Effective Electron Temperature Research Articles

Determination of electron energy distribution function shape for non-Maxwellian plasmas using floating harmonics method

A new floating harmonics method is developed to determine the electron energy distribution shape in the non-Maxwellian plasmas. The slope and curvature of the electron energy probability function (EEPF) at the floating potential can be obtained by measuring amplitude ratios among the first three sideband harmonics of the probe electron current. Together with the generalized EEPF formula, the EEPF shapes of the non-Maxwellian plasmas are able to be determined from the slope and curvature. Here, the new method is experimentally verified and its results are compared with EEPF measurements using the Langmuir probe (LP) method. The effective electron temperature and the EEPF shape parameter obtained by the method give good agreements with those of the LP measurements.

Effect of particles attachment to multi-sized dust grains present in electrostatic sheaths of discharge plasmas

The loss of electrons and ions due to their attachment to a Gauss-distributed sizes of dust grains present in electrostatic sheaths of discharge plasmas is investigated. A uni-dimensional, unmagnetized, and stationary multi-fluid model is proposed. Forces acting on the dust grain along with its charge are self-consistently calculated, within the limits of the orbit motion limited model. The dynamic analysis of dust grains shows that the contribution of the neutral drag force in the net force acting on the dust grain is negligible, whereas the contribution of the gravity force is found considerable only for micrometer particles. The dust grains trapping is only possible when the electrostatic force is balanced by the ion drag and the gravity forces. This trapping occurs for a limited radius interval of micrometer dust grains, which is around the most probable dust grain radius. The effect of electron temperature and ion density at the sheath edge is also discussed. It is shown that the attachment of particles reduces considerably the sheath thickness and induces dust grain deceleration. The increase of the lower limit as well as the upper limit of the dust radius reduces also the sheath thickness.

Effects of relativistic electron temperature on parametric instabilities for intense laser propagation in underdense plasma

Effects of relativistic electron temperature on stimulated Raman scattering and stimulated Brillouin scattering instabilities for high intensity lasers propagating in underdense plasma are studied theoretically and numerically. The dispersion relations for these instabilities are derived from the relativistic fluid equation. For a wide range of laser intensity and electron temperature, it is found that the maximum growth rate and the instability region in k-space can be reduced at relativistic electron temperature. Particle-in-cell simulations are carried out, which confirm the theoretical analysis.

The effect of ponderomotive force on the density in the interaction of two superposing fundamental modes with plasma in an elliptical waveguide

The interaction of high power microwaves and plasma is studied through the propagation of two fundamental TE11 modes in a plasma-filled elliptical waveguide, when their fields have a phase difference and they superpose at a small angle. Here we derive two nonlinear differential equations for the amplitude of the electric field in a plasma-filled elliptical waveguide. These equations are solved numerically using the fourth order Runge–Kutta method for the field amplitude of the microwave in the waveguide considering the waveguide to be made up of a perfect conductor and to be filled by plasma with a homogeneous initial density distribution. The effects of electron temperature, the microwave field and its frequency are studied on the perturbed density in relation to the ponderomotive force exerted by the resultant mode. It is shown that the field of the microwave is altered and that the density profile can be controlled by the angle of superposition, the phase difference between the fields of the two modes, the microwave frequency and the microwave field amplitude.

On the anode potential fall in a vacuum arc: PIC simulation

The interaction of plasma generated by a vacuum arc with the anode surface was numerically simulated by using a particle-in-cell (PIC) method. It has been found that the anode potential fall remains negative even at high electron drift velocities. The plasma–anode-surface interaction has been analyzed for a ‘steady-state’ regime and for a ‘turbulent’ regime with microinstabilities developing in the plasma. For the steady-state regime, approximate formulas for the anode potential fall and energy flux have been derived from the simulation results. For the turbulent regime, naturally associated with high electron drift velocities, the negative anode fall voltage and the effective plasma electron temperature have been found to be larger than that for the steady-state regime.

Investigation of structural disorder using electron temperature in VHF-PECVD on hydrogenated amorphous silicon films for thin film solar cell applications.

