Drop Of Electron Temperature Research Articles

Investigation of the afterpeaks in pulsed microwave argon plasma at atmospheric pressure

We studied the energy transport process in pulsed microwave argon plasmas at atmospheric pressure, focusing on the optical emission burst during the pulse-off time called the afterpeak. Guided by experimental observations using nanosecond time resolution imaging and spectroscopic diagnostics, we developed a global simulation model considering time-varying reaction rate coefficients and non-thermal electron energy distribution. Experimental and simulation results show that the afterpeak can be maximized by choosing an appropriate pulse period. Our analysis of the generation and consumption of excited argon species reveals that the rapid drop in electron temperature during the inter-pulse time reduces the diffusive loss of ions and enhances the recombination reactions, which produce the afterpeak. We also reveal that the radiation trapping and high energy level argon must be considered to simulate the afterpeak in atmospheric conditions. The improved understanding of the afterpeak dynamics can be utilized to optimize the power coupling and/or generation of reactive species.

I-mode pedestal relaxation events in the Alcator C-Mod and ASDEX Upgrade tokamaks

In some conditions, I-mode plasmas can feature pedestal relaxation events (PREs) that transiently enhance the energy reaching the divertor target plates. To shed light on their appearance, characteristics and energy reaching the divertor targets, a comparative study between two tokamaks — Alcator C-Mod and ASDEX Upgrade — is carried out. It is found that PREs appear only in a subset of I-mode discharges, mainly when the plasma is close to the H-mode transition. Also, a growing oscillating precursor before the PRE onset is observed in the region close to the separatrix in both devices, and a discussion on a possible triggering mechanism is outlined. The PRE relative energy loss from the confined region is found to increase with decreasing pedestal top collisionality . Similarly, also the relative electron temperature drop at the pedestal top, which is related to the conductive energy loss, rises with decreasing . Based on these relations, the PRE relative energy loss in future devices such as DEMO and ARC is estimated. Finally, the divertor peak energy fluence due to the PRE is measured on each device. Those values are then compared to the model introduced in Eich et al (2017 Nucl. Mater. Energy 12 84–90) for type-I edge localized modes. The model is shown to provide an upper boundary for PRE energy fluence data, while a lower boundary is found by dividing the model by three. These two boundaries are used to make projections of the PRE divertor energy fluence to DEMO and ARC.

Излучение молекул азота при острой фокусировке фемтосекундных лазерных импульсов в воздухе

A numerical model describing the dynamics of plasma particle density upon filamentation of femtosecond radiation in the air is presented. The simulation results are in good agreement with the experimental data. The pumping processes of the N2 and N2+ radiative levels are investigated. The model predicts a sharp drop in electron temperature and density within 1 ns. For the first positive nitrogen system, an excess of the population of the upper radiation level over the population of the lower one is observed for 550 ps.

Cooling of accretion disc coronae by Type I X-ray bursts

ABSTRACT Although accretion disc coronae appear to be common in many accreting systems, their fundamental properties remain insufficiently understood. Recent work suggests that Type I X-ray bursts from accreting neutron stars provide an opportunity to probe the characteristics of coronae. Several studies have observed hard X-ray shortages from the accretion disc during an X-ray burst implying strong coronal cooling by burst photons. Here, we use the plasma emission code eqpair to study the impact of X-ray bursts on coronae, and how the coronal and burst properties affect the coronal electron temperatures and emitted spectra. Assuming a constant accretion rate during the burst, our simulations show that soft photons can cool coronal electrons by a factor of ≳ 10 and cause a reduction of emission in the 30–50 keV band to $\lesssim 1{{\ \rm per\ cent}}$ of the pre-burst emission. This hard X-ray drop is intensified when the coronal optical depth and aspect ratio is increased. In contrast, depending on the properties of the burst and corona, the emission in the 8–24 keV band can either increase, by a factor of ≳ 20, or decrease, down to $\lesssim 1{{\ \rm per\ cent}}$ of the pre-burst emission. An increasing accretion rate during the X-ray burst reduces the coronal cooling effects and the electron temperature drop can be mitigated by $\gtrsim 60{{\ \rm per\ cent}}$. These results indicate that changes of the hard X-ray flux during an X-ray burst probe the geometrical properties of the corona.

