Ions In Hot Plasmas Research Articles

Scattering of ultrashort x-ray pulses in optically dense plasmas: Account for pulse duration and propagation effects

Scattering of ultrashort laser pulses (USP) in optically dense plasma is presented, accounting for the peculiarities of ultrashort electromagnetic interaction and absorption of radiation in plasmas. The results are presented in terms of a universal dimensionless total scattering coefficient (TSC). A universal closed expression for TSC is derived and analyzed. Numerical calculations of TSC are carried out for the special case of USP resonance scattering on H-like ions in hot plasmas. Fine structure splitting of the resonant transition and Doppler broadening are taken into account. The dependences of the TSC on USP parameters and the optical thickness of plasma layers are derived.

Interpreting ion-energy distributions using charge exchange emitted from deeply kinetic field-reversed-configuration plasmas

Energy resolving the atomic hydrogen outflux created within plasmas by charge exchange (CX) of hot plasma ions with cooler hydrogen neutrals is used to infer the ion energy distribution within the plasma, IEDp. In high-β plasma with field nulls and ion gyro-radii comparable to the plasma size, the measured ion energy distribution (IEDm) of the CX outflux will depend on the viewing angle and position of the detector. We describe the physics for this, results from a synthetic diagnostic code that contrasts the IEDm to the IEDp within relatively small and hot field-reversed-configuration plasmas, and how these data can show the presence of magnetic nulls and different orbit classes in the plasma.

Statistical Model for Quasicontinuum of Heavy Ions in Hot Plasma

A statistical description is proposed for the shape of heavy impurity spectra observed in tokamaks and stellarators, corresponding to the quasicontinuous radiation distribution, and called the quasicontinuum. For the tungsten ions, in particular, it is located in the spectral range of ∼2 to 7 nm. The method is based on the statistical plasma model of atom, which allows expressing the quasicontinuum structure through the electron density distribution of multielectron ions. The Slater distribution is used as a model density distribution. The quasicontinuum is formed by the great number of closely spaced lines of heavy ions with different degrees of ionization, and its calculation is very complicated. A comparison of the results obtained within the statistical approach with the results of the level-by-level quantum mechanical calculations shows that such a model plausibly describes the envelopes of the individual line arrays in the ion spectra. In contrast to the level-by-level calculation codes, the statistical atom model is rather simple and it can be used by a large number of users. The quasicontinuum was simulated for the tungsten, gold, lead, and gadolinium ions, which are observed in plasmas of fusion facilities. The simulation results are in fairly good agreement with the experimental data. The studies of the tungsten quasicontinuum are of interest from the point of view of plasma diagnostics, as well as determination of the tungsten impurity density and its transport in fusion facilities.

Excitation of atoms and ions in plasmas by ultra-short electromagnetic pulses

The problem of atoms and ions diagnostics in rarefied and dense plasmas by ultrashort laser pulses (USP) is under consideration. The application of USP provides: 1) excitation from ground states due to their carrier frequency high enough, 2) penetration into optically dense media due to short pulses duration. The excitation from ground atomic states increases sharply populations of excited atomic states in contrast with standard laser induced fluorescence spectroscopy based on radiative transitions between excited atomic states. New broadening parameter in radiation absorption, namely inverse pulse duration time 1/τ appears in addition to standard line-shape width in the profile G(ω). The Lyman-beta absorption spectra for USP are calculated for Holtsmark static broadening mechanism. Excitation of highly charged H-like ions in hot plasmas is described by both Gaussian shapes for Doppler broadening and pulse spectrum resulting in analytical absorption line-shape. USP penetration into optically thick media and corresponding excitation probability are calculated. It is shown a great effect of USP duration on excitation probabilities in optically thick media. The typical situations for plasma diagnostics by USP are discussed in details.

On stability of collisional coupling between relativistic electrons and ions in hot plasmas

The collisional coupling of relativistic electrons and non-relativistic ions in hot plasmas has been analysed. It is found that relativistic effects produce a new feature: while the condition Te<3Ti guarantees a stable collisional coupling between electrons and ions in low-temperature plasmas, relativistic effects shift the upper Te/Ti boundary of stability to higher values. Moreover, for sufficiently high temperatures, Te,i>75 keV, collisional decoupling between relativistic electrons and ions becomes impossible.

Spectral line-shape calculations for multielectron ions in hot plasmas submitted to a strong oscillating electric field

Spectral line shapes in hot plasmas submitted to a strong oscillating electric field have been theoretically studied by applying the standard hypotheses of plasma line broadening together with a nonperturbative Floquet treatment of the external oscillating field. The formulation has been applied to the lines of nonhydrogenic emitters like He-like and Ne-like ions. It is found that the simultaneous action of a strong oscillating field and plasma particle fields can lead to dramatic changes in the spectral emission.

