  Colliding of two high Mach-number quantum degenerate plasmas is one of the
most essential components in the double-cone ignition (DCI) inertial
confinement fusion scheme, in which two highly compressed plasma jets from the
cone-tips collide along with rapid conversion from the colliding kinetic
energies to the internal energy of a stagnated isochoric plasma. Due to the
effects of high densities and high Mach-numbers of the colliding plasma jets,
quantum degeneracy and kinetic physics might play important roles and challenge
the predictions of traditional hydrodynamic models. In this work, the colliding
process of two high Mach number quantum degenerate Deuterium-plasma jets with
sizable scale ($\sim 1000\ \si{\mu m}$, $\sim 300\ \si{ps}$, $\sim 100\
\si{g/cc}$, $\sim 300\ \si{km/s}$) were investigated with first-principle
kinetic simulations and theoretical analyses. In order to achieve high-density
compression, the colliding kinetic pressure should be significantly higher than
the pressure raised by the quantum degeneracy. This means high colliding Mach
numbers are required. However, when the Mach number is further increased, we
surprisingly found a decreasing trend of density compression, due to kinetic
effects. It is therefore suggested that there is theoretically optimal
colliding velocity to achieve the highest density compression. Our results
would provide valuable suggestions for the base-line design of the DCI
experiments and also might be of relevance in some violent astrophysical
processes, such as the merger of two white dwarfs.
