  The onset and development of electron-positron cascade in a standing wave
formed by multiple colliding laser pulses requires tight focusing in order to
achieve the maximum laser intensity. There, steep spatio-temporal gradients in
the laser intensity expel seed particles from the high-intensity region and
thus can prevent the onset of a cascade. We show that radially polarized laser
pulses ensure that the seed electrons are present at the focal plane at the
moment of the highest amplitude even in the case of extreme focusing. This
feature reduces the required laser power for the onset of a cascade 100 times
(80 times) compared to circularly (linearly) polarized laser pulses having the
same focal spot radius and duration.
