Local Electron Temperature Research Articles

R&D on a high energy accelerator and a large negative ion source for ITER

R&D pertaining to a 1 MeV accelerator and a large negative ion source has been carried out at the Japan Atomic Energy Research Institute for the ITER neutral beam system. The R&D is, at present, progressing towards: (1) 1 MeV acceleration of H− ion beams at the ITER relevant current density of 200 A m−2 and (2) improvement of uniform negative ion production over wide extraction area in large negative ion sources. Recently, H− ion beams of 1 MeV and 140 mA level have been generated with a substantial beam current density (100 A m−2). In the uniformity study, it was clarified that the electron temperature in the ion extraction region was locally high (>1 eV), which resulted in the destruction of negative ions at a high reaction rate. Interception of fast electrons leaking through a transverse magnetic field called ‘magnetic filter’ has been found to be effective in lowering the local electron temperature, followed by an improvement of negative ion beam profile.

Spatial Distributions of Electron Temperature in Quantum Hall Systems with Slowly-Varying Confining Potentials

Spatial distributions of the electron temperature perpendicular to the applied current in quantum Hall systems with slowly-varying confining potentials are calculated in the linear-response regime by employing hydrodynamic equations for number conservation and energy conservation. The electron temperature exhibits spatial pattern, reflecting the confining potential, along the Hall field induced by the current. The local electron temperature shows oscillations as a function of the filling factor. Such spatially dependent electron temperature causes a certain change in distributions of the current density and the current-induced potential even in the linear-response regime.

Electron density and temperature determination using the concept of particle confinement time uniqueness

The use of atomic hydrogen line emission to determine the particle confinement time τp of a tokamak plasma is a well-known diagnostic technique. Using such a method, for any one of the emission lines, be it from Lyman, Balmer, or Paschen series, the same (i.e., unique) value of τp must be obtained. Furthermore, this measurement is directly related to the local values of the electron temperature and density. We have developed a method based on the Hα, Hβ, and Hγ hydrogen line emissions and on the concept of τp uniqueness for a tokamak plasma, to determine the local electron density and temperature. The technique has been applied to plasma discharges generated in the NOVA-UNICAMP tokamak. The results show good agreement with measurements from multichannel Thomson scattering and Langmuir probe. A procedure to simulate the Hα emissivity radial profile using the obtained results is also discussed.

2-D imaging of electron temperature in Tokamak plasmas

By taking advantage of recent developments in millimeter wave imaging technology, an electron cyclotron emission imaging (ECEI) instrument, capable of simultaneously measuring 128 channels of localized electron temperature over a two-dimensional (2-D) map in the poloidal plane, has been developed for the TEXTOR tokamak. Data from the new instrument, detailing the magnetohydrodynamic (MHD) activity associated with a sawtooth crash, is presented.

Local effects of gas fuelling and their impact on transport processes in the plasma edge of the tokamak TEXTOR

Deuterium fuelling through a carbon test limiter has been applied to maximize the plasma density in the Radiative Improved Mode. The impact of the fuelling on the local plasma edge properties has been investigated, by analyzing the spectral emission of both deuterium atoms and molecules, which indicates the creation of a cold and dense plasma cloud with a local electron density at the last closed flux surface up to 8×1019m−3, about 4 times higher than the electron density far away from the puffing location at the same plasma radius. The local electron temperature decreases to less than 10eV. The experimental data can be reproduced by a model for the development of the cold plasma cloud and the critical fuelling rate to initiate the process based on the heat balance in the cloud. The correlation of the resulting local perturbation with the global confinement properties is discussed.

Simulation of turbulence in the divertor region of tokamak edge plasma

Results are presented for turbulence simulations with the fluid edge turbulence code BOUT [X.Q. Xu, R.H. Cohen, Contr. Plas. Phys. 36 (1998) 158]. The present study is focussed on turbulence in the divertor leg region and on the role of the X-point in the structure of turbulence. Results of the present calculations indicate that the ballooning effects are important for the divertor fluctuations. The X-point shear leads to weak correlation of turbulence across the X-point regions, in particular for large toroidal wavenumber. For the saturated amplitudes of the divertor region turbulence it is found that amplitudes of density fluctuations are roughly proportional to the local density of the background plasma. The amplitudes of electron temperature and electric potential fluctuations are roughly proportional to the local electron temperature of the background plasma.

