  Attosecond pulses, produced through high-order harmonic generation in gases,
have been successfully used for observing ultrafast, sub-femtosecond electron
dynamics in atoms, molecules and solid state systems. Today's typical
attosecond sources, however, are often impaired by their low repetition rate
and the resulting insufficient statistics, especially when the number of
detectable events per shot is limited. This is the case for experiments where
several reaction products must be detected in coincidence, and for surface
science applications where space-charge effects compromise spectral and spatial
resolution.
  In this work, we present an attosecond light source operating at 200 kHz,
which opens up the exploration of phenomena previously inaccessible to
attosecond interferometric and spectroscopic techniques. Key to our approach is
the combination of a high repetition rate, few-cycle laser source, a specially
designed gas target for efficient high harmonic generation, a passively and
actively stabilized pump-probe interferometer and an advanced 3D
photoelectron/ion momentum detector. While most experiments in the field of
attosecond science so far have been performed with either single attosecond
pulses or long trains of pulses, we explore the hitherto mostly overlooked
intermediate regime with short trains consisting of only a few attosecond
pulses.e also present the first coincidence measurement of single-photon double
ionization of helium with full angular resolution, using an attosecond source.
This opens up for future studies of the dynamic evolution of strongly
correlated electrons.
