Low-energy Electron Population Research Articles

Preliminary Exploration of Low Frequency Low-Pressure Capacitively Coupled Ar-O2 Plasma

Non-thermal plasma as an emergent technology has received considerable attention for its wide range of applications in agriculture, material synthesis, and the biomedical field due to its low cost and portability. It has promising antimicrobial properties, making it a powerful tool for bacterial decontamination. However, traditional techniques for producing non-thermal plasma frequently rely on radiofrequency (RF) devices, despite their effectiveness, are intricate and expensive. This study focuses on generating Ar-O2 capacitively coupled plasma under vacuum conditions, utilizing a low-frequency alternating current (AC) power supply, to evaluate the system’s antimicrobial efficacy. A single Langmuir probe diagnostic was used to assess the key plasma parameters such as electron density (ne), electron temperature (Te), and electron energy distribution function (EEDF). Experimental results showed that ne increases (7 × 1015 m−3 to 1.5 × 1016 m−3) with a rise in pressure and AC power. Similarly, the EEDF modified into a bi-Maxwellian distribution with an increase in AC power, showing a higher population of low-energy electrons at higher power. Finally, the generated plasma was tested for antimicrobial treatment of Xanthomonas campestris pv. Vesicatoria. It is noted that the plasma generated by the AC power supply, at a pressure of 0.5 mbar and power of 400 W for 180 s, has 75% killing efficiency. This promising result highlights the capability of the suggested approach, which may be a budget-friendly and effective technique for eliminating microbes with promising applications in agriculture, biomedicine, and food processing.

The Energy Budget in the Jet of High-frequency Peaked BL Lacertae Objects

Energy equipartition and the energy budget in the jet are important issues for the radiation mechanism of blazars. Early work predominantly concentrated on flat-spectrum radio quasars and a limited number of BL Lacertae (BL Lac) objects. In this paper, we compile 348 high-frequency peaked BL Lac objects (HBLs) based on the catalog of active galactic nuclei (4LAC-DR3) from Fermi-LAT, and employ JetSet to fit the spectral energy distributions (SEDs) of these HBLs in the framework of the one-zone lepton model. We aim to determine whether the energy budget is reasonable and whether energy equipartition is satisfied in HBLs. The results of the statistical analysis suggest that: (1) SEDs of HBLs can be reproduced well by using the one-zone lepton model; however, it cannot achieve energy equalization, and the relativistic electron energy density is far greater than the magnetic field energy density, U e ≳ 100U B ; (2) the majority of HBLs are located in the t cool < t dyn region (where the horizontal coordinate represents the jet power of electrons, while the ordinate indicates the ratio of the dynamic timescale to the cooling timescale), and the jet kinetic power of HBLs is greater than the jet power of radiation; there is a very low radiation efficiency, we deduce that HBLs may have optically thin advection-dominated accretion flows; (3) logϵB of HBLs is less than zero, which indicates that the jet kinetic power of HBLs is not affected by Poynting flux; (4) the relationships U e > U Syn ∼ U B , L e ∼ L p > L B ∼ L rad, and logϵe>0.5 are established. These relations indicate that most of the energy of HBLs is stored in the population of low-energy electrons.

Magnetic field induced electron temperature inhomogeneity effects on discharge properties in cylindrical capacitively coupled plasmas

This paper presents the discharge properties of a novel cylindrical Capacitively Coupled Plasma (CCP) discharge with an axisymmetric magnetic field. It is observed that with an increase in magnetic field strength, the electron temperature in the discharge increases steadily towards the peripheral region; however, the plasma density and potential tend to remain maximum at the center. The higher electron temperature in the peripheral region leads to a faster decay in plasma density towards the radial boundary, consistent with the analytical model. The radial ion flux distribution is uniform within a 5% range. With the application of a magnetic field, the EEPF depicts enhancement in the mid-energy range electron population in the peripheral region; however, the plasma bulk remains bi-Maxwellian with a higher population of low-energy electrons. A qualitative discussion is presented to highlight the potential applications of this device for a large-area plasma processing application.

