MSimon:
How hot is a bullet that has a temperature of 4K if it is traveling at 2,200 m/sec? And for simplicity let us assume a Newtonian Universe so we can avoid frame of reference arguments.
Actually you do need to consider frame of reference arguments even for Newtonian mechanics, the seat of modern physics is Galilean transformations of Cartesian reference frame. For instance, I could be travelling at 2,200 m/s parallel to the bullet and measure a different energy of the bullet than the someone stood still (i.e. the guy who measured 2,200).
The question is does he measure the 4K temperature of the bullet in the ref. frame stood still or the frame moving with the bullet?
Of course, the root problem with this whole confusion is the ease with which some people switch between energy and temperature. They slip into conflating the definitions of bulk properties, e.g. temperature and pressure with definitions of individual particle properties, e.g. energies and momentums.
The Maxwell-Boltzmann distribution does relate bulk to individual properties to some extent, using a statistical distribution, but it is not that simple. Using temperatures for individual particles is nonsensical generally and therefore using E~kT is a fraught minefield for the uninitiated. In the case of the Polywell (and other IEC concepts), it is safer to stick to talking about "fusion energies" of particles rather than "fusion temperatures", which are more applicable in thermalised plasma situations where there are easily predictable relationships between bulk temperature properties and particle energy distributions.
Unless someone would like to rigorously define temperature for a point particle?