Author Affiliations

Oliver Alexander,¹ LR25 collaboration,^1,2,3,4 including Thomas Barillot,¹ Bridgette Cooper,^1,2 Taran Driver,^1,3,4 Douglas Garratt,¹ Siqi Li,⁵ Agostino Marinelli,^3,4 James P Cryan,^3,4 and Jonathan P Marangoset al.¹

¹Quantum Optics and Laser Science Group, Blackett Laboratory, Imperial College London, London, SW7 2BW, UK

²Atomic, Molecular, Optical and Positron Physics Group, University College London, Gower Street, London, WC1E 6BT, UK

³Stanford PULSE Institute, SLAC National Accelerator Laboratory, USA

⁴Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California, 94025, USA

⁵Accelerator Research Division, SLAC National Accelerator Laboratory, Menlo Park, California, 94025, USA