Specific Energy Deposition Research Articles

On electric field measurements based on intensity ratio of 1 − and 2 + systems of nitrogen in discharges with high specific deposited energy

Analysis of optical emission spectroscopy (OES) in application to the measurements of the electric field has been performed for the nanosecond capillary discharge in molecular nitrogen at moderate gas pressure at high specific energy deposition. Significant discrepancy between results of electric field measurements by OES technique and capacitive probe measurements is demonstrated. Decay rates of excited species as well as rotation structure of corresponding optical transitions are used to identify possible population/depopulation channels of the () and N2(C3Π u ) states. Kinetic calculations taking into account additional quenching of the () and N2(C3Π u ) states by electrons, additional population of () by electron impact from the ground state of the molecular ion and by a reaction of excited N2() with the ground state of describes adequately behavior of experimentally measured emission of both considered systems.

Monte Carlo simulations of nanodosimetry and radiolytic species production for monoenergetic proton and electron beams: Benchmarking of GEANT4-DNA and LPCHEM codes.

In hadrontherapy, biophysical models can be used to predict the biological effect received by cancerous tissues and organs at risk. The input data of these models generally consist of information on nano/micro dosimetric quantities and, concerning some models, reactive species produced in water radiolysis. In order to fully account for the radiation stochastic effects, these input data have to be provided by Monte Carlo track structure (MCTS) codes allowing to estimate physical, physico-chemical, and chemical effects of radiation at the molecular scale. The objective of this study is to benchmark two MCTS codes, Geant4-DNA and LPCHEM, that are useful codes for estimating the biological effects of ions during radiation therapy treatments. In this study we considered the simulation of specific energy spectra for monoenergetic proton beams (10 MeV) as well as radiolysis species production for both electron (1 MeV) and proton (10 MeV) beams with Geant4-DNA and LPCHEM codes. Options 2, 4, and 6 of the Geant4-DNA physics lists have been benchmarked against LPCHEM. We compared probability distributions of energy transfer points in cylindrical nanometric targets (10nm) positioned in a liquid water box. Then, radiochemical species (· OH, , , H2 , and yields simulated between 10-12 and 10-6 s after irradiation are compared. Overall, the specific energy spectra and the chemical yields obtained by the two codes are in good agreement considering the uncertainties on experimental data used to calibrate the parameters of the MCTS codes. For 10 MeV proton beams, ionization and excitation processes are the major contributors to the specific energy deposition (larger than 90%) while attachment, solvation, and vibration processes are minor contributors. LPCHEM simulates tracks with slightly more concentrated energy depositions than Geant4-DNA which translates into slightly faster recombination than Geant4-DNA. Relative deviations (CEV ) with respect to the average of evolution rates of the radical yields between 10-12 and 10-6 s remain below 10%. When comparing execution times between the codes, we showed that LPCHEM is faster than Geant4-DNA by a factor of about four for 1000 primary particles in all simulation stages (physical, physico-chemical, and chemical). In multi-thread mode (four threads), Geant4-DNA computing times are reduced but remain slower than LPCHEM by ∼20% up to ∼50%. For the first time, the entire physical, physico-chemical, and chemical models of two track structure Monte Carlo codes have been benchmarked along with an extensive analysis on the effects on the water radiolysis simulation. This study opens up new perspectives in using specific energy distributions and radiolytic species yields from monoenergetic ions in biophysical models integrated to Monte Carlo software.

Energy deposition by H and He ions at keV energies in self-supporting, single crystalline SiC foils

The specific energy deposition of H and He ions in SiC has been studied for both random and channeling orientations. The experiments were carried out in transmission geometry using the Time-of-Flight Medium Energy Ion Scattering System at the 350 keV Danfysik Implanter at Uppsala University. The target was a self-supporting, single crystalline cubic 3C–SiC (100) foil with nominal thickness of 200 nm. The measured stopping cross sections are compared with data available from the literature and theoretical predictions. The results for random geometries reveal slightly lower values than predicted by SRIM for H projectiles whereas for He projectiles good agreement was observed over the whole energy range studied. Higher specific energy loss is observed along random trajectories in comparison to channeling geometry, for all measured energies and for both H and He ions. For H ions, however, differences are minor, whereas for He ions, they are found generally more pronounced.

