Laser-induced Fluorescence Diagnostics Research Articles

Recent development of LIF diagnostics in ASIPP

In this article, recent research progress of laser induced fluorescence (LIF) diagnostics in ASIPP is reviewed. The fundamental goal of ASIPP's effort on the LIF diagnostics is to develop a working velocity distribution function (VDF) diagnostic to monitor helium ash removal in burning plasmas. Real-time monitoring of helium ash VDF requires consideration in radiation safety, measurement response time, and compatibility of tokamak operation, as well as ion/neutral VDFs with a temperature at possibly >20 eV. These considerations, almost unique to large-scale instruments or specifically to tokamak plasmas, require renewed research and development (R&D) efforts in optical design, automation, and expansion of laser functionality for the LIF diagnostics that were often not considered in conventional applications of the LIF diagnostics. To this end, ASIPP invested a series of research effort into the development of LIF techniques. Signal evaluation has been performed on a diagnostics-test device (LTS), and the obtained signal strength has been extrapolated to tokamak edge and divertor relevant environments with promising results. The extrapolations compared the available solid angle and plasma density between the test device and the design scenario where the LIF diagnostics will potentially be implemented in a tokamak edge and divertor plasma. The estimation neglects the higher density of energetic electrons in the edge and divertor regions, making it a conservative assessment. Automated post-DAQ processing of LIF signals using both “conventional” methods and an AI-augmented method were also explored. Both approaches yielded usable results, but the AI-trained method showed a faster response, which is advantageous for real-time plasma diagnostics. Basic mechanisms of LIF diagnostics were also explored. Specifically, the limitations of lock-in modulation was demonstrated to determine the limitation of time-resolved measurements using such methods. De-modulation and degradation of the LIF signal occurred as the modulation frequency exceeded 1/10th of the fluorescence frequency. This finding sets a practical limit of the modulation frequency up to which we can benefit from lock-in amplification. These published works along with other unpublished progress will be thoroughly discussed in this article to illustrate ASIPP's effort towards the implementation of LIF diagnostics in future tokamak devices and to promote the application of LIF diagnostics in other fields of research.

Ion dynamic characterization using phase-resolved laser-induced fluorescence spectroscopy in a Hall effect thruster.

Significant information on the dynamics of the plasma constituents in Hall effect thrusters can be obtained using minimally intrusive techniques, such as laser-induced fluorescence (LIF) diagnostics. Indeed, LIF provides an excellent tool to determine the ion velocity distribution function with high spatial resolution. Even in a steady-state operation, recording time-resolved maps of the velocity distribution is relevant due to persisting time-dependent features of the thruster discharge. One of the preeminent phenomena that render the ion velocity distribution to be time dependent is commonly attributed to the breathing mode, characterized by pronounced oscillations in the discharge current. The goal of this work is to propose a new technique for plasma dynamic studies based on LIF spectroscopy with phase-resolution during the breathing period. For this purpose, the Hilbert transform is used to define the instantaneous phase of oscillation of the thruster current. Ion velocity distribution modification over assigned phases of oscillation is measured simultaneously and in real-time thanks to a fully numerical analysis of the data.

Accuracy of using metastable state measurements in laser-induced fluorescence diagnostics of xenon ion velocity in Hall thrusters

Laser-induced fluorescence measurements of singly-charged xenon ion velocities in Hall thrusters typically target metastable states due to lack of available laser technology for exciting the ground state. The measured velocity distribution of these metastable ions are assumed to reflect the ground state ion behavior. However, this assumption has not been experimentally verified. To investigate the accuracy of this assumption, a recently developed xenon ion (Xe II) collisional-radiative model is combined with a 1D fluid model for ions, using plasma parameters from higher fidelity simulations of each thruster, to calculate the metastable and ground state ion velocities as a function of position along the channel centerline. For the HERMeS and SPT-100 thruster channel centerlines, differences up to 0.5 km s−1 were observed between the metastable and ground state ion velocities. For the HERMeS thruster, the difference between the metastable and ground state velocities is less than 150 m s−1 within one channel length of the channel exit, but increases thereafter due to charge exchange (CEX) that reduces the mean velocity of the ground state ions. While both the ground state ions and metastable state ions experience the same acceleration by the electric field, these small velocity differences arise because ionization and CEX directly into these states from the slower neutral ground state can reduce their mean velocities by different amounts. Therefore, the velocity discrepancy may be larger for thrusters with lower propellant utilization efficiency and higher neutral density. For example, differences up to 1.7 km s−1 were calculated on the HET-P70 thruster channel centerline. Note that although the creation of slow ions can influence the mean velocity, the most probable velocity should be unaffected by these processes.

