  We demonstrate THz generation using the tilted pulse front method in Lithium
Niobate, driven at unprecedented high average power of more than 100 W and at
13.3 MHz repetition rate, provided by a compact amplifier-free modelocked
thin-disk oscillator. The conversion efficiency was optimized with respect to
pump spot size and pump pulse duration, enabling us to generate a maximum THz
average power of 66 mW, which is the highest reported to date from a
laser-driven, few-cycle THz source. Furthermore, we identify beam walk-off as
the main obstacle that currently limits the conversion efficiency in this
excitation regime (with moderate pulse energies and small spot sizes). Further
upscaling to the watt level and beyond is within reach, paving the way for
linear and nonlinear high-average power THz spectroscopy experiments with
exceptional signal-to-noise ratio at MHz repetition rates.
