Effect of magnetic islands on the local plasma behavior in a tokamak experiment

Taylor, E.; Cates, C.; Mauel, M. E.; Maurer, D.A.; Nadle, D.; Navratil, G. A.; Shilov, M.

doi:10.1063/1.1499715

Cited by 21 publications

(18 citation statements)

References 38 publications

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“…The parity we have assumed determines the boundary conditions for the two profile functions, K(0) = 0 and H (ψ) = 0 for ψ < ψ s . It also specifies one boundary condition for the equilibrium equation (13), namely, ϕ(0, y) = 0 for all y. For the second boundary condition on ϕ, we recall that the change in the vorticity across the island region is proportional to the total viscous force F y acting on the island:…”

Section: Equilibrium and Transportmentioning

confidence: 99%

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Nonlinear tearing mode in inhomogeneous plasma: I. Unmagnetized islands

Waelbroeck¹

2007

Plasma Phys. Control. Fusion

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A theory of the nonlinear growth and propagation of magnetic islands in the semi-collisional regime is presented. The theory includes the effects of finite electron temperature gradients and uses a fluid model with cold ions in slab geometry to describe islands that are unmagnetized in the sense that their width is less than ρ s , the ion Larmor radius calculated with the electron temperature. The polarization integral and the natural phase velocity are both calculated. It is found that increasing the electron temperature gradient reduces the natural phase velocity below the electron diamagnetic frequency and thus causes the polarization current to become stabilizing.

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Section: Equilibrium and Transportmentioning

confidence: 99%

“…In the island region, x ∼ w 1, we may solve the equilibrium equation by observing that ∇ 2 ∼ w −2 1, so that to lowest order equation (13) reduces to Laplace's equation, ∇ 2 ϕ 0 = 0. The solution is…”

Section: Thin-island Regime: Inner Region Solutionmentioning

confidence: 99%

Nonlinear tearing mode in inhomogeneous plasma: I. Unmagnetized islands

Waelbroeck¹

2007

Plasma Phys. Control. Fusion

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“…A few simulations have been used to understand the toroidal magnetic surfaces in tokomak plasma [1,2]. Using the definition of the ratio of kinetic pressure to magnetic pressure, b ¼ 2l 0 P B 2 , the toroidal and poloidal beta are given by b t ¼ 2l 0 P B 2 t and b P ¼ 2l 0 P B 2 P , respectively.…”

Section: Introductionmentioning

confidence: 99%

Experimental Measurement of Asymmetric Fluctuations of Poloidal Magnetic Field in Damavand Tokomak at Different Plasma Currents

et al. 2011

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Toroidal and Poloidal magnetic fields have an important effect on the tokomak topology. Damavand Tokomak is a small size tokomak characterized with k = 1.2, B t = 1T, R 0 = 36 cm, maximum plasma current is about 35 KA with a discharge time of 21 ms. In this experimental work, the variation of poloidal magnetic field on the torodial cross section is measured and analyzed. In order to measure the polodial magnetic field, 18 probes were installed on the edge of tokomak plasma with Dh = 18°, while a limiter was installed inside the torus. Plasma current, I p , induces a polodial magnetic field, B p , smaller than the torodial magnetic field B t . Magnetic lines B produced as a combination of B t and B p , are localized on the nested toroidal magnetic surfaces. The presence of polodial magnetic field is necessary for particles confinement. Mirnov oscillations are the fluctuations of polodial magnetic field, detected by magnetic probes. Disrupted instability in Tokomak typically starts with mirnov oscillations which appear as fluctuations of polodial magnetic field and is detected by magnetic probes. Minor disruptions inside the plasma can contain principal magnetic islands and their satellites can cause the annihilation of plasma confinement. Production of thin layer of turbulent magnetic field lines cause minor disruption. Magnetic limiter may cause the deformation of symmetric equilibrium configuration and chaotic magnetic islands reveal in plasma occurring in thin region of chaotic field lines close to their separatrix. The width of this chaotic layer in the right side of poloidal profile of Damavand Tokomak is smaller than the width in the left side profile because of Shafranov displacement. Ergodic region in the left side of profile develops a perturbation on the magnetic polodial field lines, B p , that are greater in magnitude than that in the right side, although the values of B p on the left side are smaller than that on the right side of the profile. The Left side of profile is close to the principal magnetic axis and the right side is away from Z axis of Tokamak.