Laminar burning velocities and nitric oxide formation in premixed dimethyl ether/air flames: Experiments and kinetic modeling

Adiabatic laminar burning velocities and post-flame NO mole fractions for flat, non-stretched, premixed dimethyl ether/air flames were experimentally determined with a heat flux burner combined with laser-induced fluorescence diagnostics, over equivalence ratios ranging from 0.7 to 1.6, at atmospheric pressure and initial temperatures from 298 to 338 K. The present burning velocity measurements were then compared with selected data available in the literature obtained with different techniques. The comparison showed reasonably good agreement with recent datasets obtained at different temperatures, as well as possible outliers not suitable for the validation of kinetic models. The detailed kinetic mechanism of the authors was extended by the reactions of dimethyl ether. A comparison of experimental and computational results using two contemporary detailed chemical kinetic mechanisms, along with the one from the authors presented in this work, was also conducted and discussed. Discrepancies between experiments and model predictions and among models themselves were observed, especially under rich conditions. Further numerical analyzes were performed to identify the main causes of the observed differences. Notwithstanding, the present model showed overall good performances in reproducing both laminar burning velocities and nitric oxide mole fractions. Kinetic modeling was also performed to enhance the understanding of the intrinsic NOx emission characteristics of dimethyl ether by comparing the present measurements with those obtained for ethanol/air flames under identical conditions.

Probabilistic effect of metastable excitation on lock-in amplified plasma laser-induced fluorescence diagnostics at modulation frequencies comparable to the fluorescence frequency

In this paper, effects of discharge parameters and modulation frequency on the signal of laser-induced fluorescence measurements of ion velocity distribution functions are investigated in the LIF Test Source. A maximum modulation frequency is found for each given set of parameters, beyond which the signal gradually declines. Meanwhile, this maximum modulation frequency occurred consistently at ~1/10 of the theoretical frequency limit and photon counts received by a photomultiplier tube, which indicates that as modulation frequency and the associated per-pulse-excitation-event count decrease, the transition from the macroscopic statistical signal to the microscopic probabilistic signal is a gradual process.

Single- and two-photon absorption laser-induced fluorescence spectroscopy in rare gases for gridded ion thruster diagnostics

Methods based on laser-induced fluorescence spectroscopy are widely used for spatially resolved non-intrusive diagnostics of atomic or molecular densities and velocity distributions in plasma applications. With regard to electric space propulsion, one focus is on the investigation of rare gases such as xenon or krypton, which are currently the favored propellants in gridded ion- and Hall-effect thrusters. For gridded ion engines, diagnostics of neutral atoms is of interest since charge-exchange processes between neutrals and ions are the main driver of accelerator grid erosion, which limits the lifetime of a gridded ion thruster. Extending the capabilities of the advanced electric propulsion diagnostics platform which has been developed by the IOM and partners, single- and two-photon absorption laser-induced fluorescence diagnostics have been set-up recently at our institute. Both experimental set-ups, and as a series of first applications, measurements of krypton neutrals in the plume of the radiofrequency ion thruster RIT-10 (ArianeGroup GmbH), and xenon neutrals within the discharge chamber of a gridded radiofrequency ion source developed at IOM, are presented.

Evidence of a free-space ion acceleration layer in the plume of a quad confinement plasma source

The quad confinement plasma source is a novel plasma device developed for space propulsion applications, whose core is an E×B discharge with open electron drift. The magnetic field is produced by independently powered electromagnets able to generate different magnetic field topologies with the ultimate aim of manipulating the ion flow field for achieving thrust vectoring. In this work, we map the ion velocity in the plasma ejected from the quad confinement thruster with different magnetic configurations using non-intrusive laser-induced fluorescence diagnostics. Measurements show a steep ion acceleration layer located 8 cm downstream the exit plane of the discharge channel, detached from any physical boundary of the plasma source. In this location, the ion velocity increases from 3 to 10 km/s within a 1 cm axial region. The ion acceleration profile has been characterized under multiple testing conditions in order to identify the influence of the magnetic field intensity and topology on this peculiar ion acceleration layer.

Laser induced fluorescence diagnostic for velocity distribution functions: applications, physics, methods and developments

With more than 30 years of development, laser-induced fluorescence (LIF) is becoming an increasingly common diagnostic to measure ion and neutral velocity distribution functions in different fields of studies in plasma science including Hall thrusters, linear devices, plasma processing, and basic plasma physical processes. In this paper, technical methods used in the LIF diagnostic, including modulation, collection optics, and wavelength calibration techniques are reviewed in detail. A few basic physical processes along with applications and future development associated with the LIF diagnostics are also reviewed.

