D. Pepler

Generation of terawatt pulses by use of optical parametric chirped pulse amplification

Optical parametric chirped pulse amplifiers offer exciting prospects for generating new extremes in power, intensity, and pulse duration. An experiment is described that was used to investigate the operation of this scheme up to energies approaching a joule, as a step toward its implementation at the petawatt level. The results demonstrate an energy gain of 10(10) with an energy extraction efficiency of 20% and close to diffraction-limited performance. Some spectral narrowing during amplification was shown to be compatible with the time-varying profile of the pump beam and consistent with the measured recompressed pulse durations of 260 and 300 fs before and after amplification, respectively.

Effects of front surface plasma expansion on proton acceleration in ultraintense laser irradiation of foil targets

The properties of beams of high energy protons accelerated during ultraintense, picosecond laser-irradiation of thin foil targets are investigated as a function of preplasma expansion at the target front surface. Significant enhancement in the maximum proton energy and laser-to-proton energy conversion efficiency is observed at optimum preplasma density gradients, due to self-focusing of the incident laser pulse. For very long preplasma expansion, the propagating laser pulse is observed to filament, resulting in highly uniform proton beams, but with reduced flux and maximum energy.

Binary phase zone-plate arrays for laser-beam spatial-intensity distribution conversion

We report on the theory and development of a diffractive element composed of a binary phase zone-plate array. This component conditions the intensity distribution in the focal plane of a conventional refractive lens to generate efficiently (82%) a flattop intensity envelope on target. Analysis of the design indicates that manufacturing tolerances are not critical. Experimental performances on target from x-ray emission and shock-breakout measurements are also presented.

Wave-front control of a large-aperture laser system by use of a static phase corrector

In large-aperture, ultrahigh-intensity laser systems, such as Vulcan at the Rutherford Appleton Laboratory, one of the most important factors that determines the ultimate on-target focused intensity is the wave-front quality of the laser pulse. We report on a wave-front analysis carried out on Vulcan to determine the nature and contribution of the aberrations present in the laser pulse that effectively limited the available on-target intensity. We also report on a significant improvement to the wave-front quality that was achieved by static correction of the main aberration, resulting in an increase of focused intensities by a factor of 4.

Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.

Binary phase zone plate designs using calculations of far-field distributions

Abstract Binary phase Fresnel zone plate arrays have been used recently to generate circular top hat intensity profiles for high intensity laser irradiation of targets. A simple analysis of these binary phase zone plates using diffraction theory is presented and used both to optimise the uniformity of the focal spot by modifying the Fresnel zone structure and to produce noncircular foci.

Effects of laser prepulse on proton generation: active manipulation of the distribution of laser accelerated proton beams

Laser pre‐pulse is a major issue in experiments on laser‐generation of protons, often limiting the performances of laser sources. In this paper, we show how we can actively use a low intensity prepulse (<1013 W/cm2, ns duration) to manipulate the proton beam direction or spatial energy distribution. The prepulse is focused onto the front surface of a thin foil before the arrival of the high intensity pulse (≈1019 W/cm2, ps duration). Under oblique high‐intensity irradiation and for low prepulse intensities, the proton beam is directed away from the target normal. Deviation is towards the laser forward direction, with an angle that increases with the level and duration of the ASE pedestal. Also, for a given laser pulse, beam deviation increases with proton energy. The observations are discussed in terms of Target Normal Sheath Acceleration, in combination with a laser‐controllable shock wave locally deforming the target surface. Results obtained with an annular intensity distribution of the prepulse show s...

Generation of focused intensities of 5 × 1019 Wcm[minus]2

The technique of Chirped Pulse Amplification (CPA) developed by Strickland and Mourou (1985) is now in common use on many laser systems [see, for instance, the review by Perry & Mourou (1994)] and has resulted in massive increases in focused intensities. This paper describes CPA implementation on the Vulcan laser system which has generated multi-Joule sub-picosecond pulses whilst maintaining beam quality to produce focused intensities of 5 × 10 19 Wcm −2 .