Simulating the charging of isolated free-falling masses from TeV to eV energies: Detailed comparison with LISA Pathfinder results

A model is presented that explains the charging rate of the LISA Pathfinder test masses by the interplanetary cosmic ray environment. The model incorporates particle-tracking from TeV to eV energies using a combination of GEANT4 and a custom low-energy particle generation and tracking code. The electrostatic environment of the test mass is simulated allowing for a comparison of the test-mass charging-rate dependence on local electric fields with observations made in orbit. The model is able to reproduce the observed charging behavior with good accuracy using gold surface properties compatible with literature values. The results of the model confirm that a significant fraction of the net charging current is caused by a population of low-energy ($\sim$eV) electrons produced by electron- and ion-induced kinetic emission from the test mass and surrounding metal surfaces. Assuming a gold work function of 4.2 eV, the unbalanced flow of these electrons to and from the unbiased test mass contributes $\sim$10% of the overall test mass charging rate. Their contribution to the charging-current shot noise is disproportionately higher and it adds $\sim$40% to the overall predicted noise. However, even with this increased noise contribution the overall charging-current noise is still only 40% of that measured in-orbit, and this remains an unsolved issue.

Electron energy probability function measurement in a 2 MHz and 13.56 MHz dual-frequency capacitively coupled argon plasma

The effect of low-frequency power and high-frequency power on the electron energy probability function (EEPF) and the physical and electrical characteristics of plasma are experimentally investigated in a dual-frequency capacitively coupled plasma. RF powers of 2 MHz (low-frequency) and 13.56 MHz (high-frequency) are simultaneously applied to an electrode. EEPFs and DC self-bias voltages (V DC) are measured as one of the two RF powers is increased while the other is fixed. When the 2 MHz power increases at a fixed 13.56 MHz power, the electron density decreases, and the electron temperature increases with the decrease in the population of low-energy (below 5 V) electrons in the EEPF. Note that the increase in the low-frequency power is accompanied by a large decrease in V DC, which is related to the ion energy. On the other hand, when 13.56 MHz power increases at a fixed 2 MHz power, the electron density and the electron temperature significantly increase with the increase in the population of high-energy (above 5 V) electrons in the EEPF, while V DC decreases slightly. Experimental results show that the increase in the low-frequency power enhances the ion energy, and the increase in the high-frequency power enhances electron heating and ionization efficiency.

Discharge characteristic of very high frequency capacitively coupled argon plasma* *Project supported by the National Natural Science Foundation of China (Grant No. 11665021) and the Natural Science Foundation of Gansu Province, China (Grant No. 20JR10RA078).

The discharge characteristics of capacitively coupled argon plasmas driven by very high frequency discharge are studied. The mean electron temperature and electron density are calculated by using the Ar spectral lines at different values of power (20 W–70 W) and four different frequencies (13.56 MHz, 40.68 MHz, 94.92 MHz, and 100 MHz). The mean electron temperature decreases with the increase of power at a fixed frequency. The mean electron temperature varies non-linearly with frequency increasing at constant power. At 40.68 MHz, the mean electron temperature is the largest. The electron density increases with the increase of power at a fixed frequency. In the cases of driving frequencies of 94.92 MHz and 100 MHz, the obtained electron temperatures are almost the same, so are the electron densities. Particle-in-cell/Monte-Carlo collision (PIC/MCC) method developed within the Vsim 8.0 simulation package is used to simulate the electron density, the potential distribution, and the electron energy probability function (EEPF) under the experimental condition. The sheath width increases with the power increasing. The EEPF of 13.56 MHz and 40.68 MHz are both bi-Maxwellian with a large population of low-energy electrons. The EEPF of 94.92 MHz and 100 MHz are almost the same and both are nearly Maxwellian.

