Laser Thomson Scattering Research Articles

An incoherent Thomson scattering system for measurements near plasma boundaries.

Laser Thomson scattering (LTS) is a minimally invasive measurement technique used for determining electron properties in plasma systems. Sheath model closure validation requires minimally invasive measurements of the electron properties that traverse the boundaries between the bulk plasma, the presheath, and the plasma sheath. Several studies have probed the radial properties along the surface of discharge electrodes with laser-based diagnostics and electrostatic probes. These measurements provide valuable insight into the electron properties in this dynamic region. However, sheath model calibration requires plasma property measurements perpendicular to plasma bounding surfaces, in this case, along the electrode normal vector between discharge electrodes. This work presents the development of a discharge plasma cell and laser Thomson scattering system with a measurement volume step of 1mm normal to plasma bounding surfaces. The laser Thomson scattering measurements are made between a set of discharge electrodes separated by ∼25mm that are used to generate a pulsed argon plasma. The spatial distribution of electron temperature and density is measured at several discharge voltages between 8 and 20kV at a pressure of 8 Torr-Ar. It is determined that the system is statistically stationary and resembles a classic DC discharge plasma. The results are some of the first laser diagnostic-based "between electrode" measurements made along the plasma bounding electrode normal vector. A one-dimensional sheath model is applied to determine the near cathode electron properties, and it is determined that the edge of the presheath is probed in the high-voltage cases. As the lengths of the presheath and sheath decrease with decreasing voltage, the region recedes below the closest probed point to the cathode. To improve the performance of the diagnostic, the step size of the interrogation volume should decrease by an order of magnitude from 1mm to less than 100 μm, and the data acquisition strategy should be revised to increase the signal-to-noise ratio.

Measurements of electron velocity distribution function in microwave cathode plume by incoherent laser Thomson scattering

This paper reports the development of a method of using incoherent laser Thomson scattering (LTS) to directly measure the electron velocity distribution function (EVDF) within a plasma plume. In this method, a 532 nm wavelength laser is injected into a plasma and Thomson scattering is detected with a CCD camera and a triple-monochromator. First, the measurement settings (e.g., laser power, number of integrations, and data analysis) were validated through a comparison between a microwave cathode and a hollow cathode. The results indicate that lower laser power is required as the photo-ionization from the metastable state particles increases the Thomson scattering. Second, the EVDFs of the microwave cathode at the plume were investigated for various anode currents and propellant flow rates. The results suggest that the non-Maxwellian EVDF was clearly observed at the low propellant flow rate, and the propellant flow rate is a more important parameter for the non-Maxwellian EVDFs than the anode current.

Transition from MAR to EIR of deuterium plasma detachment in NAGDIS-II

Volumetric plasma recombination in a detached deuterium (D) plasma was experimentally studied in the linear plasma device NAGDIS-II. The radial distributions of the electron temperature (T e ) and the electron density (n e ) were measured by upstream and downstream laser Thomson scattering (LTS) systems and D-atom excited states of the Balmer series (n = 3–7) were measured by optical emission spectroscopy (OES). The results indicated that the electron static pressure, particle flux and heat flux present a two-step reduction with an increasing neutral pressure, suggesting that molecule activated recombination (MAR) and electron–ion recombination (EIR) were activated, respectively. The difference in the distributions of the atomic state population densities was observed to be a feature of the transition from the MAR-dominant phase to the EIR-dominant phase. The transition between the two phases had a distinct boundary at T e ∼0.7 eV, which was considered a transition point between the phases. The ratios of the atomic state population densities showed different features in the two phases, and are expected to be useful in characterizing the phase and degree of plasma detachment.

Evidence for fireballs in bipolar HiPIMS plasmas

Using laser Thomson scattering (LTS) and 2D optical imaging, a fireball-like discharge is detected during the positive pulse period of bipolar voltage waveforms, for a circular planar unbalanced magnetron with W target operating in argon. These reverse discharges, excited for positive pulse voltages V pp from 200 to 300 V, sit primarily on the discharge centreline. Their establishment is delayed relative to the initiation of the positive pulse, with the delay time shortening with increased V pp: they are clearly attached to the target (anode in this phase) and have lifetimes extending to the end of the positive pulse. LTS measurements of the electron temperature T e show dramatic electron heating (T e rises from 1 up to 3 eV) both on the discharge centre line and above the racetrack during the fireball event. This is consistent with greater light intensities from the broad band optical imaging of the discharge. In the fireball phase, the LTS measurements also show greatly reduced election densities (by a factor of 5) in the magnetic trap (directly above the racetrack) compared to the unipolar pulse case. The existence of such anodic fireballs is quite possibly an unwanted effect in bipolar HiPIMS.

