  Significant transient beam loading effects were observed during beam
commissioning tests of prototype II of the injector for the Accelerator Driven
Sub-critical (ADS) system, which took place at the Institute of Modern Physics,
Chinese Academy of Sciences, between October and December 2014. During these
tests experiments were performed with CW operation of the cavities with pulsed
beam current, and the system was configured to make use of a prototype digital
low level radio frequency (LLRF) controller. The system was originally operated
in pulsed mode with a simple PID feedback control algorithm, which was not able
to maintain the desired gradient regulation during pulsed 10 mA beam
operations. A unique simple transient beam loading compensation method which
made use of a combination of PI feedback and feedforward control algorithm was
implemented in order to significantly reduce the beam induced transient effect
in the cavity gradients. The superconducting cavity field variation was reduced
to less than 1.7% after turning on this control algorithm. The design and
experimental results of this system are presented in this paper.