LIF diagnostics for selective and quantitative measurement of PAHs in laminar premixed flames

Laser-induced fluorescence (LIF) technique has been widely applied in PAH measurement in sooting flames, but it is still a big challenge to quantitatively identify the specific PAH species due to the complexities of spectral characteristics and calibration method. In this study, we aimed at developing a spectral-resolved LIF analytic model that can obtain PAH mole fractions in flames. A comprehensive LIF spectrum database in the wide temperature and concentration ranges was firstly established by the LIF experiments of gas-phase PAHs in an optical cell. Then, a spectral-resolved LIF analytic model was presented based on linear superposition regulation and an optimization fitting algorithm, and the LIF spectra of PAH components can be decomposed from the LIF spectra of the flame using this model. Finally, the integral fluorescence intensities of each PAH component were calibrated to their mole fractions by the calibration factors at corresponding temperatures. In this way, we can measure the mole fractions of several PAHs. Subsequently, the presented LIF method was applied to investigate the PAH formation characteristics at different flame temperatures. The experimental results showed that the mole fractions of each PAH except naphthalene decreased around 60% to 70% when the temperature increased from 1186 K to 1483 K, while the mole fraction of naphthalene kept nearly constant. The mole fractions simulated by a chemical kinetic model well reproduce the experimental results. According to the chemical kinetic analysis, C2H2 oxidation reaction was promoted and the C2H2-addition reactions were suppressed by increasing temperatures, which suppressed the PAH formation.

Facility pressure effects on a Hall thruster with an external cathode, II: theoretical model of the thrust and the significance of azimuthal asymmetries in the cathode plasma

A theoretical model of the thrust produced by a Hall thruster has been developed to provide insight into the longstanding problem of performance dependence on facility pressure. Inputs to the model have been provided by 2D (r–z) axisymmetric simulations that were guided by measurements of the ion velocity field from laser induced fluorescence diagnostics. The combined analyses yield very good agreement with thrust measurements from ground tests of the SPT-140, a Hall thruster operating with an external cathode, and suggest two new findings. First, at intermediate pressures (∼9–15 μTorr) the rise of the thrust with increasing pressure is largely due to fast atoms produced by charge exchange collisions with beam ions. The acceleration region remains spatially invariant and the thrust rise is linear with pressure. As the pressure increases further, displacements of the acceleration region towards the anode become sizeable and reduce the rate of increase in thrust. Second, the thrust increase from vacuum to the lower values of pressure (≲9 μTorr) is driven largely by transport of electrons along magnetic lines, from the cathode to the near-plume of the thruster, which explains why the rise is nonlinear with pressure. It is argued based on first-principles that this higher rate of thrust variation observed at the lowest pressures may be strongly linked to the highly asymmetric distribution of the external-cathode electrons around the discharge. This would explain the insensitivity of performance in thrusters with centrally-mounted cathodes as well as the longstanding challenges 2D axisymmetric simulations have had in uncovering the source of the thrust dependence on facility pressure.

Technology of Regeneration of the Bone Tissue in a Rotational Bioreactor: Modeling of the Fluid Flow and Laser-Induced Fluorescence Diagnostics

Technology of Regeneration of the Bone Tissue in a Rotational Bioreactor: Modeling of the Fluid Flow and Laser-Induced Fluorescence Diagnostics

Plasma beam characterization along the magnetic nozzle of an ECR thruster

Experimental characterization of plasma properties along the magnetic nozzle of an electron cyclotron resonance thruster is presented here. A permanent magnet (PM) prototype and a solenoid prototype are tested, whose main difference relies on the magnetic field strength and topology. A cylindrical Langmuir probe is used to measure plasma potential, plasma density and electron temperature. In the PM thruster setup, a laser induced fluorescence diagnostics is performed simultaneously with the Langmuir probe to measure the mean ion kinetic energy, and a Faraday gridded probe to characterize the angular plasma beam. An effective electron cooling rate has been identified, as well as the dependence of the total plasma potential drop with the mass flow rate. Results are compared with a supersonic collisionless fluid-kinetic 1D model where electron dynamics account for magnetic mirror effects and potential barriers, while ions are treated as a fluid cold species. The comparison allows to estimate the sonic transition of the plasma flow.

