Hot Plasma Core Research Articles

Dynamics of laser induced micro-shock waves and hot core plasma in quiescent air

AbstractWe present our results on spatio-temporal evolution of laser plasma produced shockwaves (SWs) and hot core plasma (HCP) created by focused second harmonic (532 nm, 7 ns) of Nd-YAG laser in quiescent atmospheric air at f/#10 focusing geometry. Time resolved shadowgraphs imaged with the help of an ICCD camera with 1.5 ns temporal resolution revealed the presence of two co-existing sources simultaneously generating SWs. Each of the two sources independently led to a spherical SW following Sedov-Taylor theory along the laser propagation direction with a maximum velocity of 7.4 km/s and pressure of 57 MPa. While the interaction of SWs from the two sources led to a planar SW in the direction normal to the laser propagation direction. The SW detaches from the HCP and starts expanding into the ambient air at around 3 µs indicating the onset of asymmetric expansion of the HCP along the z-axis. The asymmetric expansion is observed till 10 µs beyond which the SW leaves the field of view followed by a deformation of the irradiated region in the XY-plane due to the penetration of surrounding colder air in to the HCP. The deformation in the XY-plane lasts till 600 µs. The dynamics of rapidly expanding HCP is observed to be analogous to that of cavitation bubble dynamics in fluids.

Plasma core at the center of a sonoluminescing bubble

Considering high temperature and pressure during single bubble sonoluminescence collapse, a hot plasma core is generated at the center of the bubble. In this paper a statistical mechanics approach is used to calculate the core pressure and temperature. A hydrochemical model alongside a plasma core is used to study the bubble dynamics in two host liquids of water and sulfuric acid 85 wt % containing Ar atoms. Calculation shows that the extreme pressure and temperature in the plasma core are mainly due to the interaction of the ionized Ar atoms and electrons, which is one step forward to sonofusion. The thermal bremsstrahlung mechanism of radiation is used to analyze the emitted optical energy per flash of the bubble core.

Shockwave and cavitation bubble dynamics of atmospheric air

The generation and evolution of laser induced shock waves (SWs) and the hot core plasma (HCP) created by focusing 7 ns, 532 nm laser pulses in ambient air is studied using time resolved shadowgraphic imaging technique. The dynamics of rapidly expanding plasma releasing SWs into the ambient atmosphere were studied for time delays ranging from nanoseconds to milliseconds with ns temporal resolution. The SW is observed to get detached from expanding HCP at around 3μs. Though the SWs were found to expand spherically following the Sedov-Taylor theory, the rapidly expanding HCP shows asymmetric expansion during both the expansion and cooling phase similar to that of inertial cavitation bubble (CB) dynamics. The asymmetric expansion of HCP leads to oscillation of the plasma boundary, eventually leading to collapse by forming vortices formed by the interaction of ambient air.

Numerical Modeling of the Impact of Geometry and Wall Components on Transport in the Tokamak Edge

AbstractThe SOLEDGE suite of codes has been specially designed to model the transition region from the hot core plasma to the first wall of tokamak, through the Last Closed Flux Surface (LCFS). It is designed to model electrostatic fluid turbulence for an isothermal plasma or for a plasma with temperature variations. Dedicated discretization algorithms have been implemented to handle equations for ion density, electron/ion temperatures and parallel momentum, both for the realistic cross‐section of a diverted tokamak and for a three‐dimensional cylindrical annulus. The efficient penalization method introduced in Ref. [5] has been implemented, allowing straightforward handling of solid obstacles by treating them as sink regions corresponding to strong plasma recombination in the solid state material. The SOLEDGE capability is exemplified here by simulating two equilibria: (i) a 3D cylindrical annulus and (ii) the cross‐section of a diverted tokamak. In the annulus, the analysis of the impact of a secondary discrete limiter shows that the toroidal symmetry usually assumed for density and Mach profiles is broken. The density exhibits significant variations in the toroidal direction that extend over a large region of the scrape‐off layer where magnetic field lines are connected to a secondary limiter. In the diverted geometry, computations show a transition from subsonic to supersonic flow in the vicinity of the X‐point that is related to the location of particle sources and sinks between the edge connected region and the divertor region (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

First implosion experiments with cryogenic thermonuclear fuel on the National Ignition Facility**Part of the EPS 2011 special issue. Based on the plenary talk by S H Glenzer at the 38th EPS Plasma Physics meeting in Strassbourg, 2011.

