At the temperatures involved with plasma cutting of 10,000 degrees C or greater, the liquid nitrogen would become gaseous nitrogen very quickly.
From
http://www.mwsco.com/kb/kb_frameset.asp?ArticleID=64 which is a pretty good explanantion of the plasma cutting process: "The different gases used for plasma arc cutting include nitrogen, argon, air, oxygen, and mixtures of nitrogen/hydrogen and argon/hydrogen." So yes, you can use straight gaseous nitrogen for plasma cutting, although most of the "garden variety" units I have used over the years have used plain compressed air (which is still mostly nitrogen). The other gasses tend to be used for specialized applications.
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