Development of plasma sources and diagnostics for the simulation of fusion edge plasmas

Although the research on divertors and scrape-off layers (SOLs) has been not as focused on as the recent success of the Korean fusion program, a few linear plasma devices have been developed for simulating divertor and SOL plasmas: (1) diversified plasma simulator (DiPS), a versatile linear machine, has been developed for simulations of divertor and space plasmas with various electric probes, such as single, triple, and Mach Probes and gridded energy analyzer. DiPS consists of two major parts: a divertor plasma simulator with a \(\hbox {LaB}_6\) DC plasma source and a space plasma simulator with a helicon RF plasma source, (2) divertor plasma simulator-1 (DiPS-1) is a part of DiPS with only a \(\hbox {LaB}_6\) cathode, where a low-power laser-induced fluorescence (LIF) is added and more electric probe diagnostics are augmented; it is dedicated only for fusion edge and divertor plasmas, (3) Divertor Plasma Simulator-2 (DiPS-2) has been modified from the DiPS-1 by adding a magnetic nozzle with a limiter structure and by removing the helicon source and space chamber. DiPS-2 is a linear plasma device with a 4-inch \(\hbox {LaB}_6\) cathode, the same as DiPS-1, and it is focused on the development of various diagnostics, such as those used for LIF and laser Thomson scattering (LTS) along with various electric probes, on the divertor and scrape-off plasmas and on the plasma-material interaction (PMI) research, such as that of tungsten and graphite as plasma-facing components (PFCs), (4) A Multi-Purpose Plasma (\(\hbox {MP}^2\)) device is a renovation of the Hanbit mirror device [Kwon et al., Nucl. Fusion 43, 686 (2003)] with the installation of two plasma sources: \(\hbox {LaB}_6\) (DC) and helicon (RF) plasma sources. A honeycomb-like large-area \(\hbox {LaB}_6\) (HLA-\(\hbox {LaB}_6\)) cathode has been developed for the divertor plasma simulation to improve the resistance against the thermal shock fragility for large (8-inch) and high density plasma generation, (5) DiPS-2 has been augmented by adding another cylindrical device, called the Dust interaction with Surfaces Chamber (DiSC) for the generation and diagnostics of dusts. This combined system (DiPS-2+DiSC) has added two more diagnostics: Laser Photo-Detachment (LPD) for dust density and laser Mie Scattering (LMS) for dust size. Moreover, dusts or negative ions have been analyzed by using electric probes and capacitive diagram gauges in Transport and Removal of Dusts (TReD) device.

High resolution laser Thomson scattering system with automatic data analysis software platform for diagnosis of the low-temperature plasmas.

Accurate measurements of electron temperature (Te) and electron density (ne) are important for understanding the properties of plasma, especially for the low-temperature plasma dominated by the free electrons. In this work, a high resolution laser Thomson scattering (LTS) diagnosis system with a software platform for processing data is established to accurately measure the Te and ne in low-temperature plasmas. In this system, a homemade Triple Grating Spectrometer (TGS) is elaborated to suppress the intense stray light and Rayleigh scattering light. The TGS with high etendue (with an f-number of f/3.3) and high spectral resolution (0.07nm in full-width-half-maximum at 532nm) allows the LTS system to measure the plasma with the ne and Te as low as 1.0 × 1017m-3 and 0.1eV, respectively. Besides the development of the experimental setup, a software platform is specially designed to automatically process the complicated LTS spectra to determine ne and Te in real time during the acquirement of data. The error analyses indicate that the uncertainty of ne is around 12% and the uncertainty of Te is about 10% when Te > 0.2eV. Using the LTS system, we have successfully measured ne and Te in the cascaded arc plasma, nanosecond pulse discharge plasma, and inductively coupled plasma. The results demonstrate that the LTS system has significant potential for the diagnosis of various low temperature plasmas.

