Fast Electron Beam Research Articles

Controlling and modelling the wetting properties of III-V semiconductor surfaces using re-entrant nanostructures

Inorganic semiconductors such as III-V materials are very important in our everyday life as they are used for manufacturing optoelectronic and microelectronic components with important applications span from energy harvesting to telecommunications. In some applications, these components are required to operate in harsh environments. In these cases, having waterproofing capability is essential. Here we demonstrate design and control of the wettability of indium phosphide based multilayer material (InP/InGaAs/InP) using re-entrant structures fabricated by a fast electron beam lithography technique. This patterning technique enabled us to fabricate highly uniform nanostructure arrays with at least one order of magnitude shorter patterning times compared to conventional electron beam lithography methods. We reduced the surface contact fraction significantly such that the water droplets may be completely removed from our nanostructured surface. We predicted the wettability of our patterned surface by modelling the adhesion energies between the water droplet and both the patterned surface and the dispensing needle. This is very useful for the development of coating-free waterproof optoelectronic and microelectronic components where the coating may hinder the performance of such devices and cause problems with semiconductor fabrication compatibility.

About a contactless transmission of 10 keV electrons through tapering microchannels

The possibility of increasing the current density of a beam of fast electrons passes through glass cone-shaped channels was demonstrated in [1]. But the fraction of the electrons that passed through the conical channels without loss of the initial energy was not cleared up. Measurements of X-ray spectra generated by transmitted electrons in copper target mounted in vicinity of capillary output were performed for a detailed study of the contactless passage of 10 keV electrons through conical capillaries. All the measurements were made at different tilting angles with respect to the incident beam axis. It is shown that a significant part of electrons retains its initial energy even at the angles exceeding geometric opening of the capillary.

Screening of a dust particle charge in a humid air plasma created by an electron beam

A kinetic model has been developed for charged particle reactions in a humid air plasma produced by a fast electron beam. The model includes over 550 reactions with electrons, 33 positive ion species and 14 negative ion species. The model has been tested by solving 48 non-steady state equations for number densities of charged particles in humid air electron beam plasma, and by comparing with the available experimental data. The system of 48 steady state equations has been solved by iterative method in order to define the main ion species of the humid air plasma. A reduced kinetic model has been developed to describe the processes with the main ions and electrons. Screening constants have been calculated on the basis of the reduced system by means of Leverrier–Fadeev method. The dependencies of screening constants on gas ionization rates have been found for the rates from 10 to 1018 cm−3s−1 and the fraction of water molecules from 0 to 2%. The analysis of the constants has revealed that one of them is close to the inverse Debye length, and the other constants are defined by the inverse diffusion lengths passed by ions in the characteristic times of the attachment, recombination, and ion conversion. Pure imaginary screening constants appear at low rates of gas ionization.

Studies of runaway electrons via Cherenkov effect in tokamaks

The paper concerns measurements of runaway electrons (REs) which are generated during discharges in tokamaks. The control of REs is an important task in experimental studies within the ITER-physics program. The NCBJ team proposed to study REs by means of Cherenkov-type detectors several years ago. The Cherenkov radiation, induced by REs in appropriate radiators, makes it possible to identify fast electron beams and to determine their spatial- and temporal-characteristics. The results of recent experimental studies of REs, performed in two tokamaks - COMPASS in Prague and FTU in Frascati, are summarized and discussed in this paper. Examples of the electron-induced signals, as recorded at different experimental conditions and scenarios, are presented. Measurements performed with a three-channel Cherenkov-probe in COMPASS showed that the first fast electron peaks can be observed already during the current ramp-up phase. A strong dependence of RE-signals on the radial position of the Cherenkov probe was observed. The most distinct electron peaks were recorded during the plasma disruption. The Cherenkov signals confirmed the appearance of post-disruptive RE beams in circular-plasma discharges with massive Ar–puffing. During experiments at FTU a clear correlation between the Cherenkov detector signals and the rotation of magnetic islands was identified.

Transverse magnetic field effect on the transport of relativistic electrons beam in laser irradiating plasmas

A transverse gauss shape magnetic field with wide width is proposed for collimating the fast relativistic electron beam in laser irradiating plasmas, which is highlighted by the two-dimensional particle-in-cell simulations, in particular, the effects of this magnetic field on the production and transport of fast electron beam. When the axial magnetic field is also present, it is found that the energy density of fast electrons can be enhanced greatly. For example, in the presence of 30 MG axial magnetic field, it is enhanced by 3–4 times when the amplitude of the applied transverse magnetic field lies within the optimal regime 200–300 MG comparable to that without the transverse magnetic field. Meanwhile, the divergence angle of the electron beam can be controlled and even decreased a little due to the better sandwich structure of the overall weakening magnetic field. The study implies that the proposed transverse magnetic field is helpful to obtain the high quality electron beam which is beneficial to the fast ignition in inertial confinement fusion.

