Undepleted Direct Laser Acceleration

For the past two decades, intense lasers have supported new schemes for generating high-energy particle beams in university-scale laboratories. With the direct laser acceleration (DLA) method, the leading part of the laser pulse ionizes the target material and forms a positively charged ion plasma channel into which electrons are injected and accelerated. DLA has been realized over a wide range of laser parameters, using low-atomic-number target materials. A striking result is the extremely high conversion efficiency from laser energy to MeV electrons, with reported values as high as 23%, which makes this mechanism ideal for generating large numbers of photo- nuclear reactions. DLA is well understood and reproduced in numeric simulations. Specifically, the electron beam energy has been confirmed to scale with the normalized laser intensity up to values of a0~1.5. However, the electron energies obtained with the highest laser intensities available nowadays, fail to meet the prediction of these scaling laws. Here we reveal that at these higher laser intensities, the leading edge of the laser pulse depletes the target material of its ionization electrons prematurely. We demonstrate that for efficient DLA to prevail, a target material of sufficiently high atomic number is required to maintain the injection of ionization electrons at the peak intensity of the pulse when the DLA channel is already formed. Applying this new understanding to experiments on petawatt laser facilities, within the LaserNetUS network is expected to increase the electron energy overlap with the neutron production cross-sections of any material. These increased neutron yields are required to enable a wide range of research and applications, such as investigation of nucleosynthesis in the laboratory, performing non-destructive material analysis, and industrial applications.