Slower Ions Research Articles

Kinetics of ion and prompt electron emission from laser-produced plasma

We investigated ion emission dynamics of laser-produced plasma from several elements, comprised of metals and non-metals (C, Al, Si, Cu, Mo, Ta, W), under vacuum conditions using a Faraday cup. The estimated ion flux for various targets studied showed a decreasing tendency with increasing atomic mass. For metals, the ion flux is found to be a function of sublimation energy. A comparison of temporal ion profiles of various materials showed only high-Z elements exhibited multiple structures in the ion time of flight profile indicated by the observation of higher peak kinetic energies, which were absent for low-Z element targets. The slower ions were seen regardless of the atomic number of target material propagated with a kinetic energy of 1–5 keV, while the fast ions observed in high-Z materials possessed significantly higher energies. A systematic study of plasma properties employing fast photography, time, and space resolved optical emission spectroscopy, and electron analysis showed that there existed different mechanisms for generating ions in laser ablation plumes. The origin of high kinetic energy ions is related to prompt electron emission from high-Z targets.

Carbonization of polyimide by swift heavy ion irradiations: Effects of stopping power and velocity

We have studied the carbonization of polyimide Kapton-H (pyromellitic dianhydride-oxydialinine, PMDA-ODA) thin films under heavy ion irradiations in the electronic slowing down regime. Irradiations were performed with 650-MeV Ni, 843-MeV Xe, 2.6-GeV Xe, 1.1-GeV Ta, and 707-MeV Pb ions that were transmitted through the polymer films with the electronic stopping power ranging between 3.5 and 17 keV nm −1. Room-temperature ac/dc electrical conductivity measurements, together with ESR, UV–visible optical absorption, and micro-Raman spectroscopy were used to characterize the irradiated films. These new data are compared to our previous results obtained with heavy ion irradiations in the 1-MeV uma −1 energy range. At the largest fluences near 10 13 cm −2 the resulting carbonaceous material is found to be 8–9 orders of magnitude less conducting than with the slower ions. Such large dc-conductivity deviations are confirmed by the spin–lattice relaxation times deduced from the ESR line saturation measurements that are two orders of magnitude longer with the swift ions than with the slower ions. The evolution of the Raman spectra also shows that the appearance of a conjugated sp 2-bonded structure is delayed with respect to the slow ion case, and is probably incomplete in the observed fluence range. The yield of paramagnetic centers is found to decrease when the ion energy increases. This is interpreted by an ion-velocity effect on damage due to a dilution of the deposited energy in the ion tracks. We think that this also induces a smaller sp 2 cluster density impeding charge transport in the carbonaceous material.

The contribution of charge exchange ions to cylindrical Langmuir probe current

A theory for the collection of ion current by cylindrical Langmuir probes is presented which includes the effect of charge exchange collisions. These collisions replace ions accelerated into the probe sheath with slower ions having a much greater probability of collection, thus increasing the probe current. The probe ion current increase is obtained from the volume integral of the charge exchange rate weighted by the collection probability. The increase is significant even for plasma with a mean free path of ∼103 Debye lengths. The validity of the model is verified experimentally in an Ar plasma of density ∼107 cm−3 created with a hot cathode discharge. The plasma density measured by microwaves is held constant as the pressure and collision frequency are varied. The charge exchange current, obtained by subtracting the calculated orbit-motion-limited current from the measured net ion current, increases nearly linearly with probe bias and with neutral gas pressure.

Modification of surface mechanical properties of polycarbonate by ion implantation

Industrial use of polymers ranges across a broad field of structural, mechanical, electrical and optical applications. In many situations, e.g. in biomedical or optical uses, wear resistance is one of the main requirements. Ion bombardment of polymers is thought to have great potential for improving some surface mechanical and tribological properties. In particular, high-energy bombardment of light ions is reported to produce relevant increases in hardness and wear resistance. The reason seems to be cross-linking effects induced by the dominant mechanism of the stopping power: electronic loss of energy (for heavier and slower ions, nuclear loss of energy produces breakage of polymer chains and a decrease in hardness). This paper presents a study on the effects induced by implantation of light ions on polycarbonate used in optical applications. Different doses below 10 16 ions/cm 2 of hydrogen, deuterium, nitrogen and silicon were implanted and microindentation tests were carried out. Although results showed harder surfaces in all cases, except for silicon implantation, the biggest increases in hardness (by a factor three) corresponded to hydrogen and deuterium implantation at high doses. The paper studies the correlation between these results and the estimated electronic energy loss. The optical transmittance was also measured.

