  We present new observations with the Atacama Large Millimeter/sub-millimeter
Array of the 122um and 205um fine-structure line emission of singly-ionised
nitrogen in a strongly lensed starburst galaxy at z=2.6. The 122/205um [NII]
line ratio is sensitive to electron density, n_e, in the ionised interstellar
medium, and we use this to measure n_e~300cm^-3 averaged across the galaxy.
This is over an order of magnitude higher than the Milky Way average, but
comparable to localised Galactic star-forming regions. Combined with
observations of the atomic carbon (CI(1-0)) and carbon monoxide (CO(4-3)) in
the same system, we reveal the conditions in this intensely star-forming
system. The majority of the molecular interstellar medium has been driven to
high density, and the resultant conflagration of star formation produces a
correspondingly dense ionised phase, presumably co-located with myriad HII
regions that litter the gas-rich disk.
