24 research outputs found
Chirped pulse Raman amplification in plasma: high gain measurements
No full textHigh power short pulse lasers are usually based on chirped pulse amplification (CPA), where a frequency chirped and temporarily stretched ``seed'' pulse is amplified by a broad-bandwidth solid state medium, which is usually pumped by a monochromatic ``pump'' laser. Here, we demonstrate the feasibility of using chirped pulse Raman amplification (CPRA) as a means of amplifying short pulses in plasma. In this scheme, a short seed pulse is amplified by a stretched and chirped pump pulse through Raman backscattering in a plasma channel. Unlike conventional CPA, each spectral component of the seed is amplified at different longitudinal positions determined by the resonance of the seed, pump and plasma wave, which excites a density echelon that acts as a "chirped'" mirror and simultaneously backscatters and compresses the pump. Experimental evidence shows that it has potential as an ultra-broad bandwidth linear amplifier which dispenses with the need for large compressor gratings
Modification of a personal dosimetry device for logging melanopic irradiance
No full text Performance characterisations were carried out before and after a modification to the optics of the Condor Instruments’ ActTrust light and activity data loggers to improve the spectral performance for measuring melanopic-weighted irradiance in non-visual studies. The results confirm the intended improvement, so that the device provides the best-known single-sensor match to the melanopic response. In addition, the device includes a separate sensor which remained well-matched for illuminance logging. </jats:p
Chirped pulse Raman amplification in plasma
No full textRaman amplification in plasma has been proposed to be a promising method of amplifying short radiation pulses. Here, we investigate chirped pulse Raman amplification (CPRA) where the pump pulse is chirped and leads to spatiotemporal distributed gain, which exhibits superradiant scaling in the linear regime, usually associated with the nonlinear pump depletion and Compton amplification regimes. CPRA has the potential to serve as a high-efficiency high-fidelity amplifier/compressor stage
High gain ultra-short laser pulse raman amplification in plasma
No full textThis thesis presents an experimental study of broad-band Raman chirped pulse amplification in plasma. Amplification. of a short seed pulse occurs through stimulated Raman backscattering in the presence ofa counter-propagating chirped pump pulse in a preformed plasma waveguide channel. 300 and 200 Jlm diameter, 40 rom long, hydrogen-filled capillary discharge waveguides have been investigated. The plasma channel has the dual purpose of guiding the laser pulses over distances of many Rayleigh lengths and acting· as the nonlinear medium for the Raman instability. Laser energy transmission through the plasma channel, in single mode propagation, is observed ~o be better than 80 % for the pulses with intensities up to 5.8 X 1014 W cm-2. The dependence of the gain on pump intensity and plasma density is experimentally studied and confirms theoretical predictions. It is shown that higher pump intensities inside the capillary can be realised using unmatched guiding due to periodic oscillations ofthe laser beam waist along the plasma channel. Amplification of the seed pulse in the linear regime is obtained when the frequency detuning between the seed and the pump matches th~ plasma frequency. Thus the frequency difference between the pump and gain peaks enables to estimate the plasma density, measured around 1018 cm-3. The high gain regime has been investigated showing that peak gains of over 1200 % can be routinely realised. A peak gain of almost 3000 % has 'been observed under optimal conditions with the highest densities and pump energies. Several diagnostic systems have been developed for simultaneous detection of the spectral and energy gain together with the duration of the seed. Ultrashort pulse high gl;lin amplification of 80 fs, 5 mJ pulse has been demonstrated, giving a peak value of 800% and energy gain of 40 % together with 40 % increase in pulse duration. Amplification ofthe whole seed pulse spectra has been demonstrated using an optical cut-off filter to provide a sufficient detul1ed seed pulse. Future increase of the efficiency will occur in the nonlinear regime of the stimulated Raman backscattering resulting in compression ofthe seed pulse.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
An ultrashort pulse ultra-violet radiation undulator source driven by a laser plasma wakefield accelerator
No full textNarrow band undulator radiation tuneable over the wavelength range of 150–260 nm has been produced by short electron bunches from a 2 mm long laser plasma wakefield accelerator based on a 20 TW femtosecond laser system. The number of photons measured is up to 9 × 106 per shot for a 100 period undulator, with a mean peak brilliance of 1 × 1018 photons/s/mrad2/mm2/0.1% bandwidth. Simulations estimate that the driving electron bunch r.m.s. duration is as short as 3 fs when the electron beam has energy of 120–130 MeV with the radiation pulse duration in the range of 50–100 fs
