Plasma Confinement Systems Research Articles

Inelastic collision effects of high-energy neutrons in tungsten materials

Because the inelastic impact of high flux, high-energy neutrons on tungsten material in a commercial fusion reactor would drastically decrease the service life and steady-state functioning of the core plasma in the future, it is vital to investigate the damage process. In this paper, a particle sputtering theory approach is used to derive an equation for calculating the sputtering yield of high-energy neutrons by simulating the transport of primary knocked-out atoms (PKA) resulting from inelastic collisions of 14.1 MeV neutrons with tungsten materials at different depth levels, namely Y =∫E0Emaxf(E)*Y(E)dE, where f (E) is the energy probability distribution function of PKA, Y (E) is the sputtering yield at a given energy of PKA particles generated by inelastic scattering collisions, Emax is the maximum kinetic energy of PKA, and E0 is the minimum kinetic energy of PKA (generally taken as 0). According to the result, when the neutron wall load is 1 MW/m2, and the annual average equivalent material wastage is less than 1 nm, sputtering damage to tungsten material can be ignored. However, it must be kept in mind that the reflection of the PKA into the plasma will have an impact on the performance of the plasma confinement system and the damage caused by the PKA's internal energy.

Charge exchange radiation diagnostic with gas jet target for measurement of plasma flow velocity in the linear magnetic trap

The study of longitudinal fluxes of energy and particles is one of the most important fundamental problems in the physics of open magnetic traps. The solution to this problem is the key to the successful implementation of fusion power in linear magnetic systems. The plasma electrostatic ambipolar potential largely determines the physics of longitudinal transport of particles and energy. In this work, we used the spectroscopic method CXRS (Charge eXchange Recombination Spectroscopy) to measure the plasma ion velocity distribution via the analysis of light emitted in this atomic process. The experiments were carried out in the gas dynamic trap (GDT), which is a linear plasma confinement system with an axially symmetric magnetic field configuration. We measured plasma potential and ion temperature of two plasma components: the main (hydrogen and deuterium) and the helium impurity. The paper presents the emission spectra of the lines H-α (656.28 nm) and He-I (667.8 nm). This method was applied for the first time to measure the spatial profile of the ambipolar potential in an open magnetic trap.

Quasisymmetric Optimization of Nonaxisymmetry in Tokamaks.

Predictive 3D optimization reveals a novel approach to modify a nonaxisymmetric magnetic perturbation to be entirely harmless for tokamaks, by essentially restoring quasisymmetry in perturbed particle orbits as much as possible. Such a quasisymmetric magnetic perturbation (QSMP) has been designed and successfully tested in the KSTAR and DIII-D tokamaks, demonstrating no performance degradation despite the large overall amplitudes of nonaxisymmetric fields and strong response otherwise expected in the tested plasmas. The results indicate that a quasisymmetric optimization is a robust path of error field correction across the resonant and nonresonant field spectrum in a tokamak, leveraging the prevailing concept of quasisymmetry for general 3D plasma confinement systems such as stellarators. The optimization becomes, in fact, a simple eigenvalue problem to the so-called torque response matrices if a perturbed equilibrium is calculated consistent with nonaxisymmetric neoclassical transport.

The influence of the load’s geometrical characteristics on the generation of the electro-thermo-mechanical instability in a single wire Z-pinch

The Z-pinch plasma device is a type of plasma confinement system that uses electrical current to generate a magnetic field which compresses a current-carrying wire. In a previous proof-of-principle study, we demonstrated that in the interaction of a single wire with a pulsed current, the generated Electro-Thermo-Mechanical (ETM) instability in the solid phase acts as a seeding mechanism for the later developed instabilities observed in the plasma phase. In this study, the influence of the geometrical characteristics, such as length and thickness of the load-wire, on the generation of the ETM instability are investigated. Finite element multiphysics-multiphase simulations starting from the solid state are coupled with Magneto-Hydro-Dynamics (MHD) simulations to study the solid to plasma phase transition and the matter’s dynamics. The numerical results of the wire expansion dynamics prior to plasma formation are validated by experimental results from a modified Fraunhofer diffraction diagnostic, while in the plasma phase the simulated plasma dynamics is validated by shadowgraphic and interferometric experimental results. The numerical and experimental results demonstrate a satisfactory agreement for the wire expansion dynamics and the growth rate of the developed instabilities, for varying wire thickness and length.

