1. Target Test Diagnostics Richard Brownsword Engineering, Rutherford Appleton Laboratory

2. Target Surface Velocities Problem : to measure surface displacement remotely in a time-resolved manner for an arbitrary surface material

3. Velocity Interferometry (VISAR) : laser interferometric technique time resolution may be ~ 1 ns compensates for changes in surface reflectivity

4. Velocity Interferometry (VISAR) : Laser Frequency ω Sample Velocity u(t) Fixed mirror Beamsplitter Etalon Length L Refractive index n Detector Fixed mirror

5. 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

6. 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)

7. VISAR Simulations : surface velocity G. Skoro

8. VISAR Simulations : VISAR signal Detection limit

9. Background emission Expect source temperature ~ 2000 K VISAR laser = 1.55  m (1) Narrow-band interference filter  ~ 500:1 discrimination (2) Background subtraction module

10. Conclusion Velocity interferometry : has potential for remote measurement of surface displacement for a range of different materials and surface velocities 0.01 ms -1 @ 10 ns ?