Water, pure water can be supercooled under pressure without freezing it relaxes the hydrogen bond
http://www.lsbu.ac.uk/water/explan5.html#diff
run pure water to and injector with as much pressure as possible supercool it at the injector release into a plasma stream maybe intense uv first then into the plasma
Some diesel injectors have psi of 29000 close to 200 mpa, hot water freezes faster than cold water, not that you want it to freeze but I wonder if it helps with supercooling
A high pressure injector and some dry ice would give proof of concept but injectors are quit expensive 200 bucks refurbished
The water has to be pure distilled and filtered just a grain of dust could cause freezing and that would be disastrous to the injector.