Linear systems description of the CO₂ laser‐based tangential imaging system^a)

It is demonstrated that the phase variation produced by the projection of the density fluctuations onto a laser beam that is aligned tangent to the magnetic field lines in a toroidal plasma is, in fact, a convolution of the density fluctuation profile in the tangency plane with a shift‐invariant point spread function. Thus a spatial filter can be used to invert the corresponding transfer function to produce an undistorted image of the plasma density fluctuations at the tangency plane. Numerical simulations demonstrate that a spatial filter consisting of a simple and versatile step‐function form of a Zernike phase mirror will recover a reasonably accurate image of the fluctuations [Lo et al., Buld. APS 38, 2005 (1993)].