Session ET3: Laser Diagnostics I

Coherent Thomson scattering: a four-wave mixing approach to low temperature plasma diagnostics

The proposed novel four-wave mixing technique of coherent Thomson scattering (CTS) makes use of induced optical lattices within a plasma, to scatter off a third beam from them, giving rise to the coherent CTS signal beam. The technique builds on an established and demonstrated single shot diagnostic method, termed coherent Rayleigh-Brillouin scattering (CRBS), which has already been successfully applied to neutral flows and neutral species in glow discharges. Owing to its four-wave mixing nature, CTS will enable higher spatial resolution and lower detectable number densities for the electrons than conventional Thomson scattering, as applied to low and high temperature plasmas. Towards this goal, the theoretical framework for CTS has been developed, as well as the specification of the appropriate operational experimental parameters for successful CTS implementation in e.g. a low temperature plasma. The one-dimensional nonstationary Boltzmann equation was solved for electrons in the BGK approximation with allowance for the ponderomotive force, together with the Poisson equation for the potential distribution. As an example, nonequilibrium weakly ionized Argon plasma was considered. Importantly, this has led to the simulation of the anticipated CTS spectra for the case when the wavelength of the induced optical lattices is much smaller than the Debye length. Ultimately, successful experimental demonstration of CTS can find application in a multitude of areas in plasma physics, since it will allow for detailed, non-perturbative measurements of electron density and temperature, hard to be attained by other measurement techniques.   

Spatially extended high-resolution Thomson scattering diagnostics with volume Bragg grating filters

In this work, we elaborate on details of one-dimensional Thomson scattering (TS) measurement with volume Bragg grating (VBG) filters. The VBGs act as a spectral/angular filter and reject the narrow linewidth lights near the laser line such as Rayleigh scattering, Mie scattering, and stray light to access the electron TS component. The TS technique with VBGs has often been used as a point measurement technique, however, the technique can be easily extended to measure several mm with a higher resolution for a 1D measurement with a proper angle tuning. Methods to further extend the measurable region using multiple VBG filters are also proposed and demonstrated, which differs from the commonly known application of multiple filters to improve the optical density. The 1D TS measurements are carried out on a DC discharge plasma source with ~1 mm thick column and electron density in 10^11-13/cm^3. A sharp gradient of plasma properties along the laser line over a few hundred µm is successfully observed with a 26 µm spatial resolution.   

GEC Student Excellence Award Finalist Presentation - Experimental and numerical investigation of low-pressure iodine plasmas

This work investigates iodine as an alternative propellant for space plasma propulsion. Measurements are taken in a low-pressure inductively-coupled plasma chamber used as the ionization stage of an ion-gridded plasma thruster. Spatially resolved optical and electrostatic diagnostics are developed and implemented. First, charged species are characterized using Langmuir probes and laser-induced photodetachement diagnostics. Then, atomic absolute temperature and density measurements (with a focus on the two first electronic states: the ground state and the first excited state only 0.94 eV above) are conducted using laser absorption spectroscopy at several wavelengths and the TALIF (Two-photon Absorption Laser Induced Fluorescence) technique. Furthermore, the molecular dissociation rate is measured using laser absorption spectroscopy beyond 20000 cm^-1 where the diffusive part of the absorption spectrum leads exclusively to molecular photodissociation. Finally, the experimental results are gathered and compared to a global model and 1D fluid simulations to validate the existing iodine set of cross-sections.

    

Measurements of strength and fluctuation of 2D electric fields in plasmas using a fine particle trapped with laser tweezers

There is a strong need for processing of materials at nanometer scale to allow the continuous miniaturization of devices. Plasma processing is mainly used to manufacture these devices. The measurement of sheath electric field in a microscopic space is an important issue for optimizing plasma processing of materials. For example, small changes and fluctuations in the electric field have a significant impact on etching and deposition into high aspect ratio micro-/nano-structure. However, few reports have been published on sensitive measurements of electric field distributions with high spatial resolution on the micrometer scale. Therefore, in this study, we have investigated such sensitive measurements of strength and fluctuation of electric field in plasma using optically trapped fine particles by a laser-tweezer technique [1]. 

