Plasma Configuration Research Articles

Engineering Challenges of Wendelstein 7-X Mechanical Monitoring During Second Phase of Operation

The largest modular stellarator, the Wendelstein 7-X (W7-X), has completed its second phase of operation, OP1.2, in Greifswald, Germany. The inertially cooled divertor installed between mid-2016 and mid-2017 has allowed a wider range of plasma configurations in comparison with the first operation phase, OP1. The sophisticated W7-X superconducting magnet system is further loaded up to 70% of its maximum design loads for all main components. The extensive set of mechanical sensors clearly shows a highly nonlinear system response, which is in rather good correspondence with the predictions from the available advanced numerical models.However, there are also significant deviations observed in several areas. Therefore, modeling improvements and/or parameter variation analyses are necessary to clarify the issues in preparation for the upcoming, more demanding phase OP2 (2021+) with the actively cooled divertor and longer plasma pulses to guarantee safe and reliable W7-X operation.The updated strategy to release multiple new plasma configurations being compatible with W7-X component design values is described briefly. In this approach, the numerical model linearization in the vicinity of an accurately analyzed point is a key method to accelerate the process and to highlight areas for vacuum field parameters not allowed for plasma operation due to structural criticality.A brief overview of the W7-X measurement results, the observed deviations with numerical models, and the implemented improvements, as well as the lessons learned so far, are presented.

Experimental implementation of a real-time power flux estimator for the ITER first wall on the TCV tokamak

A control-oriented approach to the monitoring of wall power flux densities on ITER has been successfully developed. It is based on real-time equilibrium reconstruction in 2-D which is then used to describe the deposited heat flux as a poloidal flux function with user specified parameters for the power exhausted into the scrape-off layer (SOL) and the SOL heat flux width. To account for the real 3-D geometry of the plasma-facing components (PFC), appropriate weighting factors are derived from magnetic field line tracing in 3-D. Integration of the 3-D effect is performed with a new GUI-based software environment, SMITER, incorporating a field line tracer and permitting import and meshing of PFC CAD models. The paper discusses the experimental demonstration of the model-based wall heat flux algorithm on the TCV tokamak, reporting on the benchmarking of the new code package, SMITER against infra-red camera heat flux measurements and the derivation of the component shaping weighting factors. A comparison of the real-time estimation of the peak power flux and its spatial location against the off-line infra-red measurement for limiter plasma configurations is presented.

Controllability study of plasma vertical instability in HL-2M

Plasma vertical instability control is achieved by passive stabilization from induced currents flowing in surrounding structures and by active control using magnetic coils. Both aspects are numerically investigated for the medium-size copper tokamak HL-2 M, presently under construction in China. The double-shell vessel can provide some passive stabilization, with the longest wall penetration time of about 4 ms. We design an out-vessel active control system based on a single null divertor equilibria most demanding for vertical control and the maximum controllable vertical displacement dZmax ≥3.0 cm is required. Robust active control of the vertical instability can be achieved in HL-2 M, by the PF7U and PF7L coils connected in anti-phase and powered by an upgraded power supply with Imax = 4.0 kA, Vmax =2 kV and f = 5 kHz. Safe control is possible for representative plasma configurations with growth rate ranging from 200 s−1 to 400 s−1. Efficiency of each PF coil location for VDE control is investigated, PF7U/L is most capable for single null and double null equilibria, but control capability of PF7U/L is not as good as PF2U/L for snowflake divertor plasma, PF7U/L and PF2U/L will be combined together to provide safe control of low βp ‘close nulls’ advanced divertor plasma in the future.

Abnormal Glow Discharge between Two Electrodes on Plane with Transverse Magnetic Field

Surface glow discharge in the abnormal mode of its burning at pressure 5 Torr is numerically studied at a voltage drop of ∼650 V between electrodes, located on plate surface at a distance of 2 cm. The drift-diffusion two-dimensional model of gas discharge plasma is used for the description of electrodynamic parameters of the discharge. It is shown that external transverse magnetic field of induction about 0.2 T occurs significant influence on the gas discharge plasma configuration, squeezing or expanding electric current channel between electrodes depending on the magnetic field polarization. Two-dimensional fields of electrodynamic functions, such as the volume concentrations of charged particles, electric potential and the force lines of electric field strength without and with external magnetic field are presented and analyzed.

