Application Of Magnetic Perturbations Research Articles

Runaway electron mitigation by 3D fields application in ASDEX Upgrade, COMPASS, and RFX-mod

Disruption-generated runaway electron (RE) beams represent a potentially severe threat for tokamak plasma-facing components. Application of properly designed 3D fields can act as a mitigation mechanism, as recently investigated in ASDEX Upgrade (AUG) and COMPASS experiments and in the tokamak discharges of RFX-mod. In all of these devices, the dynamics of the disruption are affected by the application of magnetic perturbations (MPs), and the resulting RE beam current and lifetime are significantly reduced. These experiments show, in particular, that the strength of the observed effects strongly depends on the poloidal spectrum of the applied MPs, which has been reconstructed including the plasma response. This paper reports the main findings on RE mitigation from the previously mentioned three devices, highlighting the common physics behind them and their interpretation by using the guiding center code ORBIT.

Dependence of the L–H power threshold on the alignment of external non-axisymmetric magnetic perturbations in ASDEX Upgrade

Experiments at ASDEX Upgrade have been conducted to study the impact of the alignment of external magnetic perturbations (MPs) with n = 2 toroidal mode symmetry on the power threshold of the transition from L- to H-mode (PLH). This is interesting for ITER, since its operation will rely on an MP field configuration that securely suppresses edge localized modes (ELMs) while avoiding an increase in the L–H power threshold. PLH can increase up to ≈80% when the external MP field is aligned to maximize the associated plasma response at the edge. This alignment deviates from the equilibrium field pitch and is similar to the one that is required to suppress ELMs by MPs in H-mode. The relative radial magnetic field perturbation δBr/BT has to exceed a critical value, roughly 1.7−2×10−4 (at the q = 5 surface), to cause an increase in PLH. This value is above the one that is required to sustain ELM suppression at ASDEX Upgrade. Combining plasma response calculations with a critical value of δBr/BT captures well the measured dependencies of PLH on the alignment of the applied MP field. The increase in PLH is accompanied by a change in the E×B flow velocity (vE×B) profile. The application of MPs in L-mode flattens and reverses the edge vE×B profile when at least δBr/BT≈1.9×10−4 is applied. This is the same value above which PLH increases. More heating power is needed to roughly achieve the same shear in the vE×B profile at the transition to H-mode as without MPs but with an vE×B profile that is shifted into the ion diamagnetic direction.

Observation of toroidal Alfvén eigenmode excited by energetic electrons induced by static magnetic perturbations in the EAST tokamak

A toroidal Alfvén eigenmode (TAE) excited by barely trapped energetic electrons during the application of a static magnetic perturbations (MPs) is observed for the first time in tokamak ohmic heating plasmas. This TAE appears when the current of a static n = 2 MPs exceeds a threshold value, at which a forced n = 2 magnetic reconnection happens. Here, n is the toroidal mode number. This TAE is located near the plasma edge, which agrees with the calculation of the TAE gap. It propagates in the ion diamagnetic direction and has a dominant toroidal mode number n = 2. The frequency of the TAE is consistent with the precessional frequency of the barely trapped energetic electrons with energy around 150 keV. An increase of the hard x-ray emission intensity in this energy range is observed just after the n = 2 magnetic reconnection. The energetic electrons may be accelerated by the magnetic islands formed after reconnection. The TAE is stabilized after a reduction in the contents of energetic electrons. This observation suggests that the application of static MPs may provide us with a novel method to actively control Alfvén eigenmodes in fusion plasmas.

H-mode achievement and edge features in RFX-mod tokamak operation

The RFX-mod experiment is a fusion device designed to operate as a reversed field pinch (RFP), with a major radius m and a minor radius m. Its high versatility recently allowed operating it also as an ohmic tokamak, allowing comparative studies between the two configurations in the same device. The device is equipped with a state of the art MHD mode feedback control system providing a magnetic boundary effective control, by applying resonant or non-resonant magnetic perturbations (MP), both in RFP and in tokamak configurations. In the fusion community the application of MPs is widely studied as a promising tool to limit the impact of plasma filaments and ELMs (edge localized modes) on plasma facing components. An important new research line is the exploitation of the RFX-mod active control system for ELM mitigation studies. As a first step in this direction, this paper presents the most recent achievements in term of RFX-mod tokamak explored scenarios, which allowed the first investigation of the ohmic and edge biasing induced H-mode. The production of D-shaped tokamak discharges and the design and deployment of an insertable polarized electrode were accomplished. Reproducible H-mode phases were obtained with insertable electrode negative biasing in single null discharges, representing an unexplored scenario with this technique. Important modifications of the edge plasma density and flow properties are observed. During the achieved H-mode ELM-like electromagnetic composite filamentary structures are observed. They are characterized by clear vorticity and parallel current density patterns.

