High Electron Temperature Research Articles

Crystalline In2O3 nanoparticles synthesis using microwaves vaporization of metallic wires

Single crystal In2O3 nanoparticles were synthesized using microwaves vaporization of a thin metallic In wire in air. The output of an 800 W microwave generator was coupled through an antenna to a cylindrical wave guide cavity in with a metallic In wire was placed in the electromagnetic node, where a high power density was achieved. The wire strongly absorbed the microwaves, resulting in its heating, vaporization and finally a plasma plume formation. Optical emission spectroscopy investigations indicated that a high electronic temperature was reached in the plasma plume. The vaporized material was collected on a Si wafer placed near the cavity wall. Scanning electron microscopy investigations showed that the deposited material consisted of an agglomeration of In2O3 nanoparticles. X-ray diffraction and transmission electron microscopy (TEM) investigations indicated that the nanoparticles were very crystalline, with random orientation. X-ray photoelectron spectroscopy analysis confirmed the formation of stoichiometric In2O3. High resolution TEM investigations found that the deposited nanostructures were single crystals, with faceted surfaces. This simple method could have many applications for metal nanocrystal oxides synthesis.

Numerical Study on Mechanism and Optimization of Pulsed Discharges Without Voltage Plateau Phase

In recent years, pulsed discharges at atmospheric pressure have attracted extensive attention due to their remarkable discharge advantages. In this article, a fluid model is applied to investigate a new type of pulsed dielectric barrier discharge (DBD), characterized by a sharp rising phase and slow falling phase but without a plateau phase, which could be realized by using a circuit composed of a device called silicon diode for alternating current (SIDAC). Based on the computational data, the influences of voltage waveform on the plasma density and electric field are carefully discussed. The simulation results show that only one significant discharge event occurs during the voltage rising phase, which produces a large electron density with high electron temperature, but the secondary discharge current cannot be observed due to the slow voltage falling phase. We also qualitatively investigate the effects of discharge parameters on the plasma dynamics, such as the relative permittivity, barrier thickness, and repetitive frequency from simulation data, which agree well with the experimental observation. Based on the simulation results, the optimization of this pulsed discharges to enhance the electron density and improve the electron temperature is discussed, thus promoting the wide applications of the pulsed discharges.

Numerical Simulation of Multispecies Vacuum Arc Subjected to Actual Spatial Magnetic Fields

A three-dimensional (3-D) model for multispecies vacuum arc considering ionization and recombination process has been built in this article. With the model, the arc behaviors and species characteristics subjected to actual spatial magnetic fields of cup-shaped axial magnetic field (AMF) contact and transverse magnetic field-axial magnetic field (TMF-AMF) contact are investigated and compared. Furthermore, the validity of the model is verified by comparing with the experiments. Simulation results show that contact structures affect the species distributions of the arc by changing magnetic field and current density distributions. More triple charged ions are generated through ionization under higher electron temperature so that the triple charged ions gather where current constricts. The heat flux density is large in the current contraction region, where the contribution from triple charged ions is large as well. Spatial magnetic field distribution affects the Lorentz force acting on ion components. The force distribution is relatively uniform and the species are subject to inward force on the arc edge under cup-shaped AMF contact. For TMF-AMF contact, the double charged ions are subject to the force pointing to the TMF contact arms, whereas the triple charged ions are subject to the force pointing to the arc center. In addition, the consideration of arc species can improve the accuracy of the simulation results of the heat flux distribution, leading to a good agreement of the erosion condition between experiments and simulation.

Application of Non-thermal Plasmas in Medicine and Energy-Food-Water

Non-thermal plasma (NTP) due to their versatile nature have been successfully used in many applications ranging from hydrocarbon destruction to cancer treatment. The NTPs, can deliver high electron temperatures and high densities of radicals at near ambient conditions making it a unique source for applications ranging from treatment of extremely heat sensitive surfaces to complex chemical reactions which are feasible only at low temperatures. This work presents some of the recent understandings on how various NTP reactors can be designed and characterized for various applications. The results obtained from a pin-to-water plasma discharge system, in generating plasma activated water for agricultural applications are discussed. The application of NTPs in hydrocarbon reforming to obtain a tar-free gas from a waste to energy thermo-chemical process is discussed. The results of selective killing of cancer cells using a DBD is presented. Also, application of NTPs in wastewater treatment systems to treat household chemical products are presented.

