Figure 3. Betatron X-ray data.

(a) Typical radiograph of Al and Cu masks backlit by betatron X-rays from the accelerator recorded on IP_HS. Masks vary in thickness in four lateral steps: 2 mm (centre), 1 mm, 0.5 mm and 0.25 mm (edge). Vertical lines are shadows of tungsten-wire fiducials. (b) Typical X-ray spectrum. Data points were obtained by analysing transmittance of the Cu/Al masks (see Methods). Horizontal error bars correspond to the FWHM of the f_k distribution (see Methods); vertical error bars represent uncertainty in X-ray transmission caused by scattering at edges of the filter masks, and error in reconstructing the incident X-ray intensity profile from transmission through gaps in the masks. Blue curve is a cubic spline fit to the data points. (c) X-ray source locations within the 7 cm gas cell for 22 shots that yielded >1 GeV electrons, determined by triangulation from tungsten-wire fiducial shadows. Source locations for shots in Fig. 2a are labelled ‘a’ (red) and ‘b’ (yellow), respectively. Horizontal error bars (±10 mm) indicate triangulation error arising from uncertainty in measuring fiducial wire positions.