  We have developed and tested a novel type of driver for insertion devices. In
our approach, we compensate for the magnetic forces acting on the insertion
device (ID) arrays by installing a number of compact hydraulic cylinders along
the ID. These assisting cylinders are connected to a single hydraulic line and
activated by the same pressure. A computer-controlled closed-loop feedback
system maintains the target ID array position and an acceptable load on the
mechanical driver. This is accomplished by simultaneously controlling both the
hydraulic assist pressure and the mechanical driver position. The quantity and
location of the compact hydraulic cylinders are optimized to keep the
deformation of ID structural components in an acceptable range. The hydraulic
cylinders compensate for over 90% of the strong forces generated by the
magnetic field. This reduces the load applied to a conventional mechanical
driver to less than 10% of the total load. The implementation of this type of
driver can make insertion devices much more compact without compromising their
performance
  In the proof-of-concept experiments, our novel hydraulic-assist driver
demonstrated a long-term stability and reproducibility of the ID gap within 2
microns. Magnetic field stability and reproducibility was within 6e-5 of the
normalized field strength or better. Here we will describe the concept, details
of our test setup and the results of the proof-of-concept experiments. We will
also discuss the advantages of the hydraulic-assist driver over conventional
mechanical drivers, as well as possible applications and future developments.
