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7/15/2008 Page 1

Operational Experience and Hazards of Photon Doppler Velocimetry as an Alternative to VISAR
David B. Holtkamp Hydrodynamic & X-Ray Physics

This is not going to be a “Physics” talk… At least not much… Mostly gadgets, examples, and stuff…

7/15/2008 Page 2

Acknowledgements
? Ted Strand (LLNL) – it is a tribute to the quality of his advice that our first generation PDV worked the first time we turned it on ? LANL Colleagues – Pat Rodriguez, Benjie Stone, Lenny Tabaka (P-22), Brian Jensen, Paulo Rigg (DE-9), Matt Briggs, Will Hemsing, Mike Shinas (HX-3) ? NSTec Colleagues – Doug DeVore, Araceli Diaz, Adam Iverson, Vince Romero, Mike Rutkowski, Jason Young ? STL (Santa Barbara) – Bruce Marshall

7/15/2008 Page 3

Why do we need velocimetry?
? Shock waves create conditions of pressure and strain that are well beyond the limits of conventional velocity sensors ? Velocities of shocked surfaces move at 1-20 km/s ? In the past we used long pulse (or CW) lasers at visible wavelengths in Fabry Perot or VISAR systems

Courtesy http://www.dtra.mil/newsservices/photo_library/oe/CS/CS-3.cfm

7/15/2008 Page 4

What is PDV?
? Photon Doppler Velocimetry
– Direct detection of difference beat signal between unshifted & Doppler shifted light – First used for fluid flow measurements (1964-65)* – Recent improved methods developed by Ted Strand and co-workers at LLNL (1999-2008)** – Los Alamos systems developed & refined 2004-08
?Y. Yeh and H. Z. Cummins, Appl. Phys. Lett. V. 4, no. 10, p. 176 (1964); & J. W. Foreman, E. W. George, R. D. Lewis, Appl. Phys. Lett. v. 7, no. 4, p. 77 (1965); Wayne Willhite at China Lake (unpublished). ** Oliver T. Strand, Leon V. Berzins, David R. Goosman, William W. Kuhlow, Paul R. Sargis, and Tony L. Whitworth, LLNL Report UCRL-CONF-206034, 26th Int. Conf. on High-speed Photography and Photonics, Alexandria, VA September 19-24, 2004; O.T. Strand, D. R. Goosman, C. Martinez, T. L. Whitworth, and W. W. Kuhlow, “Compact system for high-speed velocimetry using heterodyne techniques,” Rev. Sci. Inst. 77, 083108 (2006).

7/15/2008 Page 5

Features of PDV
? ? ? ? Small, portable systems are fast to set up Class IV lasers, but nearly eye safe (1550 nm) Absolute accuracy demonstrated to ± 0.1% (or better) Demonstrated long tracking distances under less than ideal conditions ? Probe geometry flexible over a wide range of distances and probe geometries
– Single bare fiber per point (< 10 or 20 mm) – Lensed probes (< 100 mm) – Coupled with conventional lens optic (< 30 m (!) or more)

? Velocity range from ~few mm/s to > 14 km/s with current systems; long record lengths (> 1 ms); self triggerable

7/15/2008 Page 6

What is PDV? (Reprise)
? Direct detection of interference between unshifted and Doppler shifted light
– PDV is Ted’s original name; he’s also suggested Heterodyne Velocimeter (“Het-V”) but it wasn’

t as catchy…it’s really a homodyne velocimeter (usually) – Basically - a very fast Michelson Interferometer

Michelson Interferometer

7/15/2008 Page 7

Michelson Interferometer
? Distance can be measured by moving the mirror and counting fringes ? The distance of travel d is associated with m fringes by d = m ?/2

7/15/2008 Page 8

Beat of Shifted & Unshifted Frequencies

? If we know unshifted (f1) frequency, and beat frequency, then we can infer shifted (f2) frequency
7/15/2008 Page 9

