Schlieren diagnostics of pulsed electron beam ablation of polymethyl-methacrylate

The investigation of the interaction of pulsed electron beams with PMMA (polymethylmethacrylate) targets is reported. The electron beam of some 10^–8 s in duration is produced in a pulsed low-pressure gas discharge. The beam power density of up to 10⁸ W/cm² leads to a surface plasma formation similar to that of the pulsed laser ablation process. The propagation of the ablated material and the shock wave inside the PMMA target are observed by means of Schlieren diagnostics. An electron density gradient of over 3×10¹⁹ cm^–4 has been observed in the expanding plasma up to 1.5 s after the plasma formation. During the early stage of expansion, the expansion velocity of the plasma plume as determined by the steep electron density gradient is around 10⁵ cm/s. The pressure behind the shock front inside the PMMA target as determined from the shock velocity exceeds 0.3 Gpa.     

TEM study of ion implanted GaAs after pulsed electron beam annealing

Abstract

(001) GaAs single crystals were implanted with 150 keV Cr⁺ ions using a dose of 5 × 10¹⁵ ions cm^?2. The amorphized surface layers were subjected to pulsed electron beam annealing at energy densities in the range 0–1.3 J cm^?2. A detailed TEM investigation of the damaged and annealed surface layer was conducted. These observations were correlated with backscattering results.     

Generation of high-energy negative hydrogen ions upon the interaction of superintense femtosecond laser radiation with a solid target

The formation of a high-energy (～35 keV) beam of negative hydrogen ions was observed in the expanding femtosecond laser plasma produced at the surface of a solid target by radiation with an intensity of up to 2× 10¹⁶ W/cm². The energy spectra of the H⁺ and H^?-ions show a high degree of correlation.     

Nitriding of Stainless Steel in Electron-Beam Plasma in the Pulsed and DC Generation Modes

The influence of electron-beam parameters on the thickness and phase composition of a hardened layer formed upon the nitriding of austenitic stainless steel 12Cr18Ni10Ti in plasma produced by a beam in a low-pressure (3 Pa) nitrogen-argon mixture is studied. The results obtained in the DC and pulse-periodic modes of beam generation with the same mean current and electron energy are compared. In this case the negative bias voltage applied to the samples is 100 V. The nitriding temperature of 400°C is maintained at a mean beam current of 2.6 A and various combinations of frequency (100–500 Hz) and current pulse durations (0.1–0.3 ms) with an amplitude of 80 A. The mean ion-plasma current densities in the DC and pulsed modes are close in magnitude (2–3 mA/cm² at 400°C). The high pulsed ion-current density (35–70 mA/cm²) creates conditions under which the surface sputtering rate during the pulse exceeds the growth rate of the nitrided layer. The nitriding of steel in the pulsed and DC modes over four hours gives the same result. Hardened layers with a thickness of 7–8 μm and a microhardness of the surface component of 15 ± 1 GPa in which the main phase is a supersaturated nitrogen solid solution (expanded austenite) are formed. A possible explanation is that nitriding in an electron-beam plasma proceeds mainly under the action of long-lived active neutral nitrogen particles rather than as a result of ion bombardment.     

Stay time measurements for Xe,Kr, and CO2 physisorbing on Ni and Cu surfaces

A pulsed molecular beam apparatus is used to measure mean stay times for gases physisorbing on cooled surfaces. Most of the data are for Xe on nickel surfaces. Data are also presented for Kr and CO₂ on nickel, Xe on copper, and Xe on ion-sputter-cleaned nickel. All targets are polycrystalline. Surface temperatures range from 92 to 125 K and measured stay times range from 10^?5 to 10^?3 s. Heats of adsorption and pre-exponential factors deduced from the data indicate that the adsorption is localized (immobile) and suggest that the sputter-cleaned targets may be approximately clean. A model relating the shape of the detector signal to the mean stay time is presented and its validity is assessed. Measured speed distributions for the desorbing molecules exhibit an excess of slow molecules compared to that expected for simple effusion. At lower surface temperatures where longer stay times are observed, a peculiar detector signal dip is observed which appears to indicate that the adsorbing beam pulses temporarily reduce the steady state desorption rate of background atoms.     

