TJ-II:Transport and magnetic configuration: Difference between revisions

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TJ-II:Transport and magnetic configuration (view source)

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A significant part of the research effort at [[TJ-II]] is dedicated to the study of the interaction between transport and the (flexible) magnetic configuration.
A significant part of the research effort at [[TJ-II]] is dedicated to the study of the interaction between transport and the (flexible) magnetic configuration.
<ref>[http://dx.doi.org/10.1088/0741-3335/41/12B/307 C. Alejaldre et al, ''Confinement studies in the TJ-II stellarator'', Plasma Phys. Control. Fusion '''41''' (1999) B109]</ref>
<ref>C. Alejaldre et al, ''Confinement studies in the TJ-II stellarator'', [[doi:10.1088/0741-3335/41/12B/307|Plasma Phys. Control. Fusion '''41''' (1999) B109]]</ref>
<ref>[http://dx.doi.org/10.1088/0741-3335/45/3/304 M.A. Ochando et al, ''Emissivity toroidal asymmetries induced by ECRH driven convective fluxes in the TJ-II stellarator'', Plasma Phys. Control. Fusion '''45''' (2003) 221-231]</ref>
<ref>M.A. Ochando et al, ''Emissivity toroidal asymmetries induced by ECRH driven convective fluxes in the TJ-II stellarator'', [[doi:10.1088/0741-3335/45/3/304|Plasma Phys. Control. Fusion '''45''' (2003) 221-231]]</ref>
<ref>[http://dx.doi.org/10.1088/0029-5515/45/10/S22 C. Hidalgo et al, ''Overview of TJ-II experiments'', Nucl. Fusion '''45''' (2005) S266-S275]</ref>
<ref>C. Hidalgo et al, ''Overview of TJ-II experiments'', [[doi:10.1088/0029-5515/45/10/S22|Nucl. Fusion '''45''' (2005) S266-S275]]</ref>
The following issues contribute to the influence of the configuration on confinement:
The following issues contribute to the influence of the configuration on confinement:
* The rotational transform <ref>[http://dx.doi.org/10.1088/0029-5515/45/4/009 E. Ascasíbar et al, ''Magnetic configuration and plasma parameter dependence of the energy confinement time in ECR heated plasmas from the TJ-II stellarator'', Nucl. Fusion '''45''' (2005) 276-284]</ref>
* The [[rotational transform]] <ref>E. Ascasíbar et al, ''Magnetic configuration and plasma parameter dependence of the energy confinement time in ECR heated plasmas from the TJ-II stellarator'', [[doi:10.1088/0029-5515/45/4/009|Nucl. Fusion '''45''' (2005) 276-284]]</ref>
* Rational surfaces <ref>[http://dx.doi.org/10.1209/0295-5075/82/65002 D. López-Bruna et al, ''Tracking magnetic resonances in the effective electron heat diffusivity of ECH plasmas of the TJ-II Heliac'', EuroPhysics Letters '''82''' (2008) 65002]</ref><ref>[http://doi:10.1088/0029-5515/49/8/085016 D. López-Bruna et al, ''First dynamic magnetic configuration scans in ECRH plasmas of the TJ-II Heliac'', Nucl. Fusion '''49''' (2009) 085016]</ref><ref>[http://dx.doi.org/10.1002/ctpp.200900005 D. López-Bruna et al, ''Magnetic Resonances in ECR-Heated Plasmas of the TJ-II Heliac'', Contrib. Plasma Phys. '''50''' (2010) 600]</ref>
* Rational surfaces <ref>D. López-Bruna et al, ''Tracking magnetic resonances in the effective electron heat diffusivity of ECH plasmas of the TJ-II Heliac'', [[doi:10.1209/0295-5075/82/65002|EuroPhysics Letters '''82''' (2008) 65002]]</ref><ref>D. López-Bruna et al, ''First dynamic magnetic configuration scans in ECRH plasmas of the TJ-II Heliac'', [[doi:10.1088/0029-5515/49/8/085016|Nucl. Fusion '''49''' (2009) 085016]]</ref><ref>D. López-Bruna et al, ''Magnetic Resonances in ECR-Heated Plasmas of the TJ-II Heliac'', [[doi:10.1002/ctpp.200900005|Contrib. Plasma Phys. '''50''' (2010) 600]]</ref><ref>D. López-Bruna et al, ''Magnetic resonances and electric fields in the TJ-II Heliac'', [[doi:10.1088/0741-3335/53/12/124022|Plasma Phys. Control. Fusion 53 (2011) 124022]]</ref><ref>D. López-Bruna et al, ''Relationship between MHD events, magnetic resonances and transport barriers in TJ-II plasmas'', [[doi:10.1088/0029-5515/53/7/073051|Nucl. Fusion '''53''', 7 (2013) 073051]]</ref>