Pepper-pot emittance measurement of laser-plasma wakefield accelerated electrons
No full textThe transverse emittance is an important parameter governing the brightness of an electron beam. Here we present the first pepper-pot measurement of the transverse emittance for a mono-energetic electron beam from a laser-plasma wakefield accelerator, carried out on the Advanced Laser-Plasma High Energy Accelerators towards X-Rays (ALPHA-X) beam line. Mono-energetic electrons are passed through an array of 52 mu m diameter holes in a tungsten mask. The pepper-pot results set an upper limit for the normalised emittance at 5.5 +/- 1 pi mm mrad for an 82 MeV beam
Measurements of UV—A Exposure of Commercial Pilots Using Genesis-UV Dosimeters
No full textA number of studies suggest that pilots are at twice the risk of melanoma and keratinocyte skin cancers than the general population, and that they have a raised mortality from melanoma. No conclusive links with in-flight exposure to ionising radiationor circadian rhythm disruption due to the pilots’ shift work were found. Possible over-exposure to ultraviolet radiation (UVR) may be implicated as pilots may be exposed to higher UV-A levels at cruise altitude compared with those at ground levels. The direct method of making in-flight spectral measurements has been carried out on a limited number of flights, but this technique is challenging; the use of small wearable sensors may be more appropriate but there are a few issues that should be addressed for their use in cockpit measurements. While the spectral response of sensors for erythema effective values usually closely matches the corresponding weighting function, the response of UV-A sensors may not be spectrally flat, which, if not corrected to account for the transmission of the aircraft windshield, could potentially result in large errors. In this paper, the spectral correction method was applied to the UV-A sensor of the Genesis-UV unit to measure UVR exposure of commercial pilots on 312 flights to a range of destinations from four UK airports from September 2016 to August 2017
Sunglass Filter Transmission and Its Operational Effect in Solar Protection for Civilian Pilots.
No full textINTRODUCTION: The ocular effects of excess solar radiation exposure are well documented. Recent evidence suggests that ocular ultraviolet radiation (UVR) exposure to professional pilots may fall outside international guideline limits unless eye protection is used. Nonprescription sunglasses should be manufactured to meet either international or national standards. The mean increase in UVR and blue light hazards at altitude has been quantified and the aim of this research was to assess the effectiveness of typical pilot sunglasses in reducing UVR and blue light hazard exposure in flight. METHOD: A series of sunglass filter transmittance measurements were taken from personal sunglasses (N = 20) used by pilots together with a series of new sunglasses (N = 18). RESULTS: All nonprescription sunglasses measured conformed to international standards for UVR transmittance and offered sufficient UVR protection for pilots. There was no difference between right and left lenses or between new and used sunglasses. All sunglasses offered sufficient attenuation to counter the mean increase in blue light exposure that pilots experience at altitude, although used sunglasses with scratched lenses were marginally less effective. One pair of prescription sunglasses offered insufficient UVR attenuation for some flights, but would have met requirements of international and national standards for UV-A transmittance. This was likely due to insufficient UVR blocking properties of the lens material. CONCLUSIONS: Lenses manufactured to minimally comply with standards for UVR transmittance could result in excess UVR exposure to a pilot based on in-flight irradiance data; an additional requirement of less than 10% transmittance at 380 nm is recommended
Study of chirped pulse amplification based on Raman backscattering
No full textRaman backscattering (RBS) in plasma is an attractive source of intense, ultrashort laser pulses, which has the potential asa basic for a new generation of laser amplifiers.(1) Taking advantage of plasma, which can withstand extremely high power densities and can offer high efficiencies over short distances, Raman amplification in plasma could lead to significant reductions in both size and cost of high power laser systems. Chirped laser pulse amplification through RBS could be an effective way to transfer energy from a long pump pulse to a resonant counter propagating short probe pulse. The probe pulse is spectrally broadened in a controlled manner through self-phase modulation. Mechanism of chirped pulse Raman amplification has been studied, and features of supperradiant growth associated with the nonlinear stage are observed in the linear regime. Gain measurements are briefly summarized. The experimental measurements are in qualitative agreement with simulations and theoretical predictions
