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H-mode Access on JET

H-mode Access on JET. Y Andrew and JET EFDA Contributors UKAEA- Fusion Transport and Confinement ITPA meeting 20 – 22 October 2008.

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H-mode Access on JET

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  1. H-mode Access on JET Y Andrew and JET EFDA Contributors UKAEA- Fusion Transport and Confinement ITPA meeting 20 – 22 October 2008

  2. Y Andrew1, TM Biewer2, K Crombe3, D Keeling1, E de la Luna4, C Giroud1, NC Hawkes1, M Kempenaars1, A Korotkov1, DC McDonald1, A Meigs1, YR Martin5 A Murari6, I Nunes7, R Sartori8, T Tala9and JET EFDA contributors* JET-EFDA, Culham Science Centre, Abingdon, OX14 3DB, UK • Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK • Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA • Department of Applied Physics, Ghent University, Belgium • Asociasion EURATOM-CIEMAT para Fusion, CIEMAT, Madrid, Spain • Ecole Polytechnique Fédéral de Lausanne (EPFL), Centre de Recherche en Physique des Plasmas, Association Euratom-Confederation Suisse, CH-1015 Lausanne, Switzerland • Consorzio RFX, ENEA-Euratom Association, Padua, Italy • Associação EURATOM/IST, Centro de Fusão Nuclear, Lisbon, Portugal • EFDA CSU, Boltzmannstrasse 2, 85748 Garching, Germany • Association EURATOM-Tekes, VTT, PO Box 1000, FIN-02044 VTT, Finland • * See annex of ML Watkins et al., ‘Overview of JET Results’, (Proc. 21st IAEA Fusion Energy Conference, Chengdu, China (2006)) • Outline • Low Density Power Threshold Behaviour • Pth density dependence with divertor geometry • Hysteresis in H-mode power • Power requirements for H98 ~ 1 access • Isotope dependence of Pth • TF Ripple • ICRH versus NBI • Summary and Conclusions

  3. Low density Pth behaviour • Density scan experiments conducted on JET clearly demonstrate the existence of a minimum ne, below which the Pth does not get any lower. Righi E., et al., PPCF 42 (2000) A199 – A204. Sartori R., et al., PPCF 46 (2004) 723-750. Andrew Y et al., PPCF 48 (2006) 479 – 488.

  4. Shift in JET Pth turning point • Density scan experiments repeated with septum replacement plate divertor • Pthdependence on ne linear across scan with SRP - turning point in Pth could be shifted to lower ne or absent • Values of pedestal Te very similar for two divertors, indicating pedestal to be uninfluenced by removal of septum

  5. ICRH versus NBI auxilliary heating Y Andrew et al., PPCF 48 (2006) 479-488. • At the lower end of the density scans H-modes were achieved using ICRH additional heating only. • Reference pulses were run at similar edge ne with either NBI only or ICRH only, to examine the influence of heating method on Pth. • Generally, the ICRH only heated plasmas had sawteeth which often triggered early, short lived H-modes as shown below. • The L-H transition in these cases is taken as the transition following which the plasma remains in H-mode. • No significant difference observed for H-mode access for the two heating methods.

  6. Density dependence of Pth • ne dependence of Pth plays an important role in the predictions for ITER • Difference in the ne dependence of Pth may determine to whether it is possible to increase plasma density and maintain H-mode with available heating power • Effect of changing the magnetic and divertor configuration on ne dependence of Pth has been studied on JET

  7. Pth with different divertor configurations • Dependence of Pth on edge <ne> varies with divertor configuration: • Pth <ne>0.12(+/-0.04) ----- du/dl = 0.43/0.43 • Pth  <ne>0.82 (+/-0.04)----- du/dl = 0.43/0.33 • Pth  <ne>1.26 (+/-0.04)------ du/dl = 0.23/0.23 • Pedestal Te shows little variation across ne scan • Ti shows some variation with configuration, lower values of Ti for increased d • Variation in ne dependence may account for some of the variation in Pth in the international H-mode threshold DB Andrew Y et al., PPCF October (2008)

  8. Lowering of Pth with X-point height • Extension of earlier work on JET which demonstrated the X-point height to have a significant effect on Pth • H-mode access Pth is reduced by a factor of two when X-point height is lowered with strike points on horizontal divertor targets • No difference between Septum and SRP divertors on JET Y Andrew et al., PPCF 46 (2004) A87-A93. LD Horton et al., ‘Proc. 26th EPS Conf. on Controlled Fusion and Plasma Phys. (Maastricht, 1999). LD Horton, PPCF 42 (2000) A37-A49. DC McDonald, PPCF 46 (2004) 519-534. H Meyer et al., PPCF 50 (2008) 1-9.

  9. First JET H-mode hysteresis experiments • Power step down experiments • NBI power level varied in three steps – H-mode is lost at a similar power to that which was required to achieve L-H transition. • Density ramp-up experiment • Density is increased and eventually H-mode disappears at a power level close to that of L-H Pth at the same density. ‘Energy confinement and H-mode power threshold scaling in JET with ITER dimensionless parameters’ The JET Team presented by JG Cordey, Proc. 16th IAEA International Conference on Fusion Energy, Montreal, Canada, October 1996, FUSION ENERGY 1996, Vol. 1, paper IAEA-CN-64/AP1-2, 603 - 610 (VIENNA 1997), JET report JET-P(96)51

  10. Recent JET hysteresis experiments • Detailed studies on the L-H and H-L transitions carried out on JET using slow power ramps up and back down • No evidence of hysteresis in power threshold on JET • ITER should not rely on hysteresis in power threshold to access H-mode under conditions of limited auxiliary power • Further studies planned on hysteresis in the H-mode power on JET in 2008 Andrew Y et al., PPCF October (2008)

  11. Power requirements for H98 = 1 • Experiment performed on JET to study the power requirements relative to experimental value of L-H transition Pth for H98 = 1 • Constant input power 6 s at approximately steady plasma density • Power varied in each shot to carefully scan H-mode phases • ELMs transition from Type-III/ELM-free H-mode to Type-I ELMs with increasing power • H98 = 0.8 for H-mode with lowest power (8.6 MW) in scan Input power step up

  12. Pin = Pth is insufficient for H98 = 1 access Andrew Y et al., PPCF October (2008) • Values of total Pin for Type-III and Type-I ELMy phases are plotted along with measured Pth for L-H transition • Pin > 1.5Pth for H98 = 1.0, Type-I ELMS R Sartori et al., PPCF 46 (2004) 723 – 750. • In JET Type-III ELMy H-modes H98 is lower than for Type-I ELMy H-modes over the entire density range.

  13. Low activation phase on ITER • Log-linear fit with atomic charge and mass, including hydrogen, deuterium, deuterium-tritium, tritium and helium data results in: • where M and Z are the mean isotope mass and charge, scaled to that of a proton. • Pth scaling for helium has similar ne and Bt scaling as hydrogenic plasmas • Pth for helium plasmas was found to be ~42 % higher than deuterium plasmas –Pth ~ 73 MW at ne = 0.5x1020 m-3 in helium • Pth for hydrogen plasmas was found to be 2 x helium equivalent - Pth~ 152 MW at ne = 0.5x1020 m-3 in hydrogen E Righi et al., Nuclear Fusion, 393 (1999) 309-319. DC McDonald et al., PPCF 46 (2004) 519-534.

  14. dBt=1.1% has no influence on Pth • vf is in counter-current direction at dBt = 1.1 % • Direction of edge plasma vf does not appear to influence L-H transition Pth on JET

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