Fig. 1: FREM experimental setup and axiparabola focal spots.

a Schematic of the FREM experimental setup. Beam 1 is focused by the axiparabola onto jet 1, generating the laser wakefield. Beam 2 is focused by a parabola onto jet 2, generating a second laser wakefield which accelerates a femtosecond duration electron bunch, known as the “probe electron bunch”. This probe bunch is then allowed to expand and impinges onto the axiparabola-generated wakefield, which imposes momentum changes onto the probe electrons. After further propagation, these momentum changes turn into density modulations which are then imaged when the electrons hit a YAG screen. b 2D axiparabola focal spots, measured in vacuum, at specified points along the focal depth relative to the beginning of the focal line 480 mm away from the axiparabola.