Internal disruptions and sawtooth like activity in Large Helical Device

Varela, J.; Garcia, L.; Ohdachi, S.; Watanabe, K. Y.; Sánchez, Raúl

doi:10.1063/1.4740064

Cited by 12 publications

(12 citation statements)

References 20 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The present model was already used to study the stability of PM during sawtooth-like events and internal collapses [88,89,90,91], TAE [68] and EIC [92] activity in LHD, indicating reasonable agreement with the observations. T th,0 n th,0 < β tot > V A0 (keV) (10 20 m −3 ) (%) (10 6 m/s) 4 0.3 1.8 3.98 Table 1.…”

Section: Equations and Numerical Schemesupporting

confidence: 81%

Effect of the tangential NBI current drive on the stability of pressure and energetic particle driven MHD modes in LHD plasma

Varela¹,

Cooper²,

Nagaoka³

et al. 2020

Nucl. Fusion

Self Cite

View full text Add to dashboard Cite

The aim of the present study is to analyze the stability of the pressure gradient driven modes (PM) and Alfvén eigenmodes (AE) in the Large Helical Device (LHD) plasma if the rotational transform profile is modified by the current drive of the tangential neutral beam injectors (NBI). This study forms a basic search for optimized operation scenarios with reduced mode activity. The analysis is performed using the code FAR3d which solves the reduced MHD equations describing the linear evolution of the poloidal flux and the toroidal component of NBI current drive 2 the vorticity in a full 3D system, coupled with equations for density and parallel velocity moments of the energetic particle (EP) species, including the effect of the acoustic modes. The Landau damping and resonant destabilization effects are added via the closure relation. On-axis and off-axis NBI current drive modifies the rotational transform which becomes strongly distorted as the intensity of the neutral beam current drive (NBCD) increases, leading to wider continuum gaps and modifying the magnetic shear. The simulations with on-axis NBI injection show that a counter (ctr-) NBCD in inward shifted and default configurations leads to a lower growth rate of the PM, although strong n = 1 and 2 AEs can be destabilized. For the outward shifted configurations, a co-NBCD improves the AEs stability but the PM are further destabilized if the co-NBCD intensity is 30 kA/T. If the NBI injection is off-axis, the plasma stability is not significantly improved due to the further destabilization of the AE and energetic particle modes (EPM) in the middle and outer plasma region.

show abstract

Section: Equations and Numerical Schemesupporting

confidence: 81%

Effect of the tangential NBI current drive on the stability of pressure and energetic particle driven MHD modes in LHD plasma

Varela¹,

Cooper²,

Nagaoka³

et al. 2020

Nucl. Fusion

Self Cite

View full text Add to dashboard Cite

show abstract

“…15,16 Two different events were observed: the non resonant 1/3 sawtooth like events (if the mode 1/3 is outside the plasma, there is no 1/3 rational surface) and the resonant 1/3 sawtooth like events (if the mode 1/3 is inside the plasma, there is a 1/3 rational surface close to the magnetic axis). The effect of non resonant 1/3 sawtooth like events in the plasma performance is small and the equilibria do not suffer a large distortion after their excitation, only a slight increase in the 1/2 instability in the middle plasma, 0:4 < q < 0:6, before the pressure profile is flattened and the mode saturates.…”

Section: Introductionmentioning

confidence: 95%

The internal disruption as hard Magnetohydrodynamic limit of 1/2 sawtooth like activity in large helical device

2012

Self Cite

View full text Add to dashboard Cite

Large helical device (LHD) inward-shifted configurations are unstable to resistive MHD pressuregradient-driven modes. Sawtooth like activity was observed during LHD operation. The main drivers are the unstable modes 1/2 and 1/3 in the middle and inner plasma region which limit the plasma confinement efficiency of LHD advanced operation scenarios. The aim of the present research is to study the hard MHD limit of 1/2 sawtooth like activity, not observed yet in LHD operation, and to predict its effects on the device performance. Previous investigations pointed out this system relaxation can be an internal disruption [J. Varela et al., "Internal disruptions and sawtooth like activity in LHD," 38th EPS Conference on Plasma Physics (2011), P5.077]. In the present work, we simulate an internal disruption; we study the equilibria properties before and after the disruptive process, its effects on the plasma confinement efficiency during each disruptive phase, the relation between the n/m ¼ 1/2 hard MHD events and the soft MHD events, and how to avoid or reduce their adverse effects. The simulation conclusions point out that the large stochastic region in the middle plasma strongly deforms and tears the flux surfaces when the pressure gradient increases above the hard MHD limit. If the instability reaches the inner plasma, the iota profiles will be perturbed near the plasma core and three magnetic islands can appear near the magnetic axis. If the instability is strong enough to link the stochastic regions in the middle plasma (around the half minor radius q) and the plasma core (q < 0:25), an internal disruption is driven. V C 2012 American Institute of Physics. [http://dx.

show abstract

“…The optimization of the LHD discharges requires an operation regime without plasma collapses, namely the soft MHD limit. The operation regimen where a collapse can be driven it is called the hard MHD limit, therefore it is critical to avoid the transition between the soft and the hard MHD regimes 4 .…”

Section: Introductionmentioning

confidence: 99%

“…The plasma conductivity and density are large enough to avoid the transition to the hard MHD regime [28][29][30] . In the inner plasma region, two different 1/3 sawtooth like events were analyzed, the non resonant and the resonant versions 4 . The non resonant event is driven in the soft MHD limit and its effect in the device performance is small, a minor energy leak and a weak distortion of the plasma equilibria.…”

Section: Introductionmentioning

confidence: 99%

Hard magnetohydrodynamic limit in 1/3 sawtooth like activity in LHD

et al. 2014

Self Cite

View full text Add to dashboard Cite

The optimization of LHD discharges in inward-shifted configurations with 1/3 sawtooth like activity is an open issue. These relaxation events limit the LHD performance driving a periodic plasma deconfinement. The aim of this study is to analyze the 1/3 sawtooth like activity in plasmas with different stability properties to foreseen the best operation conditions and minimize its undesired effects. We summarize the results of several MHD simulations for plasmas with Lundquist numbers between 10 5 and 10 6 in the slow reconnection regime, studying the equilibria properties during the onset of a chain of 1/3 sawtooth like events. The research conclusions point out that the hard MHD limit can be reached in the inner plasma region after the onset of a strong 1/3 resonant sawtooth like event and trigger a plasma collapse. The collapse can be avoided if the system remains in the soft MHD limit, namely in a regime with a pressure gradient and a magnetic turbulence below the critical values to drive the soft-hard MHD transition. In the soft MHD limit the system relaxations are the non resonant 1/3 sawtooth like events or a weak version of the 1/3 resonant sawtooth like events. A system relaxation in the soft MHD regime drives a minor plasma deconfinement that doesn't limit the LHD performance if the event periodicity is not very high.

show abstract

Internal disruptions and sawtooth like activity in Large Helical Device

Cited by 12 publications

References 20 publications

Effect of the tangential NBI current drive on the stability of pressure and energetic particle driven MHD modes in LHD plasma

Effect of the tangential NBI current drive on the stability of pressure and energetic particle driven MHD modes in LHD plasma

The internal disruption as hard Magnetohydrodynamic limit of 1/2 sawtooth like activity in large helical device

Hard magnetohydrodynamic limit in 1/3 sawtooth like activity in LHD

Contact Info

Product

Resources

About