Reduction Of Electron Temperature Research Articles

Analysing the effects of heating and gas puffing in Proto-MPEX helicon and auxiliary heated plasmas *

The Material Plasma Exposure Experiment (MPEX) is being constructed at Oak Ridge National Laboratory to investigate critical fusion reactor issues, such as plasma–material interactions (PMIs) under reactor-relevant conditions and time scales. The linear device Proto-MPEX was used as a test bed to address anticipated research and development issues associated with heating scenarios and establish the physics basis for MPEX. The SOLPS-ITER code suite has been applied to understand plasma and neutral transport in Proto-MPEX and to increase confidence in predictive simulations for MPEX. Coupling between COMSOL and SOLPS is performed to implement a 2D electron heating profile of the helicon source. The simulations show reasonable agreement with the experimental data for plasma with helicon and auxiliary electron cyclotron heating (ECH). Both Bohm and constant diffusion (D: 0.5 m2 s−1 and : 1 m2 s−1) simulations show similar levels of agreement with respect to the sparse experimental data available, assuming a few per cent impurity concentration. ECH significantly increases the target electron temperature, however, the target electron density is reduced compared to helicon-only heated plasmas due to an increase in flow velocity and radial losses. The simulations show that further increasing the ECH power results in an increase in the target electron density due to increased recycling flux and ionization. The results indicate that ECH significantly enhances the target heat flux, with ECH power of 50 kW increasing the target heat flux from 0.4 to 17 MW m−2. It is found that a small amount of gas puffing (GP) near the target plate can further increase the target heat fluxes at the higher ECH power cases, but the target heat flux is reduced at higher GP conditions due to a significant reduction in electron temperature via radiation. ECH and GP scenarios can generate a higher target flux, facilitating improved PMI studies with more reactor-relevant plasma conditions.

Effects of the excited states on electron kinetics and power absorption and dissipation in inductively coupled Ar plasmas

The effects of the excited states on electron kinetics as well as plasma power absorption and dissipation are numerically studied in radio frequency low-pressure inductively coupled Ar plasmas. The model used in this work is based on the coupling of the kinetic module, the electromagnetic field module, and the global model module. The existence of excited states caused by the electron-impact excitations of the ground state of Ar decreases the electron temperature due to the significant depletion of the electron energy probability function in the inelastic energy range. The reduction in electron temperature decreases the power dissipation of an electron per unit volume and, therefore, increases the electron density for the fixed total power. The profile and maximum variations of the absorption power density indicate that the increased electron density suppresses the power deposition deeper into the plasma with inclusion of the electron-impact excitations of the ground state to excited states of Ar. However, the collision processes involving the excited states as reactants hardly affect the electron kinetics and electromagnetic field properties due to far lower densities of the excited states than that of the ground state of Ar.

Reduced Electron Temperature in Silicon Multi-Quantum-Dot Single-Electron Tunneling Devices.

The high-performance room-temperature-operating Si single-electron transistors (SETs) were devised in the form of the multiple quantum-dot (MQD) multiple tunnel junction (MTJ) system. The key device architecture of the Si MQD MTJ system was self-formed along the volumetrically undulated [110] Si nanowire that was fabricated by isotropic wet etching and subsequent oxidation of the e-beam-lithographically patterned [110] Si nanowire. The strong subband modulation in the volumetrically undulated [110] Si nanowire could create both the large quantum level spacings and the high tunnel barriers in the Si MQD MTJ system. Such a device scheme can not only decrease the cotunneling effect, but also reduce the effective electron temperature. These eventually led to the energetic stability for both the Coulomb blockade and the negative differential conductance characteristics at room temperature. The results suggest that the present device scheme (i.e., [110] Si MQD MTJ) holds great promise for the room-temperature demonstration of the high-performance Si SETs.

Experimental diagnosis of electron density and temperature in capacitively coupled argon plasmas: Triple-frequency discharges and two-dimensional spatial distributions

An approach combining optical emission spectroscopy with a collisional radiative model (OES-CRM) has been used to diagnose electron density and electron temperature in low pressure capacitively coupled argon plasmas. The electron density and electron temperature obtained by the OES-CRM show reasonable agreement with the results measured by a Langmuir probe. The method was first applied to diagnosis in triple-frequency (2, 13.56, and 27.12 MHz) plasmas. Compared with dual-frequency (2 and 27.12 MHz) discharges, it was found that in triple-frequency discharges, the high frequency source controls electron density more independently and with less influence on electron temperature as the intermediate frequency power increases. Next, the method was extended to a two-dimensional diagnosis based on the use of a charge coupled device camera and optical bandpass interference filters. The results showed that the axial and radial distributions of the electron density are more uniform at a lower radio frequency (RF) power. The axial uniformity of the electron density is better at a lower discharge frequency while the radial profiles of the electron temperature are flatter at a higher frequency. In all the cases, the electron temperature is highly uniform within the bulk plasma. Moreover, a mode transition from an α to a γ mode is observed at 13.56 MHz with the increasing RF power, and this is accompanied by a significant enhancement in electron density and a sharp reduction in electron temperature.

