LNF:Plasma Physics: Difference between revisions

A key point in stellarator/heliotron plasmas is the empirical observation of increasing impurity confinement at high densities.<ref>R. Burhenn et al., NF 49 (2009) 065005</ref>  

Nevertheless, effective impurity control is possible in some regimes like the HDH-mode in W7AS<ref>K. McCormick et al Phys. Rev. Lett. 89 (2002) 015001</ref> and impurity hole in LHD.<ref>T. Ido et al., PPCF 52 (2010) 124025</ref>  

The measurement and modelling of impurity density asymmetries are considerably simpler than those of radial impurity transport and yet can provide an indirect validation of the model prediction on radial transport. The localization of impurities in a flux surface depends on the location of magnetic and electrostatic wells as well as on ion-impurity friction.<ref>J. M. García-Regaña et al., PPCF 55 (2013) 074008</ref>  

Centrifugal effects become important in strongly rotating plasmas.<ref>M. L. Reinke et al., 2012 Plasma Phys. Control. Fusion 54 045004</ref>  

The first direct observations of plasma potential asymmetries on a flux surface, consistent with MC calculations, have been reported in the TJ-II stellarator.<ref>M.A. Pedrosa, A. Alonso, J.M. García-Regaña et al., NF 55 (2015) 052001</ref>

Furthermore, ECRH induced turbulent transport and development of plasma potential flux surface asymmetries<ref>L. C. Ingesson et al., Plasma Phys. Control. Fusion 42 (2000) 161   </ref> can play a role on impurity transport.  

* It has been proven<ref>F. Parra, M. Barnes, I. Calvo, P. Catto, PoP 19 (2012) 056116</ref> that the calculation of radial transport of toroidal angular momentum in tokamaks, and equivalently the rotation profile and long-wavelength radial electric field, requires to deal with microturbulence. In particular, high-order gyrokinetic equations are needed. During the last few years, we have worked out the electrostatic gyrokinetic equations to the required order in arbitrary magnetic geometry and applied them to the tokamak. Concretely, we have given the equations that allow to compute the long-wavelength pieces entering the formula that determines the rotation profile.<ref>I. Calvo and F. Parra, PPCF 54 (2012) 115007</ref> However, the equations for the short-wavelength pieces are still lacking. We will compute them, and give all the explicit expressions that have to be implemented in a code aiming to determine tokamak intrinsic rotation. The expressions for flux-tube formulations of gyrokinetics will also be provided. A mid-term objective involves the actual code implementation and testing of the theory.

Experimental studies have shown a reduction of the L-H power threshold by about 50% when using Deuterium and He instead of Hydrogen.<ref>F. Ryter  et al., 2013 Nuclear Fusion 53 (2012) 113003 </ref>  

Based on present ITPA scaling laws, H-mode operation is expected to be marginally feasible in H but likely in He.<ref>A. Sips et al., 25th IAEA Int. Conf. on Fusion Energy St Petersburg 2014 / EX/9-1. </ref>  

Studies in the TEXTOR tokamak<ref>Y. Xu, C. Hidalgo, I. Shesterikov et al., PRL-2013</ref> and the TJ-II stellarator<ref>B. Liu et al., Nuclear Fusion 2015 </ref> have reported the properties of local turbulence and Long-Range-Correlations (LRC) in Hydrogen and Deuterium plasmas concluding that there is a systematic increase in the LRC amplitude during the transition from H to D dominated plasmas in the TEXTOR tokamak but not in the TJ-II stellarator.  

Stellarator inter-machine studies<ref>A. Dinklage et al., Nucl. Fusion 53 (2013) 063022</ref> have shown that ion energy transport significantly affects energy confinement at medium-to-high densities (<math>n_e > 4 x 10^{19} m^{-3}</math>).  

* The singular alteration of transport at magnetically resonant plasma regions. TJ-II offers the only experimental benchmark in Europe to evaluate their impact in deviating neoclassical predictions<ref>D. López-Bruna, Plasma Phys. Control. Fusion 55 (2013) 015001</ref> and these studies can now be pursued with improved capabilities (e.g., higher NBI power and additional microwave heating in conditions of high plasma density).

* An important open question is the relation between neoclassical and turbulence optimization. It is seen that in LHD stellarator<ref>T H Watanabe, H Sugama and S Ferrando. PRL 100 (2008) 195002</ref> both actions go together. Effort is being carried out to understand the zonal flow relaxation in stellarator geometry and the role of neoclassical radial electric fields on ZFs and confinement.

Clarifying the physics behind uncoupled (heat and particle) transport channels is one of the main scientific conundrums for understanding ELM control techniques (e.g. using magnetic perturbations<ref>T. E. Evans et al. Nature Phys. 2 (2006) 419.</ref>), as part of the ITER base-line scenario, and the development of plasma scenarios without ELMs (e.g. the I-mode<ref>A. Hubbard et al 25th IAEA Int. Conf. (St. Petersburg, 2014) /[EX/P6-22]</ref>).  

Lithium Capillary-Pore-system (CPS) limiters with a closed circulation loop are under development at the T11M tokamak.<ref>A. Vertkov et al., 25th IAEA Int. Conf. on Fusion (2014) / EX/P1-49. </ref>  

CPS experiments are also in progress at the FTU tokamak<ref>G. Mazzitelli G et al., 25th IAEA Int. Conf. on Fusion Energy St Petersburg 2014 </ref> and the TJ-II stellarator.<ref>J. Sánchez et al., Nuclear Fusion 2015 / EX/P2-46 </ref>   

TJ-II experimental results have shown that stellarator stability is better than predicted by linear stability analyses.<ref>A. M. de Aguilera  et al., Nuclear Fusion 55 (2015) 113014</ref>  

This result strongly suggests that stability calculations, as those presently used in the optimization criteria of stellarators, might miss some stabilization mechanisms,<ref>S. Sakakibara et al., PPCF 50 (2008) 124014</ref> which could be explained by self-organization mechanisms between transport and gradients.<ref>K. Ichiguchi et al., NF 51 (2011) 053021; C. Hidalgo et al., PRL 108 (2012) 065001</ref>  

The observed mitigation effect of ECRH on NBI beam-driven Alfvén eigenmodes (AEs) first reported in DIII-D and later in TJ-II<ref>K. Nagaoka, T. Ido, E. Ascasibar et al., Nuclear Fusion (2013)</ref> has opened an attractive avenue for a possible control of the AEs though the physics behind this effect is yet to be understood. Fast particle can have also influence on the broadband turbulence and on several instabilities like ITGs.