Insulation and conditioning of an electrostatic bunch length monitor for the Linear IFMIF Prototype Accelerator.

The Linear IFMIF Prototype Accelerator (LIPAc) is designed to validate the technology necessary to accelerate 125 mA of Deuteron beam to 9 MeV in continuous wave. A bunch length monitor has been developed to measure the longitudinal properties of the beam. The bunch length is inferred from the arrival time distribution, Vs. the beam accelerating RF, of secondary electrons from ionization of the vacuum residual gas by the beam. To minimize the differences in time of flight of the detected electrons, they are accelerated in a first stage by plates at different potentials, bent in an electrostatic spectrometer and collimated. The high current beam accelerated in LIPAc exhibits significant non-linear forces on the secondary electrons. To compensate this effect, high voltages are applied (up to 30 kV) in gaps of few centimeters. We introduce the design choices for the voltage distribution and insulation of the device. This work focuses on the redesign of some details triggered by the suspect of arcs tracks during the visual inspection carried while relocating the monitor to a different location of the accelerator vacuum chamber. Additionally, we present potential evidence of further arcs and their reduction throughout a conditioning process.