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Using Tune Shifts to Evaluate Electron Cloud Effects on Beam Dynamics at CesrTA

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### Using Tune Shifts to Evaluate Electron Cloud Effects on Beam Dynamics at CesrTA

Jennifer Chu

Mentors: Dr. David Kreinick and Dr. Gerry Dugan

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Outline

- Review of Electron Clouds and Tune Shifts
- Simulations of New Data
- Varying the Simulation Parameters

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Electron Clouds

- ILC will collide electrons and positrons
- Accelerating charges radiate
- Photons knock electrons off walls of beampipe
- Photoelectrons are accelerated by beam and knock off more electrons, forming a cloud
- Electrons in the cloud are attracted to positive beams

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Tune Shifts

- Beams are displaced from nominal path
- Tune (Q): number of oscillations of a particle about nominal path, per turn around the ring
- Tune shift (ΔQ): difference in tune caused by electric field of electron cloud

Qy = 9.52

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Taking Data

- CesrTA is used to measure tune shifts
- Beams are set into oscillation
- BPMs measure the position for 2048 turns
- Fourier transform is used to calculate tune shifts

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POSINST

- POSINST is a simulation code used to model the electron cloud effects
- Simulations are run for different values for each of five parameters which describe the physics of electron cloud generation
- Simulations are compared to data to test accuracy of the model

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Comparing Data to Simulations

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Goals

- We want to find the optimal set of parameters that most accurately models electron clouds
- The simulation can then be used to predict the behavior of electron clouds in damping rings of future linear colliders

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Submitting Jobs to the Queue

- Simulations were run on Cornell’s batch nodes
- Each job is only allowed 48 hours of CPU time
- I wrote code to parallel process the simulations to get more statistics

I monopolized the queues for the summer:

87 data sets

x 5 simulation parameters

x 2 for x, y tune shifts

x 6 jobs per submission

-----------------------------------

> 5000 total jobs

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Calculating Tune Shifts

- I used Mathematica to post-process the results of POSINST to calculate the tune shifts
- I superimposed the tune shifts from multiple simulations onto plots of the data for comparison

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June 2011 Coherent Tune Shift Data

- 2.085 GeV e+: 20 x 0.5 mA
- 2.085 GeV e+: 45 x 0.5, 1.0, 1.5, 2.0mA
- 4.00 GeV e+: 20 x 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 mA
- 4.00 GeV e+: 45 x 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 mA
- 5.3 GeV e+: 20 x 0.5, 1.0, 2.0 mA
- 5.3 GeV e-: 20 x 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 mA
- 5.3 GeV e+: 45 x 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 mA
- 5.3 GeV e-: 45 x 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6 mA

Bunch spacing studies:

- 2.085 GeV e-: 30 x 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 mA for 12, 16, 20 ns spacing
- 2.085 GeV e-: 45 x 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 mA for 4, 8, 12 ns spacing

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2.1 GeV 4.0 GeV 5.3 GeV

0.50 mA/bunch

0.40 mA/bunch

0.50 mA/bunch

1.00 mA/bunch

1.00 mA/bunch

1.00 mA/bunch

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Simulation Parameters

- Parameters describe the physics of electron cloud generation
- When a radiated photon from the beam knocks into the wall of the beampipe, photoelectrons are generated
- (1) Quantum Efficiency: number of electrons generated for every photon

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Secondary Emission

- Photoelectrons are accelerated by the electric field of the beam and continue to produce more electrons
- (2) Secondary Emission Yield (SEY): number of secondary electrons generated for every primary electron
- (3) Energy at the SEY Peak

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Types of Secondary Electrons

- When a photoelectron hits a wall of the vacuum chamber, it can:
- Bounce off (elastic)
- Interact with material (rediffused)
- Knock off electrons in material (true)
- (4) Fraction of Secondaries that are Rediffused
- (5) Fraction of Secondaries that are Elastic

Sketch of the currents that are used to define the different components of secondary emission.

Figure taken from M. Furman and G. Lambertson, “The electron-cloud instability in the arcs of the PEP-II positron ring”

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Varying the Simulation Parameters

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Summary

- Tune shifts are used to study electron clouds
- Simulations were run and compared to data for all five parameters and all 77 new data sets
- Model seems to work reasonably well for a variety of beam energies and bunch currents
- Finding the optimal parameters will allow the model to be used for future linear colliders

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