Temporal evolution of electron energy distribution function and its correlation with hydrogen radical generation in atmospheric-pressure methane needle–plane discharge plasmas

Non-thermal plasmas show great potential in low-temperature activation of methane (CH4) owing to the abundant energetic active species. Motivated by the fact that the chemical reactions in plasma-based CH4 conversion are dominated and regulated by the energetic electrons and various radicals, the temporal evolution of the electron energy distribution function (EEDF) and its relation to hydrogen (H) radical generation in an atmospheric-pressure CH4 needle–plane discharge plasma have been investigated numerically. The simulations are carried out using one-dimensional particle-in-cell Monte-Carlo collision and fluid dynamic models. It can be shown that during the formation and development of the streamer, a characteristic time exists, before and after which the evolution characteristic of the EEDF is reversed. This is mainly attributed to the competition between the energies continuously obtained from the electric field and the increasingly strong inelastic collisions and fast-growing low-energy electron population. When the amplitude of the applied voltage is increased, the fraction of electrons with high enough energy to participate in dissociation or ionization reactions of CH4 increases, leading to an increased H density. Besides, the characteristic time decreases exponentially, and the energy efficiency of the activation of CH4 molecules is decreased. An appropriate electron energy distribution and H radical density should be chosen to ensure acceptable product selectivity and conversion rate without excessive energy consumption; this will depend on the required products. The results presented in this work provide a partial theoretical basis for effectively optimizing the content of high-energy electrons and H radicals.

Effect of dust particle and magnetic field on EEPF and plasma oscillation

The significance of dust particles for the electron energy probability function (EEPF) and plasma oscillations is studied under varying magnetic field strength in a filamentary discharge hydrogen plasma. The experimental result shows that with an increase in dust density, the electron density decreases as a result of the charging of dust grains in the plasma background. A bi-Maxwellian EEPF is computed in both a pristine hydrogen plasma and a dust-containing plasma at different magnetic field strengths. We have observed that the increase in magnetic field decreases the lower energy electron population. The electron population of the lower energy range shows nearly identical results at magnetic field, $B\leqslant 3.7$ mT whereas the behaviour of the high-energy electron population becomes identical for a field strength $B\leqslant 5.8$ mT. From the observation, we have seen that the mid energy electron population slightly decreases and the high energy electron population slightly increases due to the presence of dust particles as compared to a pristine plasma. Further, very low energy electron population remains almost unchanged. With increase in dust density, the mid energy electron population further decreases whereas the high energy electron population slightly increases for different magnetic fields. But, no changes were observed for the very low energy electron population in the presence of dust particles. From the study of plasma oscillation, it is observed that the dominant frequency associated with the plasma oscillation is matched with the ion cyclotron frequency. The amplitude of the ion cyclotron frequency reduces with the increase of dust density which might be due to the decrease of plasma density.

Modulation of electron energy distribution functions and plasma parameters in a dual-frequency cylindrical ICP source

In this paper, a new dual-frequency cylindrical inductively coupled plasma source which operates with dual-frequencies (high-frequency (HF) 13.56 MHz and low-frequency (LF) 2 MHz) and dual-antennas (a 2-turn HF antenna and a 6-turn LF antenna) is designed, and the effects on electron energy distribution function and plasma parameters (i.e., plasma density ne, electron temperature Te, plasma floating potential Vf and plasma potential Vp) in the following cases are investigated: (1) fixed HF power of 500 W and LF power varying from 300 W to 900 W, (2) fixed LF power of 500 W and HF power varying from 300 W to 900 W. It is found that increasing HF power enhances the population of high-energy electrons, while increasing LF power increases the population of low-energy electrons. This result can be explained by the different electron heating mechanisms, i.e., ohmic heating for the LF discharge and collisional-less heating for the HF discharge at an argon gas pressure of 15 mTorr. Moreover, the different influences of HF power on ne, Te, Vf, Vp at fixed LF 500 W and that of LF power on ne, Te, Vf, Vp at fixed HF power 500 W are compared. It is found that although ne increases with both HF and LF power, it increases more significantly with the change of HF power. This indicates that ne is mainly determined by the HF power and slightly influenced by the LF power. Furthermore, the Te, Vf and Vp decrease with increasing LF power at a fixed HF power of 500 W, which is contrary to the trend of Te, Vf and Vp with increasing HF power at the same LF power of 500 W.

