P.T. Simpson

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.

Low- and medium-mass ion acceleration driven by petawatt laser plasma interactions

An experimental investigation of low- and medium-mass ion acceleration from resistively heated thin foil targets, irradiated by picosecond laser pulses at intensities up to 5 × 1020 W cm−2, is reported. It is found that the spectral distributions of ions, up to multi-MeV/nucleon energies, accelerated from the rear surface of the target are broadly consistent with previously reported measurements made at intensities up to 5 × 1019 W cm−2. Properties of the backward-directed beams of ions accelerated from the target front surface are also measured, and it is found that, compared with the rear surface, higher ion numbers and charges, and similar ion energies are produced. Additionally, the scaling of the maximum ion energy as a function of ion charge and laser intensity are measured and compared with the predictions of a numerical model.

Micron-scale fast electron filaments and recirculation determined from rear-side optical emission in high-intensity laser–solid interactions

The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of mu m-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given.

High harmonics from relativistically oscillating plasma surfaces - a high brightness attosecond source at keV photon energies

An intense laser pulse interacting with a near discontinuous plasma vacuum interface causes the plasma surface to perform relativistic oscillations. The reflected laser radiation then contains very high order harmonics of fundamental frequency and-according to current theory-must be bunched in radiation bursts of a few attoseconds duration. Recent experimental results have demonstrated x-ray harmonic radiation extending to 3.3 angstrom (3.8 keV, order n > 3200) with the harmonic conversion efficiency scaling as eta(n) n(-2.5) over the entire observed spectrum ranging from 17 nm to 3.3 angstrom. This scaling holds up to a maximum order, n(RO) 81 8(1/2)gamma(3), where gamma is the peak value of the Lorentz factor, above which the harmonic efficiency decreases more rapidly. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy h nu > 1 keV is found to be into a cone angle similar to 4 degrees, significantly less than that of the incident laser cone (20 degrees).

Detection of short lived radioisotopes as a fast diagnostic for intense laser-solid interactions

As a diagnostic of high-intensity laser interactions (>1019Wcm−2), the detection of radioactive isotopes is regularly used for the characterization of proton, neutron, ion, and photon beams. This involves sample removal from the interaction chamber and time consuming post shot analysis using NaI coincidence counting or Ge detectors. This letter describes the use of in situ detectors to measure laser-driven (p,n) reactions in Al27 as an almost real-time diagnostic for proton acceleration. The produced Si27 isotope decays with a 4.16s half-life by the predominantly β+ emission, producing a strong 511keV annihilation peak.

Laser driven MeV proton beam focussing by auto-charged electrostatic lens configuration

Significant reduction of inherent large divergence of the laser driven MeV proton beams is achieved by strong (of the order of 10(9) V/m) electrostatic focussing field generated in the confined region of a suitably shaped structure attached to the proton generating foil. The scheme exploits the positively charging of the target following an intense laser interaction. Reduction in the proton beam divergence, and commensurate increase in proton flux is observed while preserving the beam laminarity. The underlying mechanism has been established by the help of particle tracing simulations. Dynamic focussing power of the lens, mainly due to the target discharging, can also be exploited in order to bring up the desired chromaticity of the lens for the proton beams of broad energy range.

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...