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4GLS BBU

4GLS BBU. Emma Wooldridge. Background Threshold equation Methods of increasing the BBU threshold Future work. 4GLS Schematic. Multipass regenerative Beam Break Up is caused by interaction of the beam with dipole Higher Order Modes. Threshold current.

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4GLS BBU

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  1. 4GLS BBU Emma Wooldridge Cockcroft Wakefields Interest Group Emma Wooldridge

  2. Background • Threshold equation • Methods of increasing the BBU threshold • Future work Cockcroft Wakefields Interest Group Emma Wooldridge

  3. 4GLS Schematic

  4. Multipass regenerative Beam Break Up is caused by interaction of the beam with dipole Higher Order Modes Cockcroft Wakefields Interest Group Emma Wooldridge

  5. Cockcroft Wakefields Interest Group Emma Wooldridge

  6. Threshold current • This is the highest current at which the beam can remain stable • Equation given is for one cavity and one HOM Cockcroft Wakefields Interest Group Emma Wooldridge

  7. Increasing the current • Decrease HOM Q • Decrease R12, R34 • Decrease HOM R/Q • Increase the injection energy Cockcroft Wakefields Interest Group Emma Wooldridge

  8. Lowering Q • Including ferrite dampers next to the cavity can increase the threshold • They remove power from higher order modes at a large range of frequencies Cockcroft Wakefields Interest Group Emma Wooldridge

  9. Position of Dampers • Tiles made from three different materials to ensure good coupling to a broad frequency range Cockcroft Wakefields Interest Group Emma Wooldridge

  10. Altering the Cavity • Introduce small defects in each cell to split the degeneracy of dipole modes • Early results suggest that alternating the perturbation in x then y produces higher thresholds • Further tests required • Could this be errors in the model? Cockcroft Wakefields Interest Group Emma Wooldridge

  11. Change Rij • R12and R34 relate the displacement to the momentum of the bunch for the x and y direction respectively • By minimising this value for each cavity the threshold can be maximised • As the number of cavities increases this criteria becomes more difficult to implement • Can require point to point focusing Cockcroft Wakefields Interest Group Emma Wooldridge

  12. Reflection • Reflection about 45°, x becomes y, y becomes x • Providing the HOMs are orientated on x and y BBU is completely suppressed • Realistic gain in threshold, 2 to 3 times the original value Cockcroft Wakefields Interest Group Emma Wooldridge

  13. Rotation • Rotation by 90°, x becomes y, y becomes –x • If the transport for x and y is identical and the transport along the linac axis is symmetric then BBU is suppressed in all cavities • Simulations suggest 100 fold increase in threshold • Realistic gain in threshold, 10 times the original value? Cockcroft Wakefields Interest Group Emma Wooldridge

  14. Future Work • Investigate the HOM damper • Lowering of HOM Q • Verify cavity perturbation results • Investigate methods to increase the threshold by reflection or rotation Cockcroft Wakefields Interest Group Emma Wooldridge

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