Modeling of fast ionization waves in pure nitrogen at moderate pressure

The fast ionization wave (FIW) discharges in pure nitrogen in a capillary tube at 27 mbar, initiated by positive polarity, high-voltage nanosecond pulses, are numerically studied by coupled two-dimensional plasma fluid modeling. The 2D fluid code based on local mean energy approximation is validated and used, an extended three-exponential Helmholtz photoionization model is proposed for pure nitrogen. The development and structure of the nitrogen FIW is analyzed, the electric field and current are calculated compared with experimental measurements. The evolution of radial distribution of electrons and N2(C 3Π u ) during the FIW development stage and in the afterglow are analyzed, the radial profiles of electron density show a ‘hollow’ structure in the FIW development stage, the temporal-spatial evolution of N2(C 3Π u ) is dominated by the competition between the pooling reaction of and the quenching by electrons. The role of photoionization on the nitrogen FIW radial morphology is discussed, the equivalent background electron density of photoionization in nitrogen discharge is suggested to be (4–6) × 1013 m−3. Spatial distribution of specific energy deposition (SED), fast gas heating (FGH) energy and temperature rise are obtained, heating efficiency varies with electric field E/N and SED and tends to be 10% at high SED. The dominating reactions responsible for FGH and their fractional contribution in space and time are analyzed, in the near axis region, pooling reactions of and N2(B 3Π g ) contribute up to 80% FGH energy, electron impact dissociation of molecular nitrogen contributes about 10%, while e- recombination and quenching of N(2 D) atoms by N2 molecules contribute rest.

Scaling of pulsed nanosecond capillary plasmas at different specific energy deposition

Nano-second, capillary discharges (nCDs) are unique plasma sources in their ability to sustain high specific energy deposition ω dep approaching 10 eV/molecule in molecular gases. This high energy loading on short timescales produces both high plasma densities and high densities of molecular exited states. These high densities of electrons and excited states interact with each other during the early afterglow through electron collision quenching and associative ionization. In this paper we discuss results from a two-dimensional computational investigation of a nCD sustained in air at a pressure of 28.5 mbar and with a voltage amplitude 20 kV. Discharges were investigated for two circuit configurations—a floating low voltage electrode and with the low voltage electrode connected to ground through a ballast resistor. The first configuration produced a single ionization wave from the high to low voltage electrode. The second produced converging ionization waves beginning at both electrodes. With a decrease of the tube radius, the velocity of the ionization fronts decreased while the shape of the ionization wave changed from the electron density being distributed smoothly in the radial direction, to being hollow shaped where there is a higher electron density near the tube wall. For sufficiently small tubes, the near-wall maxima merge to have the higher density on the axis of the capillary tube. In the early afterglow, the temporal and radial behavior of the N2(C3Πu) density is a sensitive function of ω dep due to electron collision quenching. These trends indicate that starting from ω dep ⩾ 0.3 eV/molecule, it is necessary to take into account interactions of electrons with electronically excited species during the discharge and early afterglow.

High-speed imaging of transition from fluid breakup to phase explosion in electric explosion of tungsten wires in air

High-speed visible imaging of sub-microsecond electric explosion of wires at the low specific energy deposition threshold reveals three distinct modes of wire failure as capacitor charge voltage and energy deposition are increased. For 100 μm diameter gold-plated tungsten wires of 2 cm length, deposited energies of 1.9 eV/atom produce a liquid column that undergoes hydrodynamic breakup into droplets with radii of the order of wire diameter on timescales of 200 μs. Instability growth, column breakup, and droplet coalescence follow classical Rayleigh–Plateau predictions for instability of the viscous fluid column. Above a deposited energy of 3.2 eV/atom, wires are seen to abruptly transition to an expanding mixture of micrometer scale liquid-droplets and vapor within one frame (less than 3.33 μs), which has been termed “phase explosion” in the literature. Between these two limits, at a deposited energy of 2.5 eV/atom, the wire radius remains unchanged for the first 10 μs before the onset of a rapid expansion and disintegration that resembles homogenous nucleation of mechanically unstable bubbles. Thermodynamic calculations are presented, which separate cases by temperature obtained during heating: below the boiling point, near the boiling point, and exceeding the boiling point.

