Video of debris from an amorphous carbon target shot in Gemini TA3 during Doria’s experiment 22210024 in 2024

Abstract

This video was recorded during shot 93 of an experiment called “Ion acceleration from ultra-thin foils by employing Laguerre-Gaussian laser beams” on 10 October 2024. The Principal Investigator was Domenico Doria. The target was a 15nm-thick amorphous carbon film floated over the back of a copper frame with an aluminium frontpiece (see Fig. 1 in the attached README.pdf). The copper frame featured 9 holes for 9 separate targets. The holes in the copper frame over which each target was floated were 500μm in diameter. The aluminium frontpiece featured 9 circular apertures (1mm in diameter) that tapered down to the 500μm diameter of the holes in the copper frame. The laser was therefore focused through the tapered aperture in the aluminium frontpiece, through the hole in the copper mount and onto the surface of the target. The target consisted of the area of carbon film visible through the hole in the copper mount. Shot 93 also used a radiochromic film (RCF) stack as a diagnostic of the ion spectrum (not visible in the video images). The RCF was placed behind the target at a distance of 5cm and the stack design was S53 (contact Domenico Doria for more information). Shot 93 was taken with a standard Gaussian focus (flat mirror before the parabola – not the L1 or L3 helical mirrors also used on the experiment) in a double plasma mirror configuration. The video was taken using an i-SPEED 727 camera (3.4 Mpixels, 2.4 Mfps maximum frame rate) from iX cameras. The target was “side-lit” using an expanded and recollimated 527nm green laser. This sidelighter laser had a power of ∼ 0.9 mW and was directed at 7deg to the target surface, which was the closest to transverse illumination that could be achieved with our set-up. The camera was operated at 5000 fps due to the relatively low sidelighter irradiance of 0.22 mWcm−2. The camera was placed outside of the vacuum chamber behind a lens of 50 cm focal length which magnified the image by a factor 3. The camera did not view the interaction directly – light was relayed out of the chamber using mirrors and BG39 and bandpass filters (525±25 nm) were used to shield the camera from all frequencies of light except green. Due to the width of the aluminium support, only the rear surface of the target (i.e. a thin carbon film floated over a copper mount) is visible in the video. The camera field of view was 5.3 × 5.7 mm2 and the dimple/depression visible on the rear surface of the copper mount is ~500μm in width (not depth). The energy of the laser drive for shot 93 was 11.3 J post compressor with a pulse duration of 45 fs, focused to 4 μm FWHM on the surface of the target. A double plasma mirror system was employed before the focusing parabola to improve the pulse’s temporal contrast, producing an on-target energy of ∼ 6 J and a focused intensity of ∼ 4 × 1020 Wcm−2 averaged over the spot (accounting for 33% enclosed energy in the FWHM). Based on recent measurements, the on-target laser contrast after the two plasma mirrors was ~10-8 at 1 ps prior to peak laser intensity. The purpose of the measurement was to try to assess the speed, size, phase and number of debris particles produced when the Gemini laser is fired on thin (10-100nm-thick) targets.