Hobirk, J.

Hogeweij, G.

Imbeaux, F.

Köchl, F.

Litaudon, X.

Parail, V.

Sips, A.

English

MPG.PuRe

Simulation of the current ramp-up phase of ITER dischargesG.M.D. Hogeweij1, J. Hobirk2, F. Imbeaux3, F. Köchl4, X. Litaudon3, V. Parail5,A.C.C. Sips2 and contributors to the EFDA ITM-TF work programme1 FOM-Institute for Plasma Physics Rijnhuizen, Association EURATOM-FOM,Trilateral Euregio Cluster, P.O.Box 1207, Nieuwegein, The Netherlands, www.rijnhuizen.nl2 Max-Plank-Institut für Plasmaphysik, EURATOM-Assoziation, D-85748 Garching, Germany3 Association EURATOM-CEA, CEA/IRFM, CEA Cadarache, France4 Association EURATOM-AW/ATI, Vienna, Austria5 EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon OX14 3DB, UK1. Introduction. Routine operation of ITER requires a robust current ramp-upphase. Within the narrow operational window of ITER the plasma has to be built up,preferably at low cost in terms of flux consumption, such that at the end of the currentramp-up the temperature (Ti, Te), density (ne) and current density (j) profiles have theright shape to run the wanted scenario (H-mode, hybrid mode, etc.). To perform asystematic modelling of this early phase of ITER discharges, a dedicated effort was setup within the ITER Scenarios Modelling working group. This group was formed lastyear as part of the EU Integrated Tokamak Modelling (ITM) Task Force, which has aslong term aim to provide a European suite of software tools for ITER exploitation.The primary question to be resolved is: what are the most important actuators duringthe current rise phase (plasma shape, density, heating and current drive profile, etc.),and how can one achieve the profiles of q, Te, Ti, etc. that are needed to enter the variousITER scenarios.This paper discusses choices that have been made for the simulations (Sec.2), com-parison of the transport codes in use (Sec.3) and calibration of the empirical transportmodel used so far (Sec.4). Then first results of the simulations are reported (Sec.5),followed by an outlook of future work (Sec.6).2. Choices Based on results from existing experiments [1] and on ITER referencescenarios, following choices were made for the ITER current rise simulations.:• For the first results, reported here, the ITER divertor shape is assumed from thestart of the simulations. However, recently also simulations with a time-varyingplasma shape, i.e. starting from a much smaller plasma volume and interpolatingbetween a given time series of fixed boundary snapshots, were initiated.• Three groups of simulations have been carried out, using respectively ne/nGwe =0.15, 0.25 and 0.4. The ne profile is assumed flat.• The ITER reference current ramp-up scheme is followed with ramp rate decreasingfrom 0.35 to 0.1 MA/s during the Ip ramp-up phase; this is not the fastest ramppossible, to limit edge MHD and to give time for the non-inductive actuators toshape the q profile. The current flat-top (Ip = 15 MA) is reached after 100 s.• A variation of heating schemes is tested: Ohmic, off-axis ECRH/ECCD, LHCD,NBI; the results presented here only use Ohmic and off-axis ECRH.• For the time being an empirical L-mode heat transport model is used; its calibrationagainst JET start-up plasmas is reported in Sec.3. Later on the usage of theorybased transport models is foreseen.• Carbon is the only impurity and Zeff follows the empirical prescription Zeff =1.7 + 2.3(0.5/〈ne〉)2.6.35th EPS Conference on Plasma Phys. Hersonissos, 9 - 13 June 2008 ECA Vol.32D, P-5.034  (2008)Initial and edge conditions are based on experimental evidence from existing devices.The break down phase is not treated here; simulations start at 8 s when Ip = 3 MA.Current diffusion and heat transport are solved, the other quantities are prescribed.3. Comparison of Transport Codes The three main transport codes currently inuse in Europe (JETTO [2], ASTRA [3], and CRONOS [4]) have been compared underdifferent conditions. Figures 1 and 2 summarize this comparison for a reference case:ne/nGwe = 0.25, no additional heating, fixed plasma shape. Various time traces areplotted in Fig.1; the profiles of q, Te and Ti at various time slices are plotted in Fig.2.Figure 1: Comparison of CRONOS (red), JETTO (green)and ASTRA (blue) simulation of standard ITER ramp-upwithout addition heating. Time traces of global quantities:Ip and li (first panel, full/dashed lines); line averaged den-sity and Zeff (second panel, full/dashed); ohmic power andq(0) (3rd panel, full/dashed); Te(0) and Ti(0) (4th panel,full/dashed) and Wth (5th panel).Figure 2: Profiles of q (left) and Te(right, full lines) and Ti (right, dashedlines) for the same simulations and withthe same colour coding as in Fig.1. Theprofiles are shown at 8 s (start of the simu-lations), 15 , 40 and 100 s (end of ramp-upphase).Regarding global parameters (Fig.1) there is excellent agreement between JETTOand CRONOS, whereas ASTRA deviates somewhat. The initial q profiles deviatestrongly due to a different implementation of the initial plasma parameters; this dis-agreeable point is currently under investigation. The profiles deviate significantly duringthe first phase of the current