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Studies on electron beam injection in AWAKE Run2Livio VerraBE-ABP Information Meeting07.05.2020
• Goals and setup of AWAKE Run2
• Electron-beam-seeded self modulation
• Electron beam injection studies
• Plan for upcoming measurements and diagnostics
• Conclusions
Summary
07.05.2020 L.Verra 1
Run2 goals & setup
• Acceleration of a witness electron bunch to 10s of GeV energies• Electron beam quality• 100% charge capture• emittance preservation• small energy spread
• Self-modulation (SM) of the long SPS proton bunch, seeded by a short electron bunch
2021
mid-term plan
ULTIMATE GOALS
https://arxiv.org/abs/1911.07534https://arxiv.org/abs/2002.02189
18 MeV
150 MeV
07.05.2020 L.Verra 2
MODULATOR
• We need a seeding system to trigger the proton bunch SM in a stable and reproducible way• so far, we seeded with a laser
ionization process
• We want the proton bunch to fully modulated• otherwise, the head of bunch could self-modulate in the accelerator section
with a different phase and disrupt the structure of the wakefields
Why electron beam seeding
laser pulse
Seeded Self Modulation
Self Modulation Instability
07.05.2020 L.Verra 3
18 MeV
Electron beam injection
• High charge§ Short beam precisely injected in
the accelerating & focusing phase
• Emittance preservation§ Blowout regime (nb >> npe)
• Low energy spread§ Beam loading: the witness bunch
modifies the wakefields
1
1
2
2
p+ μ-bunch
e- bunch(nb >> npe)
ion column
V. K. Berglyd Olsen, Phys. Rev. Accel. Beams 21, 011301 (2018)
07.05.2020 L.Verra 4
150 MeV
Blowout regime
• The blowout is generated by the witness bunch itself• The bulk of the electron beam travels in a pure ion
column (plasma cavity)
à the focusing force is given by Gauss’ law:
𝐸! =12𝑛" 𝑒𝜀#
𝑟
• The force is cylindrically symmetric and linear with the distance from axis
à the beam emittance can be preserved
BUT: the head of the electron bunch generates the blowout.
Is the emittance preserved there?
07.05.2020 L.Verra 5
150 MeV
Electron beam matching
• The electron beam follows the envelope equation:
𝜎$$(𝑧) + 𝐾%& −𝜀'&
𝜎( 𝑧𝜎(𝑧) = 0
𝐾! =𝜔"#𝑐 2 𝛾
focusing term, given by the ion columndefocusing term, given by the beam emittance
07.05.2020 L.Verra 6
Electron beam matching
• To satisfy the blowout and beam loading conditions everywhere, the beam size σ must not evolve along the acceleratorà beam matching to the ion column is a necessary condition
𝜎$$(𝑧) + 𝐾%& −
𝜀'&
𝜎( 𝑧𝜎(𝑧) = 0
𝜀'(
𝛾 𝜎)$1𝑛*+
=𝑞(
2𝑚+𝜖,𝑐(
𝜎 0 = 𝜀' 𝛽#𝜎$ 0 = 0
• The beam must be injected at the waist
• The plasma focusing force exactly balances the beam divergence
07.05.2020 L.Verra 7
Electron beam matching
• The beam matching condition is satisfied defining the β function at the waist:
𝛽, =2 𝜖,𝑚+ 𝑐( 𝛾
𝑛+ 𝑒(
e.g. for E = 150 MeV and npe= 7・ 1014 cm-3,𝛽0 = 4.8 mm
hence, for εN = 2 mm mradσ0 = 5.7 μm
07.05.2020 L.Verra 8
Effect of foils on the electron beam
• Electron beam injection happens on-axis, avoiding transverse plasma boundaries• Because of the setup, the electron beam crosses two foils before injection:
• laser beam dump• vacuum window
Therefore, some beam parameters are spoiled:• ε increases • β function decreases• the waist moves upstream
Laser pulse
Rb vapor
07.05.2020 L.Verra 9
https://arxiv.org/abs/1912.00779
Electron beam matching (β function)
vacuum Rb vapor
plasma
vacuum120µm aluminum
vacuum window100µm aluminum laser beam dump
(not considering acceleration and energy spread)07.05.2020 L.Verra 10
vacuum Rb vapor
plasma
vacuum120µm aluminum
