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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.This flow damping and resulting profile flattening are much stronger than expected from the Rechester-Rosenbluth model.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

Decay of ion temperature and flow velocity.Radial profiles of (a) ion temperature and (b) toroidal flow velocity in the core region (reff/a99<0.6) during the stochastization of the magnetic field. The decay of the ion temperature gradient and the toroidal flow velocity shear at reff/a99=0.27 are also plotted in log-scale. The solid lines in the radial profiles of ion temperature and toroidal rotation are polynomial fit curves to data points and the solid lines in the time evolution of the ion temperature gradient and velocity shear in the log-plot are the exponential curve and linear curve to fit data points. The error bars of toroidal rotation and ion temperature are derived from the uncertainty of the fitting parameter of the charge exchange line emission to a Gaussian profile.
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f3: Decay of ion temperature and flow velocity.Radial profiles of (a) ion temperature and (b) toroidal flow velocity in the core region (reff/a99<0.6) during the stochastization of the magnetic field. The decay of the ion temperature gradient and the toroidal flow velocity shear at reff/a99=0.27 are also plotted in log-scale. The solid lines in the radial profiles of ion temperature and toroidal rotation are polynomial fit curves to data points and the solid lines in the time evolution of the ion temperature gradient and velocity shear in the log-plot are the exponential curve and linear curve to fit data points. The error bars of toroidal rotation and ion temperature are derived from the uncertainty of the fitting parameter of the charge exchange line emission to a Gaussian profile.

Mentions: In this section, the physics mechanism of flow damping is discussed. The large effective Prandtl number observed during stochastization (μφ/χ=3) discussed in the previous section suggests the existence of an additional damping mechanism of the toroidal flow due to stochastization of the magnetic field. Figure 3 shows radial profiles of the ion temperature and toroidal flow velocity in the core region of reff/a99<0.6, and the decay of the ion temperature gradient and the toroidal flow velocity shear during the decay phase at reff/a99=0.27. The ion temperature profiles show a prompt flattening after the stochastization (the drops in ion temperature occur earlier and faster over time); however, the toroidal flow velocity profiles show a damping after the drop of ion temperature in the time scale of 100 ms. The decay rate of the central ion temperature becomes smaller with time (δTi/δt(5.278–5.307 s)>δTi/δt(5.307–5.347 s)~δTi/δt(5.347–5.382 s)), while that of the toroidal rotation becomes larger and then constant with time (δVφ/δt(5.278–5.307 s)<δVφ/δt(5.307–5.347 s)~δVφ/δt(5.347–5.382 s)). There is a clear difference in the decay of the ion temperature gradient and the toroidal flow velocity shear observed. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay, which is indicated by the solid lines in Fig. 3a,b in log-scale. The toroidal flow shear decreases more than the linear curve at t~5.4 s, as seen in Fig. 3b. This experimental observation (linear decay) cannot be explained by the increase in the diffusive term of the momentum transport, which should be proportional to the velocity shear, and it suggests a new damping mechanism.


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)

Decay of ion temperature and flow velocity.Radial profiles of (a) ion temperature and (b) toroidal flow velocity in the core region (reff/a99<0.6) during the stochastization of the magnetic field. The decay of the ion temperature gradient and the toroidal flow velocity shear at reff/a99=0.27 are also plotted in log-scale. The solid lines in the radial profiles of ion temperature and toroidal rotation are polynomial fit curves to data points and the solid lines in the time evolution of the ion temperature gradient and velocity shear in the log-plot are the exponential curve and linear curve to fit data points. The error bars of toroidal rotation and ion temperature are derived from the uncertainty of the fitting parameter of the charge exchange line emission to a Gaussian profile.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Decay of ion temperature and flow velocity.Radial profiles of (a) ion temperature and (b) toroidal flow velocity in the core region (reff/a99<0.6) during the stochastization of the magnetic field. The decay of the ion temperature gradient and the toroidal flow velocity shear at reff/a99=0.27 are also plotted in log-scale. The solid lines in the radial profiles of ion temperature and toroidal rotation are polynomial fit curves to data points and the solid lines in the time evolution of the ion temperature gradient and velocity shear in the log-plot are the exponential curve and linear curve to fit data points. The error bars of toroidal rotation and ion temperature are derived from the uncertainty of the fitting parameter of the charge exchange line emission to a Gaussian profile.
Mentions: In this section, the physics mechanism of flow damping is discussed. The large effective Prandtl number observed during stochastization (μφ/χ=3) discussed in the previous section suggests the existence of an additional damping mechanism of the toroidal flow due to stochastization of the magnetic field. Figure 3 shows radial profiles of the ion temperature and toroidal flow velocity in the core region of reff/a99<0.6, and the decay of the ion temperature gradient and the toroidal flow velocity shear during the decay phase at reff/a99=0.27. The ion temperature profiles show a prompt flattening after the stochastization (the drops in ion temperature occur earlier and faster over time); however, the toroidal flow velocity profiles show a damping after the drop of ion temperature in the time scale of 100 ms. The decay rate of the central ion temperature becomes smaller with time (δTi/δt(5.278–5.307 s)>δTi/δt(5.307–5.347 s)~δTi/δt(5.347–5.382 s)), while that of the toroidal rotation becomes larger and then constant with time (δVφ/δt(5.278–5.307 s)<δVφ/δt(5.307–5.347 s)~δVφ/δt(5.347–5.382 s)). There is a clear difference in the decay of the ion temperature gradient and the toroidal flow velocity shear observed. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay, which is indicated by the solid lines in Fig. 3a,b in log-scale. The toroidal flow shear decreases more than the linear curve at t~5.4 s, as seen in Fig. 3b. This experimental observation (linear decay) cannot be explained by the increase in the diffusive term of the momentum transport, which should be proportional to the velocity shear, and it suggests a new damping mechanism.

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.This flow damping and resulting profile flattening are much stronger than expected from the Rechester-Rosenbluth model.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