Time-resolved Electron Temperature Research Articles

Time-Resolved Analysis of the Electron Temperature in RF Magnetron Discharges with a Pulsed Gas Injection

Pulsed gas injection in a plasma can affect many fundamentals, including electron heating and losses. The case of an asymmetric RF magnetron plasma with a pulsed argon injection is analyzed by optical emission spectroscopy of argon 2p-to-1s transitions coupled with collisional-radiative modeling. For a fully detailed population model of argon 2p levels accounting for direct and stepwise electron-impact excitation in optically thick conditions, a rapid decrease in the electron temperature, Te, is observed during each gas injection with the sudden pressure rise. The opposite trend, with unrealistic Te values before and after each pulse, is observed for analysis based on simple corona models, thus emphasizing the importance of stepwise excitation processes and radiation trapping. Time-resolved electron temperature variations are directly linked to the operating parameters of the pulsed gas injection, in particular the injection frequency. Based on the complete set of data, it is shown that the instantaneous electron temperature monotonously decreases with increasing pressure, with values consistent with those expected for plasmas in which charged species are produced by electron-impact ionization of ground state argon atoms and lost by diffusion and recombination on plasma reactor walls.

Temporal and spatial properties of plasma induced by infrared femtosecond laser pulses in air

Space and time-resolved electron density and temperature of the plasma, created in air by focused femtosecond laser pulses have been investigated as a function of the pump pulse energy and duration. For the air ionization the infrared (1030 nm) femtosecond (190–500 fs) Yb:KGW laser pulses of up to 1 mJ energy were used. Based on the Stark broadening of the oxygen-I 777.19 nm line we have found that after establishing a local equilibrium the density of laser-created plasma could exceed 1017 cm−3 with the electron temperature of over 5000 °C. Obtained results agree well with the results of previously reported measurements of the plasma density created by the femtosecond near-infrared Ti:sapphire laser pulses.

Emission characteristics and dynamics of neutral, ionic and molecular species in a laser produced CaF2 plasma

The emission characteristics of a laser-ablated calcium fluoride CaF2 plasma have been investigated using time and space optical emission spectroscopy. The plasma-plume was produced by focusing 266 nm, 15 ns pulses from a Nd: YAG laser on a calcium fluoride crystal substrate placed in a vacuum chamber. The emission observed in the plasma region is due to electronic relaxation of excited neutral Ca and F, ionic fragments Ca2+, Ca+ and F+ and molecular features of CaF (A2Π- X2Σ+, B2Σ+- X2Σ+ and C2Π- X2Σ+) band systems. Plasma species were analyzed both as a function of the delay from the laser pulse incidence and location from the target surface. Spectroscopic diagnostics were employed to estimate time-resolved electron densities, electron temperatures and velocity expansion of different species. The results show a faster decay of the continuum emission, ionic F+, Ca2+, Ca+, F species than in the case of neutral Ca atoms and molecular species of CaF. Different excitation temperatures were measured by comparing Ca and F atomic/ionic Saha-local thermodynamic equilibrium (LTE) spectra with experimental ones at various delay times after the ablation event. Vibrational temperatures of CaF were assessed from the temporal analysis of the A2Π - X2Σ+ Δv = 0 band sequence molecular emission. We observed a non-equilibrium plasma although the plasma is found to maintain partial LTE.

Two events of spectral enhancement in spatially confined laser-induced Cu plasmas

This study mainly discussed the effect of spatial confinement on the line emission of laser-excited breakdown spectroscopy with cylindrical cavity. A Q-switched nanosecond laser ablated Cu target to generate plasma. When the cylindrical cavity was presented, the significant enhancement in the spectral emission was observed. And, the time-resolved spectra without spatial confinement and with different cylindrical cavities were measured. The delay time for the spectral enhancement increased as the cavity diameter increased. We also changed the Y-scale of time-resolved emission intensity plot from linear to logarithmic, so two spectral enhancement events with the delay time were observed due to the two compressions of the plasma by the reflection and re-reflection of the shockwave from the inner surface of the confinement cavity. Also, the calculated time-resolved electron temperatures showed the temperature increase events similar to that of the time-resolved emission.

