Session W3: NT.2 Novel Techniques: Microwave and Fiber Bragg Gratings

Microwave Diagnostics of Shock Wave and Detonation Processes

The physical bases of laser and microwave Doppler interferometry are the same -- measurements of the Doppler shift of probing electromagnetic frequency, reflected from a moving surface. However, using probing wavelength 4 orders of magnitude longer, microwave diagnostics has some specific advantages as compared with laser diagnostics, namely: measurements inside the microwave-transparent media, which spectrum is much more wide than the spectrum of optically transparent media; for microwave measurements the reflecting surfaces of media, but all jumps of medium parameters -- density, dielectric permittivity, conductivity; for microwave technique due to its wavelength all practically important hydrodynamical jumps are smooth. The results of application of the microwave technique were presented in the paper, which demonstrate capabilities of diagnostics of various dynamic processes using single equipment, namely: liners and massive objects launching; shock-to-detonation transition in HE; propagation of steady detonation waves; laminar HE combustion etc. In all conducted investigations the using of the microwave technique gives a big amount of interesting experimental information which is inaccessible for the other traditional experimental techniques.   

Dielectric characterization and microwave interferometry of HMX-based explosives

Microwave interferometry is a useful technique for understanding the development and propagation of detonation waves. The velocity of the front can be determined directly with the instantaneous phase difference of the reflected microwave signal from the detonation front and the dielectric constant of the explosive. However, the dielectric constant of HMX-based explosives has been measured only over a small range of wavelengths. Here we employ an open-ended coaxial probe to determine the complex dielectric constant for LX-10 and other HMX-based explosives over the full 5-50 GHz range. The development and propagation of detonation waves in both heavily- and lightly-confined cylindrical charge geometries will also be highlighted. In some experiments the microwave reflective properties of the region behind the detonation front are characterized by using a remotely-positioned microwave waveguide probe. Ionization pins and Manganin gauges were used with microwaves simultaneously to verify the technique as the detonation front progresses.   

Rubidium Atomic Line Filtered (RALF) Doppler Velocimetry

We report our progress towards adapting the well-known Global Dopper Velocimetry (GDV) technique, popular in the aerodynamics community, to the order-of-magnitude higher velocities pertinent to shock experiments. In GDV, the narrow-line illumination laser is tuned to an edge of a molecular iodine absorption line; an iodine gas cell converts the Doppler shift of the reflected light to transmitted intensity. We follow the suggestion in the original 1990 patent by Komine and broaden the absorption lines of alkali metal atoms by adding a buffer gas, thereby tuning the transmission edge spectrum to match the Doppler shift (surface velocity) range of interest. We use atomic rubidium vapor cells, with 0 to 1 atmosphere pressures of molecular nitrogen buffer gas, and coin the name ``Rubidium Atomic Line Filtered'' (RALF) Doppler velocimetry. [96ABW-2013-0036]   

Fiber Bragg Grating sensor for shock wave diagnostics

Fiber Bragg Grating (FBG) sensor response was studied in gas-gun shock wave experiments. The sensors were embedded in PMMA target subjected to planar shock waves under 1GPa. Two orientations of the FBG sensor with respect to the shock plane were examined: parallel and perpendicular. The shift of the reflected wavelength was measured with a system based on commonly available communication grade add-drop filters that covered the maximal expected wavelength swing. The FBG sensors survived the shock and their strain-to-wavelength response was determined by comparison to the calculated strain based on the known PMMA EOS and VISAR measurements.   

Fiber Grating Sensor System to Measure Velocity, Position, Pressure, and Temperature during Burn, Deflagration and Detonation of Highly Energetic Events

A novel very high speed fiber grating sensor system has been used to support velocity, position, temperature and pressure measurements during burn, deflagration and detonation of energetic materials including explosives and rocket propellant in Russian DDT tests. For the first time the system has been demonstrated in card gap testing and has allowed real time measurements of the position of the blast front into the card gap and monitoring of pressure at key locations in the card gap test. Fiber grating sensors are capable of providing a continuous measurement of the position, velocity, local pressure and temperature of energetic materials during the early stages of detonation and the transition to full detonation represents a significant advance in diagnostic capabilities. These measurements provide insight into this dynamic regime detonation physics. Continuous velocity and burn back position measurements are significantly more accurate in determining this run-up in velocity relative to single point measurements which yield only the average velocity measurement between the individual pin placement points. This work describes the first demonstration of this technology to card gap testing.   

Embedded optical probes for simultaneous pressure and temperature measurement of materials in extreme conditions

We present a new technique for simultaneous, in situ pressure and temperature measurements under dynamic conditions by using an all-optical fiber-based approach. While similar tests have been done previously in deflagration-to-detonation tests (DDT), where pressure and temperature were measured to 82 kbar and 400$^{\circ}$C simultaneously, here we demonstrate the use of embedded fiber grating sensors to obtain high temporal resolution in situ pressure measurements in inert materials under precise shock loading from a gas-gun driven plate impact. The system capitalizes on existing telecom components and fast transient digitizing recording technology. It operates as a relatively inexpensive embedded probe (single-mode 1550 nm fiber-based Bragg grating - FBG) that provides a continuous fast pressure record during shock and/or detonation. Fiber Bragg grating sensors have predictable thermal and mechanical response properties with pressure spectrally shifting the reflectance peak at $\lambda =$1550 nm to the blue and temperature shifting the peak to the red. By applying well-controlled steady shock wave pressure profiles to soft materials such as PMMA, we study the dynamic pressure response of embedded fiber Bragg gratings to extract pressure amplitude of the shock wave and compare our results with in situ particle velocity wave profiles measured simultaneously.   

Fiber Bragg Sensing of High Explosive Detonation Experiments at Los Alamos National Laboratory

An all optical-fiber-based approach to measuring high explosive detonation front position and velocity is described. By measuring total light return using an incoherent light source reflected from a fiber Bragg grating sensor in contact with the explosive, dynamic mapping of the detonation front position and velocity versus time is obtained. We demonstrate three calibration procedures and provide several examples of detonation front measurements: PBX 9502 cylindrical rate stick, radial detonation front in PBX 9501, PBX 9501 detonation along curved meridian line, and detonation along a multi-HE cylindrical rate stick containing sections of PBX 9501, Comp B, TNT, PBX 9407, PBX 9520, and inert PMMA. In the PBX 9501 cylindrical rate stick measurement, excellent agreement with complementary diagnostics (electrical pins and streak camera imaging) is achieved, demonstrating accuracy in the detonation front velocity to below the 0.3{\%} level when compared to the results from the pin data. Operating with components based on telecommunications technologies at 1550 nm, we believe this approach offers an attractive, safe, and affordable alternative for time continuous recording of HE detonation front sensing when compared existing methods.