Characterization of a laser-induced plasma by spatially resolved spectroscopy of neutral atom and ion emissions.: Comparison of local and spatially integrated measurements

The local values of the parameters that characterize a laser-induced plasma (temperature, electron density, relative number densities of neutral atoms and ions) have been obtained by spatially resolved emission spectroscopy, including the deconvolution of the measured intensity spectra. The plasma has been generated using a Nd:YAG laser with a Fe–Ni alloy in air at atmospheric pressure, and the emission in the time window 3.0–3.5 μs has been detected. The temperature values obtained from neutral atom and ion emissions have been compared in the cases of local and spatially-integrated measurements. Local Boltzmann and Saha–Boltzmann plots with high correlation to linear fittings have been obtained using two broad sets of optically thin neutral atom and ion lines (21 Fe I lines and 15 Fe II lines), resulting in local values of the electronic temperature that coincide within the error. These results of local measurements contrast with those of spatially integrated measurements, for which two different temperatures are obtained from the Boltzmann plots of neutral atoms (9100±150 K) and ions (13 700±300 K). This difference is explained according to the measured distributions of the electronic temperature and the neutral atom and ion number densities, that result in separated emissivity (or population) distributions of neutral atom and ion lines, leading to different neutral atom and ion apparent temperatures (population-averages of the local electronic temperature). Local values of the plasma parameters have been obtained at all the positions with significant emission, including the determination of the electronic temperature from Saha–Boltzmann or Boltzmann plots. The ionization degree is high- and low-varying at the inner part of the plasma, decaying only near the plasma front. The maximum of the ion density does not coincide with the temperature maximum; on the contrary, the axial variation of both the neutral atom and ion densities (that decrease towards the sample surface) is opposite to that of the temperature, a behaviour that is interpreted to result from the plasma expansion process.

Study of particle behaviour at the edge in HT-7 tokamak

The cross-field diffusion coefficient (D⊥) at the edge in the HT-7 tokamak is close to the Bohm value when the line average electron density ranges from 1.5×1019 to 3.0×1019m−3. The energy profile of the particles is derived directly from the Hα(Dα) line shape; the dissociative excitation of molecules is dominating when the local electron temperature is above 10eV. By means of the Monte Carlo method the Dα line shape is also simulated. We find that the molecular dissociation contributes to 57% of neutral atoms and 53% of emission intensity in front of the limiter and 85% of neutral atoms and 82% of emission intensity in front of the wall. The influence of atomic and molecular processes on the energy balance is discussed for the scrape-off layer (SOL) and the power loss from molecular dissociation is found to be 6×104kW at the SOL. The ion Bernstein wave (IBW) can effectively suppress the magnetohydrodynamic behaviour, the fluctuation levels and the turbulence; the D⊥ in front of the limiter declines from 0.84 to 0.2m2.s−1 and the particle confinement time rises from 9 to 12ms.

