The short ionizing photon mean free path at z=6 in Cosmic Dawn III, a
  new fully-coupled radiation-hydrodynamical simulation of the Epoch of
  Reionization
  Recent determinations of the mean free path of ionising photons (mfp) in the
intergalactic medium (IGM) at $\rm z=6$ are lower than many theoretical
predictions. In order to gain insight into this issue, we investigate the
evolution of the mfp in our new massive fully coupled radiation hydrodynamics
cosmological simulation of reionization: Cosmic Dawn III (CoDaIII). CoDaIII's
scale ($\rm 94^3 \, cMpc^3$) and resolution ($\rm 8192^3$ grid) make it
particularly suitable to study the evolution of the IGM during the Epoch of
Reionization (EoR). The simulation was performed with RAMSES-CUDATON on Summit,
and used 131072 processors coupled to 24576 GPUs, making it the largest EoR
simulation, and largest RAMSES simulation ever performed. A superior agreement
with global constraints on reionization is obtained in CoDaIII over CoDaII
especially for the evolution of the neutral hydrogen fraction and the cosmic
photo-ionization rate, thanks to an improved calibration, later end of
reionization ($\rm z=5.6$), and higher spatial resolution. Analyzing the mfp,
we find that CoDaIII reproduces the most recent observations very well, from
$\rm z=6$ to $\rm z=4.6$. We show that the distribution of the mfp in CoDaIII
is bimodal, with short (neutral) and long (ionized) mfp modes, respectively,
due to the patchiness of reionization and the co-existence of neutral versus
ionized regions during the EoR. The neutral mode peaks at sub-kpc to kpc scales
of mfp, while the ionized mode peak evolves from $\rm 0.1 Mpc/h$ at $\rm z=7$
to $\sim 10$ Mpc/h at $\rm z=5.2$. Computing the mfp as the average of the
ionized mode provides the best match to the recent observational
determinations. The distribution reduces to a single neutral (ionized) mode at
$\rm z>13$ ($\rm z<5$).
