Number Of High-energy Electrons Research Articles

The dynamical behavior of the capacitively coupled argon plasma driven by very high frequency

The dynamical behavior of the capacitively coupled Ar plasma driven by four frequencies (54, 80, 94 and 100[Formula: see text]MHz) at fixed pressure are simulated by the particle-in-cell/Monte-Carlo collisions (PIC/MCC) model based on the electromagnetic field. The magnetic field is generated by plasma current itself. The results show that the electron density, charge density, and electron temperature increase with the frequency increase when radio frequency (RF) power is 40[Formula: see text]W. The electron heating rate and argon ion heating rate near the sheath increase with the frequency increase. The high-energy electron density ([Formula: see text] [Formula: see text]eV) varies nonlinearly with the frequency increase. The high-energy electron density is highest at 80[Formula: see text]MHz among the four frequencies. The electron density, charge density, and high-energy electron density increase with RF power (30, 45, 60, and 75[Formula: see text]W) increase at 80[Formula: see text]MHz. The electron temperature decreases with RF power increase at 80[Formula: see text]MHz. The electron heating rate and argon ion heating rate near the sheath have no change with RF power increase at 80[Formula: see text]MHz. The electron energy probability distributions (EEPF) show that the number of high-energy electrons ([Formula: see text] [Formula: see text]eV) is highest at 80[Formula: see text]MHz. The reason may be the electron resonant heating that occurs in the plasma when the electron cyclotron frequency equals source frequency at 80[Formula: see text]MHz. The electron dynamics of capacitively coupled plasma is important for microelectronics etching and membrane deposition.

A study on the influence of external magnetic field on Nitrogen RF discharge using Langmuir probe and OES methods

This paper reports the study of the effects of an externally applied magnetic field (0–300 G), in the mode transition as well as in the radial and axial variation of different plasma parameters such as electron density, temperature, etc, in nitrogen RF discharge with the help of an RF compensated Langmuir probe (LP). Also, Optical Emission Spectroscopy (OES) study is performed in order to have a good understanding of the properties of plasma at different magnetic fields. Data collected from LP shows all three mode transitions (E, H, and W mode) in presence of magnetic fields whereas for no magnetic field only two modes (E and H) are visible. The measured value of electron density by using LP is further verified and compared theoretically using particle and power balance equations. However, the overall density profile attains a higher value for no magnetic field. This rise in overall density at 0 G field is further explained in terms of EEPF plot and OES analysis. The EEPF plot reveals that the number of high energy electrons is reduced with the application of magnetic fields. Also from OES analysis, it is found that the molecular excitations in N2 second positive system [C 3Πu (ν′) → B 3Πg (ν′′)] are increased in the presence of magnetic fields whereas with no magnetic field the ionization peak of N2 first negative system and the molecular dissociation peak at 746.8 nm attains the largest value at a certain power. Plasma density values calculated with the OES method at the different magnetic fields and RF power show a similar trend with respect to the density values obtained from the LP method.

Toluene removal over TiO2-BaTiO3 catalysts in an atmospheric dielectric barrier discharge

Non-thermal plasma (NTP) combined with catalysts has been regarded as a promising approach for the removal of VOCs. In the present work, a series of TiO2-BaTiO3 catalysts were prepared by simple mechanical mixing. The performance of the catalysts on degradation of high concentration of toluene was investigated in a dielectric barrier discharge (DBD) plasma system. The effects of input power, mass ratio of Ti/Ba, gas flow rate and the initial concentration of toluene on the removal of toluene and the selectivity of CO2 as well as the energy yield have been investigated. The toluene removal efficiency and CO2 selectivity increased obviously in the plasma-catalytic system compared with the single NTP system. The removal efficiency of toluene with different catalysts abided by the following rules: Ti1Ba1 > Ti2Ba1 > Ti1Ba2 > BaTiO3 > TiO2. The reason for the best performance of Ti1Ba1 is that TiO2 offered the providential number of electron-hole pairs while BaTiO3 supplied the fitting number of high energy electrons. Additionally, the synergistic effects between TiO2 and BaTiO3 also reached its maximum in this case. The concentration of O3 in the exhaust gas was also detected and it reached the lowest point when Ti1Ba1 catalyst was packed in the discharge chamber.

High-Yield High-Efficiency Positron Generation in High-Z Metal Targets Irradiated by Laser Produced Electrons from Near-Critical Density Plasmas**Supported by the National Basic Research Program of China under Grant No 2013CBA01502, the National Natural Science Foundation of China under Grant Nos 11575011 and 11535001, the National Grand Instrument Project under Grant No 2012YQ030142, and the UK EPSRC under Grant Nos EP/G054950/1, EP/G056803/1,

An improved indirect scheme for laser positron generation is proposed. The positron yields in high-Z metal targets irradiated by laser produced electrons from near-critical density plasmas and underdense plasma are investigated numerically. It is found that the positron yield is mainly affected by the number of electrons of energies up to several hundreds of MeV. Using near-critical density targets for electron acceleration, the number of high energy electrons can be increased dramatically. Through start-to-end simulations, it is shown that up to positrons can be generated with state-of-the-art Joule-class femtosecond laser systems.

