An optically shaped gas target was developed at NRL for producing high-energy electron beams. The gas target is initially formed using a simple gas jet, which consists of a nozzle and valve arranged so that when the valve is opened, a supersonic jet of gas is produced. The jet of gas is then optically shaped, i.e., a high-energy laser pulse is used to create a shock wave in the gas, altering its density profile. As the optically induced shock wave evolves, a density profile is created that is favorable for laser acceleration of electrons. At the right instant, another laser pulse, more intense than the first, is focused into the shaped gas jet. This laser produces a high-quality, high-energy, electron beam. Because of the optical shaping, the reliability and quality of the electron beam are improved.
Background: When an ultra-intense laser is focused into plasma, the laser drives a wake that can be used to accelerate electrons. In order to accelerate the electrons to high energy, the laser intensity has to be maintained over a long distance. The intensity of the laser at any point determines the strength of the wake at that point. The intensity can be maintained at a constant level by guiding the laser pulse in a plasma channel, or by relying on nonlinear self-guiding in a gas jet or slow-flow gas cell. Electrons are typically generated by a self-trapping mechanism that allows them to be collected from the background plasma. One self-trapping mechanism that has gained attention recently is down-ramp injection. In this scheme, the laser initially propagates through a localized high-density region. As it exits this region, there is a sharply decreasing density. The dynamics of the wake in this situation encourages self-trapping. After exiting the high-density region, there is a much longer low-density region where subsequent acceleration takes place.
Accomplishment: The accomplishment is the experimental realization of a novel gas jet arrangement that allows for down-ramp injection into a self-guided laser wakefield accelerator. The arrangement also could enable other types of injection. The first element of the accomplishment was the production and characterization of hydrodynamic shocks in a gas jet using nanosecond laser pulses. By observing time-gated Schlieren images of the shock as it developed, the velocity of the shock front propagation and the deformation of its shape due to the gas jet flow (which is supersonic) were measured. The experimental results were reproduced and interpreted by means of 3D hydrodynamic simulations. Next, a gas jet with a tantalum plate incorporated into the nozzle was fabricated. The tantalum plate acts as an energy absorber for the nanosecond laser used to create the shock wave, and as a partitioning wall that defines two regions of the nozzle. Schlieren and interferometry were performed on the shocks created using the new nozzle, and vacuum-plasma transitions as short as 20 microns were observed. These abrupt transition regions are useful for both external injection of electrons into a laser wakefield accelerator, and injection schemes which rely on self-trapping such as down-ramp injection or ionization impurity injection. For down-ramp injection, a highly localized high-density region is needed to trap electrons, followed by a low-density region to bring them to high energy. If the high-density region is too long, deleterious effects can disrupt the acceleration process. The optically shaped gas jet we developed avoids this difficulty by confining the high-density region to a scale of 20 microns. Using this technique, we have already observed a well-collimated beam of electrons emerging from a gas jet with a very localized high density region and much longer low density region.
Application: Compact sources of high-energy electrons can be used to produce gamma rays by means of Thomson scattering, which may be useful for detection of special nuclear materials and other applications.
Significance: This accomplishment is expected to allow electron acceleration to 200 MeV using a 10 TW laser, without the need for any external guiding structure such as a capillary discharge. Furthermore, the acceleration process is stabilized. That is, the electrons can be produced with greater reliability and consistency. Without this accomplishment, the highest energy attainable in such a configuration would be less than 100 MeV and the results from shot to shot would be erratic.