Diagnosing asymmetric bipolar HiPIMS discharges using laser Thomson scattering

The temporal evolution of the electron temperature T e and density n e has been measured at two positions on the centre-line of an asymmetrically pulsed bi-polar HiPIMS plasma using incoherent laser Thomson scattering (LTS). The magnetron was operated with a tungsten target in argon atmospheres. The results show that in the plasma afterglow when positive voltage pulses are applied (above a threshold of at least 200 V) significant heating of the electrons can occur in which T e can rise to values comparable to the those measured in HiPIMS on-time. The on-set of the rises in T e are significantly delayed relative to the start of the positive pulse, with the delay time decreasing with the magnitude of the positive voltage. The delay is only weakly dependent on the operating pressure. The presence of large positive pulses can also affect the local electron density with n e seen to decay significantly more quickly in the afterglow than for the corresponding unipolar pulsing case, in which no positive pulse is applied. The LTS measurements were complemented by a time-resolved study of the plasma optical emission (neutral argon and tungsten lines). With increasing positive potentials applied in the afterglow the Ar(I) line intensities grow consistent with increasing T e. Interestingly, W(I) line intensities are detected in the afterglow with positive voltages >200 V despite the termination of all target sputtering, suggesting that tungsten is being re-sputtered from the vessel walls. With the aid of emissive probe measurements of the spatial and temporal evolution of plasma potential profile along the centre-line we discuss the phenomena of plasma electron heating and wall sputtering in the positive pulse. This is done in terms of the existence of a non-sustained reverse discharge, in which the vessel walls become an effective cathode.

Time-resolved spatial profiles of electron density and temperature in hydrogen plasmas induced by radiation from laser-produced tin plasmas for extreme ultraviolet lithography light sources

We observed the spatial and temporal changes of the electron density (n e) and the electron temperature (T e) of hydrogen plasmas around a laser-produced Sn plasma EUV source. The plasma parameters were measured by the laser Thomson scattering (LTS) method. In the experiment, the Sn plasmas are produced in H2 gas at a pressure of 50–200 Pa and the hydrogen plasmas were induced by radiation from the Sn plasmas. The LTS measurements were performed at distances 30–90 mm away from the Sn plasmas. In all cases, the strong bremsstrahlung radiation of the Sn plasmas easily overwhelmed the weak LTS signals. To suppress noise due to the radiation, the solid angle of radiation from the Sn plasmas was restricted. The experimental results show that the n e was in the order of 1017 m−3 and T e was around 0.7 eV.

激光汤姆孙散射测量容性耦合等离子体的电子密度

激光汤姆孙散射测量容性耦合等离子体的电子密度

Measurement of electron velocity distribution function in a pulsed positive streamer discharge in atmospheric-pressure air

Laser Thomson scattering (LTS) was used for the first time to measure the non-Maxwellian electron velocity distribution function (EVDF) in an atmospheric-pressure-air pulsed positive streamer discharge. The air streamer was generated with specially designed electrode configuration which provided good spatial reproducibility. The LTS was conducted at a secondary streamer phase with a repetition rate of 2 Hz. To reduce the measurement error caused by intense rotational Raman scattering from N2 and O2, 20 000 LTS signals were accumulated. The LTS spectrum, which was a 1D-projected EVDF in the streamer, has shown a non-Maxwellian EVDF predicted by solving the Boltzmann equation. The measured EVDF exhibited good agreement with an EVDF calculated with E/N = 150 Td. Electron density at the initial phase of the secondary streamer was obtained from the LTS spectrum as 2.7 × 1014 cm−3.

The spatial-temporal distribution of the electron density and temperature in nanosecond pulsed discharge plasma measured by laser Thomson scattering

The spatial-temporal distribution of the electron temperature (Te) and density (ne) in a nanosecond pulsed discharge (NPD) argon plasma with a ball-to-plate electrode configuration at 2 × 103 Pa has been measured by Laser Thomson Scattering (LTS). The peak voltage between two electrodes is 1520 V and the current is 3 A when the ball-like electrode is connected to the negative high voltage. The results indicate that it is possible to enhance the electron density of the plasma discharge using the rising edge of pulse voltage. The measured plasma parameters: Te decreases exponentially from 1.75 eV to 0.1 eV and ne increases rapidly to 3.94 × 1020 m−3 from 15 ns to 115 ns then decreases to 1.17 × 1019 m−3. In the discharge with cylindrical geometric symmetry, Te increases from ball to plate, while ne is opposite. In the r-direction, both Te and ne decrease from central to edge at 3.4 mm. Value of ne varies by 20 times from maximum to minimum and Te is only about 1.5 times.