Electronic excitation of transition metal nitrides by light ions with keV energies

We investigated the specific electronic energy deposition by protons and He ions with keV energies in different transition metal nitrides of technological interest. Data were obtained from two different time-of-flight ion scattering setups and show excellent agreement. For protons interacting with light nitrides, i.e. TiN, VN and CrN, very similar stopping cross sections per atom were found, which coincide with literature data of N2 gas for primary energies ⩽25 keV. In case of the chemically rather similar nitrides with metal constituents from the 5th and 6th period, i.e. ZrN and HfN, the electronic stopping cross sections were measured to exceed what has been observed for molecular N2 gas. For He ions, electronic energy loss in all nitrides was found to be significantly higher compared to the equivalent data of N2 gas. Additionally, deviations from velocity proportionality of the observed specific electronic energy loss are observed. A comparison with predictions from density functional theory for protons and He ions yields a high apparent efficiency of electronic excitations of the target for the latter projectile. These findings are considered to indicate the contributions of additional mechanisms besides electron hole pair excitations, such as electron capture and loss processes of the projectile or promotion of target electrons in atomic collisions.

Studies of equation of state properties of high-energy-density matter generated by intense ion beams at the facility for antiprotons and ion research.

The work presented in this paper shows with the help of two-dimensional hydrodynamic simulations that intense heavy-ion beams are a very efficient tool to induce high energy density (HED) states in solid matter. These simulations have been carried out using a computer code BIG2 that is based on a Godunov-type numerical algorithm. This code includes ion beam energy deposition using the cold stopping model, which is a valid approximation for the temperature range accessed in these simulations. Different phases of matter achieved due to the beam heating are treated using a semiempirical equation-of-state (EOS) model. To take care of the solid material properties, the Prandl-Reuss model is used. The high specific power deposited by the projectile particles in the target leads to phase transitions on a timescale of the order of tens of nanosecond, which means that the sample material achieves thermodynamic equilibrium during the heating process. In these calculations we use Pb as the sample material that is irradiated by an intense uranium beam. The beam parameters including particle energy, focal spot size, bunch length, and bunch intensity are considered to be the same as the design parameters of the ion beam to be generated by the SIS100 heavy-ion synchrotron at the Facility for Antiprotons and Ion Research (FAIR), at Darmstadt. The purpose of this work is to propose experiments to measure the EOS properties of HED matter including studies of the processes of phase transitions at the FAIR facility. Our simulations have shown that depending on the specific energy deposition, solid lead will undergo phase transitions leading to an expanded hot liquid state, two-phase liquid-gas state, or the critical parameter regime. In a similar manner, other materials can be studied in such experiments, which will be a very useful addition to the knowledge in this important field of research.

Effect of Accelerated Electrons on the Glow Structure of Nanosecond Diffuse Discharge in the Rod–Plane Air Gap

A diffuse discharge in a rod (cathode)–plane gap with an interelectrode spacing of 10 cm is studied in air at atmospheric pressure. The cathode voltage is 370–390 kV at a pulse FWHM of about 70 ns. It is shown that a pulse-to-pulse variation in delay time of the discharge formation in an interval of 3–15 ns leads to variations in the discharge current and X-ray pulse amplitude in intervals of 100–200 A and 20–250 relative units, respectively. A relationship of the amplitude of the X-ray pulse and structural parameters of the discharge glow is obtained. It is shown that fast electrons with energies of less 30 keV that are accelerated by strong electric field in the vicinity of the cathode play the key role in the formation of the longitudinal structure of glow at relatively large delay times of the discharge formation. Fast electrons ionize peripheral regions of the discharge, increase its cross section, and, hence, provide a decrease in the current density, specific energy deposition, and radiation intensity at a distance from the cathode that corresponds to electron path in air.