* [[Magnetic shear]] <ref>[http://dx.doi.org/10.1088/0029-5515/43/6/301 J.A. Romero et al, ''Controlling confinement with induced toroidal current in the flexible Heliac TJ-II'', Nucl. Fusion '''43''' (2003) 387-392]</ref> <ref>[http://dx.doi.org/10.1088/0029-5515/44/5/008 D. López-Bruna et al, ''Effects of ohmic current in the TJ-II stellarator'', Nucl. Fusion '''44''', 5 (2004) 645-654]</ref>
* [[Magnetic shear]] <ref>J.A. Romero et al, ''Controlling confinement with induced toroidal current in the flexible Heliac TJ-II'', [[doi:10.1088/0029-5515/43/6/301|Nucl. Fusion '''43''' (2003) 387-392]]</ref> <ref>D. López-Bruna et al, ''Effects of ohmic current in the TJ-II stellarator'', [[doi:10.1088/0029-5515/44/5/008|Nucl. Fusion '''44''', 5 (2004) 645-654]]</ref>
* [[Magnetic well]] <ref>[http://dx.doi.org/10.1088/0741-3335/43/12A/324 C. Hidalgo et al, ''On the radial scale of fluctuations in the TJ-II stellarator'', Plasma Phys. Control. Fusion '''43''' (2001) A313-A321]</ref> <ref>[http://link.aip.org/link/?PHPAEN/9/713/1 J. Castellano et al, ''Magnetic well and instability thresholds in the TJ-II stellarator'', Phys. Plasmas '''9''' (2002) 713]</ref> <ref>[http://dx.doi.org/10.1088/0741-3335/44/12B/322 E. Ascasíbar et al, ''Confinement and stability on the TJ-II stellarator'', Plasma Phys. Control. Fusion '''44''' (2002) B307-B322]</ref>
* [[Magnetic well]] <ref>C. Hidalgo et al, ''On the radial scale of fluctuations in the TJ-II stellarator'', [[doi:10.1088/0741-3335/43/12A/324|Plasma Phys. Control. Fusion '''43''' (2001) A313-A321]]</ref> <ref>J. Castellano et al, ''Magnetic well and instability thresholds in the TJ-II stellarator'', [[doi:10.1063/1.1430435|Phys. Plasmas '''9''' (2002) 713]]</ref> <ref>E. Ascasíbar et al, ''Confinement and stability on the TJ-II stellarator'', [[doi:10.1088/0741-3335/44/12B/322|Plasma Phys. Control. Fusion '''44''' (2002) B307-B322]]</ref>


The picture emerging from these studies is complex. In general, the presence of low-order rational surfaces in the plasma does not seem to be deleterious for transport; rather, local reductions of transport are observed (except at very low shear). Possibly, this is associated with the formation of [[TJ-II:Internal Transport Barriers|internal transport barriers]] at or near such rational surfaces, a consequence of the modification of the radial electric field by the rational surfaces themselves. It is being investigated whether the plasmas where this has been studied have the conditions for a self-healing state of the corresponding magnetic perturbations. On the other hand, the reduction of the magnetic well clearly enhances turbulence levels.
The picture emerging from these studies is complex. In general, the presence of low-order rational surfaces in the plasma does not seem to be deleterious for transport; rather, local reductions of transport are observed (except at very low shear). Possibly, this is associated with the formation of [[TJ-II:Internal Transport Barriers|internal transport barriers]] at or near such rational surfaces, a consequence of the modification of the radial electric field by the rational surfaces themselves. It is being investigated whether the plasmas where this has been studied have the conditions for a self-healing state of the corresponding magnetic perturbations. On the other hand, the reduction of the magnetic well clearly enhances turbulence levels.
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