This Grant is a Continuation of N00014-12-1-1030 Particulate and Contamination-Induced Breakdown in Large Area Optics
Abstract
I. Investigate the interaction of particles and surfaces under high power laser illuminationDetermine the characteristics of particle heating by absorption of laser light and the transfer of heat to the substrate.a) Measurement of Parameters used in Thermal Models:One of the most striking features of micro- and nano-scale particulates is how widely their material properties can differ from their bulk parent material. While some of these differences are widely known, such as the change in melting point of gold nanoparticles, most properties of the materials that are of special interest to HELs are poorly known. This section of the program aims to correct this problem.b) Development of Particle-Surface Thermal Models for Sudden Catastrophic Failure:One of the first models that we will explore is one of evaporative heat transfer by particles onto a surface, which increases the local absorption. This begins with a microor nano-particle on the surface of a coated optical element. Incident laser light strikes the particle and some of that light is absorbed and raises the temperature of the particle to a point where significant evaporation occurs. II. Investigate the contamination-surface interactionAmong the most important contaminants are organics, water (such as in the form of rain or sea spray), soot and smog, acidic droplets, salts, and mineral particles. One of the most important features of some contaminants is that they can be concentrated in evaporating water droplets. This has been seen in many environments, even in laboratory self-assembly processes. The residue left behind by evaporated water will be intensively studied for optical/infrared constants and absorption as described in Section I using spectroscopic ellipsometry and micro-FTIR. Further the thermal conductance of the contaminated regions will be measured using nanothermal tools at Anasys. These regions will be compared to pristine regions and the differences in thermal conductance quantified. We will also look at these particles using transient absorption and spatial modulation to provide information about heat transfer (time scales) and optical constants (from spectra).The initial studies will be performed with distilled normal water as a control on coated optics, and the results of these measurements will be compared to dirty water such as sea water and water exposed to soot contamination (such as that in car exhaust). These latter two materials will leave residues that more closely match conditions seen in real HEL operation. The contaminated surfaces will be testing at Minnesota in local areas using a 250 Watt CW Nd:YAG laser and also over large areas at the Penn State Electro-optics Center using their 10kW fiber laser. Close examination of any differences in failures between the two will be made so that an assessment can be made of how well local failure with low power systems mimics more realistic conditions. III. Investigate the Impact of UV flux on Sudden Catastrophic FailureParticipants: UM, ND, NPSAnother factor that may impact the failure of HEL optics is high UV flux. This is particularly important in free-electron lasers where exposure to higher-order harmonics of the primary beam is continual during operation. UV light is ionizing radiation and many UV damaged materials have similar characteristics to materials exposed to beta or gamma rays. In this program, performer will study if and how the absorption spectra of optical coatings and elements change after UV exposure. This work will also be extended to single particles at Notre Dame, where the absorption spectra and time-resolved traces would be assessed before and after exposure. The substrate would be marked so that the same particle could be found and interrogated. The University of Minnesota will examine the coated materials for fluorescence or thermoluminescence after large UV exposure because these characteristics are excellent indicators of the formation of traps and defects by ionizing r
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 23, 2016
- Source ID
- N000141613118
Entities
People
- Joseph Talghader
Organizations
- Office of Naval Research
- Regents of the University of Minnesota
- United States Navy