Non-burning thermonuclear fuel implosion experiments have been fielded on the National Ignition Facility to assess progress toward ignition by indirect drive inertial confinement fusion. These experiments use cryogenic fuel ice layers, consisting of mixtures of tritium and deuterium with large amounts of hydrogen to control the neutron yield and to allow fielding of an extensive suite of optical, x-ray and nuclear diagnostics. The thermonuclear fuel layer is contained in a spherical plastic capsule that is fielded in the center of a cylindrical gold hohlraum. Heating the hohlraum with 1.3 MJ of energy delivered by 192 laser beams produces a soft x-ray drive spectrum with a radiation temperature of 300 eV. The radiation field produces an ablation pressure of 100 Mbar which compresses the capsule to a spherical dense fuel shell that contains a hot plasma core 80 µm in diameter. The implosion core is observed with x-ray imaging diagnostics that provide size, shape, the absolute x-ray emission along with bangtime and hot plasma lifetime. Nuclear measurements provide the 14.1 MeV neutron yield from fusion of deuterium and tritium nuclei along with down-scattered neutrons at energies of 10–12 MeV due to energy loss by scattering in the dense fuel that surrounds the central hot-spot plasma. Neutron time-of-flight spectra allow the inference of the ion temperature while gamma-ray measurements provide the duration of nuclear activity. The fusion yield from deuterium–tritium reactions scales with ion temperature, which is in agreement with modeling over more than one order of magnitude to a neutron yield in excess of 1014 neutrons, indicating large confinement parameters on these first experiments.

Fraunhofer-type absorption lines in double-pulse laser-induced plasma

We studied the confocal double-pulse laser-induced plasma in the very beginning of its life. It was found that the second laser pulse fired 0.7 to 5 µs after the first pulse produces plasma which, during the first 0 to 20 ns, resembles solar configuration. There is a very hot and compact plasma core that radiates a broad continuum spectrum and a much larger and cooler outer shell. The light from the hot core passes through the cold outer shell and is partly absorbed by atoms and ions that are in ground (or close to ground) states. This produces absorption lines that are similar to Fraunhofer lines observed in the sun spectrum. The possibility to use these absorption lines for new direct and calibration free laser-induced breakdown spectroscopy analytical applications, both in laboratory and industrial conditions, is proved.

Bounce-free spherical hydrodynamic implosion

In a bounce-free spherical hydrodynamic implosion, the post-stagnation hot core plasma does not expand against the imploding flow. Such an implosion scheme has the advantage of improving the dwell time of the burning fuel, resulting in a higher fusion burn-up fraction. The existence of bounce-free spherical implosions is demonstrated by explicitly constructing a family of self-similar solutions to the spherically symmetric ideal hydrodynamic equations. When applied to a specific example of plasma liner driven magneto-inertial fusion, the bounce-free solution is found to produce at least a factor of four improvement in dwell time and fusion energy gain.

Li experiments on T-11M and T-10 in support of a steady-state tokamak concept with Li closed loop circulation

