Effective Electron Temperature Research Articles

Ion Acoustic Waves in Weakly Relativistic Plasma—Separation of Relativistic and Electron-Temperature Effects

Ion Acoustic Waves in Weakly Relativistic Plasma—Separation of Relativistic and Electron-Temperature Effects

Two-Dimensional Cold Electron Transport for Steep-Slope Transistors.

Room-temperature Fermi-Dirac electron thermal excitation in conventional three-dimensional (3D) or two-dimensional (2D) semiconductors generates hot electrons with a relatively long thermal tail in energy distribution. These hot electrons set a fundamental obstacle known as the "Boltzmann tyranny" that limits the subthreshold swing (SS) and therefore the minimum power consumption of 3D and 2D field-effect transistors (FETs). Here, we investigated a graphene (Gr)-enabled cold electron injection where the Gr acts as the Dirac source to provide the cold electrons with a localized electron density distribution and a short thermal tail at room temperature. These cold electrons correspond to an electronic refrigeration effect with an effective electron temperature of ∼145 K in the monolayer MoS2, which enables the transport factor lowering and thus the steep-slope switching (across for three decades with a minimum SS of 29 mV/decade at room temperature) for a monolayer MoS2 FET. Especially, a record-high sub-60-mV/decade current density (over 1 μA/μm) can be achieved compared to conventional steep-slope technologies such as tunneling FETs or negative capacitance FETs using 2D or 3D channel materials. Our work demonstrates the potential of a 2D Dirac-source cold electron transistor as a steep-slope transistor concept for future energy-efficient nanoelectronics.

Hybrid model of radio-frequency low-pressure inductively coupled plasma discharge with self-consistent electron energy distribution and 2D electric field distribution

Low-pressure radio-frequency (RF) inductively coupled plasmas (ICPs) are extensively used for materials processing. In this work, we have developed a hybrid model consisting of two-dimensional (2D) Maxwell equations with an open boundary, zero-dimensional Boltzmann equation under linear and quasilinear approximations, and a power balance equation. The hybrid model is capable of achieving a self-consistent description of the electron heating mechanism and electron kinetics for the RF ICPs at low pressures. This work presents an investigation of the influence of operating conditions on 2D distributions of electric field and power density, normalized electron energy probability function (EEPF) (effective electron temperature), and plasma density in a low-pressure RF Ar ICP using the hybrid model. The results show that the RF frequency and absorption power significantly affect the 2D distributions and amplitudes of electric field and power density. The normalized EEPF is almost independent of RF frequency and weakly dependent on absorption power but significantly modulated by pressure at low RF frequency. The plasma density is also almost independent of RF frequency but increases with absorption power and pressure. In addition, we have validated the hybrid model against experimental data obtained in the driver region of a two-chamber RF Ar ICP source, where the RF frequency is 13.56 MHz, the power range is 200–1000 W and the pressure range is 0.1–1.0 Pa. The hybrid model qualitatively (and even quantitatively for some cases) reproduces the experimentally normalized EEPF and plasma density. The discrepancies in these plasma parameters could be attributed to the simplified collision processes taken into account in the hybrid model. The developed hybrid model can help us to better understand the effect of discharge conditions on electron kinetics and electron heating mechanism, and to ultimately optimize the parameters of RF ICP sources.

Effects of RF bias frequency and power on the plasma parameters and ash rate in a remote plasma source

The effects of the RF bias frequency (2–27.12 MHz) and power (0–50 W) on plasma parameters, i.e., effective electron temperatures, electron densities, and electron energy probability functions (EEPFs), were investigated in a remote plasma source. A small cylindrical Langmuir probe based on the Druyvesteyn method was used for the measurements. When the bias power was changed from 0 W to 10 W for each bias frequency, the electron density decreased and the effective electron temperature increased at a given antenna power. As the 2 MHz bias power increased to 50 W, the electron density increased remarkably, whereas the effective electron temperature decreased. Simultaneously, the EEPF evolved from a Druyvesteyn-like distribution to a nearly Maxwellian distribution. In contrast to 2 MHz, when increasing the bias power of 12.5 MHz or 27.12 MHz, there was no distinct change in the effective electron temperature by the bias power and the electron density increased slightly or barely changed. Moreover, the EEPFs retained a Druyvesteyn-like distribution during the bias power increase. These results reveal that the plasma parameters are more controllable at lower bias frequencies, and the analysis is presented in relation to the electron heating mechanism. Therewithal, the ash rate for a 2 MHz bias power was observed to be the highest among the three frequencies when the discharge was operated with pure oxygen.

