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Modelling of ITER-like current ramps in JET with ILW: lessons for ITER regarding H-mode and Li control

This study analyzes and models the ITER-like current ramp experiments in JET with the ITER-like wall (ILW), comparing them to previous experiments with a carbon wall. The results provide insights into H-mode and Li control in ITER and highlight the differences between the two wall configurations.

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Modelling of ITER-like current ramps in JET with ILW: lessons for ITER regarding H-mode and Li control

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  1. Modelling of ITER-like current ramps in JET with ILW: lessons for ITER regarding H-mode and li control • G.M.D. Hogeweij1, G. Calabrò2, C.F. Maggi3,4, N. Hawkes4, E Joffrin 5, • Loarte6, A.C.C. Sips4,7, F.G. Rimini3, E. Barbato1, M. Beurskens4, • M. Brix4, G. Cunningham4, E. Delabie1,4, I Carvalho8, • G.M. De Tommasi9, D. Frigione2, F. Maviglia4,9, J. Mlynar10, • Th. Pütterich3, E. Solano11, C. Sozzi12, I. Voitsekhovitch4, JET EFDA Contributors * and ITM-TF ITER Scenario Modelling group • JET-EFDA, Culham Science Centre, Abingdon, OX14 3DB, UK • 1FOM Institute DIFFER, Ass. EURATOM-FOM, Nieuwegein, The Netherlands 7European Commission, Brussels, B-1049, Belgium • 2Associazione Euratom-ENEA, Via Enrico Fermi 45, 0044 Frascati, Italy 8Associacao EURATOM-IST, Lisboa, Portugal • 3Max-Planck-Institut für Plasmaphysik, Garching, Germany 10 Ass. EURATOM-ENEA-CREATE, Università di Napoli Federico II, 80125 Napoli, Italy • 4EURATOM/CCFE Fusion Association, Culham Science Centre, UK 10 Association EURATOM-IPP.CR, Prague, Czech Republic • 5Association Euratom-CEA, IRFM, F-13108 St-Paul-Lez-Durance, France 11Asociacion EURATOM-CIEMAT, Madrid, Spain • 6ITER Organization, 13115 St Paul Lez Durance – France 12Istituto di Fisica del Plasma ‘P.Caldirola’, Associazione Euratom-ENEA-CNR, Milano, Italy • * See the Appendix of F. Romanelli et al., Proc. 24th IAEA FEC, San. Diego 2012 • 14th International Workshop on H-mode Physics and Transport Barriers, • Fukuoka, Japan, 2-4 October 2013 – JET rehearsal 18 September 2013

  2. MOTIVATION and SUMMARY li(3) • MOTIVATION • Due to the limitations of the PF-system of ITER, only limited range of internal • inductance (li) allowed • Also need as low as possible flux consumption (FC), to keep sufficient for long flat-top • Past ITER-like RU and RD experiments were done with C wall, • so cannot directly be extrapolated to ITER •  need to repeat these experiments with ILW • SUMMARY of ILW RU in JET • Regarding li: same range with C-wall and ILW • With H-mode li slightly lower with ILW • However, when looking more deeply, many differences between C-wall and ILW RU and RD •  this poster (analysis & modelling)

  3. INTRODUCTION • INTRODUCTION - Experiments • Dedicated ITER-like Ramp-up (RU) and Ramp-down (RD) experiments have been performed in the past with the Carbon wall [Sips NF 2009, Nunes EPS 2011] , showing that flux consumption and li could be controlled within ITER requirements. • Since the ITER-like wall (ILW) in JET came into operation, such experiments have been repeated and matched well with similar discharges with the Carbon wall. • Following parameters were varied in the experiments: input power (including L-H and H-L transitions during RU/RD), density, Ip ramp rate, elongation [Calabrò EPS 2013].

  4. INTRODUCTION • INTRODUCTION – Modelling • Interpretative modelling: • Check consistency of data • Disentangleeffects of different parameters, e.g. what is role of Zeff in lievolution • Predictive modelling: • [Differences in thermal transport C-wall ILW pulses? – not in this poster] • Role of W influx (peaked / flat W profile) in li , flux consumption, q profileevolution • Extrapolation to ITER • Models used: • Transport model: semi-empiricalBohm-gyrobohm[Erba NF 1998] • (note: firstprinciples models do notworkwell in ohmic/L-mode RU) • Radiation model, usingdetailed atomic physics [Pütterich NF 2010] • (results slightly changed compared to simple average ion model [Post 1977] ) • All modelling was done with the CRONOS suite of codes [Artaud NF 2010]

  5. INTRODUCTION • INTRODUCTION – this poster • This poster: analysis & modelling of ITER-like current ramp experiments at JET, in particular: • What are the differences with the C-wall pulses; • How to control flux consumption and li ; • Effect of W on evolution of electron temperature (Te), safety factor (q), flux consumption and li • Consequences for ITER • Only few Ramp-Down experiments were done with ILW, therefore this poster concentrates on Ramp-Up

