Nanosecond Pulsed Electron Research Articles

Harnessing Nanosecond STEM Electron Pulses with an Electrostatic Beam Blanker for Angstrom Spatial Resolution in Time-Resolved Studies of Electrically Excited Dynamics

Harnessing Nanosecond STEM Electron Pulses with an Electrostatic Beam Blanker for Angstrom Spatial Resolution in Time-Resolved Studies of Electrically Excited Dynamics

Study of nanosecond pulsed hollow-cathode electron beam generation and transportation based on PIC-MCC simulation

This work employs the PIC-MCC (Particle-in-Cell Monte-Carlo collisions) method to study the nanosecond pulsed electron beam time-varied beam current profiles through the hollow-cathode discharge plasma evolution and electron beam transportation process. The electron beam’s energy and current distribution characteristics at the anode hole are investigated, and the simulation’s grid and step sizes are set as 0.04 µm and 5×10-13s, respectively. Experimental parameters are used in the simulation, and experimental results are also used to verify the simulation accuracy. The simulations investigate the transportation characteristics of the electron beam under different conditions and obtain the effects of voltage, current, and ion density on the distribution characteristics of the electron beam. The electron beam current generated by the hollow-cathode discharge is gradually transformed from a random distribution in the central region to a Gaussian distribution. Due to the ion-forced channel formed by the runaway electron pre-ionization, the high-energy electron beam achieves focused transport over a certain distance, and the high-energy electron beam has a smaller Gaussian radius than the source input. Besides, the ion-focusing channel formed by the high-energy electron transportation process can realize the transportation of the high-current electron beam in the conduction stage.

Nanosecond Photoemission near the Potential Barrier of a Schottky Emitter

Nanosecond electron pulses are appealing for ultrafast imaging and electron-gating applications, where tunable currents and narrow energy spreads are desirable. Here, we demonstrate photoemission from a Schottky emitter triggered by nanosecond laser pulses. Using photon energies optimally tuned to the emission potential barrier, we generate pulses containing over ${10}^{5}$ electrons with energy spreads below 1 eV by means of a prompt single-photon photoemission process. These results are consistent with a theoretical model of laser-triggered electron emission and energetic broadening during propagation and can be widely implemented.

Time-resolved Analytical Electron Microscopy with Single Nanosecond Electron Pulses

Time-resolved Analytical Electron Microscopy with Single Nanosecond Electron Pulses

Non-radioactive electron source with nanosecond pulse modulation for atmospheric pressure chemical ionization.

Ion mobility spectrometers (IMSs) are well-known instruments for fast and ultrasensitive trace gas detection. In recent years, we introduced a compact nonradioactive electron source providing a defined current of free electrons with high kinetic energy at atmospheric pressure for initiating a chemical gas phase ionization of the analytes identical to radioactive sources. Besides its nonradioactivity, one major advantage of this electron source is its controlled electron emission current even in pulsed mode. By optimizing the geometric parameters and developing faster control electronics, we now achieve electron pulses with extremely short pulse widths down to 23 ns. This allows us to kinetically control the formation of reactants and analyte ions by chemical gas phase ionization (e.g., reducing discrimination processes caused by competing ionization), enhancing the analytical performance of the IMS. However, this paper concentrates on the pulsed electron source. For its characterization, we developed a measurement setup, which allows the detection of nanosecond electron pulses with amplitudes of only a few nanoamperes. Furthermore, we investigated the spatial ion distribution in the ionization region depending on several operating parameters, such as the kinetic electron energy or the ionization time.

SMALL-SIZE DIRECT-ACTION ELECTRON ACCELERATOR WITH A HIGH-EFFICIENCY NANOSECOND PLASMA-CURRENT SWITCH

The paper presents the results of a study undertaken to determine operating modes for a small-size direct-action electron accelerator with a high-efficiency nanosecond plasma-current switch (PCS). The investigations have shown that using PCS as a base it is possible to develop small-size nanosecond pulsed high-current electron accelerators with a voltage pulse sharpening coefficient of about 12, beam (flow) electron energy of 300…400 keV and current of 100 kA for pulse duration of 30 ns. The ways for improving the PCS and accelerator operating parameter stability, increasing the switching current and maximum accessible switching frequency are proposed.

Nanosecond electron pulses in the analytical electron microscopy of a fast irreversible chemical reaction

We show how the kinetics of a fast and irreversible chemical reaction in a nanocrystalline material at high temperature can be studied using nanosecond electron pulses in an electron microscope. Infrared laser pulses first heat a nanocrystalline oxide layer on a carbon film, then single nanosecond electron pulses allow imaging, electron diffraction and electron energy-loss spectroscopy. This enables us to study the evolution of the morphology, crystallography, and elemental composition of the system with nanosecond resolution. Here, NiO nanocrystals are reduced to elemental nickel within 5 µs after the laser pulse. At high temperatures induced by laser heating, reduction results first in a liquid nickel phase that crystallizes on microsecond timescales. We show that the reaction kinetics in the reduction of nanocrystalline NiO differ from those in bulk materials. The observation of liquid nickel as a transition phase explains why the reaction is first order and occurs at high rates.