Characteristics of Ar Plasma Jet Generated by 50 Hz Alternating Current

Plasma jet generated by dielectric barrier discharge system has gain importance in surface modification processes. Characteristics of plasma of the plasma jet depend on type of gas, flow rate of gas, design of electrodes, power and frequency of the power source. In this research, the argon gas is flowed through a hollow tube is investigated. Alternating high voltage power supply connected to electrodes is operated with frequency of 50 Hz. It was found that the lengths of the plasma jet generated by this system ranges from 0.5 cm and 1 cm. The minimum and the maximum electron temperature of the plasma was 0.44 eV and 0.86 eV respectively. The electron temperature increased as the applied voltage was increased. However, the electron temperature decreased when the operating voltage was increased above 6.5 kV to 7.5 kV. It can be explained by the effect of arcing between the electrodes which causes the lost in energy. Also, increasing of the flow rate over a critical limit produces turbulence that also causes reduction in electron temperature of the plasma jet.

Spectroscopic analysis of magnetized hollow cathode discharge plasma characteristics

The present work investigated the discharge characteristics in the magnetized hollow cathode system. Also, the gas pressure effect on the discharge characteristics has been examined with a gas pressure between (0.08 and 0.4) Torr. The voltage-current curve of magnetized hollow cathode discharges has the voltage become approximately constant with slight increases in current. The normal glow discharge was established with constant voltage and increasing current. When the current is increased even more, then, the discharge turns into a mode comparable to the normal glow discharge between the parallel electrodes. The increasing of the pressure causes compressing of the cathode falls inside the cathode cavity and the negative glow becomes thin and more luminous. The spectroscopic method used depends on analyzing the argon’s emission atomic spectra. The electron temperature has been evaluated with the use of Boltzmann plot approach. The influence of the pressure on electron temperature observed the reduction in electron temperature when the pressure is increased. The electron number density has been calculated with the use of the Stark broadening method. The density curve indicated that the increase in electron number density with increasing of pressure until 0.4 Torr and then reduced with pressure exceeds 0.4 Torr.

Introspection Into Reliability Aspects in AlGaN/GaN HEMTs With Gate Geometry Modification

Reliability enhancement of AlGaN / GaN HEMT is a significant thrust area due to rapidly improving material and processing technology. In this paper, a detailed analysis of gate-shaped AlGaN / GaN HEMT with field plate is presented. Although AlGaN / GaN HEMT with field-plate is well known, its blending with gate-shaping leading to a more robust and reliable behaviour is described in this paper. It is observed that the threshold voltage and transconductance invariably remain constant for various combinations of gate-shaped and field plate placements. The threshold voltage for all the devices are found to be - 5.8 V. The peak transconductance for the devices without field plate and with field plate is ~ 0.16 S/mm and ~ 0.15 S/mm, respectively. Apart from leakage current, the electric field also gets mitigated for both gate-shaped and field-plated devices by ~ 45% and ~ 68%, respectively. The moderation in electric field further assists in the reduction of electron temperature for gate-shaped and field-plated structures by ~ 12% and ~ 85%, respectively. Additionally, breakdown voltage increases for the gate-shaped devices to 133 V as compared to 120 V of conventional devices without field plate. Significant reduction in leakage current, electric field, and electron temperature is accompanied by a minor increment in capacitance for the field-plated structure, hence, the proposed structure is expected to enhance reliability of the device. Thus, it is anticipated that the proposed devices with enhanced reliability would be a step ahead of conventional devices and would find major applications in high power domain.

