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Flow damping due to stochastization of the magnetic field.

Ida K, Yoshinuma M, Tsuchiya H, Kobayashi T, Suzuki C, Yokoyama M, Shimizu A, Nagaoka K, Inagaki S, Itoh K, LHD Experiment GroupLHD Experiment Gro - Nat Commun (2015)

Bottom Line: The driving and damping mechanism of plasma flow is an important issue because flow shear has a significant impact on turbulence in a plasma, which determines the transport in the magnetized plasma.The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay.This observation suggests that the flow damping is due to the change in the non-diffusive term of momentum transport.

View Article: PubMed Central - PubMed

Affiliation: National Institute for Fusion Science, Toki, Gifu 509-5292, Japan.

ABSTRACT
The driving and damping mechanism of plasma flow is an important issue because flow shear has a significant impact on turbulence in a plasma, which determines the transport in the magnetized plasma. Here we report clear evidence of the flow damping due to stochastization of the magnetic field. Abrupt damping of the toroidal flow associated with a transition from a nested magnetic flux surface to a stochastic magnetic field is observed when the magnetic shear at the rational surface decreases to 0.5 in the large helical device. This flow damping and resulting profile flattening are much stronger than expected from the Rechester-Rosenbluth model. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. This observation suggests that the flow damping is due to the change in the non-diffusive term of momentum transport.

No MeSH data available.


Related in: MedlinePlus

Radial propagation of stochastization.(a) Time evolution of toroidal flow velocity and two liner fitting lines in the case of tfit=5.29 s at reff/a99=0.09 and 0.53 during the stochastization of the magnetic and radial profiles of (b) onset time of flow damping associated with the stochastization of magnetic field, (c) rotational transform ι/2π, and (d) radial electric field before stochastization (with nested magnetic flux surface) and after the stochastization measured with HIBP in a similar discharge. The onset time (intersections of two lines) is indicated with the arrows in (a). The typical error bar of the measurements of rotational transform in (c) is 0.01. Because the time of the stochastization itself is unknown before the fitting and the intersection depends on the separation of the data (namely tfit), tfit is scanned from well before (t=5.26 s) to well after (t=5.32 s) the stochastization by 60 ms. The error bars are determined from the standard deviation in this scan.
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f4: Radial propagation of stochastization.(a) Time evolution of toroidal flow velocity and two liner fitting lines in the case of tfit=5.29 s at reff/a99=0.09 and 0.53 during the stochastization of the magnetic and radial profiles of (b) onset time of flow damping associated with the stochastization of magnetic field, (c) rotational transform ι/2π, and (d) radial electric field before stochastization (with nested magnetic flux surface) and after the stochastization measured with HIBP in a similar discharge. The onset time (intersections of two lines) is indicated with the arrows in (a). The typical error bar of the measurements of rotational transform in (c) is 0.01. Because the time of the stochastization itself is unknown before the fitting and the intersection depends on the separation of the data (namely tfit), tfit is scanned from well before (t=5.26 s) to well after (t=5.32 s) the stochastization by 60 ms. The error bars are determined from the standard deviation in this scan.

Mentions: It is an interesting issue how the stochastic region develops in time during the stochastization. In order to study the radial propagation of stochastization, the radial profile of the time taken for the abrupt drop of toroidal flow velocity is studied. Figure 4 shows the time evolution of the toroidal flow near the ι/2π=0.5 surface and magnetic axis, and a radial propagation of the onset of the flow damping and the radial profile of the rotational transform measured with the MSE. As discussed earlier, the decay of toroidal flow velocity can be fitted well with a linear line. Therefore we can derive the onset time of flow damping from the intersection of two lines of linear fitting to the data before (t<tfit) and after (t>tfit) the stochastization. Because the time of the stochastization itself is unknown before the fitting and the intersection depends on how the data are separated (namely tfit), tfit is scanned from well before (t=5.26 s) to well after (t=5.32 s) the stochastization by 60 ms, and the onset time and its error bars are determined from the average value and standard deviation in this scan. Please note that the onset time is insensitive to tfit. These results show that the stochastization starts near the rational surface of ι/2π=0.5 at reff/a99=0.45–0.55 and propagates radially in two time scales. The thickness of the stochastic region increases slowly up to a quarter of the plasma minor radius (reff/a99=0.36–0.62), and then a rapid extension of the stochastic region to the magnetic axis takes place. The sudden extension of the stochastic region to the magnetic axis observed in this experiment indicates the non-linear growth of the perturbation field causing stochastization, which was proposed in the major disruption or sawtooth crash model1618.