Drude-Lorentz Model for Optical Properties of Photoexcited Transition Metals under Electron-Phonon Nonequilibrium

Evaluating the optical properties of matter under the action of ultrafast light is crucial in modeling laser–surface interaction and interpreting laser processing experiments. We report optimized coefficients for the Drude-Lorentz model describing the permittivity of several transition metals (Cr, W, Ti, Fe, Au, and Ni) under electron-phonon nonequilibrium, with electrons heated up to 30,000 K and the lattice staying cold at 300 K. A Basin-hopping algorithm is used to fit the Drude-Lorentz model to the nonequilibrium permittivity calculated using ab initio methods. The fitting coefficients are provided and can be easily inserted into any calculation requiring the optical response of the metals during ultrafast irradiation. Moreover, our results shed light on the electronic structure modifications and the relative contributions of intraband and interband optical transitions at high electron temperatures corresponding to the laser excitation fluence used for surface nanostructuring.

Control of electron acceleration process during merging start-up of spherical tokamak

The axial plasma merging method is one of the start-up schemes to form spherical tokamak (ST) plasma without use of a center-solenoid coil. It involves a magnetic reconnection process in the presence of high toroidal magnetic field parallel to the reconnection electric field and has the potential to accelerate electrons directly along the magnetic field lines. In the paper, the role of boundary conditions of the reconnection downstream region was investigated in the UTST experiment. Self-generated axial electric field in the reconnection downstream region was observed to grow up to 20–30 times as large as the toroidal electric field where the MHD condition holds, but was largely suppressed inside the area where the magnetic field lines are contacted to the inboard limiter conductors equipped on the center stack. This suppression effect was attributed to the short circuit connection of the magnetic field lines to hinder the axial charge separation and then a significant part of the parallel electric field remained. These experimental results suggest that the merging start-up method has the capability to enhance electron acceleration efficiency by magnetic reconnection and may have the potential to produce high electron temperature ST plasma applicable for additional heating and current drive techniques.

Relativistic effect on synergy of electron cyclotron and lower hybrid waves on EAST

In recent experiments on EAST, the electron temperature at the center can be raised to 9.7 keV by injecting electron cyclotron (EC) and lower hybrid (LH) waves simultaneously. With such strong core electron heating, the relativistic effect could play an important role in the interactions between the plasma and waves. In order to explore the relativistic effect on synergy between the EC and LH waves on EAST, ray-tracing/Fokker–Planck simulations are conducted to investigate electron heating for a typical discharge with a center electron temperature of 9.7 keV. It is found that the relativistic effect can cause the EC wave to deposit its power deeper in the plasma core, where the synergy between the EC and LH waves occurs and enhances the absorption of the LH waves. As a result, a high center electron temperature can be achieved.

Ultrafast electronic linewidth broadening in the C 1s core level of graphene

Core level binding energies and absorption edges are at the heart of many experimental techniques concerned with element-specific structure, electronic structure, chemical reactivity, elementary excitations and magnetism. X-ray photoemission spectroscopy (XPS) in particular, can provide information about the electronic and vibrational many-body interactions in a solid as these are reflected in the detailed energy distribution of the photoelectrons. Ultrafast pump-probe techniques add a new dimension to such studies, introducing the ability to probe a transient state of the many-body system. Here we use a free electron laser to investigate the effect of a transiently excited electron gas on the core level spectrum of graphene, showing that it leads to a large broadening of the C 1s peak. Confirming a decade-old prediction, the broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron system, rather than by vibrational excitations. This interpretation is supported by a line shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in agreement with electronic temperature values obtained from valence band data. Furthermore, making use of time- and momentum-resolved C 1s spectra, we illustrate how the momentum change of the outgoing core electrons leads to a small but detectable change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the narrow $\sigma$-band in the C 1s state. The results demonstrate that the XPS line shape can be used as an element-specific and local probe of the excited electron system and that X-ray photoelectron diffraction investigations remain feasible at very high electronic temperatures.