Comparative analysis and new post-processing methods for plasma tomography at tokamaks

The JET tokamak is equipped with extensive diagnostic systems for tomography, including bolometers and soft X-ray (SXR) detectors. The tomographic reconstructions presented in this contribution were obtained using an algorithm based on Tikhonov regularisation and Minimum Fisher information. A new method for subsequent interpretative analysis of the tomographic reconstructions will be introduced. It is based on a comparison of radiated power from different regions of reconstruction, that can be defined by a rectangle or by a flux surface. The method was also used for comparative analysis of data obtained by reconstructing signals from different detectors. Both combined and separate reconstructions will be analysed. Example applications are presented, showing electron temperature drop, sawteeth instability and transport after laser blow-off.

Investigation of the transition of multicycle AC operation in ISTTOK under edge electrode biasing

In this paper we present recent results obtained on plasma edge electrode biasing during AC discharges. The goal is to obtain experimental evidence on a number of plasma parameters that can play a role during the AC transition on the repeatability and reproducibility of AC operation. The control of the plasma density in the quiescent phase is made just before the AC transition by means of positive edge biasing leading to a transitory improved of density (30%–40%). Gas puff experiments show that the increase of background gas pressure during discharge led to a better success of the AC transition. The experimental results indicate that the increase of density during the AC transition induced by edge biasing is followed by an electron temperature drop. The drop in electron temperature leads in most cases the formation of runaway electrons. It has been observed that the runaway population during discharge flattop depends on the interplay between gas content and plasma density and temperature. The results also confirm that the correct balance of external magnetic fields is crucial during the AC transition phase where drift electron currents are formed. The results from the heavy ion beam diagnostic show that the formation of plasma current during consecutive AC transitions is asymmetric. Numerical simulations indicate that for some particular conditions this result could be reproduced from assuming the presence of two counter-currents during AC transition.

A comparison between a refined two-point model for the limited tokamak SOL and self-consistent plasma turbulence simulations

A refined two-point model is derived from the drift-reduced Braginskii equations for the limited tokamak scrape-off layer (SOL) by balancing the parallel and perpendicular transport of plasma and heat and taking into account the plasma–neutral interaction. The model estimates the electron temperature drop along a field line, from a region far from the limiter to the limiter plates. Self-consistent first-principles turbulence simulations of the SOL plasma including its interaction with neutral atoms are performed with the GBS code and compared to the refined two-point model. The refined two-point model is shown to be in very good agreement with the turbulence simulation results.

Validation of TOKES vapor shield simulations against experiments in the 2MK-200 facility

Measurements of tungsten (W) plasma shield dynamics have been performed in the 2MK-200 facility under ITER relevant disruptive plasma heat flux densities of ∼100GW/m2. Plasma temperature profiles at two time instants and time dependent electron density profiles are available from these experiments. These data have been used to validate TOKES code simulations of the W plasma shield which is computed to form in front of the ITER W divertor targets during disruptions. The experimental and simulated temperature profiles are in a good agreement for the measurement points at distances of 1–5cm from the target. An important feature of the W plasma shield seen in the code simulations is the sharp electron temperature drop very close to the target. This region is not resolved in the 2MK-200 facility because of unacceptably low signal/noise ratio. Further experiments in a new, larger plasma gun are planned for experimental validation of this feature.

Physics of a magnetic filter for negative ion sources. I. Collisional transport across the filter in an ideal, 1D filter

Magnetic filters are used in negative ion sources for neutral beam injection in fusion devices to reduce the electron temperature in the extraction region in order to limit negative ion destruction by fast electrons. The drop in electron temperature through the filter is due to the enhanced residence time and collisional energy losses of electrons trapped in the magnetic field. The mechanisms controlling particle and energy transport through the magnetic filter in negative ion sources of the ITER type are still not clear and the aim of this paper is to clarify and quantify these mechanisms. A particle-in-cell Monte Carlo simulation is used to revisit and analyze the role and operation of the magnetic filter in an “ideal” one-dimensional configuration and to study the stability of the one-dimensional solution in a two-dimensional configuration with periodic boundary conditions. The roles of collisions and instabilities on electron transport through the filter are discussed. The influence of a more realistic geometry on electron transport through the filter is analyzed in the companion paper [Boeuf et al., Phys. Plasmas 19, 113510 (2012)].