This paper presents a review of the last T-11M and T-10 tokamak activity in the field of Li plasma facing component (PFC) investigation. Attention is mainly paid to the realization of the concept of closed loop lithium circulation as a solution of the PFC problem of a steady-state DT volumetric neutron source on a tokamak basis. Realization of the Li PFC concept demands the decision of three main tasks: lithium injection into the plasma, Li collection before its deposition on the vacuum vessel and the return of Li to the injection zone from the collector. This emitter–collector concept assumes that the main heat flux from a hot plasma to the PFC (limiters and divertor plates) can be dissipated on the entire vessel wall surface by non-coronal Li radiation, which will smoothen the local heat load PFC. A rail limiter on the basis of a capillary porous system manufactured from tungsten felt and provided with W wings was successfully tested in the last T-11M experiments as a prototype of steady-state Li emitter–collector. A witness-sample analysis showed that the lateral sides of the rail and ring limiters crossing the plasma scrape-off layer can collect a significant (∼80%) part of Li, injected into the plasma during discharges. This can be used in the future for closing Li loop circulation. As was shown by Li pellet injection in T-10, the probability of Li penetration into the hot plasma core from its boundary is lower than that of deuterium by a factor of 5–10. This result can explain the effect of plasma cleaning (Zeff (0) ∼ 1) during T-10 Li experiments. Some different schemes of future lithium circulation loops are discussed.

Diagnosing temperature change inside sonoluminescing bubbles by calculating line spectra

With the numerical calculation of the spectrum of single bubble sonoluminescence, we find that when the maximum temperature inside a dimly luminescing bubble is relatively low, the spectral lines are prominent. As the maximum temperature of the bubble increases, the line spectrum from the bright bubble weakens or even fades away relative to the background continuum. The calculations in this paper effectively interpret the observed phenomena, indicating that the calculated results, which are closely related to the spectrum profile, such as temperature and pressure, should be reliable. The present calculation tends to negate the existence of a hot plasma core inside a sonoluminescing bubble.

Plasma-wall interactions and plasma behaviour in fusion devices with liquid lithium plasma facing components

The application of lithium as a self-recovery and renewable material of plasma facing components (PFC) can be used to solve steady state fusion reactor PFC problems. This paper is a survey of liquid Li use in current tokamaks. Liquid Li as tokamak limiter material has been tested in T-11 M tokamak (TRINITI, RF), in FTU (Italy) and in CDX-U (USA). The idea of T-11 M and FTU liquid Li limiters is based on the Capillary–Pore System (CPS) concept. The main feature of CDX-U toroidal limiter was free liquid Li surface. The crucial issue of tokamak is impurity contamination. Lithium experiments in tokamaks discovered that poor lithium penetration into hot plasma core from its periphery (lithium screening) and the development close to plasma boundary lithium non-coronal irradiative blanket. Lithium screening can be physical ground of lithium ‘emitter–collector’ limiter concept with irradiated blanket and PFC prevention from a high local power load up to level of ITER parameters.

Benchmarking of Codes for Calculating Local Net EC Power Losses in Fusion Plasmas

The codes SNECTR, CYTRAN, CYNEQ, and EXACTEC are compared in view of the calculation of the profile of the net electron cyclotron (EC) wave power density emitted for different electron temperature profiles and average temperatures of relevance for reactor-grade magnetoplasmas. The effects of either specularly or diffusely reflecting walls are assessed for a cylindrical plasma with circular cross-section, specular reflection, as assumed in EXACTEC, providing a lower bound to the net EC wave power losses in the hot plasma core (and therefore, as a rule, also to the total EC power loss) as well as to reabsorption in the edge plasma. The assumption of isotropy of the radiation intensity in the plasma that is adopted in both CYTRAN and CYNEQ (which cannot be justified a priori) is discussed and found to be adequate for strong diffuse reflection. However, it overestimates the net EC power loss in the plasma core for weakly as well as for specularly reflecting walls by up to 20%. The full transport code SNECTR (no longer in active use), for specular reflection, and the exact cylindrical code EXACTEC are in excellent agreement with each other while for strong diffuse reflection EXACTEC is found to underestimate the net EC power loss typically by 20%. EXACTEC, CYTRAN, and CYNEQ are confirmed to be well suited for use in systematic transport simulations of fusion plasmas.