Products on electrodes in an argon-methane magnetically rotating arc at atmospheric pressure

The arc discharge method in a hydrocarbon atmosphere has a promising application in the nanocarbon synthesis field due to its high yield. In this paper, products on electrodes in an argon-methane magnetically rotating arc at atmospheric pressure are investigated. Different nanocarbons are obtained on the cathode and anode surfaces, respectively. Material analysis indicates that the products on the cathode are mainly carbon tubes. Some carbon tubes are well-aligned in one direction, and the others have a coil-like structure. The products on the anode are a mixture of graphene nano-flakes and aggregate hollow carbon particles. The hollow carbon particles exhibit a chain-like structure with internal space to be connected. To the authors’ knowledge, this is possibly the first time that aggregate hollow carbon particles with a chain-like structure have been observed in the absence of any catalyst. Based on the configuration of arc plasma, the formation mechanism of electrode products is briefly proposed.

Diamagnetic field states in cosmological plasmas.

Using a generally covariant electrovortic (magnetofluid) formalism for relativistic plasmas, the dynamical evolution of a generalized vorticity (a combination of the magnetic and kinematic parts) is studied in a cosmological context. We derive macroscopic vorticity and magnetic field structures that can emerge in spatial equilibrium configurations of the relativistic plasma. These fields, however, evolve in time. These magnetic and velocity fields, self-consistently sustained in a plasma with arbitrary thermodynamics, constitute a diamagnetic state in the expanding universe. In particular, we explore a special class of magnetic and velocity field structures supported by a plasma in which the generalized vorticity vanishes. We derive a highly interesting characteristic of such "superconductor-like" fields in a cosmological plasmas in the radiation era in the early universe. In that case, the fields grow proportional to the scale factor, establishing a deep connection between the expanding universe and the primordial magnetic fields.

SPIDER in the roadmap of the ITER neutral beams

To reach fusion conditions and control plasma configuration in ITER, a suitable combination of additional heating and current drive systems is necessary. Among them, two Neutral Beam Injectors (NBI) will provide 33 MW hydrogen/deuterium particles electrostatically accelerated to 1 MeV; efficient gas-cell neutralisation at such beam energy requires negative ions, obtained by caesium-catalysed surface conversion of atoms inside the ion source. As ITER NBI requirements have never been simultaneously attained, a Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Italy), including two experiments. MITICA is the full-scale NBI prototype with 1 MeV particle energy. SPIDER, with 100 keV particle energy, aims at testing and optimising the full-scale ion source: extracted beam uniformity, negative ion current density (for one hour) and beam optics (beam divergence <7 mrad; beam aiming direction within 2 mrad). This paper outlines the worldwide effort towards the ITER NBI realisation: the main results of the ELISE facility (IPP-Garching, Germany), equipped with a half-size source, are described along with the status of MITICA; specific issues are investigated by small specific facilities and by joint experiments at QST and NIFS (Japan). The SPIDER experiment, just come into operation, will profit from strong modelling activities, to simulate and interpret experimental scenarios, and from advanced diagnostic instruments, providing thorough plasma and beam characterisation. Finally, the results of the first experiments in SPIDER are presented, aimed at a preliminary source plasma characterisation by plasma light detectors and plasma spectroscopy.

