Quasiquadrature interferometer for plasma density radial profile measurements: Achieving ten channel operation

A quadrature interferometer designed to measure radial electron density profiles in a long (3 m) small bore (2.8 cm) plasma tube, and described previously in this journal, has been further developed to have ten simultaneously operating channels. Achieving multichannel operation was essential to the accurate measurement of radial electron density profiles and required the design, fabrication, and implementation of two closely matched optical polished quartz tube sections. The first polished cylinder forms the cross tube scene beam window and is used as a sleeve to join ceramic tube sections that comprise the bulk of the 3-m plasma tube. The second matched cylindrical section is placed in the interferometer reference beam to compensate for the strong negative lens affects of the cylindrical scene beam window. Design criteria for the cylindrical tube sections is discussed, the plasma tube assembly technique is given, and typical ten channel data is presented for a laser heated 3-m-long theta-pinch.