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“…In fact, the plasma as a whole rotates with the E ∧ B velocity, so that this contribution must be subtracted from the island rotation velocity (Doppler shift). Experimental observations of magnetic islands in tokamaks, under specific conditions, show a rotation frequency closer to the ion diamagnetic frequency, ω − ω E ≈ ω * i [4,5,6]. This disagreement between the predictions of the linear theory and the experimental observations raises doubts on the validity of the most credited theoretical models describing magnetic island dynamics.…”

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confidence: 95%

A four-field gyrofluid model with neoclassical effects for the study of the rotation velocity of magnetic islands in tokamaks

Casolari

2018

Physics of Plasmas

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At equilibrium, in a tokamak, magnetic field lines lie on surfaces forming a family of nested tori, named magnetic surfaces. This structure of nested magnetic surfaces can be affected by instabilities. One of the most important ones is the so-called tearing mode, which is an instability "tearing" and reconnecting magnetic field lines. Magnetic reconnection locally breaks the topology of magnetic surfaces leading to a more energeticallyfavorable configuration. Magnetic islands result from the nonlinear evolution of tearing modes and represent a serious obstacle for obtaining nuclear fusion in magnetic confinement devices. In fact, the breaking of magnetic surfaces causes an increase in the heat and particle fluxes. The uncontrolled growth of magnetic islands can also lead to major disruptions, causing serious damage to the device. Many efforts have been made in the past decades to develop a theory of magnetic island dynamics in tokamaks. The interest in this kind of studies is to understand the conditions for the onset of the islands in the tokamak experiments and to control them to prevent their growth to large amplitudes and the consequent negative effects on confinement. Magnetic islands arise from the nonlinear evolution of tearing modes [1]. In the presence of an equilibrium density and temperature gradient, the tearing mode acquires a propagation frequency and the instability is said a drift-tearing mode [2]. According to the linear drift-tearing dispersion relation, the propagation frequency of the instability should be close to the electron diamagnetic frequency, ω − ω E ≈ ω * e [3,2], where the frequency is related to the velocity through the wave vector k, ω = k · v. The tearing mode is an instability characterized by a long wavelength, which corresponds to a small wavevector. ω E is the E ∧ B-drift frequency, due to the equilibrium electric field. In fact, the plasma as a whole rotates with the E ∧ B velocity, so that this contribution must be subtracted from the island rotation velocity (Doppler shift). Experimental observations of magnetic islands in tokamaks, under specific conditions, show a rotation frequency closer to the ion diamagnetic frequency, ω − ω E ≈ ω * i [4,5,6]. This disagreement between the predictions of the linear theory and the experimental observations raises doubts on the validity of the most credited theoretical models describing magnetic island dynamics. According to recently developed models, in the presence of significant electron temperature gradients, the introduction of the so-called "mode inductivity" [7] in the Ohm's law permits the existence of modes propagating with the ion diamagnetic frequency. This effect arises naturally in the linear regime, but the experimental observations of the island rotation concern nonlinear islands, thus a direct check of the validity of this model is not currently possible. Another widely accepted interpretation of the observed rotation velocity is that, when the island width becomes larger than the ion-acoustic radius, the ion fluid canno...

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Effect of magnetic islands on the local plasma behavior in a tokamak experiment

Cited by 21 publications

References 38 publications

Nonlinear tearing mode in inhomogeneous plasma: I. Unmagnetized islands

Nonlinear tearing mode in inhomogeneous plasma: I. Unmagnetized islands

Experimental Measurement of Asymmetric Fluctuations of Poloidal Magnetic Field in Damavand Tokomak at Different Plasma Currents

A four-field gyrofluid model with neoclassical effects for the study of the rotation velocity of magnetic islands in tokamaks

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