  Aims: We resolve the length-scales for filament formation and fragmentation
(res. <=0.1pc), in particular the Jeans length and cylinder fragmentation
scale.
  Methods: We observed the prototypical high-mass star-forming filament
IRDC18223 with the Plateau de Bure Interferometer (PdBI) in the 3.2mm continuum
and N2H+(1-0) line emission in a ten field mosaic at a spatial resolution of
~4'' (~14000AU).
  Results: The dust continuum emission resolves the filament into a chain of at
least 12 relatively regularly spaced cores. The mean separation between cores
is ~0.40(+-0.18)pc. While this is approximately consistent with the
fragmentation of an infinite, isothermal, gravitationally bound gas cylinder, a
high mass-to-length ratio of M/l~1000M_sun/pc requires additional turbulent
and/or magnetic support against radial collapse of the filament. The N2H+(1-0)
data reveal a velocity gradient perpendicular to the main filament. Although
rotation of the filament cannot be excluded, the data are also consistent with
the main filament being comprised of several velocity-coherent sub-filaments.
Furthermore, this velocity gradient perpendicular to the filament resembles
recent results toward Serpens south that are interpreted as signatures of
filament formation within magnetized and turbulent sheet-like structures.
Lower-density gas tracers ([CI] and C18O) reveal a similar red/blueshifted
velocity structure on scales around 60'' east and west of the IRDC18223
filament. This may tentatively be interpreted as a signature of the large-scale
cloud and the smaller-scale filament being kinematically coupled. We do not
identify a velocity gradient along the axis of the filament. This may either be
due to no significant gas flows along the filamentary axis, but it may partly
also be caused by a low inclination angle of the filament with respect to the
plane of the sky that could minimize such signature.