Lift-Off Ablation of Metal Thin Films for Micropatterning Using Ultrashort Laser Pulses

Laser ablation of metal thin films draws attention as a fast means of clean micropatterning. In this study, we attempt to remove only the metal thin film layer selectively without leaving thermal damage on the underneath substrate. Specifically, our single-pulse ablation experiment followed by two-temperature analysis explains that selective ablation can be achieved for gold (Au) films of 50–100 nm thickness by the lift-off process induced as a result of vaporization of the titanium (Ti) interlayer with a strong electron–phonon coupling. With increasing the film thickness comparable to the mean free path of electrons (100 nm), the pulse duration has to be taken shorter than 10 ps, as high-temperature electrons generated by the ultrashort pulses transfer heat to the Ti interlayer. We verify the lift-off ablation by implementing millimeters-scale micropatterning of optoelectronic devices without degradation of optical properties.

Structural stability and electron‐phonon coupling in two‐dimensional carbon allotropes at high electronic and atomic temperatures

Several two-dimensional carbon allotropes have been recently fabricated experimentally, triggering various innovative research and technological applications. The properties of such 2d materials under extreme conditions are to a large extent unknown. In this work, we study theoretically the effects of high electronic and atomic temperatures on free-standing graphene, pentaheptite, and biphenylene network. We analyse the limits of their structural stability with respect to thermal and nonthermal damage. With a state-of-the-art approach, we calculate the electronic heat capacity and the electron-phonon coupling parameter dependent on both electronic and atomic temperatures. The knowledge of these parameters may facilitate modelling of various two-dimensional carbon allotropes under laser pulse irradiation, improving understanding of their behaviour in experimental applications.

Experimental observations of local plasma parameters in the COMPASS divertor in NBI-assisted L-mode plasmas

This paper presents an experimental characterization of the local plasma parameters in the COMPASS open divertor in deuterium, L-mode plasmas by means of Langmuir probes during neutral beam injection (NBI) heating. The effect of NBI heating of the core plasma on the local divertor plasma parameters is investigated under different plasma configurations and different directions of the magnetic field and plasma current for different line-averaged electron densities. At low line-averaged densities below 6 × 1019 m-3, a low NBI heating power level does not lead to any significant changes in the plasma parameters in the divertor region. It is also found that for a reversed magnetic field the influence of NBI in the divertor is stronger at higher line-averaged electron density and plasma current. In the divertor, the impact of the NBI heating is more pronounced at high delivered powers (above P NBI = 320 kW) and line-averaged densities (above 7 × 1019 m-3). The electron temperature and the floating potential at the outer target increases respectively with 10–50% and 100%, while the plasma potential in the inner divertor increases twofold. At a higher line-averaged density, the NBI heating affects the edge plasma, namely, the electron temperature in the midplane scrape-off layer and the heating moves toward the divertor. Thus, a twice as high electron temperature is observed in the divertor and a bi-Maxwellian electron energy distribution function arises.

A simulation study of the self-pulsing regime of a micro hollow cathode discharge in helium using a zero-dimensional model

A volume-averaged model is used to study the self-pulsing operation of a micro hollow cathode discharge in helium. An equivalent circuit with a nonlinear resistance is employed to model the discharge in this regime. The effects of the applied voltage and the gas pressure on the self-pulsing frequency, particle densities, and electron temperature are carefully studied. The simulation results are also compared with those of the argon micro hollow cathode discharge. The results show that the time-averaged densities in the self pulsing regime are close to the steady-state densities. Both the applied voltage and the background gas pressure have remarkable effects on the self pulsing frequency and consequently on the average densities. However, the pressure effects are much stronger than those of the applied voltage. The results also indicate that there is an optimum pressure, in which PD (D is the diameter of the hole) reaches its upper limit, and the electron density is maximized. By further increase in the pressure, the electron density declines. However, the densities of the excited particles are independent of PD and always increase with the increase in pressure. It is also found that the electron density in the helium discharge is less than that of the argon. But, the reverse is true for the electron temperature. The higher electron temperature in the helium discharge is a key factor in increasing the efficiency of production of the reactive species.