Electron temperature and density profile evolution during the edge-localized mode cycle in ohmic and electron cyclotron-heated H-mode plasmas in TCV

The dynamics of electron temperature and density profiles during quasi-stationary H-mode phases in the TCV tokamak has been investigated for type III and type I edge-localized modes (ELMs) using electron cyclotron heating (ECH) to vary the collisionality. At heating power levels close to the threshold for the L–H transition, ELMs of type III were observed, with an energy loss per ELM below 10% of the total plasma energy. Electron temperature and pressure showed no significant increase during the phase before the ELM crash. ELMs of type I were characterized by a lower repetition rate, but caused fractional energy losses reaching 20%. For this ELM type, a clear increase in pedestal pressure and pressure gradient was observed before the collapse associated with the ELM event. The rapid drop in electron temperature also affected the plasma core explaining the rather large energy losses per ELM. Ideal MHD stability calculations of the edge pedestal with the KINX code showed that high-n ballooning modes restricted the achievable pressure gradient at high collisionality and ELMs of type III. With additional heating and type I ELMs, stability limits were set by medium-n kink-ballooning modes. Comparing the values of the pressure gradients and current densities obtained from experimental data with those of ideal MHD stability calculations showed good agreement. The model, however, needs to account for the radial displacement of the location of maximum pressure gradient during the ELM cycle.

Modeling of Plasma Parameters during High Pressure Gas Injection with a Three Temperature Model

AbstractA three temperature model is constructed to describe dynamics of plasma parameters during argon injection, which is aimed to mitigate disruption consequences. Model predicts fast decrease of electron and ion temperatures, distribution of ionization stages and average ion charge in time, the rate of total radiation losses and time dependent intensities of ArXVII lines, widely used in plasma diagnostics. It was found that rapid drop of electron temperature is not associated with radiation losses (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The ionospheric behavior in conjugate hemispheres during the 3 October 2005 solar eclipse

Abstract. We investigate the ionospheric behavior in conjugate hemispheres during the 3 October 2005 solar eclipse, on the basis of observations of electron temperature (Te) from the Defense Meteorological Satellites Program (DMSP) spacecraft, F2 layer critical frequency (foF2) and F2 layer peak height (hmF2) at the Grahamstown ionosonde station, and total electron content (TEC) from the Global Positioning System (GPS) station SUTH. The observations show that when the eclipse occurred in the Northern Hemisphere, there was a decrease in Te, an increase in foF2 and TEC, and an uprising in hmF2 in its conjugate region compared with their reference values. We also simulated the ionosphere behavior during this eclipse using a mid- and low-latitude ionospheric model. The simulations agree well with the observations. Because of the eclipse effect, there are far fewer photoelectrons travelling along the magnetic field lines from the eclipse region to the conjugate region, resulting in reduced photoelectron heating in the conjugate hemisphere which causes a drop in electron temperature and subsequent disturbances in the region.

Effect of low-frequency power on dual-frequency capacitively coupled plasmas

In low-pressure dual-frequency capacitively coupled plasmas driven with 60/13.56 MHz, the effect of low-frequency power on the plasma characteristics was investigated using a compensated Langmuir electrostatic probe. At lower pressures (about 10 mTorr), it was possible to control the plasma density and the ion bombardment energy independently. As the pressure increased, this independent control could not be achieved. As the low-frequency power increased for the fixed high-frequency power, the electron energy probability function (EEPF) changed from Druyvesteyn-like to Maxwellian-like at pressures of 50 mTorr and higher, along with a drop in electron temperature. The plasma parameters were calculated and compared with simulation results.

