Investigating Thermal Shock*
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37830
April 29, 1999
Fabry-Perot Interferometric Strain Sensor
Detailed in the following figures is the strain sensor data from shot 9, which occurred at Los Alamos Neutron Science Center on January 30, 1999. A beam of 2.24 x 10^13 protons with an approximate energy of 0.8 GeV hit the target with a pulse width of 30ns.
Figure 1 and 2 detail the raw strain sensor signal data from sensor 19 taken during shot 9. Figure 1 details the first millisecond of response from the strain sensor while figure 2 details approximately the next millisecond of response. The peak to peak voltage was 3.5V and the gauge factor was 12.7mm. The raw strain signal data voltage output was translated from the interferometric optical signal, using the Fiber Optic Support System, which was outputted to a digital oscilloscope. This data was categorized as a Multi Fringe event due to it being constrained within the peak to peak calibration voltage and its sinusoidal waveform. The characteristic turning points can be immediately identified within the waveform to further confirm the presence of a Multi Fringe event. The interpretive technique used for the locating of a turning point is an acquired skill that requires a lengthy study of past analyzed strain signal and the comparison of the two signals produced from the sensors located in the same strain area.
Figures 3 and 4 display the raw strain sensor signal data, which has been identified as acquired signal. The approximated signal is the result of the modification of the strain signal model, which is detailed in figures 5 and 6. By altering points surrounding the ideal strain model, the interpolating polynomial will adjust the strain model and update the approximated signal in figures 3 and 4. The changes then can be compared with the acquired signal and appropriate accuracy can be obtained. Figures 5 and 6 relate the physical strain exerted on the sensing area due to the pressure applied by the mercury when it interacts with the proton beam. From the examination of the strain signal, we see that the cavity wall oscillates between its equilibrium position. The distance between the fibers within the capillary initially expands due to pressure being applied to the cavity wall. The sensor then begins to contract after about 14 microseconds to where it reaches its equilibrium position after approximately 20 microseconds. The sensor then compresses within the capillary as the exterior wall of the cavity contracts due to the oscillation of the mercury and the negative flux of the mercury away from the wall. Thereupon, the mercury follows this transient oscillatory decay into a steady state, which approaches the equilibrium condition as long as the sensor was not permanently deformed from the maximum strain exerted.
Phosphor Temperature Sensor
Shot 14 was examined using pulse averaging to obtain temperature measurements both before and after the shot. Shot 14 occurred on January 30, 1999 where two methods were used to determine the baseline values in each case. The first used an average of the last ten points of the waveform to determine the baseline while the second used the maximum value of the signal before the lasing spike.
Figure 7 details the overall analysis made on shot 14 using the baseline averaging technique. The first column represents the numerical identifier that was assigned by the data acquisition device while the second column detailed the percentage of the data used, centered between the lasing spike and the high noise levels expected near the end of the waveform. By excluding 30 percent of the waveform on both the beginning and end extremes, the middle 40 percent accurately represented an exponential decay as seen in the following figures where the data follows a linear relationship. To ensure that no pertinent data is excluded, various other amounts of the data was also analyzed (presented is only 40 and 50 percent). In figure 7, the third column details the slope values obtained from figures 10 and 11 (representative graphs) to 29 as based on the linear regression. The following column calculates the decay time based on the slope values. Using the calibration data the temperature is determined as is change in temperature. The sixth column details the type of baseline calculation used and what the baseline value was for each data acquisition. The last two columns translate the temperature from Kelvin to Celsius and Fahrenheit, respectfully.
High Sensitivity Diaphragm Pressure Sensor
Pressure sensor data was taken on shot 34, which occurred on January 31, 1999. The data was taken with a 2ms time base and a fixed vertical gain of 500mV per division. Figure 50 displays the signal output related from the Fiber Optic Support System, which was connect to the diaphragm sensor via single mode fiber optic cables. Further analysis requires that the signal data be translated using an interferometric technique to translate the waveform into a relationship of diaphragm displacement verse time. The displacement of the diaphragm can then be compared to the cavity pressure calibration curves detailed in figure 51. That figure has one curve made up of two points with one having a very low initial value at the pressure setting of 0 psi and a value which was erroneously high at a pressure of 500 psi. Also, two other points at that pressure value of 0 psi were too low due to errors with the Fiberscan 2000. There four points were separated from the curves. Note that the bottom most curve on the graph reaches its mechanical limit around 2000 psi as would have the other curves, which had initial displacements around 100 microns.
Figure 52 details the cavity pressure calibration curves with linear regression analysis applied to the data. One can see how the difference in the three initial starting points of the fiber optic probe produced three groups of curves that were representative of a linear nature.