Effect of dust particle size on the plasma characteristics in a radio frequency capacitively coupled silane plasma

Compared with dust-free plasmas, the existence of dust particles in plasmas may greatly influence the plasma properties, such as the plasma density, electron temperature, sheath properties, electron energy distribution function (EEDF) as well as the heating mechanism. In this work, a 1D hybrid fluid/MC model has been developed to investigate the interaction between dust and plasma in a low-pressure silane discharge sustained in a radio frequency capacitively coupled plasma, in which we assume spherical dust particles with a given radius are generated by taking the sum of the production rate of Si2 and Si2 as the nucleation rate. From our simulation, the plasma may experience definite perturbation by dust particles with a certain radius (more than 50 nm) with an increase in electron temperature first, which further induces a rapid rise in the positive and negative ion densities. Then, the densities begin to decline due to the gradual lack of sufficient seed electrons. In addition, as the dust radius increases, the high energy tails of the EEDFs will be enhanced for discharge maintenance, accompanied by a decline in the population of low-energy electrons in comparison with those of pristine plasma. Furthermore, an obvious bulk heating is observed apart from the α-mode and local field reversal heating. This may contribute to the enhanced bulk electric field (also called the drift field) as a result of electron depletion via the dust. In addition, large-sized dust particles that accumulate near the sheaths tend to form two stable density peaks with their positions largely influenced by the time-averaged sheath thickness. A detailed study of the effects of the external parameters, including pressure, voltage and frequency, on the spatial distribution of dust particles is also conducted.

Studies on probe measurements in presence of magnetic field in dust containing hydrogen plasma

The accuracy of plasma parameters measured by Langmuir probe in presence of magnetic field is studied in our present work. It is observed that the ratio of electron to ion saturation current shows almost identical behavior with that of unmagnetized hydrogen plasma when r L > 10r p (here r L : Larmor radius and r p : probe radius). At magnetic field strength, B = 594 gauss, the electron temperature (T e ) shows an overestimated value up to ~35–40%, whereas at B ≤ 37 gauss, T e shows around ≤10% overestimated value w.r.t. unmagnetized case. A bi-Maxwellian electron energy probability function is observed for entire magnetic field range for both pristine and dust containing hydrogen plasma. The bulk (cold) electron collection by the Langmuir probe is strongly suppressed whereas the higher energetic electron collection remains unaffected in presence of magnetic field. In presence of dust grains, it is found that the low energy electron population decreases even more than the magnetized plasma and the high-energy tail slightly increases compared to the pristine plasma.

Electron energy distribution function in the divertor region of the COMPASS tokamak during neutral beam injection heating

This paper presents the results from swept probe measurements in the divertor region of the COMPASS tokamak in D-shaped, L-mode discharges, with toroidal magnetic field BT = 1.15 T, plasma current Ip = 180 kA and line-average electron densities varying from 2 to 8×1019 m−3. Using neutral beam injection heating, the electron energy distribution function is studied before and during the application of the beam.The current-voltage characteristics data are processed using the first-derivative probe technique. This technique allows one to evaluate the plasma potential and the real electron energy distribution function (respectively, the electron temperatures and densities).At the low average electron density of 2×1019 m−3, the electron energy distribution function is bi-Maxwellian with a low-energy electron population with temperatures 4-6 eV and a high-energy electron group 12-25 eV. As the line-average electron density is increased, the electron temperatures decrease. At line-average electron densities above 7×1019 m−3, the electron energy distribution function is found to be Maxwellian with a temperature of 6-8.5 eV.The effect of the neutral beam injection heating power in the divertor region is also studied.