Trapped-electron-mode property and role for the L-H transition in L-mode edge plasmas with relatively high electron temperature and weak ion temperature gradient

When the low-confinement mode (L-mode) edge has relatively high electron temperature and weak ion temperature gradient, for example, as observed in the ECH-heated low-density plasmas, the trapped electron mode (TEM) can play an important role in the low to high confinement (L-H) transition as well as the L-mode edge transport, instead of the resistive ballooning or ion temperature gradient mode (RBM or ITG). Motivated by this, a study is first presented about, among several types of the TEM, which one is the most dominant in the above L-mode edge condition, showing that it is dominated by a mixture of the density gradient driven and dissipative TEMs. The parameter dependence of the growth rate of this dominant TEM type is then checked, showing that it has the scaling qualitatively similar to the H-mode threshold power one observed in plasmas where the TEM can be a dominant mode. Closely related to this, a discussion is also given about the L-H transition expected in plasmas where the TEM is assumed to be dominant. Unlike the typical L-H transition that is believed to occur in the RBM-dominant plasmas, it is conjectured to take place in one-step form, with no generation or negligible role of the limit-cycle-oscillation, thus requiring a larger threshold power.

Numerical study on optimization of atmospheric pulse-modulated radio frequency discharges in the very high frequency range

In this paper, we investigated the optimization of pulse-modulated radio frequency (rf) discharges in the range of very high frequency from 50 to 800 MHz by a fluid model. A very strong Peak Current in the First Period (PCFP) during the power-on phase can be observed only when the excitation frequency is large enough, usually larger than 50 MHz, and the reversal electric field near the anode due to the accumulation of electrons contributes greatly to the formation of this peak current from the simulation data. The highest electron temperature is achieved in the first period, while the largest electron density is usually obtained in the last period during the power-on phase. By increasing the duty cycle, the value of PCFP increases initially, then it reaches the peak value at a duty cycle of approximately 70%, and later it drops to the normal value generated in a continuous rf discharge, and the maximum electron temperature also shows the similar evolution. However, as the duty cycle is increased, the electron density is always enhanced during the power-on phase. According to the simulation results, the duty cycle and modulation frequency can be effectively applied to modulate and optimize the electron density and electron temperature for applications.

An extreme ultraviolet spectrometer working at 10–130 Å for tungsten spectra observation with high spectral resolution and fast-time response in Experimental Advanced Superconducting Tokamak

A newly developed flat-field extreme ultraviolet (EUV) spectrometer working at 10–130 Å wavelength range with high spectral resolution and fast-time response has been developed to observe line emissions from highly ionized tungsten ions in the Experimental Advanced Superconducting Tokamak (EAST). This flat-field EUV spectrometer is equipped with a laminar-type varied-line-spacing (VLS) concave holographic grating (2400 grooves/mm at grating center) fixed at an angle of incidence of 88.6°, and a back-illuminated charge-coupled device (CCD) with pixel numbers of 1024 × 255 (26 × 26 μm2/pixel). The CCD is routinely operated at 5 ms per frame with full vertical binning (FVB) mode. An excellently high spectral resolution of Δλ0=4–6 pixels (full width at foot position of spectral line profile), which corresponds to full width of half maximum (FWHM) of 0.1 Å at 20 Å, has been obtained by carefully adjusting angles and positions of the grating and CCD. Thus, the line identification of complicated tungsten spectra became possible, when a precise wavelength calibration was done. Typical tungsten spectra are recorded at low (Te=1.0 keV) and high (Te=2.6 keV) electron temperatures. Tungsten unresolved transition arrays (W-UTA) existing at wavelength ranges of 15–35 and 45–65 Å were preliminarily identified as line emissions from W24+–W33+ and W27+–W45+ ions, respectively, by referring to previous experimental results and National Institute of Standards and Technology (NIST) database. Developing a set of analytical computer programs, line spectra observed with the EUV spectrometer have been automatically identified after data acquisition basically for all discharges during daily EAST experiment.

Al/Ni reactive multilayer films enhancing the properties of plasma induced by nanosecond pulsed laser

In this paper, the influence of Al/Ni reactive multilayer films on laser ablation induced plasma process is investigated. Laser ablation of single Ni layer and Al/Ni RMFs was performed by nanosecond pulsed laser beam with 1064 ​nm wavelength. The characterization methods were applied for researching the structure and chemical composition with transmission electron microscopy, X-ray photoelectron spectroscopy, and atom probe tomography. The effects of the Al/Ni RMFs on the plasma properties were then systematically investigated in terms of the electron temperature, density and ionization. Meanwhile, measurements of the plasma expansion velocity were performed using ICCD system. The results exhibit improved plasma behaviors with high electron temperature, fast expansion velocity and violent ablation phenomenon of Al/Ni RMFs in comparison with the Ni film, which is significant for broadening the potentials in military applications.

