Optimization of confocal laser induced fluorescence in a plasma

Vandervort, Robert; Elliott, Drew; McCarren, Dustin; McKee, John; Soderholm, Mark; Sears, Stephanie; Scime, Earl

doi:10.1063/1.4886424

Cited by 5 publications

(5 citation statements)

References 18 publications

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“…One way to facilitate optical alignment with a moving diagnosed volume is the adaption of a confocal optical assembly [75], focusing the laser beam and the collection optics to the diagnosed volume with a single objective lens. As the injection and collection optical paths are united, confocal LIF setups do not require the alignment of translatable collection optics to the laser's path.…”

Section: Collection Optics and Optical Sensorsmentioning

confidence: 99%

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

Yip

Jiang

2021

Plasma Sci. Technol.

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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.

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Section: Collection Optics and Optical Sensorsmentioning

confidence: 99%

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

Yip

Jiang

2021

Plasma Sci. Technol.

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“…Previous attempts to measure ion velocity distributions with confocal single-photon LIF have been reported, with longitudinal spatial localization estimated to be approximately 3 cm using an objective focal length of 20 cm. 20 The purpose of this work is to report for the first time, confocal, single-photon LIF measurements that achieve longitudinal spatial localization of the same order of magnitude as conventional LIF methods. The new technique overcomes two major limitations in previous designs: it removes the need for two, spatially separated sightlines required by conventional LIF schemes and extends the short, millimeterscale observing distance limitation of conventional confocal microscopy to hundreds of millimeters.…”

Section: Introductionmentioning

confidence: 99%

Confocal laser induced fluorescence with comparable spatial localization to the conventional method

Thompson

Henriquez

Scime

et al. 2017

Review of Scientific Instruments

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We present measurements of ion velocity distributions obtained by laser induced fluorescence (LIF) using a single viewport in an argon plasma. A patent pending design, which we refer to as the confocal fluorescence telescope, combines large objective lenses with a large central obscuration and a spatial filter to achieve high spatial localization along the laser injection direction. Models of the injection and collection optics of the two assemblies are used to provide a theoretical estimate of the spatial localization of the confocal arrangement, which is taken to be the full width at half maximum of the spatial optical response. The new design achieves approximately 1.4 mm localization at a focal length of 148.7 mm, improving on previously published designs by an order of magnitude and approaching the localization achieved by the conventional method. The confocal method, however, does so without requiring a pair of separated, perpendicular optical paths. The confocal technique therefore eases the two window access requirement of the conventional method, extending the application of LIF to experiments where conventional LIF measurements have been impossible or difficult, or where multiple viewports are scarce.

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“…This widthis about 30% broader than conventional LIF, which has a FWHM of 1.1 mm and encompasses 97.3% of the signal. Previously published state-of-the-art confocal LIF designs exhibit collection regions several centimeters wide, leading to flattened profiles of measured plasma quantities[151]. By comparison, the excellent agreement between the profiles measured confocally and in the conventional way, even on the far side of the core at x > 0 as shown inFig.…”

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confidence: 65%

Three-Dimensional Multispecies Distribution Functions in a Plasma Boundary With an Oblique Magnetic Field

Thompson¹

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The physics of a weakly magnetized boundary region are investigated by immersing a 76.2 mm diameter stainless steel disk in a helicon plasma. The velocity distributions of ions and neutral particles are mapped in the boundary region, created in 3.6 mTorr argon gas, 650 W RF power, and magnetic field B = 0.06 T, where the magnetic field lines obliquely intersect the wall (α = 16 •). These distributions are mapped using laser induced fluorescence in three spatial (3D) and three velocity (3V) dimensions, and reveal that models neglecting any one of the velocity components omit important components of the flow field. Electrostatic probe measurements are presented and establish a typical helicon plasma discharge with electron temperature T e ≈ 4.2 eV and density n ≈ 5.5 × 10 17 m −3. In addition to the observed ion temperature, T i ≈ 0.37 eV, these measurements indicate a plasma in which the electrons are highly magnetized (the ratio of the electron cyclotron frequency to the electron collision frequency is ω ce /ν e = 2.9 × 10 3) and the ions are weakly collisional (the ratio of the ion cyclotron frequency to the ion collision frequency is ω ci /ν i = 0.5 ± 0.3). Ion drift velocity measurements are presented in the E × B direction and are compared to predictions from measured plasma potential gradients. Ion flow fields are compared to predictions from collisional fluid simulations and particle-in-cell models. These models require collisions to reproduce the ion drifts well. Neutral particle distributions are observed to be in thermal equilibrium with the chamber walls (T n ≈ 0.028 eV) and essentially non-flowing on average. Charge exchange population fractions are expected to be O(10 −2), and are too small to be resolved, but are excluded to the 5% level. "O me! O life!... of the questions of these recurring; Of the endless trains of the faithless-of cities fill'd with the foolish; Of myself forever reproaching myself, (for who more foolish than I, and who more faithless?) Of eyes that vainly crave the light-of the objects mean-of the struggle ever renew'd; Of the poor results of all-of the plodding and sordid crowds I see around me; Of the empty and useless years of the rest-with the rest me intertwined; The question, O me! so sad, recurring-What good amid these, O me, O life? Answer. That you are here-that life exists, and identity; That the powerful play goes on, and you may contribute a verse." Walt Whitman, Leaves of Grass (1892) v I want to first express my gratitude for the support of my advisor, Earl Scime, who smoothed my way to WVU and who for the last two and half years has trusted me with his lab probably more than he had reason to. He has answered annoying questions at 7 am and after midnight and was available for lunch almost every day I've been in Morgantown. That kind of face-to-face relationship building is exceptional and not lost on me. He is a mentor and a friend.

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Optimization of confocal laser induced fluorescence in a plasma

Cited by 5 publications

References 18 publications

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

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

Confocal laser induced fluorescence with comparable spatial localization to the conventional method

Three-Dimensional Multispecies Distribution Functions in a Plasma Boundary With an Oblique Magnetic Field

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