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High Density Limit in RFP’s

High Density Limit in RFP’s. M Valisa and the RFX-mod team. OUTLINE. Background on density limit in RFP’s New RFX-mod experimental results Discussion. Why are we interested in High Density plasmas?. In a reactor P_fusion ~ n e 2 (provided T is around 10 keV) In RFX t E ~ n e

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High Density Limit in RFP’s

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  1. High Density Limit in RFP’s M Valisa and the RFX-mod team

  2. OUTLINE • Background on density limit in RFP’s • New RFX-mod experimental results • Discussion

  3. Why are we interested in High Density plasmas? • In a reactor P_fusion ~ ne2 (provided T is around 10 keV) • In RFX tE~ne • The density limit is a commonality of all of the magnetic configurations .

  4. Upper Density Limit in RFP’s known since many years • Costa,De Angelis, Ortolani Puiatti, Nuc. Fusion 22, 1301 (1982). Radiation limit • Ortolani & Rostagni 1983 Nucl. Instrum. Meth 207 35 Radiation limit around n/nG= 0.5 (I/N=2) • Perkins & Hulse 1985 Phys. Fluids 28 1837 • Bartiromo et al 2000 27th EPS Conf. Budapest http://epsppd.epfl.ch/Buda/pdf/p4_031.pdf Published in M. Greenwald’s review PPCF 2002 • Valisa et al IAEA Conf 2004 Villamoura • Valisa et al IAEA Conf 2006 Chengdu • MST density limited regimes ( Wyman’s Thesis 2007) • Rencent work : Puiatti et al Submitted to PoP • Puiatti et al IAEA Conf 2008 Geneve

  5. RFP and Tokamak share the same limit • The Greenwald limit is an empirical law * RFX-Mod • The Greenwald threshold is not an absolute limit but provides values in the right range • In Tokamaks nG can be exceeded by a factor 2 • by peaking the density profile • . Some ITER scenarios assume n/nG ~1.2

  6. In JET a pedestal DL is found Predicted pedestal DL (detachment) overlap and Experimental and is worse than the Greenwald limit JET Borrass NF 2004 752

  7. Stellarator : Sudo limit , much higher than nG RFX

  8. Stellarators: Sudo Scaling In LHD the critical edge density that leads to complete detachment is well reproduced by the Sudo scaling with a factor 0.8. Pellet-fuelled plasmas with large density peaking factor  Ne(0)/neSudo up to 7 ! Core density can be increased until the core plasma locally collapses, as long as the edge density is kept below the Sudo scaling. The Sudo scaling is an edge density limit scaling. The operational limit is ne_edge< neSudo J. Miyazawa . Nucl Fus 2008 and EPS 2008

  9. New RFX-mod data New current regime Greenwald limit Runaway region Old RFX space

  10. New on RFX-mod Confirmed the difficulty to keep the current derivative = 0 at high density Normalized Current derivativevs n/nG

  11. Radiation pattern: a MARFE-like structure Toroidally localized , poloidally symmetric belt

  12. RFX-mod: DL experiments with induced m=o modes rotation New experiments with induced rotation of a strong artificial m=0 n=1 mode for a “3D” diagnostic of the relevant structures

  13. induced m=o modes rotation displacement due to m=0 edge flow radiation electron density Ha m=1 Locked mode position • At the same relative position with respect to the Locked Mode (90 ° apart where the plasma shrinks): • the radiating belt develops • edge densityaccumulates due to the flow stagnation point ( and low diffusivity. • Instead at the LM (m=1 modes) : • Strong Ha signal= main particle source, plasma shrinking (Normalized to the Locked mode position) toroidal angle

  14. New . Reconstruction of local flux structure m=0 islands develop around the reversal surface ( green line). Magenta line: toroidal flux function. Region where large radiation belts develop . The relationship between specific m=0 islands and radiaon belts still unclear. Measurements of Flow and radial electric field reversal are consistent. Possible explanation: the pattern of m=0 islands conveys the electron parallel flow to the wall in certain regions only. ( Carraro et al JNM 2003) (Bartiromo, Phys. Plasmas 5, 3342 (1998) Poincaré plot.