  6. COMPARISON BETWEEN RAMP-UPWITH C-WALL AND WITH ILW COMPARISON BETWEEN RAMP-UP WITH C-WALL AND WITH ILW a. First second after break-down Within 0.5 s after break-down (BD) the ILW pulses show, compared to similar pulses with C-wall, more peaked Te and higher li with ILW. Likely explanation: with ILW a more aggressive BD was needed, i.e. higher loop voltage, hence faster initial current rise, inducing n=1 MHD, causing fast inward current and heat transport Te profiles from ECE at 41.4 s (~1 s after break-down) for C-wall pulse 72723 (blue) and ILW pulse 83223, red ) Comparison of initial phase of ohmic RU: C-wall (JPN 72723, blue) vs. ILW (JPN 83223, red ): from top to bottom Vloop , Ip, n=1 MHD signal, and li

  7. COMPARISON BETWEEN RAMP-UPWITH C-WALL AND WITH ILW b. Ohmic RU Good match was obtained regarding dIp/dt, Te(0) and line averaged ne. Ip, line averaged ne, Te (0) and Te peaking (derived from ECE profiles) for ohmic RU, JPN 72723 (blue, C) and 83223 (red, ILW), showing good match.

  8. COMPARISON BETWEEN RAMP-UPWITH C-WALL AND WITH ILW • b. Ohmic RU (ctd) • Main changes with ILW: • lower Zeff (main imp. Be in lower • concentration than main imp. C in C-wall) • <Te> is a bit lower • The two effects on current diffusion and flux consumption nearly cancel. Zeff , li, flux consumption and <Te> for the same pulses. Note: here and in following plots the dashed lines in the 2nd and 3rd frames are from interpretative modelling due to the poor quality of the data in the first few seconds, the modelled li deviates there from the measured value.

  9. COMPARISON BETWEEN RAMP-UPWITH C-WALL AND WITH ILW • c. H-mode • L-H threshold much reduced with ILW - not discussed here, ref to [Maggi EPS2012] • Such high power could not be delivered in the ramp-up experiments reported here •  no good match with C-wall pulses • Will compare ILW H-mode ramp-up case (JPN 83446) with two C-wall pulses: • one L-mode with similar power (72516) • one H-mode with ~ double input power (72512) • Density was not well controlled in the ILW pulse, hence <Te> was lower. Ip, NBI power, line averaged ne, Te (0) and Te peaking for heated RU, JPN 72512 (blue, H-mode, C), 72516 (green, L-mode, C) and 83446 (red, H-mode, ILW).

  10. COMPARISON BETWEEN RAMP-UPWITH C-WALL AND WITH ILW • c. H-mode (ctd) • With ILW only half the power was needed to get the same li reduction as with C-wall • During H-mode there is more core radiation due to W  further on this in section 5. Zeff , li, flux consumption, <Te> and <pe> for the same pulses: Left comparison ILW (red) with C-wall H-mode (blue), right with C-wall L-mode (green) To be added for full journal paper: more complete comparison, including ELM type and frequency

  11. COMPARISON BETWEEN RAMP-UPWITH C-WALL AND WITH ILW c. H-mode (ctd) Modelling shows that most of the differences in li evolution and flux consumption are due to the lower Zeff with ILW. Effect of replacing low ILW Zeff by high C-wall Zeff . Shown are modelled li and flux consumption for JPN 72516 (blue), 83446 (red) and 83446 with Zeff of 72516 (magenta).

  12. li CONTROL AND FLUX CONSUMPTION REDUCTION IN RU and RD • H-mode in RU and RD • li during RU can be controlled and flux consumption reduced by early transition • to H-mode. • Similarly sustained H-mode during RD keeps li and flux consumption low. Effect of H-mode during RU (left) and RD (right) on li and flux consumption. left: JPN 83223/83224 (blue/red); right: JPN 83224/83225 (blue/red); In lower panels both exp. data (full lines) en data from modelling (dashed) are plotted. In panel b time 0 indicates start of RD.

  13. li CONTROL AND FLUX CONSUMPTION REDUCTION IN RU and RD b. elongation reduction in ohmic RD In fast ohmic RD elongation reduction prevents uncontrolled rise of li; important for ITER when in an emergency case fast current termination is needed and additional heating is unavailable Effect of fast elongation reduction on li during fast ohmic RD: JPN 83449 (blue) and 83447 (red) . In lower panel both exp. data (full lines) en data from modelling (dashed) are plotted. Time 0 indicates start of RD.