Flow damping due to stochastization of the magnetic field.

Ida K, Yoshinuma M, Tsuchiya H, Kobayashi T, Suzuki C, Yokoyama M, Shimizu A, Nagaoka K, Inagaki S, Itoh K, LHD Experiment GroupLHD Experiment Gro - Nat Commun (2015)

Radial propagation of stochastization.(a) Time evolution of toroidal flow velocity and two liner fitting lines in the case of tfit=5.29 s at reff/a99=0.09 and 0.53 during the stochastization of the magnetic and radial profiles of (b) onset time of flow damping associated with the stochastization of magnetic field, (c) rotational transform ι/2π, and (d) radial electric field before stochastization (with nested magnetic flux surface) and after the stochastization measured with HIBP in a similar discharge. The onset time (intersections of two lines) is indicated with the arrows in (a). The typical error bar of the measurements of rotational transform in (c) is 0.01. Because the time of the stochastization itself is unknown before the fitting and the intersection depends on the separation of the data (namely tfit), tfit is scanned from well before (t=5.26 s) to well after (t=5.32 s) the stochastization by 60 ms. The error bars are determined from the standard deviation in this scan.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4308719&req=5

f4: Radial propagation of stochastization.(a) Time evolution of toroidal flow velocity and two liner fitting lines in the case of tfit=5.29 s at reff/a99=0.09 and 0.53 during the stochastization of the magnetic and radial profiles of (b) onset time of flow damping associated with the stochastization of magnetic field, (c) rotational transform ι/2π, and (d) radial electric field before stochastization (with nested magnetic flux surface) and after the stochastization measured with HIBP in a similar discharge. The onset time (intersections of two lines) is indicated with the arrows in (a). The typical error bar of the measurements of rotational transform in (c) is 0.01. Because the time of the stochastization itself is unknown before the fitting and the intersection depends on the separation of the data (namely tfit), tfit is scanned from well before (t=5.26 s) to well after (t=5.32 s) the stochastization by 60 ms. The error bars are determined from the standard deviation in this scan.
Mentions: It is an interesting issue how the stochastic region develops in time during the stochastization. In order to study the radial propagation of stochastization, the radial profile of the time taken for the abrupt drop of toroidal flow velocity is studied. Figure 4 shows the time evolution of the toroidal flow near the ι/2π=0.5 surface and magnetic axis, and a radial propagation of the onset of the flow damping and the radial profile of the rotational transform measured with the MSE. As discussed earlier, the decay of toroidal flow velocity can be fitted well with a linear line. Therefore we can derive the onset time of flow damping from the intersection of two lines of linear fitting to the data before (t<tfit) and after (t>tfit) the stochastization. Because the time of the stochastization itself is unknown before the fitting and the intersection depends on how the data are separated (namely tfit), tfit is scanned from well before (t=5.26 s) to well after (t=5.32 s) the stochastization by 60 ms, and the onset time and its error bars are determined from the average value and standard deviation in this scan. Please note that the onset time is insensitive to tfit. These results show that the stochastization starts near the rational surface of ι/2π=0.5 at reff/a99=0.45–0.55 and propagates radially in two time scales. The thickness of the stochastic region increases slowly up to a quarter of the plasma minor radius (reff/a99=0.36–0.62), and then a rapid extension of the stochastic region to the magnetic axis takes place. The sudden extension of the stochastic region to the magnetic axis observed in this experiment indicates the non-linear growth of the perturbation field causing stochastization, which was proposed in the major disruption or sawtooth crash model1618.

Bottom Line: The driving and damping mechanism of plasma flow is an important issue because flow shear has a significant impact on turbulence in a plasma, which determines the transport in the magnetized plasma.The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay.This observation suggests that the flow damping is due to the change in the non-diffusive term of momentum transport.

View Article: PubMed Central - PubMed

Affiliation: National Institute for Fusion Science, Toki, Gifu 509-5292, Japan.

ABSTRACT
The driving and damping mechanism of plasma flow is an important issue because flow shear has a significant impact on turbulence in a plasma, which determines the transport in the magnetized plasma. Here we report clear evidence of the flow damping due to stochastization of the magnetic field. Abrupt damping of the toroidal flow associated with a transition from a nested magnetic flux surface to a stochastic magnetic field is observed when the magnetic shear at the rational surface decreases to 0.5 in the large helical device. This flow damping and resulting profile flattening are much stronger than expected from the Rechester-Rosenbluth model. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. This observation suggests that the flow damping is due to the change in the non-diffusive term of momentum transport.

No MeSH data available.


Related in: MedlinePlus