
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Impurity charge provides additional handle
to identify different transport processes
[Bourdelle PoP 07]
 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 gyrocentre motions
 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 modelling
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
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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)
and
He and Impurity transport modelling
where
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
5
Application to He transport at typical Hmode parameters (ITER standard scenario)
 Input parameter of linear and nonlinear simulations provided
by a GLF23 simulation of the ITER standard scenario
 D is an actual (incremental )diffusivity,
c is a power balance conductivity
 The predicted value of D/c
does not change
significantly with increasing
values of R/LT (blue curve
20% smaller)
 Predicted values of D/c
rather constant along minor
radius and around 2, most of
experimental estimates
indicate lower values (
around 1 or less )
He and Impurity transport modelling
3
2
1
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
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b predicted to decrease ratio D/c
 Theoretically predicted
dependence to be validated
against experimental results
 Qualitatively in agreement with
observations in DIII-D [Petty PoP
04]Requires quantitative

comparisons
[ Hein & Angioni PoP 10 ]
3.2
2.4
1.6
 Could be of some concern for
very high beta scenarios in case
the drop of diffusivity becomes too
0.8
large
 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 modelling
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
7
Turbulent convection of He at typical Hmode 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]
[ GYRO linear and nonlinear]
thermodiffusion
pure convection
[ Angioni NF 09]
 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
95transport
]
He and
Impurity
modelling
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
8
b has some (limited) effect also on V /
D
[ Hein & Angioni PoP 10 ]
 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
He and Impurity transport modelling
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
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b 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 Hmodes
[ Hein & Angioni PoP 10 ]
 Effect on V/D of light impurities is weak
He and Impurity transport modelling
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
10
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
[ Angioni PPCF 07]
Hethough)
and Impurity transport modelling
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
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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
[ Angioni NF 09]
GYRO
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
He and
transport plasma
modelling
C. Angioni,
ITPA T&C,to
CCFE,
22-25.3 2010(or

In Impurity
addition,
conditions
leading
outward
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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
wayconditions for an unprecedented effort in
 comprehensive
This gives also the
validation of turbulent theory of impurity transport
 Proposed key objectives
 Investigate of size and main parametric dependences of the
ratio of the turbulent diffusivity to the effective heat
 conductivity
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
He and Impurity transport modelling
C. Angioni, ITPA T&C, CCFE, 22-25.3 2010
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