Evolution Of Electron Temperature Research Articles

Ion and electron temperature evolution during equatorial spread F

The first simulation study of ion (O+ and H+) and electron temperature evolution during equatorial spread F (ESF) is presented. The simulation results are based on the Naval Research Laboratory (NRL) SAMI3/ESF three‐dimensional code. It is found that the ions and electrons undergo both cooling and heating during bubble evolution. The main cause of cooling is adiabatic, associated with the increase of the flux tube volume as the plasma bubble rises. Ion heating is primarily caused by the compression of ions as they stream down the converging magnetic field. The electrons are heated by collisional coupling with the ions. Additionally, it is found that the electrons are heated at high altitudes (≳1200 km) because of thermal conduction, and that hydrogen ions can be heated at relatively low altitudes (≃300 km) because of ion‐neutral frictional heating. We compare the simulation results with observations from the ROCSAT and Hinotori satellites.

Crystallographic effects in the kinetic excitation of metal surfaces: A computational study

We present a statistical investigation of the kinetic excitation in atomic collision cascades induced by keV particle impact on metal surfaces. For the model system 5-keV Ag→Ag and a set of 120 impact points, we study the effect of the crystallographic orientation of the target on the average electron temperature evolution at the very surface. Results are discussed in view of secondary ion formation.

Experimental Investigation of 193-nm Laser Breakdown in Air

We present the measurements and analysis of laser-induced breakdown processes in dry air at a wavelength of 193 nm by focusing 180-mJ 10-MW high-power 193-nm UV ArF laser radiation onto a 30-mum-radius spot size. We examine pressures ranging from 40 torr to 5 atm, for laser power densities of 1 TW/cm2, well above the threshold power flux for air ionization. The breakdown threshold electric field is measured and compared with classical and quantum theoretical ionization models at this short wavelength. A universal scaling analysis of these results allows one to predict aspects of high-power microwave breakdown based on measured laser breakdown observations. Comparison of 193-nm laser-induced effective field intensities for air breakdown data calculated based on the collisional cascade and multiphoton breakdown theories is used successfully to determine the collisional microwave scaled portion with good agreement regarding both pressure dependence and breakdown threshold electric fields. Using a laser shadowgraphy diagnostic technique, the plasma and shock-wave dynamics are analyzed. Blast shock-wave expansion of the plasma and laser-heated neutral gas is observed with average velocities of 47 km/s, and the temporal shock-wave velocity variation is used to determine electron temperature evolution just behind the shock wave.

Real-time measurements for edge plasma parameters using triple probes on EAST

The achievement of the first plasma has demonstrated that the Experimental Advanced Superconducting Tokamak (EAST) engineering construction is completely successful. Divertor plasma discharge with double null configuration was achieved and verified by the fitting codes. Arrays of 60 tilted divertor probes have been installed in the lower, both inboard and outboard, divertor plates of EAST. Evolution of electron temperature and density is obtained by using triple-probe systems; real-time particle flux and power flux at divertor target plates have been measured. Tilted Langmuir triple probes (TLTPS) have been successfully used for measuring edge plasma parameters. The strike point by the fitting codes coincides well with measured edge plasma parameters.

Energy confinement and magnetic field generation in the SSPX spheromak

The Sustained Spheromak Physics Experiment (SSPX) [Hooper et al., Nuclear Fusion 39, 863 (1999)] explores the physics of efficient magnetic field buildup and energy confinement, both essential parts of advancing the spheromak concept. Extending the spheromak formation phase increases the efficiency of magnetic field generation with the maximum edge magnetic field for a given injector current (B∕I) from 0.65T∕MA previously to 0.9T∕MA. We have achieved the highest electron temperatures (Te) recorded for a spheromak with Te>500eV, toroidal magnetic field ∼1T, and toroidal current (∼1MA) [Wood et al., “Improved magnetic field generation efficiency and higher temperature spheromak plasmas,” Phys. Rev. Lett. (submitted)]. Extending the sustainment phase to >8ms extends the period of low magnetic fluctuations (<1%) by 50%. The NIMROD three-dimensional resistive magnetohydrodynamics code [Sovinec et al., Phys. Plasmas 10, 1727 (2003)] reproduces the observed flux amplification ψpol∕ψgun. Successive gun pulses are demonstrated to maintain the magnetic field in a quasisteady state against resistive decay. Initial measurements of neutral particle flux in multipulse operation show charge-exchange power loss <1% of gun input power and dominantly collisional majority ion heating. The evolution of electron temperature shows a distinct and robust feature of spheromak formation: A hollow-to-peaked Te(r) associated with q∼1∕2.

