1. RFX Programme Meeting Feb-2010 Influences of main gas choice on RFP plasmas RFX-mod Programme Planning Workshop, 9 Feb 2011 M Valisa with contributions of F Auriemma ad M Spolaore

2. RFX Programme Meeting Feb-2010 In Tokamaks various beneficial isotope effects (IE) and also certain advantages using He In RFP some isotopic/mass effect have also been documented. Can we conceive experiments to improve the RFP performance based on our knowledge on IE /mass effects? Motivation

3. RFX Programme Meeting Feb-2010 This is a collection of thoughts and results that is far from being complete Rather, this is an opportunity to trigger once more the discussion in RFP’s The isotope effect has been and still is an important subject in Tokamak and Stellarator research and a vast literature exists on the subject. Note

4. RFX Programme Meeting Feb-2010 - Confinement / Heating / Turbulence - LH transition - QSH physics - Density Limit - Plasma wall interaction Outline

5. RFX Programme Meeting Feb-2010 Confinement / Turbulence

6. RFX Programme Meeting Feb-2010  In JET Te and Ti at the top of the edge pedestal increase systematically as the isotope mass in increased (J. G. Cordey, Report No. JET-P (98)53, 1998). -With type I ELMs, the edge pedestal - pressure increases with the ion mass of the plasma ( Saibene NF 39 (1999),1133). IE in Tokamaks: Confinement improvement E. Righi et al NF 39 (1999) 309 JET

7. RFX Programme Meeting Feb-2010 IE in Tokamaks: ELM’s E. Righi et al NF 39 (1999) 309 Isotope Effect on ELM frequency JET

8. RFX Programme Meeting Feb-2010 In AUG “ deuterium plasmas always surpass hydrogen plasmas in all respects: in energy, particle and momentum confinement. “ BESSENRODT-WEBERPALS WAGNER et al. NF 33 (1993) 1205 With D : - more peaked and Te profiles - higher Te - more impurity production = more edge radiation - higher momentum confinement - easier L-H transition In He 20-30% lower confinement wrt D (because of lower density due to Z=2). IE in Tokamaks: Confinement improvement AUG

9. RFX Programme Meeting Feb-2010  Asdex : IE in Tokamaks: Confinement improvement BESSENRODT-WEBERPALS et al. NF 33 (1993) 1205 AUG

10. RFX Programme Meeting Feb-2010  TFTR Global  E thermal Increased,  I Decreased in Core of DT Supershots Compared to D (R. Haryluk,, 2007) IE in Tokamaks: Confinement improvement S. Scott, M. Zarnstorff

11. RFX Programme Meeting Feb-2010 L Rodrıguez-Rodrigo and F Castejon Plasma Phys. Control. Fusion 38 (1996) 681 Fluctuations n TJ-I Tokamak Fluctuations from Hard X-Rays hygher in Hydrogen runaway electron kinetic effects induced by magnetic turbulence H D H D

12. RFX Programme Meeting Feb-2010 IE and Zonal Flows ZF= self-generated shear flows out of a turbulent sea In helical plasmas, an equilibrium-scale Er ( driving poloidal rotation of trapped particles= reduced radial displacement of orbits) enhances the ZF response. An isotope effect occurs if Ti and Er are the same for all the different ion masses. (This happens to be the case for neoclassical transport in helical plasmas). Hence neoclasical transport generates an equiibrium Er, which enhances ZF response that is larger for D than for H. Why confinement improves with deuterium not yet fully clarified. T H Watanabe et al FEC Daejaeon 2010 THC/6-1

13. RFX Programme Meeting Feb-2010 RFX density profile RR and Harvey theory: Particle diffusivity at the edge Evidences of stochastic transport in the plasma core: isotope effect fits with RR&H theory RFX ( MH regime) H vs He: effect on transport: At the edge, in high collisionality regimes, ( transport driven by electrostatic fluctuations) : D H /D He ~2 Effect disappears when n < 610 19 m −3 H He Lorenzini, Auriemma, Canton, Carraro PoP 2006

14. RFX Programme Meeting Feb-2010 RFX: H and He: effects on the egde momentum Spolaore et al JNM 2001 Different flow profile at the edge. With He less shear at the edge.

15. RFX Programme Meeting Feb-2010 In RFP: weak Impact of a Stochastic Magnetic Field on Fast-Ion Confinement Fiksel et al PRL 2005 Magnetic field perturbations and passing fast ions can satisfy resonance conditions unlike thermal particles Gobbin et al Nucl. Fusion 48 (2008) Fast ion confinement in RFP (MST) Possibility of exploiting neutron diagnostics in D plasmas Even with small D2 doping Diagnostic NBI would help. 1ms 20 ms Isotopic effects??

