Faraday Rotation Measurements on the Scylla IV Theta Pinch

Faraday rotation measurements have been combined with Mach-Zehnder interferometer measurements to yield the value of the internal magnetic field Bi in the Scylla IV plasma as a function of time and radius. Since the Faraday rotation is proportional to ∫ Bi(l)ne(l) dl and the Mach-Zehnder interferometer provides ∫ ne(l) dl, the data give the average value of the internal magnetic field Bi(r) which exists over the plasma length l at the radius r. The measurements of Bi(r) are combined with those of the external magnetic field Be to determine the average plasma β as a function of radius, β = 1 − Bi2(r)/Be2. The Faraday rotation measurement employs a 1.5-mm diam beam of visible radiation (6328 Å) from a He-Ne gas laser as a plasma magnetic-field probe and a calcite Wollaston prism as the polarization analyzer with silicon diode detectors. In the low-pressure plasma regime (10-25 mTorr) the β values in the central plasma-core (∼5-mm diameter) range in value between 0.9 and 1.0 with initial reversed bias magnetic fields and between 0.7 and 0.95 without bias fields. Radial profiles of β averaged over the plasma length have been obtained at various times during the discharge. A combination of β, n, and Te in the simple pressure balance yields ion temperatures in the 2- 6-keV region.