IMAGING OF LASER-PRODUCED PLASMA AT 1.43 KEV USING FRESNEL ZONE-PLATEAND BRAGG-FRESNEL LENS

X-ray imaging of plasmas with a resolution on the order of 1 mu m coul d 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. Compare d to pinholes, they can have an equivalent diameter of about 100 mu m with a resolution of about 1 mu m. The two kinds of devices tested wer e: (1) a transmission phase Fresnel zone lens (PFZL) associated with a multilayer mirror; (2) a reflective Bragg-Fresnel lens (BFL) which co mbines a multilayer mirror and the grating. The PFZL works at normal i ncidence 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 inciden ce and we only use an off-axis part of the BFL in order to avoid the i llumination of the detector by zeroth order diffracted light. We compu ted the spatial response of both devices and found that the aberration s were very low within an object held of 100 mu m for the BFL device a nd 1 mm for the PFZL device. Their theoretical resolution is given by the width of their last zone: below 0.1 mu m or the BFL device and 0.4 mu m for the PFZL device. We also made images of plasma with both dev ices and using back-lighted grids, we obtained a resolution on the ord er of 4-5 mu m. In the discussion, we try to explain the difference be tween the theoretical and experimental results. (C) 1998 American Inst itute of Physics. [S0034-6748(98)01409-9]