CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
TEMPERATURE MEASUREMENT OF ISENTROPICALLY
ACCELERATED FLYER PLATES
Thomas Bergstresser and Steven Becker1
Sandia National Laboratories*, P O Box 5800, Albuquerque, NM 87185-1168
j
Bechtel Nevada, P O Box 98521, Las Vegas, NV 89193-8521
Abstract. Two frequently-used methods to accelerate flyer plates to extreme velocity (>10 km/s) are
magnetic acceleration and impact of the flyer with a high velocity, layered impactor. In either case the
temperature of the flyer is not definitely known, either because of diffusion of the magnetic field into
the flyer or the quasi-isentropic nature of the impactor's acceleration. We have measured the
temperature of flyers in both methods using radiation thermometry. Our pyrometer has four channels
in the range 1.3 to 5.1 microns. It can resolve a 20 ns rise time, a necessary feature in these
experiments where the available time can be as short as 200 ns. We have also seen the large
temperature rise due to magnetic diffusion in plates that are too thin.
An optical pyrometer is an instrument for
measuring temperature by the intensity of light
radiated by an incandescent body. The theory and
practice of such an instrument has been presented in
detail1. We have constructed an infrared pyrometer
and have used it to estimate the temperature of
hypervelocity flyer plates. The pyrometer is outlined
in Fig. 1. Radiation is brought to the pyrometer by a
single optical fiber. This radiation is divided in the
pyrometer by dichroic beamsplitters into four
spectral bands centered at these wavelengths: 1.3,
2.4, 3.5 and 5.1 microns, representing channels 1
through 4 respectively. The beamsplitters were
chosen to make use of an infrared transmissive,
chalcogenide (As2S3) glass optical fiber. The
wavelength bands are almost overlapping in order to
increase the pyrometer output. The split beams are
focussed with coated zinc selenide lenses onto
photovoltaic detectors composed of mercurycadmium telluride for channels 2 to 4 and indiumgallium arsenide for channel 1. The (Hg,Cd)Te
detectors have thermoelectric coolers. Channels 2 to
4 can resolve a risetime of 20 ns, channel 1 is
somewhat faster. This speed is required because of
the short timescale of hypervelocity experiments.
The pyrometer is calibrated while the fiber is
viewing a cavity blackbody set at a sequence of
temperatures. In this it is important to use the fiber
that is to be used for the experiment because the fiber
is somewhat lossy and also variable from batch to
batch. Sometimes a lens has been used at the sample
end of the fiber to focus the viewing onto a 1 mm
diameter spot. When it is used this lens must also be
a part of the calibration. The spectral responsivity of
each individual channel is also measured, and this
information is used to check the quality of the
blackbody calibration as well as to extend it to higher
temperatures than the blackbody can attain. The
blackbody used currently has a maximum of 1273 K.
The first use of the pyrometer on a flyer plate
occurred at Sandia's hypervelocity launcher, a threestage gun. This work has been reported2 and will
This work was supported by the U.S. DOE under contract DE-AC04-94AL85000. Sandia is a multi-program laboratory operated by
Sandia Corporation, a Lockheed Martin Company, for the U.S. DOE.
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porosity, unanticipated effects
compression, magnetic field
of
the
rapid
IE
with
FIGURE 1. Schematic of the Pyrometer
be summarized here. In this type of experiment a
composite, layered projectile strikes the third-stage
flyer plate, accelerating it to high velocity without
causing it to break up. The acceleration proceeds by
a sequence of small shocks so that the flyer remains
nearly isentropic. In two experiments the flyer-plate
velocity was 9.8 and 10.8 km/s. The temperature
was 605 ± 35 and 750 ± 70 Kelvin, respectively.
There was good agreement with later calculations.
Evidence of some stray light was apparent, possibly
from jetting at the edge of the flyer where it met the
inside portion of the target fixture. This is reflected
in the uncertainty of the temperature.