Радиационная деградация сцинтилляторов на основе LiF : W

Patterns and causes of changes in spectral-kinetic characteristics of the cathodoluminescence in LiF: W crystals are studied under irradiation by nanosecond electron pulses. The contribution of the transformation processes of pre-radiation imperfection and accumulation of color centers to cathodoluminescence distortion is revealed. Optimal operating conditions for LiF:W scintillators are determined. It is found that pre-irradiation of LiF:W with doses of 103–104 Gy causes 1.5–2 fold increase in cathodoluminescence intensity (low dose effect).

Imaging and Electron Energy-Loss Spectroscopy with Single Nanosecond Electron Pulses

Imaging and Electron Energy-Loss Spectroscopy with Single Nanosecond Electron Pulses

Imaging and electron energy-loss spectroscopy using single nanosecond electron pulses

We implement a parametric study with single electron pulses having a 7 ns duration to find the optimal conditions for imaging, diffraction, and electron energy-loss spectroscopy (EELS) in the single-shot approach. Photoelectron pulses are generated by illuminating a flat tantalum cathode with 213 nm nanosecond laser pulses in a 200 kV transmission electron microscope (TEM) with thermionic gun and Wehnelt electrode. For the first time, an EEL spectrometer is used to measure the energy distribution of single nanosecond electron pulses which is crucial for understanding the ideal imaging conditions of the single-shot approach. By varying the laser power, the Wehnelt bias, and the condenser lens settings, the optimum TEM operation conditions for the single-shot approach are revealed. Due to space charge and the Boersch effect, the energy width of the pulses under maximized emission conditions is far too high for imaging or spectroscopy. However, by using the Wehnelt electrode as an energy filter, the energy width of the pulses can be reduced to 2 eV, though at the expense of intensity. The first EEL spectra taken with nanosecond electron pulses are shown in this study. With 7 ns pulses, an image resolution of 25 nm is attained. It is shown how the spherical and chromatic aberrations of the objective lens as well as shot noise limit the resolution. We summarize by giving perspectives for improving the single-shot time-resolved approach by using aberration correction.

Development of an electron momentum spectrometer for time-resolved experiments employing nanosecond pulsed electron beam.

The low count rate of (e, 2e) electron momentum spectroscopy (EMS) has long been a major limitation of its application to the investigation of molecular dynamics. Here we report a new EMS apparatus developed for time-resolved experiments in the nanosecond time scale, in which a double toroidal energy analyzer is utilized to improve the sensitivity of the spectrometer and a nanosecond pulsed electron gun with a repetition rate of 10 kHz is used to obtain an average beam current up to nA. Meanwhile, a picosecond ultraviolet laser with a repetition rate of 5 kHz is introduced to pump the sample target. The time zero is determined by photoionizing the target using a pump laser and monitoring the change of the electron beam current with time delay between the laser pulse and electron pulse, which is influenced by the plasma induced by the photoionization. The performance of the spectrometer is demonstrated by the EMS measurement on argon using a pulsed electron beam, illustrating the potential abilities of the apparatus for investigating the molecular dynamics in excited states when employing the pump-probe scheme.

Influence of nanosized carbon forms on the properties and susceptibility of energy-saturated cobalt salt to a pulsed electron beam

The influence of fullerenes and detonation nanodiamonds on the initiation of complex cobalt perchlorate by a nanosecond pulsed electron beam has been studied. The results on the thermal compatibility of experimental compositions in nonisothermal conditions have been presented.

The Influence of Pulsed Electron Beam Treatment on Properties of PLLA Nonwoven Materials Produced by Solution Blow Spinning

One of the most important aspects in nonwoven materials fabrication in case biodegradable scaffolds is the control of their degradation rate which must correlate with a speed of tissue regeneration. Radiation treatment based on electron beam irradiation is one of the promising methods for control over polymer degradation. The effect of nanosecond pulsed electron beam irradiation on properties of poly-L-lactic acid (PLLA) nonwoven materials produced by solution blow spinning (SBS) method was investigated. Modification of nonwoven materials was performed in electron beam accelerator with absorbed dose from 26 to 260 kGy. Properties of the nonwoven materials were examined by means of viscosity measurement, X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR) spectroscopy, and scanning electron microscopy (SEM). It was shown that pulsed electron beam exposure leads to the reduction of polymer molecular weight and recrystallization. It was shown that pulsed electron beam modification of PLLA nonwoven materials changes their physical and chemical properties that allows considering this method as a potential technology for the manipulation of polymer degradation rate and fabrication of polymer materials with required properties.

