  We present an efficient and accurate energy-conserving implicit
particle-in-cell~(PIC) algorithm for the electrostatic Vlasov system, with
particular emphasis on its high robustness for simulating complex plasma
systems with multiple physical scales. This method consists of several
indispensable elements: (\romannumeral1) the reformulation of the original
Vlasov-Poisson system into an equivalent Vlasov-Amp\`ere system with
divergence-/curl-free constraints; (\romannumeral2) a novel
structure-preserving Fourier spatial discretization, which exactly preserves
these constraints at the discrete level; (\romannumeral3) a preconditioned
Anderson-acceleration algorithm for the solution of the highly nonlinear
system; and (\romannumeral4) a linearized and uniform approximation of the
implicit Crank-Nicolson scheme for various Debye lengths, based on the
generalized Ohm's law, which serves as an asymptotic-preserving preconditioner
for the proposed method. Numerical experiments are conducted, and comparisons
are made among the proposed energy-conserving scheme, the classical leapfrog
scheme, and a Strang operator-splitting scheme to demonstrate the superiority
of the proposed method, especially for plasma systems crossing physical scales.