A multi-channel x-ray temporal diagnostic for measurement of time-resolved electron temperature in cryogenic deuterium-tritium implosions at OMEGA.

Electron-temperature (Te) measurements in implosions provide valuable diagnostic information, as Te is unaffected by residual flows and other non-thermal effects unlike ion temperature inferred from a fusion product spectrum. In OMEGA cryogenic implosions, measurement of Te(t) can be used to investigate effects related to time-resolved hot-spot energy balance. The proposed diagnostic utilizes five fast-rise (∼15 ps) scintillator channels with distinct x-ray filtering. Titanium and stepped aluminum filtering were chosen to maximize detector sensitivity in the 10 keV-20 keV range, as it has been shown that these x rays have similar density and temperature weighting to the emitted deuterium-tritium fusion neutrons. Initial data collected using a prototype nosecone on the existing neutron temporal diagnostic demonstrate the validity of this diagnostic technique. The proposed system will be capable of measuring spatially integrated Te(t) with 20 ps time resolution and <10% uncertainty at peak emission in cryogenic DT implosions.

Influence of sample temperature on nanosecond laser-induced Cu plasma spectra

There are many experimental methods to increase the emission intensity of laser-induced breakdown spectroscopy (LIBS). Among these methods, pre-heating the target is a simple and effective method to improve LIBS signal intensity. This study used Nd: YAG nanosecond laser pulse to ablate Cu target and produce Cu plasma, and time-resolved spectra of the Cu plasma were measured at different Cu temperatures. The measurement suggested that the higher the target temperature, the stronger the time-resolved spectra of the Cu plasma, and the higher the time-resolved electron temperature and density. Next, the morphology of ablation craters was observed by microscope at different target temperatures. As the Cu target was heated, the melting area in the center of ablation crater became larger, which indicated that more mass targets were ablated to produce plasma, thus producing stronger spectral signal. Finally, the thermal behavior of Cu at different target temperatures was simulated by one-dimensional heat conduction equation. The simulated results show that the temperature of the Cu under laser irradiation increased with the increase of initial temperature of the Cu, and the melting depth also increased, which was consistent with the observed results of Cu target morphology.

Time-Resolved Analysis of SF6 Arc Plasmas in a Laboratory Model Chamber for Circuit Breaker

This article presents time-resolved analysis of SF6 arc plasma generated in a laboratory-scale model chamber prepared for studying the characteristics of a circuit breaker. During the arc discharge, the optical emission spectra are measured by a high-speed spectrometer under various operating conditions. The time-resolved electron temperature and density values are determined using the well-known Boltzmann plot and Stark broadening methods, respectively, for the measured Cu I, S II, and F I emission lines. Moreover, the temporal evolution of the electrical conductivity of the arc plasma is deduced from the measured electron temperature and density values, depending on the operating conditions. In our experimental conditions, it is revealed that the electrical conductivity of arc plasmas is mainly determined by Coulomb (electron–ion) collisions as opposed to electron–neutral collisions due to the much higher Coulomb collision frequency compared to the electron–neutral collision frequency. Both the electron temperature and density were found to increase when the discharge current is increased at a fixed gas pressure, resulting in an increase of the electrical conductivity. When the gas pressure inside the model chamber is increased, the electron density is increased as well, but the electrical conductivity does not change significantly due to the slight change in the electron temperature. The present study will be helpful for investigating the performance capabilities of circuit breakers because the electrical conductivity, the most important parameter to guarantee successful circuit breaking at points near the current-zero, is determined by the temperature and density of the electrons.

Time-resolved electron temperature and density of spark discharge assisted femtosecond laser-induced breakdown spectroscopy

In laser-induced breakdown spectroscopy (LIBS), spark-discharge (SP) is an effective auxiliary method to improve spectral emission intensity, and can effectively affect plasma temperature and electron density. In traditional LIBS technology, nanosecond laser is usually used as excitation source. Compared with nanosecond laser, femtosecond (fs) pulse laser has many advantages in the interaction between laser and matter. The SP can also enhance the emission intensity of fs-LIBS. However, the discussion on plasma temperature and electron density of SP assisted fs-LIBS is rare. In this study, we measured time-resolved spectra of fs-LIBS with the SP. By using the Boltzmann plot and the Stark broadening, we calculated time-resolved plasma temperatures and electron densities for different laser energies at different discharge voltages. We hope that this study can provide a better understanding for SP assisted fs-LIBS.

