  The China spallation neutron source (CSNS) linac is designed with beam energy
of 81MeV and a peak current of 15mA in the first phase. The RF power system for
the 81 MeV Linac requires 8 units of RF power sources, each unit has one
independent digital low level RF (LLRF) control system which is used to
stabilize the amplitude and phase of the RF accelerating field along the linac,
and to minimize beam loss. During beam commissioning, all of 8 on-line units of
LLRF control system were operating stably and reliable, the control precision
of accelerating fields met requirement of beam commissioning. The amplitude and
phase variations of the linac fields are less than $\pm$0.2% and
$\pm$0.2$^\circ$ without beam loading, or $\pm$0.5% and $\pm$0.5$^\circ$ with
10mA beam loading, much better than the design requirements of $\pm$1% in
amplitude and $\pm$1$^\circ$ in phase.
