Mean Ion Charge Research Articles

Equation of State of Al Based on the Thomas–Fermi Model

AbstractA wide‐range semiempirical equation of state for aluminum applicable both at near‐normal conditions and in dense plasma is presented. The finite‐temperature Thomas–Fermi model is used for the thermal contribution of electrons thus diminishing the number of fitting parameters and giving information about a mean ion charge. The constructed equation of state agrees with majority of available shock‐wave data (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Measurements of the Ion Species of Cathodic Arc Plasma in an Axial Magnetic Field

Metal and gas ion species and their charge state distributions were measured for pulsed copper cathodic arcs in argon background gas in the presence of an axial magnetic field. It was found that changing the cathode position relative to anode and ion extraction system, as well as increasing the gas pressure, did not affect much the arc burning voltage and the related power dissipation. However, the burning voltage and power dissipation greatly increased as the magnetic field strength was increased. The fraction of metal ions and the mean ion charge state were reduced as the discharge length was increased. The observations can be explained by the combination of charge exchange collisions and electron impact ionization. They confirm that previously published data on characteristic material-dependent charge state distributions (e.g., Anders and Yushkov in 2002) are not universal but valid for high vacuum conditions and the specifics of the applied magnetic fields.

Ion irradiation effects on tungsten-oxide films and charge state effect on electronic erosion

We have studied electronic- and atomic-structure modifications of polycrystalline WO 3 films (bandgap of ∼3 eV) by ion irradiation. WO 3 films were prepared by oxidation of W films on MgO substrates and of W sheets. We find disordering or amorphization, the lattice expansion of ∼1.5% and bandgap increase of 0.2 eV after 90 MeV Ni ion irradiation at ∼3 × 10 12 cm −2. A broad peak of optical absorption appears around 1.6 μm by ion irradiation. We also find that the erosion yield by high-energy ions with the equilibrium charge exceeds 10 4 and that the erosion yield under ion impact with non-equilibrium charge (90 MeV Ni +10) is ∼1/5 of that with the equilibrium charge (89 MeV Ni +19). Effects of depth dependence of the ion mean charge on the erosion yields are discussed. The erosion yield by low-energy ions is also presented.

Magnetic Field Diffusion and Enhanced Resistivity in 12-cm-Diameter 200-ns 3.5-MA <formula formulatype="inline"> <tex Notation="TeX">$Z$</tex></formula>-Pinch Implosions

Investigations of magnetic field diffusion and plasma resistivity in 12-cm-diameter triple-gas-puff Ar <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> -pinch implosions were carried out by using planar laser-induced fluorescence (PLIF), a laser shearing interferometer (LSI), and a laser wavefront analyzer (LWA) on a 3.5-MA 200-ns generator. The PLIF measurements gave the initial Ar gas distributions. The implosion velocity and electron density profiles were measured from LWA and/or LSI. From these, the implosion plasma sheath thickness, ion density, mean ion charge states, temperatures, and implosion velocity are obtained, which allows us to calculate the classical plasma resistivity. A 1-D analytic magnetic field diffusion model is constructed and used to predict the imploding plasma sheath thickness and its resistivity. Based on comparisons of the experimental measurements and the diffusion model prediction, we found out that plasma resistivity is enhanced by the cross-field diffusion above the classical value, as high as 60 times the Spitzer's value. Details are given in this paper.

An extended model for ion charge state distribution of plasmas in collisional radiative steady state

A non-Boltzmann distribution is constructed in terms of ion excitation-deexcitation equilibrium in plasmas, over which the rate coefficients of atomic processes are averaged, and an extended numerical model for charge state distribution of plasmas in collisional radiative steady state is established. The model is used to compute mean ion charge for various plasmas, ranging from low Z to high Z elements, as a function of electron temperature and density. Furthermore, the effects of non-Boltzmann distribution on mean ion charge and ion excited configuration population are investigated. The results show that the ion excited configuration population is evidently affected by the non-Boltzmann distribution.

