Slow Formation and Sustainment of Spheromaks by a Coaxial Magnetized Plasma Source

The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A = R0/a) reduced to A ∼ 1.5, well below the normal tokamak operating range of A ≥ 2.5. As the aspect ratio is reduced, the ideal tokamak beta β (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as β ∼ 1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation κ, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United States and Mega Ampere Spherical Tokamak in UK, active ST research has been conducted worldwide. More than 16 ST research facilities operating during this period have achieved remarkable advances in all fusion science areas, involving fundamental fusion energy science as well as innovation. These results suggest exciting future prospects for ST research both near term and longer term. The present paper reviews the scientific progress made by the worldwide ST research community during this new mega-ampere-ST era.

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“…139 This method has been previously successfully used to create spheromaks. 140 A schematic of the CHI set-up is shown in Fig. 55.…”

Section: Dc-helicity Injection Based Start-upmentioning

confidence: 99%

Recent progress on spherical torus research

Ono

Kaita

2015

Physics of Plasmas

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“…These losses must be reduced to a relatively low level for spheromaks to make attractive, high-temperature experiments or reactors. In addition to observations on the Sustained Spheromak Physics Experiment (SSPX) [2] which are discussed in more detail in this paper, saturation has also been seen in the Compact Toroid Experiment (CTX) [3], the Flux Amplification Toroid (FACT) [4], and the Spheromak Experiment (SPHEX) [5].…”

Section: Introductionmentioning

confidence: 99%

Achieving high flux amplification in a gun-driven, flux-core spheromak

Hooper¹,

Hill²,

McLean³

et al. 2007

Nucl. Fusion

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“…Since relaxation operates on a shorter time scale, the spheromak configuration is maintained regardless of the details of the specific helicity injection mechanism. Some particular examples are the coaxial helicity injection method (CHI) [4][5][6], the merging of helicitycarrying filaments (MHF) [7] and the helicity injected torus with steady inductive helicity injection (HIT-SI) [8].In this Letter we report the first evidence coming from nonlinear, resistive, 3D MHD numerical simulations that demonstrate the possibility of forming and sustaining a spheromak by forcing tangential flows at the plasma boundary. The method can by explained in terms of helicity injection and differs from other helicity injection methods employed in the past (CHI, MHF and HIT-SI).…”

mentioning

confidence: 92%

Spheromak formation and sustainment by tangential boundary flows

García-Martínez

Farengo

2010

Physics of Plasmas

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The nonlinear, resistive, 3D magnetohydrodynamic equations are solved numerically to demonstrate the possibility of forming and sustaining a spheromak by forcing tangential flows at the plasma boundary. The method can by explained in terms of helicity injection and differs from other helicity injection methods employed in the past. Several features which were also observed in previous dc helicity injection experiments are identified and discussed.Spheromak plasmas have been formed using several different techniques [1,2]. The existence of multiple formation methods clearly shows that the spheromak is a preferred (lowest energy) state toward which a magnetohydrodynamic (MHD) system naturally evolves when the appropriate boundary conditions are imposed. The physical process responsible for the formation is magnetic relaxation: on the time scale of MHD instabilities the plasma relaxes to the minimum energy state compatible with its magnetic helicity content (which remains approximately constant) [3]. Once formed, the spheromak will decay in a resistive time scale because resistivity does dissipate magnetic helicity. For this reason, the sustainment of the configuration during times longer than the resistive decay time requires some helicity injection method. Since relaxation operates on a shorter time scale, the spheromak configuration is maintained regardless of the details of the specific helicity injection mechanism. Some particular examples are the coaxial helicity injection method (CHI) [4][5][6], the merging of helicitycarrying filaments (MHF) [7] and the helicity injected torus with steady inductive helicity injection (HIT-SI) [8].In this Letter we report the first evidence coming from nonlinear, resistive, 3D MHD numerical simulations that demonstrate the possibility of forming and sustaining a spheromak by forcing tangential flows at the plasma boundary. The method can by explained in terms of helicity injection and differs from other helicity injection methods employed in the past (CHI, MHF and HIT-SI).An enhanced helicity injection mode was recently reported in spheromak experiments with large plasma rotation [9]. Althought not analyzed in terms of boundary flows, this observation could support the feasibility of the mechanism proposed and studied in this Letter.We model the plasma using the resistive MHD equations with finite viscosity and zero β. The evolution equations for u and B are:where E = −u × B + ηJ and Π = (∇u + ∇u T ) − (2/3) × (∇·u) (see Ref.[10] for further details). These equations are normalized with a (chamber radius), ψ G (imposed flux) and c A (Alfvèn speed). In addition, B and J are scaled with √ µ 0 . Time is expressed in units of the Alfvèn time τ A = a/c A . The normalized resistivity η and the kinematic viscosity ν are set to 5 × 10 −5 . With these values the resistive time is τ r ∼ 800 (defined as in Ref.[11]). The resulting system is solved with the Versatile Advection Code [12].The domain is a cylinder of elongation h/a = 1, with perfectly conducting wall conditions (B · n = 0 ...

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Slow Formation and Sustainment of Spheromaks by a Coaxial Magnetized Plasma Source

Cited by 139 publications

References 15 publications

Recent progress on spherical torus research

Recent progress on spherical torus research

Achieving high flux amplification in a gun-driven, flux-core spheromak

Spheromak formation and sustainment by tangential boundary flows

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