The influence of relative humidity on double dielectric barrier discharge plasma for chlorobenzene removal

The influence of relative humidity on the electrical characteristics, the generation of the active species and the chlorobenzene removal performance was studied. The results show that both the peak-to-peak current and the discharge power decreased with the increase of relative humidity under the same peak-to-peak voltage. A reduction of electron temperature was observed with the addition of water vapour, affecting the electron impact processes. It was observed that excited states of N 2 and O were affected by the water due to the quenching effect. A decrease of chlorobenzene removal efficiency was achieved from 81.7% to 70.7% as the increase of humidity. However, carbon balance analysis depicts the yields of CO 2 in humid air were higher than that in dry air due to the help of radical OH, especially in RH = 40%, and highest energy efficiency with 4.4 g kW −1 h −1 was also achieved at RH = 40%. Chlorine balance indicates that the product HCl is more easily generated in the presence of H 2 O. A difference in the chlorobenzene degradation pathway was revealed. Fewer kinds of benzenoid byproducts were found in dry air, which was in agreement with the plasma diagnostics results. Radicals generated in humid air promoted the total oxidation process, resulting in fewer kinds of ring-open byproducts and higher CO 2 yield. • A novel home-made mini dDBD reactor was used to remove chlorobenzene. • Active species generated in the discharge were affected by relative humidity. • The addition of water was helpful to improve the yield of CO 2 and the yield of HCl. • The removal pathways of chlorobenzene were affected by the water.

Influence of metastable atoms in low pressure magnetized radio-frequency argon discharges

One-dimensional particle-in-cell simulations with Monte Carlo collisions are used to investigate the influence of metastable atoms in low pressure radio-frequency argon discharges with magnetic fields ranging from 0 G to 60 G. Two metastable levels of argon species are included and tracked as particles, enabling multistep ionization and metastable pooling. At low magnetic fields, the metastable argon atoms have little influence on the discharge. At higher magnetic fields, the electron density increases and the electron temperature decreases at the center of the discharge with the inclusion of metastable atoms. The reduction in electron temperature is attributed to the depletion of energetic electrons due to low-energy-threshold reactions related to the metastable atoms. The reduced electron temperature leads to a reduction in the total ionization rate, albeit the contribution of multistep ionization increases with the magnetic field. The suppression of plasma diffusion at low electron temperatures plays a greater role than the reduction in ionization rate, resulting in a higher electron density. A transformation in metastable density profiles from parabolic to saddle type is observed with the increase in the magnetic field. Metastable atoms may play an important role in modulating the electron temperature in low pressure magnetized discharges.

Effect of fuel isotope mass on q-profile formation in JET hybrid plasmas

The initial current ramp phase of JET hybrid plasmas is used to optimise the target q-profile for main heating to allow access to high β and avoid MHD instabilities. Mixed protium-deuterium experiments, carried out at JET since the installation of the beryllium-tungsten wall, have shown that the q-profile evolution during this Ohmic phase varies systematically with average main ion isotope mass, indicating the need for re-optimisation for future T and mixed D-T experiments. Current diffusion modelling shows that the key factor was a reduction in electron temperature profile peaking as the hydrogenic isotope mass was increased. This was correlated with an increase in plasma radiation by metallic impurities, consistent with the increased sputtering yield by higher mass isotopes during the current ramp phase. Reduced electron temperature peaking can lead to magnetic shear reversal and the appearance of a 2/1 mode, which can lock, causing the JET massive gas injection system to be triggered to avoid an unmitigated disruption. The potential for a further reduction in electron temperature peaking in T and D-T plasmas could, therefore, result in an increased risk of disruptions. To mitigate this risk, electron temperature peaking measurements have been included in the real-time control system to allow this type of disruption to be avoided by central heating, density increase or early pulse termination. These experiments indicate the need for integrated modelling of impurity behaviour, including the plasma core, scrape-off layer and plasma wall interactions, to predict q-profile evolution in the current ramp phase and anticipate the effects of isotope changes.

Experimental study of detachment density threshold in L-mode plasmas on EAST

One of the critical challenges for long pulse operation of ITER and future fusion reactors is the excessively high heat and particle fluxes on the divertor targets. The divertor detachment offers an effective way for the control of steady state heat flux and erosion at the divertor target. Dedicated experiments have been systematically performed for both H-mode and L-mode plasmas on EAST with ITER-like W divertor in the last two years to further advance detachment physics understanding. The detachment is identified by the rollover of divertor peak particle flux near the strike point during the density ramping up, which correlates with the reduction of electron temperature down to 5 eV, the increase of Dδ/Dα ratio and radiated power. It has been demonstrated that the more closed divertor has a lower density threshold at the onset of detachment on EAST. Furthermore, the detachment density threshold will be reduced when the strike point moves towards the divertor corner. The onset of detachment exhibits a strong asymmetry between inner and outer targets, with the inner target accessing detachment at lower density due to the lower electron temperature, as expected. The effects of the heating power and plasma current on the detachment onset were also investigated. The experimental results show that the density at detachment onset is lower in L-mode relative to H-mode, and increases with the heating power. The detachment density threshold normalized to the Greenwald density limit is lower at a high plasma current.