A space-charge-neutralizing plasma for beam drift compression

Simultaneous radial focusing and longitudinal compression of intense ion beams are being studied to heat matter to the warm dense matter, or strongly coupled plasma regime. Higher compression ratios can be achieved if the beam compression takes place in a plasma-filled drift region in which the space-charge forces of the ion beam are neutralized. Recently, a system of four cathodic arc plasma sources has been fabricated and the axial plasma density has been measured. A movable plasma probe array has been developed to measure the radial and axial plasma distribution inside and outside of a ∼10-cm-long final focus solenoid (FFS). Measured data show that the plasma forms a thin column of diameter ∼5 mm along the solenoid axis when the FFS is powered with an 8 T field. Measured plasma density of ⩾1×10 13 cm −3 meets the challenge of n p/ Zn b>1, where n p and n b are the plasma and ion beam density, respectively, and Z is the mean ion charge state of the beam ions.

Physical limits for high ion charge states in pulsed discharges in vacuum

Short-pulse high-current discharges in vacuum were investigated with the goal to maximize the ion charge state number. In a direct extension of previous work [G. Y. Yushkov and A. Anders, Appl. Phys. Lett. 92, 041502 (2008)], the role of pulse length, rate of current rise, and current amplitude was studied. For all experimental conditions, the usable (extractable) mean ion charge state could not be pushed beyond 7+. Instead, a maximum of the mean ion charge state (about 6+ to 7+ for most cathode materials) was found for a power of 2–3 MW dissipated in the discharge gap. The maximum is the result of two opposing processes that occur when the power is increased: (i) the formation of higher ion charge states and (ii) a greater production of neutrals (both metal and nonmetal), which reduces the charge state via charge exchange collisions.

A Numerical Model for Ion Charge Distribution of Plasmas in Collisional Radiative Steady State

A numerical model for the charge state distribution of plasmas in a collisional radiative steady state (CRSS) is established by averaging over the atomic process rate coefficients in universal kinetic equations. It is used to calculate the mean ion charge and ion population for a given temperature and density of the plasmas, ranging from low Z to high Z elements. The comparisons of the calculated results with those of other non-local thermodynamic equilibrium kinetics codes show that this model possesses acceptable precision. Furthermore, the NLTE effects are investigated by virtue of the model, and the differences between CRSS and LTE models for low density plasmas are quite evident.

Continuous phase transition in the region of the vacuum arc cathode spot

A model for the near-cathode region of electric arcs is presented to investigate the liquid-plasma phase transition in the cathode spot region. Due to the high values of pressure and temperature after spot ignition, a “continuous phase transition” occurs in the liquid-vapor interface. A set of fluid equations with suitable boundary conditions have been solved to obtain diagrams of the spot plasma in the temperature-density plane during the spot evolution for a typical spot. To evaluate the model, the magnitude of some essential quantities such as the mean ion charge state of plasma and current density have been calculated, which are in accordance with experimental results.

Infrared Spectroscopy and Elastic Properties of Nanocrystalline Mg–Mn Ferrites Prepared by Co-Precipitation Technique

Nanoparticles having particle size in the range 25-40 nm for compositions x = 0.0, 0.2, 0.4 and 0.5 of Mg(x)Mn(1-x)Fe2O4 spinel ferrite system have been prepared by chemical co-precipitation route. The microstructure, infrared spectral and elastic properties have been studied by means of energy dispersive analysis of X-rays (EDAX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and infrared spectroscopic (IR) measurements, before (W) and after high temperature annealing A(w). The force constants for tetrahedral and octahedral sites determined by infrared spectral analysis, lattice constant and X-ray density values by X-ray diffraction pattern analysis; have been used to calculate elastic constants. The magnitude of force constant and elastic moduli for nanocrystalline W-samples are found to be larger as compared to coarse grained A(w)-samples. The results have been explained in the light of redistribution of cations and as a result change in mean ionic charge for such cationic sites, elastic energy and grain size reduction effect of Nanoparticles.

Effects of magnetic field on laser-generated plasma

A laser-generated plasma in vacuum was placed in an axial magnetic field directed along the normal to the target surface. The laser beam intensity, of the order of 1010 W/cm2, was obtained by a Nd:Yag operating at 1064 nm wavelength, 9 ns pulse width, and 300 mJ pulse energy. Time-of-flight measurements of ion emission were performed along the direction normal to the target surface by using an ion collector at different target distances. Investigations were performed ablating a lot of metallic elements and applying a permanent 0.1 Tesla magnetic field. An electron magnetic trap, placed in front of the target surface, acts so as a negative charge cloud, which accelerates and deflects the ions ejected from the plasma. Results demonstrate that the magnetic field trap modifies the electron charge density in front of the target surface inducing an ion focalization, increasing the ion velocity, the mean ion charge state, and the ion current.