16. RFX Programme Meeting Feb-2010 MST : momentum confinement time somewhat larger in D2 plasma.- based on a beam-based measurement Customary explanation: the lower central neutral density in D2 plasmas results in lower drag on the plasma rotation and on reduced loss of momentum due to CX. - B.E. Chapman et al., Phys. Plasmas, Vol. 11, p. 2169 (2004). MST : in He discharges plasma and modes rotate faster than in D2 ( D Den Hartog pc – only few He experiments due to high sputtering) This effect was searched in RFX but never replicated (neither in RFX nor in RFX-mod. Possible explanation. In RFX mod modes rotation is controlled by feedback system RFP’s Mass effect on rotation

17. RFX Programme Meeting Feb-2010 Fiksel et al PRL 103, 145002 (2009) Ion heating in MST Non collisional ion heating increases approximately as the square root of the ion mass. A simple model based on stochastic ion heating consistent with data: A large cross-field radial transport of ions – caused by magnetic stochasticity - through a strong fluctuating radial electric field causes fast random changes in the ion perpendicular EXB drift velocity, which results in perpendicular heating.

18. RFX Programme Meeting Feb-2010 QSH physics

19. RFX Programme Meeting Feb-2010 H and He RFX-mod QSH and plasma mass/charge No major differences in QSH persistency or duration in H and He plasmas

20. RFX Programme Meeting Feb-2010 Selezionati F e Ip H and He Neither with a selection in current RFX-mod QSH and plasma mass/charge

21. RFX Programme Meeting Feb-2010 Selezionati F e n/ngw H and He Nor with a selection in n/nG RFX-mod QSH and plasma mass/charge Is the QSH physics dominated by electron channel ??

22. RFX Programme Meeting Feb-2010 LH Transition in Tokamak

23. RFX Programme Meeting Feb-2010  IE in Tokamaks: L-H power theshold He plasmas have significantly higher P-TH than D at low densities for all heating schemes, but similar P-TH as D at high densities except for H- neutral beam injected heated discharges, which are still higher. Gohil FEC 2010 Daejeon DIII-D Complex dependence of LH transition on main plasma ions, but in general PTh_H> PTh_He> PTh_D and PTh has a minimum when plotted vs density

24. RFX Programme Meeting Feb-2010 NSTX IE in Tokamaks: L-H power theshold  NSTX the L-H threshold power for He is 20 to 40% greater than that for D Li wall conditioning can ease access to the H-mode significantly. Potential complication with utilizing an ELM suppression system which applies low-n magnetic perturbations to the plasma edge in a preventative mode prior to the transition, as this can result in at least a 50% increase in P-Th Kaye FEC 2010 EXC/2-3Rb

25. RFX Programme Meeting Feb-2010 IE in Tokamaks: L-H power theshold JET

26. RFX Programme Meeting Feb-2010 Density Limit

27. RFX Programme Meeting Feb-2010 Greenwald PPCF 2002 ASDEX, NSTX and other machines have reported significantly higher density limits for He discharges compared to D, particularly at low current The hydrogen ion isotope is not found to be particularly important for the density limit. Density limit in Tokamak

28. RFX Programme Meeting Feb-2010 Density limit in RFX, higher in He than in H H H + pelletHe In MST: limit slightly overcome in D. No experience with He. (B Chapman, p.c.)

29. RFX Programme Meeting Feb-2010 Plasma Wall Interaction

30. RFX Programme Meeting Feb-2010 PWI: erosion yield Very complex processes that depend on ion energies, experiment history Wall materials, deposited layers etc. Deuterium produces larger physical Sputtering

31. RFX Programme Meeting Feb-2010 PWI : H vs He For low energies ( <200 eV) Strong Chemical erosion for H

32. RFX Programme Meeting Feb-2010 PWI : H vs He H induced erosion mainly chemical ; DEPENDS ON SURFACE TEMPERATURE He induced erosion is physical DOES NOT DEPEND ON SURFACE TEMP.  He plasmas more reproducible

33. RFX Programme Meeting Feb-2010 Edge gas pressure In NSTX lower edge gas pressue with Li Conditioning Gas pressure dependence on mass of main fuel? If so. Consequences on - Momentum losses - Energy losses - Beam reionization before beam enters the plasma

34. RFX Programme Meeting Feb-2010 Summary Deuterium features - higher confinement ( particle, energy and momentum) in Tok - favourable PTh for L-H transition in Tok - also higher sputtering,but associated to higher radiation - higher Ti than H after reconnection events - Extended diagnostic capabilities ( neutrons) - - Possible IE on Zonal Flows and turbulence regulation in helical plasmas He -more favourable PTh for L-H transition than H in Tok -- better reproducibility Shoudl RFX_TOK work in He to search for H-mode? - lower confinement in RFX, apparently, no major impact on QSH physics Effects on mode control and RMP (through viscosity) ?