Modeling of beam-target interaction during pulsed electron beam ablation of graphite: Case of melting

A one-dimensional thermal model based on a two-stage heat conduction equation is employed to investigate the ablation of graphite target during nanosecond pulsed electron beam ablation. This comprehensive model accounts for the complex physical phenomena comprised of target heating, melting and vaporization upon irradiation with a polyenergetic electron beam. Melting and vaporization effects induced during ablation are taken into account by introducing moving phase boundaries. Phase transition induced during ablation is considered through the temperature dependent thermodynamic properties of graphite. The effect of electron beam efficiency, power density, and accelerating voltage on ablation is analyzed. For an electron beam operating at an accelerating voltage of 15kV and efficiency of 0.6, the model findings show that the target surface temperature can reach up to 7500K at the end of the pulse. The surface begins to melt within 25ns from the pulse start. For the same process conditions, the estimated ablation depth and ablated mass per unit area are about 0.60μm and 1.05μg/mm2, respectively. Model results indicate that ablation takes place primarily in the regime of normal vaporization from the surface. The results obtained at an accelerating voltage of 15kV and efficiency factor of 0.6 are satisfactorily in good accordance with available experimental data in the literature.

Study on nanosecond pulsed electron beam generation

The paper presents the findings of an investigation on volt-ampere characteristics of the diode with explosive emission cathodes of different constructions (blade metal-dielectric (MD-cathode) and solid graphite cathodes) under the change of the anode-cathode gap in wide ranges. The investigations were carried out using the TEA-500 pulsed electron accelerator. The total current of the electron beam was measured using the Faraday cup (FC). A 0.5-mm foiled glass fiber laminate was used as an emitting edge of the cathode in the experimental study with the explosive emission blade MD-cathode. Based on the obtained results, the conclusion was made that the graphite cathode has the most effective efficiency factor.

Thermoluminescence of anion-deficient aluminum oxide single crystals after high-dose irradiation by nanosecond electron pulses

The effect of deep traps filled by a pulsed electron beam on the dosimetric thermoluminescence (TL) peak at 450 K in anion-deficient aluminum oxide single crystals has been investigated. After filling deep traps, the dosimetric TL peak becomes nonelementary and is characterized by a complex dependence of the TL intensity on the crystal annealing temperature with alternating fall and rise portions. The influence of the occupancy of deep centers of different nature and different energy depths on the structure of dosimetric TL peak is analyzed. The suggestion that basically electron traps are depleted in the temperature ranges of 600–750 and 900–1000 K while holes are depleted at T = 780–900 and above 1000 K is substantiated. The possibility of using TL deep traps for high-dose dosimetry of pulsed electron beams is demonstrated.

Pulsed cathodoluminescence of calcite crystals of various origins

We have studied the luminescence of calcites from phlogopite-calcite veins, marbles, leucogranites, apatite-calcite ores, and carbonatites in the spectral range 300–800 nm under excitation with nanosecond electron pulses at an electron beam current density of ∼10 A/cm2. All of the calcite varieties have a fast emission component, with a broad spectrum in the near-UV and visible spectral regions, whose decay time is comparable to the excitation pulse duration, and a slow emission component, with a peak-emission wavelength of 610–620 nm and decay time of tens of milliseconds. The spectral composition of the emission depends on the nature of the calcite sample. We analyze the evolution of the spectrum after excitation and discuss the luminescence excitation mechanisms and the nature of the emission centers.

Pulsed cathodoluminescence of single-crystalline and pressed CaF2:Yb2+,Yb3+

We have studied the pulsed cathodoluminescence spectra and kinetics of CaF2:Yb2+,Yb3+ (3 mol % YbF3) single crystals and pressed samples under excitation with nanosecond electron pulses and determined the characteristic times and intensities of nanosecond and microsecond emission decay components at temperatures from 15 to 300 K. The results demonstrate that deformation pressing in vacuum at 1150°C followed by annealing in a CF4 atmosphere at 1180°C has an insignificant effect on the emissive properties of CaF2:Yb2+,Yb3+.

Electric breakdown and explosive decomposition of PETN monocrystals initiated by electron beam

The glowing, fracture, and explosive decomposition of pentaerythritol tetranitrate (PETN) monocrystals initiated by a nanosecond pulsed high-current electron beam (HEB) have been experimentally studied. It is established that crystallographically oriented electric (strimer) discharges are formed within the electron-beam action range in the crystal in a broad range of energy densities (0.1–75.0 J/cm2). Threshold energy density for the electric breakdown, fracture, and explosive decomposition of PETN monocrystals under HEB action have been determined. In contrast to pressed PETN samples, monocrystals exhibit incomplete explosion with expansion of the majority of unreacted substance.

Methane conversion in a plasma created by a periodic-pulsed electron beam and non-self-sustained discharge

The process of methane conversion in a mixture modeling the natural gas under the action of a periodic nanosecond pulsed electron beam and an electron-beam-induced non-self-sustained discharge has been numerically simulated and studied in experiment. The possibility of methane conversion under the action of electric pulses without significant heating of the processed gas mixture is demonstrated for the first time. The main conversion products in experiment were hydrogen, ethylene, and acetylene.