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XFEL HOM Specification . WG 1 – HOM Damping Requirements

This specification document discusses the HOM damping requirements for XFEL, focusing on the degradation of damping in the TM011 mode and exploring potential solutions. The document also analyzes the impact of mechanical fabrication and shrinkage instability on HOM damping.

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XFEL HOM Specification . WG 1 – HOM Damping Requirements

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  1. XFEL HOM Specification.WG 1 – HOM Damping Requirements Alexey Sulimov, DESY (Hamburg), July 14, 2014

  2. Requirements HOM Damping Requirements for strongest modes (typical Q-values only!) - Monopole modes Q < 105 - Dipole modes Q < 105 (published on HOMSC 2010) Dispersion curves for monopole (solid line), dipole (dashed line) and quadrupole (dash-dotted line) modes. (R. Wanzenberg, TESLA 2001 33)

  3. TM011- mode. The History of Measurements

  4. TM011- mode. Present Statistics

  5. Two Explanations of HOM Damping Degradation Inaccuracy of Cells’ Shape ! Deviation of HOM Coupler Geometry Factors of geometry inaccuracy: 1. Mechanical fabrication (of half cells or HOM couplers); 2. Shrinkage instability of equator (cells) welds; 3. Chemical surface treatment (additional); 4. Unplanned deformations.

  6. Mechanical Fabrication (data analyses) In spite of the most dimensions of HOM couplers and half cells are in tolerance according XFEL specification, some tendencies to their influence on HOM damping were found. At the moment, results of this investigations are not allowed to be published. They are stillunder discussion.

  7. Mechanical Fabrication of Cells (simulations) Form deviation: • Equator radius of cell #1 is increased at 0.2 mm (inside the tolerance, corresponding XFEL specification); • Length cell #1 will be increased at 1.5 mm to compensate the field distribution on TM010 (pi-mode). TM010 (pi-mode) |E| (r=0, z) for tuned cavity TM011 (zero-mode) |E| (r=0, z) for tuned cavities with different equator radius and length of cell#1 (right cell on the plots). Estimations are based on the simulations by 2D FEM-code (version 1.0, 2001) written by J. Sekutowicz(DESY, Hamburg)

  8. Shrinkage Instability of Equator Welds Shrinkage instability: • Cavity A: double welding of cell #3, dL(C3)  - 0.8 mm; • Cavity B: cell #1 is longer than it was planned, due to the bad contact between half-cells, dL(C1)  0.4 mm. TM011 (zero-mode) |E| (r=0, z) (cell#1 on the left) Cavity A Q = 339 000 Cavity B Q = 252 000 Cavity C Q = 70 000

  9. Acknowledgements I appreciate: 1. IFJ-PAN RF team for the measurements of XFEL cavities under cryo conditions at DESY. 2. MPY group (DESY) for very useful discussion and especially J.Sekutowicz for possibility of using his simulation code. 3. XFEL DB Team (DESY) and especially V. Gubarev for helping with data preparation. 4. WP04 XFEL Team and especially J.-H. Thie for analysis of mechanical production status.

  10. Discussion New requirements: Monopole modes Q <? x 105 Questions: • Is Q(TM011_9) > 105 really dangerous? • Where the “Real Limit”? • Degradation of TM011 damping could be explained by loosing of field (geometry) asymmetry for this mode.How does it correlate with field asymmetry for TE121 (third dipole) and its damping?

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