S. Kneip

Dynamic Control of Laser-Produced Proton Beams

The emission characteristics of intense laser driven protons are controlled using ultrastrong (of the order of 10(9) V/m) electrostatic fields varying on a few ps time scale. The field structures are achieved by exploiting the high potential of the target (reaching multi-MV during the laser interaction). Suitably shaped targets result in a reduction in the proton beam divergence, and hence an increase in proton flux while preserving the high beam quality. The peak focusing power and its temporal variation are shown to depend on the target characteristics, allowing for the collimation of the inherently highly divergent beam and the design of achromatic electrostatic lenses.

Controlling the spectrum of x-rays generated in a laser-plasma accelerator by tailoring the laser wavefront

By tailoring the wavefront of the laser pulse used in a laser-wakefield accelerator, we show that the properties of the x-rays produced due to the electron beams betatron oscillations in the plasma can be controlled. By creating a wavefront with coma, we find that the critical energy of the synchrotronlike x-ray spectrum can be significantly increased. The coma does not substantially change the energy of the electron beam, but does increase its divergence and produces an energy-dependent exit angle, indicating that changes in the x-ray spectrum are due to an increase in the electron beams oscillation amplitude within the wakefield.

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.

Applied physics: A stroke of X-ray

X-rays were discovered more than 100 years ago. They have since become a staple tool for medicine and science, so researchers are continuing their efforts to find innovative ways to produce them.

Third harmonic order imaging as a focal spot diagnostic for high intensity laser-solid interactions

As the state of the art for high power laser systems increases from terawatt to petawatt level and beyond, a crucial parameter for routinely monitoring high intensity performance is laser spot size on a solid target during an intense interaction in the tight focus regime ( 10(19) Wcm(-2) is demonstrated experimentally and shown to provide the basis for an effective focus diagnostic. Importantly, this technique is also shown to allow in-situ diagnosis of focal spot quality achieved after reflection from a double plasma mirror setup for very intense high contrast interactions (> 10(20) Wcm(-2)) an important application for the field of high laser contrast interaction science.

Compact laser accelerators for X-ray phase-contrast imaging

Advances in X-ray imaging techniques have been driven by advances in novel X-ray sources. The latest fourth-generation X-ray sources can boast large photon fluxes at unprecedented brightness. However, the large size of these facilities means that these sources are not available for everyday applications. With advances in laser plasma acceleration, electron beams can now be generated at energies comparable to those used in light sources, but in university-sized laboratories. By making use of the strong transverse focusing of plasma accelerators, bright sources of betatron radiation have been produced. Here, we demonstrate phase-contrast imaging of a biological sample for the first time by radiation generated by GeV electron beams produced by a laser accelerator. The work was performed using a greater than 300 TW laser, which allowed the energy of the synchrotron source to be extended to the 10–100 keV range.

Investigation of the role of plasma channels as waveguides for laser-wakefield accelerators

The role of plasma channels as waveguides for laser-wakefield accelerators is discussed in terms of the results of experiments performed with the Astra-Gemini laser, numerical simulations using the code WAKE, and the theory of self-focusing and self-guiding of intense laser beams. It is found that at a given electron density, electron beams can be accelerated using lower laser powers in a waveguide structure than in a gas-jet or cell. The transition between relativistically self-guided and channel-assisted guiding is seen in the simulations and in the behaviour of the production of electron beams. We also show that by improving the quality of the driving laser beam the threshold laser energy required to produce electron beams can be reduced by a factor of almost 2. The use of an aperture allows the production of a quasi-monoenergetic electron beam of energy 520 MeV with an input laser power of only 30 TW.

Comparative study of betatron radiation from laser-wakefield and direct-laser accelerated bunches of relativistic electrons

The dynamics of relativistic electrons in a laser driven plasma cavity are studied via measurements of their radiation. For ultrashort laser pulses at comparatively low focused laser intensities (3 < a0 < 10), low density and long f-number of 10, electrons are predominantly accelerated in the wakefield leading to quasi-monoenergetic collimated electron beams and well collimated (< 12 mrad) beams of comparatively soft x-rays (1-10 keV) with unprecedented small source size (2-3 μm). For laser pulses with increasing laser intensity (10 < a0 < 30), density and short f-number (< 5), electrons are accelerated directly by the laser, leading to divergent quasimaxwellian electron beams and divergent (50-95°) beams of hard x-rays (20-50 keV) with relatively large source size (> 100 μm). In both cases, the measured x-rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. At low laser intensity transverse oscillations are small as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser. A betatron resonance leads to a tenfold increase in transverse oscillation amplitude and electrons enter a highly radiative regime with up to 5% of their energy converted into x-rays.

Tuning the electron energy by controlling the density perturbation position in laser plasma accelerators

A density perturbation in an underdense plasma was used to improve the quality of electron bunches produced in the laser-plasma wakefield acceleration scheme. Quasi-monoenergetic electrons were generated by controlled injection in the longitudinal density gradients of the density perturbation. By tuning the position of the density perturbation along the laser propagation axis, a fine control of the electron energy from a mean value of 60 MeV to 120 MeV has been demonstrated with a relative energy-spread of 15 ± 3.6%, divergence of 4 ± 0.8 mrad, and charge of 6 ± 1.8 pC.

Self-Guided Wakefield Experiments Driven by Petawatt-Class Ultrashort Laser Pulses

We investigate the extension of self-injecting laser wakefield experiments to the regime that will be accessible with the next generation of petawatt-class ultrashort pulse laser systems. Using nonlinear scalings, current experimental trends, and numerical simulations, we determine the optimal laser and target parameters, i.e., focusing geometry, plasma density, and target length, that are required to increase the electron beam energy (to >1 GeV) without the use of external guiding structures.