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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

Radial profiles of flow velocity, temperatures, and density.Radial profiles of (a) toroidal flow velocity, (b) electron temperature, (c) ion temperature and (d) electron density before (t=5.64, 5.61 s) and after (t=6.44, 6.41 s) the stochastization of the magnetic field. The solid lines in the radial profiles of electron density are polynomial fit curves to 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. The error bars of electron temperature are derived from the standard deviations of the signal.
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f2: Radial profiles of flow velocity, temperatures, and density.Radial profiles of (a) toroidal flow velocity, (b) electron temperature, (c) ion temperature and (d) electron density before (t=5.64, 5.61 s) and after (t=6.44, 6.41 s) the stochastization of the magnetic field. The solid lines in the radial profiles of electron density are polynomial fit curves to 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. The error bars of electron temperature are derived from the standard deviations of the signal.

Mentions: In the core plasma of the LHD, the electron thermal diffusivity evaluated from heat pulse is comparable to that evaluated from the power balance in the steady state13. These experimental results suggest that the heat pinch14 or other non-linearities of electron transport are small enough to be neglected in this experiment. Therefore, the effective transport coefficients (electron thermal diffusivity, ion thermal diffusivity and viscosity) are evaluated from the ratio of radial flux normalized by density to gradient for simplicity. Here, the radial flux of electron ion heat transport and momentum transport are calculated from the power deposition and torque profiles driven by the MECH and the NBIs. Figure 2 shows the radial profiles of toroidal flow velocity, electron temperature, ion temperature and electron density before (t=5.64 s, 5.61 s) and after (t=6.44 s, 6.41 s) the stochastization of the magnetic field. Before the stochastization, the toroidal flow velocity is very peaked at the plasma centre, because the toroidal viscosity due to helical ripple increases sharply towards the plasma edge and hence significant damping of the toroidal flow occurs there. After the stochastization, a clear flattening of the toroidal flow, ion temperature and electron temperature profiles is observed. Since the density profile is already flat even before the stochastization, the effect of stochastization on particle transport is not clear in this experiment. The increase of electron density is gradual and not due to the stochastization of the magnetic field.


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 profiles of flow velocity, temperatures, and density.Radial profiles of (a) toroidal flow velocity, (b) electron temperature, (c) ion temperature and (d) electron density before (t=5.64, 5.61 s) and after (t=6.44, 6.41 s) the stochastization of the magnetic field. The solid lines in the radial profiles of electron density are polynomial fit curves to 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. The error bars of electron temperature are derived from the standard deviations of the signal.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Radial profiles of flow velocity, temperatures, and density.Radial profiles of (a) toroidal flow velocity, (b) electron temperature, (c) ion temperature and (d) electron density before (t=5.64, 5.61 s) and after (t=6.44, 6.41 s) the stochastization of the magnetic field. The solid lines in the radial profiles of electron density are polynomial fit curves to 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. The error bars of electron temperature are derived from the standard deviations of the signal.
Mentions: In the core plasma of the LHD, the electron thermal diffusivity evaluated from heat pulse is comparable to that evaluated from the power balance in the steady state13. These experimental results suggest that the heat pinch14 or other non-linearities of electron transport are small enough to be neglected in this experiment. Therefore, the effective transport coefficients (electron thermal diffusivity, ion thermal diffusivity and viscosity) are evaluated from the ratio of radial flux normalized by density to gradient for simplicity. Here, the radial flux of electron ion heat transport and momentum transport are calculated from the power deposition and torque profiles driven by the MECH and the NBIs. Figure 2 shows the radial profiles of toroidal flow velocity, electron temperature, ion temperature and electron density before (t=5.64 s, 5.61 s) and after (t=6.44 s, 6.41 s) the stochastization of the magnetic field. Before the stochastization, the toroidal flow velocity is very peaked at the plasma centre, because the toroidal viscosity due to helical ripple increases sharply towards the plasma edge and hence significant damping of the toroidal flow occurs there. After the stochastization, a clear flattening of the toroidal flow, ion temperature and electron temperature profiles is observed. Since the density profile is already flat even before the stochastization, the effect of stochastization on particle transport is not clear in this experiment. The increase of electron density is gradual and not due to the stochastization of the magnetic field.

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