Taylor relaxation and λ decay of unbounded, freely expanding spheromaks

Yee, J.; Bellan, Paul M.

doi:10.1063/1.1287137

Cited by 27 publications

(41 citation statements)

References 21 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…It is well established empirically in coaxial gun spheromak experiments that, under proper conditions, a spheromak "magnetic bubble" will be formed by the gun discharge. 21,22,23,24 In the simulations reported here, the bubble structure is similar to the one given in Li et al…”

Section: B Magnetized Bubble Plasmasupporting

confidence: 70%

Ideal magnetohydrodynamic simulation of magnetic bubble expansion as a model for extragalactic radio lobes

Liu

Hsu

et al. 2008

Physics of Plasmas

View full text Add to dashboard Cite

Nonlinear ideal magnetohydrodynamic (MHD) simulations of the propagation and expansion of a magnetic "bubble" plasma into a lower density, weakly-magnetized background plasma are presented. These simulations mimic the geometry and parameters of the Plasma Bubble Expansion (2007)], which is studying magnetic bubble expansion as a model for extra-galactic radio lobes. The simulations predict several key features of the bubble evolution. First, the direction of bubble expansion depends on the ratio of the bubble toroidal to poloidal magnetic field, with a higher ratio leading to expansion predominantly in the direction of propagation and a lower ratio leading to expansion predominantly normal to the direction of propagation. Second, an MHD shock and a trailing slow-mode compressible MHD wavefront are formed ahead of the bubble as it propagates into the background plasma. Third, the bubble expansion and propagation develop asymmetries about its propagation axis due to reconnection facilitated by numerical resistivity and to inhomogeneous angular momentum transport mainly due to the background magnetic field. These results will help guide the initial experiments and diagnostic measurements on PBEX.

show abstract

Section: B Magnetized Bubble Plasmasupporting

confidence: 70%

Ideal magnetohydrodynamic simulation of magnetic bubble expansion as a model for extragalactic radio lobes

Liu

Hsu

et al. 2008

Physics of Plasmas

View full text Add to dashboard Cite

show abstract

“…Additionally, the mode was shown to couple power from the central column to the spheromak [14]. The present work offers a significant new result in that a nonlinear n = 1 geometric axis kink is directly observed to produce the dynamo and poloidal flux amplification leading to spheromak formation.Building on a prior Caltech spheromak formation experiment [18], the present experiment simplifies, and makes more accessible, the spheromak formation process by using a novel coaxial gun [19]. A schematic of the gun is shown in Fig.…”

mentioning

confidence: 99%

Experimental Identification of the Kink Instability as a Poloidal Flux Amplification Mechanism for Coaxial Gun Spheromak Formation

