Experimental study of the post‐implosion phase of a theta pinch

Using 90° Thomson scattering and magnetic probes, the time and space resolved electron temperature and density and magnetic field profiles were obtained for a small, 15 kJ theta pinch. At times after the implosion phase, t≳0.45–0.5 μsec, the plasma is characterized by a high β, 0.5≲β≲1.0, Te/Ti<1, and vEϑ≲ vi (where vEϑ is the E×B drift speed and vi is the ion thermal speed). The current sheath width, ΔLs, and plasma resistivity in the sheath ηeff, were obtained from the measured temperature, density, and field profiles at these late times. The results show that ΔLs≲ 3c/ωpi≫c/ωpe, ηeff≈100 η⊥, and the effective collision frequency, ν′eff, ω1h≲ν′eff≪ωpi, where η⊥ is the classical, Spitzer–Harm resistivity, and ω1h is the lower hybrid frequency. These results indicate that plasma turbulence is an important factor when considering heating and transport, even in the low drift velocity regime. Theoretical estimates based on the lower‐hybrid‐drift instability are in reasonably good agreement with the measured r...