Chaojie Zhang, D. Storey, A. Knetsch, Brendan O’Shea, Robert Ariniello, G. Cao, S. Corde, Thamine Dalichaouch, Claudio Emma, O. G. Finnerud, Spencer Gessner, Claire Hansel, Elias Hansen, Valentina Lee, C. A. Lindstrøm, M. Litos, Nathan Majernik, K. A. Marsh, W. B. Mori, Ivan Rajković, Mark Hogan, C. Joshi
High-energy particle colliders and X-ray free-electron lasers demand electron beams with qualities currently achieved only in kilometer-scale radio-frequency accelerators. Plasma accelerators promise a compact alternative but have faced challenges in delivering the needed beam quality at relevant energies. Here, we demonstrate that a plasma-wakefield accelerator operating in the nonlinear regime acts as a transformer to simultaneously boost the energy and brightness of an electron bunch injected from the plasma. Using a 10-GeV drive bunch and a three-stage meter-scale plasma source, we generated electron bunches exceeding 20 GeV with sub-percent energy spread, 2 mm·mrad normalized emittance, and multi-kA peak current. A significant number of drive-bunch electrons lost over 90% of their energy, a prerequisite for high energy-conversion efficiency. This demonstration of an energy transformer ratio exceeding two and a brightness enhancement over an order of magnitude opens a path towards cost-effective accelerators for future colliders and light sources.