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Title: Superdense core mode in the Large Helical Device with an internal diffusion barrier

Abstract

In reduced recycling discharges using a local island divertor in the Large Helical Device [O. Motojima, H. Yamada, A. Komori et al., Phys. Plasmas 6, 1843 (1999)], a stable high-density plasma develops in the core region when a series of pellets is injected. A core region with {approx}5x10{sup 20} m{sup -3} and temperature of 0.85 keV is maintained by an internal diffusion barrier (IDB). The density gradient at the IDB (r/a{approx}0.6) is very high, and the particle confinement time in the core region is {approx}0.4 s. Because of the increase in the central pressure, a large Shafranov shift up to {approx}0.3 m is observed. The critical ingredients for IDB formation are a strongly pumped divertor to reduce edge recycling, and multiple pellet injection to ensure efficient central fueling. No serious magnetohydrodynamics activity and impurity accumulation have been observed so far in this improved discharge.

Authors:
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;  [1]
  1. National Institute for Fusion Science, Toki 509-5292 (Japan) (and others)
Publication Date:
OSTI Identifier:
20975051
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2718530; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BOUNDARY LAYERS; CONFINEMENT TIME; DIFFUSION BARRIERS; DIVERTORS; ELECTRON TEMPERATURE; ION TEMPERATURE; KEV RANGE; LHD DEVICE; MAGNETIC ISLANDS; MAGNETOHYDRODYNAMICS; PELLET INJECTION; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; PLASMA IMPURITIES; STELLARATORS