vacuum window100µm aluminum laser beam dump
(not considering acceleration and energy spread)
Electron beam matching (β function)
07.05.2020 L.Verra 11
vacuum Rb vapor
plasma
vacuum120µm aluminum
vacuum window100µm aluminum laser beam dump
(not considering acceleration and energy spread)
Electron beam matching (β function)
07.05.2020 L.Verra 12
Oscillation of size induces oscillation in nb
à blowout and beam loading changesNote: plasma density ramp may lower exit angle
Emittance growth of the electron beam head
07.05.2020 L.Verra 13
In the blowout: preserved Ahead of the blowout: from 2 to 3 mm mrad
à v = cV. K. Berglyd Olsen, Phys. Rev. Accel. Beams 21, 011301 (2018)
Emittance growth of the electron beam head
07.05.2020 L.Verra 14
• The first slices of the electron beam generate the blowoutà they propagate in the focusing non-linear (with r) transverse fields generated by the train of proton microbunches.
• The beam size is much smaller than the proton bunch beam size• ~ linear force but un-matchedà emittance grows up to a saturation value
Transverse wakefields
Emittance growth
ε N/ ε
in
r [mm]
Wr
[MV/
m]
z [m]
• Electron bunch position and direction measurements before the electron beam injection(very compact and complex area, µm transverse size, mm β-function)
• Diagnostics for final emittance measuremente.g. butterfly measurement at the spectrometer; 3-screen measurement
• Diagnostics for matching:measurement of the beam size, varying the injected beam properties
Plan for Run2 diagnostics
P. MUGGLI, PRL 93, 014802 (2004)
07.05.2020 L.Verra 15
• 2020: studies on the current 18 MeV electron beamline• reach the required parameters for electron beam seeding and matching to the plasma, as
determined from theory (Gaussian bunch) and simulations (real transverse shape)• stability (energy, energy spread, charge, pointing, etc..)
• 2021: Proton bunch seeded self-modulation (SSM) with the electron bunch (18 MeV, same injector as for Run1, with possible changes to the transport line)• SSM phase reproducibility• comparison with ionization front seeding (Run1)• no Hosing Instability
Upcoming measurements
07.05.2020 L.Verra 16
• Tight schedule to meet the AWAKE Run2 goals
1. Demonstration of electron beam seedingPhase reproducible self-modulation of the whole proton bunch
2. Plasma density step measurementsNeeded to keep the wakefields constant at high amplitude
3. Build the new setupNew beamline, new vapor sources, new diagnostics, CNGS dismantling
4. Electron beam injection with blowout and beam loadingThe electron beam must be matched to the ion columnWe must consider the effect of foils, acceleration, time delay on the electron beam parameters, etc.
Conclusions
07.05.2020 L.Verra 17
Thank you for your attention!
07.05.2020 L.Verra 18
Backup slides
07.05.2020 L.Verra 19
Effect of foils on the electron beam
Crossing the foils:• ε increases• β function decreases• the waist moves upstream
waist position
07.05.2020 L.Verra 20
Effect of foils on the electron beam
Goal parameters at the injection point:• β0 = 5.1 mm• εN = 20 mm · mrad
We calculate backwards the necessary incoming beam parameters and the maximum foils thickness
07.05.2020 L.Verra 21
Electron beam matching (acceleration)
𝑞&
2𝑚"𝜖#𝑐&6 𝑛)"(𝑧) −
𝜀*&
𝛾(𝑧)𝜎((𝑧)= 0
no plasma
matched
(not considering energy spread)
If the beam is matched at the injection and accelerated, the beam size σ is reduced and small oscillations occur
à the beam keeps being matched
07.05.2020 L.Verra 22
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