Time-Resolved Measurements of Plasma Parameters for Nanosecond-Pulsed Argon Plasmas

This article aims to understand the physics of pulsed plasmas by investigating the ionization and recombination of pulsed plasmas on the nanosecond timescale and how operating conditions affect the time-resolved electron temperature and electron density of a pulsed plasma. A nanosecond-pulsed argon plasma discharge was generated at various pulse frequencies, widths, and pressures. The argon emission lines were analyzed with a time-synchronized, intensified charge-coupled device (ICCD) spectrometer assembly that gated at 4 ns, and relative intensities of strong argon neutral and ion lines were used in line-ratio calculations. These experimentally determined ratios were compared to theoretical ratios generated from PrismSPECT, a collisional-radiative spectral analysis software, to obtain time-resolved electron temperature ( $\sim 1$ eV) and electron density ( $10^{14}-10^{15}$ cm $^{-3}$ ) over the various operating conditions as well as to discover trends of these plasma parameters over the lifetime of the pulsed plasma. Increasing pulse repetition frequency of the plasma increased the value of maximum $T_{e}$ , shortened the $T_{e}$ rise and decay times, and increased the excited argon neutral population. While $n_{e}$ was relatively independent of pulse frequency, higher pulse frequencies resulted in a faster $n_{e}$ decay. Maximum $T_{e}$ and $n_{e}$ were weakly dependent on applied discharge voltage, but both parameters decayed sooner in time with increasing voltage. Lastly, $T_{e}$ was inversely proportional to pressure, but $n_{e}$ was approximately linear. All three pressures had similar but time-shifted temporal profiles for $n_{e}$ . Plasmas in all operating conditions had ionization and recombination times in the tens of nanoseconds.

Laser-induced breakdown spectroscopy of ammonia gas with resonant vibrational excitation.

In this work, laser-induced breakdown spectroscopy (LIBS) of gaseous ammonia (NH3) molecules on- and off-resonant vibrational excitation was studied in open air. A wavelength-tunable, continuous wave (CW), carbon dioxide (CO2) laser tuned at a resonant absorption peak (9.219 µm) within the infrared radiation (IR) range was used to resonantly excite the vibration of the N-H wagging mode of ammonia molecules. A pulsed Nd:YAG laser (1064 nm, 15 ns) was used to break down the ammonia gas for plasma imaging and spectral measurements. In this study, plasmas generated with the ammonia molecules without additional CO2 laser beam irradiation and with additional CO2 laser beam irradiation with the wavelengths on- and off-resonant vibrational excitation of ammonia molecules were investigated and referred as LIBS, LIBS-RE-ON and LIBS-RE-OFF, respectively. The experimental results showed that the temporal and spatial evolution as well as electron temperature and density of plasmas induced with LIBS and LIBS-RE-OFF were consistent but differed from LIBS-RE-ON. Compared with LIBS and LIBS-RE-OFF, plasmas in LIBS-RE-ON showed larger spatial expansion and enhanced emission after a delay time of 1 µs in this study, as well as significantly enhanced electron temperature by ∼ 64%. Time-resolved electron temperatures and densities showed that the emission signal enhancement in LIBS-RE-ON can be primarily attributed to the electron temperature enhancement. Signal enhancement in LIBS indicated improved detection sensitivity. This study could inspire future works on LIBS for gas detection with improved sensitivity and selectivity probably by using ultrafast/intense laser-induced molecular breakdown/ionization with resonant vibrational excitation of molecules.

Absolute calibration of a time-resolved high resolution x-ray spectrometer for the National Ignition Facility (invited).

