transport simulation of current ramp-up & ramp-down by f. imbeaux* presented by x. litaudon*

ISM Working Group 1 ITPA Naka meeting 2009 31st March 2009

Transport simulation of current ramp-up & ramp-down

by F. Imbeaux* presented by X. Litaudon*

F. Imbeaux, F. Köchl, V. Basiuk, J. Fereira, J. Hobirk, D. Hogeweij, X. Litaudon, J. Lönnroth, V. Parail, G. Pereverzev, Y. Peysson, G. Saibene, M. Schneider, G. Sips, G. Tardini, I. VoitsekhovitchOn behalf of : JET-EFDA contributors, Tore Supra work programme, ITER Scenario Modelling group (ITM-TF)

AssociationEuratom-cea

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Modelling of current ramp

2

• Aim of the working group: model current ramp-up (and down) in ITER

– Implications on PF system design,

– H&CD methods for current profile shaping

– flux consumption

• Main issues are related to the transport model

try to validate a model against present experiments

• Validation: li , Vloop, Te, Ti test against several JET, Tore Supra AUG, experiments (ohmic, NBI, LHCD, ECCD)

• Up to now, energy and current diffusion are modelled

• L mode edge plasmas


Consideration on current ramp transport modelling

3

• Choice of the transport model : scaling-based, empirical, 1st principles

– Li prediction and Flux consumption strongly depend on the Te at >0.5

– Model has to predict Te up to = 1 with L-mode edge

– Scaling-based transport model are

• a priori less sensitive to the assumptions on the boundary conditions (stiffness issue, drift wave models not accurate close to the edge)

• Have hopefully a correct dependence on Ip and machine size for extrapolation

• May miss several physical effects try to validate on extensive range of machines / heating schemes

– empirical models: Bohm/gyro-Bohm and Coppi-Tang have been tried as well

• Validation on existing experiments is essential

• Excellent opportunity for code-to-code benchmarking :

– Jetto and Cronos used in comparison with experimental data

– Astra, Jetto, Cronos used in ITER predictions


Database from JET, Tore Supra, AUG with various H&CD mix

4

• Current ramp-up for base-line (q95~3) & AT scenario (q95~5) :

Pulse H&CD scheme

JET 72823 LHCD+ ~1MW NBI for MSE&CXSJET 72818 ~1MW NBI for MSE &CXSJET 71828 OhmicJET 71827 OhmicJET 70497 OhmicTS 40676 ECCDTS 40679 LH+ECCD

• Current ramp-down from base-line scenario (2.4T/2.7MA, q95~3):

Pulse ramp-down additional heating

JET 72200 slow noneJET 72202 fast noneJET 72210 fast NBI heating until ramp-downJET 72241 fast NBI heating after ramp down (in low Ip phase)

TORE SUPRA

AUG 22110 (ohmic) just added last week

To be transferred to ITPA database }


Scaling-based model

5

• Scaling-based model: energy content of ohmic or heated Ip ramps with L-mode edge correctly modelled by either

– H mode scaling with H98 = 0.4 – 0.5

– L mode scaling with H97 = 0.6

• i = e, renormalised so that

• Fixed () shape : power balance chi’s during ramp-up tend to be rather flat, then strong increase towards the plasma edge : (,t) = A(t)(1+6 2 + 80 20)

• Boundary Te ( = 1) taken from experiment (guessed from ECE)

• Ne profile taken from experiment (JET: inversion of interferometry data)

• Flat Zeff assumed, <Zeff> taken from experiment (Bremsstrahlung)

thlossth WPHW 9898


Calibration on JET shot 70497 (constant q95 ~3 ohmic ramp-up)

6

• Model i = e, renormalised to IPB98 scaling, H98 = 0.5 (mimics L mode), radial shape 1+6 2 + 80 20 (adjusted to fit experimental Te profile peaking).

• Te is correctly reproduced.

• CRONOS simulation

t = 0.5 s

red - : Simulation

blue * : ECE

Purple * : Thomson scattering

t = 4 s t = 5 st = 2 s

2.6T/2.6MA, q95~3


• Model i = e, renormalised to IPB98 scaling, H98 = 0.5 (mimics L mode), radial shape 1+6 2 + 80 20 (adjusted to fit experimental Te profile peaking).

