  Laser and beam driven wakefields promise orders of magnitude increases in
electric field gradients for particle accelerators for future applications. Key
areas to explore include the emittance properties of the generated beams and
overcoming the dephasing limit in the plasma. In this paper, the first in-depth
study of the self-injection mechanism into wakefield structures from
non-homogeneous cluster plasmas is provided using high-resolution two
dimensional particle-in-cell simulations. The clusters which are typical
structures caused by ejection of gases from a high-pressure gas jet have a
diameter much smaller than the laser wavelength. Conclusive evidence is
provided for the underlying mechanism that leads to particle trapping,
comparing uniform and cluster plasma cases. The accelerated electron beam
properties are found to be tunable by changing the cluster parameters. The
mechanism explains enhanced beam charge paired with large transverse momentum
and energy which has implications for the betatron x-ray flux. Finally, the
impact of clusters on the high-power laser propagation behavior is discussed.