Tokamak research at the Ioffe Institute

Recent research at three small tokamaks with different parameters located at the Ioffe Institute—the spherical tokamak Globus-M, the large aspect ratio tokamak FT-2 and the compact tokamak TUMAN-3M—are reviewed. This overview covers energy confinement (Globus-M and FT-2), L–H transition (TUMAN-3M and FT-2), Alfvén waves (Globus-M and TUMAN-3M), ion cyclotron emission (TUMAN-3M), major plasma discharge disruption (Globus-M) and scrape-off layer (Globus-M) studies. A full-f global gyrokinetic modeling benchmark using synthetic diagnostics in FT-2 is described. Anomalous absorption and emission in electron cyclotron resonance heating experiments due to the parametric excitation of localized upper hybrid waves are analyzed theoretically. Progress in the development of the neutral particle analysis, gamma-ray spectrometry and divertor Thomson scattering combined with laser-induced fluorescence diagnostics for ITER is discussed. The status of the new Globus-M2 spherical tokamak is reported.

Evaluation of emission contributions from charge-exchange between the excited states of deuterium with He+ during diagnostic of thermal helium gas beam injection and laser-induced fluorescence

Emission contributions from charge-exchange of excited deuterium (n = 2, 3) with He+ are evaluated in a 1-D kinetic collisional radiative model in order to analyze their effects on the Thermal Helium Beam (THB) line-ratio diagnostic on ASDEX Upgrade and Laser Induced Fluorescence (LIF) He I density measurements in ITER. Recent charge-exchange calculations show that cross sections from excited deuterium (n = 2, 3) with He+ are over 4-orders of magnitude higher than those from the ground state (n = 1) and occur at very low energies where they are more likely to interact with the thermal He+ ions introduced by ionization of the diagnostic helium gas-puff injection. Higher densities of excited deuterium are typically present in the Scrape-Off Layer (SOL), divertor, and edge regions of tokamaks, where the LIF and THB helium diagnostics are typically used for nHeI and simultaneous determination of electron temperatures and densities and where contributions from charge-exchange emission may offset these values if not taken into account. The analysis presented in this work shows that due to the higher density of deuterium in the ground rather than in excited states and the divergent behavior of deuterium and He+ density profiles along the SOL and edge regions, the deuterium-He+ charge-exchange contributions to the helium puff emission are 3-orders of magnitude lower than those from electron-impact excitation. Similar plasma conditions are expected in the ITER divertor, with the exception that in the area near the strike-points and targets, the electron temperature is not high enough to excite from the ground state but deuterium, electron, and He+ densities are high enough to dominate the emission from charge-exchange and recombination. These findings strengthen the assumption made in the present line-ratio model that helium emission from gas-puff into plasma is mainly dominated by electron-excitation. It is also shown that, in general, charge-exchange helium emission is 2-orders of magnitude higher than the emission due to recombination. These findings suggest the importance of including charge-exchange processes as a source of neutrals in ionic fractional abundance calculations in plasmas and helium-ash transport modeling in fusion reactors.

Analysis and comparison of ion-acoustic wave reflection using laser-induced fluorescence and Langmuir probes

The first laser-induced fluorescence measurements of ion acoustic wave reflection are presented. The experiment is performed in a multi-pole cylindrical chamber using a singly ionized argon (ArII) plasma produced by means of a hot cathode. Ion-acoustic waves are launched from a mesh antenna and reflected/absorbed by a biased, solid boundary (electrode). A comparison between the Langmuir probe and laser-induced fluorescence diagnostics is presented, with complementary Electric-field probe measurements.

Time-resolved laser-induced fluorescence diagnostics for electric propulsion and their application to breathing mode dynamics

Several techniques have been developed recently for performing time-resolved laser-induced fluorescence (LIF) measurements in oscillating plasmas. One of the primary applications is characterizing plasma fluctuations in devices like Hall thrusters used for space propulsion. Optical measurements such as LIF are nonintrusive and can resolve properties like ion velocity distribution functions with high resolution in velocity and physical space. The goals of this paper are twofold. First, the various methods proposed by the community for introducing time resolution into the standard LIF measurement of electric propulsion devices are reviewed and compared in detail. Second, one of the methods, the sample-hold technique, is enhanced by parallelizing the measurement hardware into several signal processing channels that vastly increases the data acquisition rate. The new system is applied to study the dynamics of ionization and ion acceleration in a commercial BHT-600 Hall thruster undergoing unforced breathing mode oscillations in the 44–49 kHz range. A very detailed experimental picture of the common breathing mode ionization instability emerges, in close agreement with established theory and numerical simulations.

