Target Test Diagnostics Richard Brownsword Engineering, Rutherford Appleton Laboratory
Target Surface Velocities Problem : to measure surface displacement remotely in a time-resolved manner for an arbitrary surface material
Velocity Interferometry (VISAR) : laser interferometric technique time resolution may be ~ 1 ns compensates for changes in surface reflectivity
Velocity Interferometry (VISAR) : Laser Frequency ω Sample Velocity u(t) Fixed mirror Beamsplitter Etalon Length L Refractive index n Detector Fixed mirror
Velocity Interferometry (VISAR) : recombines beams at beamsplitter with delay = (2 L / c)( n - 1/n ) beat frequency (t) = ω [u(t) – u(t - )] / c time and velocity resolution determined by etalon (L, n) optical fibre etalons offer flexibility
Derivation of velocity : VISARs measure acceleration : Δu in time fringe constant K f = o / (ms -1 ) fringe count f c Δu u (t) = u (t - ) + Δu (t)
VISAR Simulations : surface velocity G. Skoro
VISAR Simulations : VISAR signal Detection limit
Background emission Expect source temperature ~ 2000 K VISAR laser = 1.55 m (1) Narrow-band interference filter ~ 500:1 discrimination (2) Background subtraction module
Conclusion Velocity interferometry : has potential for remote measurement of surface displacement for a range of different materials and surface velocities 0.01 ms 10 ns ?