Energy Spectra Of Ions Research Articles

Ion energy spectrum control in modified cross-beam pulsed laser deposition method

The time-of-flight curves of the ion current to a probe in a plasma beam formed by crossing plumes from two laser-ablated silicon targets have been measured using the Langmuir probe technique. It is established for the first time that the ion energy spectrum of the plasma beam formed by the two laser erosion plumes and oriented in the direction of the bisector of the angle between the initial plumes can be modified by changing the angle between them (i.e., between the erosion plume). A modified cross-beam pulsed laser deposition method is proposed, which allows the energies of deposited particles to be controlled in a broad range.

Neutralization phenomena observed with secondary ions originating from inner‐shell excitation

AbstractThe observation of comparatively high yields of low‐energy secondary ions, ejected from clean ion‐bombarded solids, has been a long‐standing miracle. The measured velocity dependence of the ionization probability is at variance with the predicted exponential velocity dependence. It has been proposed that the recorded high yields are due to electronic excitations initiated by the collision cascade. Here, the issue is addressed by exploring energy spectra of ions generated by inner shell electron promotion during energetic collisions. Using new experimental results and literature data, ionization probabilities were determined using two different approaches. The evaluated data suggest that doubly charged ions of Mg, Al and Si (and possibly, Ca) can escape neutralization, presumably as a consequence of Auger deexcitation taking place at sufficiently large distances from the surface. The ion fractions for Sc, Ti, V and Cr are in accordance with an exponential velocity dependence at energies > 300 eV, but approach a constant level at low energies. Given the fact that the electronic‐excitation model does not apply in the case of ionization by electron promotion, it is suggested that the unusually high ionization probabilities observed at low energies are due to suppressed neutralization. The idea is that in a small fraction of impact events the collision cascade may generate a very high perturbation of the surface (‘dynamic randomization’) so that a well‐defined band structure may not exist, locally and for a short period of time. In these rare cases, the conduction electrons required for neutralization are not available. Copyright © 2010 John Wiley & Sons, Ltd.

Laser driven shock accelerated ion energy spectrumbroadening mechanisms in over-dense plasmas

The broadening mechanisms are investigated for the energy spectrum of ions accelerated by electrostatic shocks in overdense plasmas irradiated by intense ultra-short laser pulses. It is found that the width of the shock-accelerated ion spectrum could be ineluctably broadened by three mechanisms, which are the continuously decreasing speed of the shock front due to the energy dissipation into ions, the collisions between the particles, and the further acceleration by the sheath field at the rear of target where when energetic ions arrive. Other effects, such as effect of driving laser pulse duration, are also studied.

An ion acceleration mechanism in laser illuminated targets with internal electron density structure

1D and 2D PIC simulations of light ion acceleration by irradiation of structured thin foils by short pulse, high intensity lasers have been performed. When the back coating material is thicker than the typical scale length for TNSA fields the ion energy spectra contain peaks. These peaks are found to be due to the acceleration of ions by an electric field which builds up at internal interfaces between materials with different electron number densities. The electric field is generated by incomplete neutralization of the charge built up at the head of the hot electron beam as it crosses the interface between the two materials. In these simulations the back material layer is sufficiently thick that the sheath field drops to zero before reaching the vacuum boundary. The acceleration responsible for the peaks is therefore from within the material and is superimposed on the normal TNSA acceleration from the back material–vacuum surface, i.e. there are two distinct sites of particle acceleration. It is possible that this mechanism may explain features previously observed in simulations. A comparison is also made with experiments with thick rear coatings.

Charging of submicron structures during silicon dioxide etching in one- and two-frequency gas discharges

A model that combines the Monte Carlo method for calculating electron and ion trajectories in three-dimensional geometry and an analytic approach developed for calculating an electric field in two-dimensional geometry is used to simulate the charging of the surface of periodic submicron SiO2 structures by electron and ion fluxes in the plasma of a one- and a two-frequency capacitive RF discharge. The energy distribution function of the electrons and ions that come to the bottom of a submicron structure in an argon and an argon-containing plasma is calculated for structures with a width of 11–45 nm and an aspect ratio of d/w = 1–10 (where d and w are the depth and width of the structure). It is shown that secondary electronelectron emission plays an important role in the redistribution of the electric charge and, accordingly, of the electric potential in a submicron structure. It is demonstrated that, when the secondary electron-electron emission mechanism is taken into account, the ion energy spectrum at the bottom of a submicron structure is shifted toward lower energies and becomes broader in comparison with the spectrum of an ion flux from an RF discharge plasma. Moreover, the shift and broadening depend only on the secondary electron-electron emission coefficient, the energy of the charged particles, and the aspect ratio.

