Fast Heavy Particles Research Articles

Investigation of the characteristics and mechanism of subnanosecond switching of a new type of plasma switches. I. Devices with counter-propagating electron beams—kivotrons

Novel opportunities of waveform tailoring for controlling plasma parameters based on the development of a high-voltage gas-discharge switch with a subnanosecond breakdown time and high pulse repetition frequency are discussed. The studies of characteristics and breakdown development mechanisms of the switch based on an ‘open’ discharge—kivotron are summarized. The discharge in the switch is carried out in conditions when counter-propagating electron beams in high electric field are generated. In this case, when using helium as an operating medium, firstly, atoms are effectively excited into the resonance state by fast particles. Secondly, due to the Doppler effect, resonant photons without reabsorption reach the cathode surface, maintaining the discharge current due to photoemission. Thirdly, fast heavy particles modify the cathode surface, thereby significantly (up to an order of magnitude) increasing the photoemission coefficient. The combination of these processes leads to an increase in the switching rate with an increase in the operating voltage U and helium pressure pHe. At U > 20 kV and pHe > 10 Torr, the switching time becomes less than 100 ps both in the experiment and according to the simulation. It is preferable to use planar geometry without a drift space as a switching device, in which, on the one hand, the most complete use of EB energy is realized in creating a plasma with a high charge density, on the other hand, a small wave impedance of the switch is realized. As a result, currents of tens of kiloamperes are achieved at voltages up to 100 kV. In an interpulse period plasma in the discharge gaps recombinates fast. As a result, switches can operate up to pulse repetition frequency 100 kHz. Together these achievements open new opportunities to control plasma parameters.

Mechanism of High-Efficiency Electron Beam Generation in a High-Voltage Discharge in Helium and Its Mixtures with Oxygen and Nitrogen

The results of investigation of the current-voltage characteristics (CVCs) and electron beam generation efficiency in continuous discharges in helium, its mixtures with oxygen and nitrogen, as well as in pure oxygen and helium are presented. Peculiarities of CVCs are singled out and interpreted in terms of a changed role of the principal emission mechanisms with an increase in the voltage. It is shown that a high efficiency of the electron beam generation of more than 80% can be achieved in glow discharges in helium, oxygen, and nitrogen. In helium, it is provided by the predominating photoemission, while in oxygen and nitrogen and their mixtures with helium – mainly by the kinetic emission under the impact of fast heavy particles.

Study of the Properties of an Anomalous Glow Discharge Generating Electron Beams in Helium, Oxygen, and Nitrogen

The current–voltage characteristics (CVCs) and efficiency of electron beam generation in glow discharges in helium and its mixtures with oxygen and nitrogen, as well as in pure oxygen and nitrogen, are studied experimentally. Special attention is paid to creating clean conditions for a discharge operating in helium. It is shown that, under clean conditions and pressures above 10 Torr, the CVC first rapidly grows. Then, the growth slows down and the CVC begins to decrease; however, at voltages above 1.5 kV, it rapidly grows again. These features are explained via changes in the mechanisms of electron emission and electron runaway from the cathode sheath, which lead to a highly efficient (up to 85%) generation of electron beams. In the presence of molecular admixtures, the CVC changes and begins to smoothly grow, the current being substantially higher than in pure helium. In pure oxygen and nitrogen, the CVC also grows smoothly and electron beam generation is highly efficient, but its mechanism is different. In pure helium, electrons are generated primarily due to photoemission, whereas in pure oxygen and nitrogen, electron emission from the cathode is mainly caused by the bombardment by fast heavy particles. In helium mixtures with oxygen and nitrogen, other emission mechanisms can also take place.

Solid-state track detectors in laser plasma investigations

The possibility of application of the solid-state track detectors, developed earlier for experimental nuclear physics, in laser plasmas investigations is considered. A review of the main physical and metrological properties of the solid-state track detectors is given. Special attention to the detector on the basis of the CR- 39 polymer plastic is made. The experimental layouts of the CR-39 applications for investigation of both the properties of the fast heavy particles generated in the femtosecond laser plasma and the properties of the various materials by irradiation by the flows of the fast particles emitted by the laser plasmas are investigated.

