Temporal Evolution Of Electron Temperature Research Articles

Population inversion mechanism of optically pumped metastable rare gas based on laser-induced preionization

A 532 nm laser was used to induce ionization of Ar–He mixture, and metastable 4s [3/2]2 level Ar atoms were prepared. The interaction mechanism between laser, Ar atoms, He atoms and free electrons in laser-induced plasma (LIP) was revealed. Through time-resolved atomic emission spectroscopy, two regimes separated by a few microseconds were found during temporal evolution of Ar I 811.5 nm (4p [5/2]3→4s [3/2]2) line. It was demonstrated that the first regime was generated by the “photoexcitation + radiation + collisional relaxation” process, while the second regime was generated by the “ion-electron recombination” process. The above conclusion was confirmed again by the temporal evolution of electron temperature and electron density, and it was also demonstrated that the ionization of Ar atoms induces the ionization of He atoms. A 811.5 nm laser was utilized to excite the metastable 4s [3/2]2 level Ar atoms to the 4p [5/2]3 level, when changing the delay time of the 811.5 nm laser lagging behind the 532 nm laser, the generated 912.3 nm (4p [1/2]1→4s [3/2]2) fluorescence intensity also showed different pattern of variations. The analysis indicated that “photoexcitation + radiation + collisional relaxation”, “ion-electron recombination” and quenching processes will affect the density of metastable 4s [3/2]2 level Ar atoms. Amplified spontaneous emission (ASE) signal at 912.3 nm was obtained along the direction of pump laser. This work verifies the feasibility of optically pumped metastable rare gas laser (OPRGL) based on laser-induced preionization, and provides an alternative scheme for the future development of OPRGL.

The self-absorption temporal evolution of spectral lines emitted from laser-induced plasmas

The interference of self-absorption (SA) in measurement of spectral lines in laser-induced plasmas (LIPs) needs to be addressed urgently, since the SA affects the spectral line shape, i.e., the line intensity decreases and line broadening changes. Here, the time resolved SA coefficients of six lines (Ca I 422.67 nm, Ca II 396.84 nm, Mg I 285.21 nm, Mg II 280.27 nm, Al I 394.40 nm, Al II 781.23 nm) emitted from laser induced soil plasmas are investigated based on the investigation of the temporal evolution of electron temperature (Te) and electron density (Ne). The SA coefficient of line Ca I 422.67 nm decreases sharply in the delay time of 0.2–0.6 μs, and tends to be maintained in the 0.6–2.0 μs temporal window. While other five lines (Ca II 396.84 nm, Mg I 285.21 nm, Mg II 280.27 nm, Al I 394.40 nm, Al II 781.23 nm) SA coefficients decrease sharply in the 0.2–1.2 μs delay time due to the decreasing Stark broadening of those lines with the decreasing Te and Ne correspondingly. While, in the 1.2–1.6 μs temporal window, the changes in SA coefficients of these five lines become steady because the plasmas property parameters (Te and Ne) are stable after the delay time of about 1.0 μs. This indicates that the best time window for the accurate analysis of element concentration using LIPs is from 1.2 μs to 1.6 μs.

Investigation of heat transport using directly driven gold spheres

Recently, heat transport was investigated using a directly driven beryllium sphere [Farmer et al., Phys. Plasmas 27, 082701 (2020)]. Models that overly restrict heat transport were rejected. This paper extends work to directly driven gold spheres where radiation loss is more important. Here, gold coated spheres are directly driven at the OMEGA laser facility at intensities of 5×1014 W/cm2. Plasma conditions, laser coupling, and x-ray flux are all measured. Comparisons to 2D radiation-hydrodynamic simulations are performed. Simulations use three common heat transport models: local transport with flux limiters of f = 0.15 and f = 0.03, and the nonlocal Schurtz–Nicolai–Busquet (SNB) model. It is shown that both the SNB model and f = 0.15 match the measured plasma conditions with the SNB model better capturing the temporal evolution of electron temperature. The f = 0.03 model predicts too low of an electron density and too hot of a temperature. The measured scattered light is roughly 6% of the incident energy, the f = 0.15 and SNB models predict 0.5% uncoupled light, and f = 0.03, 38% uncoupled light. The x-ray fluxes in the f = 0.15 and SNB simulations rise too quickly and are just outside the measurement's error, while the x-ray flux in the f = 0.03 simulation is low by a factor of two-three. For these reasons, the f = 0.03 model is rejected.