Implementation and assessment of an extended hydrogenic molecular model in UEDGE

AbstractTwo sets of pure deuterium plasmas are simulated using UEDGE, one including atoms and molecules and the other including atoms only as separate fluid species. Simulation results in one and two dimensions are reported to assess the role of molecules in tokamak plasmas. It is shown that thermal coupling of the molecules to the plasma can be relevant for the onset of detachment under highly collisional conditions in simple geometries, but play a modest role at low collisionality. Ion‐molecule equipartition presents an additional ion energy loss channel, dissipating ion energy that would otherwise heat the electrons by equipartition. The resulting reduced electron temperature increases the ion‐electron recombination by an order of magnitude as the plasma density is increased, yielding earlier and deeper detachment.

Thomson scattering near the high-fluence target surface of the Magnum-PSI linear plasma generator

In the quest to long-term operation of high-power magnetically confined fusion devices, it is crucial to control the particle and heat loads on the wall. In order to predict these loads, understanding of the plasma-wall interaction is important. Near the wall surface, the plasma is accelerated towards the Debye sheath edge. In plasma conditions with high density and low temperature, the interaction between the incoming plasma and recycled neutrals can become important. In this paper, we present incoherent Thomson Scattering (TS) measurements in the near-surface region of the Magnum-PSI linear plasma generator. To enable TS measurements close to the plasma target of Magnum-PSI, a stray light suppression up to a factor 104 was achieved, while retaining high transmission. By incrementally moving the target along the magnetic field, this adapted system was used down to 1.9 mm from the target. In the last 10–15 mm in front of the surface, the electron density as well as temperature were observed to decrease significantly. Under the assumption of constant particle flux in this region, the density drop indicates plasma acceleration. In that case, the measurements can be interpreted to show the plasma presheath, and its lengthscale: ∼ 1 cm. The electron cooling indicates an energy loss channel for the electrons near the wall. A reduced electron temperature near the sheath entrance leads to lower estimates of particle and energy flux, as well as ion impact energy, on the target surface.

Temporal Evolution and Spatial Distribution of the Plume Plasma from an MPD Arc Generator

We used a triple Langmuir probe to measure the temporal evolution and spatial distribution of the plasma parameters of a newly designed steady-state applied-field magnetoplasmadynamic arc generator. Varying the discharge current, mass flow rate, and applied magnetic field strength during a 170-s steady discharge, we investigated the corresponding instantaneous variations of electron temperature and density. The temporal profiles show that both the discharge current increase and mass flow rate decrease result in a reduction of electron temperature and density. On the contrary, the application of the external magnetic field, confining the plume expansion, can raise the electron temperature and density by 10 times. The centerline electron temperature and density show a linear decrease from 20- to 34-cm downstream along the thruster axis. The radial distribution exhibits an adiabatic expansion behavior with the ratio of specific heat $\gamma $ in the range of 1.1–1.5.

Spatial Temperature Profile in a Magnetised Capacitively Coupled Discharge

A floating emissive probe has been used to obtain the spatial electron temperature (Te) profile in a 13.56 MHz parallel plate capacitive coupled plasma. The effect of an external transverse magnetic field and pressure on the electron temperature profile has been discussed. In the un-magnetised case, the bulk region of the plasma has a uniform Te. Upon application of the magnetic field, the Te profile becomes non-uniform and skewed. With increase in pressure, there is an overall reduction in electron temperature. The regions adjacent to the electrodes witnessed a higher temperature than the bulk for both cases. The emissive probe results have also been compared with particle-in-cell simulation results for the un-magnetised case.

Effect of working power and pressure on plasma properties during the deposition of TiN films in reactive magnetron sputtering plasma measured using Langmuir probe measurement

The ion, electron density and electron temperature during formation of TiN films in reactive magnetron sputtering system have been investigated for various settings of radio frequency (RF) power and working pressure by using Langmuir probe measurements. The RF power and working pressure able to affect the densities and plasma properties during the deposition process. In this work, a working pressure (100 and 20 mTorr) and RF power (100, 150 and 200 W) have been used for data acquisition of probe measurement. Fundamental of studied on sputter deposition is very important for improvement of film quality and deposition rate. Higher working pressure and RF power able to produce a higher ion density and reduction of electron temperature.

