studies on electron beam injection in awake run2€¦ · §beam loading: the witness bunch modifies...

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

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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

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https://arxiv.org/abs/1911.07534


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

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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)

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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?

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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

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• 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


• 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

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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

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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



(not considering acceleration and energy spread)


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vacuum Rb vapor

plasma



(not considering acceleration and energy spread)


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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

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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

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• 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)

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• 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

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• 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

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Thank you for your attention!

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Backup slides

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Crossing the foils:• ε increases• β function decreases• the waist moves upstream

waist position

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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

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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

studies on electron beam injection in awake run2€¦ · §beam loading: the witness bunch modifies...

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