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Transport simulation of current ramp-up & ramp-down by F. Imbeaux* presented by X. Litaudon*

Transport simulation of current ramp-up & ramp-down by F. Imbeaux* presented by X. Litaudon*. Association Euratom- cea. *.

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Transport simulation of current ramp-up & ramp-down by F. Imbeaux* presented by X. Litaudon*

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  1. Transport simulation of current ramp-up & ramp-down by F. Imbeaux* presented by X. Litaudon* AssociationEuratom-cea * 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. Voitsekhovitch On behalf of : JET-EFDA contributors, Tore Supra work programme, ITER Scenario Modelling group (ITM-TF)

  2. Modelling of current ramp • 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 2

  3. Consideration on current ramp transport modelling • Choice of the transport model : scaling-based, empirical, 1st principles • Li prediction and Flux consumption strongly depend on the Te at r >0.5 • Model has to predict Te up to r = 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 3

  4. Database from JET, Tore Supra, AUG with various H&CD mix TORE SUPRA • Current ramp-up for base-line (q95~3) & AT scenario (q95~5) : AUG 22110 (ohmic) just added last week } To be transferredto ITPA database • Current ramp-down from base-line scenario (2.4T/2.7MA, q95~3): • Pulse ramp-down additional heating • JET 72200 slow none • JET 72202 fast none • JET 72210 fast NBI heating until ramp-down • JET 72241 fast NBI heating after ramp down (in low Ip phase) 4

  5. Scaling-based model • 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 • ci = ce, renormalised so that • Fixed c(r) shape : power balance chi’s during ramp-up tend to be rather flat, then strong increase towards the plasma edge : c(r,t) = A(t)(1+6 r2 + 80 r20) • Boundary Te (r = 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) 5

  6. Calibration on JET shot 70497 (constant q95 ~3 ohmic ramp-up) • Model ci = ce, renormalised to IPB98 scaling, H98 = 0.5 (mimics L mode), radial shape 1+6 r2 + 80 r20 (adjusted to fit experimental Te profile peaking). • Te is correctly reproduced. • CRONOS simulation red - : Simulation blue * : ECE Purple * : Thomson scattering 2.6T/2.6MA, q95~3 t = 0.5 s t = 2 s t = 4 s t = 5 s 6

  7. Apply same model on another ohmic ramp for AT scenario (JET #72818) • Model ci = ce, renormalised to IPB98 scaling, H98 = 0.5 (mimics L mode), radial shape 1+6 r2 + 80 r20 (adjusted to fit experimental Te profile peaking). • Te is well reproduced for r > 0.6. Even if larger deviations occur inside r = 0.6, they almost do not affect the li evolution. • Li slightly overestimated, Dli ~ 0.08, ~ measurement accuracy. • Vloop good, slightly underestimated 2.7T/1.8MA, q95~5 t = 6 s Simulation ECE Thomson scattering time (s) time (s) 3 4 5 6 3.5 4 4.5 5 5.5 7 ITPA meeting Milan, October 2008, F. Imbeaux

  8. Ohmic Ip Ramp-down (JET 72202) 2.4T/2.7MA down to 1.0MA JETTO Experimental Bohm/gyro-Bohm* Scaling, H98 = 0.5 • Both models follow equally well li and the volume averaged Te time (s) *without non-local Bohm multiplier 14 16 18 20 22 24 8

  9. Ohmic Ip Ramp-down (JET 72202) r • 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 ? JETTO Experimental Bohm/gyro-Bohm Scaling, H98 = 0.5 9 r

  10. LHCD assisted ramp-up in AT scenario(JET 72823) • Scaling base model, H98 = 0.4 • LHCD calculated during interpretative run • Li slightly overestimated, Dli ~ 0.1 2.7T/1.8MA, q95~5 3 4 5 6 2 time (s) time (s) 5 6 3 4 3 4 5 6

  11. LHCD assisted ramp-up for AT scenario(JET 72823) Excellent fit of the volume averaged temperatures, both electron and ion Simulation ECE Thomson Scattering 2.7T/1.8MA, q95~5 t = 3 s t = 4 s t = 5.5 s

  12. LHCD assisted ramp-up for AT scenario(JET 72823) • NBI blips (MSE & CXS) during current rise : • CXS & MSE measurements 2.7T/1.8MA, q95~5 Ion temperatures from CXS TI TI t = 4 s t = 5 s Simulation *CXS

  13. LHCD assisted ramp-up for AT scenario(JET 72823) • NBI blips (MSE & CXS) during current rise : • CXS & MSE measurements 2.7T/1.8MA, q95~5 t = 4.5 s t = 5.5 s • Comparison to MSE q-profile (EFTM)

  14. First attempts to test GLF23 in JET ramp-up phase Te Ti • 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 (r = 1)  barrier forms and non monotonic Te profiles appear (2.6T/2.6MA, q95~3 at flat top ) JET 77251 5.5s JET 71828 t=1s After breakdown Exp Normalised radius • When trying to patch the edge (impose c = 10 m2/s from r = 1 to r = 0.75), the same problem appears at r = 0.75 • Possibility to use GLF23 ? : on the whole radius ? At high q ?

  15. Tore Supra ramp-up experiments TORE SUPRA • 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 Blue : usual scaling-based model H98 = 0.5 Green / red: two experimental measurements of li TS40676 : ECCD TS40679 : LH+ECCD

  16. 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 Ip3MA: 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). 16 ISM Working group

  17. Modelling of ITER ramp with various heating & densities <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 17 Working group D. Hogeweij et al, EPS 2008

  18. Modelling of ITER ramp with various heating at <ne>/nGW = 0.15 D. Hogeweij et al, EPS 2008 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) 18 Working group

  19. 2nd set of simulation: ITER ramp with evolving boundary shape ASTRA CRONOS JETTO JET like Tore Supra like 19 ISM Working group

  20. 2nd series: ITER with prescribed evolving boundary shape • 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 20 Working group

  21. Perspectives • 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

  22. JET ohmic ramp-up 71828 • 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 Black dots : experimental data from LIDAR (Thomson scattering) G. Sips et al, EPS 2008

  23. First attempts to test GLF23 in JET ramp-up phase Te Ti • 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 (r = 1)  barrier forms and non monotonic Te profiles appear (2.6T/2.6MA, q95~3 at flat top ) Normalised radius • When trying to patch the edge (impose c = 10 m2/s from r = 1 to r = 0.75), the same problem appears at r = 0.75 • Possibility to use GLF23 ? : on the whole radius ? At high q ?

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