RUI: Collaborative Research: Collisionless Heating During Strong Field Irradiation of Wavelength-scale Particles

Harvey Mudd College, Claremont CA

Investigators

Abstract

The physics of the interaction of intense laser light with plasma when wavelength-scale particles are irradiated is investigated. While the nature of strong field interactions with atoms, molecules, small clusters and planar solids has been studied extensively for many years, how intense laser pulses interact with objects that are of a spatial scale comparable to the light?s wavelength is largely unexplored. These interactions are likely to exhibit properties which are quite distinct from the interactions of intense pulses with single atoms and molecules on one hand and large planar solid plasmas on the other. This work fills this gap in strong field physics with careful, well-designed experiments that isolate the effects which are peculiar to wavelength-scale targets. Previously, our investigations showed that strong field interactions could be enhanced by an appropriate choice of target particle size. Boundary conditions imposed by the particles create Mie enhancements in the local laser field and thereby increase the nonlinear response of the interaction. The specific focus of the current work is motivated by an important question that arose from our previous experimental and computation studies: is the nature of collisionless absorption by hot electrons around wavelength-scale plasmas different from collisionless absorption from a simple planar solid? A number of theories have recently surmised that such absorption is different at these scales, dominated by what has been termed multi-pass stochastic heating, in which hot electrons can absorb energy from the laser field by passing back and forth multiple times through the micron-scale plasma. This novel heating mechanism has been suggested as being important in a number of experiments and particle-in-cell simulations but there has as yet been no systematic experimental investigation. The current studies are designed to address this shortcoming. The experiments are designed to directly measure the electron and ion distributions generated in the collisionless heating process, and, by studying specific scalings in particles of well defined size, ascertain the importance or existence of this stochastic heating mechanism. The studies utilize a 20 TW, high temporal contrast laser at UT, electron and ion energy diagnostics, and a novel electrostatic particle injector as a target. These experiments are complemented by simulations using codes available at UT. The work is done through a unique collaboration between an academic research lab at the University of Texas and at Harvey Mudd College (HMC), an undergraduate institution. Application of these kinds of studies may aid in the development of novel bright, laser-driven x-ray sources for radiography and time resolved diffraction, or compact neutron sources for imaging of other scientific applications. In addition to these scientific impacts, this collaborative work promises to make a significant and somewhat novel impactin graduate and undergraduate education. In addition to its scientific merit, and support of graduate student research at UT, this research significantly adds to the scope and number of research opportunities available to physics undergraduate students at HMC. This work represents a unique situation in which an undergraduate group is involved directly and in a critical way in larger scale strong field optics research, a field which is otherwise prohibitively resource intensive for undergraduate-level research alone. The PIs have an excellent record of working together and meaningfully involving undergraduates in their collaborations. Undergraduates have the opportunity to do research at HMC, travel to UT in the summers to participate in research using equipment they have developed, and travel to conferences to present their work to the scientific community. This collaboration will continue to be an effective way to motivate undergraduates to seeking advanced degrees in science. Furthermore the PIs will continue to disseminate their work through publication in appropriate peer-reviewed journals and international conference as they have done actively during the past funding period. Funds for this award are provided by the Physics Division within the NSF's Mathematics and Physical Sciences Directorate and the Office of Fusion Energy Sciences of the DoE within the context of the NSF/DOE Partnership in Basic Plasma Science and Engineering.