Substantiation of the Possibility of Predicting Behavior of Solids under Extreme Conditions at Various High-Intensity Impacts

The paper is devoted to the data analysis on the amplitude-time regularities of the dynamic failure process of solids under various types of high-intensity impact in the ranges of nonequilibrium states from 3 × 10^?10 to 10^?5 s and establishing general regularities of behavior of unstudied materials under extreme conditions. We have analyzed the process of dynamic destruction of solids of different nature using the method of magnetic-pulse loading in the microsecond range of nonequilibrium states, as well as the dynamic failure process for a number of metals in the mode of pulsed volume heating under the action of pulsed relativistic electron beams in the nanosecond and subnanosecond range of nonequilibrium states. It has been shown that, upon using different methods of pulsed loading in the dynamic longevity range, the failure time as a function of amplitude of applied load has an exponential form for various solid materials. This indicates the scaling nature of the destruction process. The foregoing determines the possibility of predicting the behavior of unstudied solid bodies in the dynamic range of nonequilibrium states.     

Nanostructuring of surfaces of metalloceramic and ceramic materials by electron-beams

An electron-emitting source generating a low-energy beam measuring 1–3 cm in diameter, with current up to 300 A, pulse duration within 50–200 μs, and pulse repetition frequency up to 10 Hz is investigated in a gas-filled diode with a mesh plasma cathode at the accelerating voltage up to 25 kV. The beam is transported in a longitudinal pulsed magnetic field to a distance of up to 30 cm towards the region of its interaction with a solid. For the current densities up to 100 A/cm², it provides the power density as high as 10–100 J/cm² sufficient to melt surfaces of metals, alloys, and composite (metalloceramic) materials within one or a few pulses. This makes this beam useful for modification of material surfaces and articles made thereof. Using the methods of optical, scanning and diffraction electron microscopy, by building micro-and nanohardness profiles, and via identification of the treated surface roughness, the phase composition and the substructure state of the materials subjected to pulsed low-energy e-beam of sub-millimeter durations are investigated. Formation of submicro-and nanocrystalline multi-phase structure is observed, which ensures a multiple increase in physico-mechanical and tribological characteristics of the treated material. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 60–70, May, 2008.     

Low beam current density Auger spectroscopy and surface analysis

Auger and secondary electron spectroscopy become a more and more routine technique in surface characterization. Even with primary electron beam current density as low as 10^?2 or 10^?3 A cm^?2 beam damage were reported in both Auger and LEED experiments. So we developed and compared counting method, brightness modulation and Harris' modulation techniques in terms of signal to noise ratio. The two first methods offer the advantage of a primary beam current density decreasing about 10⁴ times. So various mechanisms of beam damage were identified as thermal, chemical and electrical. The advantage of the method is shown with hydrocarbons adsorption layer; the beam cracking of the organic chain produces a chemical shift of the C_KLL maximum Auger line about 5 eV. This progressive shift is observed with current densities of 10^?5 A cm^?2 order of magnitude. The reproducibility of this low current density Auger spectroscopy allowed the study of the background and the true secondary electron yield modifications when adsorbed layers are built up.     

Device for Producing and Investigating an Electron Beam Plasma Discharge

A survey on the properties and applications of the beam plasma discharge is given. A device for the experimental investigation of this discharge is described in which a magnetically guided (H = 0.05?0.1 T), ribbonlike electron beam (eU = 0.5–5 KeV, I = 10^?3?1 A) passes through a working chamber (p = 0.1–10 Pa; l = 10 cm, ø = 22 cm). The beam plasma discharge is sustained by collective beam plasma interaction. First results concerning the wall current and the ignition of the discharge as dependent on beam parameters are presented.     

Spectroscopic Investigation of a Double Discharge Pulsed Electron Beam Generator

ABSTRACT

The wide range of applications of the plasma-based electron beam generator make it necessary to diagnose the device with a noninterfering method. The results of experimental and modeling studies of neutral helium and hydrogen beta spectral lines emitted from the double discharge pulsed electron beam generator are presented in this paper. Neutral helium lines emitted from the plasma in the pressure range 0.1–0.4 torr are studied and compared with results of the collisional radiative model. The duration of the electron beam is shorter than 100 ns, and the peak current intensity is of order amperes. The full width at half maximum of the H _β spectral line is used for the determination of the plasma electron density, found as 3.16 × 10²¹ m^?3 at 0.3 torr, and good agreement is obtained by comparing with the full computer simulation method.     