A Fluorescence Spectrum Analyzer Based on a Fiber-Optic Y-Circulator

We consider a method of probe analysis employing fiber-optic a composite Y-circulator design for the implementation of laser-induced fluorescence (LIF) spectroscopy diagnostics. The proposed principle of the LIF analyzer is based on spatially separated directed optical beams of excitation and fluorescence radiations. Conditions of the effective Y-circulator operation as a passive fiber-optic switch are determined. Experimental investigation of the switched radiation losses is performed. An example of using the proposed analyzer for LIF diagnostics of biological media is presented.

On limitations of laser-induced fluorescence diagnostics for xenon ion velocity distribution function measurements in Hall thrusters

Hall thruster operation is characterized by strong breathing oscillations of the discharge current, the plasma density, the temperature, and the electric field. Probe- and laser-induced fluorescence (LIF) diagnostics were used to measure temporal variations of plasma parameters and the xenon ion velocity distribution function (IVDF) in the near-field plasma plume in regimes with moderate (<18%) external modulations of applied DC discharge voltage at the frequency of the breathing mode. It was shown that the LIF signal collapses while the ion density at the same location is finite. The proposed explanation for this surprising result is based on a strong dependence of the excitation cross-section of metastables on the electron temperature. For large amplitudes of oscillations, the electron temperature at the minimum enters the region of very low cross-section (for the excitation of the xenon ions); thus, significantly reducing the production of metastable ions. Because the residence time of ions in the channel is generally shorter than the time scale of breathing oscillations, the density of the excited ions outside the thruster is low and they cannot be detected. In the range of temperature of oscillations, the ionization cross-section of xenon atoms remains sufficiently large to sustain the discharge. This finding suggests that the commonly used LIF diagnostic of xenon IVDF can be subject to large uncertainties in the regimes with significant oscillations of the electron temperature, or other plasma parameters.

Анализатор спектра флуоресценции на основе оптоволоконного Y-циркулятора

AbstractWe consider a method of probe analysis employing fiber-optic a composite Y-circulator design for the implementation of laser-induced fluorescence (LIF) spectroscopy diagnostics. The proposed principle of the LIF analyzer is based on spatially separated directed optical beams of excitation and fluorescence radiations. Conditions of the effective Y-circulator operation as a passive fiber-optic switch are determined. Experimental investigation of the switched radiation losses is performed. An example of using the proposed analyzer for LIF diagnostics of biological media is presented.

Planar laser-induced fluorescence diagnostics of water droplets heating and evaporation at high-temperature

Gas-steam-droplet technologies are widely used in systems operating at high temperatures ranging between 400 and 2000°C, namely: fire-fighting systems, thermal fluid cleaning, fuel compounding, industrial waste gasification and evaporation systems for advanced fuel components, cleaning of thermally loaded surfaces of power equipment, etc. System parameters are usually selected empirically via multiple tests and continuous trial operation of appropriate units, aggregates, assemblies, and installations. This situation is caused by insufficient basic knowledge of conditions and parameters of high-temperature (over 500°C) heating and evaporation of water and water-based emulsions, solutions, and slurries. Limited information is available regarding the evaporation rates dependent on the temperature of gaseous medium. Consequently, the up-to-date evaporation models allow the researchers to achieve adequate values (in good agreement with the experiment) of evaporation rates at air temperatures not exceeding 300–400°C. The paper presents a set of experiments on water droplets with the size ranging from 1 to 2mm, which is used to create the information database on high-temperature evaporation parameters. The approach to measuring the evaporation rate involves observation of the droplet size or more exactly its mean radius, and recording the time of its existence. A high-speed video camera and Tema Automotive software with different tracking algorithms are used for experimental observations. During gas heating, the distribution of highly non-homogeneous and non-steady temperature field in evaporating water droplets is detected by the hardware and software cross-correlation system and Planar Laser-induced Fluorescence optical diagnostics. Instantaneous and medium evaporation rates are computed for the whole period of the droplet lifetime. Highly nonlinear evaporation rate dependences are suggested for gas temperatures and the water droplet surface, size, and time of gas heating. Approximate relationships are described for the prediction of evaporation rates depending on the basic parameters. The evaporation rate is shown to increase several-fold during the heating of a water droplet in a high-temperature gaseous medium. The experimental data are described most accurately by exponential dependences of water evaporation rate versus the airflow temperature and velocity. This result makes it possible to predict the corresponding highly nonlinear dependences of evaporation rate versus the density of convective heat flux and the overall thermal conditions in the industrial chambers, evaporators, heat exchangers, etc. The mathematical expressions obtained can be used to identify the effective conditions of water droplet heating in a large group of high-temperatures applications as well as for developing high-temperature liquid evaporation models.