Quantifying noise sources in the KSTAR 2014 Thomson Scattering system from the measured variation on electron temperature

With the Thomson scattering (TS) system in KSTAR, temporal evolution of electron temperature (Te) is estimated using a weighted look-up table method with fast sampling (1.25 or 2.5 GS/s) digitizers during the 2014 KSTAR campaign. Background noise level is used as a weighting parameter without considering the photon noise due to the absence of information on absolute photon counts detected by the TS system. Estimated electron temperature during a relatively quiescent discharge are scattered, i.e., 15% variation on Te with respect to its mean value. We find that this 15% variation on Te cannot be explained solely by the background noise level which leads us to include photon noise effects in our analysis. Using synthetic data, we have estimated the required photon noise level consistent with the observation and determined the dominant noise source in KSTAR TS system.

Measurement of Electron-Ion Relaxation in Warm Dense Copper.

Experimental investigation of electron-ion coupling and electron heat capacity of copper in warm and dense states are presented. From time-resolved x-ray absorption spectroscopy, the temporal evolution of electron temperature is obtained for non-equilibrium warm dense copper heated by an intense femtosecond laser pulse. Electron heat capacity and electron-ion coupling are inferred from the initial electron temperature and its decrease over 10 ps. Data are compared with various theoretical models.

Ignition delay of a pulsed inductively coupled plasma (ICP) in tandem with an auxiliary ICP

Plasma ignition delays were observed in a ‘main’ inductively coupled plasma (ICP), in tandem with an ‘auxiliary’ ICP. The Faraday-shielded ICPs were separated by a grounded metal grid. Power (13.56 MHz) to the main ICP was pulsed with a frequency of 1 kHz, while the auxiliary ICP was operated in continuous wave (cw) mode. In chlorine plasmas, ignition delay was observed for duty cycles greater than 60% and, in contrast to expectation, the delay was longer with increasing duty cycle up to ~99.5%. The ignition delay could be varied by changing the auxiliary and/or main ICP power. Langmuir probe measurements provided the temporal evolution of electron temperature, and electron and positive ion densities. These measurements revealed that the plasma was ignited shortly after the decaying positive ion density (n+), in the afterglow of the main ICP, reached the density () prevailing when only the auxiliary ICP was powered. At that time, production of electrons began to dominate their loss in the main ICP, due to hot electron injection from the auxiliary ICP. As a result, increased from a value below , improving inductive power coupling efficiency, further increasing plasma density leading to plasma ignition. Plasma ignition delay occurred when the afterglow of the pulsed plasma was not long enough for the ion density to reach during the afterglow. Besides Cl2, plasma ignition delays were also observed in other electronegative gases (SF6, CF4/O2 and O2) but not in an electropositive gas (Ar).

Emission analysis of a laser-produced barium plasma plume.

In the present work we report the characteristic emission features of a laser-produced barium plasma plume. The time-resolved analysis for the different spectral lines of neutral and singly charged ionic barium has been carried out. It has been observed that the temporal evolution of electron temperature and density shows a peculiar behavior which is significantly different from the reported results of laser ablation of materials. The electron density increases with increase in delay time but the temperature does not change to any significant extent. Strong self-reversal in the emission of a resonant singly charged barium ionic line (455.4nm) with time delay indicates the increase of population of singly charged barium ion with time. The results are explained on the basis of the increased population of barium metastables and subsequent ionization (Penning type).