Nuclear fusion: bringing a star down to Earth

Nuclear fusion offers the potential for being a near limitless energy source by fusing together deuterium and tritium nuclei to form helium inside a plasma burning at 100 million K. However, scientific and engineering challenges remain. This paper describes how such a plasma can be confined on Earth, and discusses the similarities and differences with fusion in stars. It focuses on the magnetic confinement technique and, in particular, the method used in a tokamak. The confinement achieved in the equilibrium state is reviewed and it is shown how the confinement can be too good, leading to explosive instabilities at the plasma edge called edge localised modes (ELMs). It is shown how the impact of ELMs can be minimised by the application of magnetic perturbations and discusses the physics behind the penetration of these perturbations into what is ideally a perfect conducting plasma.

Investigations on the edge kinetic data in regimes with type-I and mitigated ELMs at ASDEX Upgrade

The behaviour of profiles and gradients of electron density, temperature and pressure at the edge of ASDEX Upgrade was studied in regimes with type-I and small edge localized modes (ELMs) of discharges with and without applied magnetic perturbations (MPs). Estimation of the edge kinetic parameters was performed by means of integrated data analysis for joint reconstruction of electron density and temperature profiles via combination of data from different diagnostics. The MP fields for ELM mitigation were produced by 16 in-vessel coils allowing to execute this survey with large variations in poloidal spectrum and resonant component of the error field. With several dedicated discharges the effect of MPs on the edge kinetic data and ELMs was determined in dependence of heating power, gas puff and MP-coil configuration. Small ELMs are dominant—with and without MPs—in regimes with reduced pedestal top electron temperatures and flattened edge electron pressure gradients compared to type-I ELM phases. Furthermore, application of MPs opens an additional small ELM regime in the high temperature range at reduced electron pressure gradient.

Structural transformations in magnetorheological slurries induced by perturbations

The pattern formation produced by the aggregation of the particles in magnetorheological dispersions when they are in the presence of a static magnetic field, and the process of reconstruction of these structures induced by the application of magnetic perturbations, are investigated experimentally and theoretically. Under the influence of a static magnetic field the aggregation of the dispersed particles generates a multifractal structure, whose corresponding hierarchical structure has been characterized by means of its mass fractal dimensions and other correlations. When this system is perturbed by an oscillatory magnetic field, in addition to the static one, the dispersion rearranges becoming, for certain values of the amplitude and frequency of the perturbation, a more compact and relatively more ordered structure. Here, the analysis of these phenomena is approached as if the pattern formation were a kind of glassy transformation in a supercooled liquid, and the effect of the perturbation were an annealing process. The role of the temperature is taken here by the ratio of the intensities of the applied fields. From high resolution photographs taken at different stages of the structure evolution, it is possible to calculate some complexity measures such as the singularity spectrum, the lacunarity index, enthalpy and the configurational entropy. On this basis it is possible to describe quantitatively these glassy-like transformations. The relaxation time that distinguishes between a cooling process that leads the structure to a glassy or a crystalline one, is experimentally obtained.

Strike point splitting induced by the application of magnetic perturbations on MAST

Divertor strike point splitting induced by resonant magnetic perturbations (RMPs) has been observed on MAST for a variety of RMP configurations in a plasma scenario with Ip=750kA where those configurations all have similar resonant components. Complementary measurements have been obtained with divertor Langmuir probes and an infrared camera. Clear splitting consistently appears in this scenario only in the even configuration of the perturbation coils, similarly to the density pump-out. These results present a challenge for models of plasma response to RMPs.

Application of magnetic-perturbation Mössbauer spectroscopy to bi-nuclear complexes with inequivalent iron atoms

57 Fe Mossbauer spectra of diamagnetic binuclear iron complexes in applied magnetic fields have been computed for a number of cases of interest. Analysis of the spectra shows that one can deduce the signs of the quadrupole coupling constants at both iron sites, regardless of the complexity of the zero-field spectrum, and in many cases the asymmetry parameters can be evaluated as well. The magnetically perturbed spectra also provide completely unambiguous assignments of the Mossbauer parameters for the two iron atoms.