Physical Kinetics of Electrons in a High-Voltage Pulsed High-Pressure Discharge with Cylindrical Geometry

Results of theoretical modeling of the phenomenon of a high-voltage discharge in nitrogen at atmospheric pressure are presented, based on a consistent kinetic theory of the electrons. A mathematical model of a nonstationary high-pressure discharge has been constructed for the first time, based on a description of the electron component from first principles. The physical kinetics of the electrons are described with the help of the Boltzmann kinematic equation for the electron distribution function over momenta with only ionization and elastic collisions taken into account. A detailed spatiotemporal picture of a nonstationary discharge with runaway electrons under conditions of coaxial geometry of the gas diode is presented. The model describes in a self-consistent way both the process of formation of the runaway electron flux in the discharge and the influence of this flux on the rate of ionization processes in the gas. Total energy spectra of the electron flux incident on the anode are calculated. The obtained parameters of the current pulse of the beam of fast electrons correlate well with the known experimental data.

Fast Rise Time High Current Electron Beam: Emission, Acceleration, and Drift Motion

Fast processes associated with emission, acceleration, and drift motion of a high current, moderately magnetized electron beam (particles energy ~300 keV; current ~3 kA) were studied experimentally when tubular explosive-emission cathode was supplied by subnanosecond rise time voltage pulse. Kinematic effect causes the sharpening of the observed beam front which was proven by particle-in-cell numerical modeling. The angular structure of tubular beam was studied through the current waveform records from collector probe after beam propagation through a radial-slit collimator. Current waveforms had time resolution no worse than 10 ps and provided the analysis of the beam temporal structure after its acceleration as well as in the process of further electrons drift motion in a finite guiding magnetic field.

Ionization and reflux dependence of magnetic instability generation and probing inside laser-irradiated solid thin foils

When an intense laser accelerated electron beam, with large current density on the order of 1012 A/cm2, enters a solid density plasma, it is well-known to be subject to a number of different types of instabilities that cause it to filament. In this work, we investigate the transport instability of a fast electron beam that is imprinted on the self-generated magnetic filaments inside the solid density plasmas using particle-in-cell simulations. By varying collisional ionization models, our simulations show that the atomic ionization process is crucial to determine the structure of the magnetic filaments. We further attribute the generation of bulk magnetic filaments to Weibel-like instability mechanism caused by counter-propagating hot forward-bulk return current streams and counter-propagating hot forward-reflux current streams. It is found that the magnetic fields in the filament channels near the rear surface are around one order of magnitude higher than those near the front surface of the thin solid target. This asymmetry is likely induced by the very different properties of bulk electron stream and hot reflux electron stream in terms of density and velocity distribution. Finally, we propose to probe the magnetic fields inside the solid density plasmas by X-Ray polarimetry via Faraday rotation using X-Ray free electron lasers (XFELs). The synthetic simulations show that XFELs are capable to detect the magnetic fields from relativistic laser-solid interactions.

Ionic composition of a humid air plasma under ionizing radiation

A kinetic model is proposed for ion–molecular processes involving charged particles of a humid air plasma produced by a fast electron beam. The model includes more than 600 processes involving electrons and 41 positive and 14 negative ions, including hydrated ions H3O+ (H2O) n and O 2 − (H2O) n with n = 1, 2, …, 12. The energy costs of production of electron–ion pairs and electronic and vibrational (for water molecules, also rotational) excitation of molecules are calculated in nitrogen, oxygen, water vapor, air, and humid air. A method is proposed for calculating the energy costs in mixtures by the calculation data in pure gases. The evolution of the plasma composition is studied by the numerical solution of a system of 56 time-dependent balance equations for the number of charged particles of plasma by the fourth-order Runge–Kutta method. The steady-state composition of plasma is determined by solving nonlinear steady-state balance equations for the ionization rates of humid air from 10 to 1016 cm–3/s and the fraction of water molecules from 10–3% to 1.5%. It is established that, for water vapor content (the ratio of the number density of water molecules to the total number density of air molecules) of 0.015–1.5% in air at atmospheric pressure and room temperature, the main ion species are two types of positive ions H3O+ (H2O) n with the number of water molecules n = 5, 6 and three species of negative ions O 2 − (H2O) n with n = 5, 8, 9.

Propagation of 10-keV electrons through glass macrocapillaries

The features of 10-keV electron propagation through a cylindrical channel made of borosilicate glass are experimentally studied. Experimental results indicate that a fast electron beam can be controlled using tapered capillaries fabricated from the given material. The charge distribution formed inside the channel provides conditions under which no less than 20% of beam electrons pass through the channel without considerable energy losses even if the inclination angles exceed the geometric angle of transmission.