To generate Ar plasmas, a rf voltage with a frequency 13.56MHz was applied between a powered ring electrode and ground planar electrode. A fine particle (a diameter of 20 µm) was injected into the plasmas. The laser trapped fine particle was suspended at plasma/sheath boundary region. At the levitation position of a fine particle, the electrostatic force and the laser light force on the particle were balanced by the gravity. The force of the laser on the particle was obtained from a ray optical model [2], and a particle charge was deduced from Orbit Motion Limited model with ion collision [3]. Because the trapped particle was negatively charged, it can be a probe of electric fields. Therefore, we deduced vertical electric field strengths Ez from these derivations. Moreover, strengths and fluctuations of horizontal electric fields Er were deduced by deriving horizontal force balance. As the results, we investigated 2D profiles of electric field vector with high spatial resolution in micrometer scale. Details are shown in the conference.    

Vibrational excitation measurements by CARS in a nanosecond discharge

A broadband, dual-pump CARS (coherent anti-Stokes Raman scattering) setup is realized for simultaneous measurements of nitrogen and carbon dioxide vibrational distribution functions in the electronic ground state. Measurements are performed in a 1 mm discharge gap between two plane-parallel molybdenum electrodes at sub-atmospheric pressure (0.1 to 1 bar). In a pure nitrogen discharge it is found that for vibrational states with v < 8 a two-temperature distribution function is a very good approximation to the vibrational distribution. The excitation conditions for vibrational states are constant during most of the discharge pulse and agree very well with the excitation rates from the literature for the given electric field value, which was measured by E-FISH (electric field induced second harmonic generation). The development of the vibrational states in the afterglow is compared to a state-to-state kinetic model, which is dominated by vibrational-vibrational transfer and transport losses. Here too, good agreement was found for rates available in the literature. It is planned to extend the measurements to nitrogen/carbon dioxide gas mixtures, to investigate the vibrational interaction between both species.

    

GEC Student Excellence Award Finalist Presentation - Electric-field-vector-profile measurement in gases based on electric-field-induced second-harmonic generation

Electric-field measurement based on the electric-field-induced second-harmonic generation (E-FISHG) method has attracted attention because of its noninvasive field measurement in plasmas and gases [1]. In the E-FISHG method, the E-FISHG signal changes depending on the polarization of the probe laser beam. It is expected that this characteristic can be used to measure the direction of the electric field. In this paper, we investigate the optimal laser energy for the measurement and measure an electric-field-vector profile. We use an optical geometry such that it is sensitive only to the electric field component parallel to the polarization of the laser beam and acquire an E-FISHG signal by rotating the polarization. The dependence of the E-FISHG signal on the polarization angle corresponds to the simulated result below the filamentation threshold energy. We have proposed a method to restore a one-dimensional electric-field-profile from a sequence of E-FISHG signals along the probing laser path [2]. We measure an electric field vector profile under the filamentation threshold energy by this method. We successfully restore the one-dimensional electric-field-vector profile from the E-FISHG signal along the laser path.

    

High Spatial Resolution Measurement of Electric Field Vector in Positive Secondary Streamer Discharge under Atmospheric-Pressure Air

Electric field vector measurement of a streamer discharge under atmospheric-pressure air is crucially important for systematic understanding of the production mechanisms of reactive species. However, the measurement has never been achieved for the streamer discharge, due to its irreproducible, complex branching structure. Further, measurement-accuracy uncertainty and/or low spatial resolution of conventional electric-field sensors has prevented reliable determination. Here, the reliable electric field vector measurement was first achieved for the streamer discharge by the development of two original apparatuses: a multi-anode plasma generator capable of reproducibly realizing a non-branching, straight streamer discharge and a measurement-accuracy-quantified electric-field induced second harmonic (E-FISH) generation sensor with 20-mm spatial resolution, which is one-order superior. The vector measurement demonstrated that in an initial phase of the secondary streamer discharge, the electric field inside the discharge was as high as 150±30 Td. This suggested the occurrence of ionization inside the discharge, which was supported by previous electron density measurement but inconsistent with earlier numerical simulations predicting insignificant ionization. The contradiction could be caused by the fact that the measured electron density distribution at the primary streamer contact with the cathode was different from the numerical simulations.