I. INTRODUCTION Ultrastrong field science encompasses a broad range of topics including the ultimate energy limit of11. M. Klaiber, K. Z. Hatsagortsyan, J. Wu, S. S. Luo, P. Grugan, and B. C. Walker, “Limits of strong field rescattering in the relativistic regime,” Phys. Rev. Lett. 118, 093001 (2017). https://doi.org/10.1103/physrevlett.118.093001 coherent x-ray generation by high harmonic generation,22. M. Lewenstein, Ph. Balcou, M. Yu. Ivanov, A. L’Huillier, and P. B.