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A simple Model to describe Smoke Ring shaped Beam Profile - - PowerPoint PPT Presentation

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A simple Model to describe Smoke Ring shaped Beam Profile Measurements with Scintillating Screens at the European XFEL G. Kube, S. Liu, A. Novokshonov, M. Scholz DESY (Hamburg) OTR based profile measurements Optical Transition Radiation (OTR)

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A simple Model to describe Smoke Ring shaped Beam Profile Measurements with Scintillating Screens at the European XFEL

  • G. Kube, S. Liu, A. Novokshonov, M. Scholz

DESY (Hamburg)

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OTR based profile measurements

Optical Transition Radiation (OTR) principle

  • backward OTR: reflected electron beam field is measured
  • single shot measurement
  • full transverse (2D) profile information

courtesy:

  • K. Honkavaara (DESY)

Coherent OTR observation at LCLS (SLAC)

  • R. Akre et al., Phys. Rev. ST Accel. Beams 11 (2008) 030703, H. Loos et al., Proc. FEL 2008, Gyeongju, Korea, p.485.

20 40 60 20 40 60 20 40 60 20 40 60 20 40 60 20 40 60 20 40 60 20 40 60 20 40 60 20 40 60 20 40 60 20 40 60

  • strong shot-to-shot fluctuations
  • donut structure
  • measured spot isn’t a beam profile
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Scintillator based monitors @ XFEL

Scintillator based profile monitor

e-Beam Mirror Schneider macro symmar HM lens LYSO:Ce, 200 μm CCD

courtesy:

  • Ch. Wiebers (DESY)

e-Beam Scintillator 45° CCD

~ 70 monitors are used along the machine

Injector upstream L1 downstream L3 downstream Bunch Compressor 1 Collimation Section TLD upstream SASE1 downstream Bunch Compressor 2

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LYSO:Ce as the material

Scintillator based profile monitor

10

  • 2

10

  • 1

10 10

1

10

2

10

3

25 30 35 40 45 50 55 60

I / nA

 y /  m

BGO 0.5mm PWO 0.3mm LYSO 0.5mm LYSO 0.8mm YAG phosphor YAG 0.2mm YAG 1.0mm

Wire Scanner @ 31 nA

  • G. Kube et al., Proc. IPAC’10, Kyoto (Japan), 2010, p.906

OTR OTR CRY19 CRY19 LYSO LYSO BGO BGO CRY18 CRY18 LuAG LuAG YAG YAG

10 20 30 40 50 60 horizontal beam size vertical beam size [μm]

  • G. Kube et al., Proc. IPAC’12, New Orleans (USA), 2012, p.2119

LYSO:Ce best spatial resolution

  • 40
  • 30
  • 20
  • 10

10 20 30 40 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

y / m intensity / a.u.

σy = 1.44 μm

  • G. Kube et al., Proc. IBIC’15,

Melbourne (Australia), 2015, p.330 beam size in excellent agreement with independent OTR measurement

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“Smoke-ring” shape profile @ XFEL

“Smoke-ring“ shaped beam profiles

600 650 700 750 800 600 650 700 750 800 850 900 950 1000 600 650 700 750 800 500 1000 1500 2000 x [pixel] Intensity

courtesy: M. Scholz (DESY)

  • projected emittances larger than expected ~ 1 - 4 mm.mrad
  • same origin of large emittance and „smoke-ring“ shaped profiles ?

Excluded options

  • COTR contribution
  • Space charge effects from gun might lead to depopulation of bunch center
  • CCD saturation effects

suspicious:

effect of scintillator

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Screen saturation as the material

  • A. Murokh et al., in The Physics of High Brightness Beams, World Scientific (2000), p. 564.
  • A. Murokh et al., Proc. PAC‘01, Chicago (USA), 2001, p. 1333

𝐽 𝑦 = 𝐽𝑛𝑏𝑦 1 − exp − 1

2𝜌 𝜇𝑗0 𝜏 exp − 𝑦 2𝜏2

  • T. F. Silva et al., Proc. PAC‘09 , Vancouver (Canada), 2009, p. 4039

model for saturated beam profiles:

