1. Y Andrew 1 (14) Transport and Confinement ITPA meeting 20-22 October 2008 H-mode Access on JET Y Andrew and JET EFDA Contributors UKAEA- Fusion Transport and Confinement ITPA meeting 20 – 22 October 2008

2. Y Andrew 2 (14) Transport and Confinement ITPA meeting 20-22 October 2008 Y Andrew 1, TM Biewer 2, K Crombe 3, D Keeling 1, E de la Luna 4, C Giroud 1, NC Hawkes 1, M Kempenaars 1, A Korotkov 1, DC McDonald 1, A Meigs 1, YR Martin 5 A Murari 6, I Nunes 7, R Sartori 8, T Tala 9 and JET EFDA contributors* JET-EFDA, Culham Science Centre, Abingdon, OX14 3DB, UK 1.Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK 2.Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 3.Department of Applied Physics, Ghent University, Belgium 4.Asociasion EURATOM-CIEMAT para Fusion, CIEMAT, Madrid, Spain 5.Ecole Polytechnique Fédéral de Lausanne (EPFL), Centre de Recherche en Physique des Plasmas, Association Euratom-Confederation Suisse, CH-1015 Lausanne, Switzerland 6.Consorzio RFX, ENEA-Euratom Association, Padua, Italy 7.Associação EURATOM/IST, Centro de Fusão Nuclear, Lisbon, Portugal 8.EFDA CSU, Boltzmannstrasse 2, 85748 Garching, Germany 9.Association EURATOM-Tekes, VTT, PO Box 1000, FIN-02044 VTT, Finland * See annex of ML Watkins et al., ‘Overview of JET Results’, (Proc. 21 st IAEA Fusion Energy Conference, Chengdu, China (2006)) Outline Low Density Power Threshold Behaviour P th density dependence with divertor geometry Hysteresis in H-mode power Power requirements for H 98 ~ 1 access Isotope dependence of P th TF Ripple ICRH versus NBI Summary and Conclusions

3. Y Andrew 3 (14) Transport and Confinement ITPA meeting 20-22 October 2008 Density scan experiments conducted on JET clearly demonstrate the existence of a minimum n e, below which the P th does not get any lower. Low density P th behaviour 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. Y Andrew 4 (14) Transport and Confinement ITPA meeting 20-22 October 2008 Density scan experiments repeated with septum replacement plate divertor P th dependence on n e linear across scan with SRP - turning point in P th could be shifted to lower n e or absent Values of pedestal T e very similar for two divertors, indicating pedestal to be uninfluenced by removal of septum Shift in JET P th turning point

5. Y Andrew 5 (14) Transport and Confinement ITPA meeting 20-22 October 2008 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 n e with either NBI only or ICRH only, to examine the influence of heating method on P th. 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. Y Andrew 6 (14) Transport and Confinement ITPA meeting 20-22 October 2008 Density dependence of P th n e dependence of P th plays an important role in the predictions for ITER Difference in the n e dependence of P th 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 n e dependence of P th has been studied on JET

7. Y Andrew 7 (14) Transport and Confinement ITPA meeting 20-22 October 2008 Dependence of P th on edge varies with divertor configuration: P th  0.12(+/-0.04) -----  u /  l = 0.43/0.43 P th  0.82 (+/-0.04) -----  u /  l = 0.43/0.33 P th  1.26 (+/-0.04) ------  u  l = 0.23/0.23 Pedestal T e shows little variation across n e scan T i shows some variation with configuration, lower values of T i for increased  Variation in n e dependence may account for some of the variation in P th in the international H-mode threshold DB P th with different divertor configurations Andrew Y et al., PPCF October (2008)

8. Y Andrew 8 (14) Transport and Confinement ITPA meeting 20-22 October 2008 Extension of earlier work on JET which demonstrated the X-point height to have a significant effect on P th H-mode access P th 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. 26 th 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. Lowering of P th with X-point height

9. Y Andrew 9 (14) Transport and Confinement ITPA meeting 20-22 October 2008 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 P th 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 First JET H-mode hysteresis experiments

10. Y Andrew 10 (14) Transport and Confinement ITPA meeting 20-22 October 2008 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 Recent JET hysteresis experiments Andrew Y et al., PPCF October (2008)

11. Y Andrew 11 (14) Transport and Confinement ITPA meeting 20-22 October 2008 Experiment performed on JET to study the power requirements relative to experimental value of L-H transition P th for H 98 = 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 H 98 = 0.8 for H-mode with lowest power (8.6 MW) in scan Power requirements for H 98 = 1 Input power step up

12. Y Andrew 12 (14) Transport and Confinement ITPA meeting 20-22 October 2008 Values of total P in for Type-III and Type-I ELMy phases are plotted along with measured P th for L- H transition P in > 1.5P th for H 98 = 1.0, Type-I ELMS P in = P th is insufficient for H 98 = 1 access R Sartori et al., PPCF 46 (2004) 723 – 750. In JET Type-III ELMy H-modes H 98 is lower than for Type-I ELMy H-modes over the entire density range. Andrew Y et al., PPCF October (2008)

13. Y Andrew 13 (14) Transport and Confinement ITPA meeting 20-22 October 2008 E Righi et al., Nuclear Fusion, 39 3 (1999) 309-319. DC McDonald et al., PPCF 46 (2004) 519-534. 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. P th scaling for helium has similar n e and B t scaling as hydrogenic plasmas P th for helium plasmas was found to be ~42 % higher than deuterium plasmas –P th ~ 73 MW at n e = 0.5x10 20 m -3 in helium P th for hydrogen plasmas was found to be 2 x helium equivalent - P th ~ 152 MW at n e = 0.5x10 20 m -3 in hydrogen Low activation phase on ITER

14. Y Andrew 14 (14) Transport and Confinement ITPA meeting 20-22 October 2008 v  is in counter-current direction at  Bt = 1.1 % Direction of edge plasma v  does not appear to influence L-H transition P th on JET  B t =1.1% has no influence on P th  B t = 1.1% has no influence on P th