Interstellar Flight: Perspectives and Patience

>To get around thousand-year generation ships, we are examining some beamed energy solutions that could drive a small sail to Proxima in 20 years.

The odds of a spacecraft hitting a single particle of dust while in space are 100%.

A spacecraft hitting a single particle of dust at 0.2c will impart tens of millions of joules into the body of the spacecraft, the equivalent of getting hit with hundreds of pulses from the most powerful laser ever created by humanity-- simultaneously.

Or concentrating several kilogram's worth of TNT into the size of a particle of dust and detonating it.

Only true if the dust grain is stopped by the craft. For a thin lightsail the grain will probably pass right through without depositing much energy

So it passes through the sail and then hits the spacecraft attached to the sail. Now what? kaboom? small holes in the hull would not be good for the occupants.