On the Fractional Diffusion-Advection Equation for Fluids and Plasmas

The problem of studying anomalous superdiffusive transport by means of fractional transport equations is considered. We concentrate on the case when an advection flow is present (since this corresponds to many actual plasma configurations), as well as on the case when a boundary is also present. We propose that the presence of a boundary can be taken into account by adopting the Caputo fractional derivatives for the side of the boundary (here, the left side), while the Riemann-Liouville derivative is used for the unbounded side (here, the right side). These derivatives are used to write the fractional diffusion–advection equation. We look for solutions in the steady-state case, as such solutions are of practical interest for comparison with observations both in laboratory and astrophysical plasmas. It is shown that the solutions in the completely asymmetric cases have the form of Mittag-Leffler functions in the case of the left fractional contribution, and the form of an exponential decay in the case of the right fractional contribution. Possible applications to space plasmas are discussed.

SMITER: A field-line tracing environment for ITER

Built around the SMARDDA modules for magnetic field-line tracing [IEEE Tr. Plasma Sc. 42 (2014) 1932], the SMITER code package (SMARDDA for ITER) is a new graphical user interface (GUI) framework for power deposition mapping on tokamak plasma-facing components (PFC) in the full 3-D CAD geometry of the machine, taking as input a user-defined specification for parallel heat flux in the scrape-off layer (SOL) and a description of the equilibrium magnetic flux. The software package provides CAD model import and integration with the ITER Integrated Modelling and Analysis Suite (IMAS), parametric CAD components catalogue and modelling, CAD de-featuring for PFC surface extraction, meshing, visualization (using an integrated ParaView module), Python scripting and batch processing, storage in hierarchical data files, with several simulation cases in one study running in parallel and using message passing interface (MPI) for code speed-up. An integrated ParaView module can combine CAD geometry, magnetic field equilibrium, meshes and results for detailed setup analysis and a module is under development for full finite element computation of surface temperatures resulting from the power deposition patterns on 3-D PFCs. The code package has been developed for ITER, but can be deployed for similar modelling of any tokamak. This paper presents and discusses key features of this field-line tracing environment, demonstrates benchmarking against existing field-line tracing code and provides specific examples of power deposition mapping in ITER for different plasma configurations.

1-D Modeling of the Screw-Pinch Plasma in PROTO-SPHERA

A simple steady-state model for a 3-species mixture (ions, electrons, and neutrals) in a screw-pinch plasma configuration is developed. The model is applied to the central plasma column of the PROTO-SPHERA experiment. Degree of ionization, azimuthal current density, and azimuthal ion velocity are calculated. Full ionization is found at plasma temperatures above 1.5 eV, with neutrals confined in an outer shell where radial plasma flow develops and drives both azimuthal current and azimuthal flow.

Coherent magnetic structures in self-organized plasmas

The concept of Lagrangian coherent structures (LCS) has been recently applied to complex magnetic configurations in plasmas in order to find and characterize their main structural features. LCS make it possible to separate regions inside these configurations where field lines exhibit a different kind of behavior. In the present article, first we review the main features and uses of this technique and then apply it to the study of configurations that evolve into a self-organized quasi-single helicity state referring in particular to results obtained in the reversed-field pinch experiment in Padua.

Resonant micro-instabilities at quasi-parallel collisionless shocks: Cause or consequence of shock (re)formation

A case of two interpenetrating, cold and quasi-neutral ion-electron plasmas is investigated with the multi-fluid approach. We consider that one plasma flows quasi-parallel to the lines of a background magnetic field embedded in another static plasma. If the flow turns super-Alfvénic, we show that parallel R/L-modes and perpendicular X/O-modes become unstable and grow in amplitude. Within the linear theory, we find that the growth rate curve of an unstable mode has a maximum at some wavenumber specific to each mode. If we consider a shock-like plasma configuration, we find that the fastest growing mode is the resonant one (with k∼rgi−1) which strongly interacts with ions. In Particle-In-Cell (PIC) simulations, we observe that a resonant wave with the same properties is excited during the early phases of shock formation. Once the wave becomes non-linear, it efficiently scatters ions and triggers the initial shock formation. This implies that the typical compression ratio of ∼4 could naturally arise as a consequence of a highly resonant micro-physical process. We model the interaction of ions reflected from the reforming shock barrier in a weak-beam case, and we show that the upstream wave now matches the instability we expect from the equations. By using PIC simulations, we explain how the strong-beam resonant instability triggers shock formation in the non-linear stage, and how the weak-beam instability reforms and transmits the shock afterwards. The micro-instabilities that we study here could largely contribute to shock triggering as well as to the reformation and transmission of the shock itself.