A novel high performance LDMOS transistor with high channel density

In this paper, we propose a novel high channel density LDMOS transistor (HCD-LDMOS) with low specific on-resistance and high transconductance. This is achieved primarily through increased channel density to improve transistor performance via the creation of a U-shaped structure. The channel density is significantly increased when one extra etching process is added to create the U-shaped structure, resulting in a 68.42% reduction in specific on-resistance compared with a conventional structure (C-LDMOS). In addition, the gate control over the channel of the HCD-LDMOS is improved, and a marked increase in the peak transconductance value is achieved. Another key advantage is the lower electron temperature relative to the C-LDMOS. Using the U-shaped formation of the HCD-LDMOS structure, increases the uniformity of the electric field. This smoothing effect leads to a reduction in electron temperature and greater device reliability. The only difference between the C-LDMOS and the proposed structure is the addition of one extra etching process.

Single-shot electron diffraction using a cold atom electron source

Cold atom electron sources (CAESs) are a promising alternative to traditional photocathode sources for use in ultrafast electron diffraction due to greatly reduced electron temperature at creation, and the potential for a corresponding increase in brightness. Here we demonstrate single-shot, nanosecond electron diffraction from monocrystalline gold using cold electron bunches generated in a CAES. The diffraction patterns have sufficient signal to allow registration of multiple single-shot images, generating an averaged image with significantly higher signal-to-noise ratio than obtained with unregistered averaging. Reflection high-energy electron diffraction was also demonstrated, showing that CAESs may be useful in resolving nanosecond dynamics of nanometre scale near-surface structures.

Effect of Ti-Al cathode composition on plasma generation and plasma transport in direct current vacuum arc

DC arc plasma from Ti, Al, and Ti1-xAlx (x = 0.16, 0.25, 0.50, and 0.70) compound cathodes was characterized with respect to plasma chemistry and charge-state-resolved ion energy. Scanning electron microscopy, X-ray diffraction, and Energy-dispersive X-ray spectroscopy of the deposited films and the cathode surfaces were used for exploring the correlation between cathode-, plasma-, and film composition. Experimental work was performed at a base pressure of 10−6 Torr, to exclude plasma-gas interaction. The plasma ion composition showed a reduction of Al of approximately 5 at. % compared to the cathode composition, while deposited films were in accordance with the cathode stoichiometry. This may be explained by presence of neutrals in the plasma/vapour phase. The average ion charge states (Ti = 2.2, Al = 1.65) were consistent with reference data for elemental cathodes, and approximately independent on the cathode composition. On the contrary, the width of the ion energy distributions (IEDs) were drastically reduced when comparing the elemental Ti and Al cathodes with Ti0.5Al0.5, going from ∼150 and ∼175 eV to ∼100 and ∼75 eV for Ti and Al ions, respectively. This may be explained by a reduction in electron temperature, commonly associated with the high energy tail of the IED. The average Ti and Al ion energies ranged between ∼50 and ∼61 eV, and ∼30 and ∼50 eV, respectively, for different cathode compositions. The attained energy trends were explained by the velocity rule for compound cathodes, which states that the most likely velocities of ions of different mass are equal. Hence, compared to elemental cathodes, the faster Al ions will be decelerated, and the slower Ti ions will be accelerated when originating from compound cathodes. The intensity of the macroparticle generation and thickness of the deposited films were also found to be dependent on the cathode composition. The presented results may be of importance for choice of cathodes for thin film depositions involving compound cathodes.

Reduced electron temperature in a magnetized inductively-coupled plasma with internal coil

The effect of magnetic filtering on the electron energy distribution function is studied in an inductive discharge with internal coil coupling. The coil is placed inside the plasma and driven by a low-frequency power supply (5.8 MHz) which leads to a very high power transfer efficiency. A permanent dipole magnet may be placed inside the internal coil to produce a static magnetic field around 100 Gauss. The coil and the matching system are designed to minimize the capacitive coupling to the plasma. Capacitive coupling is quantified by measuring the radiofrequency (rf) plasma potential with a capacitive probe. Without the permanent magnet, the rf plasma potential is significantly smaller than the electron temperature. When the magnet is present, the rf plasma potential increases. The electron energy distribution function is measured as a function of space with and without the permanent magnet. When the magnet is present, electrons are cooled down to low temperature in the downstream region. This region of low electron temperature may be useful for plasma processing applications, as well as for efficient negative ion production.