RFD Cavity Frequency Fix Plan HyeKyoung Park SRF Engineering, Jefferson Lab 06/23/2016

Facts One cavity has about 1 MHz higher frequency from the measurement. Measured MHz, target frequency at subassembly state is MHz. Cavity center body is about 1.5 mm concave from CMM measurement. From tuning simulation (by both CERN 1 and ODU 2 ) tuning sensitivity is about.7-1 MHz/mm. Frequency increases as the surface pushed inward. Frequency decreases as the surface pushes outward. Cavity pole area is the most important area for the cavity performance. Pole should be the last place to touch to tune the cavity. Also the symmetry of the poles should be preserved. Deforming pole area will most likely change the current symmetry wrong way. As the concave surface flattens (in other words, getting close to the design), the frequency is moving to the right direction. Frequency decreases approximately 1.3 MHz. The most logical fix is to correct the deformed area. 1. Crab cavities cryomodule review “Tuner”, Kurt Artoos, November RF Dipole Cavity SPS Prototype Engineering, H. Park, May 2014

Main Body Scan 1 Main Body Serial Number (Top surface) Total form and location deviation = 1.569mm Max = 0.171, Min = Courtesy of Brian Carpenter at JLab

Main Body Scan 2 Main Body Serial Number (Bottom surface) Total form and location deviation = 1.176mm Max = 0.212, Min = Courtesy of Brian Carpenter at JLab

Plan –step 1 Elastically flatten top and bottom area by fixture (when fixture is out cavity will resume the deformed form). Machine the tuner attachment groove and weld the tuner attachment. Jlab machine shop has been consulted and they do not see any difficulty. Machine shop also assured to keep the weld seam profile. These steps will have to happen in the future anyways.

Plan –step 2 Once the attachment is welded, subassemblies will be clamped together and frequency measurement will take place to verify the frequency sensitivity. Less than 0.5 kN force (or 0.1 mm deflection) will be applied. After the sesitivity confirmation, plastically flatten the top and bottom surface. Tuner tab will NOT be used to apply force. Anneal to relieve residual stress at 800 deg. To be able to perform stress relief, cavity main body before end cap weld is the best stage (avoid risk of knife edge softening).

Fixture Concept Polished SS plate Cavity length Jack mechanism Example of inner fixture used for 499 MHz deflecting cavity Profile hold at the edge

Fixture Options Preserving profile of main body during fix – all four edge will be monitored with indicator. Pole side is much more rigid, top and bottom edge will tend to be stretched out. Constrain these two edges from outside of cavity. Worst case, use machinable epoxy cast to provide the perfect profile holder.