Development of Focusing Plasma Mirrors for Ultraintense Laser-Driven Particle and Radiation Sources

Abstract

Increasing the peak intensity to which high power laser pulses are focused can open up new regimes of laser-plasma interactions, resulting in the acceleration of ions to higher energies and more efficient generation of energetic photons. Low f-number focusing plasma mirrors, which re-image and demagnify the laser focus, provide an attractive approach to producing higher intensities, without requiring significant changes to the laser system. They are small, enhance the pulse intensity contrast and eliminate the requirement to expose expensive optics directly to target debris. We report on progress made in a programme of work to design, manufacture and optimise ellipsoidal focusing plasma mirrors. Different approaches to manufacturing these innovative optics are described, and the results of characterisation tests are presented. The procedure developed to align the optics is outlined, together with initial results from their use with a petawatt-level laser.

Influence of spatial-intensity contrast in ultraintense laser–plasma interactions

“…Further details on the design and operation of this optic are provided in Refs. 13 , 14 . The second uses a planar plasma mirror (PPM) to maintain the F /3 OAP focusing after reflection.…”

“…These characterisation measurements were obtained using a testing set-up (described in Refs. 13 , 14 ), that utilises a 532 nm light source, and thus smaller spots/wings are obtained when compared to FPM usage on the Vulcan laser (operating at 1054 nm). As the effect of unoptimised FPM alignment results in decreased focal spot intensity and increased spot wing size (thus irradiation area), this gives us a method to measure not only the effect of spot size at intensities similar to those achieved by the larger spot ( , when used on the Vulcan laser), but crucially in this investigation the influence of focal spot spatial-intensity contrast, particularly in the case of tight focus, when the wings are relativistically intense.…”

“…In recent years, efforts to boost the intensity have therefore focused on generating shorter pulses or achieving tighter focusing. The latter, due to the costs involved in producing large diameter short focal length optics and potential damage from target debris, has led to innovative approaches, such as the use of disposable focusing plasma mirrors (FPM) 12 – 14 . These efforts and the employment of extremely tight focusing optics drive the need to understand how the resulting decrease in focal spot size influences the laser–plasma interaction physics and properties of the secondary sources of energetic particles and radiation.…”

Enhanced laser intensity and ion acceleration due to self-focusing in relativistically transparent ultrathin targets

“…1(b). This optic reimages the f /3 focal spot with a demagnification of ×3, reducing φ L to 1.5 μm (FWHM) and increasing the nominal intensity by up to ×9, for the same plasma reflectivity [30,33]. Compared to the f /3 configuration, the encircled energy within focal spot FWHM was reduced by between 25% and 30%.…”

“…The encircled energy value as measured at low power for each PPM and FPM was corrected to account for any beam wavefront irregularity measured on the corresponding full power shot on the same plasma mirror. The reduction in encircled energy for the f /1 case arises due to the increased sensitivity to small shot-to-shot variations in the laser pointing, which change the energy balance between the focal spot and the lower-intensity wings [30,33]. Factoring this in, together with the measured laser pulse parameters, the calculated intensity using the FPM was (1.5 ± 0.8) × 10 21 W cm −2 .…”