A high resolution, Diagnostic Instrument Manipulator (DIM)-based x-ray Bragg crystal spectrometer has been calibrated for and deployed at the National Ignition Facility (NIF) to diagnose plasma conditions in ignition capsules near stagnation times. The spectrometer has two conical crystals in the Hall geometry focusing rays from the Kr Heα, Lyα, and Heβ complexes onto a streak camera, with the physics objectives of measuring time-resolved electron density and temperature through observing Stark broadening and the relative intensities of dielectronic satellites. A third von Hámos crystal that time-integrates the Kr Heα, Heβ and intervening energy range provides in situ calibration for the streak camera signals. The spectrometer has been absolutely calibrated using a microfocus x-ray source, an array of CCD and single-photon-counting detectors, and multiple K- and L-absorption edge filters at the Princeton Plasma Physics Laboratory (PPPL) x-ray laboratory. Measurements of the integrated reflectivity, energy range, and energy resolution for each crystal are discussed. These calibration data provide absolute x-ray signal levels for NIF measurements, enabling precise filter selection and comparisons to simulations.

Implementing time resolved electron temperature capability at the NIF using a streak camera.

A new capability at the National Ignition Facility (NIF) has been implemented to measure the temperature of x-ray emitting sources. Although it is designed primarily for Inertial Confinement Fusion (ICF), it can be used for any hot emitting source that is well modeled. The electron temperature (Te) of the hot spot within the core of imploded ICF capsules is an effective indicator of implosion performance. Currently, there are spatially and temporally integrated Te inferences using image plates. A temporally resolved measurement of Te will help elucidate the mechanisms for hot spot heating and cooling such as conduction to fuel, alpha-heating, mix, and radiative losses. To determine the temporally resolved Te of hot spots, specific filters are added to an existing x-ray streak camera "streaked polar instrumentation for diagnosing energetic radiation" to probe the emission spectrum during the x-ray burn history of implosions at the NIF. One of the difficulties in inferring the hot spot temperature is the attenuation of the emission due to opacity from the shell and fuel. Therefore, a series of increasingly thick titanium filters were implemented to isolate the emission in specific energy regions that are sensitive to temperatures above 3 keV while not significantly influenced by the shell/fuel attenuation. Additionally, a relatively thin zinc filter was used to measure the contribution of colder emission sources. Since the signal levels of the emission through the thicker filters are relatively poor, a dual slit (aperture) was designed to increase the detected signal at the higher end of the spectrum. Herein, the design of the filters and slit is described, an overview of the solving technique is provided, and the initial electron temperature results are reported.

Time-resolved study of the electron temperature and number density of argon metastable atoms in argon-based dielectric barrier discharges

A combination of optical emission spectroscopy and collisional-radiative modelling is used to determine the time-resolved electron temperature (assuming Maxwellian electron energy distribution function) and number density of Ar 1s states in atmospheric pressure Ar-based dielectric barrier discharges in presence of either NH3 or ethyl lactate. In both cases, Te values were higher early in the discharge cycle (around 0.8 eV), decreased down to about 0.35 eV with the rise of the discharge current, and then remained fairly constant during discharge extinction. The opposite behaviour was observed for Ar 1s states, with cycle-averaged values in the 1017 m−3 range. Based on these findings, a link was established between the discharge ionization kinetics (and thus the electron temperature) and the number density of Ar 1s state.

Electron density and electron temperature measurements in nanosecond pulse discharges over liquid water surface

Time-resolved electron density, electron temperature, and gas temperature in nanosecond pulse discharges in helium and O2–He mixtures near liquid water surface are measured using Thomson/pure rotational Raman scattering, in two different geometries, (a) ‘diffuse filament’ discharge between a spherical high-voltage electrode and a grounded pin electrode placed in a reservoir filled with distilled water, with the tip exposed, and (b) dielectric barrier discharge between the high-voltage electrode and the liquid water surface. A diffuse plasma filament generated between the electrodes in helium during the primary discharge pulse exhibits noticeable constriction during the secondary discharge pulse several hundred ns later. Adding oxygen to the mixture reduces the plasma filament diameter and enhances constriction during the secondary pulse. In the dielectric barrier discharge, diffuse volumetric plasma occupies nearly the entire space between the high voltage electrode and the liquid surface, and extends radially along the surface. In the filament discharge in helium, adding water to the container results in considerable reduction of plasma lifetime compared to the discharge in dry helium, by about an order of magnitude, indicating rapid electron recombination with water cluster ions. Peak electron density during the pulse is also reduced, by about a factor of two, likely due to dissociative attachment to water vapor during the discharge pulse. These trends become more pronounced as oxygen is added to the mixture, which increases net rate of dissociative attachment. Gas temperature during the primary discharge pulse remains near room temperature, after which it increases up to T ~ 500 K over 5 µs and decays back to near room temperature before the next discharge pulse several tens of ms later. As expected, electron density and electron temperature in diffuse DBD plasmas are considerably lower compared to peak values in the filament discharge. Use of Thomson scattering for measurements of electron density and electron temperature in near-surface plasmas is hindered by strong light scattering off the surface.

