Time-resolved Electron Density Measurements Research Articles

Time-resolved measurements of electron density and plasma diameter of 1 kHz femtosecond laser filament in air

Time-resolved measurements of electron density and plasma diameter of 1 kHz femtosecond laser filament in air

Laser absorption measurements of electron density in nanosecond-scale atmospheric pressure pulsed plasmas

We report on time-resolved measurements of electron number density by continuous-wave laser absorption in a low-energy nanosecond-scale laser-produced spark in atmospheric pressure air. Laser absorption is a result of free-free and bound-free electron excitation, with the absorption coefficient modeled and evaluated using estimates of the time-variation in electron temperature and probe laser absorption path length. Plasma electron number densities are determined to be as high as ne=7×1019 cm−3 and decay to 1/e of their peak values over a period of about 50 ns following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled Nd:YAG laser. The measured plasma densities at later times are shown to be in reasonable agreement with Stark broadening measurements of the 3s[5So]-3p[5P] electronic transition in atomic oxygen at 777 nm. This study provides support for the use of such continuous wave laser absorption for time resolved electron density measurements in low energy spark discharges in air, provided that an estimate of the electron temperature and laser path length can be made by accompanying diagnostics.

First time-resolved electron density measurements in the C-2W advanced field-reversed configuration plasmas from long-path compact second-harmonic interferometer

Characterization of the plasma structure and density is critical for the diagnosis and control of C-2W plasma equilibria. To this end, two compact, highly portable, turnkey second harmonic interferometers are used to make measurements with greater flexibility than available from other diagnostics, providing important information in areas otherwise inaccessible to more complicated systems. The systems are based on a fiber-coupled 1064 nm Nd:YAG laser and provide a sensitivity of a few 1018 m-2 with a time resolution of a few microseconds. System upgrades were made to allow for beam paths in excess of 5 m. Initial data from two system configurations are presented, showing plasma translation and merged equilibria.

Time-resolved measurements of electron density in nanosecond pulsed plasmas using microwave scattering

In this work, microwave scattering was utilized to measure the electron number density produced in a nanosecond-long high-voltage breakdown of air between two electrodes in a pin-to-pin configuration. The approach allows tracing the absolute electron number in the plasma volume during the high-voltage pulse and the plasma decay for wide range of plasma and discharge parameters. Electron number density decayed on the timescale of about several μs governed by plasma-chemical processes including dissociative recombination, three-body attachment and formation of cluster ions.

Processes controlling properties of high-voltage nanosecond discharge plasma in combustible mixtures

We present the results of the experimental study of high-voltage nanosecond repetitively pulsed discharge and of its afterglow in ethane, dimethyl ether and their mixtures with oxygen. The measurements were made for room temperature and pressures from 2 to 6 Torr. The measured specific deposited energy and mean discharge current varied with the number of voltage pulses monotonously in ethane and dimethyl ether and nonmonotonously in the hydrocarbon:oxygen mixtures. A microwave interferometer was used for time-resolved electron density measurements in the discharge afterglow. The effective recombination coefficients were obtained from the analysis of the measured data. These coefficients varied with the number of voltage pulses monotonously in ethane and peaked in pure dimethyl ether and its mixture with oxygen. Possible mechanisms of the nonmonotonous behavior of the discharge characteristics were discussed.

Temporal variations of electron density and temperature in Kr/Ne/H2 photoionized plasma induced by nanosecond pulses from extreme ultraviolet source

Spectral investigations of low-temperature photoionized plasmas created in a Kr/Ne/H2 gas mixture were performed. The low-temperature plasmas were generated by gas mixture irradiation using extreme ultraviolet pulses from a laser-plasma source. Emission spectra in the ultraviolet/visible range from the photoionized plasmas contained lines that mainly corresponded to neutral atoms and singly charged ions. Temporal variations in the plasma electron temperature and electron density were studied using different characteristic emission lines at various delay times. Results, based on Kr II lines, showed that the electron temperature decreased from 1.7 to 0.9 eV. The electron densities were estimated using different spectral lines at each delay time. In general, except for the Hβ line, in which the electron density decreased from 3.78 × 1016 cm−3 at 200 ns to 5.77 × 1015 cm−3 at 2000 ns, most of the electron density values measured from the different lines were of the order of 1015 cm−3 and decreased slightly while maintaining the same order when the delay time increased. The time dependences of the measured and simulated intensities of a spectral line of interest were also investigated. The validity of the partial or full local thermodynamic equilibrium (LTE) conditions in plasma was explained based on time-resolved electron density measurements. The partial LTE condition was satisfied for delay times in the 200 ns to 1500 ns range. The results are summarized, and the dominant basic atomic processes in the gas mixture photoionized plasma are discussed.

