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Reference: Jacobs, Robert Michael James., (2000). Laser studies of plasmas. Dphil. University of Oxford.Citable link to this page:

 

Laser studies of plasmas

Abstract: Measurement of the intrinsic properties of processing plasmas is critically important in understandingdischarges so that the optimum conditions can be achieved. Several different diagnostic methods havebeen developed and tested.A planar probe has been used to measure the ion flux and electron temperature in both inductivelycoupled and capacitively coupled plasma systems, at various pressures and applied powers, using theassumption that the electron energy distribution is a Maxwellian.Frequency modulation spectroscopy (FMS) has been used to detect species in a plasma. It has beenshown to be very effective, giving a significantly increased S/N ratio compared to both single passabsorption and low frequency mechanical modulation techniques. It has been used to measure excitedargon atom concentrations, in both capacitively and inductively produced plasmas. The argon atom4s[3/2]1 level concentration was found to be between 2 x 108 and 1 x 1011 atom/cm3 and to generallyincrease with increasing applied power and to decrease with increasing total pressure. The temperatureof the atoms was also measured and was found to be approximately 323 ± 17 K.A simple compact laser source at 308 nm has been produced from a frequency doubled cooledcommercial diode laser. This has been used to detect the OH radical, by absorption, within theafterglow of a microwave discharge, produced either directly or chemically. Simple kinetic modelshave provided explanations of the variations in OH concentration with discharge conditions.A novel method, cavity laser induced fluorescence (CLIP), that combines the advantages of both laserinduced fluorescence (LIF) and cavity ring down spectroscopy CRDS, has been shown to increase thesensitivity of a diagnostic system compared to absorption. This method could be used to followconcentration variations of a reaction in a single laser shot. Although such variation can be observedusing LIF, it requires a calibration and a many laser shot experiment with a signal recorded at each timepoint. Whilst CRDS allows temporal information about the absolute concentrations of the speciesobserved to be obtained, it is not as sensitive as LIF. By combining the two in CLIP, it may be possibleto retain the sensitivity of LIF with the advantage of CRDS so that absolute and time varyingconcentrations can be obtained in a single pulsed laser shot.LIF and CRDS signals have been observed using the A 2?u ? X 2Sg+ transition of the N2+ ion. The lifetime of the A 2?u state in the discharge was found to be sufficiently long for a time of flightexperiment to be contemplated (an ion with a velocity of 10 kms-1 on average would travel 5 mm beforeradiating). Although the preliminary tests for the time of flight experiment have shown that this methodis not feasible with a pulsed laser, the basic cavity locking procedures required for an analogouscontinuous wave experiment have been successfully demonstrated.A frequency doubled diode laser source has been constructed and tested, with the eventual aim ofdetecting N2+ via the B 2Su ? X 2Sg+ electronic transition. This has been found to be insufficientlyintense to be used for a proposed two dimensional velocity mapping experiment, but several strategiesto improve its performance are suggested.

Type of Award:Dphil Level of Award:Doctoral Awarding Institution: University of Oxford Notes:The digital copy of this thesis has been made available thanks to the generosity of Dr Leonard Polonsky

Contributors

Hancock, G. (Gus)More by this contributor

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Gus HancockMore by this contributor

RoleSupervisor

 Bibliographic Details

Issue Date: 2000Identifiers

Urn: uuid:3de1c919-d7ce-485c-94c8-e5ab9e8e4a4b

Source identifier: 603838383 Item Description

Type: Thesis;

Language: eng Subjects: Modulation spectroscopy Fluorescence spectroscopy Tiny URL: td:603838383

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Autor: Jacobs, Robert Michael James. - institutionUniversity of Oxford facultyMathematical and Physical Sciences Division - - - - Contri

Fuente: https://ora.ox.ac.uk/objects/uuid:3de1c919-d7ce-485c-94c8-e5ab9e8e4a4b



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