Experiments have been done using Sandia's Zmachine as a source of intense electrical current
with a short rise time, less than 200 ns. The current
and resultant magnetic field produce a magnetic
pressure which accelerates the conductor carrying
the current, see Fig. 2 and refer to Ref. 3 for an
extended discussion. The compression wave will
ultimately shock up, but a counterbore with a thin
floor provides the opportunity to observe the effects
of rapid, shockless compression. The counterbore
floor can be accelerated to high velocity, in the
present case 12 km/s, and then used as an impactor
onto a target plate. In this application the density of
the impactor must be known. A high, unknown
temperature with its associated density decrease
would cause errors. An unacceptable temperature
rise could be caused by cold working, near-surface
aluminum
FIGURE 2. Magnetic Acceleration
("B-field") penetration to the surface with its
accompanying current density, etc. Calculations
indicated that B-field penetration was sufficiently
delayed and that the other effects were not
consequential, but measurements had not been done.
To remove the pyrometer from the intense
electromagnetic noise nearby to the Z-machine, a
long (41 m) optical fiber carried the signal to the
pyrometer stationed in a screen room. The required
length of fiber made impossible the use of the lossy
chalcogenide fiber for which the pyrometer was
designed. Accordingly, we used a low-OH silica
fiber fabricated for use in the infrared. This fiber
entirely blocked channels 3 and 4, leaving the
shortest wavelength channel 1 and part of channel 2.
The measured response of the pyrometer is shown
in Fig. 3. Starting at -0.4 u,s is the noise caused by
the operation of Z, picked up mainly by channel 2.
At 0.589 jj-s there is a fiducial time-mark on channel
2. Centered at 0.95 |is is a spike due to fiber
fluorescence, discussed in the next paragraph.
Judging from visar measurements on similar
experiments, first motion of the counterbore surface
occurs at about 1 |is.
There was no visar
measurement on this particular counterbore. After
the fluorescence spike there is a short time interval
before the temperature rise due to B-field penetration
at 1.288 jus.
The optical fiber used in the experiment was
sheathed in a plastic tube, providing protection from
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light but not x-rays or charged particles. On two
subsequent runs at Z similar fibers were installed
temperature from these results, but it is possible to
find an upper bound. Take as an upper bound for the
pyrometer output the average value plus the standard
deviation. This is to be compared to the blackbody
calibration result multiplied by the emissivity. To
increase the emissivity, the counterbore surface had
been prepared with a 32 finish rather than the usual
diamond-turned, 20 nm rms finish. The emissivity
of similar pieces was measured, yielding an
emissivity of 0.255 for channel 1 and 0.22 for
channel 2.
This could be changed by the
acceleration process, but we cannot estimate this
effect. The resulting temperature upper bounds are
790 Kelvin for channel 1 and 695 K for channel 2.
'20
DC
1
0.0
Time (/is)
2«4
25
2.6
2.7
2.8
Time (/^s)
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but the tips were blocked off. The same spike was
seen in these experiments, demonstrating that the
light did not come from the counterbore surface.
These tests also revealed that there was a long-lived,
low-level fluorescence that continued after the
spike, visible only in the shorter-wavelength channel
1. In yet another test the fiber was sheathed in a
light-weight hydraulic tubing with a thin, flexible
steel layer.
this completely eliminated the
fluorescence spike.
That such insubstantial
protection eliminated the problem suggests that the
fluorescence was due to low-energy electrons rather
than x-rays.
Data from the short time interval between the
fluorescence spike and field penetration is available
for estimating the temperature of the counterbore.
The average signal and standard deviation for
channel 1 between 1.096 and 1.248 (is is 1.4 ± 1.6
mV, The corresponding information for channel 2
is -0.2 ± 1 . 4 mV. It is not possible to find a
50
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w
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O
FIGURE 3. Pyrometer output from the bare floor of
an aluminum counterbore. Top: channel 1, Bottom:
channel 2
il Radiance (W/m2
-1.0
i
1 ; :
o
1
°2 4
25
26
Time
27
2J
(MS])
FIGURE 4. Pyrometer output from the aluminized
coating on a LiF anvil glued to a copper counterbore
surface. Top: channel 1, Bottom: channel 2
Recently three experiments were completed at Z,
one with an Al panel, two with Cu, one of which is
discussed here. The purpose was to measure B-field
penetration; pyrometry was an add-on. The B-field
was measured4 by a Faraday-law loop, and to slow
down the destruction of the loop by the advancing
counterbore surface a 1 mm thick LiF anvil was
glued to the counterbore. The glued side was
aluminized, 1 jiim thick, so that visar measurements
could be made. This thermally complex system is
not ideal for pyrometry. The pyrometer output,
converted to spectral radiance, is presented in Fig. 4.
