by Jeffrey EldredData Analysis Workshop March 13th 2013

Intro to Electron Cloud:An experimental summary

Outline

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

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.

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.

Cloud Electron Acceleration Electron crossing on the

trailing edge of a positive bunch receives a net acceleration.

“Resonance” behavior.

secondary electrons

Beam

Net acceleration

e pipe wall

WC41

E-Detector x 4

LANL PSR

Electron Cloud Threshold Effect

Fermilab

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.

SEY Testing

Fermilab & Cornell

Electron Energy & SEY

Fermilab & Cornell

Fermilab Main Injector steel beampipe material

(eV)

Electron DensityMeasurement Techniques

Retarding Field Analysizer (RFA) Several layers of mesh

at different nonnegative potentials.

Collects electrons and measures current.

Partially sorts the electrons by energy.

Fermilab

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.

Secondary Electron Yield Mitigation

Clearing Electrodes Clearing electrodes

can localized or distributed.

Localized: Charged plate in special outlet.

Distributed: Wire hanging in beampipe.

DAFNE INFN ECLOUD Simulation

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

Surface Grooves

Fermilab

Beampipe Conditioning

Fermilab

Surface Coating

TiN conditions faster and better.

Amorphous carbon coating under testing.Fermilab

Beam Instability andFeedback Damping

Characteristics of EC Instability

LANL PSR

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

BP

M p

ositio

n

Coherent Tune Shift

LANL PSR

Analog Feedback Damping

fiber optic delay

BPM

rf switch

low pass filter

vertical difference

hybridatten

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

Comb Filtering

Harmonics of revolution frequency damped. Damping at revolution frequency doesn't

seem to affect instability, just wastes power.

Frequency response of a comb filter locked to 1 MHz

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12

Frequency(MHz)

Y(w

)

A test of EC damping system

LANL PSR

electron density

Dampening switch

Proton intensity

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?

Electron Cloud Simulation Software

ORBIT Code

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

0 200 4000.01

0.1

1

10

PSR beam line density (scaled) complete SE model (0)=0.5 (Pivi and Furman) ORBIT E-Cloud module =

ini ORBIT E-Cloud module =

ini*0.95

ele

ctro

n's

de

ns

ity

(n

C/m

)

t, nsec

ORBIT EC Simulation results

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.

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.

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.

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?

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