High-repetition rate attosecond pulse sources are indispensable tools for time-resolved studies of electron dynamics

such as coincidence spectroscopy and experiments with high demands on statistics or signal-to-noise ratio

especially in the case of solid and big molecule samples in chemistry and biology. Although with the high-repetition rate lasers

such attosecond pulses in a pump-probe configuration are possible to achieve

until now

only a few such light sources have been demonstrated. Here

by shaping the driving laser to an annular beam

a 100 kHz attosecond pulse train (APT) is reported with the highest energy so far (51 pJ/shot) on target (269 pJ at generation) among the high-repetition rate systems (>10 kHz) in which the attosecond pulses were temporally characterized. The on-target pulse energy is maximized by reducing the losses from the reflections and filtering of the high harmonics

and an unprecedented 19% transmission rate from the generation point to the target position is achieved. At the same time

the probe beam is also annular and low loss of this beam is reached by using another holey mirror to combine with the APT. The advantages of using an annular beam to generate attosecond pulses with a high-average power laser are demonstrated experimentally and theoretically. The effect of nonlinear propagation in the generation medium on the annular-beam generation concept is also analyzed in detail. The ELI-ALPS project (GINOP-2.3.6-15-2015-00001) is supported by the European Union and cofinanced by the European Regional Development Fund.