Abstract
X-ray imaging of plasmas with a resolution on the order of 1 μm could not be achieved with pinholes because the light flux on the detector would be too low. We tested two different types of diffractive lenses derived from the circular grating based on the Fresnel zones. Compared to pinholes, they can have an equivalent diameter of about 100 μm with a resolution of about 1 μm. The two kinds of devices tested were: (1) a transmission phase Fresnel zone lens (PFZL) associated with a multilayer mirror; (2) a reflective Bragg–Fresnel lens (BFL) which combines a multilayer mirror and the grating. The PFZL works at normal incidence by transmission; an additional mirror is used to reflect only a small bandwidth within the spectrum; the angle of reflection of the multilayer of the imaging beam on the mirror is set as to adjust the center of the useful bandwidth. The BFL works at fixed grazing incidence and we only use an off-axis part of the BFL in order to avoid the illumination of the detector by zeroth order diffracted light. We computed the spatial response of both devices and found that the aberrations were very low within an object field of 100 μm for the BFL device and 1 mm for the PFZL device. Their theoretical resolution is given by the width of their last zone: below 0.1 μm for the BFL device and 0.4 μm for the PFZL device. We also made images of plasma with both devices and using back-lighted grids, we obtained a resolution on the order of 4–5 μm. In the discussion, we try to explain the difference between the theoretical and experimental results.
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