Certain recent semi-classical theories of spin-half quantum plasmas are
examined with regard to their internal consistency, physical applicability and
relevance to fusion, astrophysical and condensed matter plasmas. It is shown
that the derivations and some of the results obtained in these theories are
internally inconsistent and contradict well-established principles of quantum
and statistical mechanics, especially in their treatment of fermions and spin.
Claims of large semi-classical effects of spin magnetic moments that could
dominate the plasma dynamics are found to be invalid both for single-particles
and collectively. Larmor moments dominate at high temperature while spin
moments cancel due to Pauli blocking at low temperatures. Explicit numerical
estimates from a variety of plasmas are provided to demonstrate that spin
effects are indeed much smaller than many neglected classical effects. The
analysis presented suggests that the aforementioned `Spin Quantum Hydrodynamic'
theories are not relevant to conventional laboratory or astrophysical plasmas.Comment: 11 pages, To appear in Contributions to Plasma Physics. Minor
correction on page 3 to electron spin magnetic momen