High time resolved electron temperature measurements by using the multi-pass Thomson scattering system in GAMMA 10/PDX.

High time resolved electron temperature measurements are useful for fluctuation study. A multi-pass Thomson scattering (MPTS) system is proposed for the improvement of both increasing the TS signal intensity and time resolution. The MPTS system in GAMMA 10/PDX has been constructed for enhancing the Thomson scattered signals for the improvement of measurement accuracy. The MPTS system has a polarization-based configuration with an image relaying system. We optimized the image relaying optics for improving the multi-pass laser confinement and obtaining the stable MPTS signals over ten passing TS signals. The integrated MPTS signals increased about five times larger than that in the single pass system. Finally, time dependent electron temperatures were obtained in MHz sampling.

Time-resolved electron temperature and electron density measurements in a nanosecond pulse filament discharge in H2–He and O2–He mixtures

Time evolution of electron density and electron temperature in a nanosecond pulse, diffuse filament electric discharge in H2–He and O2–He mixtures at a pressure of 100 Torr is studied by Thomson/pure rotational Raman scattering and kinetic modeling. The discharge is sustained between two spherical electrodes separated by a 1 cm gap and powered by high voltage pulses ~150 ns duration. Discharge energy coupled to the plasma filament 2–3 mm in diameter is 4–5 mJ/pulse, with specific energy loading of up to ~0.3 eV/molecule. At all experimental conditions, a rapid initial rise of electron temperature and electron density during the discharge pulse is observed, followed by the decay in the afterglow, over ~100 ns–1 µs. Electron density in the afterglow decays more rapidly as H2 or O2 fraction in the mixture is increased. In He/H2 mixtures, this is likely due to more rapid recombination of electrons in collisions with and ions, compared to recombination with ions. In O2/He mixtures, electron density decay in the afterglow is affected by recombination with and ions, while the effect of three-body attachment is relatively minor. Peak electron number densities and electron temperatures are ne = (1.7–3.1) · 1014 cm−3 and Te = 2.9–5.5 eV, depending on gas mixture composition. Electron temperature in the afterglow decays to approximately Te ≈ 0.3 eV, considerably higher compared to the gas temperature of T = 300–380 K, inferred from O2 pure rotational Raman scattering spectra, due to superelastic collisions. The experimental results in helium and O2–He mixtures are compared with kinetic modeling predictions, showing good agreement.

Time-resolved electron density and electron temperature measurements in nanosecond pulse discharges in helium