Recent Electron Cyclotron Emission Results on TCV

Electron cyclotron emission (ECE) diagnostics on Tokamak à Configuration Variable (TCV) allow study of the electron temperature evolution in time with good spatial and temporal resolution at the high field side and low field side at various lines of sight. That is why ECE is being widely used to obtain both qualitative and quantitative information on heat transport, magnetohydrodynamics (MHD) phenomena, and fast electron dynamics. In this paper, a new regime on TCV with regular oscillations of the electron temperature in electron cyclotron current drive (ECCD) driven fully noninductive discharges and in discharges with a combination of ohmic/ECCD driven current is discussed. These oscillations are reminiscent of the oscillations of the central electron temperature (O-regime) seen on Tore Supra in fully noninductive lower hybrid current drive plasmas. A link between evolutions of the electron temperature, the MHD modes, and the current density profile on TCV is considered. In order to yield information on the properties of microturbulence of electrostatic and magnetic origin on TCV, a correlation ECE radiometer is currently under development. A technical description of the diagnostic is presented in this paper.

502 細管内に形成された大気圧非平衡プラズマ流のラジカル輸送機構(2)(OS5-1 機能性流体工学の先端融合化,OS5 機能性流体工学の先端融合化,オーガナイズドセッション)

For improvement of the performance of sterilization in a tube, it is very important to clarify the discharge processes such as electron avalanche and streamer propagation. We clarified the time and spatial evolution of electron number density, electron temperature and number density of chemical species using computational analysis. The distribution of the oxygen atom depends on electron number density and the ozone generates after increasing of oxygen atoms. The electrons diffuse and charge on the interior surface of the tube after the streamer strikes it.

Probe Diagnostics of Microwave Plasma at Frequency 2.45 GHz in CW and Pulse Regime

AbstractThe paper is focused on diagnostics of newly developed surfatron‐based plasma source working at frequency 2.45 GHz. This plasma source of jet type can be operated in cw as well as in PC controlled pulse regime. We carried out diagnostic of the plasma exiting the nozzle by single and double Langmuir probe in cw as well as in pulsed regime of plasma source operation. The basic plasma parameters ‐ electron density and electron temperature ‐ are estimated from single probe measurements at low pressures. The electron temperature in wide range of pressures is evaluated from double probe measurements. Time evolution of electron temperature along a single period in pulsed mode of operation is measured and discussed. The exiting plasma stream is directly applied on a plastic material sample (PE) with the aim to increase its hydrophility. Degree of hydrophility is characterized by contact angle measurements. Relations between process plasma parameters and results of contact angle measurements are discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical Simulation of the Thomson Scattering Experiment Performed with 0.351μm Laser-Produced Aluminum Plasmas

The evolutions of the electron temperatures of aluminum plasmas produced with 0.351 μm laser are simulated by means of one-dimensional hydrodynamic code. The simulations show that the plasma geometry has strong influence on the electron temperature's evolution while the effect of the flux limiter is not so significant. The simulations are in good agreement with the experiments only at some spatial points. A full comparison between the simulations and experiments indicates that the one-dimensional code is not accurate enough to characterize the laser-produced plasmas. A post-processor code based on the hydro code is developed to generate the streak image of the Thomson scattering spectra, which can be directly compared with the experimental data.

