Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the alpha-particle heating expected to trigger a burn wave in the hydrogen pellet and the resulting thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where the theory agrees with data. Here, we report experimental data at low projectile velocities where the stopping force reaches its maximum (Bragg peak). This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to clearly rule out several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.