Fig. 1. The main principle behind maximising field strength starting from laser sources with optical frequencies.

Fig. 2. (a) The numerical result for the dipole focusing of XUV pulse. (b) The total laser power of 200 PW is split into six beams and each is focused to at from the focus, where the plasma converters provide an amplitude boost by a factor of 15 and frequency upshift by a factor of approximately . The conversion is followed by the MCLP (e-dipole) focusing using six beams at . (c) The dependency of the field strength on the -coordinate (green curve), -coordinate (blue curve) and time (red curve) is shown in panel (c) together with the fit (black solid curves) and the threshold for cascaded pair-generation (dashed black line).

Fig. 3. The prospects of reaching high field strength using tight focusing, multiple laser colliding pulses, the plasma conversion and their combination on the map of the attainable field strength and total power of the laser facility. The two outlined options correspond to the use of the plasma conversion at and , respectively. The labels show the results of simulations by Gonoskov et al.^[41] (1), by Baumann et al.^[33] (2) and by Vincenti^[34] (3).