The Cassini Radio and Plasma Wave Investigation

The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn. Three nearly orthogonal electric field antennas are used to detect electric fields over a frequency range from 1 Hz to 16 MHz, and three orthogonal search coil magnetic antennas are used to detect magnetic fields over a frequency range from 1 Hz to 12 kHz. A Langmuir probe is used to measure the electron density and temperature. Signals from the electric and magnetic antennas are processed by five receiver systems: a high frequency receiver that covers the frequency range from 3.5 kHz to 16 MHz, a medium frequency receiver that covers the frequency range from 24 Hz to 12 kHz, a low frequency receiver that covers the frequency range from 1 Hz to 26 Hz, a five-channel waveform receiver that covers the frequency range from 1 Hz to 2.5 kHz in two bands, 1 Hz to 26 Hz and 3 Hz to 2.5 kHz, and a wideband receiver that has two frequency bands, 60 Hz to 10.5 kHz and 800 Hz to 75 kHz. In addition, a sounder transmitter can be used to stimulate plasma resonances over a frequency range from 3.6 kHz to 115.2 kHz. Fluxes of micron-sized dust particles can be counted and approximate masses of the dust particles can be determined using the same techniques as Voyager. Compared to Voyagers 1 and 2, which are the only spacecraft that have made radio and plasma wave measurements in the vicinity of Saturn, the Cassini radio and plasma wave instrument has several new capabilities. These include (1) greatly improved sensitivity and dynamic range, (2) the ability to perform direction-finding measurements of remotely generated radio emissions and wave normal measurements of plasma waves, (3) both active and passive measurements of plasma resonances in order to give precise measurements of the local electron density, and (4) Langmuir probe measurements of the local electron density and temperature. With these new capabilities, it will be possible to perform a broad range of studies of radio emissions, wave-particle interactions, thermal plasmas and dust in the vicinity of Saturn.

Upper limit on turbulent electron temperature fluctuations in the core of Alcator C-Mod

Electron cyclotron emission correlation radiometry is used to measure the local turbulent electron temperature fluctuations in the plasma core of the Alcator C-Mod tokamak. Using standard analysis techniques, we see no evidence of the broadband fluctuations seen in other devices. From an analysis of the diagnostic resolution, coupled with these data, we can place an upper limit on the measurable fluctuation amplitude and set bounds for the wave number spectrum required for the electrostatic turbulence model of anomalous heat flux to be valid.

Electron-beam-generated plasmas for materials processing

The results of investigations aimed at characterizing pulsed, electron-beam-produced plasmas for use in materials processing applications are discussed. In situ diagnostics of the bulk plasma and at the plasma/surface interface are reported for plasmas produced in Ar, N 2, and mixtures thereof. Langmuir probes were employed to determine the local electron temperature, plasma density, and plasma potential within the plasma, while ion energy analysis and mass spectrometry were used to interrogate the ion flux at an electrode located adjacent to the plasma. The results illustrate the unique capabilities of electron-beam-produced plasmas and the various parameters available to optimize operating conditions for applications such as nitriding, etching, and thin film deposition.

Turbulence in the Divertor Region of Tokamak Edge Plasma

AbstractResults of recent modeling of tokamak edge plasma with the turbulence code BOUT are presented. In previous studies with BOUT, the background profiles of plasma density and temperature were set as flux surface functions. However, in the divertor region of a tokamak, the temperature is typically lower and density is higher than those at the mid‐plane. To account for this in the present study, a poloidal variation of background plasma density and temperature is included to provide a more realistic model. For poloidally uniform profiles of the background plasma, the calculated turbulence amplitude peaks near outer mid‐plane, while in the divertor region the amplitude is small. However, present simulations show that as the background plasma profiles become more poloidally non‐uniform, the amplitude of density fluctuations, ñi, starts peaking in the divertor. It is found that in the divertor region the amplitude of ni fluctuations grows approximately linearly with the local density of the background plasma, ni0, while the amplitudes of Te and ϕ fluctuations are positively correlated with the local electron temperature, Te0. Correlation analysis shows that plasma turbulence above and below the x‐point is largerly uncorrelated. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ELM driven divertor target currents on TCV

