AS part of their responsibilities
for stewardship of the nation’s nuclear stockpile, Livermore
researchers study the behavior of materials detonated with high
explosives or struck with projectiles at extreme velocities. In
diagnosing these experiments, researchers must measure velocities
as great as 3,000 meters per second over distances from less than
0.5 millimeter to more than 50 millimeters.
The ability to measure continually changing velocities is important.
Within a few microseconds, shock waves cause objects, especially
metal samples shocked by high explosives, to accelerate, decelerate,
and then accelerate again. Precise velocity data gathered from
experiments are used to refine the computer codes that model weapons
physics. These data are especially important for improving the
hydrodynamics codes used to simulate materials under the extreme
pressures and temperatures generated by a high-explosives shock,
when metals seem to flow as if they were liquid.
Advanced Diagnostics Group, part of the Defense and Nuclear Technologies
Directorate, is tasked with diagnosing
hydrodynamics tests and other experiments that shock materials.
To characterize these experiments, researchers use radiation detectors,
imaging cameras, temperature probes, and velocimeters. They are
continually in search of better diagnostic equipment to measure
the extreme velocities. One recent Livermore development is a cost-effective
and easy-to-operate technique called the photonic Doppler velocimeter
(PDV). Equipped with two new telecommunication
devices, the PDV takes advantage of a basic phenomenon that’s
taught in high school physics—the beat frequency.
Laser technician Tony Whitworth adjusts
the settings on the digitizer component of the photonic Doppler
velocimeter (PDV). The PDV consists of a fiber laser, a detector
chassis, and a digitizer with 20-gigahertz sample oscilloscopes.
In this setup, the laser light is split into seven beams,
with four feeding a detector and digitizer, and another three
feeding a second detector and digitizer. The yellow optical
fibers at the top of the case (see inset) are routed to an
adjoining room and secured to the back end of a target assembly.
In the experiment, a projectile from a gas gun hits a sample
of aluminum placed on the opposite side of the target assembly.
Making Use of Doppler Shift
method to determine the velocity of a moving surface is to measure
the Doppler-shifted frequency of light reflected off that
surface. The Doppler shift is the difference between the frequency
at which sound or light waves leave a source and the frequency
seen by an observer. The difference is caused by the relative motion
of the observer and the source. A well-known example of the Doppler
shift from sound waves is the falling pitch of a train as it moves
away from a stationary observer.
same relationship occurs with light waves. If laser light is shone
on a stationary metal surface, light reflected from the
surface will have the same frequency, or color, as the incident
laser light. However, if the surface is moving, the Doppler shift
slightly changes the frequency (color) of the reflected light.
As the surface moves, the shift in frequency of the reflected light
is proportional to the change in velocity of the illuminated surface.
The greater the velocity, the greater the Doppler shift and frequency
researchers use several types of velocimeters that measure the
Doppler effect. The most sophisticated is the multibeam Fabry–Perot
velocimeter, developed in the mid-1990s by physicist David Goosman,
who leads the Advanced Diagnostics Group. This device splits a
laser light into five individual beams with very high efficiency,
and five streak cameras then record the reflected laser light.
In this way, the time history of material at five different spots
on a metal’s surface is recorded with extreme accuracy. (See
S&TR, July 1996, The
Multibeam Fabry–Perot Velocimeter: Efficient Measurement of High
“The multibeam Fabry–Perot is a robust system that really
tells us what is going on,” says Livermore physicist Ted
Strand, who also is a member of the Advanced Diagnostics Group.
Unfortunately, the Fabry–Perot instrument must be custom
manufactured, it takes up a lot of space, and several people are
needed to operate it—all of which limit the number of channels
that can be fielded on an experiment. In addition, many experimenters
cannot afford to build a Fabry–Perot system.
diagnostic system, the commercially available VISAR (Velocity Interferometer
System for Any Reflector), is less expensive than
the Fabry–Perot and performs well for measuring a single
velocity. However, a VISAR is inadequate for many of the experiments
that could potentially produce multiple velocity signatures.
overcome these problems, Strand searched for a more cost-effective
method to measure velocity—one that would offer about the
same accuracy as Fabry–Perot. That search led him to the
photonic Doppler velocimeter detects the beat frequency between
two slightly different frequencies of light. Laser light (red)
from a probe is shone on a moving surface. The Doppler-shifted
light (blue) is collected and sent to a detector. At the same
time, some of the original light is sent to the same detector.
