  The landscape of ultrafast structured light pulses has recently evolved
driven by the capability of high-order harmonic generation (HHG) to up-convert
orbital angular momentum (OAM) from the infrared to the extreme-ultraviolet
(EUV) spectral regime. Accordingly, HHG has been proven to produce EUV vortex
pulses at the femtosecond timescale. Here we demonstrate the generation of
attosecond vortex pulse trains, i.e. a succession of attosecond pulses with a
helical wavefront, resulting from the synthesis of a comb of EUV high-order
harmonics with the same OAM. By driving HHG with a polarization tilt-angle fork
grating, two spatially separated circularly polarized high-order harmonic beams
with order-independent OAM are created. Our work opens the route towards
attosecond-resolved OAM light-matter interactions.