There is currently both considerable interest in the physics of ELM transport in the scrape-off layer (SOL) and concern over the impact of the ELM power and particle loads on the divertor targets of future fusion reactors. This paper describes some experimental observations of the reaction of target floating potentials and currents during ELMs in TCV, relying principally on fast measurements of these parameters using tile embedded Langmuir probe arrays. Clear evidence is presented for rapid modifications in the local target floating potentials occurring long before the characteristic rise of hydrogenic excitation emission due to local recycling provoked by the arrival at the target of the ELM ion flux. This precursor activity appears to be synchronous with the growth of MHD modes in the main chamber. Simple conditional averaging is used to derive a ‘coherent ELM’ and thus generate the radial distribution of the current to probes held at the target potential. At some locations, the coherent ELM can also be used to estimate the time evolution of the local target electron temperature, density and power flux, even though these quantities are not directly measured. The time delays between the reactions of currents, floating potentials and derived temperature are consistent with the expections of recently published kinetic simulations of ELM energy transport down a one-dimensional SOL plasma. The strong potential variations observed during the ELM are the result of current flows at the targets. These currents are generally of opposite sign at inner and outer divertors and are thus consistent, at least in part, with a thermoelectric origin in which the driven current is produced by an in/out divertor temperature asymmetry near the strike point of nearly a factor 2, probably due to the generation of divertor asymmetries by the ELM heat pulse. Such asymmetries are commonly observed in low to medium density L-modes for the particular TCV magnetic equilibrium studied in this paper. The total current balance during the ELM is satisfied only to within a factor 2, so that, whilst some of the driven current flows parallel to field lines in the SOL, there is an apparent additional negative current to the inner divertor during the ELM whose origin remains unexplained. It might, however, be due, in part, to increased plasma–wall interaction in the main chamber during the ELM event.

Local thermometry technique based on proximity-coupled superconductor/normal-metal/superconductor devices

In mesoscopic superconductor/normal-metal/superconductor heterostructures, it is known that the resistance of the normal metal between the superconductors has a strong temperature dependence. Based on this phenomenon, we have developed a type of thermometer, which dramatically enhances our ability to measure the local electron temperature Te at low temperatures. Using this technique, we have been able to measure small temperature gradients across a micron-size sample, opening up the possibility of quantitatively measuring the thermal properties of mesoscopic devices.

Energy Distribution of Recycling Particles in Front of theLimiter and the Wall on the HT-7 Tokamak

The Hα (Dα) line shape in front of the limiterand the wall is measured by two sets of optical spectroscopymulti-channel analysers. The energy distribution is derived directlyfrom the Hα (Dα) line shape, especially showingseveral molecular processes. The dissociative excitation of moleculesis dominating when the local electron temperature is above 10 eV. TheDα line shape is also simulated by the Monte Carlo method,the result shows that the molecular dissociation contributes 57%neutral atoms, 53% emission intensity in front of the limiter, and85% neutral atoms, 82% emission intensity in front of the wall. Thepower loss from molecular dissociation is about 5.6×104 kW/m-3 and the molecular dissociation is an importantcooling mechanism at the plasma edge.

Femtosecond pump–probe nondestructive examination of materials (invited)

Ultrashort-pulsed lasers have been demonstrated as effective tools for the nondestructive examination (NDE) of energy transport properties in thin films. After the instantaneous heating of the surface of a 100 nm metal film, it will take ∼100 ps for the influence of the substrate to affect the surface temperature profile. Therefore, direct measurement of energy transport in a thin film sample requires a technique with picosecond temporal resolution. The pump–probe experimental technique is able to monitor the change in reflectance or transmittance of the sample surface as a function of time on a subpicosecond time scale. Changes in reflectance and transmittance can then be used to determine properties of the film. In the case of metals, the change in reflectance is related to changes in temperature and strain. The transient temperature profile at the surface is then used to determine the rate of coupling between the electron and phonon systems as well as the thermal conductivity of the material. In the case of semiconductors, the change in reflectance and transmittance is related to changes in the local electronic states and temperature. Transient thermotransmission experiments have been used extensively to observe electron-hole recombination phenomena and thermalization of hot electrons. Application of the transient thermoreflectance (TTR) and transient thermotransmittance (TTT) technique to the study of picosecond phenomena in metals and semiconductors will be discussed. The pump–probe experimental setup will be described, along with the details of the experimental apparatus in use at the University of Virginia. The thermal model applicable to ultrashort-pulsed laser heating of metals will be presented along with a discussion of the limitations of this model. Details of the data acquisition and interpretation of the experimental results will be given, including a discussion of the reflectance models used to relate the measured changes in reflectance to calculated changes in temperature. Finally, experimental results will be presented that demonstrate the use of the TTR technique for measuring the electron–phonon coupling factor and the thermal conductivity of thin metallic films. The use of the TTT technique to distinguish between different levels of doping and alloying in thin film samples of hydrogenated amorphous silicon will also be discussed briefly.

