3D Magnetic Configuration Research Articles

Displacement of the electron cyclotron resonance heating region and time evolution of the characteristics of short-wavelength turbulence in the 3D magnetic configuration of the L-2M stellarator

Reflection of the heating extraordinary microwave incident obliquely onto the surface of the electron cyclotron resonance (ECR) at the second harmonic of the electron gyrofrequency in the 3D magnetic configuration of the L-2M stellarator was studied experimentally. The plasma was heated using two gyrotrons with a total power of 600–700 kW, the specific heating power being 2.4–2.8 MW/m3. The displacement of the ECR region in the course of heating was monitored by measuring the phase of the reflected extraordinary wave. It is found that the growth of the plasma density is accompanied by the displacement of the ECR heating region from the center of the plasma column toward its periphery. The coefficient of reflection of the heating microwave beam from the ECR region was measured. The spectra of short-wavelength (k s ≈ 30 cm−1) plasma density fluctuations were explored by analyzing backscattered microwave radiation. A tenfold increase in the energy of short-wavelength density fluctuations and the growth of the spectral density of fluctuations in the frequency range of 0.3–1.5 MHz were observed.

Three-dimensional linear peeling-ballooning theory in magnetic fusion devices

Ideal magnetohydrodynamics theory is extended to fully 3D magnetic configurations to investigate the linear stability of intermediate to high n peeling-ballooning modes, with n the toroidal mode number. These are thought to be important for the behavior of edge localized modes and for the limit of the size of the pedestal that governs the high confinement H-mode. The end point of the derivation is a set of coupled second order ordinary differential equations with appropriate boundary conditions that minimize the perturbed energy and that can be solved to find the growth rate of the perturbations. This theory allows of the evaluation of 3D effects on edge plasma stability in tokamaks such as those associated with the toroidal ripple due to the finite number of toroidal field coils, the application of external 3D fields for elm control, local modification of the magnetic field in the vicinity of ferromagnetic components such as the test blanket modules in ITER, etc.

Time evolution of the plasma spatial structure during the formation of a current sheet in argon according to holographic interferometry

The spatiotemporal dynamics of plasma sheets formed in argon plasma in two- and three-dimensional magnetic configurations with an X-type singular line was studied using holographic interferometry. An analogy is revealed between the development of plasma sheets and the time evolution of the current sheet structure, which was previously studied using magnetic measurements. In the late stage of the sheet evolution, a change in the rotation direction of plasma sheets formed in 3D magnetic configurations was observed. Apparently, this is caused by a change in the direction of the Hall currents at the side edges of the current sheet.

Plasma acceleration in current sheets formed in helium in two- and three-dimensional magnetic configurations

The processes of heating and acceleration of plasma in current sheets formed in 2D and 3D magnetic configurations with an X-line in helium plasma have been investigated using spectroscopic methods. It is found that, in 2D magnetic configurations, plasma flows with energies of 400–1000 eV, which are substantially higher than the ion thermal energy, are generated and propagate along the width (the larger transverse dimension) of the sheet. In 3D configurations, the influence of the longitudinal (directed along the X-line) component of the magnetic field on the plasma parameters in the current sheet has been studied. It is shown that plasma acceleration caused by the Ampere force can be spatially inhomogeneous in the direction perpendicular to the sheet surface, which should lead to sheared plasma flows in the sheet.

Barriers to field line transport in 3D magnetic configurations

The transport properties of magnetized plasma configurations are studied that arise from a one-dimensional, current layer that is unstable to reconnecting modes. These magnetic configurations are partially stochastic. It is shown that ridges in the Finite Time Lyapunov Exponents (FTLE) distribution are aligned with the invariant manifolds related to the lines of uniform hyperbolicity. It is shown that these ridges form approximate Lagrangian Coherent Structures (LCS) and act as barriers to the transport of magnetic field lines.

Formation of electron clouds during particle acceleration in a 3D current sheet

AbstractAcceleration of protons and electrons in a reconnecting current sheet (RCS) is investigated with the test particle and particle-in-cell (PIC) approaches in the 3D magnetic configuration including the guiding field. PIC simulations confirm a spatial separation of electrons and protons towards the midplane and reveal that this separation occur as long as protons are getting accelerated. During this time electrons are ejected into their semispace of the current sheet moving away from the midplane to distances up to a factor of 103 – 104 of the RCS thickness and returning back to the RCS. This process of electron circulation around the current sheet midplane creates a cloud of high energy electrons around the current sheet which exists as long as protons are accelerated. Only after protons gain sufficient energy to break from the magnetic field of the RCS, they are ejected to the opposite semispace dragging accelerated electrons with them. These clouds can be the reason of hard X-ray emission in coronal sources observed by RHESSI.