Self-sputtering runaway in high power impulse magnetron sputtering: The role of secondary electrons and multiply charged metal ions

Self-sputtering runaway in high power impulse magnetron sputtering is closely related to the appearance of multiply charged ions. This conclusion is based on the properties of potential emission of secondary electrons and energy balance considerations. The effect is especially strong for materials whose sputtering yield is marginally greater than unity. The absolute deposition rate increases ∼Q1∕2, whereas the rate normalized to the average power decreases ∼Q−1∕2, with Q being the mean ion charge state number.

Numerical simulations of the projectile ion charge difference in solid and gaseous stopping matter

AbstractThe dependence of calcium ion subshell populations on the target density during the ion stopping process was analyzed using a five charge-state collisional-radiative model. The model, which consists of the ground and three excited states for every ion charge, was successfully compared with the experiment. The gas-solid difference of calcium ion charge state distribution has been numerically demonstrated. For Ca projectiles with energies of 4–11 MeV/u, the increase of the mean ion charge in solid target is explained by the increase of the total ionization rate and by suppression of the bound electron capture process at high densities of the stopping medium.

Production of neutrals and their effects on the ion charge states in cathodic vacuum arc plasmas

Cathodic arc plasmas are considered fully ionized and they contain multiply charged ions, yet gaseous and metal neutrals can be present. It is shown that they can cause a significant reduction of the ion charge states as measured far from the cathode spots. Several cathode materials were used to study the evolution of the mean ion charge state as a function of time after arc ignition. The type of cathode material, arc current amplitude, intentionally increased background gas, additional surfaces placed near the plasma flow, and other factors influence the degree of charge state reduction because all of these factors influence the density of neutrals. In all cases, it was found that the mean ion charge state follows an exponential decay of first order, Q¯(t)=A exp(−t∕τ)+Q¯ss, where A is a parameter describing the importance of the decay, τ is the characteristic decay time, and Q¯ss is a steady-state value approached for continuous arc operation. The extrapolated values Q¯(t→0) indicate surprisingly high mean charge states as produced at cathode spots and not “skewed” by charge exchange collisions with neutrals.

Ionic Charge States of Solar Energetic Particles: A Clue to the Source

The ionic charge of solar energetic particles (SEP) as observed in interplanetary space is an important parameter for the diagnostic of the plasma conditions at the source region and provides fundamental information about the acceleration and propagation processes at the Sun and in interplanetary space. In this paper we review the new measurements of ionic charge states with advanced instrumentation onboard the SAMPEX, SOHO, and ACE spacecraft that provide for the first time ionic charge measurements over the wide energy range of ∼0.01 to 70 MeV/nuc (for Fe), and for many individual SEP events. These new measurements show a strong energy dependence of the mean ionic charge of heavy ions, most pronounced for iron, indicating that the previous interpretation of the mean ionic charge being solely related to the ambient plasma temperature was too simplistic. This energy dependence, in combination with models on acceleration, charge stripping, and solar and interplanetary propagation, provides constraints for the temperature, density, and acceleration time scales in the acceleration region. The comparison of the measurements with model calculations shows that for impulsive events with a large increase of Q Fe(E) at energies ≤1 MeV/nuc the acceleration occurs low in the corona, typically at altitudes ≤0.2 R S .

