IHC Application to HEL Heating
Jonathan W. Woolley
Inverse Problems Symposium 2012
East Lansing, Michigan, June 2012
There are currently no technologies available to directly measure the incident laser irradiance on airborne targets. Determining lethality effects on the target, short of total destruction, is problematic. Off-board infrared detection techniques such as thermal imaging and pyrometry are limited due to the obscuring/interfering effects of target degradation products and the poor resolution at large distances provided by current imaging systems. Off-board techniques have a limited engagement angle. Infrared-based techniques require knowledge of the material emissivity, which changes with temperature and surface conditions. With HEL heating, it can be anticipated that oxidation and metallurgical changes will take place, which change the emissivity of the surface. The inaccuracies associated with unknown and changing emissivity of the target material during HEL heating make infrared detection techniques impractical.
To quantify the effectiveness of an HEL weapon system, test instrumentation is required that can make high spatial resolution measurements of irradiance on the target. It would be beneficial if the instrument could also provide the corresponding temperature increase on the surface of the target. These sensors must respond to the laser wavelengths of interest and ideally be able to survive laser energies of several kW/cm2. The sensors should have minimal interference of the laser interaction with the target.
We have successfully demonstrated an integrated system for measuring high resolution temperature profiles and processing the data with solution of the inverse heat conduction (IHC) problem. Mature fabrication techniques have been established for building high-resolution (5mm pitch) RTD arrays (up to 20x20 nodes per tile, Figure 1) that are survivable to high temperatures (around 1000ºC). The arrays are fabricated on a flexible metal substrate that allows them to conform to target surfaces. The RTD arrays can be scaled down to smaller arrays or multiple tiles can be used to build larger arrays. The sensor arrays are constructed in a row/column architecture that requires specialized custom electronics to read the data. Early generation electronics have been developed and tested that can sample 20x20 sensor arrays (400 sensors) at 2.5 kHz. These electronics are built using a combination of commercial off-the-shelf (COTS) components and custom electronics. The inverse heat conduction solver has been verified for both flat plate and curved (Figure 2) geometries.
Figure 1. A 20x20 micro-fabricated RTD array (sensors on a 5mm pitch, a close up of a single node inset).
Figure 2. The temperature distribution from an IHC calculation for a curved geometry. The front surface temperatures (left) were found using the back surface temperature inputs (right).
The purpose of this presentation is to share some experimental data from a High Energy Laser test. The data to be presented is awaiting the approval of the U.S. government. Upon approval, the data will be presented at the Inverse Problems Symposium. We will also present some validation of the IHC solution based on laser heating experimentation.
