  We demonstrate an injection-seeded thin-disk Yb:YAG laser at 1030 nm,
stabilized by the Pound-Drever-Hall (PDH) method. We modified the PDH scheme to
obtain an error signal free from Trojan locking points, which allowed robust
re-locking of the laser and reliable long-term operation. The single-frequency
pulses have 50 mJ energy (limited to avoid laser-induced damage) with a beam
quality of $\text{M}^2$ < 1.1 and an adjustable length of 55-110 ns. Heterodyne
measurements confirmed a spectral linewidth of 3.7 MHz. The short pulse
build-up time (850 ns) makes this laser suitable for laser spectroscopy of
muonic hydrogen, pursued by the CREMA collaboration.