Coefficient of Radiation-Stimulated Diffusion of Interstitial Atoms in a Flux of Fast Electrons

An expression for the coefficient of radiation-stimulated diffusion of interstitial atoms, which arises in a flux of fast electrons, is obtained on the basis of a solution of Boltzmann’s equation. As a specific example, a relation for approximate evaluation of the coefficient of radiation-stimulated diffusion of interstitial atoms in a flux of fast electrons formed when fast heavy charged particles decelerate in materials is presented and analyzed.

Fluorescent nuclear track detector technology – A new way to do passive solid state dosimetry

This paper is a review of the latest progress in development of the next generation luminescent dosimetry technique-Fluorescent Nuclear Track Detectors (FNTD) that have important advantages in measuring fast neutrons and high energy heavy charge particles. New Mg-doped aluminum oxide crystals and novel imaging instrumentation for FNTD technology are discussed with regard to application in mixed neutron-gamma fields, medical dosimetry and radiobiological research. Dosimetric characteristics of new detectors are summarized.

On the role of high-energy electrons in the formation of a plasma-beam discharge structure in a diode with a slit cathode

The spatial structure of visible emission from plasma of a transverse nanosecond pulsed electric discharge in a gas-filled diode with a hollow (slit) cathode has been experimentally studied. A relation is established between the regime of electron energy relaxation and the plasma-beam discharge structure formation. High values of the electron emission coefficient are observed in the experiment, which cannot be explained using the notion of electron emission from a cathode bombarded by fast heavy particles.

Mechanism for efficient electron beam generation in a pixel of an open-discharge plasma display

A concept is proposed of a plasma pixel based on an open-discharge microstructure. The concept employs the capability of an open discharge to generate an electron beam at moderate (1–3 kV) discharge voltages with an efficiency close to 100%. To determine the possible application of this type of discharge, the parameters of the electron beams generated in open discharges operating in different working gases at various geometries of the discharge cell and various dimensions of the discharge channel were investigated. The electric potential distributions in the dielectric plate channel and in the cathode cavity were measured. The effect of additional illumination by radiation generated in the drift space on the current-voltage characteristic of the discharge is studied. Based on the results obtained, a noncontradictory model of a discharge capable of very efficiently generating an electron beam is proposed. According to this model, the main contribution to the electron beam comes from the photoelectron emission from the cathode under the action of radiation from the working-gas atoms excited by fast heavy particles in a highly nonuniform electric field in the cathode cavity. Such a field also scatters ions and fast atoms, thus reducing their fluxes toward the cathode. The results obtained indicate that highly efficient light sources and plasma panels can be created on the basis of open-discharge microstructures with a cathode cavity. Such microstructures allow very efficient conversion of electric energy into light.

Computer program TRACK_TEST for calculating parameters and plotting profiles for etch pits in nuclear track materials

Computer program TRACK_TEST for calculating parameters and plotting profiles for etch pits in nuclear track materials

Electron emission from a cathode doped by fast particles of a working gas

The effect of fast charged and neutral particles on the emission properties of different materials is investigated. In many plasma devices, a flux of fast atoms and ions produces a specific self-sustained state of the layer near the cathode surface. In particular, this layer is saturated by working gas atoms to a depth of several monolayers. This state variously modifies different types of emission. Potential emission, taking place under the action of metastable atoms and ions, weakens because of a rise in the work function. On the contrary, kinetic emission, which is due to fast heavy particles, is enhanced, since energy losses in the modified near-surface layer increase. Photoemission resulting under the action of resonance radiation increases considerably, and its mechanism changes.

