The topic of advanced accelerators includes a wide variety of mechanisms for achieving high-gradient particle acceleration, ranging from the relatively well-established to the exotic. At the Neptune photoinjector facility, the PBPL has the capability to investigate various acceleration methods by injecting a relativistic electron beam into nearly any device, from structures to plasmas. Beam parameters can be varied to optimize acceleration or test theoretical dependencies. Because the facility also contains a terawatt CO2 laser (part of the laser-plasma physics program led by Prof. Chan Joshi, a codirector of the Neptune lab) the experimental program is weighted toward laser-driven acceleration concepts. Theoretical work is also an essential component of the PBPL program, and is used to guide the experimental effort.
Currently, the PBPL is investigating several potential high-gradient accelerators, both in the Neptune lab and offsite. Neptune was first conceived as a lab dedicated to laser-driven plasma acceleration, one of the original goals being the demonstration of electron acceleration by injection into a plasma wave driven by two beating laser frequencies (the plasma beatwave accelerator, or PBWA). The first phase of this experiment was completed in fall 2002, with a broad spectrum of accelerated electrons observed, and a second phase is being planned. Another plasma-based acceleration technique, the plasma wakefield accelerator (PWFA), uses intense electron beams to excite plasma waves. It has been experimentally and theoretically investigated at UCLA and collaborating labs for over 15 years. A variant of the PWFA involving the trapping and acceleration of a high-charge beam in a relativistic plasma at a density transition, has been studied at UCLA for several years, and an experimental proof-of-principle is currently underway at the Fermilab/NICADD Photoinjector Laboratory, as is an experiment which demonstrates over 130 MV/m acceleration in plasma, and deceleration of the drive beam to near stopping. In Neptune, an inverse free-electron laser (IFEL) experiment is currently running. This version of the IFEL uses a strongly tapered planar undulator driven by 10-micron laser radiation; we are working toward energy tripling via the IFEL interaction. Finally, one proposed future experiment in the PBPL falls under the category of advanced accelerators: a slab-symmetric laser-driven accelerator structure, constructed of dielectric material, operating at the laser beat wavelength of 340 microns. This effort is in the early stages of design.