The plasma device for the high-heat plasma testing of refractory metals and inventing of new highly porous materials

A unique plasma device has been constructed at the NRU “MPEI” for the study of plasma-surface interaction and the high-heat plasma testing of refractory metals, such as tungsten, molybdenum, steel and other plasma facing materials used in fusion reactor including the ITER. This plasma device is a multi-cusp linear stationary plasma confinement system. It has power-saving characteristics as well as compactness due to the employment of the 8-pole multicusp magnetic field configuration instead of a strong axial magnetic field. Experiments are planned to develop a novel technology for highly porous surface structure of the refractory metal with a pore size and nanofibers of 50 nanometers including tungsten “fuzz”.

25 Years of Self-organized Criticality: Space and Laboratory Plasmas

Studies of complexity in extended dissipative dynamical systems, in nature and in laboratory, require multiple approaches and the framework of self-organized criticality (SOC) has been used extensively in the studies of such nonequilibrium systems. Plasmas are inherently nonlinear and many ubiquitous features such as multiscale behavior, intermittency and turbulence have been analyzed using SOC concepts. The role of SOC in advancing our understanding of space and laboratory plasmas as nonequilibrium systems is reviewed in this article. The main emphasis is on how SOC and related approaches have provided new insights and models of nonequilibrium plasma phenomena. Among the natural plasmas the magnetosphere, driven by the solar wind, is a prominent example and extensive data from ground-based and space-borne instruments have been used to study phenomena of direct relevance to space weather, viz. geomagnetic storms and substorms. During geomagnetically active periods the magnetosphere is far from equilibrium, due to its internal dynamics and being driven by the turbulent solar wind, and substorms are prominent features of the complex driven system. Studies using solar wind and magnetospheric data have shown both global and multiscale features of substorms. While the global behavior exhibits system-wide changes, the multiscale behavior shows scaling features. Along with the studies based on observational data, analogue models of the magnetosphere have advanced the understanding of space plasmas as well as the role of SOC in natural systems. In laboratory systems, SOC has been used in modeling the plasma behavior in fusion experiments, mainly in tokamaks and stellarators. Tokamaks are the dominant plasma confinement system and modeling based on SOC have provided a complementary approach to the understanding of plasma behavior under fusion conditions. These studies have provided insights into key features of toroidally confined plasmas, e.g., the existence of critical temperature gradients above which the transport rates increase drastically. The SOC models address the transport properties from a more general approach, compared to those based on turbulence arising from specific plasma instabilities, and provide a better framework for modeling features such as superdiffusion. The studies of space and laboratory plasmas as nonequilibrium systems have been motivated by features such as scaling and critical behavior, and have provided new insights by highlighting the properties that are common with other systems.

Detection of magnetic barriers in a chaotic domain: first application of finite time Lyapunov exponent method to a magnetic confinement configuration

Magnetic field lines embedded in a plasma confinement system are often characterized by a chaotic motion. This weakens the confinement properties of any magnetic configuration. However, even in case of chaotic domains, magnetic barriers can emerge and limit the field line motion itself. In the context of the numerical simulation of a Reversed-Field Pinch configuration a new magnetic topology analysis, borrowed from previous fluid dynamic studies, is discussed. This methodology relies on the behavior of the Finite Time Lyapunov Exponent (FTLE) associated with the magnetic field. By referring to a previous work in which the magnetic field is given in terms of analytical function (Borgogno et al 2011 Phys. Plasmas 18 102307) the FTLE field shows the presence of ridges, special gradient lines normal to the direction of minimum curvature, forming magnetic barriers. These ridges can be recognized as Lagrangian Coherent Structures (LCSs) for the system, actually opposing the penetration of magnetic field lines across them. In this article a more general numerical scheme for the detection of the LCSs has been adopted that allows analysis of realistic cases in which the magnetic fields are numerically known on a discrete mesh. After a validation test performed on the analytical case, a first application to a numerical magnetohydrodynamics simulation of the RFP, characterized by a broad chaotic region, has been performed. A strong magnetic barrier has been observed that effectively limits the field lines motion inside the chaotic sea.

Classical trajectory Monte Carlo simulations of particle confinement using dual levitated coils

The particle confinement properties of plasma confinement systems that employ dual levitated magnetic coils are investigated using classical trajectory Monte Carlo simulations. Two model systems are examined. In one, two identical current-carrying loops are coaxial and separated axially. In the second, two concentric and coplanar loops have different radii and carry equal currents. In both systems, a magnetic null circle is present between the current loops. Simulations are carried out for seven current loop separations for each system and at numerous values of magnetic field strength. Particle confinement is investigated at three locations between the loops at different distances from the magnetic null circle. Each simulated particle that did not escape the system exhibited one of four modes of confinement. Reduced results are given for both systems as the lowest magnetic field strength that exhibits complete confinement of all simulated particles for a particular loop separation.