Analysis of ECRH switch on/off events in ASDEX Upgrade

In this paper transient phenomena related to the electron heat transport when electron cyclotron resonance heating (ECRH) power is switched on or off are investigated by predictive and interpretative transport analysis. In the latter two numerical approaches—one based on the transport code ASTRA and the other by COBRA code—are compared. The transport simulations were done in the ASTRA code based on a critical ∇Te/Te model. The results for the evolution of the electron temperature in steady state and transient phases are compared with the experiment. The changes of the heat diffusivity with the ECRH power are compared with the results of the transport calculations from the interpretative analysis. The critical ∇Te/Te model reproduces well both the heat diffusivity and the electron temperature evolution in the region outside the ECRH deposition, and the results show good agreement with the experimental data there. The model cannot recover the electron temperature changes inside the ECRH deposition region with accuracy due to the simple assumption made in transport below the threshold.

Two-dimensional simulation of a miniaturized inductively coupled plasma reactor

A two-dimensional self-consistent simulation of a miniaturized inductively coupled plasma (mICP) reactor was developed. The coupled equations for plasma power deposition, electron temperature, and charged and neutral species densities, were solved to obtain the spatial distribution of an argon discharge. The effect of control parameters, such as power and pressure, on the evolution of plasma density and electron temperature was investigated. Strong ion density gradients were observed which can make spatially resolved Langmuir probe measurements particularly challenging. Simulation results were in reasonable agreement with available experimental data. The neutral gas temperature was predicted to be close to the wall temperature, due to the small length scale of the mICP, allowing for efficient heat transfer.

Three-Field Model for Drift Waves in a Simple Magnetized Torus

A three-field model for resistive drift waves in a weakly ionized plasma, comprising the evolution of particle density, vorticity, and electron temperature, is investigated in a simple toroidal geometry. A derivation of the model is presented, along with a linear normal mode analysis. The non-linear evolution and saturation are studied by means of two-dimensional computations for a double-periodic domain. The analysis is focused in particular on the dependence of the behavior of the system on the parameter C which is inversely proportional to the electron-neutral collision frequency and which couples potential, density and electron temperature fluctuations along the magnetic field. In the adiabatic regime, corresponding to C >> 1 it is found that many of the linear fluctuation characteristics are preserved, including cross phases and a clear wave-like feature with propagation in the electron diamagnetic drift direction. Electron temperature fluctuations in this regime turn out to be strongly damped due to the large parallel heat flux. In the hydrodynamic regime (C << 1) the wave motion is less evident and non-linear behavior prevails. The convective heat transport is seen to dominate the conductive one for all values of the coupling parameter.

Transport and reconnection in tokamak sawteeth.

The core of a tokamak discharge often undergoes periodic relaxation oscillations, sawteeth, as the steepening current and temperature profiles are flattened by fast reconnection events. Careful analysis of the electron temperature evolution over this cycle gives an estimate of the energy dissipated in the electrons during reconnection and a measure of the transport characteristic (energy flux versus temperature gradient) over the range of parameters occurring over the remainder of the cycle. The energy dissipated is consistent with estimates of the loss of poloidal magnetic energy. The transport characteristics exhibit a wide range of behaviors.

Laser ablation of Mg, Cu, and Pb using infrared and ultraviolet low-fluence lasers

The modified two-dimensional hydrodynamic code POLLUX has been used to simulate the ablation of magnesium, copper, and lead targets in the early stage of expansion (&amp;lt;100 ns) by 30 ns, 0.248 μm KrF excimer and 1.064 μm Nd–YAG lasers at fluences of 3–10 J/cm2. The results of magnesium ablation using KrF laser with different fluences are compared and found to be in reasonably good agreement with the experimental results. A comparative study of laser ablation for magnesium, copper, and lead has been made by simulation using Nd–YAG lasers at fluence 10 J/cm2. The temporal evolution of surface temperature of the target, ablated vapor mass, and ionization fraction of plasma plume is studied for each material. The simulated temporal evolution and spatial distribution of electron density, electron temperature, and axial velocity of ablated plume of each material are studied and the results are compared. It is found that the plume expansion follows adiabatic behavior and is slowed down with increasing atomic mass. In addition to that, the effect of laser wavelength on ablation has also been investigated by comparing ultraviolet KrF excimer and infrared Nd–YAG lasers. It is found that shorter-wavelength lasers favor ablation and plasma screening is more effective for longer-wavelength lasers.