Electrode distances and gas flow ratios are important parameters for fabricating intrinsic (i-type) layers of hydrogenated amorphous silicon (a-Si:H) films using a very high frequency plasma-enhanced chemical-vapor deposition (VHF-PECVD) system. In this work, we investigated the relationship between the electrode distances and gas flow ratios on the properties of i-type a-Si:H films. The electrical, chemical and structural properties are improved with decreasing electrode distances (20-40 mm) at a hydrogen ratio [R (H2/SiH4) = 4], due to the low electron temperature and heating effect. A low electron temperature generates silane-related-reactive species (SiH3) and decreases structural disorder resulting in high quality i-type a-Si:H films. The electrical, chemical and structural properties of the a-Si:H films are confirmed using Al coplanar electrodes, FTIR, Raman spectroscopy, and spectroscopy ellipsometry (SE). When a solar cell is fabricated using the a-Si:H film, J(sc) of 13.2-14.8 mA/cm2, photoconductivity of 1.5 x 10(-5)-8.6 x 10(-6) S/cm, Si--H2 content of 0-1.24 at.%, and hydrogen content of about 10 at.% are obtained. These results together with a model of the plasma chemistry indicate that H atoms and SiH3 radicals play an important role in the deposition process.

Radial profiles and emissivity ratio analysis of Ne-like FeXVII n = 3−2 transitions in Large Helical Device

The radial profiles of the FeXVII 3s-2p and 3d-2p transitions emitted in the wavelength range of 15 − 17 A have been observed in the Large Helical Device (LHD). The chord-integrated radial profiles are converted into radial emissivity profiles by means of Abel inversion. The emissivity ratios among FeXVII n = 3−2 transitions calculated from the radial emissivity profiles are compared with calculations based on a collisional-radiative (CR) model developed for Fe ions. The result reasonably confirms the effect of electron temperature and density on the emissivity ratio. However, the emissivity of the 3C (2p 53d 1 P 1 → 2p 6) transition is obviously lower than the prediction from the CR model. This discrepancy is consistent with measurements in the solar corona and other laboratory plasmas.

Electronic dynamic behavior in inductively coupled plasmas with radio-frequency bias

The inflexion point of electron density and effective electron temperature curves versus radio-frequency (RF) bias voltage is observed in the H mode of inductively coupled plasmas (ICPs). The electron energy probability function (EEPF) evolves first from a Maxwellian to a Druyvesteyn-like distribution, and then to a Maxwellian distribution again as the RF bias voltage increases. This can be explained by the interaction of two distinct bias-induced mechanisms, that is: bias-induced electron heating and bias-induced ion acceleration loss and the decrease of the effective discharge volume due to the sheath expansion. Furthermore, the trend of electron density is verified by a fluid model combined with a sheath module.

The influence of the repetition rate on the nanosecond pulsed pin-to-pin microdischarges

The effect of repetition rate on a nanosecond atmospheric pressure discharge is investigated. The discharge is generated between two pins in a mixture of Ne and Ar. The voltage, current, power waveforms and the temporally and spatially resolved electron density and an ‘effective’ electron temperature are measured, with a pulse interval between 1.5 and 200 µs. It is found that not only does the repetition rate have a strong influence on the breakdown voltage and the peak discharge power, but it can also affect the rise rate of the volume averaged electron density and its peak value. Temporally and spatially resolved measurement of the electron density and the effective electron temperature show that the spatial distributions of both quantities are also influenced by the repetition rate. In the initial discharge period of all cases, the sharp rise of the electron density correlates with the drastic drop of the effective electron temperature. It is suggested that the residual charges have a strong impact on the axial distribution of the electric field and energetic electrons between the electrodes during the breakdown period, as illustrated by a simple sheath model.

Energy-filtered cold electron transport at room temperature.

Fermi-Dirac electron thermal excitation is an intrinsic phenomenon that limits functionality of various electron systems. Efforts to manipulate electron thermal excitation have been successful when the entire system is cooled to cryogenic temperatures, typically <1 K. Here we show that electron thermal excitation can be effectively suppressed at room temperature, and energy-suppressed electrons, whose energy distribution corresponds to an effective electron temperature of ~45 K, can be transported throughout device components without external cooling. This is accomplished using a discrete level of a quantum well, which filters out thermally excited electrons and permits only energy-suppressed electrons to participate in electron transport. The quantum well (~2 nm of Cr2O3) is formed between source (Cr) and tunnelling barrier (SiO2) in a double-barrier-tunnelling-junction structure having a quantum dot as the central island. Cold electron transport is detected from extremely narrow differential conductance peaks in electron tunnelling through CdSe quantum dots, with full widths at half maximum of only ~15 mV at room temperature.