Heavy-Particle Hybrid Simulation of a High-Voltage Glow Discharge in Helium

A high-voltage glow discharge – operated at several thousand Volts – is described by a hybrid model in which we apply kinetic simulation for the fast electrons and fast heavy particles (He+ ions and neutral atoms). In the model we take into account the increase of the gas temperature, and calculate the apparent secondary electron yield at the cathode from the flux-energy distributions of heavy particles. The results of the simulations show that electron production at the cathode is dominated by the impact of fast neutral atoms (ejecting ≈75% of the primary electrons) and that ionization of the gas atoms by fast heavy particles contributes significantly to the ionization balance of the discharge. A fair agreement is obtained between measured and calculated electrical characteristics and potential distributions, and the calculations confirm the existence of the high-energy electron beam observed experimentally.

The effect of compensation currents of the open discharge

Previous conclusions concerning a general mechanism of electron beam formation in glow discharges of various types, including the open discharge, are confirmed. The discharge behavior is determined by ionization processes and by the bombardment of cathode with fast heavy particles, in agreement with well-established notions about the glow discharge. Neither compensation currents nor the electron beam current can account for the photoelectron mechanism of the open discharge, despite still existing opposite assumptions.

The role of ions(N＋２) in the dissociation of nitrogen in a direct-current glow discharge

A model is developed for a nitrogen direct-current glow discharge by a combination of Monte Carlo model of fast electrons and various heavy particles(N＋２，N＋，Nf),in which N+2-N２ dissociative charge transfer is incorporated,and the role of this process in nitrogen glow discharge is investigated .It is found that this process is dominant in the space of close to the cathode,and that this becomes important with increasing voltage,especially that this plays an important role at higher voltages for the production of fast atomic species(N＋，Nf) at the cathode.We have also compared our calculated results with experimental data.

Au-II 282 nm segmented hollow-cathode laser-parametric studies and modeling

Laser operation on the Au-II 282.3 nm ultraviolet transition is obtained using a high-voltage segmented hollow-cathode discharge tube. The metal vapor is produced by means of cathode sputtering. A small amount of argon is added to the helium buffer gas in order to achieve higher sputtering yield. Measurements of the laser power and small signal gain indicate that the optimal partial concentration of argon is in the range of 0.25%–0.75%. Quasi-continuous wave output power of 100 mW is obtained from a 34-cm-long active region while the highest small-signal gain is 52% m−1. To explain the basic features of the laser operation we present a model of the segmented hollow-cathode discharge. All the discharge characteristics are calculated in a self-consistent way except the temperature of slow electrons. The trajectories of fast electrons emitted from the cathode are followed by Monte Carlo simulation. Rate equations of ion, metastable and metal atom densities are solved in the negative glow, while another Monte Carlo code is applied for the fast heavy particles in the cathode sheath. The spatial distribution of the gas temperature and the thermalization of sputtered metal atoms are calculated as well. The laser characteristics predicted by the model are in reasonable agreement with the experimental results.

Modeling the response of thermoluminescence detectors exposed to low- and high-LET radiation fields.

Lithium fluoride thermoluminescence (TL) detectors, with different Li composition (Li-6 and Li-7) and various activators (LiF:Mg,Ti, LiF:Mg,Cu,P), are widely used for dosimetry in space. The primary radiation field in space is composed of fast electrons, protons and heavy charged particles (HCP). By its interaction with the structures of the spacecraft, this field may be modified inside the crew cabin. Therefore, calibration of TL detectors against a dose of gamma-rays is not sufficient for relating the TL readout to absorbed dose or to quantities relevant in radiation protection, without suitable correction. We introduce and calculate the detection efficiency, eta, relative to gamma-ray dose, of lithium fluoride detectors after proton and heavy charged particle (HCP) irradiation. We calculate eta for MCP-N (LiF:Mg,Cu,P) and for MTS-N (LiF:Mg,Ti) using microdosimetric models. The microdosimetric distributions used in these models (for HCP of charges between Z=1 to Z=8 and in the energy range between 0.3 MeV/amu and 20 MeV/amu) are calculated using an analytical model, based on the results of Monte Carlo simulated charged particle tracks using the MOCA-14 code. The ratio etaMCP-N/etaMTS-N for protons of stopping power (in water) below 10 keV/microm lies in the range between 0.65 and 1.0 and for HCP with Z>1--between 0.3 and 0.6. The stopping power of the particle is found not to be a unique parameter to scale the response of TL detectors. The combination of response of LiF:Mg,Cu,P and LiF:Mg,Cu,P detectors can be more suitable for a dose correction in space radiation fields.