  14. ROLE OF W IN ILW RU • a. Experimental observations • After H-mode transition increased core radiation NBI power and radiation time traces for ohmic (JPN 83223, blue) and H-mode (83224, red) RU. Lower panels: SXR emissivity profiles PNBI PRAD,bulk Note: signal in ohmic case is too low for analysis of W concentration

  15. ROLE OF W IN ILW RU • a. Experimental observations (ctd) • Peaked W concentration in H-mode • However, sawteeth flatten W-profile Upper panel left: SXR radiation of JPN 83224 around ST collapse, total (magenta), inside r=0.4/0.15 (orange/red). Lower panel left: W concentration at r=0 / 0.15 / 0.45 (red, blue, orange). Right panels: emissivity and W profiles around ST crash Note: this figure will be replaced by one plot, overlaying the curves of both pulses

  16. ROLE OF W IN ILW RU • Experimental observations (ctd) • Few cases of “W-event”: a sudden influx of W, causing increased core radiation, reduction of core Te , end of sawteeth and hence even more peaked W profile. • After such event, the plasma stays in regime with peaked W and hollow Te profile. Left panels: SXR radiation and W concentration of JPN 83224; right panels: same for JPN 83444, showing W-event around 45.5 s. Note: this figure will be replaced by one plot, overlaying the curves of both pulses

  17. ROLE OF W IN ILW RU • Experimental observations (ctd) IP Surprisingly, such “W-event” hardly affects li evolution and flux consumption. Same PNBI Prad,bulk Increase in Prad,bulk W event occurs in 83444 at 45.4 sec Te(0) ne Sawteeth (ST) die out in 83444 after W event occurs  strong decrease in Te(0) li(3) 83224 (red), 83444 (blue)

  18. ROLE OF W IN ILW RU b. Modelling of effect of W on profile evolution Predictive modelling was performed to assess the critical W density, above which the sawteeth disappear. This was done both for ohmic RU (not shown here) and during H-mode RU, both assuming flat and peaked nW/ne profiles. The W-scan in H-mode was based on JPN 83224, i.e. NBI switched on at 45 s. A low W density was assumed in the ohmic phase; in the H-mode phase higher densities were assumed. Modelling was done with CRONOS suite of codes [Artaud NF 2010] , starting at 41.5 s with initial conditions from experiment; Density was taken from experiment, and evolution of j, Te and Ti was simulated, using Bohm-gyroBohm model; For reference run Zeff was taken from experiment (flat profile), assuming Be the only impurity; then W was added in increasing concentrations, thus increasing Zeff slightly

  19. ROLE OF W IN ILW RU b. Modelling of effect of W on profile evolution – assuming flat nW/ne Left: time traces of predictive modelling of various W concentrations assuming flat nW/ne Shown from top to bottom: nW/ne in the centre, Te(0), q(0), li , flux consumption and Prad. Right: profiles of nW/ne, Te, q and Prad at 48 s for the same cases. Conclusion: Significant effect on flux consumption and onTe; However, hardly effect on q(0) and li

  20. ROLE OF W IN ILW RU b. Modelling of effect of W on profile evolution (ctd) Same plots as previous page, predictive modelling of various W concentrations , now assuming peaked nW/ne Also exp. time traces of Te(0) and li plotted: blue/green dashed lines for JPN 83224/83444. Conclusion: Significant effect on core q and Te; however, apart from most extreme case, hardly effect on flux consumption and li Note: central nW/ne of JPN 83444 is very close to green curve; this case indeed shows end of sawteeth and hollow Te without strong effect on flux consumption and li

  21. ROLE OF W IN ILW RU b. Modelling of effect of W on profile evolution (ctd) • Conclusions from modelling: • When flat nW/ne is assumed, increasing nW leads to lower Te but the q profile is not modified; • On the other hand with peaked nW/ne, the sawtooth disappear and the q profile is spoiled; • Note that the case with edge / central nW/ne is 0.2 / 1 10-4 (green dashed line) is just at the margin: it has a few sawteeth and then has q>1. • This critical nW/ne for the peaked case agrees very well with the experimental findings as given earlier (JPN 83224/83444). Modelling of the effect of W radiation was also done for ITER, with similar result (not shown here) - will be added in the full journal paper

  22. DISCUSSION AND OUTLOOK • Discussion and outlook: • Although limited parameter range was covered in ILW ITER-like RU/RD experiments • so far, useful insight for ITER was gained. • Regarding Ramp-up, some significant differences with C-wall: • lower Zeff • lower L-H threshold • in H-mode significant and peaked W-radiation • With H-mode RU and RD li can be kept within acceptable ITER-limits, and strong • reduction of flux consumption is obtained – consistent with modelling • Above a critical central W concentration the q and T evolution is seriously affected; • this is a sharp threshold, nW/ne(0) ~1.0 10-4 – consistent with modelling • Even with this high central W concentration, li and flux consumption are hardly • affected - consistent with modelling • Sawteeth appear crucial in the H-mode phase of RU to avoid deleterious W peaking • Also for ITER H-mode pulses, it might be essential to have a ST plasma; this might • put an extra constraint on the ITER operational space

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