Influence of magnetic field configuration on plasma characteristics and thin film properties in dual magnetron reactive high power impulse magnetron sputtering discharge with Al in Ar/O2 mixture

In this work, the influences of the magnetic field configuration (mirror-field and closed-field) on plasma parameters and ion flux dynamics of dual magnetron reactive high power impulse magnetron sputtering (R-HiPIMS) with Al targets in Ar/O2 mixture were studied by time-resolved Langmuir probe and time-averaged mass spectrometer. Compared with mirror-field configuration, the electron density (ne) in the closed-field discharge is obviously higher while the plasma potential (Vp), floating potential (Vf) and effective electron temperature (Teff) were lower. Time-averaged mass spectrometer results showed that the plasma compositions in both configurations were similar. The ion energy distribution functions (IEDFs) recorded at both discharges consisted of three components: a main narrow peak at low energy followed by a broad shoulder and then a high energy tail. Compared with the closed-field discharge, the mirror-field discharge exhibited higher peak energies but lower ion fluxes. For both discharges, the ions intensity increased immediately after the start of the negative voltage pulse. The signal for the mirror-field discharge peaked exactly at the end of the pulse, whereas fluxes of the ion species in the closed-field configuration reached the highest value 10μs after the voltage pulse termination. To investigate how changes in the magnet arrangement affect the film properties, Al2O3 thin films were deposited and characterized. It is confirmed that the magnet configuration appeared to have little influence on the deposition rate. In addition, high average energy per deposited atom in the closed-field discharge resulted in a well densified and stoichiometric crystalline Al2O3 film.

Effect of electron temperature in a magnetized plasma sheath using kinetic trajectory simulation

The understanding of the properties of magnetized plasma sheath has been various beneficial applications in surface treatment, electron emission gun, ion implantation, and nuclear fusion, etc. The effect of electron temperature on the magnetized plasma sheath has been studied for a fixed magnetic field and ion temperature. It has been observed that various plasma sheath parameters can be prominently altered by the varying temperature of the electron. The density of ion is influenced more by the change in electron temperature rather than the electron density. The temperature of the electron has a great effect at the wall, when electron temperature increases, the ion and electron densities at the wall decreases. This shows the potential at the wall also decreases follows the Poisson’s equation. Similarly, the electric field also decreases but total charge density increases when the electron temperature is increased.
 BIBECHANA 18 (2021) 58-66

Wavebreaking amplitudes in warm, inhomogeneous plasmas revisited

The effect of electron temperature on the space–time evolution of nonlinear plasma oscillations in an inhomogeneous plasma is studied using a one-dimensional particle-in-cell code. It is observed that, for an inhomogeneous plasma, there exists a critical value of electron temperature beyond which the wave does not break. These simulation results, which are in conformity with the purely theoretical arguments presented by Trines [Phys. Rev. E 79, 056406 (2009)], represent the first numerical elucidation of the effect of plasma pressure on wave breaking amplitude. The results presented are of relevance to experiments where moderate to large amplitude plasma waves are excited, e.g., laser–plasma interaction experiments.

A combined actinometry approach for medium pressure N2–O2 plasmas

An argon actinometry approach which uses the 777 nm and 844 nm atomic oxygen lines along with the common N2/ line ratio method is used to experimentally investigate the excitation mechanisms of the O(3p5P) state in medium pressure (0.75 Torr) N2–O2 plasmas. The method provides strong evidence that metastable excitation from the O(3s5S◦) state is an important excitation mechanism for O(3p5P) state for these conditions. The strong dependence of this additional mechanism on electron density along with the use of the N2/ line ratio method allow for the simultaneous determination of effective electron temperature, dissociation fraction, metastable O(3s5S◦) density and electron density. Predicted electron temperatures, electron densities and dissociation fractions are consistent with those found for similar discharges in prior studies. Additionally, we investigate the efficacy of using the observed 616 nm atomic oxygen line from the O(4d5D◦) state for actinometry, which could open the door to a novel and comprehensive actinometry technique.

