  We conduct a theoretical study of the propagation of few-cycle, ultrafast
vortices (UFVs) carrying orbital angular momentum (OAM) in free space. Our
analysis reveals much more complex temporal dynamics than that of few-cycle
fundamental Gaussian-like beams, particularly when approaching the single-cycle
regime and the magnitude of the topological charge $l$ is high. The recently
described lower bound $\sqrt{|l|}$ to the number of oscillations of UFVs with
propagation-invariant temporal shape (isodiffracting UFVs) is found to hold on
average also for UFVs of general type, with variations along the propagation
direction above and below that bound, even vanishing locally. These variations
are determined by the so-called Porras factor or $g_0$-factor characterizing
the dependence of the Rayleigh distance of the spectral constituents with
frequency. With a given available bandwidth, UFVs must widen temporally with
increasing magnitude of the topological charge, and must widen or may shrink
temporally during propagation as a result of the axially varying,
$g_0$-dependent lower bound. Under very restrictive conditions in their
generation, an UFV can be shrunk below the lower bound $\sqrt{|l|}$ at a focus
into a kind of locally compressed state of OAM, but it broadens well-above
$\sqrt{|l|}$ and distorts in a tiny fraction of the depth of focus because of
the dispersions introduced by Gouy's phase and wave front mismatch. These
propagation phenomena have implications and should be taken into account in
experiments and applications of UFVs, such as the generation of high-harmonics
and attosecond pulses with high OAM, or in OAM-based ultrafast communications
systems, as well as in other areas of physics such as acoustics or electron
waves.
