This dissertation addresses electron acceleration and the associated betatron X-ray radiation generated by laser wakefield inside dielectric capillary tubes. Focusing the state-of-the-art multi-terawatt laser pulses, high peak intensity, of the order of 1018 W/cm2, can be achieved in the focal plane, where a plasma bubble free of electron is formed just behind the laser. Owing to space charge separation ultrahigh electric fields, of the order of 100 GV/m, occur inside the plasma bubble, providing the possibility to accelerate electrons up to GeV-class over merely a centimetre-scale distance. Furthermore, ultra-short synchrotron-like X-ray radiation, known as betatron radiation, is produced simultaneously when the accelerated electrons are transversely wiggled by the radial electric field inside the plasma bubble. This thesis reports experimental results on the generation and optimization of electron and X-ray beams, particularly when a capillary tube is used to collect the energy of laser halos in the focal plane to facilitate the laser keeping self-focused over a long distance. Employing the 40 fs, 16 TW Ti:sapphire laser at the Lund Laser Centre (LLC) in Sweden, either peaked or widely-spread accelerated electron spectra with a typical beam charge of tens of pC were measured with a maximum energy up to 300 MeV in 10 mm long capillary tubes. Meanwhile, betatron X-ray radiation consisting of 1-10 keV photons was measured with a peak brightness of the order of 1021 photons/s/mm2/mrad2/0.1%BW, which is around 30 times higher than that in the case of a 2 mm gas jet without external optical guiding. When the laser pointing fluctuation is compensated, exceptionally reproducible electron beams are obtained with fluctuations of only 1 mrad RMS in beam pointing, a few percent in electron energy, and around 20% RMS in beam charge. The relatively large instability of beam charge is found to be essentially correlated to laser power fluctuation. Moreover, betatron radiation is able to provide the diagnostics about electron acceleration process and average number of betatron oscillations fulfilled by electrons inside the plasma bubble. The typical X-ray source size (waist of Gaussian distribution at 1/e2 intensity) is quantified to be ~2.5 μm using Fresnel diffraction induced by a razor blade, which furthermore yields the corresponding normalized RMS emittance of electron beam 0.83π mm mrad. Three dimensional particle-in-cell (PIC) modelings are in good agreement with the experimental findings. The PIC simulations also reveal the generated electron bunches (or X-ray bursts) have pulse durations as short as 10 fs.
| Identifer | oai:union.ndltd.org:CCSD/oai:tel.archives-ouvertes.fr:tel-00861267 |
| Date | 27 June 2013 |
| Creators | Ju, Jinchuan |
| Publisher | Université Paris Sud - Paris XI |
| Source Sets | CCSD theses-EN-ligne, France |
| Language | English |
| Detected Language | English |
| Type | PhD thesis |
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