  15. Edge Diffusivity is low with increasing n/nG Diffusion coefficient estimated from turbulence plotted as a function of normalized density This confirms old RFX measurements (Gregoratto et al NF 96)..

  16. Prad/P_input ratio LOCALLY very high (open dots)Prad=Pohm as a function of n=nG; m = 1 mode normalized amplitude vs n=nG: Squares : low-n modes n = 7 ¡ 9), Dots: high-n modes (n = 10 ¡ 18). (full dots) Prad=Pohm taking into account the fraction of input power impinging on the toroidal portion of the radiating layer Radiating layers tend to manifest and grow beyond the dashed line

  17. The ULq case In the ULq case, appraoching and slightly exceeding the Greenwald limit is quite simple with flat current waveforms. Well sustained current.

  18. Summary of the RFX-mod experimental evidence With induced rotation of artificially produced modes • At high density in the vicinity of m=0 modes ( not of the m=1 locked mode ) the composition of the radial Er procures a stagnation point for the toroidal flow • Diffusion is sufficiently low to allow local density increase favoured by the RFP topology ( around the reversal parallel flow has no or little toroidal component) • Density ( not necessarily impurity ) accumulation causes radiation to locally build up • At high density localized radiation dissipation is experimenatlly comparable to the power flow to that region  possibility of a thermal instability.

  19. Question What in absence of artificially induced m=0 modes (i.e. pure Clean Mode Control) ? • Fast TV images (1 kHz) show PWI patterns rapidly changing in space • 1D Simulation can be made to check the RFP response to an increase density in an axi-symmetric topology • MST is more m=0 stable. Experiments could clarify the role of m=0 modes • In RFX shallow F experiments at high desnity will be performed

  20. 1D Simulation of high density discharge ( Riport code) “Riport” code: - 1D ( cilindrical symmetry – does not include 3D effects of plasma deformations - Heat and particle equations ( including impurities solved self-consistently with Spitzer resistivity and a-dynamo term See Predebon et al submitted to PoP

  21. 1D Simulation od high density discharge ( Report code) Simulated RFP discharge with a linearly increasing influx Increased resistivity causes the current to decrease unless the dynamo term is proportionally augmented

  22. Conclusions 1 • New experimental evidence in RFX—mod confirms that localized radiation belts develop in the region where m=0 islands are formed. • The evolution is favoured by converging toroidal flow of particles and low edge diffusivity. • ULq reaches quite easily the Greenwald DL with well sustained current • ( despite general transport is worse) . • NB: In ULq’s the m=0 resonance is no longer in the plasma • In RFX-mod best performance is with QSH which is instead prefers • regimes at high Lundquist ~ Te3/2/ ne1/2 • We must learn how to to make QSH and high ne compatible. • .

  23. Conclusions 2 • In Tokamak , Stellarators and RFP’s exceeding a density threshold triggers some kind of instability processes at the egde: typically involving radiation cooling.at the edge(except TEXT : DL due to transport change.D. Brower et al PRL 67, 200 (1991) ) • In Tokamaks MARFE structures can lead to a disruption. • In the Stellarator and in the RFP (including ULq) the consequences are not disruptive • In the RFP current sustainement becomes critical even before nG is reached unlike the ULq • The density limit is originated by similar physics , but modulated by the topological and stability properties of each configuration.

  24. Conclusions 3 • Questions: • Would an m=0 controlled RFP plasma avoid the Greenwald density limit? • Is the RFP intrinsically limited in density due to its topology and • the need of an internal dynamo? • In Tokamaks and Stellarators average densities well above the “DL” • have been achieved. • Can we conceive a high density RFP with peaked profiles and sufficiently • low edge density to overcome the Greenwald limit? • Try with internal fuelling (pellet/ power beams )& edge recycling control

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