Investigation and optimization of the magnetic field configuration in high-power impulse magnetron sputtering

An effort to optimize the magnetic field configuration specifically for high-power impulse magnetron sputtering (HiPIMS) was made. Magnetic field configurations with different field strengths, race track widths and race track patterns were designed using COMSOL. Their influence on HiPIMS plasma properties was investigated using a 36 cm diameter copper target. The I–V discharge characteristics were measured. The temporal evolution of electron temperature (Te) and density (ne) was studied employing a triple Langmuir probe, which was also scanned in the whole discharge region to characterize the plasma distribution and transport. Based on the studies, a closed path for electrons to drift along was still essential in HiPIMS in order to efficiently confine electrons and achieve a high pulse current. Very dense plasmas (1019–1020 m−3) were generated in front of the race tracks during the pulse, and expanded downstream afterwards. As the magnetic field strength increased from 200 to 800 G, the expansion became faster and less isotropic, i.e. more directional toward the substrate. The electric potential distribution accounted for these effects. Varied race track widths and patterns altered the plasma distribution from the target to the substrate. A spiral-shaped magnetic field design was able to produce superior plasma uniformity on the substrate in addition to improved target utilization.

Plasma Characterization in Divertor Simulation Experiments with a V-Shaped Target on GAMMA 10/PDX

The divertor simulation experimental module (Dmodule) has been installed in the west end region in GAMMA 10/PDX. By use of Langmuir probes and spectroscopic measurement of intensity ratios of He I lines, temporal evolution of electron temperature and that of electron density of the plasma in the D-module with the V-shaped tungsten target are obtained. When the additional ICRF heating is applied to the anchor cell, the electron temperature evaluated with He I intensity ratios decreases from ~60 eV to ~25 eV and that from the probe measurement decreases from ~27 eV to ~14 eV. The difference between both measurements seems to be attributed to the difference of their measurement positions. The electron density measured by the Langmuir probe increases 2.3 times due to the RF3 power but it is rather low (< 1017 m-3). The electron density at the end region is expected to be increased by enhancement of ICRF heating and additional gas puffing at the plug/barrier cell which is the upstream cell of the end region.

An investigation of the temporal evolution of plasma potential in a 60 MHz/2 MHz pulsed dual-frequency capacitively coupled discharge

Using an electron-emitting probe, time-resolved plasma potential (Vp) measurements are carried out in a pulsed dual-frequency 60 MHz/2 MHz capacitively coupled discharge. The discharge is produced using argon gas at two chosen pressures of 20 and 40 mTorr, a duty ratio of 50% and pulse powers ranging from 100 to 500 W. The pulsing frequency is set at 1 kHz. The plasma potential measurements are carried out at 10 mm above the centre of the substrate.It is observed that Vp follows the source discharge pulse voltage and remains positive during the whole pulse cycle. Without substrate biasing, the prominent features observed in the Vp profile, under all the operating conditions, remain similar; however, the magnitude of Vp increases with the applied RF source power. For further analysis, three distinguishable features in the Vp profile, a transient spike at the beginning of the discharge pulse, a stable ‘on-phase’ and a ‘stable-off’ phase, are identified. For typical operating conditions (20 mTorr and 500 W), the transient spike in Vp of ∼30 V appears for 30 µs, then it attains a stable value of ∼12 V during the rest of the pulse on-period. Vp decreases up to ∼3 V as the pulse is switched off.It is also observed in this study that a continuous wave RF biasing of the substrate significantly modulates the plasma potential evolution, specifically when the pulse is switched off and the magnitude of modulation depends on the substrate biasing power. The temporal evolution of electron temperature derived from the plasma and floating potentials is also reported.

Plasma generation in an organic molecular gas by an ultraviolet laser pulse

Plasmas are generated in a low ionization potential gas, tetrakis(dimethylamino)ethylene (TMAE) vapor through a one-photon ionization process by an ultraviolet laser beam at a 193 nm wavelength. The TMAE plasma characteristics are studied by means of a Langmuir probe and microwave scattering. A new method is used to measure the 193 nm ultraviolet (UV) photon absorption cross section in TMAE. It is determined to be 1.1±0.3×10−17 cm2 from the axial profile of electron density. The temporal evolution of electron temperature is measured. A peak electron density of ne=5×1013/cm3 and peak electron temperature of Te=1 eV are measured at 500 mTorr TMAE pressure. The plasma decay process is studied, and the electron-ion recombination coefficient is measured to be 5.4±0.5×10−6 cm3/s. A theoretical model is derived to describe the photon flux and the one photon ionization process. An application of a TMAE plasma as a mirror for microwave reflections is proposed.