Comparative study of electron temperature in cascaded arc Ar/N2 plasma using laser Thomson scattering combined optical emission spectroscopy approach

The expanding cascaded arc Ar/N2 plasma has been investigated by both the active and passive optical diagnostic technologies. In the investigation, the laser Thomson scattering (LTS) and optical emission spectroscopy (OES) have been adopted to measure electron temperature (Te) and electron excitation temperature (Texc), respectively. The LTS measurements show that a remarkable nonlinear behavior of Te as a function of the N2/(Ar + N2) ratio is found, which is caused by the collective interaction between the superelastic collision and the electron-impact excitation. The superelastic collisions by the highly excited vibrational nitrogen molecules can effectively heat the free electrons, while the electron kinetic energy can be transferred to N2 molecular internal energy via the electron impact with N2 in the ground state. The difference between Texc and Te demonstrates that the cascaded arc Ar/N2 plasma significantly deviates from the local thermodynamic equilibrium. This would be useful for improving our further understanding of nonequilibrium plasma and extending applications of the cascaded arc Ar/N2 plasma.

Effect of Nitrogen Addition on Electron Density and Temperature of Cascaded Arc Argon Discharge Plasma Diagnosed by Laser Thomson Scattering

The electron density and electron temperature of the recombining argon plasma from a cascaded arc discharge source were measured by the laser Thomson scattering approach. In this paper, a small amount of nitrogen gas was introduced into the argon discharge source. The addition of the nitrogen gas was applied to investigate the change of the plasma parameters in terms of electron density and temperature. The measurements showed that the electron density dropped by 2 orders of magnitude at 10% nitrogen gas addition compared with no nitrogen addition. The dissociative recombination following the charge transfer between atomic ions and nitrogen molecules was responsible for the decrease of electron density. In addition, with the increase of nitrogen gas addition, the measurements indicated that the electron temperature first increased to a maximum and then decreased. This was due to that the superelastic collision between the electrons and the highly excited vibrational nitrogen molecules could heat the electrons leading to the increase of electron temperature while the electron-vibrational energy transfer by electron–nitrogen molecules impact excitation resulted in that electron temperature decreased due to electrons’ kinetic energy losing processes.

Thomson Scattering Measurement of Two Electron Temperature Components in Transition to Detached Plasmas

We have performed laser Thomson scattering (LTS) measurements during the transition between attached and detached helium plasmas in the linear divertor simulator NAGDIS-II. In the detached plasma, the LTS spectrum shows a discrepancy with a single Gaussian function. The discrepancy is resolved by the spectrum fitting with a sum of two Gaussian functions, indicating that the electron energy distribution contains two different temperature components. (C) 2018 The Japan Society of Plasma Science and Nuclear Fusion Research

Helium line emission spectroscopy in recombining detached plasmas

Line emissions from helium (He) plasmas have been used for measurement of electron density and temperature in various situations. We performed measurements of He line intensities in recombining plasmas in a divertor simulator, followed by discussion of some potential difficulties of using the line intensity ratio (LIR) method in a divertor environment under detached conditions. When using four line emissions (501.6, 667.8, 706.5, and 728.1 nm), the discrepancy of the results compared to results from the laser Thomson scattering (LTS) method increased along with increasing gas pressure. However, the LIR method agreed well with LTS when a recombination sensitive line emission (447.1 nm) was added. The influence of the transport of metastable atoms is also discussed. Additionally, anomalous characteristics of the LIR method were revealed in hydrogen helium mixture plasmas when the temperature was in the range of 1–4 eV. These anomalies were likely attributable to the dissociative recombination of HeH+.