High-resolution transmission electron microscopy study of the radiation damage defects in high temperature superconductors

High resolution transmission electron microscopy (HREM) reveals that MeV heavy ions like Au generate radiation damage defects, a few nanometers in size, in surface regions (about 100 Å) of the superconductors. Much larger defects have been found in the deeper regions of the superconductor sample, which is believed to be due to the larger damage cascade created by the slower ions after their electronic energy loss along the track. Monte Carlo simulation (trim) shows the same result in damage size as the HREM study. Light ions, like protons, generate mostly point defects, which are not visible to current electron microscopy. These point defects, although they can pin the flux, are not ideal for flux pinning, due to their small sizes.

Modelling of the hollow cathode plasma for thw pre-breakdown phase of a pseudospark discharge

The ionization growth during the pre-breakdown phase of a pseudospark discharge is numerically simulated within a hollow cathode region. A swarm model based on the local-field approximation is applied. The continuity equations for electrons and ions are solved simultaneously with Poisson's equation as a function of time in a two-dimensional axially symmetric geometry. The distortion of the electric field due to the hollow-cathode geometry and due to space-charge effects is incorporated. The numerics of the coding is outlined, and problems, like boundary conditions, and solutions adopted for this type of geometry are described. The validity and the robustness of the numerical scheme are discussed. The results of detailed tests proof that errors related to the limited physical model and to the numerics are tolerable. According to the simulations, the plasma density inside the hollow-cathode region at first grows exponentially in time during several 100 ns, mainly because of electron impact ionization and photon emission releasing secondary electrons at the surfaces by photoeffect. The electrons follow the electric field lines as a nearly incompressible fluid, resulting in a focusing towards the axis combined with an increase in velocity. Then they are extracted quickly into the main discharge gap, leaving the much slower ions behind nearly stationary. Consequently a positive space charge builds up, which has its maximum on axis at the cathode hole. During a transition phase of several 10 ns this charge starts to shield off the electric field from the main gap and enhances it inside the hollow cathode, causing increased ionization locally. Further the force related to the positive charge attracts the electrons by means of the modified electric field towards its maximum, thereby compressing the electron fluid. During several ns this leads to an overexponential growth in electron density in this vicinity, triggering a similar growth in the positive-ion density. The result is the onset of the pseudospark as an extremely high-current-density electron beam with a very small diameter leaving the hollow cathode on axis. Such electron beams can be extracted experimentally in pseudospark discharges through a hole in the anode. The pre-breakdown phase in a hollow cathode geometry was also studied experimentally by means of high-speed streak photography. It is shown that the light-emitting region begins in time from a point on axis close to the hole, corresponding to the predictions of the simulation.

Electron sub-cycling in particle simulation of plasma

A straightforward modification which reduces by half the computational cost of standard particle-in-cell algorithms for simulation of plasmas is described. The saving is obtained by integrating only the electrons through a number of time steps (sub-cycles) in order to resolve their evolution, while integrating the much slower ions only once per cycle, i.e., to match the time step of each species to their characteristic frequencies. A dispersion relation is derived which describes the numerical instabilities expected by sampling frequency arguments. Simulations support the broad features of the analytical results, viz., the maximum growth rate and domain of the instability, and its stabilization by the addition of weak damping. An implicit sub-cycling algorithm is suggested which may provide further saving while avoiding a limitation of implicit algorithms described elsewhere.

The energy spectra of secondary ions emitted during ion bombardment

Abstract Secondary ion energy spectra have been measured for singly charged ions emitted from targets irradiated with 43 keV A+ ions. Targets studied include the 3d transition metals (Sc, Ti, V, Cr, Fe, Ni) Cu and Zn, Zr, Al and Si and the compounds SiO2, Al2O3, NaCl, KCl. Energy spectra were measured in the energy range 1–600 eV. In several cases a peak in the energy spectrum in the region around 200 eV has been found. This is in addition to the usual low energy peaks in the region of 5–10 eV. In many cases the low energy peak was observed to decay steadily with irradiation time or to increase with oxygen pressure. In the case of the cleanest Zn spectrum, only the high energy peak can be detected. The data are discussed in relation to current models of secondary ion emission. We conclude that, in general, elemental metal targets which are clean are characterised by the high energy peak in the secondary ion energy spectrum. The slower ions emitted have been neutralised by electron exchange processes. The ...

