Laser induced fluorescence of Ar-I metastables in the presence of a magnetic field

Laser-Induced Fluorescence (LIF) provides the temperature and flow velocity of a target species by direct measurement of its velocity distribution via Doppler Shift. A LIF diagnostic has been developed at the Caltech Water-Ice Dusty Plasma Experiment that uses an ultra-narrow tuneable diode laser to pump the λ vac = 696.735 nm argon neutral transition. A photomultiplier detects fluorescence emission at λ vac = 772.633 nm. Signal to noise ratios in excess of 100 are achieved along with a high degree of reproducibility between measurements. A Labview program fully automates data collection throughout the three-dimensional plasma volume by controlling four stepper motors and recording measured data. Argon neutral temperature is measured to be slightly above room temperature. Challenges such as the lack of absolute calibration of diode lasers and wavelength drift due to slight changes in ambient room conditions are overcome to measure bulk neutral flow speeds on the order of 1-2 m/s with resolution on the order of 2/3 of a meter per second. Highspeed video shows that introducing an argon flow to a cloud of ice grains causes the cloud of ice grains to move and change shape. Ice grain motion is analyzed and found to show agreement with neutral LIF flow measurements. Surprisingly, when the flow ceases, the ice grain cloud reverts to its original location and shape.

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“…One subtle issue is that the linewidth of the 696.735 nm transition in neutral argon is ∆f = 5.6 MHz [10]. One might therefore presume as in Ref.…”

Section: Resolving Details Smaller Than the Natural Linewidthmentioning

confidence: 99%

“…PLIF (Planar LIF) imaging using a camera sensor to detect LIF emission from a planar excitation beam has been used to build an image of density [9]. The shift between the double-peaked LIF spectrum from Zeeman splitting can even be used to measure magnetic fields [10].…”

Section: Introductionmentioning

confidence: 99%

Laser-induced fluorescence measurement of very slow neutral flows in a dusty plasma experiment

Marshall

Bellan

2020

Review of Scientific Instruments

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“…The work of C. Csambal et al [10] provides an interpretation of the laser absorption profiles for a cylindrical magnetron discharge based on a frequency integrated absorption coefficient over the whole absorption structure and not distinguishing between sub-states with different magnetic quantum numbers m. Several recent studies were done using laser-induced fluorescence [11] and high resolution optical emission spectroscopy [4] to evaluate effects of a magnetic field on transition profiles in argon. However evaluation of the densities of individual magnetic sub-levels and their impact on optical properties were not discussed.…”

Section: Introductionmentioning

confidence: 99%

Optical properties of magnetized transient low-pressure plasma

Bergert¹,

Mitic²

2019

Plasma Sources Sci. Technol.

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A plasma under the influence of an external magnetic field changes the optical properties due to the Zeeman splitting of the energy levels. This splitting degenerates an initial single spectral line into a system of spectral lines with different transition frequencies defined by the electronic structure of the energy levels. Newly created magnetic sub-levels redefine the spectral profile of the line emission and therefore radiation transport mechanism in optically thick plasma.Self-absorption which defines the excited state-densities is an important mechanism and can be used with other methods to describe the state densities for an optically thick plasma. This method is an established tool to retrieve state-density and plasma parameters. To measure each magnetic sub-level density of argon 1s 4 and 1s 5 (in Paschen's notation) a tunable diode laser absorption spectroscopy (TDLAS) was used. Based on reconstructed excited state densities, the self-absorption coefficient was calculated for individual magnetic sub-levels. A decrease in self-absorption with an external magnetic field was noticed indicating a higher transparency of the plasma. Furthermore a polarization dependent self-absorption was found. The presented results can help to model optical properties and interpret the absorption of a low-pressure optically thick magnetized plasma.

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“…Considering the zeroth order Doppler shift v = λ 0 Δf, this implies the need of a frequency (wavelength) tunable range of an exciting laser of well beyond 100 GHz, and preferably >200 GHz. This is a much boarder VDF than ones typically obtained in LTPs with which lasers with tunable range ∼20 GHz were often more than sufficient [9,12,13,[40][41][42][43][44][45][46][47][48]. Typical LTPs obtains VDFs with a temperature smaller than 0.1 eV, which is compared to helium neutral/ion VDFs with 1 eV and 50 eV in figure 1.…”

Section: Developing a Working He I And He Ii Lif Systemsmentioning

confidence: 69%

Recent development of LIF diagnostics in ASIPP

Yip,

Jiang,

Jin

et al. 2023

J. Inst.

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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.

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Laser induced fluorescence of Ar-I metastables in the presence of a magnetic field

Cited by 15 publications

References 46 publications

Laser-induced fluorescence measurement of very slow neutral flows in a dusty plasma experiment

Laser-induced fluorescence measurement of very slow neutral flows in a dusty plasma experiment

Optical properties of magnetized transient low-pressure plasma

Recent development of LIF diagnostics in ASIPP

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