Energy and Particle Loss from a Short Theta Pinch

The axial and temporal variation of the electron density and temperature have been measured in a 0.5-m-long theta pinch with a 14.5-μsec half-cycle duration by means of laser light scattering. The plasma diameter and diamagnetic flux signal were also measured. A fairly flat peak of the electron temperature of ∼ 160 eV is found from 2 to 6 μsec inside the mirror coil. The density is more sharply peaked, both in axial and temporal variation, with a maximum of 3.2 × 1016electrons/cm3 at the centerplane from 3 to 4 μsec. The predominant energy loss is the energy transport associated with the particle loss (e-folding time of 1.5 μsec) in this relatively short theta pinch, but electron heat conductivity does result in an energy loss of 50-150 eV/μsec per electron. Ion-electron heat transfer from the hotter ions to the cooler electrons nearly balances the electron heat flow. From the electron temperature and density profiles and using the electron momentum balance equation, the plasma potential was derived and found to be approximately 450 V at the centerplane from 2 to 6 μsec. A magnetohydrodynamic model computation for this experiment has been made which shows that the parallel electron heat conductivity as given by Spitzer and Landshoff is correct to no worse than a factor of 10, and possibly a factor of 3. The computations also show that the amount of magnetic flux trapped in the plasma cannot be constant but either decreases with time or is nonuniformly imbedded in the plasma at the end of the initial implosion.