**1.3. Requirements for high repetition rate**

With the light sources of 10–100 PW peak power, the accelerated electron beams can reach the energy up to TeV and ion beams up to GeV, (see **Figure 1**) as well as by using these secondary sources the ultra-bright X and Y-rays can be obtained [21]. These results could be widely applied into many areas of science, industry, medicine, homeland security, and so on. Nevertheless, it will be possible if the ultra-high peak power laser systems also will be able to combine with high repetition rate (hundreds of Hz to kHz) or high average power (kWs). In the petawatt class laser amplifiers, a pump pulse energy exceeds a few hundred J regime, which means significant thermal load in the gain medium even at low repetition rates.

Thin disk laser technology (TDT) is able to eliminate thermal distortions and damages of the laser crystals in the systems with both high peak and average output power [22]. However, conventionally used in TDT, Nd:YAG and Yb:YAG possess the narrow emission spectra and the low emission cross-section that lead to very complicated multipass amplification schemes which is practically acceptable only for low peak power systems with the ps-level pulse duration. The most promising crystal with required characteristics for ultra-high peak power laser is Ti:Sa, especially if one is taking into account its higher emission cross-section and thermal conductivity (compare 10 W/(mK) for YAG to 40 W/(mK) for sapphire at the room temperature and more than two orders higher when cryogenically cooled). But the attempt to increase the longitudinal gain in TD-amplifier by rising the concentration of active ions and/or using crystals with higher emission cross-section leads to dramatic increase of the gain in the transverse direction and consequently to the big losses and inability to store pump energy due to TASE. Technology for solving this problem will be presented below in this chapter.