Formation and time development of spark channel in strong magnetic field

The characteristics of the spark channel plasma formed in the external longitudinal magnetic field are studied experimentally. In experiment, the current-voltage, optical, and spectral characteristics of the discharge were measured and analyzed. Synchronization of the pulsed electric and magnetic fields was performed. The effect of the magnetic field on the radial expansion of the spark channel, the shock wave formation, as well as the changes in the balance of energy and temperature in the ionized plasma of the high-pressure gas discharge were analyzed. It was found out that, in the strong longitudinal magnetic field, the temperatures of electrons and ions in the spark channel (ne ∼ 1018 cm−3) become equal. The current density, the plasma channel conductivity of the spark channel, and the specific energy deposition into the discharge increased. The channel plasma parameters were analyzed as functions of the external magnetic field.

Propagation of circularly polarized laser in strongly magnetized over-dense collisional plasma

Permittivity of a circularly polarized (CP) laser in magnetized collisional plasma is obtained in simplified form. The propagation of a CP laser in magnetized homogeneous over-dense collisional plasma was investigated, and characteristics such as attenuation length, specific energy deposition and blue shift are obtained. Potential applications in fields such as inertial confined fusion are discussed.

Discharge in a Subthreshold Microwave Beam as an Effective Means for Mercaptan Decomposition

A subthreshold microwave discharge in atmospheric-pressure air is used to process mixtures of mercaptans (thiols) with air and with air and methane. It is found that, at microwave intensities of ~10 kW/cm2 and pulse durations of 1–3 ms, the mercaptan concentration decreases 100-fold in mixtures with air and 20-fold in mixtures with air and methane. It is shown that, at an average specific energy deposition in a single discharge of ~0.4 J/cm3, the specific microwave energy spent on mercaptan decomposition is ~10 J/cm3. It is concluded that the active zone of the discharge with high specific energy deposition occupies less than 5% of the microwave beam volume.

Experimental investigations on energy deposition and morphology of exploding aluminum wires in argon gas

Experiments demonstrate a strong dependence of the energy deposition and morphology of exploding Al wires in argon gas on ambient pressures, charging voltage, and wire sizes. The specific energy deposition, before voltage collapse, increases with increasing ambient pressures and applied voltage but decreases with increasing wire sizes. The observation of the spatiotemporal distribution of exploding wires indicates that increased energy deposition suppresses the axial inhomogeneity, especially for large-sized wires, and improves the expansion velocity of the metal core. The expansion velocity of the wire core (100 μm in diameter, 2 cm in length) varies from ∼0.49 km/s to ∼2.3 km/s when the specific energy deposition increases from 1.95 eV/atom to 3.01 eV/atom. Decreased surrounding pressures also improve the expansion velocity. Furthermore, the analysis of photographs and emission spectrums demonstrates that the surrounding gases promote the formation of coronal plasmas when the pressure is no more than 50 kPa.

UV Radiation of High-Voltage Multi-Electrode Surface Discharge in Gaseous Medium

UV radiation of high-voltage pulsed multispark surface discharge in atmospheric-pressure air in the spectral range of 200–380 nm was investigated. The discharge was a sequence of microplasma objects with a specific energy deposition of ~1 kJ/cm3 and electron density of 1017 cm–3. Copper and stainless steel were used as the electrode materials. It is found that lines of atoms and ions of the electrode material dominate in the discharge UV spectrum. The UV radiation intensity was measured using the actinometry technique. The UV radiation efficiency relative to the energy deposited in the discharge is found to be ~1%.

Experimental Investigations on Specific Energy Deposition and Burst Characteristics in Electrically Exploded Single Copper Wires

Experimental study has been carried out to look for the effect of varying parameters such as diameter, the rate of rise of current, and coating on the behavior of copper wire explosion in the air with an aim to, especially, observe the effect of parameters which has significance in confinement of exploded wire plasmas by inhibiting its expansion. Single-wire experiments are performed on a capacitor of 2-μF charged to voltages in the range of 11-40 kV. Effects of parameters such as dielectric coating and change in diameter (58-233 μm) of coated wires on specific energy deposition are experimentally observed at varying rates of rise of current (peak current 28-104 kA in a quarter period of 1.1 μs). Current in the circuit and the voltage across the load are measured using Rogowsky coil and voltage divider, respectively. The results indicate that the specific energy deposition can be enhanced by optimizing radial dimensions of wire, charging voltage, and surface coating. Analysis of results also suggests that the action integral is affected by the diameter of the wire and the rate of rise of currents as reported by some theoretical works so far.

