A digital seismic measuring chain is an electromechanical system able to record the lowest natural ground motions
observable on Earth but also to measure signals from largest earthquakes. Its cornerstones are an inertial seismometer
and a digitizer. As equipments available on the market don't answer to all seismological applications CEA/DAM/DIF
(Commissariat a l'Energie Atomique/Direction des Applications Militaires/ Ile De France) is interested in, it has
developed the adequate digital seismic measuring chains. Today, the technologies used have reached their maturity. New
sensing techniques need to be developed.
Optical sensors are now widely used in vibrometry and motion measurements. Such devices generally use interferometry
and frange interpolation to achieve high resolution with a large dynamic range. We have developed macroscopic
prototype digital motion transducer from a Michelson interferometer in order to evaluate the potential of this technology
for seismological applications.
First, tests were carried out to validate the operation of these transducers and to estimate their main characteristics for
seismological applications. We focused on transducer motion range, intrinsic noise and temperature sensitivity.
We have evaluated and compared performances of such interferometers in visible (632 nm) and in infrared (1550 nm)
range. Then, we have experimentally evaluated noise sources in displacement measurement. Last, we designed
Michelson interferometer with integrated optics component with a company which is specialized in integrated optics.
Expected results are presented in this document.
A digital seismic measuring chain is an electromechanical system able to record the lowest natural ground motions
observable on Earth but also to measure signals from largest earthquakes. Its cornerstones are an inertial seismometer
and a digitizer. As equipments available on the market don't answer to all seismological applications CEA/DASE
(Commissariat a l'Energie Atomique/Departement analyse, surveillance, environnement) is interested in, it has
developed the adequate digital seismic measuring chains. Today, the technologies used have reached their maturity. New
sensing techniques need to be developed.
Optical sensors are now widely used in vibrometry and displacement measurements. Such devices generally use
interferometry to achieve subnanometric resolution with a large dynamic range. We have developed a prototype digital
motion transducer from a Michelson interferometer in order to evaluate the potential of this technology for seismological
applications.
Tests were carried out to validate the operation of this transducer and to estimate its main characteristics for
seismological applications. We focused on transducer motion range and intrinsic noise. Results are promising. Prototype
intrinsic noise reaches levels as small as 100 fm/√Hz around 8 Hz and is better than that of present transducers all over the bandwidth of interest, motion range also. Interesting seismological applications can be considered leading to more accurate seismic measuring chains, easier to manufacture, deploy and operate.
CEA is interested in radiative transfer induced by hypersonic flow for the study of extern layer ablation phenomena. Material ablation during atmospheric re-entry generally depends on object shape, incidence angle, speed and atmospheric composition at actual altitude. To improve the knowledge of these aero-thermo-chemical phenomena, we use specific instrument inside representative flying object. For this, we must observe the emission of certain chemical components in the shock layer through spectrometric measurements in the visible range. In this paper, we present a characterisation of two commercial spectrometers of Ocean Optics and Avantes trademark. Some tests we made in vibration and acceleration were representative of the different phase of the life cycle of these instruments in a ballistic flight environment. Then, temperature and saturation effects are observed according to the photoelectrical performances of the spectrometer and a transfer function model is proposed. At last, spectrometers have shown they could withstand these harsh environments.
Advances in understanding laser-matter interaction physical processes require extending instrumentation observations limits. In the case of high-speed photography by means of streak analyzers, the main contribution in improving instruments comes from high-speed HV electronics and new tube architectures. However, the cost of these technologies is still a limiting factor. The alternative to instrumentation renewal is based on using conventional technology (Centrally symmetric electron lens streak tube) associated to a convenient image processing. This option, relying on prior metrological characterization of the degradations generated by streak cameras, can be performed thanks to the affordable powerful computing capabilities and the widespread use of CCD readout. This paper describes a simple method for measuring static Point Spread Function (PSF) on streak tube. Experimental results are discussed and then introduced for image restoration purpose. A discussion of a deconvolution method based on Singular Value Decomposition (SVD) for a shift variant blur is briefly sketched. Application of this method shows an actual increase of streak images spatiotemporal resolution.
The CEA is developing a high energetic laser Facility for large scale investigations on plasma physics. This facility named Laser Megajoule (LMJ) will offer 240 laser beams and 1.8 MJ pulse energy is 3 ns. These laser beams will impact a target at the center of a large spherical chamber, for experiments by means of fusion by inertial confinement. We describe here a prototype of insertable Streak camera for hard X-ray experiments specifically designed to operate in vacuum (< 5 10-6 mbar) near the target chamber. The Streak tube is based on an open and modular structure of cylindrical electrodes with a large photocathode (approximately 20 mm). Time window ranges from 1 ns up to 25 ns with triggering jitter less than 10 ps rms. The readout is made by a specific high dynamic cooled CCD camera. Cooling system (Peltier stage combined with water flow) allows a -10°C stabilized temperature on chip. The CCD camera is an ATMEL product comprising a (2048 x 2048) matrix of 14 μm square pixels. Digitizing is performed by a 16 bits conversion chain with 6 electrons readout noise. First results obtain on the Streak tube electronics and CCD camera are presented (MTF, CCD, S/N, flat field, temporal jitter and linearity or temporal resolution...). Time bandwidth estimated for this camera for the 1 ns base time will be of 45 GHz at -3 dB.
The first part of the work descried here deals with the metrology of image reducer systems like fiber optic tapers and relay lenses. These components are very useful to adapt the image on the phosphor screen of a streak camera on a limited size CCD array. We measured the collimated and lambertian transmissions, geometric distortions and the Modulation Transfer Function (MTF) of two SCHOTT tapers and of a specific VANNIER relay lens. The second part of this paper is related to a space-time distortions measurement of a nanosecond streak camera with an appropriate CCD readout system (THOMSON TSN506N). A correction method based on two 2D polynomials calculation was also tested. If some experimental conditions are verified, this method could be very efficient and useful when a streak camera is used as a multichannel chronometer (with a fiber optic adapter).
One of the most important objectives in nuclear tests is the precise and accurate measurement of the physical phenomena involved. Very fast and wide dynamic range imaging is among the multiple and other diagnostics used. A major development in scientific imaging is the increasing capabilities of diagnostic systems for gamma ray, X ray, visible and neutron spectra. However, the different sub-assemblies making up an imaging chain introduce a number of attenuations and distortions in the acquired image. The main objective of the work described here was the accurate measurement of these perturbations to correct the raw recorded image. For this, a test bench was set up and calibration methods were used to precisely adjust the spatial position of the imaging detector. To provide automation, all the instruments were controlled by a microcomputer. The quantities measured were: spatial uniformity, amplitude transfer function (linearity), background noise and noise on signal, geometric distortions, spectral response, 2D time aperture and 2D Modulation Transfer Function.
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