  Using plasma mirror injection we demonstrate, both analytically and
numerically, that a circularly polarized helical laser pulse can accelerate
highly collimated dense bunches of electrons to several hundred MeV using
currently available laser systems. The circular-polarized helical
(Laguerre-Gaussian) beam has a unique field structure where the transverse
fields have helix-like wave-fronts which tend to zero on-axis where, at focus,
there are large on-axis longitudinal magnetic and electric fields. The
acceleration of electrons by this type of laser pulse is analysed as a function
of radial mode number and it is shown that the radial mode number has a
profound effect on electron acceleration close to the laser axis.Using
three-dimensional particle-in-cell simulations a circular-polarized helical
laser beam with power of 0.6 PW is shown to produce several dense attosecond
bunches. The bunch nearest the peak of the laser envelope has an energy of 0.47
GeV with spread as narrow as 10\%, a charge of 26 pC with duration of $\sim
400$ as, and a very low divergence of 20 mrad}. The confinement by longitudinal
magnetic fields in the near-axis region allows the longitudinal electric fields
to accelerate the electrons over a long period after the initial reflection.
Both the longitudinal E and B fields are shown to be essential for electron
acceleration in this scheme. This opens up new paths towards attosecond
electron beams, or attosecond radiation, at many laser facilities around the
world.

Plasma Physics;Accelerator Physics