Spatial and temporal distribution characteristics of an exploding aluminum wire immersed in argon gas

Experiments involving exploding aluminum wires immersed in argon gas and driven by a pulsed current source (in the microsecond time scale) were carried out in this work. Apart from electrical measurements, a multi-frame optical camera was used to observe the spatial and temporal distributions of the plasmas produced by the explosion. A time-integrated spectrometer and a photo-electric multiplier were used to analyze the light emitted from exploding aluminum wires. Electrical explosion of wires has been classified into three types according to current and voltage waveforms. By using photographic evidence and spectrum-based measurements, characteristics of the spatial and temporal distributions of the exploding wires associated with different experimental parameters were obtained and analyzed. Furthermore, the expansion trajectories of the exploding Al wires with different specific energy depositions are presented. It was found that more deposited energy leads to faster expansion velocities of the exploding products.

Optical actinometry of O-atoms in pulsed nanosecond capillary discharge: peculiarities of kinetics at high specific deposited energy

The density of O-atoms was studied in a nanosecond capillary discharge in air with 5.3% additions of Ar at 28.5 mbar. Time-resolved electrical current, longitudinal electric field, optical emission of O(3p3P) at λO = 844.6 nm, Ar(2p1) λAr = 750.4 nm and their ratio, and emission of N2(C3Πu) at were measured. A kinetic scheme describing consistent behavior of the set of the experimental data was developed. The main processes responsible for population and decay of the species of interest were selected on the basis of sensitivity and rate analysis. The electric field was taken as input data; all other experimentally obtained signals were modeled; experimental data and results of calculations were in good agreement. The role of the reactions between excited, charged species and electrons in the early afterglow for pulsed discharges at high reduced electric fields and high specific deposited energy was discussed. The density of O-atoms in the ground state was calculated. It was concluded that Ar-based traditional actinometry demands advanced kinetic modeling with regards to the nanosecond discharge with a high specific energy deposition and high reduced electric field.

Features of Structural Phase Transitions of Aluminum Hydroxides under Laser Processing

The results of the studies of laser processing of alkoxide aluminum hydroxides with micrometer and nanometer particle sizes are presented. It is shown that the pseudo-boehmite processing process and phase composition of formed oxides are controlled by particle packing, laser radiation propagation in powder, and specific energy deposition. The main phases formed upon laser heating are γ, α-Al2O3; the content of δ, θ-Al2O3 is low. The minimum corundum crystallite size is ∼50 nm.

Characteristics of the electrical explosion of fine metallic wires in vacuum

The experimental investigations on the electrical explosion of aluminum, silver, tungsten and platinum wires are carried out. The dependence of the parameters related to the specific energy deposition on the primary material properties is investigated. The polyimide coatings are applied to enhance the energy deposition for the exploding wires with percent of vaporized energy less than unit. The characteristics of the exploding wires of different materials with and without insulating coatings are studied. The effect of wire length on the percent of vaporization energy for exploding coated wires is presented. A laser probe is employed to construct the shadowgraphy, schlieren and interferometry diagnostics. The optical diagnostics demonstrate the morphology of the exploding products and structure of the energy deposition. The influence of insulating coatings on different wire materials is analyzed. The expansion trajectories of the exploding wires without and with insulating coatings are estimated from the shadowgram. More specific energy is deposited into the coated wires of shorter wire length, leading to faster expanding velocity of the high-density products.

Gain distribution in the volume of a solid-state active element of a diode-pumped high-power laser

We propose a technique for calculating the spatial gain profile in the volume of a solid active element of a high-energy laser pumped by laser diode arrays, which takes into account the properties of the radiation pattern of semiconductor lasers. This technique is employed to calculate the profile of the gain in a neodymium glass active element. By the example of a rod of rectangular section 4.5 by 4.5 by 25 cm we show that, by varying the pump system parameters, it is possible to define the profile of the gain: for instance, a profile uniform over the cross section (with its nonuniformity under 2.5%), with a peak at the centre or in peripheral regions, with a specific energy deposition of ∼1 J cm−3.