rise; these differences tend to relax towards the end of thecurrent rise phase at 100 s. Therefore we will not address them further here, and we willuse both JETTO and CRONOS modelling results in the rest of the paper.4. Transport calibration In the modelling an empirical formula is used for the heat35th EPS 2008; G.M.D.Hogeweij et al. : Simulation of the current ramp-up phase of ITER discharges 2 of 4Figure 3: Comparison of predictive CRONOSsimulation of Ip ramp-up phase (blue) of two JETdischarges (see text) with experimental data (red).From top to bottom the panels show time traces ofIp, li and Vloop for JET pulses 71827 (left) and71828 (right).Figure 4: This plot shows profiles for the samecomparison as in Fig.3, and with the same colourcoding. Shown are profiles of q (dashed lines) andTe (full lines) 2, 5 and 8 s after break-down. Ex-perimental data are from the EFIT equilibrium re-construction (q) and ECE (Te).transport: χe(ρ) = χi(ρ) = f ∗ (1+6ρ2 +80ρ20). where f is dynamically adapted duringthe run such that the energy content of the plasma (Wth) satisfies Wth = Hτ98E (Ploss −dWth/dt) where H = 0.5 and τ98E is given by the ITER-H98(y,2) H-mode scaling [5]. Thisformula has been calibrated against ohmic JET pulses with ITER-like Ip ramp-up phase.Figures 3 and 4 show the results of this calibration for two JET pulses with differentIp ramp rate. Both the current penetration (li time trace) and the heat transport (Teprofiles) are very well reproduced by this model.5. ITER simulations Two important questions to be addressed are: (i) How muchheating power and current drive is needed and where should it be deposited in order tokeep li below 1 (required by the available flux of the transformer), and to reach the targetq profiles for the various scenarios? (ii) How can one minimize the flux consumption?All three transport codes have been used; here only results from CRONOS will beshown. In Figures 5 and 6 we present the results of two scans: (a) an ECH power scanfrom 0 till 20 MW; the power is assumed to be localized at mid-radius with a narrowGaussian deposition profile without current drive; (b) a ne scan from 0.15 till 0.4nG;Regarding (i): It is seen that the q profile at the start of the flat-top is flat withOhmic heating, and is hollow with 10 MW of off-axis ECH; moreover, off-axis heating iscrucial to keep li below 1. Note that the heat transport model used in the simulationshas been tested against ohmic experiments only. Being non-stiff, it may amplify theeffect of strongly localized heating methods like ECRH.35th EPS 2008; G.M.D.Hogeweij et al. : Simulation of the current ramp-up phase of ITER discharges 3 of 4Figure 5: Effect of varying input power and den-sity during ITER Ip ramp-up. The reference simu-lation (full red lines) has 〈ne〉 = 0.25 ∗ nG, withoutadditional input power. Shown are simulations: (a)with 10 and 20 MW of ECRH power at mid-radiusstarting at 30 s, at the same density (green and blue,respectively); with 〈ne〉 = 0.15 and 0.4 ∗ nG withoutadditional input power (dashed, dotted red lines, re-spectively). The panels from top to bottom give 〈ne〉,Pecrh, li, q(0), Vloop and the flux consumption.Figure 6: Final profiles of q (left) and Te, Ti (right,full/dashed lines) after 100 s for the power scan ofFigure 5, with the same colour coding as in Fig.5.Regarding (ii): Both lowering the density and applying additional heating lead to aflux saving of 10-20 Wb, which is significant; with a typical loop voltage during the Ipflat-top phase of 0.05 V this would imply an enhancement of this phase by 200-400 s.6. Discussion and Outlook The ITER start-up modelling effort so far has alreadyyielded some important results, in particular regarding the necessity of off-axis heatingalready early during the ramp-up phase.In the near future, in order to be more realistic, these simulations will be repeatedstarting earlier in the Ip ramp-up phase and addressing the variation of the plasmaboundary during the ramp. Moreover, first principle based transport models will beexploited. Finally, other heating and current drive schemes will be used, with morerealistic parameters regarding power deposition and current drive profile.References[1] A.C.C. Sips et al, this conference [2] G. Cenacchi et al, Report JET-IR(88) 03[3] G.V. Pereverzev et al, ASTRA: Automated System for Transport Analysis in a Tokamak,Max-Planck Institute Report, IPP 5/98[4] V. Basiuk et al, Nuc.Fus. 43 (2003) 822 [5] ITER Physics Basis, Nuc.Fus. 39 (1999) 1975Acknowledgements. This work, supported by the European Communities under thecontract of Association between EURATOM/FOM, was carried out within the framework ofthe European Fusion Programme with financial support from NWO. The views and opinionsexpressed herein do not necessarily reflect those of the European Commission.35th EPS 2008; G.M.D.Hogeweij et al. : Simulation of the current ramp-up phase of ITER discharges 4 of 4

Simulation of the current ramp-up phase of ITER discharges

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Simulation of the current ramp-up phase of ITER discharges

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