**5. Conclusion**

cooled, multislab ceramic Yb:YAG amplifier technology. A prototype amplifier is delivering up to 10.8 J pulse energy at 1030 nm wavelength with 10 Hz repetition rate. The optical-optical conversion efficiency is 22.5% [42]. The long-term energy stability was observed as 0.85% RMS with 7 J pulse energy for 48 h operation (2 million shots). An extension of the cryogenic technology is now under test in the DiPOLE 100 to confirm the cryogenic concept at 100 J, 10 Hz region (kW average power). The present laser system is built for the HiLASE project and will deliver 100 J temporally shaped ns pulses at 10 Hz with a fully integrated control system. A second system is also under development for the high-energy density (HED) beamline of

**Figure 16.** Strehl ratio for the four passes of the probe beam through the Yb:YAG slab and absorbed pump power per single pass as a function of temperature. Lines represent a linear fit with slopes of –6 × 10−4/K and −0.064%/K, respec‐

The 10 J amplifier architecture is based on the multislab approach. The gain medium is composed of four circular Yb:YAG slabs with two different Yb doping levels as 1.1 and 2.0 at % to confirm a uniform temperature distribution among each slab. The diameter of the circular

beam is supplied from 939 nm diodes in stack with a pumping time duration of 700 μs at 10 Hz. The Yb:YAG circular slab is cladded with a 5 mm wide Cr:YAG absorber with 6 cm−1 absorption coefficient. This is effective to prevent amplified spontaneous emission (ASE) and parasitic oscillations. A cryogenic He flow cools the slab and keeps the temperature as 150– 170 K. The pressure of the He flow is 10 bar. **Figure 17** shows the optical arrangement for amplification. A seed beam is injected into the amplifier through a dichroic mirror and then image relayed by a spatial filter (*f* = 1 m) to a back reflector and reflected back to the amplifi‐ er module. One spatial filter locates on each side of the amplifier head. Each pass is com‐ posed by a set of separate mirrors. A deformable mirror is placed in the amplifier after the third pass for the aberration compensation. After seven passes, the beam is extracted from the

. The pump

.

slab is 45 mm with a 5 mm thickness, and the pump area is square of 23 × 23 mm2

amplifier with pulse energy increased to 9 J with a size of 21 mm × 21 mm2

the European XFEL project.

122 High Energy and Short Pulse Lasers

tively.

Recent progress of thin-disc lasers is promising to realize a high-brightness pumping source of laser plasma or laser Compton short-wavelength sources. Further progress is possible by an advanced cryogenic technology with its higher thermal conductivity. These laser progress‐ es are contributing in the practical applications in short-wavelength imaging and material processing. Picosecond thin-disc laser technology is now in the stage of 1 kW level with >100 kHz repetition rate. Furtherresearch and developments are aiming at 1 kWwith kHz repetition rate (1 J, ps, kHz), pulse length reduction into subpicosecond region with MHz repetition rate (mJ, fs, MHz), and increase in the average powerto 10 kW region. These challenges require further improvements of the achieved technology bases and evaluation of new schemes. Cryogenic technology is now offering an option for these challenges in the solid-state laser technology.
