Laser-wakefield accelerators as hard x-ray sources for 3D medical imaging of human bone

A bright μm-sized source of hard synchrotron x-rays (critical energy Ecrit > 30 keV) based on the betatron oscillations of laser wakefield accelerated electrons has been developed. The potential of this source for medical imaging was demonstrated by performing micro-computed tomography of a human femoral trabecular bone sample, allowing full 3D reconstruction to a resolution below 50 μm. The use of a 1 cm long wakefield accelerator means that the length of the beamline (excluding the laser) is dominated by the x-ray imaging distances rather than the electron acceleration distances. The source possesses high peak brightness, which allows each image to be recorded with a single exposure and reduces the time required for a full tomographic scan. These properties make this an interesting laboratory source for many tomographic imaging applications

Direct fluorescence characterisation of a picosecond seeded optical parametric amplifier

The temporal intensity contrast of high-power lasers based on optical parametric amplification (OPA) can be limited by parametric fluorescence from the non-linear gain stages. Here we present a spectroscopic method for direct measurement of unwanted parametric fluorescence widely applicable from unseeded to fully seeded and saturated OPA operation. Our technique employs simultaneous spectroscopy of fluorescence photons slightly outside the seed bandwidth and strongly attenuated light at the seed central wavelength. To demonstrate its applicability we have characterised the performance of a two-stage picosecond OPA pre-amplifier with 2.8×105 gain, delivering pulses at 1054 nm. We show that fluorescence from a strongly seeded OPA is reduced by ~500× from the undepleted to full pump depletion regimes. We also determine the vacuum fluctuation driven noise term seeding this OPA fluorescence to be 0.7±0.4 photons ps−1 nm−1 bandwidth. The resulting shot-to-shot statistics highlights a 1.5% probability of a five-fold and 0.3% probability of a ten-fold increase of fluorescence above the average value. Finally, we show that OPA fluorescence can be limited to a few-ps pedestal with 3×10−9 temporal intensity contrast 1.3 ps ahead of an intense laser pulse, a level highly attractive for large scale chirped-pulse OPA laser systems

An ultrafast optical parametric laser for driving high energy density science

This thesis describes the development of a multi-mJ, few-cycle, absolute-phase controlled laser system based on optical parametric chirped pulse amplification (OPCPA) operating at a kHz repetition rate. A laser system with these specifications will provide a table-top platform to enable a broad range of experiments in demanding research areas, including laser electron acceleration and the creation of exotic highenergy density plasmas from solid targets. The approach of the work is a combination of both experimental effort and numerical simulations used to guide and aid interpretation of laboratory studies. The non-collinear parametric gain stages of the laser have been optimised using detailed numerical simulations. A comparison is given on phase matching conditions in BBO and LBO crystals along with a novel nonlinear material BiBO. The production of 600 μJ pulses with a bandwidth that supports a transform limited temporal duration of 8.5 fs is presented in a three stage BBO based design. An all optical, low-jitter synchronisation scheme for the OPCPA pump and signal pulses has been designed and implemented by use of solitonic wavelength shifting in a photonic crystal fiber (PCF). Commercially available fibers with various core sizes have been assessed. The propagation of few-cycle pulses in the PCF has been studied by numerically solving the generalised Schr¨odinger equation with the splitstep Fourier method. An OPA pump laser with excellent spatial and temporal qualities has been developed. Amplification of the PCF output at 1053 nm is achieved in a regenerative diode pumped Nd:YLF amplifier and a multipass power amplifier. Self-phase modulation and gain narrowing is greatly reduced using a customised 500 μm low-finesse etalon in the regenerative amplifier cavity. Spectral modulation was found to increase both frequency doubling and parametric amplification efficiency and stability. The construction of an alternative 10 Hz, high-energy pump beam line is also presented.Open Acces

A fluktuáció csökkentése és munkaerő-megtartás menedzsment - esettanulmány egy vendéglátással foglalkozó cégről

Ez a dolgozat a fluktuáció problémáját, a dolgozói elégedettséget, és annak felmérését, valamint a munkaerő-megtartást vizsgálja. A felmérési folyamatot egy vendéglátással foglalkozó cégen keresztül vezeti le. A mérést két alkalommal, online kérdőívvel végezték. A felmérés eredményeit összevetve a kilépési kérdőívek eredményével, kirajzolódik, hogy a fizetés, és a fizikai terhelés a problémás területek. A fejlesztési folyamat eredetileg tervezett ideje eltolódik, a projekt félbeszakad

A tűzvédelmi hatósági jogosítványok gyakorlása

INST: L_08

Change the rules of hammer throw, please !

Propositions de modification de l'engin et du cercle de lancer pour réduire les distances de jet