Effects of relativistic frequency downshift on measurements of electron cyclotron emission in Large Helical Device plasma

The effects of relativistic down shift frequency on the determination of the electron temperature profile from electron cyclotron emission (ECE) in a large helical device(LHD) have been studied by a numerical calculation. since the value of toroidal magnetic field has maximum at the magnetic axis along the sight line in the LHD, the emissivity at the magnetic axis decreases due to the relativistic effect. We found an apparent drop in electron temperature around the magnetic axis due to the relativistic effect. The parametric dependences of the relativistic effect on electron temperature measurement have been studied. When the electron temperature is higher and/or optical thickness is thinner, the gap from the nonrelativistic electron cyclotron frequency is wider. In the case of Te(0)=5 keV and ne=1×1019 m−3, the frequency gap is about 0.5 GHz. Calculation shows that the electron temperature at the central region (ρ<0.1) apparently decreases less than one-half even at 5 keV, in the case of ne=1×1019 m-3 due to the its unique magnetic configuration. Therefore, it is necessary for precise measurement of the electron temperature profile to take into account the relativistic downshift frequency in LHD plasma. To verify the relativistic effect on electron temperature measurement from ECE in the LHD experimentally, the parameter region is evaluated numerically. It is possible to verify the relativistic effect in LHD experimentally.

Pressure dependent mode transition in an electron cyclotron resonance plasma discharge

Despite the wide range of applications of the electron cyclotron resonance (ECR) plasma sources, the ECR plasma processing control is tricky at certain operation parameters. There are several reports of regimes where abrupt changes and instabilities in plasma parameters occur. In the present work we report extensive probe measurements of plasma potential, electron temperature, ion beam energy, and velocity as well as plasma density over a mode change appearing when the neutral argon gas pressure is changed. The parameters were measured over the entire pressure range from 0.15 to 7 mTorr. We found a large drop in electron temperature and plasma potential when the pressure increased from 0.15 to 0.4 mTorr. At 0.4 mTorr the temperature reached a minimum and the density a local maximum, while at 1 mTorr the density reached a minimum. When increasing the pressure above 1.2 mTorr the temperature decreased and the density increased rapidly. While the plasma appearance at low and high pressure can be explained by a global conservation model, the behavior in the intermediate pressure between 0.4 and 1.2 mTorr needs a deeper investigation. We discuss the possibility that the mode change is connected to the competition between stepwise and direct ionization of neutral argon and excitations to metastable argon atoms.

Sub-Micron Normal-Insulator-Superconductor Tunnel Junctions for Sensitive Thermometry and Micro-Refrigerators

We report on measurements of normal-insulator-superconductor tunnel junctions at temperatures below 1K. The junctions have been fabricated by shadow mask technique and e-beam lithography and have submicron dimensions. The measurements on our junctions show that their characteristics are very sensitive to temperature changes making them suitable to be used as a sensitive secondary thermometer. To demonstrate this we use this tunnel structure for sensing the electron temperature drop in a copper strip of an electronic micro-refrigerator, that was made by arrays of tunnel junctions, as well.

Electron cooling by arrays of submicron tunnel junctions

We cooled down the electrons of a 3 μm3 copper strip by using an on-chip electronic microcooler made by a superconductor-insulator-normal-insulator-superconductor (SINIS) structure. We obtained an electron temperature drop ΔT≃−10 mK at a temperature of the bath slightly above Tb=500 mK and we estimated a cooling power slightly larger than 20 pW at this temperature. In contrast to previous experiments, we obtained this result operating with a simple He3 refrigerator. The copper strip temperature change was sensed by measuring the voltage change across a current-biased SINIS junction, with a temperature responsivity ΔV/ΔT≃−500 μV/K. On-chip microcooler and thermometer have been fabricated by two-angle shadow mask evaporation and e-beam lithography.

Transient transport phenomena induced by cold pulses in W7-AS

Cold-pulse experiments were carried out in the W7-AS stellarator for the first time. Carbon was injected by a laser blow-off system into the plasma edge. The electron density increase due to the injected carbon is found to be responsible for the edge electron temperature drop. In all cases, the propagation of the edge temperature perturbation to the plasma centre could be modelled with a local parameter-dependent electron heat diffusivity. Unlike in tokamaks, non-local transport effects were not observed in these experiments.

On the Role of Neutral Particles on Edge Turbulence in the CT-6B Tokamak

A positive limiter biasing on the CT-6B tokamak resulted in a slight increase of edge density and an obvious drop of electron temperature, which is in the temperature range where the rate coefficients for ionization processes are strongly temperature dependent. Concomitantly, edge fluctuations are enhanced, suggesting that the presence of neutral particles may play an important role in modifying edge turbulence.