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

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 & Cornell Fermilab Main Injector steel beampipe material (eV)

10. Electron Density Measurement 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 INFNECLOUD 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. Amorphous carbon coating under testing. Fermilab

19. Beam Instability and Feedback 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. There is also significant variation in instability between pulses. LANL PSR BPM position

22. Coherent Tune Shift LANL PSR

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

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 LANL PSR electron density Dampening switch Proton intensity

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 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. Acknowledgements