Developing Single Layer MOS Quantum Dots for Diagnostic Qubits.

The design, fabrication and characterization of single metal gate layer, metal-oxide-semiconductor (MOS) quantum dot devices robust against dielectric breakdown are presented as prototypes for future diagnostic qubits. These devices were developed as a preliminary solution to a longer term goal of a qubit platform for intercomparison between materials or for in-line diagnostics, and to provide a testbed for establishing classical measurements predictive of coherence performance. For this stage, we seek a robust MOS design that is compatible with wafer and chip architectures, that has a reduced process overhead and is sufficiently capable of challenging and advancing our measurement capabilities. In this report, we present our initial batch of silicon MOS devices using a single gate layer, which have not exhibited any failures with gate voltage excursions > 10 V, but do exhibit the reduced electrostatic control expected of a single gate layer design. We observe quantum dot formation, capacitive charge sensing between channels, and reasonable effective electron temperatures that enable spin qubit studies. The costs and benefits of the trade-off between device performance and fabrication efficiency will be discussed, as well as opportunities for future improvements.

EFFECTS OF ARGON ATOMS IN EXCITED STATES ON PROPERTIES OF ARGON-ACETYLENE DUSTY PLASMA

It is studied how dissociation and ionization of acetylene molecules in their collisions with argon atoms in excited states Ar* may affect properties of argon-acetylene plasma with growing inside of plasma volume dust particles. The study is carried out using a volume-averaged model. To analyze the effects of Ar* atoms on the electron and ion densities, the effective electron temperature and the densities of radical and nonradical neutral species, the values of ionization and dissociation rates for collisions of acetylene molecules with Ar* atoms are varied in numerical calculations. It is found that the collisions of Ar* atoms with acetylene molecules affect essentially the argon-acetylene dusty plasma.

Spatio-temporal measurements of overshoot phenomenon in pulsed inductively coupled discharge* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11675039, 11875101, 11935005, and 12075049) and the Fundamental Research Funds for the Central Universities of China (Grant Nos. DUT18TD06 and DUT20LAB201).

Pulse inductively coupled plasma has been widely used in the microelectronics industry, but the existence of overshoot phenomenon may affect the uniformity of plasma and generate high-energy ions, which could damage the chip. The overshoot phenomenon at various spatial locations in pulsed inductively coupled Ar and Ar/CF4 discharges is studied in this work. The electron density, effective electron temperature, relative light intensity, and electron energy probability function (EEPF) are measured by using a time-resolved Langmuir probe and an optical probe, as a function of axial and radial locations. At the initial stage of pulse, both electron density and relative light intensity exhibit overshoot phenomenon, i.e., they first increase to a peak value and then decrease to a convergent value. The overshoot phenomenon gradually decays, when the probe moves away from the coils. Meanwhile, a delay appears in the variation of the electron densities, and the effective electron temperature decreases, which may be related to the reduced strength of electric field at a distance, and the consequent fewer high-energy electrons, inducing limited ionization and excitation rate. The overshoot phenomenon gradually disappears and the electron density decreases, when the probe moves away from reactor centre. In Ar/CF4 discharge, the overshoot phenomenon of electron density is weaker than that in the Ar discharge, and the plasma reaches a steady density within a much shorter time, which is probably due to the more ionization channels and lower ionization thresholds in the Ar/CF4 plasma.

Nonthermal excitonic condensation near a spin-state transition

We consider a two-orbital Hubbard model with Hund coupling and crystal-field splitting and show that in the vicinity of the high-spin/low-spin transition, crystal-field quenches can induce an excitonic condensation at initial temperatures above the highest ordering temperature in equilibrium. This condensation is the effect of an increase in the spin entropy and an associated cooling of the effective electronic temperature. We identify a dynamical phase transition and show that such quenches can result in long-lived nonthermal excitonic condensates which have no analogue in the equilibrium phase diagram. The results are interpreted by means of an effective pseudo-spin model.