2003

Self Cite

View full text Add to dashboard Cite

The magnetohydrodynamic kink instability is observed and identified experimentally as a poloidal flux amplification mechanism for coaxial gun spheromak formation. Plasmas in this experiment fall into three distinct regimes which depend on the peak gun current to magnetic flux ratio, with (I) low values resulting in a straight plasma column with helical magnetic field, (II) intermediate values leading to kinking of the column axis, and (III) high values leading immediately to a detached plasma. Onset of column kinking agrees quantitatively with the Kruskal-Shafranov limit, and the kink acts as a dynamo which converts toroidal to poloidal flux. Regime II clearly leads to both poloidal flux amplification and the development of a spheromak configuration.PACS numbers: 52.55. Ip,52.35.Py,52.30.Cv The spheromak [1, 2] is a simply-connected plasma configuration in which the magnetic fields are largely determined by dynamo-driven plasma currents. Because of its topological simplicity and ease of formation, the spheromak is of interest as a magnetic fusion confinement scheme [3]. Spheromak formation has traditionally been explained by Taylor's hypothesis [4] that a turbulent magnetohydrodynamic (MHD) system relaxes to a state of minimum magnetic energy subject to the constraint of constant magnetic helicity. While this hypothesis has successfully explained the existence and many equilibrium properties of spheromaks, it says nothing about the actual 3D dynamics underlying the relaxation process, e.g., for coaxial gun spheromaks, the dynamo mechanism which converts injected toroidal flux into required poloidal flux [5]. The dynamics must be 3D because Cowling's theorem [6] shows that purely axisymmetric processes cannot accomplish this. This Letter experimentally identifies the MHD kink instability [7] as a 3D mechanism which converts toroidal to poloidal flux in a coaxial gun system, thereby leading to spheromak formation. This mechanism should also be of fundamental importance to coaxial helicity injection in spherical tori [8], relaxation in reversed-field pinches [4], solar coronal plasma instabilities [9], and astrophysical jets [10].Plasmas in this experiment fall into three regimes depending on peak λ gun = µ 0 I gun /ψ gun (where I gun is the gun current and ψ gun is the bias poloidal magnetic flux intercepting the inner gun electrode), with (I) low values resulting in a straight plasma column with helical magnetic field along the symmetry axis, (II) intermediate values leading to kinking of the column axis, and (III) high values leading immediately to a detached plasma with B tor ≫ B pol . Onset of column kinking agrees quantitatively with the Kruskal-Shafranov limit [7], and the kink acts as a dynamo which converts toroidal to poloidal flux. Regime II clearly leads to both poloidal flux amplification * Now at: Los Alamos National Laboratory, Los Alamos, NM 87545 and magnetic field profiles consistent with spheromak formation. These results are qualitatively consistent with recent time-dependent resistive MHD nu...

show abstract

“…There is no flux conserver so the plasma gun is effectively surrounded by empty space. Four distinct operational regimes were observed (Yee and Bellan, 2000) and these regimes were distinguished from each other by the value of λ gun compared to the nominal linear dimension L of the gun (the radial dimension).…”

Section: Spheromak Formation Experimentsmentioning

confidence: 99%

“…Measurements of internal magnetic fields (Yee and Bellan, 2000) show that there is a significant difference between the internal λ = μ 0 J φ /B φ profile in Regimes II and III. The internal λ is peaked on the magnetic axis of the spheromak in Regime II, indicating that the spheromak is detached from the gun (not being driven by the gun).…”

Section: Spheromak Formation Experimentsmentioning

confidence: 99%

See 1 more Smart Citation

Spheromaks, solar prominences, and Alfvén instability of current sheets

2014

Self Cite

View full text Add to dashboard Cite

Three related efforts underway at Caltech are discussed: experimental studies of spheromak formation, experimental simulation of solar prominences, and Alfvén wave instability of current sheets. Spheromak formation has been studied by using a coaxial magnetized plasma gun to inject helicity-bearing plasma into a very large vacuum chamber. The spheromak is formed without a flux conserver and internal λ profiles have been measured. Spheromak-based technology has been used to make laboratory plasmas having the topology and dynamics of solar prominences. The physics of these structures is closely related to spheromaks (low β, force-free, relaxed state equilibrium) but the boundary conditions and symmetry are different. Like spheromaks, the equilibrium involves a balance between hoop forces, pinch forces, and magnetic tension. It is shown theoretically that if a current sheet becomes sufficiently thin (of the order of the ion skin depth or smaller), it becomes kinetically unstable with respect to the emission of Alfvén waves and it is proposed that this wave emission is an important aspect of the dynamics of collisionless reconnection.

show abstract

Taylor relaxation and λ decay of unbounded, freely expanding spheromaks

Cited by 27 publications

References 21 publications

Ideal magnetohydrodynamic simulation of magnetic bubble expansion as a model for extragalactic radio lobes

Ideal magnetohydrodynamic simulation of magnetic bubble expansion as a model for extragalactic radio lobes

Experimental Identification of the Kink Instability as a Poloidal Flux Amplification Mechanism for Coaxial Gun Spheromak Formation

Spheromaks, solar prominences, and Alfvén instability of current sheets

Contact Info

Product

Resources

About