Generating intense beams of low-energy molecules

A method for obtaining intense pulsed beams of molecules possessing low kinetic energies is proposed. The method is based on the formation of a cold pressure shock (shock wave) in an intense pulsed molecular beam interacting with a solid surface, which serves as a source of the secondary beam of low-energy molecules. The proposed method was successfully used to obtain intense beams of H₂, He, CH₄, and Kr molecules with kinetic energies not exceeding 10 meV, and H₂/Kr and He/Kr beams with kinetic energies of H₂ and He molecules below 1 meV.     

Applications of Laser-Induced Breakdown Spectrometry

INTRODUCTION

When a high-powered laser beam is focused onto a small area or spot of a solid surface, the temperature of the locally heated region rises rapidly to the vaporization temperature of the solid material and an optically induced plasma, frequently called a laser-induced plasma (LIP) or laser-ablated plasma (LAP) or laser spark is formed at the surface. The plasma will be formed when the laser power density exceeds the breakdown threshold value of the solid surface. Although different materials have different breakdown thresholds, an optical plasma is produced when the laser power density exceeds several megawatts per centimeter squared (10⁶ - 10⁹ W/cm²). This plasma has been used for sampling, atomization, excitation, and ionization in analyhcal atomic spectroscopy. It has also been frequently used and proposed as a source for atomic emission spectrometry (AES). In this case the technique is most ofien referred to as laser microprobe optical emission spectrometry (LM-OES) developed by Brech and Cross [1] in early nineteen-sixties or more recently called laser-induced breakdown spectrometry (LIBS) [2,3]. Generally, this analyhcal technique involves two steps; the pulsed focused laser beam directed into a gaseous sample or the surface of a solid or liquid, to produce a transient LIP, followed by the measurement of a characteristic atomic emission signal related to some species present in the plasma. The LIP formed is tightly focused and consists of vaporized atoms, ions, electrons, and molecular fragments. The application of LIBS for direct spectrochemical analysis is a rapidly growing field ranging from the detection of atmospheric pollutants to monitoring of material production processes, and even to “clean-room” technology. Laser ablation techniques have also been applied for solid sample introduction into other plasma sources [4–9]. In recent years, the powerful technique of LIBS as an analytical tool has been recognized by a number of research groups, and has led to an increasing number of publications on the applications of LIBS both in the laboratory and in industry. This growing success of LIBS is a result of thorough research carried out to understand the related plasma physical processes, aided by marked improvements in laser systems and photodetector technology.     

The interference of an electron beam with the surface reaction between oxygen and germanium

Electron beam assisted adsorption and desorption of oxygen was studied by Auger electron spectroscopy (AES). Beam assisted adsorption was observed on clean as well as on oxidized surfaces. After an oxygen exposure of 1000 × 10^?7 Torr min and continuous irradiation with beam voltage of 1.5 kV and beam current density 2 microA mm^?2, the oxygen 510 eV signal amplitude from the point of beam impact was 2.5 times greater than the signal from the non-irradiated region. The Ge 89 eV signal showed a corresponding decrease. Enhanced adsorption occurred at beam energies as low as 16.5 eV. After irradiation, the oxidized surface was not carbon contaminated. Following an oxygen exposure of 30 min at 0.1 Torr and 550°C and subsequent additional beam assisted exposure of 1000 × 10^?7 Torr min, the maximum oxide thickness was about 18 Å. Beam assisted desorption did not occur from thin oxygen layers (0–510 eV signal strength less than 5 units, calculated oxide thickness about 6 Å), but occurred from thick oxides and stopped after the signal amplitude had decreased to 5 units. Based on these results, a model for the structure of the oxygen layer covering the Ge(111) surface is proposed. Mechanisms for adsorption and desorption are discussed. The implications of beam assisted adsorption and desorption on electron beam operated surface measurements (LEED, AES, ELS, APS etc.) are stressed.     

