Intro to Electron Cloud: An experimental summary

1. by Jeffrey Eldred Data Analysis Workshop March 13th 2013 Intro to Electron Cloud:An experimental summary

2. Outline • Electron Cloud Formation Process. • Electron Density Measurement Techniques. • Secondary Electron Yield Mitigation. • Beam Instability and Feedback Damping. • Electron Cloud Simulation Software.

3. Electron Cloud Formation Process • Initial seed electrons are generated. • Electrons accelerated by beam bunches. • Electrons collide into beampipe and generate secondary electrons. • The cycle repeats until the maximum concentration of electrons is reached. • Simultaneously, instabilities in beam can be seen coinciding with rising electron density.

4. Seed Electron Generation • Ionization by high-intensity beam. • Order of one electron generation per meter, per torr, per particle, per pass. • High-energy beam particle strikes beampipe. • Especially for grazing incidence, on the order of hundreds per particle lost. • Synchrotron radiation strikes beampipe. • Electron machines, LHC, muon machines.

5. WC41 E-Detector x 4 Cloud Electron Acceleration LANL PSR • Electron crossing on the trailing edge of a positive bunch receives a net acceleration. • “Resonance” behavior.

6. Electron Cloud Threshold Effect Fermilab

7. Secondary Electron Yield (SEY) • The number, characteristics, and process of electron production from various materials is not completely characterized. • If an electron striking a beampipe generates on average more than one secondary electron than the number of electrons in the cloud is amplified beyond the initial seed. • This is called multipactoring.

8. SEY Testing Fermilab & Cornell

9. Electron Energy & SEY Fermilab Main Injector steel beampipe material Fermilab & Cornell (eV)

10. Electron DensityMeasurement Techniques

11. Retarding Field Analysizer (RFA) • Several layers of mesh at different nonnegative potentials. • Collects electrons and measures current. • Partially sorts the electrons by energy. Fermilab

12. Microwave Phase Measurements • A microwave transmitter placed in the beampipe and BPM used as a receiver. • This setup allows measurement over a larger section of the beamline. • The delays in microwave phase proportional to electron-density x path-length. • Microwaves that have anomalous pathlengths are noise, therefore microwave reflectors are used to suppress those.

13. Secondary Electron Yield Mitigation

14. Clearing Electrodes • Clearing electrodes can localized or distributed. • Localized: Charged plate in special outlet. • Distributed: Wire hanging in beampipe. DAFNE INFN ECLOUD Simulation

15. Solenoidal Fields • Confines keV electrons without affecting MeV or GeV protons. • But need to avoid resonance- when time of flight is equal to the bunch to bunch time. resonance effect

16. Surface Grooves Fermilab

17. Beampipe Conditioning Fermilab

18. Surface Coating • TiN conditions faster and better. Fermilab • Amorphous carbon coating under testing.

19. Beam Instability andFeedback Damping

20. Characteristics of EC Instability LANL PSR

21. Characteristics of EC Instability • Broad-band mode excitation in frequency range of 25-250 MHz. • Rapid instability growth ~50us. BPM position LANL PSR • There is also significant variation in instability between pulses.

22. Coherent Tune Shift LANL PSR

23. vertical difference hybrid power amplifiers comb filter 180-deg splitter fiber optic delay low-level amp low pass filter atten rf switch BPM kicker Analog Feedback Damping LANL PSR • BPM position signal can be filtered, amplified, and delayed. • Apply pi/2 phase shift to signal in order to damp beam frequency with kicker.

24. Comb Filtering • Harmonics of revolution frequency damped. • Damping at revolution frequency doesn't seem to affect instability, just wastes power.

25. A test of EC damping system Dampening switch Proton intensity LANL PSR electron density

26. Why does the instability return after damping? • Problems with electronic implementation? • Enough power to kickers? • Dispersion in signal cables? • From instability along other axis? • Horizontal Instability → EC → Vertical • Beam accumulation between bunches. • Does it drive the betatron oscillation?

27. Electron Cloud Simulation Software

28. ORBIT Code • EC module written for ORBIT. • ORBIT allows 2D & 3D accelerator sim. • Set up for parallel computation.

29. ORBIT EC Simulation results

30. POSINST & VORPAL • POSINST & VOROAL attempt to model SEY in addition to electron movement in beampipe. • POSINST written exclusively for simulation of electron cloud by CERN. Available for free. • VORPAL new & proprietary, applicable to wider-range of plamsa physics problems.

31. POSINST & VORPAL results • In this Main Injector simulation, discrepancy traced to a bug in the POSINST code. • Now there is a pretty good agreement between VORPAL and POSINST.

32. Other Simulation Code • ECLOUD • Essentially rendered obsolete by more sophisticated codes. • only simulates 2D electron trajectory. • CLOUDLAND • Another free 3D code developed by CERN, distinct from POSINST. • WARP • “Particle in Cell” code, lattice approximation.

33. Active Areas of EC Research • How can we predict the features of electron clouds in the fullest range of accelerator parameters and operating conditions? • What is the most cost effective strategy to mitigate ECs and/or the resulting instability? • How can we measure EC effectively? • How much can we trust EC simulation? Can we improve on the simulation code?

34. Acknowledgements Acknowledgements • Much of these plots and information was taken from the IU Electron Cloud Feedback Workshop in 2007. • EC studies conducted at Fermilab Main Injector, Los Alamos Proton Storage Ring.