  We demonstrate a silicon-based electron accelerator that uses laser optical
near fields to both accelerate and confine electrons over extended distances.
Two dielectric laser accelerator (DLA) designs were tested, each consisting of
two arrays of silicon pillars pumped symmetrically by pulse front tilted laser
beams, designed for average acceleration gradients 35 and 50 MeV/m
respectively. The DLAs are designed to act as alternating phase focusing (APF)
lattices, where electrons, depending on the electron-laser interaction phase,
will alternate between opposing longitudinal and transverse focusing and
defocusing forces. By incorporating fractional period drift sections that alter
the synchronous phase between $\pm 60^\circ$ off crest, electrons captured in
the designed acceleration bucket experience half the peak gradient as average
gradient while also experiencing strong confinement forces that enable long
interaction lengths. We demonstrate APF accelerators with interaction lengths
up to 708 ${\mu}$m and energy gains up to 23.7 $\pm$ 1.07 keV FWHM, a 25$\%$
increase from starting energy, demonstrating the ability to achieve substantial
energy gains with subrelativistic DLA.