Establishing nonthermal regimes in pump-probe electron relaxation dynamics

Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the electronic structure of solids under optical excitation, and is a powerful technique for studying the coupling between electrons and collective modes. One approach to infer electron-boson coupling is through the relaxation dynamics of optically-excited electrons, and the characteristic timescales of energy redistribution. A common description of electron relaxation dynamics is through the effective electronic temperature. Such a description requires that thermodynamic quantities are well-defined, an assumption that is generally violated at early delays. Additionally, precise estimation of the non-thermal window -- within which effective temperature models may not be applied -- is challenging. We perform TR-ARPES on graphite and show that Boltzmann rate equations can be used to calculate the time-dependent electronic occupation function, and reproduce experimental features given by non-thermal electron occupation. Using this model, we define a quantitative measure of non-thermal electron occupation and use it to define distinct phases of electron relaxation in the fluence-delay phase space. More generally, this approach can be used to inform the non-thermal-to-thermal crossover in pump-probe experiments.

Benchmark of the KGMf with a coupled Boltzmann equation solver

The Kinetic Global Model framework (KGMf) is an open-source general-purpose global model (spatially averaged) simulation code developed to explore the reaction kinetics and pathways in plasma discharge systems. It contains species continuity and electron energy balance equations, with a time-dependent evaluated electron energy distribution function (EEDF) for electron impact reactions. The EEDF is utilized to determine the rate coefficients for electron impact reactions, which can have profound impact on the temporal evolutions of plasma parameters. Previously, the EEDF was commonly assumed as Maxwellian or an analytical function of the effective electron temperature. In this work, the KGMf is coupled with a Boltzmann equation (BE) solver to self-consistently compute the EEDF. The EEDF evolution frequency is determined based on relative changes of the reduced electric field. The KGMf is benchmarked with the ZDPlasKin code based on high-pressure low-temperature argon plasma discharge cases. The temporal evolutions of reduced electric field, electron temperature, EEDF, reaction rates, and species densities, are obtained and compared under different discharge conditions, showing good agreement between the KGMf and the ZDPlasKin simulations. The application of the KGMf for predicting breakdown times in high power microwave discharges is also presented, which shows qualitative agreement with particle-in-cell simulations. The KGMf can be further applied for more complicated plasma discharge systems, where the reaction kinetics are more intricate (e.g., plasma-assisted combustion systems).

Argon/dust and pure argon pulsed plasmas explored using a spatially-averaged model

The properties (densities of electrons and metastable argon atoms, effective electron temperature and dust charge) of argon/dust and pure argon pulsed plasmas are studied using a spatially-averaged model. The calculated time-dependencies for the densities of electrons and metastable atoms are compared with the experimental measurements and are found to be in a good qualitative agreement. It is analyzed how the plasma properties depend on the shape of the electron energy probability function (EEPF), the pulsing frequency and the duty cycle for both dust-free and dusty plasma. The analysis reveals that the agreement between theory and experiment is better with Druyvesteyn EEPF than the Maxwellian EEPF. Further, the variation in the pulsing frequency ν p differently affects the metastable density n m in a dust-free and in a dusty plasma. For large ν p, the metastable density in the dust-free pulsed plasma is larger than in the continuous-wave (CW) discharge, while the opposite is obtained in the presence of dust particles. This difference probably arises because of faster variation in the effective electron temperature in the dusty plasma due to collection of electrons by dust particles. Our calculations also show that dust particles may affect the behavior of electron density in the beginning of the on-period due to an enhancement in electron collection by dust particles.

Impact of hot electrons on optical excitation of terahertz radiation by beating of supergaussian lasers in electron-positron collisional plasma

The mechanism of generation of terahertz (THz) radiation with beating of two supergausssian lasers in corrugated density plasma containing hot electron-positron (e-p) is revisited. Resonance condition for optical excitation of THz emission is reported to be modified due to finite temperature of plasma species (electrons-positrons to the few order keV range) in the presence of weak collisions. The collision frequency is optimized to the range ν ≤ 0.5ω p in the e-p plasma depending on the temperature and collisional rates. The effect of electron temperature, collision frequency and suitability of supergaussian laser is investigated on phase modulation to achieve highest emitted THz field with 6% efficiency of the mechanism.

