Bright betatron x-ray radiation from a laser-driven-clustering gas target

Hard X-ray sources from femtosecond (fs) laser-produced plasmas, including the betatron X-rays from laser wakefield-accelerated electrons, have compact sizes, fs pulse duration and fs pump-probe capability, making it promising for wide use in material and biological sciences. Currently the main problem with such betatron X-ray sources is the limited average flux even with ultra-intense laser pulses. Here, we report ultra-bright betatron X-rays can be generated using a clustering gas jet target irradiated with a small size laser, where a ten-fold enhancement of the X-ray yield is achieved compared to the results obtained using a gas target. We suggest the increased X-ray photon is due to the existence of clusters in the gas, which results in increased total electron charge trapped for acceleration and larger wiggling amplitudes during the acceleration. This observation opens a route to produce high betatron average flux using small but high repetition rate laser facilities for applications

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 observation of the injection dynamics of a laser wakefield accelerator using few-femtosecond shadowgraphy

International audienceWe present few-femtosecond shadowgraphic snapshots taken during the non-linear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma wave period is induced by a lengthening of the first plasma period. Three dimensional particle in cell simulations support our observations

Bright X-ray radiation from plasma bubbles in an evolving laser wakefield accelerator

We show that the properties of the electron beam and bright X-rays produced by a laser wakefield accelerator can be predicted if the distance over which the laser self-focuses and compresses prior to self-injection is taken into account. A model based on oscillations of the beam inside a plasma bubble shows that performance is optimised when the plasma length is matched to the laser depletion length. With a 200~TW laser pulse this results in an X-ray beam with median photon energy of 20 keV, $> 10^{9}$ photons per shot and a peak brightness of $4 \times 10^{23}$ photons s$^{-1}$ mrad$^{-2}$ mm$^{-2}$ (0.1 % BW)$^{-1}$

Determination of the Carrier-Envelope Phase of Few-Cycle Laser Pulses with Terahertz-Emission Spectroscopy

The availability of few-cycle optical pulses opens a window to physical phenomena occurring on the attosecond time scale. In order to take full advantage of such pulses, it is crucial to measure and stabilise their carrier-envelope (CE) phase, i.e., the phase difference between the carrier wave and the envelope function. We introduce a novel approach to determine the CE phase by down-conversion of the laser light to the terahertz (THz) frequency range via plasma generation in ambient air, an isotropic medium where optical rectification (down-conversion) in the forward direction is only possible if the inversion symmetry is broken by electrical or optical means. We show that few-cycle pulses directly produce a spatial charge asymmetry in the plasma. The asymmetry, associated with THz emission, depends on the CE phase, which allows for a determination of the phase by measurement of the amplitude and polarity of the THz pulse

K-edge structure in shock-compressed chlorinated parylene

We have carried out a series of experiments to measure the Cl K-absorption edge for shock-compressed samples of chlorinated parylene. Colliding shocks allowed us to compress samples up to four times the initial density with temperatures up to 10 eV. Red shifts in the edge of about 10 eV have been measured. We have compared the measured shifts to analytical modelling using the Stewart–Pyatt model and adaptions of it, combined with estimates of density and temperature based on hydrodynamic modelling. Modelling of the edge position using density functional theory molecular dynamics (DFT-MD) was also used and it was found that good agreement was only achieved when the DFT simulations assumed conditions of lower temperature and slightly higher density than indicated by hydrodynamic simulations using a tabular equation of state

K-edge structure in shock-compressed chlorinated parylene

We have carried out a series of experiments to measure the Cl K-absorption edge for shock-compressed samples of chlorinated parylene. Colliding shocks allowed us to compress samples up to four times the initial density with temperatures up to 10 eV. Red shifts in the edge of about 10 eV have been measured. We have compared the measured shifts to analytical modelling using the Stewart–Pyatt model and adaptions of it, combined with estimates of density and temperature based on hydrodynamic modelling. Modelling of the edge position using density functional theory molecular dynamics (DFT-MD) was also used and it was found that good agreement was only achieved when the DFT simulations assumed conditions of lower temperature and slightly higher density than indicated by hydrodynamic simulations using a tabular equation of state

A proposal to measure iron opacity at conditions close to the solar convective zone-radiative zone boundary

A major problem in stellar modelling is the discrepancy between solar models and helioseismology data in the position of the convective zone-radiative zone boundary in the sun. This could be explained by a large uncertainty in the calculated opacity data and recent experimental data on iron using the Sandia National Laboratory Z facility have shown large differences, up to a factor of 4, between measurement and prediction at plasma conditions close to the convective zone-radiative zone boundary. This paper describes a proposal for a radiative burn-through experiment to be fielded on NIF to observe if a radiation wave transit through a Fe2O3 sample is consistent with the factor of 2-4 change in the iron opacity seen in the Z experiments. A target design and the diagnostic method are described. A detailed radiation-hydrodynamic model has been used to generate synthetic results and explore the sensitivities and experimental accuracy needed for the proposed measurement