Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency.

Control of the collective response of plasma particles to intense laser light is intrinsic to relativistic optics, the development of compact laser-driven particle and radiation sources, as well as investigations of some laboratory astrophysics phenomena. We recently demonstrated that a relativistic plasma aperture produced in an ultra-thin foil at the focus of intense laser radiation can induce diffraction, enabling polarization-based control of the collective motion of plasma electrons. Here we show that under these conditions the electron dynamics are mapped into the beam of protons accelerated via strong charge-separation-induced electrostatic fields. It is demonstrated experimentally and numerically via 3D particle-in-cell simulations that the degree of ellipticity of the laser polarization strongly influences the spatial-intensity distribution of the beam of multi-MeV protons. The influence on both sheath-accelerated and radiation pressure-accelerated protons is investigated. This approach opens up a potential new route to control laser-driven ion sources.

Transmission grating stretcher for contrast enhancement of high power lasers.

We propose, for the first time, a transmission grating stretcher for high power lasers and demonstrate its superiority over conventional, reflective gold grating stretchers in terms of pulse temporal quality. We show that, compared to a conventional stretcher with the same stretching factor, the transmission-grating based stretcher yields more than an order of magnitude improvement in the contrast pedestal. We have also quantitatively characterized the roughness of the grating surfaces and estimated its impact on the contrast pedestal.

Improving coherent contrast of petawatt laser pulses.

We report on an experimental study of the "coherent" contrast feature that frequently appears in petawatt(PW)-class laser pulses as an exponentially-rising pedestal within a few tens of picoseconds of the compressed pulse. We show that scattering from the diffraction gratings in the stretcher is the principal source of this feature. Replacing the gratings by new, higher-quality components resulted in an order-of-magnitude reduction in the intensity of the pedestal.

ASE suppression in a high energy Titanium sapphire amplifier.

The energy required to generate ultrashort pulses with petawatt peak power from a Ti:sapphire laser system is a few tens of joules. To achieve this, the final amplifier must have a gain region of around 5 cm diameter that is uniformly pumped at high fluence. The high level of amplified spontaneous emission (ASE) in such an amplifier will seriously degrade its performance unless care is taken to minimise the transverse gain and the internal reflections from the crystal edges. In developing the amplifiers for the Astra Gemini laser system, we have combined the techniques of beam homogenisation and double-pass pumping of a lightly-doped crystal with a new index-matched absorber liquid. Our results demonstrate that this combined approach successfully overcomes the problem of gain depletion by ASE in a high-energy Ti:sapphire amplifier.

Characterization of the backscattered radiation from petawatt laser matter interactions.

The development of high peak power and energy laser systems require the assurance that any backscattered radiation will not lead to damage of the laser system. We present the characterization of the backscattered radiation for different target types and conditions at petawatt power levels and intensities (>10(20)W/cm2). We observe that radiation is generated between 700 and 900 nm, as well as the expected self emission and laser fundamental. The percentage of the incident light backscattered reduces as a function of the incident energy and is typically <1% for petawatt laser interactions.

A closer look at truncal acne vulgaris: prevalence, severity, and clinical significance.

Data is very limited on the demographics, epidemiology, grading, and management of truncal acne vulgaris. In this study completed in 5 cities across the US, 696 patients were assessed to evaluate the frequency and severity of truncal acne vulgaris. Approximately 50% of patients who presented with acne vulgaris demonstrated involvement on the chest and/or back, with more than 3% presenting with truncal acne lesions alone. Interestingly, approximately 1 out of 4 patients who presented with both facial and truncal acne involvement did not voluntarily mention the presence of truncal acne as part of their presenting complaint. In such cases, the presence of truncal acne lesions was detected by clinical examination. A grading system for truncal acne vulgaris is also presented. The majority of patients presenting with truncal acne vulgaris exhibited mild to moderate severity and more than 75% were interested in treatment for truncal acne lesions.