  Large-scale magnetic fields are key ingredients of magnetically driven disk
accretion. We study how large-scale poloidal fields evolve in accretion disks,
with the primary aim of quantifying the viability of magnetic accretion
mechanisms in protoplanetary disks. We employ a kinematic mean-field model for
poloidal field transport and focus on steady states where inward advection of a
field balances with outward diffusion due to effective resistivities. We
analytically derive the steady-state radial distribution of poloidal fields in
highly conducting accretion disks. The analytic solution reveals an upper limit
on the strength of large-scale vertical fields attainable in steady states. Any
excess poloidal field will be diffused away within a finite time, and we
demonstrate this with time-dependent numerical calculations of the mean-field
equations. We apply this upper limit to large-scale vertical fields threading
protoplanetary disks. We find that the maximum attainable strength is about 0.1
G at 1 AU, and about 1 mG at 10 AU from the central star. When combined with
recent magnetic accretion models, the maximum field strength translates into
the maximum steady-state accretion rate of $\sim 10^{-7} M_\odot {\rm
yr}^{-1}$, in agreement with observations. We also find that the maximum field
strength is ~ 1 kG at the surface of the central star provided that the disk
extends down to the stellar surface. This implies that any excess stellar
poloidal field of strength >~ kG can be transported to the surrounding disk.
This might in part resolve the magnetic flux problem in star formation.
