positron source 30042020 - cern

30/04/2020 I. Chaikovska IJCLab/IN2P3/CNRS

Contents for a parametric study around positron source


FCC-ee positron source

e-

1.4 mm 3.4X0 (12 mm)1.5 m

4.5X0 (16 mm)

The complete filling for Z running (most demanding) => Requirement @ DR: ~2.1 ⨯ 1010 e+/bunch (4.3 nC)~0.5 e+/e- without safety factor

Primary e- beam for e+ productionBeam energy 4.46 GeVBunch charge 4.2 ⨯ 1010

Bunch length (rms) 1 mmBunch transv. size (rms) 0.5 mm

Bunch separation ~ß60 nsNb of bunches per pulse 2

Repetition rate 100-200 HzBeam power 12 kW

*Alternative option: 20 GeV linac as the FCC-ee injector => higher energy for e+ production

Conventional scheme Hybrid scheme

Present baseline: SuperKEKB-like scheme => fixed target (has a hole for e- beam passage), the positrons from the target share the same linac with electrons => additional degradation of the positron yield. Design a bypass line for e+ generation/capture.

•Current studies: both schemes can provide the comparable e+ yield (> 1 Ne+/Ne− ) accepted by the DR.

• As far as reliability of the target is concerned, the hybrid scheme is more attractive allowing lower deposited power and Peak Energy Deposition Density (PEDD) in the production target.

Goal: robust design of the e+ source for a stable long-term operation


Positron source performancesDemonstrated (a world record for the existing accelerators): SLC e+ source: ~0.08e14 e+/s

SLC CLIC (380 GeV)

ILC (250 GeV)

LHeC (pulsed)

LHeC (ERL)

LEMMA FCC-ee

e- beam energy(GeV) 45.6 380 250 140 60 45 (e+) 45.6Bunches/macropulse 1 352 1312 105 2Repetition Rate 120 50 5 10 CW 200 (Inj)Bunches/second 120 17600 6560 106 20×106 400e+/second (1014) 0.08 1.1 1.3 18 440 100 0.1(Inj)Polarization No No/Yes Yes Yes Yes No No


Positrons sources: classical schemeElectron primary beam source

Electron primary beam accelerator

Capture section

AMD/QWTLens

ConverterTarget

e+-e- separatore+

e-DUMP

Injection Line

Post acceleration

Beam shaping

Damping ring

e-

e+

Accepted e+ flux is a function of target + capture system + primary beam characteristics!


e- primary beam

Positrons sources: parametric studiesenergy, charge, time structure, beam size@target

Options for production energy:3 specifically interesting points for e+ production from presently considered transfer energies: • 6 GeV-DR energy ~4.5 GeV (present baseline) • 20 GeV-DR energy ~18.5 GeV (if circular pre-booster (SPS) replaced by higher energy linac) • X GeV to achieve 1:1 production which might be best for operation reasons


e+ production

Positrons sources: parametric studiesproduction scheme, targets characteristics (material, thickness), deposited power, Peak Energy Deposition Density (PEDD)…

Options for production scheme:• Conventional scheme (bremsstrahlung and pair conversion)• Hybrid scheme (oriented crystal as a photon radiator + target-converter or just oriented thick crystal as a radiator/converter) => collaboration with INFN-Ferrara to reoptimize the crystal target.

This phase provides the input for target design, reliability studies (mechanical stress, fatigue limit, shock waves and thermal dynamics), choice of stationary or moving target (pendulum, rotating wheel...), target cooling system, the bypass line… Collaboration with CERN.


Positrons sources: parametric studiesmatching device (peak and low field, aperture, length), its realistic design and field profile, DC solenoid field (realistic field profile), capture linac (L- or S-band, acc. gradient, phase, aperture), different techniques of e+ capture, beam steering and diagnostics…

Options for the matching device:• Large-band system: Adiabatic Matching Device (AMD) => Flux Concentrator (SLAC, KEK, BINP) or SC solenoid (considered at KEK in 2009 - 2011)

• Narrow-band system: Quarter Wave Transformer (QWT) => a short pulsed lens with a high magnetic field just after the target (Orsay, CERN/LEP, LNF, KEK…)

e+ collection/capture (matching device + capture linac)

This phase provides the input for target, magnet and RF design (collaboration with CERN and PSI). Collaboration with BINP on the FC solution. Estimation of the accepted e+ yield.


e+ linac and injection into the DR

Positrons sources: parametric studiesmatching section between the capture linac (solenoid focusing) and following FODO system, e+ linac design, beam steering and diagnostics, energy compressor if needed, DR design (energy, injection acceptance)

This phase provides the accepted e+ yield. e+ linac design (CERN and PSI). Collaboration with the LNF for the DR parameters.


Positrons source: goals

General approach: start optimisation work with parameters for the nominal FCC-ee. Later, based on the obtained results, we can deduce the parameters for the experiment.

• Input for the FCC-ee injector general parameter table •Start-to-end beam dynamics simulation and optimisation of the beam transport through the whole e+ injector. Investigation of the bypass line for e+ generation/capture.

• Evaluation of radiation level/environment in the e+ source region and required shielding.

• Cost estimation and optimization for the proposed e+ source design.• Specifications for the beam test at PSI.


SLC LEP (LIL) KEKB/SKEKB FCC-ee*Incident e- beam energy 33 GeV 200 MeV 4.3/3.5 GeV 4.46 GeV

e-/bunch [1010] 3-5 0.5 - 30 (20 ns) pulse) 6.25/6.25 4.2Bunch/pulse 1 1 2/2 2

Rep. rate 120 Hz 100 Hz 50 Hz/50 Hz 100-200 HzIncident Beam power ~20 kW 1 kW (max) 4.3 kW/3.3 kW 12 kWBeam size @ target 0.6 - 0.8 mm < 2 mm />0.7 mm

Target thickness 6X0 2X0 /4X0

Target size 70 mm 5 mm 14 mmTarget Moving Fixed Fixed/Fixed Moving

Deposited power 4.4 kW /0.6 kWCapture system AMD λ/4 transformer /AMD AMDMagnetic field 6.8T->0.5T 1 T->0.3T /4.5T->0.4T

Aperture of 1st cavity 18 mm 25mm/18 mm /30 mmGradient of 1st cavity 30-40 MV/m ~10 MV/m /10 MV/m

Linac frequency 2855.98 MHz 2998.55 MHz 2855.98 MHze+ yield @ CS exit ~4 e+/e- ~3 ⨯10-3 e+/e- (linac exit) ~0.1/~0.5 e+/e-

Positron yield @ DR ~1.2 e+/e- NO/0.4 e+/e-DR energy acceptance +/- 2.5 % +/- 1 % (EPA) +/- 1.5 % (1 σ) +/- 4 %

Energy of the DR 1.15 GeV 500 MeV NO/1.1 GeV 1.54 GeV

*FC

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positron source 30042020 - cern

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