Optimization Study of ICRF Heating in the LHD and HSX Configurations

AbstractTwo global simulation codes, TASK/WM (a full wave solver) and GNET (a 5‐D drift kinetic equation solver), are combined to simulate the ICRF heating in the 3D magnetic configuration. The combined code is applied to the ICRF minority heating in the LHD configuration. An optimization of the ICRF heating is considered in changing the magnetic configurations and the resonance surfaces in the LHD plasmas using GNET code. It is found that the heating efficiency is improved about 30% with the heating power of 10MW in the optimized heating scenario from that of the present standard off‐axis heating scenario. Also the ICRF minority heating is studied in the HSX plasma and it is found that the ICRF heating of about 100kW is still effective to heat the plasma even ϱ /a ∼ 1/7.5 for tail ions (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of the guide field during the current sheet formation in the three-dimensional magnetic configuration with an X line

Results are presented from studies of the formation of current sheets during exciting a current aligned with the X line of the 3D magnetic configuration, in the CS-3D device. Enhancement of the guide field (parallel to the X line) was directly observed for the first time, on the basis of magnetic measurements. After the current sheet formation, the guide field inside the sheet exceeds its initial value, as well as the field outside. It is convincingly demonstrated that an enhancement of the guide field is due to its transportation by plasma flows on the early stage of the sheet formation. The in-plane plasma currents, which produce the excess guide field, are comparable to the total current along the X line that initiates the sheet itself.

Ray Tracing Simulations of ECR Heating and ECE Diagnostic at W7-X Stellarator

This report shows the progress achieved in simulations of thermal and non-thermal effects which may appear for high power ECRH in the W7-X stellarator (under construction in Greifswald, Germany). Simulations are carried out with the new ray tracing code TRAVIS which was developed for electron cyclotron studies in arbitrary 3D magnetic configurations, with emphasis on heating, current drive and ECE diagnostic.

Estimation of the bias of the Minimum Variance technique in the determination of magnetic clouds global quantities and orientation

Magnetic clouds (MCs) are highly magnetized plasma structures that have a low proton temperature and a magnetic field vector that rotates when seen by a heliospheric observer. More than 25 years of observations of magnetic and plasma properties of MCs at 1 AU have provided significant knowledge of their magnetic structure. However, because in situ observations only give information along the trajectory of the spacecraft, their real 3D magnetic configuration remains still partially unknown. We generate a set of synthetic clouds, exploring the space of parameters that represents the possible orientations and minimum distances of the satellite trajectory to the cloud axis, p. The synthetic clouds have a local cylindrical symmetry and a linear force-free magnetic configuration. From the analysis of synthetic clouds, we quantify the errors introduced in the determination of the orientation/size (and, consequently, of the global magnetohydrodynamic quantities) by the Minimum Variance method when p is not zero.

Structure of the current sheet plasma in the magnetic field with an X line as evidence of the two-fluid plasma properties

Experimental results are reported on the structure of the plasma in current sheets in 3D magnetic configurations with a singular X line and the guide field along the X line. It is found for the first time that the sheets formed in plasmas with heavy ions: Ar, Kr and Xe, take the unusual shape: the sheet become tilted and asymmetric. Such plasma sheets differ noticeably from the sheets formed in the plasma with light ions. It is accounted for that the two-fluid effects come to play in plasmas with heavy ions. The observed sheet deformations provide an evidence of generation of the Hall currents and their interaction with the guide field.

A software package to manipulate space dependencies and geometry in magnetic confinement fusion

Improvement in the performance of magnetic confinement devices for nuclear fusion relies on the optimization of the geometry of the plasma: either the two-dimensional (2D) cross-section shape in tokamaks with toroidal symmetry or the 3D magnetic configuration in stellerators. The variation in time and space of the plasma parameters in these devices is measured using tomographic or imaging systems with a large number of detectors. To integrate the geometrical manipulations required for the analysis of experimental data, the description of the confining magnetic field configuration and the modeling and simulation of the physical processes within the plasma, an object oriented software package has been developed. Classes in this package are used to describe several coordinate systems, including magnetic flux coordinates, the geometry of the measurement systems, the configuration of the magnetic field and space, and time dependent functions representing plasma parameters. Methods applied on these classes can then easily implement coordinate system transformations, as well as interpolation of and integro-differential calculus on, space and time dependent functions. The geometrical description and characteristics of the magnetic flux surfaces have a natural representation in this environment, allowing the ready computation of the intersection of measurement viewing lines with a coordinate mesh and with flux surfaces, as well as the calculation of the corresponding transfer matrix used in tomographic inversion. The selected numerical methods used in these manipulations and their performances are also presented.

Experimental study of plasma compression into the sheet in three-dimensional magnetic fields with singular X lines

The formation and evolution of the plasma sheets resulting from the plasma compression in diversified three-dimensional (3D) magnetic configurations with singular X lines are reported on. The research was focused on the correlation between the structure of a plasma sheet and the topology of the initial 3D magnetic configuration, especially on the impact of the guide field aligned with the X line. It has been demonstrated experimentally that plasma compression and formation of extended plasma sheets can take place in configurations with the X lines in the presence of a strong guide field. The electron density distributions in the plasma sheets were found to be rather sensitive to the magnetic field topology. The experiments revealed the effect of progressive decrease of the plasma compression ratio in response to increasing guide field. This effect has two basic manifestations: a decrease of the maximum plasma density and an enlargement of the sheet thickness. Based on the experimental data we advanced a concept that the deterioration of plasma compression into the sheet is due to enhancement of the guide field inside the sheet over its initial value, and due to excitation of additional currents in the plane perpendicular to the singular X line and to the original current in the sheet.