The output shows three features: a low radiance
from 2.45 to 2.59 (is, an almost-linear rise to 2.67
|is, and thereafter a more rapid rise. These features
are present for channel 2 although the figure hardly
shows it, and these features are qualitatively present
in all three experiments. The output of channel 1
after 2.67 jus can not be used. Although this
detector can resolve better than a 20 ns rise time, it
is subject to a maximum slue rate, and the output is
at that rate from 2.67 JLLS until the detector has begun
to saturate after 2.7 jis. Only channel 2 remains for
estimating the temperature.
4000
unexplained. The pressure history at the Al-LiF
interface is known; using visar records of the
velocity. The Hugoniot or isentrope of glue or Al are
incapable of explaining the temperature. Channels 1
and 2 together imply that the radiance is from a small
hot spot or stray light from a hot object. In this case
the "maximum" temperature does not apply and the
minimum applies to the hot spot, not the aluminum
surface. The dotted line is an estimate of the effect
of the B-field on the temperature. It is T = 0.5 B2 / (
|Uo c), where T is the temperature, B is the B-field, jio
is the permeability of free space and c is the heat
capacity of aluminum. This can be given an
heuristic justification as a approximation, but suffice
it to say here that it is dimensionally correct.
It is desirable to improve these results. More
responsive detectors are available. Metal surfaces
can be coated to increase the emissivity5. A more
sophisticated use of the LiF anvil6 can eliminate the
possibility of stray light as well as suppress ejecta in
a shock experiment. Other techniques may be
possible.
REFERENCES
2.45
2.50
2.55
2.60
2.65
2.70
Time (/xs)
FIGURE 5. Temperature corresponding to Fig. 4.
Middle solid line is the estimated temperature, and
minimum and maximum values are shown (but see
text). Dotted line is an approximate temperature due
to magnetic field penetration.
The estimated temperature appears in Fig. 5,
prepared assuming that the aluminum surface had an
emissivity of 0.1. Minimum and "maximum"
temperatures are produced using an emissivity of 1.
and 0.03 respectively. An approximate temperature
due to the B-field penetration is shown by the dotted
line. The third of the mentioned three features is
due to the B-field.
The other two remain
1. D. P. DeWitt and G. D Nutter, eds., Theory and
Practice of Radiation Thermometry,
WileyInterscience, New York, 1988
2. W. D. Reinhart, L. C. Chhabildas, D. E. Carroll, T.
Bergstresser, T. F. Thornhill and N. A. Winfree,
"Equation of State Measurements of Materials using a
Three-Stage Gun to Impact Velocities of 11 km/s," in
Int. J. Impact Eng. Vol. 26, Hypervelocity Impact,
Proc. 2000 Symp., Galveston, TX, 6-11 Nov. 2000, in
press.
3. C. A. Hall, J. R. Asay, M. D. Knudson, W. A. Stygar,
R. B. Spielman, T. D. Pointon, D. B. Reisman, A. Toor
and R. C. Cauble, Rev. Sci. Instr. 72(9), 1 - 9 (2001).
4. Greg Sharp, Sandia National Laboratories, Private
Communication.
5. M. Perez, "Residual Temperature Measurements of
Shocked Copper and Iron Plates by Infrared
Pyrometry," in Shock Compression of Condensed
Matter 1991, edited by S. C. Schmidt, R. D. Dick, J.
W. Forbes and D. G. Tasker, Elsevier, Amsterdam,
1992,pp.73.-740.
6. D. Partouche-Sebban, D. B. Holtkamp, J. L. Pelissier,
J. Taboury and A. Rouyer, "An investigation of shock
induced temperature rise and melting of Bismuth using
high-speed optical pyrometry", to be published.
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