Hi Suren,

Here are my answers to your questions:

- What is your experience by using of the tuning slugs. Are they used on the end poles or also in the regular poles? 

We use the tuning slugs both for trajectory tuning and for the field integrals tuning. They are installed all along the undulator.
The slugs have different shim signatures from the pole motion signatures. Playing on the shim signature differences, doing pole adjustments and installing slugs at the same location, we could do tuning more efficiently.

- Which is the tuning gap for the undulator? 12mm (Why the Phase Jitter minimum at 7.2mm ( LCLS-II-HE Final Design Report, Beam Transfer Lines and Undulator Systems, Figure 8-22(left)- Measured RMS phase error)?).

The tuning gap is 12mm (12.75 from PRD but I like the round number). The tuning gap was chosen so that the r.m.s phase error deviations will be more or less equal going in both directions from tuning gap (vs.Gap). The change in the phase jitter happens mostly due to jaw's deformations along the undulator vs gap. It's just happened that the phase jitter is close to max at 12mm gap and goes to minimum at small and big gaps. We do the final check of the phase jitter vs gap when we do the final dataset to make sure we met the requirements.

- What is the thickness of the strong back?

120mm

- What is the main improvement to get field integrals and phase jitter significantly below the specification requirements?

The main improvements is the significant reduction of pole saturation at small gap due to the pole geometry optimization + better magnet sorting  + tighter mechanical tolerances on pole and magnet positioning.

- What is the feedback of the undulator motion system? - (Rotary encoders on the motors and the linear encoders measuring the gap on the both sides of the undulator?)

The undulator has both absolute motor rotary encoders and gap linear encoders.
Top jaw motion is controlled by the rotary encoders and sets the center line position. The bottom jaw move is based on the gap linear encoders' readings and sets the gap correctly.

- How much is the magnetic middle plane deviation measured by centerline encoders? - (100-150µm?)

130um.

- At which gap the force reversal (at the lead screw) turns out? 

The force would reverse at 13mm gap without the compensation springs. Because of the springs, the force doesn't reverse at all gaps.