3D Coronal magnetic field from vector magnetograms: non-constant-αforce-free configuration of the active region NOAA 8151

The Active Region 8151 (AR 8151) observed in February 1998 is the site of an eruptive event associated with a fila- ment and a S-shaped structure, and producing a slow Coronal Mass Ejection (CME). In order to determine how the CME occurs, we compute the 3D coronal magnetic field and we derive some relevant parameters such as the free magnetic energy and the relative magnetic helicity. The 3D magnetic configuration is reconstructed from photospheric magnetic magnetograms (IVM, Mees Solar Observatory) in the case of a non-constant- force-free (nl) field model. The reconstruction method is divided into three main steps: the analysis of vector magnetograms (transverse fields, vertical density of electric current, ambiguity of 180), the numerical scheme for the nl magnetic field, the interpretation of the computed magnetic field with respect to the observations. For AR 8151, the nl field matches the coronal observations from EIT/SOHO and from SXT/Yohkoh. In particu- lar, three characteristic flux tubes are shown: a highly twisted flux tube, a long twisted flux tube and a quasi-potential flux tube. The maximum energy budget is estimated to 2:6 10 31 erg and the relative magnetic helicity to 4:7 10 34 G 2 cm 4 .F rom the simple photospheric magnetic distribution and the evidence of highly twisted flux tubes, we argue that the flux rope model is the most likely to describe the initiation mechanism of the eruptive event associated with AR 8151.

Experimental study of current sheet evolution in magnetic configurations with null-points and X-lines

A review is presented on experimental research of current sheet formation in 3D magnetic configurations containing null-points and/or singular lines of the X-type. Formation of current sheets is revealed to occur in various 3D configurations, both with and without isolated magnetic null-points, specifically in configurations with X-lines. Local characteristics of 3D magnetic configuration define the parameters of the current sheet, which forms usually a twisted surface with an angular orientation determined by a local value of the transverse magnetic field gradient. A degree of plasma compression into the sheet decreases with a rise of longitudinal magnetic field component, displaying a transition to a behavior of uncompressible plasma. It is established that current sheet formation and magnetic reconnection processes can take place within a limited range of initial conditions, while a gradient of transverse magnetic field is the most important parameter.

Current sheets in magnetic configurations with singular X-lines

The possibility of the formation of current sheets in 3D magnetic configurations with singular X-lines was studied experimentally. It is shown that a sheet can be formed in the presence of the longitudinal magnetic-field component directed along the X-line, in which case the longitudinal component can exceed the transverse component everywhere inside the plasma. Characteristic of the CS formation in 3D magnetic configurations with X-lines are an increase in the longitudinal magnetic-field component inside the sheet and a decrease in the plasma compression ratio as compared to 2D configurations with null-lines. If the longitudinal component exceeds a certain critical value, a sheet cannot be formed: instead of a sheet, there appear two sheaths separated by a cavity with a local minimum in the electron density.

Magnetic reconnection and current sheet formation in 3D magnetic configurations

The problem of magnetic reconnection in three-dimensional (3D) magnetic configurations has been studied experimentally. The research has concentrated on the possibilities of formation of current sheets, which represent crucial objects for a realization of magnetic reconnection phenomena. Different types of 3D magnetic configurations were examined, including configurations with singular lines of the X-type, non-uniform fields containing isolated magnetic null-points and without null-points. It was revealed that formation of quasi-one-dimensional current sheets is the universal process for plasma dynamics in 3D magnetic fields both with null-points and without. At the same time the peculiarities of current sheets, plasma dynamics and magnetic reconnection processes depend essentially on characteristics of 3D magnetic configurations. The result of principal significance obtained was that magnetic reconnection phenomena can take place in a wide range of 3D magnetic configurations as a consequence of their ability to form current sheets.

Plasma neutraliser for ITER NBI

Abstract The maximal neutral fraction from the passage of a D beam through a gas, as proposed for a gas neutraliser of reference ITER NBI concept, is 60%. The replacement of the gas neutraliser by a plasma one must increase the maximum neutral yield to 80%. This is an attractive improvement of the ITER NBI system. As the plasma neutraliser must be able to replace the gas neutraliser in the ITER NBI of the reference concept, its length is defined and equal to 3 meters. So, the necessary plasma density ne ∼ 7 × 1012 cm−3. We assume that PN should spend less than 2 MW of electrical power per one NBI module. The multicusp 3D magnetic wali configuration seems to be the best choice for PN plasma confinement system. This system can provide the necessary plasma confinement and an acceptable deviation effect on the beam ions. The plasma will be produced in steady – state mode of operation by the electrodeless microwave discharge at gas pressure less than 10−4 torr at mainly collisionless character of microwave ene...