Telescopic refractive index gradient diagnostic of an ion diode anode plasma

Abstract

We present a general analysis of possible variants of refractive index gradient (RING) diagnostics with a laser beam probe. Using a differential bicell photodiode as a detector, the sensitivity, dynamic range, and geometric restrictions of RING deflectometry have been found for lensless, one lens, and “three-telescope” optical schemes. The three-telescope method is found to be the most flexible and easily aligned. If the refracted/deflected laser beam cross section in the back focal plane of the output lens is also recorded using a fast framing camera, measurements of the beam deflection and its spatial frequency spectrum after passing through the refractive medium can be obtained simultaneously. A general relation is presented between the Fourier transform of a Gaussian beam by the output lens and the spatial frequency spectrum of the inhomogeneities. From these considerations, we present the specific design of a RING diagnostic for study of anode plasma evolution in a magnetically insulated ion diode on the Cornell Beam Research Accelerator (COBRA) (800 kV, 80 ns pulse). The maximum density gradient was found to be located about 0.4 mm from the anode surface at the peak of the diode voltage pulse. The electron density at this position was about 2×1015 cm−3. The transverse spatial frequency distribution of the probe laser beam after passing through the anode plasma was recorded using the optical Fourier transform technique. This experiment demonstrates that the combined integrated deflection. (RING) and Fourier transform optical techniques can give a great deal of information about this thin (∼1 mm) anode plasma layer.