• Te is well reproduced for > 0.6. Even if larger deviations occur inside = 0.6, they almost do not affect the li evolution.

• Li slightly overestimated, li ~ 0.08, ~ measurement accuracy.

• Vloop good, slightly underestimated

Apply same model on another ohmic ramp for AT scenario (JET #72818)

7ITPA meeting Milan, October 2008, F. Imbeaux

t = 6 s

Simulation

ECE

Thomson scattering

3 6 5 4

time (s)

4 5 5.5 3.5 4.5

time (s)

2.7T/1.8MA, q95~5


Ohmic Ip Ramp-down (JET 72202)

8

• Both models follow equally well li and the volume averaged Te

Experimental

Bohm/gyro-Bohm*

Scaling, H98 = 0.5

JETTO

*without non-local Bohm multiplier

2.4T/2.7MA down to 1.0MA

14 16 18 20 22 24

time (s)

ISM Working Group 9 ITPA Naka meeting 2009 31st March 2009 9

• Comparison of the two models : some difference in Te profile peaking, that Bohm/gyro-Bohm provides a better agreement (for JET)

• A possible approach consists in combining: profile dependence as B/gB + scaling renormalisation for multi-machine capability ?

Ohmic Ip Ramp-down (JET 72202)

Experimental

Bohm/gyro-Bohm

Scaling, H98 = 0.5

JETTO


LHCD assisted ramp-up in AT scenario (JET 72823)

• Scaling base model, H98 = 0.4

• LHCD calculated during interpretative run

• Li slightly overestimated, li ~ 0.1

3 6 5 4

time (s)

3 6 5 4

time (s)

3 6 5 4 2

2.7T/1.8MA, q95~5


t = 3 s t = 4 s t = 5.5 s

Excellent fit of the volume averaged temperatures, both electron and ion

LHCD assisted ramp-up for AT scenario (JET 72823)

Simulation

ECE

Thomson Scattering 2.7T/1.8MA, q95~5


Ion temperatures from CXS


Simulation

* CXS

NBI blips (MSE & CXS) during current rise : • CXS & MSE measurements

t = 4 s t = 5 s

TI TI

2.7T/1.8MA, q95~5



t = 4.5 s

NBI blips (MSE & CXS) during current rise : • CXS & MSE measurements

t = 5.5 s

• Comparison to MSE q-profile (EFTM)

2.7T/1.8MA, q95~5


First attempts to test GLF23 in JET ramp-up phase

• When applied from the edge at high q (JET 71828, ohmic, 1 s after breakdown, 5 < q < 15 )

• Very low transport predicted near the edge ( = 1) barrier forms and non monotonic Te profiles appear

• When trying to patch the edge (impose = 10 m2/s from = 1 to = 0.75), the same problem appears at = 0.75

• Possibility to use GLF23 ? : on the whole radius ? At high q ?

Te

Ti

Normalised radius

(2.6T/2.6MA, q95~3 at flat top )

5.5sJET 77251JET 71828 t=1s

ExpAfter breakdown


Tore Supra ramp-up experiments

• Fast current ramp (0.7 MA/s), plateau Ip = 0.9 MA reached at t = 1 s

• off-axis co-ECCD (.7 MW) and/or LHCD (.8 MW) during ramp at t = 0.25s

• Same li evolution, but different Te evolution & time of first sawtooth

TS40679 : LH+ECCD

TORE SUPRA

TS40676 : ECCD

Blue : usual scaling-based model H98 = 0.5

Green / red: two experimental measurements of li

ISM Working Group 16 ITPA Naka meeting 2009 31st March 200916ISM Working group

1st set of simulation: ITER ramp with constant boundary shape

• L-mode transport model: (r) validated on JET & TS with H98 = 0.5.

• The edge temperature set to Tb=20*Ip[MA] eV, although with the chosen transport model Tb assumption is not that important.

• Use a formula for Zeff (used by the ITER team):

– Zeff = (1.7+2.3x(0.5/ne)2.6)), Carbon is main impurity

• The current ramp follows the ITER reference one with 15MA at 100s:

– 1.5MA t=4s, 3.0MA t=8s, 7.5MA t=30s, 13MA t=75s, and 15MA t=100s.

