  Three-dimensional (3D) simulations of electron beams propagating in high
energy density (HED) plasmas using the quasi-static Particle-in-Cell (PIC) code
QuickPIC demonstrate a significant increase in stopping power when beam
electrons mutually interact via their wakes. Each beam electron excites a
plasma wave wake of wavelength $\sim2\pi c/\omega_{pe}$, where $c$ is the speed
of light and $\omega_{pe}$ is the background plasma frequency. We show that a
discrete collection of electrons undergoes a beam-plasma like instability
caused by mutual particle-wake interactions that causes electrons to bunch in
the beam, even for beam densities $n_b$ for which fluid theory breaks down.
This bunching enhances the beam's stopping power, which we call "correlated
stopping," and the effect increases with the "correlation number" $N_b \equiv
n_b (c/\omega_{pe})^3$. For example, a beam of monoenergetic 9.7 MeV electrons
with $N_b=1/8$, in a cold background plasma with $n_e=10^{26}$ cm$^{-3}$ (450 g
cm$^{-3}$ DT), has a stopping power of $2.28\pm0.04$ times the single-electron
value, which increases to $1220\pm5$ for $N_b=64$. The beam also experiences
transverse filamentation, which eventually limits the stopping enhancement.
