The question of what beam quality and efficiency are ultimately achiev
able in plasma-based accelerators is addressed analytically and throug
h self-consistent particle-in-cell simulations. A strategy for phasing
and beam loading to minimize energy spread while at the same time ach
ieving high energy extraction efficiency is proposed. Preservation of
beam emittance is facilitated by the use of a hollow channel. [S0031-9
007(98)07381-5].