Intro to Electron Cloud:
An experimental summary
by Jeffrey Eldred
Data Analysis Workshop March 13th 2013
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.
–

High-energy beam particle strikes beampipe.
–

Order of one electron generation per meter,
per torr, per particle, per pass.
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.
WC41
“Resonance” behavior.
LANL
PSR
E-Detector x 4
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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 Main Injector steel beampipe material
(eV)
Fermilab
& Cornell
Electron Density
Measurement 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.

Fermilab
Amorphous carbon coating under testing.
Beam Instability and
Feedback 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.
BPM position

LANL
PSR
There is also significant variation in instability
between pulses.
Coherent Tune Shift
LANL
PSR
Analog Feedback Damping
power
amplifiers
180-deg
splitter
low-level
amp
comb
filter
fiber optic
delay
rf
switch
low pass
filter
vertical
difference
hybrid
atten
kicker


BPM position signal can be filtered,
amplified, and delayed.
Apply pi/2 phase shift to signal in order to
damp beam frequency with kicker.
BPM
LANL
PSR
Comb Filtering
Frequency response of a comb filter locked to 1 MHz
2.5
2
Y(w)
1.5
1
0.5
0
0
2
4
6
8
10
12
Frequency(MHz)


Harmonics of revolution frequency damped.
Damping at revolution frequency doesn't
seem to affect instability, just wastes power.
A test of EC damping system
Dampening switch
Proton
intensity
electron density
LANL
PSR
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.
e l e c t r o n 's d e n s i t y ( n C / m )
ORBIT EC Simulation results
P S R b ea
c o m p l e te
O R B IT E
O R B IT E
m
S
-C
-C
lin e d e n s it y ( s c
E m o d e l (0 )= 0
lo u d m o d u l e  =
lo u d m o d u l e  =
a le d )
. 5 ( P iv i a n d F u r m a n )

in i
 * 0 .9 5
in i
1 0
1
0 .1
0.0 1
0
2 00
t, ns e c
40 0
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.