Laser produced plasmas as a source of ions, protons and X-rays

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Title: Laser produced plasmas as a source of ions, protons and X-rays
Authors: Stefanuik, Robert
Advisor: Dunne, Padraig
O'Reilly, Fergal
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Date: 2014
Online since: 2015-08-27T08:50:42Z
Abstract: The work presented in this thesis is primarily focused on the use of a laserproduced plasma as a source of protons, ions and X-rays. It explores highimpact applications of both high power ultrafast lasers and nanosecond lasers.Section 1 gives a general introduction to the physics governing the experimentsand the lasers in the following sections. The physics of plasmas isdescribed in Section 1.1. The physics of ultrafast lasers is described in detailin Section 1.2 from the theory of mode locking of oscillators to the compressionof high power ultrafast systems. The physics and mechanisms of X-raylasers are then explained in Section 1.3. Both the collisional excitation mechanismand the Linford gain equation are described along with a summery ofthe challenges in pumping X-ray lasers. Next in Section 1.4 laser acceleratedprotons are described in detail. Acceleration mechanisms are described andthe interaction mechanism with insulators is introduced. Finally, a numberof tools for photoabsorption studies are described in Section 1.5. In this Sectionthe dual laser plasma technique for obtaining photoabsorption spectra isdescribed along with the atomic structure codes (the Cowan and RTDLDAcodes) used to simulate the spectra.Section 2 describes all the laser systems used in this thesis. The variouscomponents of the Quantronix ultrafast laser system from UCD includingthe oscillator, the Odin-II amplifier and the Thor amplifier are described inSection 2.2, Section 2.3 and Section 2.4. The operation of the Surelite III andthe Spectron SL805 Q-switched nanosecond lasers are described in Section2.5 and Section 2.6. Finally, the TARANIS multi-terrawatt system and itsdifferent components are described in Section 2.7.Section 3 is covers two different experiments involving the generation ofX-rays from a laser produced plasma. The first technique described in Section3.3 is the generation of coherent X-rays by pumping a preformed laser plasmawith the TARANIS laser system. A Ni-like Mo laser was successfully pumpedyielding energies of 2 x 10-7 J with a gain length product of 3000.The second technique described in Section 3.5, involved inner shell X-rayemission from an indium target irradiated by the Quantronix laser system.The indium K series emission was observed using a lithium-drifted silicondetector. Seven counts of K_ emission was detected under the followingconditions (12000 shots through a 0.5 mm pin hole filtered with a 60_m Alfoil and a laser energy of 30 mJ). Due to the laser reliability this experimentis only a preliminary one which is intended to be continued at a later stage.Section 4 describes a laser accelerated proton experiment conducted in theCenter for Plasma Research in Queens University Belfast, using the TARANISlaser system. In this experiment 13 MeV protons were accelerated from10 μm gold foil targets into a sample of BK-7 glass. The interaction of theprotons with the glass were observed by taking spatially resolved images ofthe transient opacity induced by the protons interacting with the BK-7 Glassand an optical probe beam. These spatially resolved images are presented inSection 4.3.In Section 4.4.2 an optical streaking technique experiment is described tocalculate the lifetime of the transient opacity induced by the proton interaction.This process showed that the opacity in the glass began to occur 62ps after the TARANIS main pulse was _red and reaches 30% transmission88 ps after the firing of the TARANIS main pulse. Due to the limited timewindow available, the exact lifetime of the opacity is not known, howeverglass is seen to de-excite after 138 ps and has returned to 50 % transmissionby 175 ps.Section 5 describes the refurbishment of the 1-m normal incidence VUVspectrometer. It describes the replacement of a photographic plate based detectionsystem with a linear CCD array. The CCD array can detect the VUVradiation through a sodium salicylate phosphor coating which emits at 410nm on interacting with VUV radiation. Different phosphors are compared interms of sensitivity and ease of coating and the grounds for choosing sodiumsalicylate are explained. The adaptations to the spectrometer to use the linearCCD array are described and the details on calibrating the spectrometerare explained.Finally, Section 6 describes a set of spectroscopic experiments which usethe refurbished 1-m normal incidence spectrometer. First Section 6.2 describesa repeat of the photoabsorption of indium and indium plus whichwas conducted previously on the spectrometer. This was designed as a proofof principle of the working of the new phosphor based linear CCD arraysystem.Next, Section 6.3, describes the photoabsorption of thulium in the 23 eVto 40 eV region. The experimental spectrum is compared to the RTDLDAcalculations and to the autoionized Cowan code calculations. The simulationssuccessfully describe the observed absorption structure and 5p → nd and5p → ms transitions are identified.In Section 6.4 an indium laser produced plasma is successfully reheatedusing the Odin-II first amplifier of the Quantronix laser system. An experimentis described which shows emission from an indium plasma fromtransitions which are normally observed in absorption. This reheating wasoptimized with a specific set of target parameters (a delay of Δτ= 500 nsbetween the lasers, the Odin was focused into the center of the absorbing lineplasma, which was set at a height of Δz = -0:2 mm from the optical axis of the spectrometer).
Type of material: Doctoral Thesis
Publisher: University College Dublin. School of Physics
Qualification Name: Ph.D.
Copyright (published version): 2014 the author
Keywords: LaserPlasmaSpectroscopyX-rays
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Language: en
Status of Item: Peer reviewed
Appears in Collections:Physics Theses

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