Nonlinear collisional absorption and induced anisotropy in plasmas heated by an intense laser field

The inverse bremsstrahlung absorption of a laser wave by electrons in homogeneous plasmas is investigated in the range α=v02/vt2≤2, where v0 is the electron quiver velocity and vt is the thermal velocity. For α≪1, previous results are recovered. In the range α≥1, the interplay of collisional absorption and induced plasma anisotropy led to significant new results regarding the electron distribution function and the laser wave damping rate. By increasing α, the low-energy electron population increases, while the collisional absorption rate is still reduced. The temperature anisotropy induced by electron heating is also estimated.

Measurement of electron energy probability function in weakly magnetized plasma

Knowledge of the real EEDF is of great importance in understanding the underlying physics of processes occurring at the magnetized plasma, such as the formation of transport barriers, cross-field diffusion coefficients and plasma–substrate interactions. In the present experiment, the application of Langmuir probe to evaluate EEPF in presence of magnetic fields within the range 594–32 G is investigated. The data recorded for EEPFs in magnetic fields and in dust is acquired using current–voltage characteristics measured in low pressure hydrogen plasma. The values of plasma density, electron temperature and EEPF are evaluated with a single cylindrical Langmuir probe at different axial positions (1 cm to 6 cm) from the magnet. From the recent EEPF observations in presence of magnetic field, it shows a bi-Maxwellian EEPF structure at different magnetic fields. But at different magnetic field, it is observed that the low energy electron population changes whereas the high-energy electron population remains almost constant. EEPF measurement shows almost identical behaviour with the unmagnetized plasma when the larmour radius of electron is greater than or equal to 10 times of the probe radius.

Determination of the plasma potential and the EEDF by Langmuir probes in the divertor region of COMPASS tokamak

This paper reports experimental data obtained on the COMPASS tokamak by an array of Langmuir probes embedded in the divertor tiles. The measured current-voltage probe characteristics were processed by the recently published first-derivative probe technique for precise determination of the plasma potential and the electron energy distribution function (EEDF). The measurements were performed during L-mode hydrogen and deuterium plasma with a toroidal magnetic field Bt = 1.15 T, plasma current Ip = 180 kA and average electron density ne = 8*1019 m-3. The spatial profile of the electron temperatures shows that in the vicinity of the inner and outer strike points in hydrogen plasma the EEDF can be approximated by a bi-Maxwellian distribution, with a dominating low-energy electron population (4 - 7 eV) and a minority of electrons with higher energies (12 - 18 eV). In the private flux region between the two strike points, the EEDF is found to be Maxwellian with temperatures in the range of 7 - 9 eV. In the case of deuterium plasma under similar discharge conditions, the EEDFs in the vicinity of the inner and outer strike points, as well as in the private flux region between the two strike points, are found to be bi-Maxwellian.

Trapped Photoelectrons During Spacecraft Charging in Sunlight

For a dielectric spacecraft charging in sunlight, the potentials are different on the sunlit and dark sides. Differential charging of spacecraft surfaces can trap low-energy electrons by means of potential wells and barriers. The low-energy electrons are mostly photoelectrons and secondary electrons. Motivated by the recent interest in trapped photoelectrons measured by the Van Allen Probes in the radiation belts, we calculate the extent of the trapped photoelectron area and the potential barrier as a function of the dipole strength and sun angle using the monopole–dipole model. We find that the dipole strength is an important parameter in controlling the behavior of the potential wells and barriers. The usual inequality, $1/2 \le A \le 1$ where $A$ is the dipole strength, used in the monopole–dipole model can be relaxed and amended for finite sun angles. We then use a simple method to estimate the density of the trapped low-energy electrons in these areas. In sunlight charging, the low-energy electron population around the spacecraft is enhanced by the photoelectrons trapped inside the potential barrier.

