  Rearranging the six-dimensional phase space of particles in plasma can
release energy. The rearrangement may happen through the application of
electric and magnetic fields, subject to various constraints. The maximum
energy that can be released through a rearrangement of a distribution of
particles can be called its available or free energy. Rearrangement subject to
phase space volume conservation leads to the classic Gardner free energy. Less
free energy is available when constraints are applied, such as respecting
conserved quantities. Also, less energy is available if particles can only be
diffused in phase-space from regions of high phase-space density to regions of
lower phase-space density. The least amount of free energy is available if
particles can only be diffused in phase space, while conserved quantities still
need to be respected.