  • U. Iriso et al., Proc. DIPAC‘09 , Basel (Switzerland), 2009, p. 200

YAG:Ce / OTR measurements at ALBA

XFEL

600 650 700 750 800 500 1000 1500 2000 x [pixel] Intensity

  • R. Ischebeck, FEL2017 Santa Fe (USA), 2017, WEP039 (unpublished)

saturation of scintillators in profile monitors

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Scintillator experience

Application of inorganic scintillators in HEP Calorimetry → nonlinearity in energy measurents Explanation in terms of energy loss:

  • creation of el.magn. shower in target
  • end of shower: low energy particles
  • low energy: high energy loss

→ high ionization density track → quenching effects

Critical parameter is an ionization density

XFEL has up to 1010 particles / bunch

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Light generation inside scintillator

Application of inorganic scintillators in HEP

A.N. Vasil‘ev, Proc. SCINT’99, Moscow (Russia), 1999, p.43

Stage responsible for density effects, non-linearity effects, …

  • energy conversion
  • thermalization
  • localization
  • transfer to

luminescent centers

  • radiative relaxation

high density in ionization track (calorimetry: @ low shower particle energies) Auger-like non-radiative recombination of excitation states (e/h pairs, excitons)

Quenching effects

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9

Transfer to beam profile diagnostics

Collisional stopping power

𝑆𝐺 = ℏ𝑑 ℏ𝜕𝑞 ħωp: plasma energy

Fermi plateau:

  • saturation polarization of target material by

particle field

  • transverse field range → Fermi radius

RF: radius of ionization track → RF(LSO) ~ 3.85 nm Radiative stopping power (thin targets) LYSO screen thickness @ XFEL → t = 200 μm Bremsstrahlung mean free path length → λBS = 1.24 mm no el. magn. shower evolution Ionization track density essentially determined by primary beam particle density → not by secondary particle energies

Bethe-Bloch ~1/β2 Minimum Ionizing Particle rise of transverse particle field „Fermi plateau“

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Ionization track density

Electron passage through scintillator Low charge density beam High charge density beam

2D representation

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Beam profile model

  • 400
  • 300
  • 200
  • 100

100 200 300 400 1 2 3 4 5 6 7 8 9 x 10

4

x /  m intensity / a.u. horizontal central cut undistorted distorted

Distorted beam profile (α = 6.4×10-5) Starting point: Gaussian beam profile

J.B. Birks, Proc. Phys. Soc. A64 (1951) 874

with dE dx ∝ 𝑜𝑢 3 𝑥 = 1 1 + 𝛽 dE dx Weight factor for each point of beam profile Birks-type weight factor for scintillator saturation

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Model calculations

Qb = 0.5 nCb σx = 100 μm σy = 50 μm Qb = 0.1 nCb σx = 100 μm σy = 50 μm Qb = 1.0 nCb σx = 100 μm σy = 50 μm Qb = 0.5 nCb σx = 90 μm σy = 50 μm Qb = 0.5 nCb σx = 75 μm σy = 50 μm Qb = 0.5 nCb σx = 50 μm σy = 50 μm

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Comparison screen monitor / wire scanner

Bunch charge: Qb = 500 pCb

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Conclusions and outlook

Development of quenching model caused by high ionization track denisty due to primary beam density → quenching of excitation centers could explain appearance of smoke ring shaped beams XFEL screen monitors: perturbed beam profiles measured emittance values larger than expectet Lu2(1-x)Y2xSiO5:Ce as scintillator material

  • recent studies showed that LYSO has very low Birks parameter α → non-linear light yield
  • property of silicate based scintillators → oxygen is intimately bound to the silicon as a SiO4

4- moiety

Quest for best scintillator material: fall back on experience in HEP

  • Gadolinium-based scintillators

→ expected that charge carriers/excitons rapidly transfer their energy to excited state of gadolinium → should improve linearity

  • Yttrium Aluminium Perovskite (YAP)

→ high mobility of exciton carriers → reduced quenching probability

  • ngoing investigation at DESY (both theoretical and experimental)
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YAG / LYSO comparison

First test experiments @ XFEL both scintillators mounted in screen station OTRBW.1635.L3 E = 14 GeV, Qb = 1 nCb series of measurements → changing beam sizes in both dimensions

(measurement No. 12)

“smoke-ring” shaped beam profile and profile widening only for LYSO