Investigation of effect of electron temperature on intensity and efficiency of terahertz generated by laser beating in inhomogeneous plasma

We report terahertz (THz) wave generation based on mixing of two radially polarized hollow Gaussian laser beams in periodic density plasma. We incorporate the effect of plasma electron temperature on THz intensity and efficiency. Our investigations reveal that emitted THz is radially polarized. Brightness and efficiency of THz significantly increase with plasma electron temperature. Our scheme predicts upto 2.4-fold enhancement in THz conversion efficiency with increase in plasma electron temperature from 0–10 KeV. We observe that effect of plasma electron temperature is more prominent when difference of laser frequency matches with plasma frequency i.e. The profile of THz depends on the laser parameters and it is independent of plasma electron temperature. A high THz field (∼10 MV cm−1) and high conversion efficiency (∼6%) is predicted for optimized pump laser and plasma parameters.

High-current laser-driven beams of relativistic electrons for high energy density research

We report on enhanced laser driven electron beam generation in the multi MeV energy range that promises a tremendous increase of the diagnostic potential of high energy sub-PW and PW-class laser systems. In the experiment, an intense sub-picosecond laser pulse of ∼1019 Wcm−2 intensity propagates through a plasma of near critical electron density (NCD) and drives the direct laser acceleration (DLA) of plasma electrons. Low-density polymer foams were used for the production of hydrodynamically stable long-scale NCD-plasmas. Measurements show that relativistic electrons generated in the DLA-process propagate within a half angle of 2 ± 1° to the laser axis. Inside this divergence cone, an effective electron temperature of 10–13 MeV and a maximum of the electron energy of 100 MeV were reached. The high laser energy conversion efficiency into electrons with energies above 2 MeV achieved 23% with a total charge approaching 1 μC. For application purposes, we used the nuclear activation method to characterize the MeV bremsstrahlung spectrum produced in the interaction of the high-current relativistic electrons with high-Z samples and measured top yields of gamma-driven nuclear reactions. The optimization of the high-Z target geometry predicts an ultra-high MeV photon number of ∼1012 per shot at moderate relativistic laser intensity of 1019 Wcm−2. A good agreement between the experimental data and the results of the 3D-PIC and GEANT4-simulations was demonstrated.

Multiparametric study of tearing modes in thin current sheets

We investigate the asymptotic scaling of the growth rate and of the characteristic layer width of reduced-MHD tearing modes occurring in thin current sheets when reconnection depends on two non-ideal parameters. For this purpose, we use a new multi-precision finite difference eigensolver. The viscous-resistive regime, the warm-resistive regime that includes both resistivity and electron temperature effects, the warm-inertial regime in which a finite electron inertia replaces resistivity in allowing reconnection, and the inertial-resistive regime that includes both electron inertia and resistivity are investigated. Previous analytical results of the first three regimes are recovered. For all regimes, the scalings of the width of the reconnecting layer are provided in the different limits of the wavelength spectrum, and general estimates for the fastest growing modes are obtained and generalized to different magnetic equilibria. Implications for the disruption of evolving current sheets are discussed.

Nonlinear electron plasma waves in fully relativistic plasmas

Propagation of nonlinear electron plasma waves is investigated in a fully relativistic election-ion plasma using two-fluid hydrodynamic model. The relationship between pressure and density is assumed to be of polytropic form for relativistic electrons. The conditions for the validity of adiabatic equation of state for electron fluid is also discussed. The well-known pseudopotential approach is employed for arbitrary amplitude wave analysis and superluminous and subluminous wave phase speed cases are discussed in two special frames of references transformed under Lorentz-transformation. The set of model equations is solved analytically as well as numerically to obtain nonlinear periodic wave structures of electron plasma waves in a fully relativistic plasmas. A special case of solitary wave structure formed in subluminous phase speed case is also discussed in detail, which has not been done in the previous published literature. The effects of electron temperature and phase speed of the plasma wave (i.e., for subluminous and superluminous wave phase speed cases) on the amplitude of nonlinear periodic wave and solitary wave structures are also presented with its possible applications to the astorphysical environment.