Neutral Rydberg matter clusters from K: extreme cooling of translational degrees of freedom observed by neutral time-of-flight

Neutral electronically excited clusters K N * with N=2–7 are formed and studied by neutral time-of-flight (TOF) caused by laser induced Coulomb explosions in a cloud of Rydberg Matter (RM). A thermal velocity distribution would give TOF peaks with widths of the order of 100 μs . Instead, the minimum half-width is 5 μs , indicating supersonic velocities with an effective temperature of <20 K. This cooling is proposed to be caused by energy transfer from the core ions to the electrons in the conduction band in RM and by evaporation of thermal electrons over the work function barrier of ≲0.5 eV. The small size of the clusters also implies that normally no electrons exist above the Fermi level, corresponding to very low local electron temperatures. Evidence for the geometrical extension of the cloud of RM in the chamber is given.

Inclusion of Radiation Effects in Plasma-Neutral Codes

Experiments on Alcator C-Mod have demonstrated edge plasma conditions with optically thick hydrogenic Lyman lines [1, 2]. In these high opacity plasma regions, the total neutral particle density and excited-state populations depend on the radiation field. Current edge plasma-neutral fluid codes ignore this dependence in their atomic physics models. For instance, collisional-radiative models are commonly used to define effective ionization, recombination and energy loss rates exclusively as functions of the local electron density and electron temperature [3]. To include radiation effects in plasma-neutral codes, we are working towards two goals: (1) extending the local atomic physics model to account for a spatially varying radiation field; and (2) driving the extended model with results from a non-local thermodynamic equilibrium atomic kinetics and radiation transfer code.

Inclusion of Radiation Effects in Plasma-Neutral Codes

Experiments on Alcator C-Mod have demonstrated edge plasma conditions with optically thick hydrogenic Lyman lines [1, 2]. In these high opacity plasma regions, the total neutral particle density and excited-state populations depend on the radiation field. Current edge plasma-neutral fluid codes ignore this dependence in their atomic physics models. For instance, collisional-radiative models are commonly used to define effective ionization, recombination and energy loss rates exclusively as functions of the local electron density and electron temperature [3]. To include radiation effects in plasma-neutral codes, we are working towards two goals: (1) extending the local atomic physics model to account for a spatially varying radiation field; and (2) driving the extended model with results from a non-local thermodynamic equilibrium atomic kinetics and radiation transfer code.

Interior and Exterior Laser-Induced Fluorescence and Plasma Measurements within a Hall Thruster

Abstract : We describe results of a study of emissive-probe-based plasma potential measurements and laser-induced fluorescence velocimetry of neutral and singly ionized xenon in the plume and interior portions of the acceleration channel of a Hall thruster plasma discharge operating at powers ranging from 250 to 725 W. Axial ion and neutral velocity profiles for four discharge voltage conditions (100, 160, 200, and 250V) are measured as are radial ion velocity profiles in the near-field plume. Axial ion velocity measurements both inside and outside the thruster as well as radial velocity measurements outside the thruster are performed using laser-induced fluorescence with nonresonant signal detection. Neutral axial velocity measurements are similarly performed in the interior of the Hall thruster with resonance fluorescence collection. Optical access to the interior of the Hall thruster is provided by a 1-mm-wide axial slot in the outer insulator wall. The majority of the ion velocity measurements used partially saturated fluorescence to improve the signal-to-noise ratio. Probe-based plasma potential measurements extend from 50 mm outside the thruster exit plane to the near anode region for all but the highest discharge voltage condition. For each condition, the axial electric field is calculated from the plasma potential, and the local electron temperature is determined from the difference between the floating and plasma potentials. These two sets of measurements delineate the structure of the plasma and indicate that the ionization and acceleration regions are somewhat separated. Also, these measurements indicate a region of low electric field near the thruster exit, especially at the higher discharge voltages. This region of near constant potential (low electric field) may be a result of oscillations, which enhance the local plasma conductivity.