Investigation of the electron kinetics in O2 capacitively coupled plasma with the use of a Langmuir probe

A Langmuir probe was used to measure various electron plasma parameters in O2 capacitively coupled plasma. It was shown that the variation in these plasma parameters was due to changes in the electron heating mechanisms as the discharge conditions varied. The so called ‘α–γ’ mode transition in O2 plasma (100 mTorr) was identified from the power evolution (30–600 W) of the electron energy probability function (EEPF), electron density (ne) and effective electron temperature (Teff). The EEPF evolved from Druyvesteyn to bi-Maxwellian with increasing applied power which resulted in a rapid decrease and an abrupt increase in Teff and ne respectively. Comparisons were made to the same mode transition for similar conditions in Ar plasma. The EEPFs were Druyvesteyn in the α mode and evolved into a Maxwellian like EEPF in the γ mode of an Ar plasma. Two distinct trends of ne versus power was observed, it was shown that the measured rf current and rf voltage had a similar behavior. The pressure evolution of the EEPF, ne, and Teff was also investigated in O2 plasma operated at both 30 and 200 W. At 30 W the number of high energy electrons decreased and flattening of the low energy portion of the EEPF occurred with increasing gas pressure (10–100 mTorr) which indicates a collisionless to collisional heating transition. However, at 200 W the right combination of rf voltage and pressure was met for the discharge to evolve into the γ mode as the pressure increased. This was evident from significant narrowing of the EEPF as the pressure increased.

Ultra-intense laser pulses in near-critical underdense plasmas – radiation reaction and energy partitioning

Although, for current laser pulse energies, the weakly nonlinear regime of laser wakefield acceleration is known to be the optimal for reaching the highest possible electron energies, the capabilities of upcoming large laser systems will provide the possibility of running highly nonlinear regimes of laser pulse propagation in underdense or near-critical plasmas. Using an extended particle-in-cell (PIC) model that takes into account all the relevant physics, we show that such regimes can be implemented with external guiding for a relatively long distance of propagation and allow for the stable transformation of laser energy into other types of energy, including the kinetic energy of a large number of high energy electrons and their incoherent emission of photons. This is despite the fact that the high intensity of the laser pulse triggers a number of new mechanisms of energy depletion, which we investigate systematically.

Polymethyl methacrylate glow under the influence of runaway electron beams generated in a gas diode

Spectral and amplitude–time characteristics of PMMA radiation under the impact of runaway electrons with subnanosecond duration are studied. The PMMA radiation spectra for a subnanosecond electron beam pulse duration are determined for the first time. The studies show that radiation of the band with a maximum at about 490 nm the intensity of which decreases toward the short-wave spectral region is recorded in the glow spectra. The glow intensity of this band varies proportionally to the number of electrons in the beam, which allows the possibility of using this radiation for determination of the number of high energy electrons in electron beams.

In situ monitoring of silicon nanocrystal formation with pulsed SiH4 supply by optical emission spectroscopy of Ar plasma

The time evolution of the optical emissions of Ar plasma during silicon nanocrystal (SiNC) fabrication using a pulsed SiH4 supply is studied. The enhancement of Ar emission with a duration longer than the ON time of the pulsed SiH4 supply is observed owing to the formation of SiNCs in the plasma. This enhancement can be explained in terms of the increase in the number of high-energy electrons caused by electron attachments to SiNCs. The size and generation rate of SiNCs clearly correlate with the duration and intensity of the enhanced emission even when the Ar flow rate or the plasma power is changed. On the basis of this relationship, the size and density of SiNCs can be predicted during the fabrication by monitoring the enhanced emission.

Spatially Resolved Energetic Electron Properties for the 21 May 2004 Flare from Radio Observations and 3D Simulations

We investigate in detail the 21 May 2004 flare using simultaneous observations of the Nobeyama Radioheliograph, Nobeyama Radiopolarimeters, Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar and Heliospheric Observatory (SOHO). The flare images in different spectral ranges reveal the presence of a well-defined single flaring loop in this event. We have simulated the gyrosynchrotron microwave emission using the recently developed interactive IDL tool GX Simulator. By comparing the simulation results with the observations, we have deduced the spatial and spectral properties of the non-thermal electron distribution. The microwave emission has been found to be produced by the high-energy electrons ($>100$ keV) with a relatively hard spectrum ($\delta\simeq 2$); the electrons were strongly concentrated near the loop top. At the same time, the number of high-energy electrons near the footpoints was too low to be detected in the RHESSI images and spatially unresolved data. The SOHO Extreme-ultraviolet Imaging Telescope images and the low-frequency microwave spectra suggest the presence of an extended "envelope" of the loop with lower magnetic field. Most likely, the energetic electron distribution in the considered flare reflects the localized (near the loop top) particle acceleration (injection) process accompanied by trapping and scattering.