Control of electron energy distribution by adding a pulse inductive field in capacitive discharge

Drastic changes in the electron energy distribution (EED) and the plasma parameter caused by a pulse-modulated (PM) inductive field were observed in a low-gas pressure capacitively coupled plasma (CCP). By applying a small amount of the PM inductive power (20 W) in the CCP, the effective electron temperature (Teff) was increased without a variation of the plasma density (ne) due to the low energy electron heating by the inductive field. When a large amount of the PM inductive power of 50 W was applied to the CCP, both the Teff and the ne were increased because of the electron heating through the coupling effect of the inductive and capacitive fields. These results show that Teff and ne can be controlled independently or simultaneously in the plasmas by adding the pulse inductive field.

Existence of the kinetic Alfvén solitons in a plasma with non-extensive electrons including complete ion nonlinearity

The existence of the shear Alfven solitons in a moderate β plasma with non-extensive electrons including complete ion nonlinearity is investigated. The characteristic of super-Alfvenic dip and hump type solitons, and the sub-Alfvenic dip solitons depends on the electrons non-extensive parameters q. The effects of the q on the kinetic Alfven solitons can be explained from the effective electron temperature and the electron distribution in a potential field modified by the non-extensive distribution.

Flush-mounted probe diagnostics for argon glow discharge plasma.

A comparison is made between plasma parameters measured by a flush-mounted probe (FP) and a cylindrical probe (CP) in argon glow discharge plasma. Parameters compared include the space potential, the plasma density, and the effective electron temperature. It is found that the ion density determined by the FP agrees well with the electron density determined by the CP in the quasi-neutral plasma to better than 10%. Moreover, the space potential and effective electron temperature calculated from electron energy distribution function measured by the FP is consistent with that measured by the CP over the operated discharge current and pressure ranges. These results present the FP can be used as a reliable diagnostic tool in the stable laboratory plasma and also be anticipated to be applied in other complicated plasmas, such as tokamaks, the region of boundary-layer, and so on.

Spatial Evolution Study of EEDFs and Plasma Parameters in RF Stochastic Regime by Langmuir Probe

An RF compensated cylindrical Langmuir probe system has been developed and used to characterize an RF capacitive two temperature plasma discharge in a stochastic mode. The novelty of the work presented here is the use of the driven electrode (cathode) without ground shield. Measurements of the electron energy distribution function (EEDF) and plasma parameters were achieved under the following conditions: 50 W of RF power and 5×10−2 mbar of argon pressure. The probe measurements are performed at 3 cm above the electrode and the probe was shifted radially (r direction) from the center (r = 0 cm) of the inter-electrodes region towards the chamber wall (R = 10.75 cm). The results show that the EEDF is bi-Maxwellian and its shape remains the same through the scanned region. The farther the probe from the central region, the lower the EEDF maximum. The plasma density is observed to decrease according to a Gaussian profile along the radial direction and falls to 50% of its maximum when close to the cathode edge (r = 5.5 cm). At the same time the effective electron temperature remains constant for r>4 cm and increases for r≥4 cm. The high-temperature and low-temperature electrons' densities and temperatures are also discussed in the article.

Cyclotron resonant photoresponse of a multisubband two-dimensional electron system on liquid helium

We report an experimental study of surface electrons on liquid helium-4 under cyclotron resonant excitation. Significant resonance-induced redistribution of electron density due to overheating was observed. Analysis of redistributed charges indicates an unusually large expansion of the electron system in a lateral direction, which cannot be understood in the framework of the generally accepted effective electron temperature approximation. To interpret the data we suggest that under resonant cyclotron pumping, a fraction of nonthermalized electrons is formed, in which the average energy associated with electron in-plane motion can significantly exceed the average energy associated with motion perpendicular to the helium surface.

