Multichannel interferometry using high-order rotating diffraction gratings

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Abstract
The phase shift suffered by a probing far-infrared laser beam on passage through a plasma is a measure of the plasma electron density integrated along the line-of-sight.' Interferometric measurements of the phase shift for beams at a number of viewing angles and impact parameters al- low, in principle, the density profile to be recovered ap- proximately using tomographic techniques.2 Having in mind application of such techniques to the H-l helical-axis stellarator under construction at the Australian National University (ANU), two new types of multichannel density interferometer based on scanning and multibeam rotating diffraction gratings have been recently proposed.3-5 The first type of interferometer makes use of a multi- sectored blazed disc grating. Upon wheel rotation, the beam incident on the circumferential edge is diffracted se- quentially through a range of discrete angles determined by the grating constant of the illuminated sector. Because of this multiplexing in time, many spatial channels can be obtained using a single laser beam and detector. An inter- ferometer based on this principle has been recently success- fully installed on the TORTUS tokamak at the University of Sydney.6 Time resolution for the scanning interferom- eter is, however, necessarily limited by the maximum at- tainable wheel speed. In this article we report the successful construction and operation of a multiorder dual-polarization rotating grating interferometer. The beam incident on the circum- ference in this case is diffracted simultaneously into a range of orders determined by the grating profile design. The &h-order angle of diffraction 8, for a plane electromag- netic wave (A,,= 2r/ko, wo= ck,) incident at angle 8, on an infinite plane grating having groove spacing d= 2?r/K is given by