Laser-Plasma Interactions

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Intense Laser-Plasma Interactions...

.The Institute's research in intense laser-plasma interactions focuses on understanding, and eventually controlling, the plasma response, which can be used for compact accelerators for fundamental research and medical applications. This effort involves developing physical models that represent the salient plasma features, realistically describing the laser structure, and building numerical algorithms that accurately solve the governing equations. We have been the leaders in bringing novel approaches and fresh ideas to these challenging questions.

Over the last two years, we have developed an entirely new collisionless kinetic model to describe the low-temperature nature of the plasma. Our model has a finite pressure force term that evolves in a novel way; the equations governing the pressure evolution are consistent with ordinary thermodynamics in the equilibrium limit, but contain highly non-equilibrium effects resulting from the collisionless nature of the plasma. With our new model, we are beginning to study the finite temperature effects on the evolution of the laser pulse and plasma wake; our preliminary results indicate that the plasma does not heat significant in the short-pulse case, and that long pulses can heat the plasma, but not significantly. These results are significant because they indicate that the cold-fluid approximation is a very good model for the plasma wake, and that particle-in-cell (PIC) numerical methods have very stringent computational constraints to control the grid-heating inherent in the method.

Augmenting our physical modeling, we have examined several new numerical methods for solving the model equations. We have looked at explicit higher-order method-of-lines techniques and have seen performance improvements anywhere from a factor of 6 to 100, depending on the type of problem. This performance improvement arises mostly from the increased size of the stability region, allowing us to use a much larger time step. We have made extensive use of symbolic methods to carry out our analysis of these methods, and we are working to extend this work to implicit methods.

Over the next two years, we will use the tools we have developed so far to investigate two critical questions: "What is the maximum electric field a plasma can support?" (known as the "wave-breaking limit" ) and "Where do the trapped electrons come from?" These questions are fundamental to accelerator design because they drive directly to the energy, efficiency and quality of the beam produced.