Formation and energy relaxation of a fast electron beam in cathode regions of a glow discharge in helium

Results of a three-dimensional direct statistical simulation of formation and energy relaxation of a group of high-energy electrons in cathode regions of the low pressure (p < 1 Torr) glow discharge in helium are given. It is demonstrated that the electron distribution function at the exit from the cathode sheath contains a group (beam) of high-energy electrons which did not suffer inelastic collisions and had energy close to the cathode fall potential. The beam is shown to be a main source of charged particle production in the negative glow region. The calculated electron energy distribution function is compared with experimental data.

Experimental and theoretical study of the near IR emission of xenon excited by a fast electron beam

Emission of xenon excited by a 120-keV electron beam at gas pressures of 100, 200, 500, and 760 Torr nm was studied experimentally and theoretically. More than 30 spectral lines were identified in the wavelength range of 750–1000 nm. A self-consistent kinetic model is developed to calculate the emission intensity of xenon atoms in the near IR range. The model includes balance equations for the number densities of electrons, ions and excimer molecules; equations for the populations of electron levels; and the Boltzmann equation for the low-energy part of the electron energy distribution function with a source of slow electrons. Excitation and ionization rates of xenon by the beam electrons and the energy spectrum of slow electrons are calculated by the Monte Carlo method. It is shown that, under these conditions, the main mechanism of xenon atom excitation is dissociative recombination of Xe3 + ions.

An installation for studying luminescence excited by high-energy charged particles in dense xenon-containing gas mixtures with the possibility of regeneration and repeated use of xenon

An installation for studying the spectral, temporal, and generation characteristics of xenon-containing high-pressure excimer mixtures is described. The mixtures are excited by a pulsed beam of fast electrons with energy Е e = 220 keV, pulse duration Т p = 5 ns, and maximum beam current density I max = 20 A/cm2. Spectral and temporal measurements are taken in the wavelength range of 200–1200 nm with a wavelength resolution of 1 nm (the Maya-2000Pro spectrometer) and a temporal resolution of 2 ns/cm (a Tektronics TDS1024 oscilloscope and a FEU-106 photomultiplier tube). To reduce the xenon consumption when carrying out experimental investigations, the installation is equipped with a system of xenon peeling and regeneration, which allows the xenon loss to be reduced by factors of 50–100. The use of one additional external mirror for determining the gain characteristics of high-pressure excimer mixtures upon pumping with fast charged particles is considered.

Diagnosis of Weibel instability evolution in the rear surface density scale lengths of laser solid interactions via proton acceleration

It is shown for the first time that the spatial and temporal distribution of laser accelerated protons can be used as a diagnostic of Weibel instability presence and evolution in the rear surface scale lengths of a solid density target. Numerical modelling shows that when a fast electron beam is injected into a decreasing density gradient on the target rear side, a magnetic instability is seeded with an evolution which is strongly dependent on the density scale length. This is manifested in the acceleration of a filamented proton beam, where the degree of filamentation is also found to be dependent on the target rear scale length. Furthermore, the energy dependent spatial distribution of the accelerated proton beam is shown to provide information on the instability evolution on the picosecond timescale over which the protons are accelerated. Experimentally, this is investigated by using a controlled prepulse to introduce a target rear scale length, which is varied by altering the time delay with respect to the main pulse, and similar trends are measured. This work is particularly pertinent to applications using laser pulse durations of tens of picoseconds, or where a micron level density scale length is present on the rear of a solid target, such as proton-driven fast ignition, as the resultant instability may affect the uniformity of fuel energy coupling.

Laser-driven fast electron ionization wave propagation in a dielectric target

Ultrafast shadowgraphy with sub-picosecond resolution is applied to investigate the propagation of laser-driven fast electron beams inside a dielectric target. Time-resolved expansion of an ionization sphere caused by hot electron transportation in the target is observed. An abnormal absorption area with a width of approximately 10 μm is observed near the ionization front and is consistent with the one-dimensional electron transport model. The observed distortions of the edge diffraction fringes near the surface of the target are analyzed to qualitatively retrieve the phase shift caused by the ionization front. A simplified three-dimensional model is proposed to analyze the underlying physics and indicates that the valley in the ionization front appears due to the unstable propagation of hot electrons in a self-induced fountain electric field, which is induced by the collective effect of the free electrons and the ions.

The weakened Weibel instability of collimated fast electron beam in nanotube array

AbstractThe Weibel instability of the collimated MeV fast electron beams in a nanotube array target is researched in this work. It is found that the filamentation of the fast electrons is significantly suppressed. When fast electrons propagate the nanotube array, a strong magnetic field is created near the surface of tubes to obstruct the transverse movement of the fast electrons and bend them into the inner vacuum spaces between the successive tubes. In consequence, the positive feedback loop between the magnetic field perturbation and the electrons density perturbation is broken and the Weibel instability is thus weakened. Furthermore, the calculated results by a hybrid particle-in-cell code have also proven this weakening effect on the Weibel instability. Because of the high-energy density delivered by the MeV electrons, these results indicate some significant applications in the high-energy physics, such as radiography, fast-electron beam focusing, and perhaps fast ignition.

