P. Dhez

New UV detectors for solar observations

BOLD (Blind to the Optical Light Detectors) is an international initiative dedicated to the development of novel imaging detectors for UV solar observations. It relies on the properties of wide bandgap materials (in particular diamond and Al-Ga-nitrides). The investigation is proposed in view of the Solar Orbiter (S.O.) UV instruments, for which the expected benefits of the new sensors -primarily visible blindness and radiation hardness- will be highly valuable. Despite various advances in the technology of imaging detectors over the last decades, the present UV imagers based on silicon CCDs or microchannel plates exhibit limitations inherent to their actual material and technology. Yet, the utmost spatial resolution, fast temporal cadence, sensitivity, and photometric accuracy will be decisive for the forthcoming solar space missions. The advent of imagers based on wide-bandgap materials will permit new observations and, by simplifying their design, cheaper instruments. As for the Solar Orbiter, the aspiration for wide-bandgap material (WBGM) based UV detectors is still more sensible because the spacecraft will approach the Sun where the heat and the radiation fluxes are high. We describe the motivations, and present the program to achieve revolutionary flight cameras within the Solar Orbiter schedule as well as relevant UV measurements.

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

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 di...

Numerical and experimental study of grating efficiency for synchrotron monochromators

The development of third-generation synchrotron sources has stimulated efforts toward high-resolution monochromators. A good knowledge of grating efficiency is needed to achieve an optimal compromise between resolution and photon flux. Because simple geometric models fail to describe correctly the gratings properties in the UVtosoft-X-ray range, we have developed a simulation software based on differential theory. A simplified R-matrix propagation algorithm assures the numerical stability of the code for deep gratings. Our numerical results are compared with previous research on deep gratings. Experimental and numerical studies have been performed on some test cases at a synchrotron source. Very good agreement between numerical prediction and measurement has been found.

The LURE grazing incidence spectromonochromator: Two years operating experience

Abstract The grazing incidence spectromonochromator specially designed to use the synchrotron radiation emitted by the ACO storage ring in the soft X-ray region (30–200 eV) has been in operation for more than two years on the four degree beam line of LURE. This is one of the most versatile apparatus, since it can work as a monochromator or as a spectrometer, with one or two gratings, with a mobile or a fixed exit slit. The unique features of this instrument will be discussed, including the possibility of varying continuously, in an extended angular range, the incidence angle on the gratings, as well as the angle between the entrance slit and the exit slit. Actual performance figures will be given, including absolute output photon flux, order sorting and rate of scattered light. A large variety of experiments have already made use of this monochromator in various fields of atomic physics: some examples of new results recently obtained will be given.

Institut d'Astrophysique Spatiale (IAS) 0.1- to 15-keV Synchrotron Radiation Facility beam lines

Two beam lines have been built at the Institute dAstrophysique Spatiale (IAS) dOrsay to perform absolute calibration of the EPIC (European photon imaging camera). EPIC consists of three x-ray charge coupled device (CCD) cameras having imaging and spectroscopic performances set at the Wolter telescope focal planes on board the x-ray multi mirror mission (XMM) planned to be launched by ESA in August 1999. To cover the desired 0.1 - 15 keV range a dedicated beam line has been built on each synchrotron sources of the Laboratoire pour lUtilisation du Rayonnement Synchrotron (LURE): SACO (0.8 GeV) and DCI (1.5 GeV). Both beam lines are merging in a clean 23 m3 vacuum tank containing the camera to calibrate. (1) The SACO windowless beam line is equipped with a grating monochromator. Four plane VLS gratings are used to cover the low energy range (0.1 - 1.2 keV). A triple grazing incidence mirror system set in front of the entrance slit removes the overlapping orders. (2) The high energy beam line on DCI has a 50 micrometer beryllium window and a double flat crystals monochromator equipped with four different crystal pairs. A double grazing incidence mirror system set close to the source absorbs the high energy photon spectra. CCD calibrations will be performed during 1997 second semester and years 1998.

CARPEM: a computer code for predicting the diffraction efficiency of soft x-ray gratings and its application to VGD gratings optimization

We have developed a LURE a code to predict the efficiency of gratings. The code is based on differential theory and uses a simplified R-matrix propagation algorithm to obtain numerical stability on the whole range from visible to hard x-rays. Experimental and numerical studies have been performed on some test cases at a synchrotron source. A good agreement between numerical prediction and measurements has been found. The code is a rigorous application of electromagnetic theory and gives exact results as long as accurate optical constants can be attributed to grating materials. Such rigorous calculations provide an important tool for the optical engineering of modern synchrotron monochromator gratings. We give an example of application of this code to the engineering of a modern beam line and for the optimization of harmonic rejection.

X-ray laser using lithium-like recombination scheme: effects of plasma inhomogeneity

Difficulties involved in maintaining large gain coefficients in plasma columns longer than 1-2 cm are examined with particular reference to the potential use of lithium-like recombination schemes for pumping X-ray lasers with a moderate pumping power. The discussion focuses on the role of plasma nonuniformity along the amplification path due to large-scale and small-scale inhomogeneities of the pumping optical laser. Two methods of smoothing nonuniformities are considered: the use of random phase plates and wall-confined plasmas.