Large-scale gyrokinetic particle simulation of microturbulence in magnetically confined fusion plasmas

As the global energy economy makes the transition from fossil fuels toward cleaner alternatives, nuclear fusion becomes an attractive potential solution for satisfying growing needs. Fusion, the power source of the stars, has been the focus of active research since the early, 1950s. While progress has been impressive--especially for magnetically confined plasma devices called tokamaks--the design of a practical power plant remains an outstanding challenge. A key topic of current interest is microturbulence, which is believed to be responsible for the unacceptably large leakage of energy and particles out of the hot plasma core. Understanding and controlling this process is of utmost importance for operating current devices and designing future ones. In addressing such issues, the Gyrokinetic Toroidal Code (GTC) was developed to study the global influence of microturbulence on particle and energy confinement. It has been optimized on the IBM Blue Gene/L™ (BG/L) computer, achieving essentially linear scaling on more than 30,000 processors. A full simulation of unprecedented phase-space resolution was carried out with 32,768 processors on the BG/L supercomputer located at the IBM T. J. Watson Research Center, providing new insights on the influence of collisions on microturbulence.

Gas jet disruption mitigation studies on Alcator C-Mod and DIII-D

High-pressure noble gas jet injection is a mitigation technique which potentially satisfies the requirements of fast response time and reliability, without degrading subsequent discharges. Previously reported gas jet experiments on DIII-D showed good success at reducing deleterious disruption effects. In this paper, results of recent gas jet disruption mitigation experiments on Alcator C-Mod and DIII-D are reported. Jointly, these experiments have greatly improved the understanding of gas jet dynamics and the processes involved in mitigating disruption effects. In both machines, the sequence of events following gas injection is observed to be quite similar: the jet neutrals stop near the plasma edge, the edge temperature collapses and large MHD modes are quickly destabilized, mixing the hot plasma core with the edge impurity ions and radiating away the plasma thermal energy. High radiated power fractions are achieved, thus reducing the conducted heat loads to the chamber walls and divertor. A significant (2 × or more) reduction in halo current is also observed. Runaway electron generation is small or absent. These similar results in two quite different tokamaks are encouraging for the applicability of this disruption mitigation technique to ITER.

The Hall dynamo effect and nonlinear mode coupling during sawtooth magnetic reconnection

During magnetic reconnection associated with sawtooth activity in a reversed field pinch, we observe a large fluctuation-induced Hall electromotive force, ⟨δJ×δB⟩∕nee, which is capable of modifying the equilibrium current. This Hall dynamo effect is determined in the hot plasma core by laser Faraday rotation which measures equilibrium and fluctuating magnetic field and current density. We find that the Hall dynamo is strongest when nonlinear mode coupling between three spatial Fourier modes of the resistive tearing instability is present. Mode coupling alters the phase relation between magnetic and current density fluctuations for individual Fourier modes leading to a finite Hall effect. Detailed measurements of the spatial and temporal dynamics for the dominant core resonant mode under various plasma configurations are described providing evidence regarding the origin of the Hall dynamo.

Understanding of edge plasmas in magnetic fusion energy devices

A limited overview is given of the theoretical understanding of edge plasmas in fusion devices. This plasma occupies the thin region between the hot core plasma and material walls in magnetic confinement devices. The region is typically formed by a change in magnetic topology from close magnetic field lines (i.e. the core region) and open field lines that contact material surfaces (i.e. the scrape-off layer (SOL)), with the most common example being magnetically diverted tokamaks. The physics of this region is determined by the complex interaction of plasma with neutral species in the presence of plasma turbulence, and impurity radiation is often an important component. Recent advances in modelling strong, intermittent micro-turbulent edge-plasma transport are given, as is the closely coupled self-consistent evolution of the edge-plasma profiles in tokamaks. In addition, selected new results are given for the characterization of edge-plasmas behaviour in the areas of edge-pedestal relaxation and SOL transport via edge-localized modes, impurity formation including dust and magnetic field-line stochasticity at the edge of tokamaks.

