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Influences of main gas choice on RFP plasmas

RFX-mod Programme Planning Workshop, 9 Feb 2011. Influences of main gas choice on RFP plasmas. M Valisa with contributions of F Auriemma ad M Spolaore. Motivation. In Tokamaks various beneficial isotope effects (IE) and also certain advantages using He

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Influences of main gas choice on RFP plasmas

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

  2. Motivation • 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?

  3. Note 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.

  4. Outline • - Confinement / Heating / Turbulence • - LH transition • QSH physics • - Density Limit • - Plasma wall interaction

  5. Confinement / Turbulence

  6. IE in Tokamaks: Confinement improvement • 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). JET E. Righi et al NF 39 (1999) 309

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

  8. IE in Tokamaks: Confinement improvement • 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). AUG

  9. IE in Tokamaks: Confinement improvement • Asdex : AUG BESSENRODT-WEBERPALS et al. NF 33 (1993) 1205

  10. IE in Tokamaks: Confinement improvement • TFTR Global tE thermal Increased, cI Decreased in Core of • DT Supershots Compared to D (R. Haryluk,, 2007) S. Scott, M. Zarnstorff

  11. 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 L Rodrıguez-Rodrigo and F Castejon Plasma Phys. Control. Fusion 38 (1996) 681

  12. IE and Zonal Flows Why confinement improves with deuterium not yet fully clarified. 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. T H Watanabe et al FEC Daejaeon 2010 THC/6-1

  13. RFX ( MH regime) H vs He: effect on transport: • Lorenzini, Auriemma, Canton, Carraro PoP 2006 Particle diffusivity at the edge • At the edge, in high collisionality regimes, ( transport driven by electrostatic fluctuations) : DH/DHe~2 • Effect disappears when n < 61019 m−3 H RR and Harvey theory: He RFX density profile • Evidences of stochastic transport in the plasma core: isotope effect fits with RR&H theory

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

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

  16. RFP’s Mass effect on rotation 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

  17. Ion heating in MST Fiksel et al PRL 103, 145002 (2009) 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. QSH physics

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

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

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

  22. LH Transition in Tokamak

  23. IE in Tokamaks: L-H power theshold 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 DIII-D 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

  24. IE in Tokamaks: L-H power theshold NSTX • 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. IE in Tokamaks: L-H power theshold JET

  26. Density Limit

  27. Density limit in Tokamak 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.

  28. Density limit in RFX, higher in He than in H H + pellet He H In MST: limit slightly overcome in D. No experience with He. (B Chapman, p.c.)

  29. Plasma Wall Interaction

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

  31. PWI : H vs He For low energies ( <200 eV) Strong Chemical erosion for H

  32. 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. 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. 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) ?

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