A self‐consistent mechanism for electron cyclotron maser emission and its application to type III solar radio bursts

AbstractType III solar radio bursts (SRBs) produced by fast electron beams (FEBs) traveling along solar magnetic fields are the best known and the most important kind of SRBs because of their clearest association with FEBs as well as most frequent observations during solar activities. However, the physics of their emitting mechanism has been a controversial issue. Based on the electron cyclotron maser (ECM) instability driven directly by a magnetized FEB, whose physics is fairly well known from the Earth's auroral kilometric radiation, this paper proposes a self‐consistent mechanism for type III SRBs, in which the Alfvén wave (AW) produced by the current instability of the beam‐return current system associated with the FEB, called the self‐generated AW, plays an important and crucial role. Taking into account the return‐current effect of the FEB, the growth rate and the saturation intensity of the self‐generated AW are estimated. Then the effects of the self‐generated AW on the ECM emission via the ECM instability driven by the magnetized FEB are further investigated. The results show that the self‐generated AW can significantly influence and change the physical properties of the ECM emission. In particular, this novel ECM emission mechanism can effectively overcome the main difficulties of the conventional ECM emission mechanism in application to type III SRBs and may potentially provide a self‐consistent physics scenario for type III SRBs.

The characteristics of the grazing interaction of fast electrons with dielectric surfaces

Experimental data on the mechanism of the grazing interaction of fast electron beams with planar dielectric surfaces of various materials are analyzed. New experimental results regarding the grazing interaction between electrons with an energy of 10 keV and metalized structured surfaces are reported. The difference between the processes of the interaction of fast electron beams with planar and structured surfaces is demonstrated.

Oscillation Maps in the Broadband Radio Spectrum of the 1 August 2010 Event

We search for indications of waves in the 25-2000 MHz radio spectrum of the 1 August 2010 event (SOL2010-08-01T08:57:00L075C013), where fast propagating waves in the solar corona with the periods of 181, 69, and 40 seconds were detected in UV observations. Using the wavelet technique we construct a new type of map of oscillations for selected periods in the whole domain of the radio spectrum. While the oscillation with the period of 181 seconds was recognized in the whole 25-2000 MHz radio spectrum, oscillations with periods of 69 and 40 seconds were confirmed only in the 250-870 MHz frequency range. In the 800-2000 MHz range we found periods of 50 and 80 seconds. Moreover, in the 250-870 MHz frequency range, the oscillation with the period of about 420 seconds was detected. We also made maps of phases of the 181-second oscillations in order to analyze their frequency drift. At the beginning of the radio event, in the 2000-500 MHz frequency range the phase of the 181-second oscillation drifts towards lower frequencies. On the other hand, at frequencies 25-500 MHz we found that the phase is nearly synchronous. While the phase drift at higher frequencies can be interpreted as being caused by the UV wave, the synchronization of the phase on lower frequencies is explained by the fast-electron beams, whose acceleration is modulated by the UV wave. Owing to this modulation, the electron beams are accelerated with the period of the UV wave (181 seconds). These beams propagate upwards through the solar corona and generate the 25-500 MHz radio emission with the 181-second period. Due to high beam velocity (~c/3, where c is the light speed) the 25-500 MHz radio emission, corresponding to a large interval of heights in the solar corona, is nearly synchronous.

Interferometry and X-ray diagnostics of pinched helium plasma in a dense plasma focus with an Al-wire on the axis

The paper describes the evolution of the ordered dense toroidal- and plasmoidal-like structures in a pinch column and the hard X-ray emission from mega-ampere dense plasma-focus discharges performed at the helium (He) filling. Some shots were carried out with an Al-wire of 270 μm in diameter, which was placed along the z-axis in the front of the anode face. The evolution of the considered structures was investigated by means of a multi-frame laser interferometer system as well as the X-ray diagnostics, and it was compared with their evolution observed earlier at the deuterium (D2) filling. In He-plasma, the ions had a higher mass and Z-number, and at the same initial filling pressure, the velocity of plasma transformations was decreased, but the stability of the investigated structures, as well as the self-generated azimuthal current component and the soft X-ray radiation were increased. The distribution of the plasma electron density (determined from the interferometric images) made it possible to estimate closed currents during the quasi-stationary phases. It was found that the internal toroidal- and plasmoidal-currents reached the level of hundreds of kA. The plasma corona around the Al-wire (penetrating through the internal structures) impeded the formation of a small diameter of the pinch constriction, but it did not prevent the production of fast electron beams with energies above 100 keV, similar to those observed at the D2-filling.