Observation of a pressure limit in a gun-driven spheromak

Woodruff, S.; Hooper, E. B.; Pearlstein, L. D.; Bulmer, R.H.; Hill, D. N.; Holcomb, C. T.; McLean, H. S.; Moller, J.M.; Stallard, B. W.; Wood, R. D.

doi:10.1063/1.2188086

Cited by 6 publications

(5 citation statements)

References 13 publications

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“…4b of Ref. 25. The corresponding a profile decreases radi-ally outwards from the magnetic axis and is comparable to optimized SSPX profiles, such as the reconstruction shown in Fig.…”

Section: Linear Analysis Of Straight Spheromak Equilibriasupporting

confidence: 77%

“…This finding may be relevant to the apparent ideal-MHD -limit observed in SSPX discharges during periods when the safety-factor profile lies above 1/2. 25 A tendency for symmetry-breaking modes and reduced confinement in nonlinear resistive-MHD simulations of the quiescent phase of SSPX discharges has been noted in previous work, 33 and it is possible that consistent twotemperature extended MHD modeling would yield results that better represent the experiment's confinement properties.…”

Section: Discussionmentioning

confidence: 74%

“…13 The parameters of our straight-spheromak computations are based on conditions achieved in SSPX, 15 where consistency of pressure profiles before and after MHD activity imply a pressure-driven limit. 25 We list the dimensional parameters in Table I The safety factor q and normalized parallel current, a = µ 0 aJ || /B, profiles of our straight-spheromak equilibria are shown in Figure 14. The parallel current profile only depends on the safety factor profile and the physical dimensions of the cylinder.…”

Section: Linear Analysis Of Straight Spheromak Equilibriamentioning

confidence: 99%

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Parameter-space survey of linear G-mode and interchange in extended magnetohydrodynamics

Howell

Sovinec

2017

Physics of Plasmas

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The extended magnetohydrodynamic stability of interchange modes is studied in two configurations. In slab geometry, a local dispersion relation for the gravitational interchange mode (g-mode) with three di↵erent extensions of the MHD model [P. Zhu, et al., Phys. Rev. Lett. 101, 085005 (2008)] is analyzed. Our results delineate where drifts stablize the g-mode with gyroviscosity alone and with a two-fluid Ohm's law alone. Including the two-fluid Ohm's law produces an ion drift wave that interacts with the g-mode. This interaction gives rise to a second instability at finite k y. A second instability is also observed in numerical extended MHD computations of linear interchange in cylindrical screw-pinch equilibria, the second configuration. Particularly with incomplete models, this mode limits the regions of stability for physically realistic conditions. However, applying a consistent two-temperature extended MHD model that includes the diamagnetic heat flux density (q ⇤) makes the onset of the second mode occur at larger Hall parameter. For conditions relevant to the SSPX experiment [E.B. Hooper, Plasma Phys. Controlled Fusion 54, 113001 (2012)], significant stabilization is observed for Suydam parameters as large as unity (D s. 1).

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“…4b of Ref. 25. The corresponding a profile decreases radi-ally outwards from the magnetic axis and is comparable to optimized SSPX profiles, such as the reconstruction shown in Fig.…”

Section: Linear Analysis Of Straight Spheromak Equilibriasupporting

confidence: 77%

Section: Discussionmentioning

confidence: 74%

Section: Linear Analysis Of Straight Spheromak Equilibriamentioning

confidence: 99%

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Parameter-space survey of linear G-mode and interchange in extended magnetohydrodynamics

Howell

Sovinec

2017

Physics of Plasmas

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“…The configuration shown in figure 29 was found to be unstable to an n φ = 2, m = 4 mode at a peak β = 6% [64]. The high-frequency, magnetic fluctuation level also increased relative to that before the instability onset.…”

