Figure 1: Schematic diagram of PW laser-driven wakefield accelerator.
From: Quasi-monoenergetic laser-plasma acceleration of electrons to 2 GeV
The main components were enclosed in a vacuum chamber, highlighted in green, which was kept at 10−6 Torr. The PW laser pulse, entering from the left and linearly polarized perpendicular to the plane of the drawing, was focused into the gas cell, where it created a He plasma and wake that captured and accelerated electrons to 2 GeV. Electrons and betatron X-rays emerging from the cell exit aperture passed through a magnetic field, then through two linear arrays of eight 127 μm diameter tungsten-wire fiducials located 1.256 and 1.764 m, respectively, downstream from the cell exit. A 25-μm thick Al foil deflected the transmitted laser pulse to a beam dump. Undeflected X-rays and energy-dispersed electrons above 0.5 GeV passed through this foil, and exposed in sequence a high-sensitivity (HS) imaging plate (IPHS), a high-resolution (HR) IP (IPHR), a phosphorescing screen (LANEX) and a plastic scintillator. An additional IPLE recorded low-energy (LE) electrons (<0.35 GeV) after they passed through a third array of fiducials. Surrounding panels highlight various diagnostics and details, clockwise from upper left: (a) transversely scattered light, spectrally filtered and imaged to a CCD camera (the dashed rectangle shows the region near the cell exit from which betatron X-rays originated, as determined by X-ray triangulation); (b) trajectories of 2 GeV electrons for shots that yielded the results in Fig. 2a (labelled ‘a’ and ‘b’, respectively) relative to the fiducial arrays (labelled 1–1 through to 1–8 for the first array and 2–1 through to 2–8 for the second array); (c–f), unprocessed data showing electrons up to 2.3 GeV and fiducial shadows for the shot that yielded the results in Fig. 2a, as detected on (c) scintillator, (d) LANEX, (e) IPHS (also showing undeflected X-rays) and (f) IPHR; (g) He pressure versus time, and an acoustic shock when the laser pulse arrived, as recorded by a fast pressure transducer; (h) a typical laser focal spot.
