  We report on an Yb:YAG thin-disk multipass amplifier delivering 50 ns long
pulses at a central wavelength of 1030 nm with an energy of 330 mJ at a
repetition rate of 100 Hz. The beam quality factor at the maximum energy was
measured to be $\text{M}^2 = 1.17$. The small signal gain is 20, and the gain
at 330 mJ was measured to be 6.9. The 20-pass amplifier is designed as a
concatenation of stable resonator segments in which the beam is alternately
Fourier transformed and relay-imaged back to the disk by a 4f-imaging optical
scheme stage. The Fourier transform propagation makes the output beam robust
against spherical phase front distortions, while the 4f-stage is used to
compensate the thermal lens of the thin-disk and to reduce the footprint of the
amplifier.
