**5. Conclusion**

The obtained results demonstrate the capacity to build a room temperature cooled final amplifier, providing few Joules energy of the seed laser pulses with in a 100 s TW/100 s Hz

Numerical modeling of scaling larger peak power amplifier modules with double channel cooling and double crystals (three cooling channels) design were also conducted in Ref. [43]. Double channel cooled disks with diameters ranging from 6 to 20 cm and corresponding thicknesses of 0.6 to 2 cm, were investigated (**Figure 19a**). The inlet flow velocity was 4 m/s in all cases to ensure high levels of heat extraction from the disk gain modules. Every gain module would remain under 45°C up to 40 Hz of operation relative to the coolant temperature. When the repetition rate is growing up further, one can see significant rise in the temperature increase (TI), nevertheless, the temperature profile would remain smooth and flat in the region of the laser amplification. Peak power could reach 8.5 PW and an average power 17 kW with compressor efficiency of 60% and pulse duration of 20 fs [**Figure 19a** (inset)]. The maximal repetition rate when amplifiers could operate safely and without serious beam degradation can be estimated based on the obtained TIs. Extremely efficient heat extraction can be obtained by increasing the diameter, while maintaining the aspect ratio of the gain disks with two coolant channels and thus flat temperature profiles with high

Further increase of the average power could be achieved by splitting the gain disk to multiple plates with reduced thickness and increasing the number of coolant channels. Four gain modules with double disk sizes ranging from 6 to 20 cm was investigated, with three coolant channel arrangement (**Figure 19b**). These simulations demonstrate 2 kW output average power with a TI in the disks of 21.5°C, and 28 kW average power at TI of 36°C using multiple

**Figure 19.** (a) Temperature dependence on repetition rates for various single disk sizes. Pump energy 40, 57, 77, 101, 127, 308, 567 J, for the diameters starting from 6 cm respectively. Peak power of compressed pulses listed in inset (compressor efficiency-60%, repetition rate-100 Hz and pulse duration-20 fs); (b) temperature in the single and double disk modules (Ti:Sa crystals of 6, 10, 15 and 20 cm diameters, 100 Hz for all cases) cooled by three channels using 4 m/s flow velocity

disks and cooling surfaces with proper coolant flow conditions.

CPA laser systems.

86 High Power Laser Systems

repetition rate operation.

at the inlet boundary of the channels.

In this chapter, several ideas for innovation of the ultra-high peak power CPA laser systems were presented. Exploiting these ideas, one is able to significantly increase the output energy (up to KJ-level), reduce pulse duration (down to few fs) and so increase output peak power up to 100 s of PW. At the same time, the possibilities of average power growing of these systems up to 10 s kW was also demonstrated.

EDP-method for Ti:Sa final amplifiers was revealed as easiest way to reach a very high output energy [25, 27, 28]. EDP amplifier, when operated under the optimal conditions, is capable of significantly increasing the extracted energy and reducing the losses connected with TASE and TPG. With the existing large aperture of Ti:Sa crystals and index-matched liquid absorbers, it is possible to approach the sub-kJ level of extracting energy. With 70% compressor transmission efficiency and 15 fs pulse duration, about 30 PW power level could be reached. The powerfulness of EDPCPA technology was proved by spreading the method in the many world class laboratories and reaching recently the output energy about 200 J and world record peak power of 5 PW. Next steps of the output energy ~ 500–800 J could be done with the existing now Ti:Sa crystals of 20–30 cm diameter.

Two recently developed method of pulse shortening have been discussed in the subtitle 3. The ability to obtain a greatly broadened spectral bandwidth in Ti:Sa laser amplifiers was shown using both π- and σ-axis and shaping the spectral gain via engineering the spectral polarization of amplified pulses [34]. Amplification bandwidth exceeding 85 nm at a gain of 200 was demonstrated in a proof-of-principle experiment. These experiments have shown also that active pre-shaping of the pulse spectrum with PE amplification preceding saturated amplification in conventional CPA amplifiers can be successfully used to compensate the spectral red-shifting and gain narrowing that accompany amplification in Ti:Sa CPA systems. The computer modeling revealed that a polarization-encoded chirped pulse amplification scheme can be scaled to higher energies and produce multi-Joule pulses with bandwidth close to 200 nm, making few-cycle petawatt Ti:Sa systems feasible.

The multiple stage compression method based on spectral broadening using SPM in the bulk of material with the further recompression of the chirped pulse is able to deliver even shorter pulse duration below 10 fs without energy sacrificing [37]. Further development of this idea, with SPM in thin film below 1 mm and much higher intensity (up to tens of TW/cm<sup>2</sup> ) was suggested [38]. Numerical simulations of two stage thin film compressor were done with the thicknesses of thin film elements 0.5 mm and 0.1 mm. The fundamental peak intensity after the first and second stages with temporal recompression procedure 16.6 and 43 TW/cm<sup>2</sup> at pulse durations 6.4 and 2.1 fs correspondingly are expected. This shortening of the pulse duration without energy losses allows to increase the output peak power to an additional order and achieve few hundred PW from the single channel of CPA laser systems.

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The combination of EDP technique with TD Ti:Sa crystals for power amplifiers [39, 41, 43] also lets the ultra-high peak power amplifiers increase as well as the average power. In a proofof-principle experiment, high-energy broadband amplification in a room temperature watercooled EDP-TD head was demonstrated at a 10 Hz repetition rate instead of performing a traditional cryogenically cooled multipass scheme. Therefore, the limits associated with thermal effects and transverse amplified spontaneous emission can be overcome by the EDP-TD combination, enabling Ti:Sa laser systems to have a petawatt peak power and hundreds Hz repetition rates or kWs of average power.
