  A technical note is given regarding our previous laboratory
plasma-astrophysical studies [C.-S. Jao et al., High Energy Density Physics 32,
31-43 (2019) and Y. Chen et al., Nucl. Instrum. Methods Phys. Res., Sect. A
903, 119 (2018)]. In this note, an upgraded accelerator beamline design is
proposed based on a feasible experimental setup in a realistic laboratory
environment. The improved design aims to provide milliampere (mA)
mega-electron-volt (MeV) quasi-continuous (cw) electron beams for
plasma-astrophysical applications. Such a design utilizes a so-called
mixed-guiding-field magnetic system right after the cut disk structure (CDS)
booster cavity to provide a periodic longitudinal focusing field. The
transportation of the produced cw beam with large energy spread to the plasma
cell location is improved. The magnetic field serves as well as a seeding field
in the plasma environment for the growth of electromagnetic instabilities. In
conjunction with the appliance of a circular collimator at the exit of the CDS,
the new design allows production of quasi-cw beams with a three orders higher
number density at the entrance of the plasma cell compared to the previous
design for a seeding magnetic field of about 50 mT while the locally enhanced
electric field at the cathode is up to 8 GV/m. The associated beam dynamics
simulation results are presented. As proof of principle studies, the produced
electron beams are applied in nonlinear plasma-astrophysical simulations for
exploring the growth of the instabilities. The extracted parameters and/or
distributions from the generated electron beams in the laboratory environment
are used in these particle-in-cell simulations. The obtained results are
presented and discussed.