Effects of energetic particles on the density-gradient-driven collisionless trapped electron mode instability in tokamak plasmas

We investigate analytically the effects of energetic particles (EPs) on the instability of the density-gradient-driven collisionless trapped electron mode (CTEM) through linear gyrokinetic theory and bounce kinetic theory in tokamak plasmas. The effects of EPs, including fusion-born alpha particles and neutral-beam-injection-driven beam ions, on the CTEM instability are compared for the dynamic model with slowing-down (SD) and equivalent Maxwellian (EM) equilibrium EP distribution functions and dilution model. It is found that the density-gradient-driven CTEM instability in the long wavelength regime can be further destabilized by EPs mainly due to the downshift in the real frequency of the mode by dilution effects. This is attributed to more resonant electrons around the smaller phase velocity of the drift wave and the consequent stronger excitation of CTEM instability. The growth rate is slightly higher for the dilution model as compared to that for the dynamic model since the Landau damping effects of EPs are neglected in the dilution model. Moreover, there is no significant difference in the growth rate between the cases of SD and EM equilibrium EP distribution functions, except for the case of alpha particles and with relatively higher electron temperature.

Rapid ionization of Xe/Ar mixtures in nanosecond discharges exploiting post-pulse field reversals

Enhanced post-pulse electric field reversals of Ar, Xe, and XeAr mixture gases in capacitively coupled nanosecond discharges are investigated with Particle-In-Cell simulations in the context of maximizing electron density. The electric field reversal occurs at the falling edge of the voltage pulse and induces electron oscillatory movement in the plasma bulk region. The amplitude of field reversals is affected by driven voltage and the ratio of bulk length to gap distance. Exploiting the field reversal with a so called Plasma frequency dependent Square Wave (PSW) in an optimal gas mixture leads to the highest electron density. Specifically, for a 250 V PSW XeAr mixture case, the electron density is 2.2 times higher compared to a 1 kV DC pure Xe case even if the driven voltage is 4 times less than DC voltage. In 250 V PSW cases, XeAr mixture plasma has 1.2 times higher average electron density and 1.2 times electron temperature in the sheath region than a pure Xe plasma. With a narrower bulk region, the XeAr plasma has an enhanced field reversal and this leads to higher and faster growing electron density and electron temperature than a Xe plasma. For applications using Xe plasmas, XeAr mixture plasmas with PSW can be exploited for high electron density and temperature at reduced costs.

One-Step Etching Characteristics of ITO/Ag/ITO Multilayered Electrode in High-Density and High-Electron-Temperature Plasma.

This paper presents the dry etching characteristics of indium tin oxide (ITO)/Ag/ITO multilayered thin film, used as a pixel electrode in a high-resolution active-matrix organic light-emitting diode (AMOLED) device. Dry etching was performed using a combination of H2 and HCl gases in a reactive ion etching system with a remote electron cyclotron resonance (ECR) plasma source, in order to achieve high electron temperature. The effect of the gas ratio (H2/HCl) was closely observed, in order to achieve an optimal etch profile and an effective etch process, while other parameters—such as the radio frequency (RF) power, ECR power, chamber pressure, and temperature—were fixed. The optimized process, with an appropriate gas ratio, constitutes a one-step serial dry etch solution for ITO and Ag multilayered thin films.

Ultrafast multi-cycle terahertz measurements of the electrical conductivity in strongly excited solids

Key insights in materials at extreme temperatures and pressures can be gained by accurate measurements that determine the electrical conductivity. Free-electron laser pulses can ionize and excite matter out of equilibrium on femtosecond time scales, modifying the electronic and ionic structures and enhancing electronic scattering properties. The transient evolution of the conductivity manifests the energy coupling from high temperature electrons to low temperature ions. Here we combine accelerator-based, high-brightness multi-cycle terahertz radiation with a single-shot electro-optic sampling technique to probe the evolution of DC electrical conductivity using terahertz transmission measurements on sub-picosecond time scales with a multi-undulator free electron laser. Our results allow the direct determination of the electron-electron and electron-ion scattering frequencies that are the major contributors of the electrical resistivity.