Fluid Model of Plasma Sheath Involving Ion Energy Spectrum

A common fluid model of ion drift in a strong electric field, appearing, e.g., in the sheath, describes ions by the Newtonian equation of motion, with ion-neutral collisions involved by an effective friction force. Such an approach implicitly anticipates a monoenergetic ion beam. We demonstrate that, due to nonlinear effects, the concept based on “average” ions is too simplified. In the suggested model, the conventional equation of ion motion is replaced by a formula considering the energy spectrum of ion flux. The formula is based on the 1-D description of charge-exchange collisions of ions with neutrals. The Newtonian equation of motion is then interpreted from the suggested collision model. It is shown that the ion fluid velocity should be identified with the rms velocity in the ion beam and that the effective ion mean free path appearing in the friction term should be chosen as a double of the ion mean free path for the charge-exchange collisions.

Analysis of a hollow-cathode reflex discharge in a hydrocarbon flow

Variation of parameters of a reflex discharge with a hollow cathode operating continuously in propane with a flow rate of 1.3–5.6 (m3 mPa)/s and a discharge current of 0.1–0.4 A is analyzed. It is shown that for a hydrocarbon flow rate of 2.4 (m3 mPa)/s and higher, an increase in the discharge voltage takes place after a time interval depending on the discharge current and gas pressure; this is explained by the formation of coating of the dissociation products of hydrocarbon molecules on the electrodes of the discharge chamber. An increase in the thickness of the carbon coating of the cathodes with time and their charging with ions lead to electric breakdown of coatings and the formation of cathode spots. The oscillograms of the discharge current and voltage indicate a short-term transformation of the glow discharge into the arc discharge. The energy spectra of ions emerging from the discharge are measured, and the effect of the discharge current and the gas flow rate on the energy spread of ions is analyzed. The operation time of the discharge in hydrocarbon after which the cleaning of the discharge chamber is required is determined. The possibility of using an ion source based on the reflex discharge with a hollow cathode for technological purposes is established.

Broadening of the energy spectrum of ions accelerated by laser-driven shocks in over-dense slab plasmas

We investigated numerically the broadening mechanisms of the energy spectrum of ions accelerated by electrostatic shocks in over dense plasmas irradiated by intense ultra-short laser pulses. It was found that the width of the shock-accelerated ion spectrum could be ineluctably broadened by two mechanisms, which are the continuously decreasing speed of the shock front due to the energy dissipation into ions, and the further acceleration by the sheath field at target rears when energetic ions arrive there. Other effects, such like that of driving laser pulse duration, were also studied.

Significance of time-of-flight ion energy spectrum on energy deposition into matter by high-intensity pulsed ion beam

AbstractEnergy deposition by high-intensity pulsed ion beam into a metal target has been studied with time-of-flight (TOF) of ions which can be related to the original ion kinetic energy E0 and the ion mass with $t_{\rm TOF} \propto 1/\sqrt{2E_{0}/m_{i}}$. It is found that the TOF effect has a profound influence on the kinetic energy distribution of implanted ions and subsequent energy deposition process into the target. The HIPIB of mixed H+ and C+ was extracted from a magnetically insulated ion diode at a peak accelerating voltage of 350 kV, leading to an ion current density of 300 A/cm2 at the target. The widespread ion energy spectrum remarkably varied in shape as arriving at the target surface, from the original Gaussian-like of 80-ns duration to a pulse form of a sharp front and a long tail extending to about 140-ns duration. Energy loss of the mixed ions into a Ti target was simulated utilizing a Monte Carlo method. The energy deposition generally showed a shallowing trend and could be divided into two phases proceeded with sequent arrivals of H+ and C+. Note that, the peak value of deposited energy profile appeared at the beginning of mixed ion irradiation phase, other than the phase of firstly arrived H+ with peak kinetic energy and peak ion current. This study indicated that TOF effect of ions greatly affects the HIPIB-matter interaction with a kinetic energy spectrum of impinging ions at the target, noticeably differing from that of original output of the ion source; consequently, the specific energy deposition phenomena of the widespread ion energy can be studied with the TOF correlation of ion energy and ion current, otherwise not obtainable in common cases assuming fixed ion energy distribution in accordance with the original source output.

