Cruise | Interstellar medium and cosmic rays
The mean free path and Larmor radius of interstellar plasma particles is far greater than the size of the nanocraft, meaning that they would impact the nanocraft walls independently rather than forming a bow shock.
Protons from the interstellar plasma would impact a nanocraft travelling at 20% of the speed of light with kinetic energies of 18MeV, and electrons would have kinetic energies of 10.2keV. Whether the protons and electrons are combined in a hydrogen atom or arrive separately is not very significant. There would be some erosion of the surface of the nanocraft by sputtering; the number of sputtered atoms per area would be of the order 1000 per cm2. The net loss of mass from the forward-facing surface would only amount to a few monolayers.
The 18MeV protons would travel several millimeters into the target before stopping. A protective layer sufficient to stop 18 MeV protons would be required to avoid damage to the electronics by proton implantation.
Cosmic rays are much rarer than interstellar protons and hence can be ignored. Impact by heavier elements would need to be mitigated by means of protective shielding: helium nuclei carry 72 MeV and are 10% as numerous as single protons. Hits by CNO group nuclei carry 200-300 MeV and are ~0.1% as common, and hits by iron nuclei carry ~1GeV of energy and are ~0.01% as common. The depth of damage per nucleus scales roughly as the total energy to the 1/3 power.
Laboratory experiments on ions traveling at 20% of the speed of light (from a modest particle accelerator) and impinging on a solid could study the impact of interstellar particles on the nanocraft walls.
Collisions with interstellar ions and electrons might conceivably also have a benefit: they could charge the nanocraft to a potential of up to 10kV (the kinetic energy per electron). The forward-facing surface of the nanocraft would be heated at a rate of 6mW per cm2, in principle providing a thermoelectric energy source during the cruising phase in the interstellar medium.
Knowledge and experience in the interstellar medium might be gained on a precursor mission to the ultimate Alpha Centauri one, venturing beyond hundreds of AU within the solar system.