“Baby Pictures”
Beat frequency = fb = fd – f0 = 2 (v/c)f0
Illustration from Ted Strand

@ 1550 nm and v = 1 mm/μs f0 = 193414.49 GHz fd = 193415.78 GHz fb = 1.29 GHz V = λ/2 x F V(mm/μs) = 0.775 x F (GHz)

? Single channel shown (2 watt laser can support ~4 pts) – typically 200-500 mW each
P-22 Single Point PDV (Oct 2004)
7/15/2008 Page 10

Krakatau Installation
4 Channel Optic & Detector Boxes

5W Lasers

? 12 point system (10 used) with two 5 W lasers & recording
7/15/2008 Page 11

Unicorn Installation
? 8 channels (2 x 4 point systems) ? 2 lasers (not shown) ? 4 scopes (2 voltage coverages per channel)

7/15/2008 Page 12

“Handheld” PDV
? Miniature PDV
– 12 V @ < 1 A – Ready < 100 ms after power on – Multimode fiber – Up to 5 km/s – Analog downlink for recording on ground – Patent pending

7/15/2008 Page 13

First Data at Santa Barbara (2004)
? 3 shots with bare fiber in tube + bare fiber VISAR alongside ? 2 mm standoff from polished or ball rolled surface
Asay Foil VISAR Xmit & Rcv PDV Xmit & Rcv

VISAR Bare Fiber

PDV Bare Fiber

Foil

7/15/2008 Page 14

Ball Rolled Surface (2004)

? Blue is raw signal, green is high passed digitally ? Note that raw signal is > ± ~100 mV
7/15/2008 Page 15

Raw PDV Signal (2004)
File:1A194.dig Mon Nov 01 13:07:52 2004 FFT:Overlap:1024:512 0.25 0.2 0.15 0.1

Signal (V)

0.05 0 -0.05 -0.1 -0.15 -0.2 3.91 3.911 3.912 3.913 3.914 3.915 3.916 3.917 3.918 3.919 Time (?s) 3.92

? Shock break out to << 1 ns ? Elastic precursor evident ? Can use various methods to estimate velocity “fast” when SNR is good

7/15/2008 Page 16

Bare Fiber + Ball Rolled Surface
~2.6 GHz
Voltage Magnitude for file:1A194.dig Mon Nov 01 13:07:52 2004 FFT:Overlap:1024:512 2000 20 1800 1600 10 1400 5 0 -5 -10 400 200 0 3.5 -15 -20 15

Velocity (m/s)

1200 1000 800 600

4

4.5

5

5.5

6

? ? ?

Time (?s) 1024 running FFT with 50% overlap Note SNR is ~40dB in places (less elsewhere) VISAR also shows impact of debris on bare fibers between 5 and 5.5 μs

7/15/2008 Page 17

SNR @ 1 Time
Velocity Distribution near 4 us 180 160 140 120 100 80 60 40 20 0 800
X: 1203 Y: 173.4

Squared Amplitude of Complex Spectrogram

? Picked a point near 4 μs ? SNR ? 104:1 in frequency/ velocity domain

X: 1332 Y: 0.004729

900

1000

1100 1200 1300 Velocity (m/s)

1400

1500

1600

7/15/2008 Page 18

Oz & Lightpath Probes

Oz Optics Probes

? Trying to standardize on bare fiber

(& Lightpath) probe tube diameters, length, etc for multiple users
7/15/2008 Page 19

Summary
? PDV is being used at LLNL, LANL, SNL, Universities, commercial (?) ? Likely to replace VISAR in most applications ? Intrinsically safer than high power visible laser velocimetry approaches (high MPE) ? However:
– Hazardous conditions exist at probes (when exposed) – Distance between laser systems and probes (and high power fiber connections) require interlocks to prevent accidental exposure

7/15/2008 Page 20

PDV is the up and coming method for velocimetry because it really works great

If you do dynamic experiments at your lab – expect to see one soon!
7/15/2008 Page 21


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