Optimization for deuterium ion acceleration in foam targets by ultra-intense lasers

AbstractIn this article, we investigate the effects of foam target composition and laser parameters on deuterium ion energy spectra with particle-in-cell simulations. We find that localized electrostatic fields with multi peaks around the surfaces of lamellar layers inside foam target are induced. These fields accelerate deuterium ions from thin foam layers by restricting the flow of hot electrons. This mechanism of ion acceleration called as bulk ion acceleration generates large number of high energy deuterium ions. Deuterons inside foam target are accelerated up to 126 MeV in case of oblique optimal angle of 30° where it is much greater than the normal laser incidence energy of 88 MeV.

In-flight Performance and Initial Results of Plasma Energy Angle and Composition Experiment (PACE) on SELENE (Kaguya)

MAP-PACE (MAgnetic field and Plasma experiment—Plasma energy Angle and Composition Experiment) on SELENE (Kaguya) has completed its ∼1.5-year observation of low-energy charged particles around the Moon. MAP-PACE consists of 4 sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measured the distribution function of low-energy electrons in the energy range 6 eV–9 keV and 9 eV–16 keV, respectively. IMA and IEA measured the distribution function of low-energy ions in the energy ranges 7 eV/q–28 keV/q and 7 eV/q–29 keV/q. All the sensors performed quite well as expected from the laboratory experiment carried out before launch. Since each sensor has a hemispherical field of view, two electron sensors and two ion sensors installed on the spacecraft panels opposite each other could cover the full 3-dimensional phase space of low-energy electrons and ions. One of the ion sensors IMA is an energy mass spectrometer. IMA measured mass-specific ion energy spectra that have never before been obtained at a 100 km altitude polar orbit around the Moon. The newly observed data show characteristic ion populations around the Moon. Besides the solar wind, MAP-PACE-IMA found four clearly distinguishable ion populations on the dayside of the Moon: (1) Solar wind protons backscattered at the lunar surface, (2) Solar wind protons reflected by magnetic anomalies on the lunar surface, (3) Reflected/backscattered protons picked-up by the solar wind, and (4) Ions originating from the lunar surface/lunar exosphere.

Charging and the secondary electron–electron emission on a trench surface: broadening and shift of ion energy spectrum at plasma trench etching

Trench surface charging at the plasma etching of dielectrics and semiconductors is a negative phenomenon because it leads to non-uniform etching of the trench bottom, undesirable etching of its wall, etch stop and breakdown of lower level device elements. To investigate the charging of a SiO2 trench surface by argon radio frequency discharge plasma we applied the 3D Monte Carlo method for modelling the electron and ion trajectories inside a trench and used the 2D analytical method to calculate electric fields and potentials produced by the deposited charges. The secondary electron–electron emission was taken into account as a really important mechanism of electrical charge redistribution on the trench surface. The ion energy spectra were calculated for the trench aspect ratios (depth d/width w) of 1–20 and trench widths of 11, 22 and 45 nm for 180 eV ion flux. The transformation of an initial ion energy spectrum from a delta function at 180 eV into bell-shaped curves with peak shifts of 10–60 eV and broadening of 5–30 eV is obtained.

Preliminary studies on space charge compensation by analyzing residual argon gas ion signals

An experimental method is related to research the space charge compensation (SCC) effect in low energy intense proton beams by analyzing residual gas (RG) ion signals. The signal curves were measured with an energy spectrometer under the RG pressure from 1.2x10(-3) to 1.6x10(-2) Pa. Most of the data showed a similar trend with our theoretical predicts. From the RG ion energy spectra the potential distribution in the beam was calculated both with and without the SCC effect. Moreover, as a preliminary result, a best compensating point is achieved for the low energy beam transport transmission of 40 KeV, 60 mA H(+) beam in Peking University.

Particle in cell analysis of a laser-cluster interaction including collision and ionization processes

A new particle-in-cell (PIC) code which includes collisional and ionization processes has been developed to study laser-plasma interaction. Using the new code, the dynamics of a cluster plasma in a strong laser field has been analyzed and the threshold intensity of the resonant heating, which was previously predicted is accurately evaluated. The angular dependence of ion energy spectrum has also been simulated. As a result, it is found that the anisotropic energy spectrum depends strongly on the presence or absence of collisional processes.