A study of electron beams extracted from a plasma produced by laser interaction with a solid target

In this work, wave formation in laser-produced plasma is investigated by an analysis of time-of-flight signal of the electron pulse. Electrons are extracted from a non-equilibrium plasma, generated by pulsed laser ablation on a solid Ge target. The process is represented by ion-acoustic waves, which are generated from an external perturbation, given by the positive bias voltage of a Faraday cup. The characteristics of the waves depend substantially on the geometry of the plasma expansion chamber and on laser fluence, but are independent on bias potential. A KrF excimer UV laser was employed for plasma generation. Measurements were performed at two different laser fluences, 4 and 7 J/cm². The plasma created propagates with a mean velocity of about 1.1?×?10⁴ m/s. A movable Faraday cup was employed in order to collect electrons at different bias voltage values.     

Effect of carbon on accumulation of deuterium in beryllium irradiated with stationary plasma

Beryllium targets placed in the PLAST beam-plasma discharge facility were irradiated with a flux of stationary deuterium (D) plasma with a deuterium ion energy of 200 eV and plasma flux density of 3 × 10²⁰ m^?2 s^?1 at temperatures of 370 and 670 K. The irradiation doses varied from 5 × 10²¹ to 10²⁴ m^?2. To heat the target and to ionize impurities near its surface the target was irradiated with an electron beam. The deuterium concentration at the target center exceeds its concentration at the periphery by a factor of more than two under all irradiation conditions. The target center was enriched with carbon up to 16–24 at %, as compared to 4–6 at % at the target periphery. The [D]: [Be] atomic concentration ratios at the target center were equal to 0.054 and 0.036 against 0.024 and 0.016 at the periphery at temperatures of 370 and 670 K, respectively. It has been found that these ratios depend on the concentration of carbon atoms which trapped deuterium atoms.     

Measurement of the HeI 4471 Å profile at an electron density of 1015 cm−3

The profile of the HeI 4471 Å line and its forbidden component was measured using a pulsed arc plasma (electron density 10¹⁵ cm^?3, temperature 1·5 eV) as the light source. The profile is in good agreement with recent calculations in which the ion motion has been taken into account.     

Desorption of halogens from some Nb,Mo, Ta and W single crystal surfaces

A systematic investigation of the thermal desorption of halogens from well characterized (111), (100) and (110) 4d (Nb, Mo) and 5d (Ta, W) transition metal surfaces has been carried out under low coverage conditions (θ < 10^?2 of a monolayer). Characterization of the surfaces was achieved by LEED, AES and work function determinations while the desorption kinetics were recorded in a large temperature range (1700–2300 K) using a pulsed ionic beam method. The new data concerning some Ta and W surfaces are presented and the results of this systematic study are discussed. It is shown that the halogen desorption parameters, e.g., desorption energies and preexponential factors, are independent of both surface structure and d bond filling of the substrate; E(F) ～4.75 eV, E(Cl) ～4.1 eV, E(Br) ～3.7 eV and τ₀ ～10^?13 ?10^?14 S. The halogen behaviour is compared with that of other adsorbates and with the predictions of a general chemisorption model.     

Gas mixture effects on ion‐beam flux characteristics from dense plasma focus device: Investigation by the Vlasov–Maxwell equations and experiments

The Vlasov–Maxwell equations were numerically solved to calculate the ion‐beam flux from the plasma of argon and the plasma of mixtures of argon and neon. Some experiments were performed to measure the ion beam from the Amirkabir plasma focus (APF) device. The calculations have shown that the argon ion‐beam flux peaked up to 1.928 × 10³⁰ ions m^?2 s^?1 at the optimum pressure of 1.866 mbar while the neon‐argon mixture's ion‐beam flux reached a maximum of 4.301 × 10³⁰ ions m^?2 s^?1 for 15% neon admixture at the optimum pressure of 1.866 mbar. The calculated kinetic energy of the ion beam has shown a maximum value of 708.7 J for the mixture of 85% argon‐15% neon at the mentioned optimum pressure.     

Changes in the silicon microhardness induced by a low-intensity electron beam

It is shown that nonequilibrium point defects are of primary importance in the changes in the silicon microhardness induced by a low-intensity (I ～ 10⁵ cm^?2 s^?1) electron beam. It is found that the necessary condition for softening under low-intensity electron irradiation is the presence of an oxide layer on the surface. The thickness of the surface layer in which anomalous changes in the microhardness are observed is determined by the layer-by-layer etching technique.