Numerical studies on intense laser-generated relativistic high-energy electrons via a thin cone target

It is proposed that with a mini-cone target, an enormous number of high-energy collimated electrons can be produced, which may be used for fast ignition research. The effect of different laser and cone target diameters on high-energy electrons are studied with two-dimensional particle-in-cell simulations. When the open angle of the mini-cone is 10 degree, the number of generated hot electrons is relatively larger. With the increase of the open angle, both the energy and number of hot electrons decrease. When preplasma is added to the cone surface, the amount of hot electrons increases, while the peak energy of the hot electrons decreases. With the increase of the laser pulse duration, the number of high-energy electrons increases linearly.

Interaction between glow discharge plasma and dust particles

The effect of dust particle concentration on gas discharge plasma parameters was studied through development of a self-consistent kinetic model which is based on solving the Boltzmann equation for the electron distribution function. It was shown that an increase in the Havnes parameter causes an increase in the average electric field and ion density, as well as a decrease in the charge of dust particles and electron density in a dust particle cloud. Self-consistent simulations for a wide range of plasma and dust particle parameters produced several scaling laws: these are laws for dust particle potential and electric field as a function of dust particle concentration and radius, and the discharge current density. The simulation results demonstrate that the process of self-consistent accommodation of parameters of dust particles and plasma in condition of particle concentration growth causes a growth in the number of high-energy electrons in plasma, but not to depletion of electron distribution function.

Effect of Dust Particle Growth on the Emission Spectrum of a Complex Plasma

The growth of dust particles in a plasma can drastically change the plasma's properties; importantly, it can affect the electron energy distribution function. We have performed optical emission spectroscopy on an RF sputtering discharge in argon during dust particle growth to analyze this change. We find that the intensities of all of the argon emission lines increase, indicating an increase in the number of high-energy electrons as a result of dust growth.

Numerical simulation of atmospheric-pressure helium discharge driven by combined radio frequency and trapezoidal pulse sources

Atmospheric-pressure capacitive discharges driven by combined radio frequency (rf) and trapezoidal pulse sources are investigated using a one-dimensional self-consistent fluid model. The results show that the plasma intensity in the rf discharge can be enhanced drastically when a low duty ratio short pulse source is additionally applied. The mechanism for the increase in the plasma density can be attributed to a strong localized electric field induced by the applied short pulse; the strong electric field generates a great number of high energy electrons and chemically active particles, which subsequently generate more electrons and ions. The rf capacitive discharges with the aid of externally applied short pulses can achieve a high plasma density with better power efficiency.

Prepulse effects on the generation of high energy electrons in fast ignition scheme

The energy distribution of the produced high energy electrons in the interaction of ultraintense picosecond laser pulses with high-Z solid targets is shown to be sensitive to the preformed plasma created by the prepulse and the amplified spontaneous emission pedestal. The created preformed plasmas, which are obtained by radiation hydrodynamic simulations for the present heating laser system at ILE, Osaka University, are seen to extend up to 30–100 μm just before the arrival of the main pulse. The dependences of the coupling efficiency of the laser energy to high energy electrons, and the energy spectra of these accelerated electrons, on this preformed plasma, are studied via a two-dimensional particle-in-cell simulation code. It is found that in a small preformed plasma case, J×B heating is dominant and the produced electron temperature agrees well with Haines’ scaling law [Haines et al., Phys. Rev. Lett., 102, 045008 (2009)]. While in a large preformed plasma case, in addition to J×B heating and/or vacuum heating, other acceleration mechanisms, such as stochastic heating, can accelerate electrons to very high energies, carrying a significant fraction of input laser energy. Even after several picoseconds, the number of high energy electrons (0.5 MeV<E<5 MeV) generated in a small preformed plasma case can be several times larger than that of a large preformed plasma case.