Collapsing bubbles have hot plasma core

Collapsing bubbles have hot plasma core

Plasma formation and temperature measurement during single-bubble cavitation

Single-bubble sonoluminescence (SBSL) results from the extreme temperatures and pressures achieved during bubble compression; calculations have predicted the existence of a hot, optically opaque plasma core with consequent bremsstrahlung radiation. Recent controversial reports claim the observation of neutrons from deuterium-deuterium fusion during acoustic cavitation. However, there has been previously no strong experimental evidence for the existence of a plasma during single- or multi-bubble sonoluminescence. SBSL typically produces featureless emission spectra that reveal little about the intra-cavity physical conditions or chemical processes. Here we report observations of atomic (Ar) emission and extensive molecular (SO) and ionic (O2+) progressions in SBSL spectra from concentrated aqueous H2SO4 solutions. Both the Ar and SO emission permit spectroscopic temperature determinations, as accomplished for multi-bubble sonoluminescence with other emitters. The emissive excited states observed from both Ar and O2+ are inconsistent with any thermal process. The Ar excited states involved are extremely high in energy (>13 eV) and cannot be thermally populated at the measured Ar emission temperatures (4,000-15,000 K); the ionization energy of O2 is more than twice its bond dissociation energy, so O2+ likewise cannot be thermally produced. We therefore conclude that these emitting species must originate from collisions with high-energy electrons, ions or particles from a hot plasma core.

Spectroscopic Measurements of Hot Plasma Temperature on the Multimirror Trap GOL-3

Study of a dense (~1021 m-3) plasma heating by a relativistic electron beam and confinement of the plasma in multimirror magnetic field is carried out at the GOL-3 facility. The high-resolution spectral system in the visible region is used for investigation of plasma parameters.The analysis of the shape of Dα spectral line shows that wings of this line are determined by the Doppler broadening from atoms being in the hot plasma core at high-temperature operation regime. A survey spectrometer with spatial and time resolution was used in addition to high-resolution spectrometer for detail study of this regime. Results of radial distribution measurements of Dα line radiation show the presence of neutral deuterium in the hot plasma core. Ion temperature of the hot plasma achieves 2 keV.The evolution of radial distribution of Dα line radiation, diagnostics features and results on ion temperature dynamics in the hot plasma core are presented in the paper.

Formation of a Z-pinch during electromagnetic compression of a nonquasineutral current filament

A study is made of the structure of a relativistic current filament with the azimuthal magnetic field Bθ in the range 4πnemec2≪Bθ24πnimic2, when the plasma quasineutrality near the filament axis is violated and a narrow peak in electron density is formed there. The ion dynamics in a strong radial electric field of the filament on time scales of about several inverse ion plasma frequencies ωpi−1 is investigated. The initial ion pressure prevents the ion plasma component from compression to infinitely high densities under the action of the electric field and leads to the formation of a dense hot plasma core near the axis of the Z-pinch on time scales of about a dozen ωpi−1. The compression of the ion component in the axial region gives rise to a collisionless “unloading” shock wave that propagates away from the axis and is accompanied by the vanishing of the radial ion velocity behind the shock front, the accumulation of positive charge near the axis, and the formation of a steady-state ion density profile. It is shown theoretically that ion-ion dissipation manifests itself as the destruction of the hot core of the formed Z-pinch on picosecond time scales. This may serve to explain the explosions of “hot points” in a current-carrying plasma.

Nanokristalline Werkstoffe hergestellt mit dem Thermischen HF‐Plasma

AbstractDas induktiv gekoppelte Plasma (ICP) eignet sich unter atmosphärischen Bedingungen besonders für das Aufschmelzen und Verdampfen von Werkstoffen. Das elektrodenlose ICP kann ohne Ausnahme mit beliebigen Gasgemischen betrieben werden. Mit der Verwendung eines Argon‐Sauerstoff Hüllgasgemisches des ICP werden Reaktion metall‐organischer, flüssiger Precursoren, die in den heißen Plasmakern injiziert werden, nanokristalline Schichten erzeugt. Für den Beschichtungsprozess wir ein Überschallplasmastrahl eingesetzt. Durch den Aufprall der Partikel auf dem Substrat, das direkt am Düsenausgang des Plasmatrons positioniert ist, werden dünne Schichten hoher Dichte mit Kristallitgrössen von 30‐40 nm erzeugt. Die Struktur und Korngrössen der abgeschiedenen Schichten werden mittels XRD analysiert.