Studies on hydrogen plasma and dust charging in low-pressure filament discharge

The effect of working gas pressure and dust charging on electron energy probability function has been studied for hydrogen plasma in a multi-dipole dusty plasma device. A cylindrical Langmuir probe is used to evaluate the plasma parameters and electron energy probability function (EEPF) for different working pressures. For lower energy range (below 10 eV), the EEPF follows a bi-Maxwellian shape at very low pressure (6 × 10−5 mbar), while elevating the working pressure up to ∼2 × 10−3 mbar, the shape of the EEPF transforms into a single Maxwellian. Some dip structures are observed at high energy range (ε &amp;gt; 10 eV) in the EEPF of hydrogen plasma at all the working conditions. In presence of dust particles, it is observed that the shape of the EEPF changes due to the redistribution of the high and low-energy electron populations. Finally, the effect of working pressure on charge accumulation on dust particles is studied with the help of a Faraday cup and electrometer. From the observations, a strong influence of working pressure on plasma parameters, EEPF and dust charging is observed.

Investigation of the Boltzmann relation in plasmas with non-Maxwellian electron distribution

An experimental investigation of the Boltzmann relation at various radio frequency (RF) powers was performed in a low pressure RF inductively coupled plasma (ICP). The measured electron energy probability functions (EEPFs) show a high population of low energy electrons at low RF powers, while the population of high energy electrons is high at high RF powers. In these bi-Maxwellian EEPFs, radial variations of the electron density and the plasma potential from the center to the edge of the discharge chamber were plotted according to the Boltzmann relation with the effective temperature Teff and the screening temperature Ts, and it was found that the Boltzmann relation with the screening temperature, rather than with the effective temperature, describes the relation between the electron density and the plasma potential.

Diffusion of cosmic-ray electrons in the Galactic centre molecular cloud G0.13–0.13

AbstractThe Galactic center (GC) molecular cloud G0.13–0.13 exhibits a shell morphology in CS J = (1 − 0), with ∼ 105 solar masses and expansion speed ∼ 20 km s−1, yielding a total kinetic energy ∼ 1051 erg. Its morphology is also suggestive of an interaction with the nonthermal filaments of the GC arc. 74 MHz emission indicates the presence of a substantial population of low energy electrons permeating the cloud, which could either be produced by the interaction with the arc or accelerated in the shock waves responsible for the cloud's expansion. These scenarios are explored using time dependent diffusion models.With these diffusion models, we determine the penetration of low-energy cosmic-ray electrons accelerated into G0.13–0.13 and calculate the spatial distribution of the cosmic-ray ionization and heating rates. We show that the 6.4 keV Fe Kα line emission associated with the electron population provides an observational diagnostic to distinguish these two acceleration scenarios.We discuss the implications of our results for understanding the distinct character of clouds in the central molecular zone compared to clouds in the Galactic disk, and how GC nonthermal filaments interact with molecular clouds.

Modulation of electron energy distributions and discharge parameters in a dual frequency ICP discharge

Using a radio frequency (RF) compensated Langmuir probe, modulations in electron energy distribution (EED) and plasma potential are investigated in a discharge produced by a large-area dual frequency/dual antenna inductively coupled plasma source. The discharge is ignited using two frequencies (2 and 13.56 MHz). It is observed that the EEDs can be tailored by varying the power ratio of the two frequencies. Increasing the power level of the low frequency (P2 MHz) enhances the population density of high-energy electrons; however, increasing the high-frequency power (P13.56 MHz) increases the low-energy electron population density. At a fixed total power (P2 MHz + P13.56 MHz), the higher the low-frequency power (P2 MHz) content, the higher the population density of high-energy electrons; however, this trend reverses with high-frequency power (P13.56 MHz). The influence of power ratio on plasma density (ne), plasma temperature (Te) and plasma potential (Vp) has also been studied. It is found out that the plasma parameters have similar trends with RF power irrespective of its frequency. The value of ne increases, and Te and Vp decrease with increasing power. At a fixed P2 MHz, Vp increases with increasing P13.56 MHz. However, Vp decreases with increasing P2 MHz at a fixed value of P13.56 MHz.