Using tomographic (“ART”) methods with an interferometer to diagnose asymmetrical gas flow

Sumamry form only given. A variety of plasma applications make use of pulsed gas jets. In some cases like a z-pinch¹, one requirement is for cylindrical symmetry in the mass distribution delivered by a nozzle system. In other cases like astrophysical simulations² or laser wakefield accelerators (LWA)³, controlled asymmetry is desired. Hence there is a need for diagnostic methods to assess the 3D geometry of the cold gas flow from a nozzle. This paper describes the use of an interferometer and tomographic techniques to derive the 3D characteristics of the flow. A key feature of the 2D interferometer is that with even a single firing ("puff") of the nozzle, the areal density as a function of chordal distance (X) and axial distance (Z) can be derived with reasonable accuracy. With rotation of the nozzle about its axis, a series of 2D maps of areal density are acquired. Then an algebraic reconstruction technique⁴ ("ART") is used to extract a full 3D map of the flow. Unlike Abel-inversion methods that assume cylindrical symmetry, ART methods make no such assumption. Also, it is easy to impose the constraint that the derived density is never negative. Most medical applications of tomography have relatively low resolution in each view but a large number of views at different angles taken around the object of interest. We describe a interferometer system with quite high resolution per image (5 megapixel) but with limited sampling in angle about the axis of the gas flow. We show that this system can provide reasonable levels of accuracy for the gas flows of interest.