Experimental and numerical analysis of atmospheric air plasma induced by multi-MeV pulsed X-ray

Quantification of electromagnetic stresses on electronic systems, following irradiation of the air by ionizing radiations, requires a thorough study of the plasma generated. In this work, the temporal evolution of non-equilibrium air plasmas self-induced by energetic X-rays is experimentally and theoretically investigated at atmospheric pressure. Time resolved electron density measurements are based on transmission measurements of an electromagnetic wave in the microwave range. The electromagnetic wave is launched into a wave guide, which is irradiated by a high flux of multi-MeV pulsed X-rays. For different X-ray fluxes, the electron density is determined from the comparison between the transmitted microwave signal at the waveguide output, and the result of the calculation of the propagation of an electromagnetic wave through time varying plasma contained in a waveguide. These measurements require a priori assumptions on electron temperature, which is obtained and confirmed by a reaction kinetics model of the evolution of the electron energy and the densities of the different humid air plasma species inside the waveguide. The considered chemical kinetics scheme involves 39 influent species (electrons, positive ions, negative ions, and neutral atoms and molecules in their ground or metastable excited states) reacting following 265 selected reactions. A good agreement is observed between the calculated and measured time evolution of the transmitted signal for specific profiles of electron energy and density. In our experiments, the maximum electron density is of the order of few 1012 cm−3, for a mean electron energy of about 0.5 eV. For doses range from 3 Gy to 21 Gy, the discrepancies between the measurements and the model for the maximum of the electron density are within a factor of 2.

Time-resolved curling-probe measurements of electron density in high frequency pulsed DC discharges

A plasma-induced shift in the resonance frequency of a curling probe measured by using a network analyzer (NWA) yields the electron density. This technique was applied here for measuring time-varying electron density in pulsed DC glow discharges. Using the NWA in an on-sweep synchronization mode with the discharge pulse allows measuring at pulse frequencies below 0.5 kHz. For higher pulse frequencies, an on-point mode was introduced which enabled time-resolved measurements of electron density at pulse frequencies reaching 25 kHz, with the minimal time interval of 2 µs, typically for nitrogen discharge at 10 Pa. In the afterglow regime, the decay time constant of electron density was measured for nitrogen and argon discharges at 40 Pa. In the case of argon, the electron density was observed to decrease in three steps. This characteristic behavior was tentatively attributed to a bi-Maxwellian electron energy distribution and Ramsauer effect, supported by Langmuir probe measurements.

Time-resolved electron density measurements in PF-1000 device by means of the Mechelle® 900 optical spectrometer

The report presents the main results of the recent spectroscopic investigations of plasma produced within the PF-1000 facility. That experimental device was operated mostly with the pure deuterium filling. The initial pressure was 5 hPa D2 or H2 and the charging voltage of a capacity bank was 28 kV. The electron density was estimated by means of three different techniques: the linear Stark broadening of the Balmer series, the quadratic Stark broadening of copper and carbon spectral lines (including the lines of multi-charged ions), and using the Inglis-Teller correlation for a shift of the boundary serial. At all the stages of the data processing there was applied a convenient and effective software of the GRAMS-32 type (attached to the MECHELLE®900 spectrometer). A critical evaluation of the obtained results, taking into consideration their accuracy and reliability, has been performed.

Radially and time-resolved measurements of electron density in an operating copper vapour laser

A practical technique for the measurement of free electron density, as a function of time and radial position, in the plasma of an operating copper vapour laser (CVL) is described. The method employs a two-colour interferometer to measure the refractive index of the plasma at both and 670 nm. Results of such measurements of electron density in a 40 W commercial CVL are presented. A high residual electron density present prior to the discharge is observed, together with a relatively modest increase during the discharge. These observations are related to the operating characteristics of the laser, in particular the phenomenon known as the `phantom current'. The source and propagation of errors are considered and a maximum error of is calculated.

Study of oscillating magnetized hollow cathode arcs by time-resolved Thomson scattering measurements

A spatially resolving incoherent Thomson scattering technique has been used for time-averaged and time-resolved investigations of a magnetized hollow cathode arc burning in hydrogen and helium. In these plasmas strong self-excited oscillations with frequencies between typically 10 and 60 kHz are found generating modulated plasma emission intensities and discharge voltage signals. In the case of periodically oscillating plasmas, space- and time-resolved measurements of electron density and temperature are performed over the arc cross section by proper triggering of the pulsed laser system with the periodically oscillating voltage signal from the plasma. The evaluation of the electron density and temperature contour maps reveals that the region of maximum electron density is shifted eccentrically and rotating azimuthally around the arc axis. The region of maximum electron temperature shows a nearly annular shape, following the dimensions of the hollow cathode. When increasing the external magnetic field and neutral gas pressure, the arc motion and time behaviour of individual plasma parameters become irregular and chaotic.

Time-resolved electron density measurements in an ArF excimer laser discharge

The electron density in an ArF excimer laser discharge has been measured by means of a time-resolved optical interferometric method. By comparing the behavior of electron density in the absence of F 2 gas with that in the presence of F 2 gas, the dissociative attachment rate is estimated from graphical calculation.

Time-resolved electron density measurements in rare-gas-halide laser discharges

Time-resolved spectroscopic measurements are described of the evolution of electron density and other parameters in a UV-preionised XeCl laser discharge. Measurements of electron density by line-broadening techniques indicate the existence of a transition in the nature of the discharge from glow-like to arc-like. Independent measurements of intensities of ionic spectral lines support the existence of a phase of constriction during a period of falling global electron density.

Two-channel technique for Stark measurements of electron density within a laser-produced sodium plasma

In this paper we have shown that time-resolved electron density measurements can be undertaken using a simple two-channel optical system to sample a Stark broadened emission line. The technique has been demonstrated in a sodium plasma produced by laser resonance saturation.

Time Resolved Microwave Interferometry as Diagnostic Tool for Decaying Plasma Afterglows

Two methods for obtaining time resolved electron density measurements with a microwave interferometer are described. Precision phase measurement techniques permit these methods to be used in weak afterglow plasmas with a high resolution such that decaying afterglow can be followed down to ωp/ω∼0.01 with a time resolution of 10 μsec.