A novel code for the simulation of plasma equilibrium and evolution implemented in ANSYS

The simulation of the plasma equilibrium and its evolution is important for the study of plasma physics and for the correct design of fusion devices. For this purpose, a novel code, based on the solution of the Grad-Shafranov equation, has been fully implemented in ANSYS. It exploits the finite element method using the magnetic potential vector formulation. In this approach plasma pressure and current density profiles are described by means of two main parameters: the internal plasma inductance li and the poloidal beta βp. Several ITER equilibria of limiter and diverted plasma configurations have been reproduced and benchmarked with validated codes (e.g. DINA). Being fully implemented in ANSYS this code will easily allow the coupling of the 2D axisymmetric plasma equilibrium equations, describing the plasma behavior, with the full 3D eddy currents equations, describing the surrounding three-dimensional structures. Moreover it will also allow the coupling with the ANSYS mechanical module to perform self-consistent coupled electro-mechanical analyses.

Influence of plasma characteristics on the efficacy of Cold Atmospheric Plasma (CAP) for inactivation of Listeria monocytogenes and Salmonella Typhimurium biofilms

Abstract The biofilm mode of growth protects bacterial cells from applied disinfection methods for abiotic (food) contact surfaces. Therefore, new inactivation technologies such as Cold Atmospheric Plasma (CAP) should be considered. However, the influence of different plasma characteristics on the CAP efficacy for biofilm inactivation requires further study. In this research, the influence of (i) the applied plasma configuration (Dielectric Barrier Discharge (DBD) and Surface Barrier Discharge (SBD)), (ii) the oxygen level of the gas flow (He + 0.0/0.5/1.0 (v/v) % O2), and (iii) the plasma intensity (13.88, 17.88, and 21.88 V input voltage) on the CAP efficacy for inactivation of L. monocytogenes and S. Typhimurium biofilms was investigated. Depending on the applied plasma characteristics, log10-reductions up to approximately 3.5 log(CFU/cm2) were obtained. Nevertheless, it could be concluded that the highest log-reductions were in general obtained while using the DBD electrode, 0.0 (v/v) % O2, and an input voltage of 21.88 V. Industrial relevance This study demonstrated the potential application of CAP for inactivation of pathogenic biofilms developed on abiotic (food) contact surfaces. The effect of different plasma characteristics on the CAP inactivation efficacy was investigated and determined optimal conditions resulted in promising reductions of the biofilm-associated cells. By incorporating this novel technology in a complete cleaning and disinfection process, the risk of (cross) contamination of food products might extensively be reduced.

Improved plasma position detection method in EAST Tokamak using fast CCD camera

To control the steady-state operation of Tokamak plasma, it is crucial to accurately obtain its shape and position. This paper presents a method for use in rapidly detecting plasma configuration during discharge of the Experimental Advanced Superconducting Tokamak device. First, a visible/infrared integrated endoscopy diagnostic system with a large field of view is introduced, and the PCO.edge5.5 camera in this system is used to acquire a plasma discharge image. Based on the analysis of various traditional edge detection algorithms, an improved wavelet edge detection algorithm is then introduced to identify the edge of the plasma. In this method, the local maximum of the modulus of wavelet transform is searched along four gradient directions, and the adaptive threshold is adopted. Finally, the detected boundary is fitted using the least square iterative method to accurately obtain the position of the plasma. Experimental results obtained using the EAST device show that the method presented in this paper can realize expected goals and produce ideal effects; this method thus has significant potential for application in further feedback control of plasma.