• The simulations start at 3MA (t=8s), with an initial condition for the q-profile (li=1) and a temperature profile.

• ITER divertor shape from Ip3MA: full bore plasma.

• densities <ne>/nGW = 0.15; 0.25; 0.4 (low to medium density).

• Use a variation of heating: Ohmic, 10MW and 20MW of ECRH with power deposition at mid-radius (heating only, no current drive).


Modelling of ITER ramp with various heating

& densities

17Working group D. Hogeweij et al, EPS 2008

<ne>/nGW = 0.25

<ne>/nGW = 0.4

<ne>/nGW = 0.15

10 MW ECRH <ne>/nGW = 0.25

20 MW ECRH <ne>/nGW = 0.25


Modelling of ITER ramp with various heating at <ne>/nGW = 0.15

18Working group

Te profile evolution. Hollow profiles achieved transiently with off-axis ECRH, still present at the end of the ramp

Te ITB may be obtained with such hollow q-profiles (not in the model)

D. Hogeweij et al, EPS 2008

ISM Working Group 19 ITPA Naka meeting 2009 31st March 200919ISM Working group

2nd set of simulation: ITER ramp with evolving boundary shape

ASTRA

CRONOS

JETTO

Tore Supra like

JET like


• Ohmic ITER ramp-up, time-dependent plasma boundary (preset), using the same model as before, H98 = 0.5

• Code comparison: rather close agreement between CRONOS and JETTO, in spite of the differences in the treatment of equilibrium, transport coefficient renormalisation, …

• Detailed code comparison starting from basic simulations going on between ASTRA and JETTO

Working group

2nd series: ITER with prescribed evolving boundary shape


• Up to now, mainly the scaling-based model was used, work being extended to empirical models like Bohm/gyro-Bohm and Coppi-Tang

– Test on multiple machine data : JET, Tore Supra and Asdex Upgrade

– Rather good agreement with experimental data (on Te, li and Vloop prediction) has been found in several cases. None of these models can be ruled out yet.

– Analysis still in progress, more detailed trends / observations to be given when the full database analysis will be completed

– Multiple transport codes working on the same dataset allow detecting bugs / sensitivity to unexpected parameters / assumptions

– Coppi-Tang implementation : still doubts on a few details (definitions of some quantities) need to have the same as in TSC

• Extend data base to other heating schemes, other machines (ITPA) should contribute to further validate the models

• Continue testing other models : – introduce more sophisticated radial dependence in scaling-based ? Try further

GLF23, through first attempt was quite unsuccessful

• Include free-boundary equilibrium calculations : try for the end of 2009

Perspectives


JET ohmic ramp-up 71828

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

42 43 44 45 46 47 48 49 50 51 520,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

Ave

rage

El.

Tem

p. [

keV

]

H98y~0.4, CRONOS L97y~1.0, JETTO L97y~0.6, CRONOS old Bohm/gyroBohm, JETTO H98y~0.5, JETTO Coppi-Tang, JETTO

The

rm.

El.

Ene

rgy

Con

tent

[M

J]

Time [s]

Black dots : experimental data from LIDAR (Thomson scattering)

• Electron energy content well fitted by H98 = 0.4 or H97 = 0.6

• Also by “old Bohm/gyro-Bohm model”, i.e. without the edge “H-mode” factor. However, can we expect that it would work so well on other tokamaks ?

• Coppi-Tang model not accurate

G. Sips et al, EPS 2008


First attempts to test GLF23 in JET ramp-up phase

• When applied from the edge at high q (JET 71828, ohmic, 1 s after breakdown, 5 < q < 15 )

• Very low transport predicted near the edge ( = 1) barrier forms and non monotonic Te profiles appear

• When trying to patch the edge (impose = 10 m2/s from = 1 to = 0.75), the same problem appears at = 0.75

• Possibility to use GLF23 ? : on the whole radius ? At high q ?

Te

Ti

Normalised radius

(2.6T/2.6MA, q95~3 at flat top )

transport simulation of current ramp-up & ramp-down by f. imbeaux* presented by x. litaudon*

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