Thomson scattering is used to study temporal evolution of electron density and electron temperature in nanosecond pulse discharges in helium sustained in two different configurations, (i) diffuse filament discharge between two spherical electrodes, and (ii) surface discharge over plane quartz surface. In the diffuse filament discharge, the experimental results are compared with the predictions of a 2D plasma fluid model. Electron densities are put on an absolute scale using pure rotational Raman spectra in nitrogen, taken without the plasma, for calibration. In the diffuse filament discharge, electron density and electron temperature increase rapidly after breakdown, peaking at ne ≈ 3.5 · 1015 cm−3 and Te ≈ 4.0 eV. After the primary discharge pulse, both electron density and electron temperature decrease (to ne ~ 1014 cm−3 over ~1 µs and to Te ~ 0.5 eV over ~200 ns), with a brief transient rise produced by the secondary discharge pulse. At the present conditions, the dominant recombination mechanism is dissociative recombination of electrons with molecular ions, . In the afterglow, the electron temperature does not relax to gas temperature, due to superelastic collisions. Electron energy distribution functions (EEDFs) inferred from the Thomson scattering spectra are nearly Maxwellian, which is expected at high ionization fractions, when the shape of EEDF is controlled primarily by electron–electron collisions. The kinetic model predictions agree well with the temporal trends detected in the experiment, although peak electron temperature and electron density are overpredicted. Heavy species temperature predicted during the discharge and the early afterglow remains low and does not exceed T = 400 K, due to relatively slow quenching of metastable He* atoms in two-body and three-body processes. In the surface discharge, peak electron density and electron temperature are ne ≈ 3 · 1014 cm3 and Te ≈ 4.25 eV, attained after the surface ionization wave reaches the grounded electrode. The sensitivity of the present diagnostics is too low to measure electron density in the plasma during surface ionization wave propagation (estimated to be below ne ≈ 1013 cm−3). After peaking during the primary current pulse, the electron density decays due to dissociative recombination. Electron temperature decreases rapidly over ~150 ns after the primary current pulse rise, to Te ≈ 0.5 eV, followed by a much more gradual electron cooling between the primary and the secondary discharge pulses, due to superelastic collisions providing moderate electron heating in the afterglow.

Time-Resolved Electron Temperature in an Argon Theta Pinch by Line Ratio Methods

Pulsed inductive argon plasma in an 80-J, 15-kV, 490-kHz theta pinch is interrogated using spectroscopic methods. Time-resolved electron temperature is obtained by coupling line intensity ratios with steady-state corona and collisional-radiative (CR) models. Neutral excited state argon transitions from the 2p to 1s subshells were utilized. The backfill pressures of 50 and 100 mtorr are of primary focus. Time-resolved electron temperature estimates are presented ranging from 1 to 11.1 eV for the corona model up to an excess of 80 eV for the CR model. Near-IR spectral emission is seen to increase rapidly near the first zero crossing of the oscillating discharge current while electron temperature increase lags by roughly one full discharge cycle later near the third zero crossing. An analysis of long exposures provides an account of substantial second-order diffracted spectra. Weak spectral signal quality for the short exposures of 0.25-μs yielded time-resolved spectral intensity trend lines of a low accuracy with an average percent difference of 69% between raw data and trend lines.

Time resolved Thomson scattering diagnostic of pulsed gas metal arc welding (GMAW) process

In this work a Thomson scattering diagnostic technique was applied to obtain time resolved electron temperature and density values during a gas metal arc welding (GMAW) process. The investigated GMAW process was run with aluminum wire (AlMg 4,5 Mn) with 1.2 mm diameter as a wire electrode, argon as a shielding gas and peak currents in the range of 400 A. Time resolved measurements could be achieved by triggering the laser pulse at shifted time positions with respect to the current pulse driving the process. Time evaluation of resulting electron temperatures and densities is used to investigate the state of the plasma in different phases of the current pulse and to determine the influence of the metal vapor and droplets on the plasma properties.

Determination of the rate coefficients of the electron-impact excitation from the metastable states to 2p states of argon by the emission line ratios in an afterglow plasma

The rate coefficients of electron-impact excitation from the argon metastable to 2p states are measured in the afterglow of an rf pulsed capacitive discharge. In the afterglow, the 2p state densities are measured by optical emission spectroscopy, the time-resolved 1s state densities are measured by laser absorption, the time-resolved electron temperature (Te) and electron density are measured by a Langmuir probe. By employing these measured parameters, the electron-impact excitation rate coefficients are obtained from the rate balance equations of 2p states in the Te range from 0.7 to 1.2 eV. The measured rate coefficients are in good agreement with those obtained from the cross sections measured by Boffard et al. In addition, we obtain rate coefficients of the transitions from the metastable states to 2p1,7,10, whose measured values have not been reported before. These rate coefficients are also compared with the calculated ones (Zatsarinny et al with the R-matrix method, Srivastava et al with the Distorted wave method).