Magnetohydrodynamic Stabilization of Axisymmetric Mirror Systems Using Pulsed ECRH

This paper, part of a continuing study of means for the stabilization of magnetohydrodynamic interchange modes in axisymmetric mirror-based plasma confinement systems, represents a preliminary look at a technique that would employ a train of plasma pressure pulses produced by electron cyclotron resonance heating (ECRH) to accomplish the stabilization. The use of sequentially pulsed ECRH rather than continuous-wave ECRH facilitates the localization of the heated-electron plasma pulses in regions of the magnetic field with positive field-line curvature, e.g., in the “expander” region of the mirror magnetic field, outside the outermost mirror. The technique proposed relies on the time-averaged effect of plasma pressure pulses generated in regions of positive field-line curvature to overcome the destabilizing effect of plasma pressure in regions of negative field-line curvature within the confinement region. The plasma pulses, when produced in regions of the confining field having a negative gradient, create transient ambipolar electric potentials, an effect studied in 1964 in the PLEIADE experiment in France. These electric fields preserve the localization of the hot-electron plasma pulse for times determined by ion inertia. It may be possible to use this aspect of pulsed ECRH not only to stabilize the plasma but also to plug mirror losses in a manner similar to that employed in the tandem mirror.

Minimum dissipative relaxed states applied to laboratory and space plasmas

AbstractThe usual theory of plasma relaxation, based on the selective decay of magnetic energy over the (global) magnetic helicity, predicts a force-free state for a plasma. Such a force-free state is inadequate to describe most realistic plasma systems occurring in laboratory and space plasmas as it produces a zero pressure gradient and cannot couple magnetic fields with flow. A different theory of relaxation has been proposed by many authors, based on a well-known principle of irreversible thermodynamics, the principle of minimum entropy production rate which is equivalent to the minimum dissipation rate of energy. We demonstrate the applicability of minimum dissipative relaxed states to various self-organized systems of magnetically confined plasma in the laboratory and in the astrophysical context. Such relaxed states are shown to produce a number of basic characteristics of laboratory plasma confinement systems and solar arcade structure.

Possible mechanism for the onset of vertical asymmetry of diverted plasma fluxes in a torsatron

Another mechanism is proposed to explain the onset of vertical asymmetry of diverted plasma fluxes in toroidal plasma confinement systems with a helical divertor configuration. The mechanism is governed by the possible regular displacement of plasma particles in a certain direction, e.g., due to the rigid rotation of the plasma column.

Comparison of a Deuterium-Helium-3 FRC and Mirror Trap

Field reversed configurations (FRC), as mirror machines have been developed as simple plasma confinement systems. FRC formed by reversing of magnetic field and reconnection and sustained by plasma current. Several mirrors may be linked together with solenoids - tandem mirrors (TM), end losses in the axisymmetric tandem mirror are reduced by ambipolar trapping - ambipolar trap (AT). Power balance of deuterium - helium-3 (D-3He) plasma in a prolate FRC power plant is compared with the power balance for a central cell of D-3He tandem mirror reactor (ambipolar trap). Deuterium - tritium (D-T) ITER, kinetically stabilized tandem mirror (KSTM) and D-3He FRC parameters are demonstrated.

Divertor Stabilization in Mirrors

Divertor stabilization is considered as an alternative MHD stability concept for mirror-based plasma confinement systems. The discussed concept is based on thermodynamics and self-organization of magnetized plasmas, contrary to the conventional MHD stabilization, based on the presence of averaged “magnetic well”. We show that self-organized plasma maintenance near a relaxed marginally-stable state with decreasing pressure profile can be realized in essentially non-paraxial longitudinally linked mirror cells restricted by a joint divertorlike magnetic separatrix with one or several magnetic field nulls at the plasma edge. Brief history, basic principles, and main advantages of the divertor stabilization concept are discussed.

Heating and current drive by electron cyclotron waves

The physics model of electron cyclotron heating (ECH) and current drive (ECCD) is becoming well validated through systematic comparisons of theory and experiment. This work has shown that ECH and ECCD can be highly localized and robustly controlled in toroidal plasma confinement systems, leading to applications including stabilization of magnetohydrodynamic instabilities like neoclassical tearing modes, control and sustainment of desired profiles of current density and plasma pressure, and studies of localized transport in laboratory plasmas. The experimental work was supported by a broad base of theory based on first principles which is now well encapsulated in linear ray tracing codes describing wave propagation, absorption, and current drive and in fully relativistic quasilinear Fokker–Planck codes describing in detail the response of the electrons to the energy transferred from the wave. The subtle balance between wave-induced diffusion and Coulomb relaxation in velocity space provides an understanding of the effects of trapping of current-carrying electrons in the magnetic well. Strong quasilinear effects and radial transport of electrons, which may broaden the driven current profile, have also been observed under some conditions and appear to be consistent with theory, but in large devices these are usually insignificant. The agreement of theory and experiment, the wide range of established applications, and the technical advantages of ECH support the application of ECH in next-step tokamaks and stellarators.