Section: Pressure Limit In a Large-aspect Ratio Spheromakmentioning

confidence: 92%

Sustained Spheromak Physics Experiment (SSPX): design and physics results

Hooper

Bulmer

Cohen

et al. 2012

Plasma Phys. Control. Fusion

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The Sustained Spheromak Physics Experiment (SSPX) was a high-temperature (T e up to 0.5 keV) spheromak formed by coaxial helicity injection (CHI) and with plasma duration of a few milliseconds following the high-current formation stage. Clean walls and low impurity operation were obtained by a combination of baking, discharge cleaning and titanium deposition on the walls, allowing the generation of high-quality plasmas. Resistive-magnetohydrodynamic simulations, benchmarked to the experiment, were used to elucidate the physics. The detailed characteristics of the n φ = 1 toroidal mode associated with CHI were determined as was the physics of the nonlinear current drive and magnetic reconnection that formed and sustained the spheromak. If the helicity injection rate was reduced following formation the plasma became relatively quiescent and magnetic surfaces formed. The measured thermal diffusivity in the core was as low as ∼1 m 2 s −1 . However, reconnection events during buildup or sustainment of the plasma current by CHI were found to open magnetic surfaces throughout the plasma allowing rapid energy loss to the walls. As a result, experiments and simulations in SSPX found no path to simultaneous sustainment by CHI and good energy confinement. Additional physics results are also presented in this review.

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“…It is possible that an interchange mode is triggered which may have been seen in experimental observations on sawteeth [25]. It may also be possible that turbulent transport is increased so that the Mercier mode is held at marginal stability as observed in spheromaks by Woodruff et al [26]. Given that the Mercier criterion is violated only at some rational surfaces and that the drive for the instability is the pressure gradient it seems likely for rational surfaces in the core that the Mercier mode would result in enhanced turbulence which would hold the mode at marginal stability.…”

Section: Mercier Stabilitymentioning

confidence: 90%

Tokamak equilibria and edge stability when non-axisymmetric fields are applied

Ham¹,

Chapman²,

Simpson³

et al. 2015

Plasma Phys. Control. Fusion

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Tokamaks are traditionally viewed as axisymmetric devices. However this is not always true, for example in the presence of saturated instabilities, error fields, or resonant magnetic perturbations (RMPs) applied for edge localized mode (ELM) control. We use the VMEC code (Hirshman and Whitson 1983 Phys. Fluids 26 3553) to calculate three dimensional equilibria by energy minimization for tokamak plasmas. MAST free boundary equilibria have been calculated with profiles for plasma pressure and current derived from two dimensional reconstruction. It is well known that ELMs will need to be controlled in ITER to prevent damage that may limit the lifetime of the machine (Loarte et al 2003 Plasma Phys. Control. Fusion 45 1549). ELM control has been demonstrated on several tokamaks including MAST (Kirk et al 2013 Nucl. Fusion 53 043007). However the application of RMPs causes the plasma to gain a displacement or corrugation (Liu et al 2011 Nucl. Fusion 51 083002). Previous work has shown that the phase and size of these corrugations is in agreement with experiment (Chapman et al 2012 Plasma Phys. Control. Fusion 54 105013). The interaction of these corrugations with the plasma control system (PCS) may cause high heat loads at certain toroidal locations if care is not taken (Chapman et al 2014 Plasma Phys. Control. Fusion 56 075004). VMEC assumes nested flux surfaces but this assumption has been relaxed in other stellarator codes. These codes allow equilibria where magnetic islands and stochastic regions can form. We show some initial results using the HINT2 code (Suzuki et al 2006 Nucl. Fusion 46 L19). The Mercier stability of VMEC equilibria with RMPs applied is calculated. The geodesic curvature contribution can be strongly influenced by helical Pfirsch-Schlüter currents driven by the applied RMPs. ELM mitigation is not fully understood but one of the factors that influences peeling-ballooning stability, which is linked to ELMs, is a three dimensional corrugation of the plasma edge (Chapman et al 2013 Phys. Plasmas 20 056101). The infinite n ideal ballooning stability of the equilibria with and without the RMPs applied has been calculated using the COBRA code (Sanchez et al 2000 J. Comput. Phys. 161 576-88). When RMPs are applied, the most unstable ballooning mode growth rate is increased.

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Observation of a pressure limit in a gun-driven spheromak

Cited by 6 publications

References 13 publications

Parameter-space survey of linear G-mode and interchange in extended magnetohydrodynamics

Parameter-space survey of linear G-mode and interchange in extended magnetohydrodynamics

Sustained Spheromak Physics Experiment (SSPX): design and physics results

Tokamak equilibria and edge stability when non-axisymmetric fields are applied

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