We report on experimental studies of divergence of proton beams from nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser with high contrast. Proton beams with extremely small divergence (half angle) of 2° are observed in addition with a remarkably well-collimated feature over the whole energy range, showing one order of magnitude reduction of the divergence angle in comparison to the results from μm thick targets. We demonstrate that this reduction arises from a steep longitudinal electron density gradient and an exponentially decaying transverse profile at the rear side of the ultrathin foils. Agreements are found both in an analytical model and in particle in cell simulations. Those novel features make nm foils an attractive alternative for high flux experiments relevant for fundamental research in nuclear and warm dense matter physics.
In view of their properties, laser-driven ion beams have the potential to be employed in innovative applications in the
scientific, technological and medical areas. Among these, a particularly high-profile application is particle therapy for
cancer treatment, which however requires significant improvements from current performances of laser-driven
accelerators. The focus of current research in this field is on developing suitable strategies enabling laser-accelerated
ions to match these requirements, while exploiting some of the unique features of a laser-driven process. LIBRA is a
UK-wide consortium, aiming to address these issues, and develop laser-driven ion sources suitable for applicative
purposes, with a particular focus on biomedical applications. We will report on the activities of the consortium aimed to
optimizing the properties of the beams, by developing and employing advanced targetry and by exploring novel
acceleration regimes enabling production of beams with reduced energy spread. Employing the TARANIS Terawatt
laser at Queen's University, we have initiated a campaign investigating the effects of proton irradiation of biological
samples at extreme dose rates (> 109 Gy/s).
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.
Radiation pressure acceleration (RPA) theoretically may have great potential to revolutionize the study of laserdriven
ion accelerators due to its high conversion efficiency and ability to produce high-quality monoenergetic ion
beams. However, the instability issue of ion acceleration has been appeared to be a fundamental limitation of the
RPA scheme. To solve this issue is very important to the experimental realization and exploitation of this new
scheme. In our recent work, we have identified the key condition for efficient and stable ion RPA from thin foils
by CP laser pulses, in particular, at currently available moderate laser intensities. That is, the ion beam should
remain accompanied with enough co-moving electrons to preserve a local "bunching" electrostatic field during
the acceleration. In the realistic LS RPA, the decompression of the co-moving electron layer leads to a change
of local electrostatic field from a "bunching" to a "debunching" profile, resulting in premature termination of
acceleration. One possible scheme to achieve stable RPA is using a multi-species foil. Two-dimensional PIC
simulations show that 100 MeV/u monoenergetic C6+ and/or proton beams are produced by irradiation of a
contaminated copper foil with CP lasers at intensities 5 × 1020W/cm2, achievable by current day lasers.
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.
It has become apparent in the last few years that the light ion surface contamination on short-pulse laser targets is a major impediment to the acceleration of heavier target ions. Mitigation strategies have been tested in experiments at the Los Alamos Trident Laser facility using one arm of the Trident laser at 150 ps to ablatively clean a large area of heated targets in a single short that are subsequently irradiated by the Trident 30 TW short-pulse arm to accelerate the bulk target ions to high energies. This process was used on targets consisting of 15 microns of vanadium. The 150 ps pulse rids the rear of the target of its omnipresent surface contamination layer, consisting mainly of water vapor and hydrocarbons, and allows the Trident 30 TW short-pulse arm to illuminate the target and accelerate ions via the Target Normal Sheath Acceleration (TNSA) mechanism. Because this mechanism relies on a laser generated electrostatic sheath, the ions with the lightest charge to mass ratio (i.e. protons) would be accelerated preferentially at the expense of heavier ions. However with the contamination layer removed, and hence the bulk of the available protons, the TNSA mechanism is able to accelerate the bulk material ions to high energies. Our experimental results are discussed and compared to the LASNEX rad-hydro code to validate and improve our predictive capabilities for future acceleration experiments.
Multi-shot damage tests were performed of gold coated mirrors in the femtosecond and in the nanosecond laser pulse regime. Sputtered gold films from different suppliers of various thicknesses were investigated. Considerable differences in the optical quality and the damage threshold are reported. The best films withstand a maximum fluence of 0.7 J/cm2 for 50-fs Ti:sapphire laser irradiation (804 nm) and 7 J/cm2 for 8-ns Nd:YAG irradiation (1064 nm). For gold films with poor optical quality a permanent surface modification one order of magnitude below the damage threshold was observed.