Transport properties of overheated electrons trapped on a helium surface

An ultra-strong photovoltaic effect has recently been reported for electrons trapped on a liquid helium surface under a microwave excitation tuned at intersubband resonance [D. Konstantinov, A.D. Chepelianskii, K. Kono, J. Phys. Soc. Jpn 81, 093601 (2012)]. In this article, we analyze theoretically the redistribution of the electron density induced by an overheating of the surface electrons under irradiation, and obtain quantitative predictions for the photocurrent dependence on the effective electron temperature and confinement voltages. We show that the photo-current can change sign as a function of the parameters of the electrostatic confinement potential on the surface, while the photocurrent measurements reported so far have been performed only at a fixed confinement potential. The experimental observation of this sign reversal could provide a reliable estimation of the electron effective temperature in this new out of equilibrium state. Finally, we have also considered the effect of the temperature on the outcome of capacitive transport measurement techniques. These investigations led us to develop, numerical and analytical methods for solving the Poisson-Boltzmann equation in the limit of very low temperatures which could be useful for other systems.

Emission spectroscopy of Ar + H2 + C7H8 plasmas: C7H8 flow rate dependence and pressure dependence

We have studied toluene flow rate dependence and pressure dependence of Ar emission intensities at 425.9 nm, 750.4 nm and 811.5 nm for Ar + H2 + C7H8 plasmas. Electron density and the electron impact excitation rate coefficients are nearly the same for Ar(100%) and Ar+H2(33%) plasmas. Toluene addition leads to a decrease in the high energy electron density, whereas the effective electron temperature and low energy electron density remain almost the same. The effective electron temperature, and Ar metastable density and/or low energy electron density decrease significantly with increasing the total pressure from 0.1 to 5 Torr.

Persistence of magnetic order in a highly excited Cu2+ state in CuO

We use ultrafast resonant x-ray diffraction to study the magnetic order in CuO under conditions of high electronic excitation. By measuring changes in the spectral shape of the Cu2+ magnetic (1/2 0 −1/2) reflection we investigate how an intense optical pump pulse perturbs the electronic and magnetic states. We observe an energy shift in the magnetic resonance at short times after the pump pulse. This shift is compared with expectations from band structure calculations at different electronic temperatures. This spectral line shift indicates that although the electrons are heated to effective electron temperatures far above TN on a time scale faster than the experimental resolution, magnetic order persists in this highly excited state for several hundred femtoseconds.Received 17 September 2013Revised 24 March 2014Corrected 25 June 2014DOI:https://doi.org/10.1103/PhysRevB.89.220401©2014 American Physical Society

Effects of emitted electron temperature on the plasma sheath

It has long been known that electron emission from a surface significantly affects the sheath surrounding that surface. Typical fluid theory of a planar sheath with emitted electrons assumes that the plasma electrons follow the Boltzmann relation and the emitted electrons are emitted with zero energy and predicts a potential drop of 1.03Te/e across the sheath in the floating condition. By considering the modified velocity distribution function caused by plasma electrons lost to the wall and the half-Maxwellian distribution of the emitted electrons, it is shown that ratio of plasma electron temperature to emitted electron temperature significantly affects the sheath potential when the plasma electron temperature is within an order of magnitude of the emitted electron temperature. When the plasma electron temperature equals the emitted electron temperature the emissive sheath potential goes to zero. One dimensional particle-in-cell simulations corroborate the predictions made by this theory. The effects of the addition of a monoenergetic electron beam to the Maxwellian plasma electrons were explored, showing that the emissive sheath potential is close to the beam energy only when the emitted electron flux is less than the beam flux.

Different shape normal metal interlayers between niobium based SIS junctions and niobium titanium nitride leads and their influence on the electron temperature

In this paper we demonstrate the reduction of heating in a niobium superconductor-insulator-superconductor (SIS) junction with aluminum-oxide tunnel barrier embedded in a niobium-titanium-nitride circuit. Nonequilibrium quasiparticles which are created due to the Andreev trap at the interface between the niobium and the niobium-titanium-nitride layers are relaxed by inserting a normal-metal conductor of gold between these two layers. In an earlier work we explained the observed relaxation of nonequilibrium quasiparticles due to the geometrically assisted cooling effect. In this paper we investigate this cooling effect in dependence of the normal-metal layer shape and size. We expect that an adapted normal-metal layer is necessary for implementation in practical terahertz SIS heterodyne mixer circuits. We observe in DC-measurements of a large number of devices a clear relation between the volume of the gold layer and the effective electron temperature in the device. Our central finding is that the shape of the gold layer does not influence the cooling provided that the volume is sufficient.