R&D for the development of negative ion beams of halogens

Ion beams of halogens can be produced either positively or negatively charged, depending on the employed ion source. At CERN-ISOLDE, although positively charged fluorine and astatine can be obtained from a hot plasma ion source, they are often contaminated by isobars and molecular sidebands. This has generated a request from the scientific community for fluorine and astatine negative ion beams free of contaminants. The high selectivity ensured by the surface ionisation process also makes negative beams of chlorine, bromine and iodine highly attractive. However, the efficiency figures for negative surface ionisation of fluorine and astatine were lower than the ones from the hot plasma. Here we report our R&D on new surface ion sources suitable for the production of negative halogen beams.

X-ray spectroscopic study of nonequilibrium laser produced plasma in porous targets of low average density

New experimental results on laser irradiation (I ≤ 10 14 W/cm 2 , λ = 1.053 μm) of low-density fibrous agar are presented. X-ray spectrometers with spherically bent mica crystals were used for measuring with high spectral resolution the line spectra of multicharge ions. Detailed analysis of the measured spectra made it possible to determine the temperature of electrons and ions in hot plasma created in laser irradiated low-density samples in dependence on average material density and average intensity within a focal spot. Significant difference between ion and electron temperatures is found for the whole range of experimental conditions.

Release of Kr, Ag, Sn, I and Xe from UC x targets

The release properties of a UC x thick target associated with a hot plasma ion source have been studied. Measurements have been performed for various elements: Kr, Ag, Sn, I and Xe. The analysis is made assuming pure diffusion or pure effusion as the release processes. The results obtained are compared to the data available in the literature.

A model of cusp magnetic-field compression by an expanding plasma fireball

The outward expansion of a hot plasma fireball initiated by a laser-driven pellet microexplosion in a spindle-cusp magnetic-field configuration is studied. The cusp magnetic field can protect the first wall of future inertial fusion energy containment vessels from the hot plasma ions and divert the expanded charged particles (waste energy) out of the system along open field lines. Closed-form solutions are derived for the compressed magnetic field in the case of a quasispherical expansion, and the maximum (stopping) radius of the field-free plasma cavity is calculated. For a 100-MJ-charged particle energy release per laser pulse, a rather low magnetic-field strength in the line cusp of 0.1 T can protect the first wall by stopping the fireball at a radius slightly less than the 6 m radius of the (spherical) target chamber wall.

Axially inhomogeneous plasma in fast discharges for creation of population inversion in soft x-ray region

Self-organizing ‘neck’ type instabilities in capillary discharges are believed to be responsible for the population inversion in vacuum ultraviolet (VUV) spectral region. Selective population of the lasing levels originates as a result of charge–exchange recombination of the hot plasma ions during their flow from ‘necks’ into the cold surrounding gas. We propose a new approach of the ‘induced neck’ exploiting the axially inhomogeneous shock wave, generated in a capillary with a special wall profile and with high discharge current time derivative (about 1012 A s−1). A series of experiments with argon filling of the profiled capillary tube were performed. Time resolved pinhole VUV images of the discharge volume demonstrate the presence of the hot and compressed plasma objects on the discharge axes. All of them reproducibly appear at the same time and strictly on the axis of the capillary, in accordance with the wall structure. The VUV spectra, emitted by the compressed plasma, indicate a rather high electron temperature (50–100 eV), which must be sufficient for the production of hot plasma outflow.

X-ray emission of a xenon gas jet plasma diagnosed with Thomson scattering.

We present the results of a benchmark experiment aimed at validating recent calculation techniques for the emission properties of medium and high-Z multicharged ions in hot plasmas. We use space- and time-resolved M-shell x-ray spectroscopy of a laser-produced gas jet xenon plasma as a primary diagnostic of the ionization balance dynamics. We perform measurements of the electron temperature, electron density, and average charge state by recording simultaneous spectra of ion acoustic and electron plasma wave Thomson scattering. A comparison of the experimental x-ray spectra with calculations performed ab initio with a non-local-thermodynamic-equilibrium collisional-radiative model based on the superconfiguration formalism, using the measured plasma parameters, is presented and discussed.

Ion source improvement by electron injection from a ferroelectric cathode

With increasing rf power, the electron concentration in the plasma of electron cyclotron resonance (ECR) ion sources is decreasing in comparison to the ion concentration, so that the plasma is charging up positively. When reviewing the basic performance requirements of ECR sources it becomes evident that the direct injection of electrons into the ECR plasma is increasing the electron charge density and the ion current yield. Ferroelectric ceramics was used as very robust, electron emitting cathode material under the heavy-duty conditions inside the plasma chamber of an ECR ion source. The electron emission from the ferroelectric cathode is turned on by a high repetition-rate bipolar pulse of ±1.2 to 1.6 kV amplitude to the electrodes deposited on both sides of the disk-shaped cathode. Lead–barium–zirconium-titanate (30/70/30) cathodes doped with 2 mol % Bi2O3 were installed and tested in the Ar–ion plasma of the ECR ion source CAESAR at INFN-LNS, Catania. The aim was to visibly increase the yield of the ion output current by electron injection. The plasma was heated by rf in continuous mode, while the ferroelectric cathode was pulsed at a repetition rate of 50 to 250 Hz. Pulsed electron injection at 250 Hz led to a noticeable increase of 30% of the Ar8+ output current at a rf power level of 400 W. In addition, magnetohydrodynamic instabilities were damped during and after electron injection. These results strongly encourage further research and development on the application of ferroelectric cathodes for increasing the ion current yield from hot plasma ion sources.

