Telescopic refractive index gradient diagnostic of an ion diode anode plasma

We present a general analysis of possible variants of refractive index grad ient (RING) diagnostics with a laser beam probe. Using a differential bicel l photodiode as a detector, the sensitivity, dynamic range, and geometric r estrictions of RING deflectometry have been found for lensless, one lens, a nd "three-telescope" optical schemes. The three-telescope method is found t o 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 reco rded using a fast framing camera, measurements of the beam deflection and i ts spatial frequency spectrum after passing through the refractive medium c an be obtained simultaneously. A general relation is presented between the Fourier transform of a Gaussian beam by the output lens and the spatial fre quency spectrum of the inhomogeneities. From these considerations, we prese nt the specific design of a RING diagnostic for study of anode plasma evolu tion in a magnetically insulated ion diode on the Cornell Beam Research Acc elerator (COBRA) (800 kV, 80 ns pulse). The maximum density gradient was fo und to be located about 0.4 mm from the anode surface at the peak of the di ode voltage pulse. The electron density at this position was about 2x10(15) cm(-3). The transverse spatial frequency distribution of the probe laser b eam after passing through the anode plasma was recorded using the optical F ourier 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 (similar to1 mm) anode pla sma layer. (C) 2001 American Institute of Physics.