Rf discharge dissociative mode in NF3 and SiH4

This paper shows that the rf capacitive discharge in NF3 and SiH4 can burn in three possible modes: weak-current α-mode, strong-current γ-mode and dissociative δ-mode. This new dissociative δ-mode is characterized by a high dissociation degree of gas molecules (actually up to 100% in NF3 and up to 70% in SiH4), higher resistivity and a large discharge current. On increasing rf voltage first we may observe a weak-current α-mode (at low NF3 pressure the α-mode is absent). At rather high rf voltage when a sufficiently large number of high energy electrons appear in the discharge, an intense dissociation of gas molecules via electron impact begins, and the discharge experiences a transition to the dissociative δ-mode. The dissociation products of NF3 and SiH4 molecules possess lower ionization potentials, and they form an easily ionized admixture to the main gas. At higher rf voltages when near-electrode sheaths are broken down, the discharge experiences a transition to the strong-current γ-mode.

An optical trap for relativistic plasma

The first optical trap capable of confining relativistic electrons, with kinetic energy ⩽350 keV was created by the interference of spatially and temporally overlapping terawatt power, 400 fs duration laser pulses (⩽2.4×1018 W/cm2) in plasma. Analysis and computer simulation predicted that the plasma density was greatly modulated, reaching a peak density up to 10 times the background density (ne/n0∼10) at the interference minima. Associated with this charge displacement, a direct-current electrostatic field of strength of ∼2×1011 eV/m was excited. These predictions were confirmed experimentally by Thomson and Raman scattering diagnostics. Also confirmed were predictions that the electron density grating acted as a multi-layer mirror to transfer energy between the crossed laser beams, resulting in the power of the weaker laser beam being nearly 50% increased. Furthermore, it was predicted that the optical trap acted to heat electrons, increasing their temperature by two orders of magnitude. The experimental results showed that the number of high energy electrons accelerated along the direction of one of the laser beams was enhanced by a factor of 3 and electron temperature was increased ∼100 keV as compared with single-beam illumination.

Comparison between ICPs and MIPs in Terms of Excitation Mechanism and Deviation from LTE

It is suggested that plasma forming and heating processes determine the plasma mode in which partially ionized discharges such as ICPs and MIPs should remain. In turn, the plasma mode, either ionizing or recombining, dictates the ionization and excitation mechanisms in the source, as well as how the plasma deviates from local thermodynamic equilibrium (LTE). It is shown that ICPs, due to relatively high electron number density, are excellent excitation sources for most metallic elements; on the other hand, owing to their extremely high electron temperature and substantial number of high energy electrons, MIPs are able to ionize or excite most nonmetallic elements efficiently. A detailed comparison between ICPs and MIPs is given concerning fundamental physical parameters, energy transfer, excitation mechanism, deviation from LTE, and analytical characteristics.

Computation of drain and substrate currents in ultra-short-channel nMOSFET's using the hydrodynamic model

The authors develop a robust and efficient numerical solution of the hydrodynamic model, which solves the energy balance equation, and compare predictions of this model, using one set of parameters, with experimental nMOSFET characteristics for a range of channel lengths down to ultrashort channels. The substrate current is calculated by direct integration of the energy distribution function, which uses the computed temperature to obtain the number of high energy electrons. The drain current calculated using this method is accurate for a range of channel lengths and biases, and correctly predicts the observed enhanced transconductance for ultrashort-channel devices. The substrate current matches the experimental data for a range of channel lengths and biases above threshold with one set of physically reasonable parameters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Kinetic Model for Plasma Etching Silicon in a SF6/O2 RF Discharge

Time-dependent Boltzmann electron distribution calculations have been made at constant power and pressure in a SF6/O2 plasma with a varying oxygen mole fraction. The results show that as the oxygen fraction increases in a SF6/O2 plasma, the number of high-energy electrons in the tail of the electron distribution and the mean electron energy both increase significantly while the plasma is kept at the same reduced electric field E/N. Rate coefficients have been computed for the electron kinetic processes of these plasmas and merged within a kinetic equilibrium model for the plasma etch process, including neutral gas-phase chemistry, ion chemistry, and surface reactions. Model simulations show good agreement with experimental results for SF6/O2 etching of polysilicon and demonstrate that the anisotropic character of dilute SF6 plasma etching is related to the shift in the electron distribution with increasing oxygen fraction. Competition between F and O species for adsorption to silicon etching sites is also shown to be a factor in determining etch rates, but this competition is not significant until very large (> 80 percent) oxygen concentrations are present. Ionization rates and ion transport to the surface are shown to be much more important. The model simulations provide a rationale for explaining the very high etch rates observed at low-SF6 partial pressures and the increasing anisotropic etch character with greater oxygen dilution of SF6.

A new type of multiplicative core selector for extensive air shower arrays

We have developed a new type of core selector for extensive air shower arrays which is able to locate in a short time the axis for each individual shower event. Basically, the device consists in two plastic scintillators separated by a layer of lead where the large number of high-energy electrons, characteristic of the core region, undergoes multiplication. The theory of the device and some preliminary experimental results are presented.