Magnetic field perturbations in closed-field-line systems with zero toroidal magnetic field

In some plasma confinement systems [e.g., field-reversed configurations (FRC) and levitated dipoles] the confinement is provided by a closed-field-line poloidal magnetic field. The influence of the magnetic field perturbations on the structure of the magnetic field in such systems is considered and it is found that the effect of perturbations is quite different from that in the systems with a substantial toroidal field. In particular, even infinitesimal perturbations can, in principle, lead to large radial excursions of the field lines in FRCs and levitated dipoles. Under such circumstances, particle drifts and particle collisions may give rise to significant neoclassical transport. Introduction of a weak regular toroidal magnetic field reduces radial excursions of the field lines and neoclassical transport.

A new technique to optimize the coil winding path for the arbitrarily distributed magnetic field and application to a helical confinement system

A new technique to calculate the optimum modular coil winding path for a helical plasma confinement system has been developed. The technique is based on current potential on an arbitrary surface with finite elements and singular value decomposition (SVD). No function is assumed for the current potential distribution, and then, little error occurs from modeling. The SVD is applied to the response matrix from the current potential to the magnetic field on the plasma surface. The current distribution is constructed from an eigen distribution obtained through SVD so that the normal field component is small. The coil winding path is determined along the flow lines or equi-contour lines of current potential. The current on the obtained coil winding path was confirmed able to generate good magnetic surfaces.

AMBIGUITIES IN DETERMINATION OF SELF-AFFINITY IN THE AE-INDEX TIME SERIES

The interaction between the Earth's magnetic field and the solar wind plasma results in a natural plasma confinement system which stores energy. Dissipation of this energy through Joule heating in the ionosphere can be studied via the Auroral Electrojet (AE) index. The apparent broken power law form of the frequency spectrum of this index has motivated investigation of whether it can be described as fractal coloured noise. One frequently-applied test for self-affinity is to demonstrate linear scaling of the logarithm of the structure function of a time series with the logarithm of the dilation factor λ. We point out that, while this is conclusive when applied to signals that are self-affine over many decades in λ, such as Brownian motion, the slope deviates from exact linearity and the conclusions become ambiguous when the test is used over shorter ranges of λ. We demonstrate that non self-affine time series made up of random pulses can show near-linear scaling over a finite dynamic range such that they could be misinterpreted as being self-affine. In particular, we show that pulses with functional forms such as those identified by Weimer within the AL index, from which AE is partly derived, will exhibit nearly linear scaling over ranges similar to those previously shown for AE and AL. The value of the slope, related to the Hurst exponent for a self-affine fractal, seems to be a more robust discriminator for fractality, if other information is available.

Some theoretical problems of the toroidal plasma equilibrium

Two problems of toroidal magnetic plasma confinement systems are considered. The first one is connected with advanced helical systems. The peculiarity of geometry of these stellarator systems with the improved confinement is discussed. Some additional ways for further optimization are marked too. The second problem is concerned with the island structure in toroidal systems. From the very beginning, the theoretical research on plasma equilibrium in stellarators was based mainly on the assumption that the toroidal magnetic surfaces are nested. It means that there is a unique magnetic axis which serves as the edge of the azimuthal coordinate surface θ = const. To avoid the difficulties with such a coordinate in the case of many magnetic axes that result from islands, the tokamak-like representation of the magnetic field B which does not use the coordinate θ is proposed.

Status of fusion technology development in JAERI stressing steady-state operation for future reactors

This paper reports on the progress of the fusion reactor technologies developed at the Japan Atomic Energy Research Institute (JAERI) and expected to lead to a future steady state operation reactor. In particular, superconducting coil technology for plasma confinement, NBI and RF systems technology for plasma control and current drive, fueling and pumping systems technology for particle control, heat removal technology, and development of long life materials are highlighted as the important key elements for the future steady state operation. It will be discussed how these key technologies have already been developed by the ITER (International Thermonuclear Experimental Reactor) technology R&D as well as by the Japanese domestic program, and which technologies are planned for the near future.

Energy production in ambipolar reactors with D-T, D-3He, and D-D fuel cycles

Kinetics of the D-T, D-3He, and D-D (catalyzed) fuel cycles in an ambipolar reactor were analyzed. The plasma characteristics and the plasma confinement system parameters necessary for the effective energy production in the ambipolar reactor operated with these fuels were calculated.