Changing the characteristics of the nitrided layer using three different plasma configurations

Direct current (DC) plasma nitriding offers some disadvantages, especially in parts with complex geometries, where the presence of edges is significant, such as gears, molds, and needle punched parts are strongly affected by this effect, leading to irregular formation of nitrided layers and high variation in surface hardness. To solve these problems two techniques were developed as alternatives, namely: pulsed plasma nitriding and cathodic cage nitriding. In this work, these three nitriding techniques were compared using AISI 1045 steel as substrate. Steel discs were nitrided with the purpose of evaluating the influence of these different plasma configurations on the characteristics of the nitrided layer. The treatments in the three techniques were carried out at the same pressure, temperature, time and gas atmosphere. The samples were characterized for the microstructure, microhardness and crystalline phases present. X-ray diffraction with grazing incidence was used to verify the order of the nitrites formed and to estimate the thickness of the single-phase layers. Differences between the configurations with respect to the occurrence of restriction rings, total thickness (xe + xγ') and relative (xe/xγ') compound layers were observed. Keywords: Plasma DC; cathodic cage; pulsed plasma; nitriding; GIXRD.

Cleaning of the Eddy Current Effects From Magnetic Diagnostics

The plasma configuration in tokamaks is created and controlled by magnetic fields generated by the external coils. Similarly, plasma macroscopic parameters are determined using sensors measuring magnetic field or magnetic flux. Some algorithms require separation of the plasma contribution from the contribution of poloidal field coils and currents induced in passive structures. In this paper, an efficient, real-time applicable method is introduced. The method is based on the identification of a state-space model in vacuum shots using both experimental and finite-element model data. The tests were performed for ISTTOK and RFX-mod. In Section II, a simple method for plasma position determination for ISTTOK is proposed. We show that the centroid position measurement by magnetic diagnostics is feasible using our algorithm for separation of the plasma signal. The correctness of the resulting method is proven by comparison with the results of the heavy-ion beam.

Time resolved x-ray emission from nanosecond vacuum discharge with virtual cathode

Design features and parameters of a modernized installation of inertial electrostatic confinement based on a low-energy nanosecond vacuum discharge (∼ 1 J) are described when operating in diode geometry in the virtual cathode formation mode and the corresponding potential well. The device is used to study the processes of collisional DD synthesis in the interelectrode space, the processes of x-ray generation in complex plasma at various stages of the discharge: from the very initial stage, when the beam of autoelectrons only begins to irradiate the nonideal anode surface, to oscillating plasma configurations in later stages of the discharge. Here, the first steps were done to introduce x-ray spectral diagnostics, as well as to obtain the integral x-ray spectra for visualizations of interelectrode complex plasmas.

Comparative Analysis of Plasma Equilibrium Computations in Toroidal and Cylindrical Magnetic Traps

Toroidal magnetic traps for plasma confinement make up an extended object of controlled nuclear fusion investigations. Mathematical simulation of equilibrium plasma configurations in the traps often deals with their analogues straightened into a cylinder. This paper presents a comparative analysis of their numerical investigations in both geometry variants. Mathematical tool of the models use two-dimensional boundary problems with the Grad-Shafranov differential equation for the magnetic flux function. As the investigation result, we present some quantitative characteristics of differences between toroidal and cylindrical configurations by two examples: a plasma torus with longitudinal electrical current and the Galathea-Belt toroidal trap with two ring-shaped current-carrying conductors immersed into the plasma.

Application of the Morozov–Solov’ev Equations to a Toroidal Magnetic Trap

In 1974, A.I. Morozov and L.S. Solov’ev derived a general system of time-independent equations of two-component ideal plasma hydrodynamics [1]. This system is extremely complicated and cumbersome. For the axisymmetric case, the authors succeeded in writing it in a more compact form by introducing three flux functions for the magnetic field, electrons, and ions. The situation resembles the derivation of the Grad–Shafranov equation, when the problem of static plasma equilibrium is reduced to one second-order equation for the magnetic flux. In this case, in order to close the problem, it was required to specify two arbitrary parameter functions. When considering flows instead of static equilibrium, the number of parameter functions increases. In the present work, the Morozov–Solov’ev equations are used to describe steady-state plasma configurations in toroidal magnetic traps.