Densitytailored Capillary Discharge

Advanced high energy electron accelerators based upon laser plasma acceleration rely on using capillary discharges for both forming the plasma and providing a means for guiding the intense laser beam over distances much longer than the Rayleigh range. Scaling these devices to high energy levels requires using long capillary discharges with relatively low onaxis electron densities while still maintaining a small matched laser spot size throughout the capillary. Tapering of the plasma density along the capillary is also important for optimizing the energy gain. Conventional capillary discharges cannot satisfy these requirements. Conventional capillary discharges also typically rely on selffocusing to achieve guiding of the laser beam, which is undesirable because selffocusing is a nonlinear process and difficult to control. Therefore, being able to achieve true matched guiding is another critical need. We have developed a new type of capillary discharge that can operate with: 1) true matched guiding without reliance on selffocusing, 2) relatively low onaxis electron densities, 3) large enough capillary diameters to avoid damage by the highpower laser beam, and 4) longitudinal density tapering. During Phase I, we successfully performed experiments that verified the model predictions for our new capillary discharge and designed a prototype. During Phase II, we will build and test the prototype, including characterizing the capillary discharge and confirming guiding of a laser beam. Our densitytailored capillary discharge will help laser plasma experiments reach their ultimate goal of TeV energies as well as enable them to advance the field by utilizing true matched guiding of the laser beam. Our device can also help laser plasma accelerators and other plasmabased technologies become marketable products, for example, to create tabletop linear accelerators or xray lasers. Compact accelerators can generate tunable, intense electromagnetic radiation for industrial, medical, defense, and homeland security applications, such as for nondestructive testing and to generate gammarays for detection of hidden nuclear materials. KEY WORDS: Capillary discharge, laser plasma acceleration, tabletop accelerators, xray lasers, NDT. Capillary discharges are key components for laser plasma accelerators and other plasmabased technologies, but these discharges have fundamental limitations. Our densitytailored capillary discharge overcomes these limitations, thereby allowing these devices to scale to high electron energies and to improve their performance while maintaining a compact size.