Stark-profile calculations for spectral lines of hydrogenic ions in plasmas submitted to a strong oscillating electric field

Spectral line shapes of ions in hot plasmas submitted to a strong oscillating electric field have been theoretically studied by applying standard hypotheses of plasma line broadening along with a nonperturbative (Floquet) treatment of the (external) oscillating field. The formulation has been applied to the two first resonance lines of various hydrogenic ions in hot plasmas subjected to a strong Langmuir oscillation or submitted to an intense laser field. It is shown that the simultaneous action of a strong oscillating field and the plasma particle fields can lead to large modifications of the spectral line shapes.

Measurements of hot plasmas in the magnetosphere of Jupiter

This paper presents an overview of a number of the principal findings regarding the hot plasmas ( E ≳ 50 keV) in Jupiter's magnetosphere by the HISCALE instrument during the encounter of the Ulysses spacecraft with the planet in February 1992. The hot plasma ion fluxes measured by HI-SCALE in the dayside magnetosphere are similar to those measured in the same energy range in this region by the Voyager spacecraft in 1979. Within the dayside plasma sheet, the hot-ion energy densities are comparable with, or larger than, the magnetic field energy densities; these hot ions are found to corotate at about one-half the planetary corotational speed. For ions of energies ≳ 500 keV/nucleon, the protons contributed from 50–60% to as much as 80% of the energy content of these plasmas. Strong, magnetic-field-aligned streaming was found for both the ions and electrons in the high-latitude duskside magnetosphere. The ion and electron pitch-angle distributions could be characterized by cos 25 α throughout many of the high anisotropy intervals of the outbound pass. There is some evidence in the ion pitch-angle distributions for a corotational component in the hot plasmas at high Jovian latitudes. While there are limitations owing to the finite geometries of the detector telescope systems on the determination of the angular spreads of the ion and electron beams, the measurements show that there are intervals when the particle distributions are not bidirectional. At such times, locally the hot plasmas could be carrying currents of ∼ 10 −4 μAm −2. The temporal variations in the streaming electron fluxes are substantially larger than the variations measured for the fluxes that are more locally mirroring. The temporal variations contain periodicities that may correspond to hydromagnetic wave frequencies in the magnetosphere as well as to larger scale motions of magnetospheric plasmas. On nearly half of the days for about a 130 day interval around the time of the Ulysses encounter with the planet, particles of Jovian origin were measured in the interplanetary medium. An event discussed herein shows evidence of an energy dependence of the particle release process from the planetary magnetosphere into the interplanetary medium.

New production systems at ISOLDE

New target systems for the ISOLDE on-line mass separator facility are presented. Targets of carbides, metal/graphite mixtures, foils of refractory metals, molten metals and oxides have been tested. Beams of high intensity of neutron-rich isotopes of a large number of elements are obtained from a uranium carbide target with a hot plasma-discharge ion source. A target of ZrO 2 has been shown to provide high intensity beams of neutron-deficient isotopes of Mn, Cu, Zn, Ga, Ge, As, Se, Br, Kr and Rb, while a SiC target with a hot plasma ion source gives intense beams of radioactive isotopes of a number of light elements. All these systems are rather chemically unselective. Chemically selective performance has been obtained for several systems, i.e.: the production of neutron-deficient Au from ( 3He, pχn) reactions on a Pt/graphite target with a hot plasma ion source; the production of neutron-deficient Lu and LuF + and Hf and HfF 3 + from a Ta-foil target with a hot plasma ion source under CF 4 addition; the production of neutron-deficient Sr as SrF + and Y as YF 2 + form a Nb-foil target with a W surface ionizer under CF 4 addition; the production of neutron-deficient Se as COSe + from a ZrO 2 target with a hot plasma ion source under O 2 addition; and the production of radioactive F from a SiC target with a hot plasma ion source operating in Al vapour.

Design and calibration of the fast ion diagnostic experiment detector on the poloidal divertor experiment

A special purpose charge-exchange analyzer was constructed to measure the spatial distribution of hot-plasma ions, as a function of energy and time, in the poloidal divertor experiment (PDX). The fast neutrals produced by charge exchange within the tokamak are reionized as they pass through a helium stripping cell in the detector. The energies of these ions are determined by the trajectories they follow between cylindrical deflection plates which are set at known electrostatic potentials. We describe the technique used to calibrate the response of this system as it depends on the energies and the masses of the particles which are being detected.