Measurement of electron density and electron temperature of a cascaded arc plasma using laser Thomson scattering compared to an optical emission spectroscopic approach

As advanced linear plasma sources, cascaded arc plasma devices have been used to generate steady plasma with high electron density, high particle flux and low electron temperature. To measure electron density and electron temperature of the plasma device accurately, a laser Thomson scattering (LTS) system, which is generally recognized as the most precise plasma diagnostic method, has been established in our lab in Dalian University of Technology. The electron density has been measured successfully in the region of 4.5 × 1019 m−3 to 7.1 × 1020 m−3 and electron temperature in the region of 0.18 eV to 0.58 eV. For comparison, an optical emission spectroscopy (OES) system was established as well. The results showed that the electron excitation temperature (configuration temperature) measured by OES is significantly higher than the electron temperature (kinetic electron temperature) measured by LTS by up to 40% in the given discharge conditions. The results indicate that the cascaded arc plasma is recombining plasma and it is not in local thermodynamic equilibrium (LTE). This leads to significant error using OES when characterizing the electron temperature in a non-LTE plasma.

Measurement of Electron Density and Temperature Using Laser Thomson Scattering in PANTA

Measurement of Electron Density and Temperature Using Laser Thomson Scattering in PANTA

Non-equilibrium Studies in Switching Arc Plasmas in Japan

This paper briefly introduce research work examples of non-equilibrium studies in switching arcs. In understanding arc behavior, one often assumes local thermodynamic equilibrium (LTE) condition in the arc plasma. However, actual arc plasmas are not completely and not always in LTE state because of strong temperature change temporally and spatially, and high electric field application etc. Recently, we have a collaboration work in numerical simulations and experimental approaches for decaying arcs without LTE assumption. First, our numerical model is presented for decaying arcs without chemical equilibrium assumption. Secondly, two experimental methods are introduced for measuring electron density in decaying arcs without LTE assumption: Laser Thomson Scattering method and the Schack-Hartmann method. Finally, comparison results is shown between the LTE simulation, the chemically non-equilibrium simulation, and the above experimental measurements.

Influence of the gas flow rate on the nonchemical equilibrium N2 arc behavior in a model nozzle circuit breaker

The influence of the gas flow rate on the N2 arc behavior was investigated based on a previously established nonchemical equilibrium (non-CE) model. This numerical non-CE model was adopted in the N2 nozzle arc in a model circuit breaker. The arc behaviors of both the arc burning and arc decay phases were obtained at different gas flow rates in both the non-CE and local thermal equilibrium (LTE) model. To better understand the influence of the gas flow rate, in this work we devised the concept of the nonequilibrium parameter. Additionally, the influences of convection, diffusion, and chemical reactions were examined separately to determine which one contributed most to the non-CE behavior. Finally, laser Thomson scattering (LTS) measurements at different gas flow rates were adopted to further demonstrate the validity of the non-CE model. The results of the macroscopic behaviors indicate that the deviations between the non-CE and LTE models during the arc burning phase are much fewer than those during the arc decay phase. By the nonequilibrium parameters, it clearly indicates that with an increase in the gas flow rate, the non-CE effect will be greatly enhanced. During the arc burning phase, this non-CE effect is mainly caused by radial diffusion of the particles. During the arc decay phase, for the charged particles, the chemical reactions had the greatest effect on the time variations of the particle number densities; however, for the neutral particles the time variations of the number densities were mutually influenced by convections, diffusions, and chemical reactions. Finally, the LTS results further demonstrate the validity of the non-CE model at different gas flow rates.

Application of maximum likelihood methods to laser Thomson scattering measurements of low density plasmas

Laser Thomson scattering (LTS) is an established plasma diagnostic technique that has seen recent application to low density plasmas. It is difficult to perform LTS measurements when the scattered signal is weak as a result of low electron number density, poor optical access to the plasma, or both. Photon counting methods are often implemented in order to perform measurements in these low signal conditions. However, photon counting measurements performed with photo-multiplier tubes are time consuming and multi-photon arrivals are incorrectly recorded. In order to overcome these shortcomings a new data analysis method based on maximum likelihood estimation was developed. The key feature of this new data processing method is the inclusion of non-arrival events in determining the scattered Thomson signal. Maximum likelihood estimation and its application to Thomson scattering at low signal levels is presented and application of the new processing method to LTS measurements performed in the plume of a 2-kW Hall-effect thruster is discussed.