Theory of the Aurora Based on Magnetic Self-Focusing of Solar Ion Streams

It is assumed that the sun emits a jet of an electrically neutral mixture of fast electrons and positive ions, and that the space between the sun and earth is filled with an ionized, electrically neutral gas. As the jet proceeds the fast electrons are slowed down and scattered by encounters with the space ionization, being continually replaced by slow electrons from space. The stream of positive ions continues as an electric current, but at all times the stream and surrounding gas are closely electrically neutral. Due to the magnetic field of the electric current, the fast moving positive ions are continually compressed into the stream and the fast electrons and slower ions are ejected from the stream. This is "magnetic self-focusing." The stream develops into a narrow core which if correctly aimed enters the earth's magnetic field and is diverted to auroral zones according to the calculations of St\"ormer. Meinel's observations indicate that the primary solar particles in the stream are mainly protons of speed greater than 3\ifmmode\times\else\texttimes\fi{}${10}^{8}$ cm ${\mathrm{sec}}^{\ensuremath{-}1}$. A speed of order of ${10}^{10}$ cm ${\mathrm{sec}}^{\ensuremath{-}1}$ of the protons is required for correct St\"ormer orbits into auroral latitudes and penetration to 60- to 80-km levels in the upper atmosphere. For a stream which carries sufficient energy for an aurora, the density of space ionization between the sun and earth is calculated to be at least ${10}^{\ensuremath{-}5}$ electron ion pairs ${\mathrm{cm}}^{\ensuremath{-}3}$ to support magnetic self-focusing and not more than ${10}^{9}$ pairs ${\mathrm{cm}}^{\ensuremath{-}3}$ for the fast protons to be able to reach the earth. It is suggested that the auroral rays are not the solar proton stream but are formed when the protons are stopped in the upper atmosphere. Due to the charges and absorbed energy of the protons, oxygen and nitrogen particles of the atmosphere are ejected outward to great heights along the lines of magnetic force and form the auroral rays and streamers.

On Polarization in Liquid Dielectrics

A theory of polarization originally developed for crystals is extended in such a way as to be applicable to dc measurements in liquid dielectrics. In Sec. II the general time dependent solution of the relevant differential equation is obtained by the method of eigenvalues. It leads to expressions for the ``charging current'' in Eq. (22) and the ``discharge current'' in Eq. (29), which are characteristic of liquids. In Sec. III the measurements and their results are described. They refer to solutions in hexane, and to pure hexane rendered conductive by radium radiation. Attempts to obtain photoconductivity were futile. In Sec. IV it is shown that the observations can be correlated by the theory of Sec. II provided the presence of two species of ions of different mobilities is assumed. A few cases of quantitative analysis are carried out. They lead to the determination of the mobility of the slower ions, and of the thickness of the polarization layers. In Sec. V some aspects of polarization in crystals are discussed in the light of the more recent development.

The Aging of Ions in Air and Nitrogen

Previous work on the mobility of aged ions in air has been extended to the measurement in air and in nitrogen of mobilities of ions at different short ages. The ions were produced by alpha-rays directly in the field used for their measurement and the average age was obtained from the time taken by the ions to cross between the two electrodes. In air containing 4 mg of water per liter, both kinds of ions of short age had the same mobility of 2.0 ${\mathrm{cm}}^{2}$/volt\ifmmode\cdot\else\textperiodcentered\fi{}sec while at an average age of 0.18 sec the negative ions moved 8 percent faster than the positive. In air partially dried by KOH the mobility of the negative ions decreased from 2.3 (above units) at 0.004 sec to 2.1 at 0.34 sec while the positive ion mobility decreased from 2.0 at 0.004 sec to 1.7 at 0.26 sec. In air dried by aid of liquid air traps the negative ion mobility decreased from 2.44 at 0.004 sec to 2.35 at 0.28 sec. The positive ions in this dry air were found to be very sensitive to minute changes in conditions that had no effect on negative ions. These changes are supposed to arise from small traces of some unknown impurity. Positive mobilities as high as 2.3 at 0.05 sec were obtained. The new information is applied to the explanation of the previous results for older ions. No transformation of one type of positive ion into another single type as observed by Erikson was found under the conditions of these measurements. The ions were all found to have the same mobility at the shortest ages but with increase of time the ion mobilities became spread over a range of values, the numbers given above corresponding to the peak of the distribution curves. The nitrogen used contained 0.3 percent of oxygen. The negative ions of short age all had a very high mobility which could only be roughly estimated as of the order of 100. With increase of age these fast ions gradually transformed into slower ions which were not all alike and had a peak mobility that decreased from 3.0 at 0.04 sec to 2.5 at 0.25 sec. The fast negative ions presumably remain free electrons for a large portion of their path and since their mobility does not appear to change with time it is necessary to suppose that these ions alternate frequently between being free electrons and monomolecular ions, but after more molecules become attached, the ion can no longer revert to the electron stage. The positive ions in nitrogen were always contained in one group only, the peak mobility of which ranged from 2.2 at 0.007 sec to 1.8 at 0.17 sec. The slow decrease of mobility of the ions with age and the large effect of slight changes apparently of gas composition upon the rate of this decrease for positive ions especially, indicate that the ions are gradually undergoing change of size owing to the accretion of scarce molecules or the exchange of such molecules for those which first gather about the central charge to form an ion cluster.