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Recent Results on the Plasma Wakefield
Acceleration at FACET
E 200 Collaboration
1) Beam loading due to distributed injection of charge
in the wake reduces the transformer ratio
2) Local ionization injection produces monoenergetic
20+ GeV bunches containing 20-40 pC charge.
Why use a Rubidium source
• Compared to previously used Li, the Rb plasma mitigates:
• 1)beam head erosion problem - v etch α IP 1.73
• 2)emittance growth due to ion motion – use heavier
atoms
Singly ionized Rb plasma is created by the electric field of
the beam. Periodic pinching of the drive beam can lead to
ionization and injection of the second Rb electron into the
wake.
This distributed injection of dark current can load the wake
and reduce the transformer ratio T = E+/E-
Experimental Set-up
Foils of different thickness and composition used
to increase the beam emittance
RbI
No Dark Current
Unmatched Beam Undergoes
Envelope Oscillations in Plasma
Dark Current
Region
RbII
ArII
Ar I
Beam radius
Rb
Neutral Rb Density
Ar
Distance (m)
The electric field at the tightly focused regions of the beam
can further ionize Rb and Ar
RbII/
ArII
Emittance of the beam used to
vary length of the wake and
thereby vary energy loss
• As the length increases
so does the number of
envelope oscillations
the beam makes.
• Each time the beam
pinches down to a
minimum it produces
Rb 2+. These new
electrons (excess
charge) are injected
into the wake
Beam Loading reduces Transformer ratio
<T> = E+/E- = ΔW+/ΔW-
PIC Simulations Confirm that
Beam Loading by Distributed Injection
of Rb 2+ Electrons Reduced T
Peak Accelerating field decreases from 44 GeV/m to 35 GeV/m
due to beam loading
Summary
• Use of Rb plasma explored for mitigating head
erosion and ion motion
• For the beam and plasma parameters used,
ionization of Rb 1+ ions leads to injection of RbII
electrons in the wake in distributed fashion
• Beam loading of the wake reduces the average
transformer ratio <T> from 1 to 0.85
• Simulations confirm the cause of beam loading as
trapping of Rb II electrons.
• Can we use ionization injection in a controlled
manner to get narrow energy spread beams??
. to Generate
Use Ionization Injection
Monoenergetic Bunches
Concept of Ionization Injection
Into a PWFA
Use Li plasma (not Rb) to
ensure no dark current. Use
He:Ar mix as buffer to
control ionization-injection
trapped charge
Simulations of Ionization Injection
(Li plasma -50/50% He-Ar Buffer Gas)
Simulations Show Monoenergetic
Bunchlets with ~1% Energy spread
Experiments Used various
Configurations
1) Li Plasma with
a) Pure He buffer (He electron injection)
b) He buffer with 10%, 22% and 50% Ar (Inject Ar II &/ He )
c) He buffer with 30% Ne ( He or Ne injection)
2) Pure Ar gas column (Ar II or Ar III injection)
All showed monoenergetic beamlets under certain beam
conditions
Mono-energietic bunchlets produced by
ionization injection
Energy (GeV)
20 24
30
50/50 Ar/He buffer gas, 30-cm long Li vapor flattop region
Bunchlets
Accelerated tail
electrons
6 images with spectometer set to image at 24 GeV
Initial
energy
1 Bunchlets are from injected electrons (0 to 20+ GeV in
30 cm)
2 Bunchlet energy gain ≥ gain of the FACET beam tail e-’s
Histogram of Trapped Charge
• Mean charge about
30 pC
The larger charge
beamlets often had
two blobs.
The lower charge
beamlets traversed
a 1mm thick W foil
placed to rule out coherent
Cherenkov emission.
Trapped Charge Histogram
60
50
40
30
20
10
0
Charge (nC)
Energy Spread of Monoenergetic
Beamlets
Energy spread of ~
0.5 GeV (FWHM) on a
25 GeV beamlet.
This corresponds to
a ~ < 1% energy
spread
Summary of Observations
• Ionization Injection has produced narrow
energy spread beamlets with energies on the
order or exceeding the beam energy
• 25 GeV energy gain from rest is observed in
just 30 cm long Li plasma
• Bunchlets typically have 30 pC of charge and a
1 % energy spread
• Process appears to be robust with rms charge
and energy spread variation of less than a
factor of 2 and a success rate of ~ 80%.