Monte Carlo simulation of fast electrons and heavy particlesin the CDS of nitrogen dc glow discharge

The characteristics of fast electrons (e-) and heavy particles (N2+, N+, N2f, Nf) in the cathode dark space (CDS) of nitrogen dc glow discharge are simultaneously studied by Monte Carlo simulation. The calculated energy and angular distributions of these particles at different positions from the cathode provide a clear picture of their transport behaviours within the CDS. The density and mean energy of these particles indicate that the electrons and the atomic ions (N+) are the main high-energy species and the molecular ions (N2+) are the major ions in the CDS. It can be seen from the energy distributions of the bombarding particles at the cathode surface that the molecular ions and the fast atoms (Nf) are the main active species participating in the cathode nitride material synthesis process. The influence of the backscattering of the electrons from the negative glow to the CDS is also investigated. All the calculated results provide good information on the spatial characteristics of the particles considered in this paper and also their internal connections in the CDS of nitrogen dc glow discharge.

Apparent secondary-electron emission coefficient and the voltage-current characteristics of argon glow discharges.

The accuracy of secondary-electron emission coefficients, that are used as input data of discharge models, seriously influences the calculated discharge characteristics. As it is very difficult to consider all possible electron emission processes of a cold cathode separately, in most of the recent models an apparent secondary coefficient gamma is applied, which is often assumed to be constant, even for a wide range of discharge conditions. In contrast with this common assumption, the present calculations-based on a heavy-particle hybrid model-show that in abnormal glow discharges gamma varies considerably with changing discharge conditions: a factor of 3 change of gamma has been found in the range of reduced current densities (0.04 mA cm(-2) Torr(-2)< or =j/p(2)< or =4 mA cm(-2) Torr(-2)) covered in this study. The present simulations also confirm that ionization by heavy particles plays a significant role in the ion production at the abnormal cathode fall. Moreover, it is shown, that the fast heavy particles reflected from the cathode surface play the dominant role in the gas heating.

Formation of an effective electric field in track regions during the stopping of fast heavy charged particles in materials

A microscopic approach is used to study the formation of an electric field near the trajectories of fast heavy charged particles propagating in various materials. The analysis is based on determining the space—time distribution function of the fast δ-electrons generated when heavy ions are stopped in materials and the electric current produced by them. The spatial dependence of the electric field strength is determined at various times. The results are used to analyze the process of electric field energy transfer to the ionic subsystem. The spatial distribution of the energy acquired by the ionic subsystem from the electric field is determined over its characteristic lifetime. A mechanism is proposed to explain the formation of track regions both as a result of the higher local heating of the ionic subsystem and as a result of the possible irreversible displacement of the atoms from nodal points.

Nitrogen gas-discharge electron source with secondary-emission cathode

A mathematical model of a high-voltage electron source with a secondary-emission cathode and anodic plasma is constructed. Nitrogen is used as the working gas. The model is based on a kinetic description of the physical processes in the source, approximations of elementary-process cross-sections are given, and calculations are performed in the 2.5-400-kV range. The need to allow for charge exchange and the reflection of fast heavy particles from the cathode is shown.

Electron emission from multielectron atoms in impulse and semiclassical approximation

The angular and energy distribution of the electrons ejected in collisions between fast heavy charged particles and atoms has been studied in the simple Born approximation using the semiclassical approximation for atoms. As a result of this approach an analytical solution of the problem is obtained. Comparisons are made with reported experimental results.