Sponsor: Air Force Office of Scientific Research DURIP (AFOSR)
DoD assets orbiting the Earth are continuously subjected to a wide range of particle sizes and energies. The energies may range from relatively low energy reactive species in the upper atmosphere, such as atomic oxygen, to very high-energy protons in the van Allen belts in excess of 100 MeV. Particle sizes may range from as small as individual protons and electrons to micron-sized particles that have resulted from the collisions of both natural and manmade objects in orbit. Much larger objects are also present, derelict spacecraft up to 1000s of kgs, however as the sizes increase from “Dusts” to “Grains” (above the micron level) the probability of collision drops rapidly and the damage incurred from a single collision event increases dramatically – no longer really a micro-scale interaction.
Assessment and mitigation of the impact damage of these plasma environments hinges on a complete understanding of the physics of the interactions at the atomic level. This, in turn, requires a rich set of experimental data to both lead the development of theoretical models as well as to verify their predictions. The experimental capability of the research team at the time of proposing the “Tightly Coupled Mechanistic Study” program was limited to the plumes of electric propulsion systems, generally below several hundred eV and comprising primarily noble gases. Additional facilities have recently been acquired that will extend the capabilities to a variety of chemically reacting species at energies up to ~5 keV.
Laser ablation studies, primarily focused on space propulsion, have achieved specific impulse values of over 6000 secondsi, corresponding to a beam energy of nearly 1.9 keV. However, some laser-driven plasma accelerators have shown that a proton beam can be accelerated up to 58 MeVii, which corresponds to a specific impulse of ~ 10 Msec. This is from a very high peak energy and short pulse laser (femtosecond) that can produce in excess of a petaWatt of power. At more easily achievable pulse widths in the picosecond range, the particle energy may end up in the 100’s of keV range.
In addition, a variety of particle sizes can be achieved, from individual ions to nano- and micro-meter sized particles, depending on the total amount of energy in the pulse and the rate of its deposit. For example, a 5 msec pulse can liberate over 0.3 micrograms of material, with the majority of particles in the 1-2 nm range (40%-60%) and having an observed particle flux of 9e13 particles/m^2. The actual energy of the individual particles is quite low, and in terms of specific impulse is less than 13 seconds as compared to the previous value of 6000 seconds.
Finally, the charge state of the plasma can also be varied, with ionization fractions ranging from 1e-9 to nearly unity, with densities that vary from 1e22-1e24 particles per cubic meter. The level of ionization can typically be increased by using a second, time-delayed pulse immediately following the primary ablation pulse.
The point of these examples is to demonstrate that laser ablation can generate plasma environments that span the majority of the energy range, particle size and ionization fraction that would be found on-orbit, and thereby provides an ideal mechanism for simulating these environments and their effect on various materials. The purpose of this DURIP is to build a laser ablation facility that will first be used to describe the plume characteristics (size, charge and energy distributions) and then to expose materials to these plumes to establish their impact.
[i] Horisawa, H., Y. Sasaki, I. Funaki, and I. Kimura, “Electromagnetic Acceleration Characteristics of a Laser-Electric Hybrid Thruster,” 44th AIAA/ASME/SAE/ ASEE Joint Propulsion Conference and Exhibit, AIAA Paper 2008-4818, 2008.
[ii] Snavely, R., Key, M., Hatchett, S., Cowan, T., Roth, M., Phillips, T., et al., “Intense High-Energy Proton Beams from Petawatt-Laser Irradiation of Solids,” Physical Review Letters, Vol. 85, No. 14, 2000, pp. 2945–2948.