Ultrasonic vibration modal analysis of ICF targets using a photorefractive optical lock-in

Abstract

A photorefractive optical lock-in is discussed in relation to ultrasonic vibration modal analysis of inertial confinement fusion (ICF) targets. In this preliminary report, the method is used to analyze specimens with similiar response characteristics to ICF targets with emphasis on both the displacement and frequency resolution ofthe technique. The experimental method, based on photorefractive frequency domain processing, utilizes a synchronous detection approach to measure phase variations in light scattered from optically rough, continuously vibrating surfaces with very high, linear sensitivity. In this photorefractive four-wave mixing technique, a small, point image ofthe object surface (containing frequency modulation due to vibration) is made to interfere with a uniform, frequency modulated reference beam inside a Bismith Silicon Oxide crystal. Optical interference and the photorefractive effect of electronic charge redistribution leads to the formation of a refractive index grating in the medium that responds to the modulated beams at a frequency equal to the difference between the signal and reference frequencies. By retro-reflecting the reference beam back into the crystal, a diffracted beam, counter-propagating with respect to the original transmitted beam, is generated. Using a beamsplitter, the counter-propagating beam can be picked-off and deflected toward a photodetector. The intensity ofthis diffracted beam is shown to be a function of the firstorder ordinary Bessel function, and therefore linearly dependent on the vibration displacement induced phase modulation depth for small S (8 < 4irc/2 << 1) where is the vibration displacement and % is the source wavelength; analytical description and experimental verification of this linear response are given. The technique is applied to determine the modal characteristics of a rigidly clamped disc (sand-blasted stainless steel) from 10 kHz to 100 kFIz, a frequency range similar to that used to characterize ICF targets. The results demonstrate the unique capabilities of the photorefractive optical lock-in to detect and to measure vibration signals with very narrow bandwidth (< I Hz) and high displacement sensitivity (-2 picometer at a signal-to-noise ratio of 1). This level ofdisplacement sensitivity is particularly important in detecting changes in vibrational mode shapes and frequencies that might be associated with asymmetries in ICF targets.Keywords: vibration modal analysis, inertial confinement fusion, resonant ultrasonic spectroscopy, photorefractive lock-in INTRODUCTIONMany optical techniques for vibration modal analysis are based on time domain processing using homodyne or heterodyne interferometry. Vibration displacement amplitudes are recorded through interference at the photodetector and subsequent signal processing. Wide bandwidth is typically employed to obtain real-time surface motion under transient conditions. Some applications, such as resonant ultrasound spectroscopy and structural analysis, are better served by measurements in the frequenc...

Solid deuterium–tritium surface roughness in a beryllium inertial confinement fusion shell

“…Beryllium shells are preferred over the plastic shells for NIF ignition experiments because their higher density results in lower Rayleigh-Taylor hydrodynamic instability growth. However, characterization of solid D-T layers inside the optically opaque beryllium shell has proven difficult [12][13][14]. It was recently suggested [15] and demonstrated [16,17] that solid D-T could be characterized using phase-contrast enhanced x-ray imaging.…”