Fast plasma heating in the high-frequency skin-effect regime

Dense plasma heating by an ultrashort laser pulse is investigated in the regime of the high-frequency skin effect. The electron temperature evolution on the plasma surface and the properties of the classical heat transfer into the plasma interior are studied. Three possible plasma heating mechanisms are considered. At relatively not large temperatures the plasma heating, due to radiation absorption, is controlled by the electron–ion collisions in the skin layer. The others two pathways are related to high temperatures and take place in the regime of collisionless radiation absorption when the electrons are reflected by the plasma surface either specularly or diffusely. It is shown that, compared with the specular reflection, the diffuse one yields a larger increase of the electron temperature on the plasma surface and a faster propagation of the heat flux in the plasma interior, even when the fraction of diffusely reflected electrons is not high.

Evolution of electron temperature during the energy quench of a major plasma disruption

An electron cyclotron emission (ECE) heterodyne radiometer and a Thomson scattering (TS) system were used to measure the electron temperature (T/sub e/) evolution in plasmas of the Rijnhuizen Tokamak Project (RTP) during the abrupt loss of energy confinement in high-density limit disruptions with unprecedented high resolution.

Optical sensor for real-time monitoring of CO_2 laser welding process

An optical sensor for real-time monitoring of laser welding based on a spectroscopic study of the optical emission of plasma plumes has been developed. The welding plasma's electron temperature was contemporarily monitored for three of the chemical species that constitute the plasma plume by use of related emission lines. The evolution of electron temperature was recorded and analyzed during several welding procedures carried out under various operating conditions. A clear correlation between the mean value and the standard deviation of the plasma's electron temperature and the quality of the welded joint has been found. We used this information to find optimal welding parameters and for real-time detection of weld defects such as crater formation, lack of penetration, weld disruptions, and seam oxidation.

Characterization of plasmas produced by laser–gas jet interaction

An experiment has been performed with one of the six nanosecond beams of the Laboratoire pour l’Utilisation des Lasers Intenses laser facility in order to create long scale uniform plasmas over a wide range of electron density (1×1019–1.6×1020 cm−3) and electron temperature (0.5–1.3 keV). Electron density and temperature evolution have been measured using Thomson scattering. Numerical simulations obtained by using a simple model are presented. Scaling law related electron density and electron temperature have been established in agreement with experimental data.

The recombination rate coefficient of a protonatedacetone dimer with electrons: indication of a temperature dependence

The formation and recombination of a protonated acetone dimer withelectrons was studied in the flowing afterglow. H3O+ ionswere formed in the early post-discharge region. The subsequent additionof acetone leads to the formation of the protonated ions CH3COCH3H+. These ions further associate with acetone formingthe cluster ions H+·(CH3COCH3)2 that react furtherwith acetone, but very slowly. This facilitates the creation of anafterglow plasma with a dominant population of H+·(CH3COCH3)2. The evolution of electron number density (ne) and electron temperature (Te) is measured using theLangmuir probe. The recombination rate coefficients that are obtainedindicate a negative temperature dependence: α = (3.4±1)×10-6 cm3 s-1 at Te = (580±150) K andα = (7±2.5)×10-6 cm3 s-1 atTe = (450±100) K.

YAG laser Thomson scattering diagnostic on the JT-60U

A YAG laser Thomson scattering system has been developed to measure the time evolution of electron temperature (Te) and density (ne) profiles in the JT-60U. Presently this system provides Te and ne periodically every 100 ms (20 ms at the minimum) at 15 spatial channels. A six spectrum channel polychromator, which is a successive interference filter type, was designed to measure a wide Te range (20 eV–20 keV) by using five channels. The sixth channel is used for calibration by Rayleigh scattering. However, we also used the Rayleigh channel to measure Te and ne since the stray light level was very low. As a result, we were able to measure temperatures below 20 eV. The temperature of the silicon avalanche photodiode (APD) is stabilized by a Péltier element so that the gain is kept constant within ±0.5%. The stable performance of the APD enables accurate Te and ne measurement. During plasma operation, the electron temperature is measured over a wide range from the low temperature plasmas in the runaway plasma current tail generated by a killer pellet to the high temperatures of high performance negative shear plasmas.