The difference in frequency between the two—the beat
frequency—is much slower than either the original laser
light or the Doppler-shifted light, allowing researchers to
infer the velocity of the metal surface and construct a graph
of velocity versus time.
Beat Frequency a Matter of Subtraction
beat frequency is the difference in frequency between two waves.
It is easily evident in sound waves: Strike two tuning forks, each
tuned to a slightly different frequency. The sound from the two
forks will go up and down in volume rather slowly. In a similar
manner, two guitar strings tuned almost identically and struck
at the same time will also produce a beat frequency identified
by an up-and-down volume. The “beats” are caused by
the constructive interference, or combined amplitude, of two different
waves as they pass the same point—in these examples, the
listener’s ear. When the two waves are in phase, their combined
amplitude is larger—or the noise is louder—than it
is when the waves are out of phase.
PDV detects the beat frequency from two slightly different frequencies
of light by shining laser light at 1,550-nanometer
wavelength (in the infrared spectrum) on the surface of a moving
target. The Doppler-shifted laser light reflected off the target
surface is collected and sent to a detector. At the same time,
some of the original light is sent back to the same detector. The
incident laser light has a frequency of 1.93 ¥ 1014 hertz,
or 193,414.49 gigahertz. If the target is moving at 1,000 meters
per second, the Doppler-shifted laser light will have a frequency
of 193,415.78 gigahertz. Both frequencies are so rapid that no
existing detector can directly measure them. However, high-speed
detectors can measure the difference in frequencies—the beat
frequency—because it is much slower than either the original
laser light or the Doppler-shifted light.
this example, the beat frequency measures 1.29 gigahertz, or less
than 1/200,000 of the original frequency. “Once we know
the beat frequency,” says Strand, “we can infer the
velocity of the metal surface. Then we have data to construct a
graph of velocity versus time.”
had once considered using the beat frequency as the basis for a
velocimeter, but developing such an instrument for an explosive
experiment was impractical until about a year ago. That’s
when Strand noticed two new and relatively inexpensive telecommunication
devices: an extremely fast digitizer and a compact fiber laser.
digitizer used in the PDV can measure 20 billion samples per second. “That’s an extraordinary sampling rate, but
it’s just barely fast enough to follow an exploding metal
sample,” says Strand. The 20-gigahertz sampling rate generates
1 million data points in a 50-microsecond experiment, which is
fast enough to provide at least 4 data points for every recorded
(a) This example shows the beat frequency
(converted to amplitude) recorded during 50 microseconds
of a high-explosive experiment. (b) The waves from a
10-nanosecond period of the beat frequency are extracted
Fiber Laser Keeps Things Simple
telecommunications product featured in the PDV is compact and affordable
high-power fiber lasers. The optical fibers in these
lasers, which have 9-micrometer-diameter cores, are doped with
rare-earth metals to amplify the laser light. In addition, the
laser power can be adjusted over a broad range. Livermore researchers
use light of 1,550-nanometer wavelength because that wavelength
is used by the telecommunications industry. “The fiber becomes
the laser,” says Strand. “This design does away with
a lot of optics and other components that traditional lasers require.”
the PDV, the originating laser light can be split into as few as
four beams or as many as eight. The four to eight fibers deliver
light from the control room out to the experiment area, where they
have been placed between a few millimeters to 25 centimeters away
from the target. During the fleeting experiment, some of the reflected
light returns to the control room on these same fibers. The reflected
light and a sample of the original light are converted to an electrical
signal. This beat signal is then amplified and digitized.
PDV has been used in more than 20 experiments conducted at Livermore’s
remote experimental site and its High Explosives Applications Facility.
It has also been used at the Big Explosives
Experimental Facility, located at the Nevada Test Site (NTS). Plans
are under way to use the diagnostic at the underground U1a complex,
also at NTS, where subcritical experiments involving small amounts
of plutonium are performed. Tests conducted in April 2004 at Livermore’s
small gas-gun facility demonstrated that PDV would be a valuable
diagnostic at JASPER—the much larger Joint Actinide Shock
Physics Experimental Research Facility gas gun at NTS. (See S&TR, June
Plutonium to Reveal Its Secrets.)
is promising to become a standard element in the arsenal of diagnostic
techniques available to Livermore researchers. Commercial
products make the technique easy to set up, simple to operate,
and cost effective. If the early successes are any indication,
more Livermore experiments will focus on the beat.
Key Words: beat frequency, Fabry–Perot velocimeter, fiber
laser, photonic Doppler velocimeter (PDV).
For further information contact Ted Strand
(925) 423-2062 (firstname.lastname@example.org).
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