Flux of multiple charged metal ions of high energy from plasma produced by a moderate energy laser pulse

Studies of the energy spectrum and charge state distribution of ions produced from an Al target irradiated with a laser pulse of moderate energy (90 mJ) and light intensity (4.2 × 1012 W cm−2) and of 27 ps pulse duration are presented. It is established that highly charged ions of the target material up to Al11+ are emitted from the laser produced plasma and evidence of the presence of Al12+ ions is also established. The maximum of the bulk of ion energy distributions approaches 7 keV/Z, which exceeds the values typically observed earlier in plasma produced by laser radiation of the intensity mentioned above. Theoretical examination of the experimental results reveals that the relatively high ion energies can be explained by the specific conditions of the experiments, namely the relatively large diameter of focal spot of the laser beam, appropriate length of the laser pulse, and so on.

Study of spatial distributions of F, H and CF2 radicals in DF CCP discharge in Ar/CF4 and Ar/CHF3 mixtures

The comparative research of production and loss of atoms (F, H) and radicals (CF2), important for plasma processing, is carried out in DF CCP plasma in Ar/CF4 and Ar/CHF3 mixtures modeling two etching plasmas (fluorine-rich) and (fluorine-poor) respectively. Electron density, electron temperature and ion energy spectra are measured for single and dual frequency discharge modes. Spatial profiles of radical and atom densities are measured in both mixtures by using spatially resolved actinometry and UV absorption techniques. The opportunity for plasma-surface interaction control using spatial radical and atom density distributions is shown. The influence of high and low frequency powers on balance between surface and volume processes is discussed.

Chemical‐Picture‐Based Modeling of Thermodynamic Properties of Dense Multicharged‐Ion Plasmas Using the Superconfiguration Approach

AbstractUsing the chemical‐picture representation of plasmas as a mixture of various ions and free electrons, a consistent description of thermodynamics of dense multicharged‐ion plasmas is being developed that involves the effects of Coulomb non‐ideality and degeneracy of plasma electrons; contribution of the excited ion states (on the base of the superconfiguration approach) that may exist under an appropriate truncation of ion energy spectra due to plasma effects; hard‐sphere‐model representation of the finite‐volume effects of plasma ions with the model parameters (effective ion sizes) corresponding to superconfigurations yielding the greatest contribution to partition functions. We present the calculated data for average ionization, Grüneisen coefficient, and specific heat of aluminum and iron plasmas at temperatures of 0.03–3 keV and densities 10–3 – 10–5 of their normal material densities. Calculated thermodynamic functions and shock Hugoniots are compared with other theoretical and experimental data (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Energy spectra of sputtering ions induced by Ar7, 9+ ions on graphite collisions

The Sputtering Ion Energy Spectra (SIES) produced by Ar7, 9+ ions at velocities of 0.9 vBohr, 1.35 vBohr and 1.8 vBohr bombardment at 45o to high oriented graphite target were obtained for the first time.

Measurements of fast deuterons from plasma accelerator by means of PM-355 track detectors

This paper reports on studies of fast ions (mostly deuterons) emitted from an RPI (Rod-Plasma-Injector) plasma accelerator. The first aim was the verification of applicability of PM-355 track detectors for dosimetry of lower-energy (<200 KeV) deuterons. The second aim was information about energy spectra of deuterons from RPI-IBIS device in different operational modes. Mass- and energy spectra of ions were investigated with a Thomson-analyzer and PM-355 detectors. On the recorded deuteron-parabolas we chose points, and from numbers of tracks we determined the deuteron energy distribution. In the slow-mode the energy distribution peak was observed at about 40 keV, while the maximum energy amounted to about 150 keV. Those measurements confirmed an influence of the initial gas-conditions on energy distributions of the deuteron streams. The results are of importance for plasma-physics and applications. Another result was the confirmation that PM-355 detectors might be used for accurate measurements of low-energy (<200 keV) deuterons.

Accurate control of ion bombardment in remote plasmas using pulse-shaped biasing

This paper deals with a pulsed biasing technique employed to a downstream expanding thermal plasma. Two pulsed biasing approaches are presented: asymmetric rectangular pulses and modulated pulses with a linear voltage slope during the pulse, and their applicability is discussed on the basis of the intrinsic capacitance of the processed substrate-layer system. The substrate voltage and current waveforms are measured, and the relation to the obtained ion energy distributions is discussed. Accurate control of the ion bombardment is demonstrated for both aforementioned cases, and the cause of broadening of the peaks in the ion energy spectra is determined as well. Moreover, several methods to determine the modulated pulse duration, such that the sloping voltage exactly compensates for the drop of the substrate sheath potential due to charging, are presented and their accuracy is discussed.