Citation Formats

Morisaki, T., Ohyabu, N., Masuzaki, S., Kobayashi, M., Sakamoto, R., Miyazawa, J., Funaba, H., Ida, K., Ikeda, K., Kaneko, O., Morita, S., Mutoh, S., Nagaoka, K., Nagayama, Y., Nakajima, N., Narihara, K., Oka, Y., Osakabe, M., Peterson, B. J., and Sakakibara, S.. Superdense core mode in the Large Helical Device with an internal diffusion barrier. United States: N. p., 2007. Web. doi:10.1063/1.2718530.
Morisaki, T., Ohyabu, N., Masuzaki, S., Kobayashi, M., Sakamoto, R., Miyazawa, J., Funaba, H., Ida, K., Ikeda, K., Kaneko, O., Morita, S., Mutoh, S., Nagaoka, K., Nagayama, Y., Nakajima, N., Narihara, K., Oka, Y., Osakabe, M., Peterson, B. J., & Sakakibara, S.. Superdense core mode in the Large Helical Device with an internal diffusion barrier. United States. doi:10.1063/1.2718530.
Morisaki, T., Ohyabu, N., Masuzaki, S., Kobayashi, M., Sakamoto, R., Miyazawa, J., Funaba, H., Ida, K., Ikeda, K., Kaneko, O., Morita, S., Mutoh, S., Nagaoka, K., Nagayama, Y., Nakajima, N., Narihara, K., Oka, Y., Osakabe, M., Peterson, B. J., and Sakakibara, S.. Tue . "Superdense core mode in the Large Helical Device with an internal diffusion barrier". United States. doi:10.1063/1.2718530.
@article{osti_20975051,
title = {Superdense core mode in the Large Helical Device with an internal diffusion barrier},
author = {Morisaki, T. and Ohyabu, N. and Masuzaki, S. and Kobayashi, M. and Sakamoto, R. and Miyazawa, J. and Funaba, H. and Ida, K. and Ikeda, K. and Kaneko, O. and Morita, S. and Mutoh, S. and Nagaoka, K. and Nagayama, Y. and Nakajima, N. and Narihara, K. and Oka, Y. and Osakabe, M. and Peterson, B. J. and Sakakibara, S.},
abstractNote = {In reduced recycling discharges using a local island divertor in the Large Helical Device [O. Motojima, H. Yamada, A. Komori et al., Phys. Plasmas 6, 1843 (1999)], a stable high-density plasma develops in the core region when a series of pellets is injected. A core region with {approx}5x10{sup 20} m{sup -3} and temperature of 0.85 keV is maintained by an internal diffusion barrier (IDB). The density gradient at the IDB (r/a{approx}0.6) is very high, and the particle confinement time in the core region is {approx}0.4 s. Because of the increase in the central pressure, a large Shafranov shift up to {approx}0.3 m is observed. The critical ingredients for IDB formation are a strongly pumped divertor to reduce edge recycling, and multiple pellet injection to ensure efficient central fueling. No serious magnetohydrodynamics activity and impurity accumulation have been observed so far in this improved discharge.},
doi = {10.1063/1.2718530},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • The electron internal transport barrier (eITB) formation in the Large Helical Device (LHD) is studied with the transport code TOTAL and a GyroBohm-like model. The reduction of anomalous transport by the ExB shear has been introduced by means of the factor [1+({tau}{omega}{sub ExB}){sup {gamma}}]{sup -1}. Simulation results show a clear critical transition between plasma regimes with rather flat electron temperature profiles (non-eITB) to a steeped one (with eITB) when average density is low enough. With the aim of studying the eITB formation as a phase transition phenomenon, the electron average density is taken as the control parameter and the ExBmore » shearing rate as the order parameter. Results show how the eITB formation in LHD is compatible with a continuum phase transition with critical exponent {beta}=0.40.« less
  • Extremely hollow profiles of impurities (denoted as 'impurity hole') are observed in the plasma with a steep gradient of the ion temperature after the formation of an internal transport barrier (ITB) in the ion temperature transport in the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. The radial profile of carbon becomes hollow during the ITB phase and the central carbon density keeps dropping and reaches 0.1%-0.3% of plasma density at the end of the ion ITB phase. The diffusion coefficient and the convective velocity of impurities are evaluated from the time evolution of carbon profilesmore » assuming the diffusion and the convection velocity are constant in time after the formation of the ITB. The transport analysis gives a low diffusion of 0.1-0.2 m{sup 2}/s and the outward convection velocity of {approx}1 m/s at half of the minor radius, which is in contrast to the tendency in tokamak plasmas for the impurity density to increase due to an inward convection and low diffusion in the ITB region. The outward convection is considered to be driven by turbulence because the sign of the convection velocity contradicts the neoclassical theory where a negative electric field and an inward convection are predicted.« less
  • The aim of this study was to analyze the feedback process between the magnetic turbulence and the pressure gradients in Large Helical Device (LHD) inward-shifted configurations as well as its role in the transition between the soft-hard magnetohydrodynamic (MHD) regimes for instabilities driven by the mode 1/2 in the middle plasma. In the present paper, we summarize the results of two simulations with different Lundquist numbers, S=2.5×10{sup 5} and 10{sup 6}, assuming a plasma in the slow reconnection regime. The results for the high Lundquist number simulation show that the magnetic turbulence and the pressure gradient in the middle plasmamore » region of LHD are below the critical value to drive the transition to the hard MHD regime, therefore only relaxations in the soft MHD limit are triggered (1/2 sawtooth-like events) [Phys. Plasmas 19, 082512 (2012)]. In the case of the simulation with low Lundquist number, the system reaches the hard MHD limit and a plasma collapse is observed.« less
  • A fully kinetic assessment of the stability properties of toroidal drift modes has been obtained for a case for the Large Helical Device [A. Iiyoshi , Nucl. Fusion 39, 1245 (1999)]. This calculation retains the important effects in the linearized gyrokinetic equation, using the lowest-order ''ballooning representation'' for high toroidal mode number instabilities in the electrostatic limit. Results for toroidal drift waves destabilized by trapped particle dynamics and ion temperature gradients are presented, using three-dimensional magnetohydrodynamic equilibria reconstructed from experimental measurements. The effects of helically trapped particles and helical curvature are investigated.
  • Large Helical Device (LHD) inward-shifted configurations are unstable to resistive magnetohydrodynamic (MHD) pressure-gradient-driven modes. These modes drive sawtooth like events during LHD operation. In this work, we simulate sawtooth like activity and internal disruptions in order to improve the understanding of these relaxation events and their effect over the device efficiency to confine the plasma, with the aim to improve the LHD present and future operation scenarios minimizing or avoiding the disadvantageous MHD soft and hard limits. By solving a set of reduced non-linear resistive MHD equations, we have studied the evolution of perturbations to equilibria obtained before and aftermore » a sawtooth like event in LHD. The equilibrium {beta} value is gradually increased during the simulation until it reaches the experimental value. Sawtooth like events and internal disruption events take place in the simulation for {beta}{sub 0} values between 1% and 1.48%. The main driver of the sawtooth like events is the resonant and non-resonant effect of the (n = 1, m = 3) mode. The instability is stronger for resonant events, and they only appear when {beta}{sub 0} = 1.48%. Internal disruptions are mainly driven by the (n = 1, m = 2) mode, and they extend throughout the whole plasma core. Internal disruption events do not show up when resonant sawtooth like events are triggered.« less