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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