  In the context of the Lamb shift measurement in muonic helium we developed a
thin-disk laser composed of a Q-switched oscillator and a multi-pass amplifier
delivering pulses of 150 mJ at a pulse duration of 100 ns. Its peculiar
requirements are stochastic trigger and short delay time (< 500 ns) between
trigger and optical output. The concept of the thin-disk laser allows for
energy and power scaling with high efficiency. However the single pass gain is
small (about 1.2). Hence a multi-pass scheme with precise mode matching for
large beam waists (w = 2 mm) is required. Instead of using the standard 4f
design, we have developed a multi-pass amplifier with a beam propagation
insensitive to thermal lens effects and misalignments. The beam propagation is
equivalent to multiple roundtrips in an optically stable resonator. To support
the propagation we used an array of 2 x 8 individually adjustable plane
mirrors. Astigmatism has been minimized by a compact mirror placement. Precise
alignment of the kinematic array was realized using our own mirror mount
design. A small signal gain of 5 for 8 passes at a pump power of 400 W was
reached. The laser was running for more than 3 months without the need of
realignment. Pointing stability studies is also reported here.