Cathodic Vacuum Arc Plasma of Thallium

Thallium arc plasma was investigated in a vacuum arc ion source. As expected from previous considerations of cathode materials in the Periodic Table of Elements, thallium plasma shows leadlike behavior. Its mean ion charge state exceeds 2.0 immediately after arc triggering and diminishes to the predicted 1.60 and 1.45 after about 100 and 150 mus, respectively. The most likely ion velocity is initially 8000 m/s and decays to 6500 and 6200 m/s after 100 and 150 mus, respectively. Both ion charge states and ion velocities decay further toward steady-state values, which are not reached within the 300-mus pulses used here. It is argued that the exceptionally high vapor pressure and charge exchange reactions are associated with the establishment of steady-state ion values

Control of ion charge states in vacuum arc ion source by imposition of a non-uniform magnetic field

The influence of the magnetic field geometry configuration on electron temperature and ion charge state in the vacuum arc ion source has been studied theoretically. It is found that a high value of electron temperature and ionization rate is retained at the exit from the annular anode (where cathodic plasma jet becomes current-free) if a strong magnetic field in the order of 1 T is imposed on this part of jet. It is shown that the maximum value of the mean ion charge can be obtained by imposing on the inter-electrode gap an external magnetic field with the slightly divergent field lines. The divergence angle of the field lines yielding a maximum of ion charge depends on the vacuum arc parameters and usually is in the order of 2–5°.

Micropinches in plasma flame expanding into vacuum ambient

Results of experimental and theoretical studies of plasma parameters in a current-carrying plasma flame expanding into the vacuum ambient are presented. It is shown that at the initial stage of the discharge burning the short-run beams of multiply charged ions of the cathode material (with mean ion charge state up +9 for Cu) were emitted from the cathode plasma flame. The X-ray pinhole measurements showed that a small-size area of hot electron with a temperature of 100 ÷ 200 eV was formed at a range of about 2÷3 mm from the cathode. A theoretical model is developed of the process of the current-carrying cathode plasma flame expansion. A set of hydrodynamic equations was solved numerically by the particle-in-cell method. It was shown that due to the Ampere forces a neck was formed at a distance from the cathode that was developed to a micropinch, where electron temperature attained of about 100 eV and the beams of multiply charged ions of the cathode material were produced. Both the temporal and space parameters of the micropinch formation are consistent with results of the measurements. The mean ion charge state of the ions also is in agreement with the experimental data.

A Model for Spectral and Compositional Variability at High Energies in Large, Gradual Solar Particle Events

Shocks driven by fast coronal mass ejections (CMEs) are generally believed to be the dominant accelerators in large, gradual solar energetic particle (SEP) events. A key challenge for this notion has been the highly variable spectral and compositional characteristics of these events above a few tens of MeV per nucleon. We have recently proposed that this variability results from the interplay of two factors: evolution in the shock-normal angle as the shock moves outward from the Sun; and a compound seed population, typically comprising at least suprathermals from the corona (or solar wind) and suprathermals from flares. We present here a simple analytical implementation of these ideas. Our calculations semiquantitatively reproduce key features of the observed variability, including spectral morphologies and energy dependence in Fe/O, 3He/4He, and mean ionic charges, in ways that are consistent with correlations in the data. The model makes a prediction for the average high-energy Fe/O enhancement that is borne out by 30 years of observations; the model also provides a quantitative explanation for the Breneman & Stone fractionation effect, a fundamental but previously unexplained aspect of SEP phenomenology. Our calculations must be bolstered by future efforts incorporating realistic CME-shock simulations and a rigorous treatment of particle transport. Suprathermal densities in the corona, as well as details of the injection process at shocks of arbitrary obliquity, require further investigation. Nevertheless, these first results suggest a comprehensive framework for understanding the complexity of high-energy variability in terms of shock physics for most, if not all, large SEP events.

Observation of energy-dependent ionic charge states in impulsive solar energetic particle events

We measured the mean ionic charge state of Fe in several 3He-rich and heavy-ion rich energetic particle events during the time period 1998–2000. We combined the ionic charge measurements of CELIAS/STOF onboard SOHO (10–100 keV/nuc) with the measurements of SEPICA onboard ACE (180–550 keV/nuc) to investigate the ionic charge distribution of Fe over a wide energy range. Whereas the mean ionic charge of Fe, Q m(Fe), increases strongly with energy from ∼14–16 at 180–250 keV/nuc to ∼16–20 at 350–550 keV/nuc, the mean ionic charge at lower energies of 10–100 keV/nuc was found to be significantly lower (12.5 ± 0.9). A comparison of the results with steady-state models that include impact ionization by protons and electrons in the low corona shows that the measured increase of Q m(Fe) with energy is often steeper than predicted by the model, and other effects, as for example adiabatic deceleration during the propagation, may play an important role.