C. A. J. Palmer

Monoenergetic proton beams accelerated by a radiation pressure driven shock.

We report on the acceleration of impurity-free quasimononenergetic proton beams from an initially gaseous hydrogen target driven by an intense infrared (λ=10 μm) laser. The front surface of the target was observed by optical probing to be driven forward by the radiation pressure of the laser. A proton beam of ∼MeV energy was simultaneously recorded with narrow energy spread (σ∼4%), low normalized emittance (∼8 nm), and negligible background. The scaling of proton energy with the ratio of intensity over density (I/n) confirms that the acceleration is due to the radiation pressure driven shock.

X-ray phase contrast imaging of biological specimens with femtosecond pulses of betatron radiation from a compact laser plasma wakefield accelerator

We show that x-rays from a recently demonstrated table top source of bright, ultrafast, coherent synchrotron radiation [Kneip et al., Nat. Phys. 6, 980 (2010)] can be applied to phase contrast imaging of biological specimens. Our scheme is based on focusing a high power short pulse laser in a tenuous gas jet, setting up a plasma wakefield accelerator that accelerates and wiggles electrons analogously to a conventional synchrotron, but on the centimeter rather than tens of meter scale. We use the scheme to record absorption and phase contrast images of a tetra fish, damselfly and yellow jacket, in particular highlighting the contrast enhancement achievable with the simple propagation technique of phase contrast imaging. Coherence and ultrafast pulse duration will allow for the study of various aspects of biomechanics.

Characterization of transverse beam emittance of electrons from a laser-plasma wakefield accelerator in the bubble regime using betatron x-ray radiation

We propose and use a technique to measure the transverse emittance of a laser-wakefield accelerated beam of relativistic electrons. The technique is based on the simultaneous measurements of the electron beam divergence given by v(perpendicular to)/v(parallel to), the measured spectrum gamma, and the transverse electron bunch size in the bubble r(perpendicular to). The latter is obtained via the measurement of the source size of the x rays emitted by the accelerating electron bunch in the bubble. We measure a normalized rms beam transverse emittance <0.5 pi mm mrad as an upper limit for a spatially Gaussian, spectrally quasimonoenergetic electron beam with 230 MeV energy in agreement with numerical modeling and analytic theory in the bubble regime.

Carbon ion acceleration from thin foil targets irradiated by ultrahigh-contrast, ultraintense laser pulses

In this study, ion acceleration from thin planar target foils irradiated by ultrahigh-contrast (10 10 ), ultrashort (50fs) laser pulses focused to intensities of 7◊10 20 Wcm 2 is investigated experimentally. Target normal sheath acceleration (TNSA) is found to be the dominant ion acceleration mechanism when the target thickness is >50nm and laser pulses are linearly polarized. Under these conditions, irradiation at normal incidence is found to produce higherenergyions thanobliqueincidenceat35 withrespectto thetargetnormal. Simulations using one-dimensional (1D) boosted and 2D particle-in-cell codes support the result, showing increased energy coupling efficiency to fast electrons for normal incidence. The effects of target composition and thickness on the acceleration of carbon ions are reported and compared to calculations using analytical models of ion acceleration. 5 Author to whom any correspondence should be addressed.

Relativistic plasma surfaces as an efficient second harmonic generator

We report on the characterization of the specular reflection of 50fs laser pulses in the intensity range 10 17 -10 21 Wcm 2 obliquely incident with p-polarization onto solid density plasmas. These measurements show that the absorbed energy fraction remains approximately constant and that second harmonic generation (SHG) achieves efficiencies of 22±8% for intensities approaching 10 21 Wcm 2 . A simple model based on the relativistic oscillating mirror concept reproduces the observed intensity scaling, indicating that this is 8 Author to whom any correspondence should be addressed.

The FLASHForward facility at DESY

The FLASHForward project at DESY is a pioneering plasma-wakefield acceleration experiment that aims to produce, in a few centimetres of ionised hydrogen, beams with energy of order GeV that are of quality sufficient to be used in a free-electron laser. The plasma is created by ionising a gas in a gas cell with a multi-TW laser system. The plasma wave will be driven by high-current-density electron beams from the FLASH linear accelerator. The laser system can also be used to provide optical diagnostics of the plasma and electron beams due to the <30 fs synchronisation between the laser and the driving electron beam. The project will explore both external and internal witness-beam injection techniques. The operation parameters of the experiment are discussed, as well as the scientific programme.

Scintillator-based ion beam profiler for diagnosing laser-accelerated ion beams

Next generation intense, short-pulse laser facilities require new high repetition rate diagnostics for the detection of ionizing radiation. We have designed a new scintillator-based ion beam profiler capable of measuring the ion beam transverse profile for a number of discrete energy ranges. The optical response and emission characteristics of four common plastic scintillators has been investigated for a range of proton energies and fluxes. The scintillator light output (for 1 MeV > Ep < 28 MeV) was found to have a non-linear scaling with proton energy but a linear response to incident flux. Initial measurements with a prototype diagnostic have been successful, although further calibration work is required to characterize the total system response and limitations under the high flux, short pulse duration conditions of a typical high intensity laser-plasma interaction.

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.

Buffered high charge spectrally-peaked proton beams in the relativistic-transparency regime

Spectrally-peaked proton beams of high charge (E-p approximate to 8 MeV, Delta E approximate to 4 MeV, N approximate to 50 nC) have been observed from the interaction of an intense laser (> 10(19) W cm(-2)) with ultrathin CH foils, as measured by spectrally-resolved full beam profiles. These beams are reproducibly generated for foil thicknesses 5-100 nm, and exhibit narrowing divergence with decreasing target thickness down to approximate to 8 degrees for 5 nm. Simulations demonstrate that the narrow energy spread feature is a result of buffered acceleration of protons. The radiation pressure at the front of the target results in asymmetric sheath fields which permeate throughout the target, causing preferential forward acceleration. Due to their higher charge-to-mass ratio, the protons outrun a carbon plasma driven in the relativistic transparency regime.