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) 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. • strong shot-to-shot fluctuations 20 20 20 40 40 40 • donut structure 60 60 60 • measured spot isn’t a beam profile 20 40 60 20 40 60 20 40 60 20 20 20 40 40 40 60 60 60 2 20 40 60 20 40 60 20 40 60
Scintillator based monitors @ XFEL Scintillator based profile monitor courtesy: Ch. Wiebers (DESY) LYSO:Ce, 200 μ m e-Beam e-Beam 45 ° Scintillator Mirror CCD CCD Schneider macro symmar HM lens ~ 70 monitors are used along the machine downstream Bunch Compressor 1 downstream Bunch Compressor 2 downstream L3 upstream SASE1 Injector Collimation Section TLD upstream L1 3
LYSO:Ce as the material Scintillator based profile monitor 60 YAG 60 CRY18 LYSO 50 LuAG 55 BGO CRY19 BGO 0.5mm YAG CRY18 PWO 0.3mm 40 50 LYSO LYSO 0.5mm OTR LuAG OTR LYSO 0.8mm [ μ m] BGO 30 YAG phosphor CRY19 45 y / m YAG 0.2mm YAG 1.0mm 20 40 35 10 30 Wire Scanner @ 31 nA 0 horizontal beam size vertical beam size 25 -2 -1 0 1 2 3 10 10 10 10 10 10 I / nA G. Kube et al., Proc . IPAC’10, Kyoto (Japan), 2010, p.906 G. Kube et al., Proc . IPAC’12, New Orleans (USA), 2012, p.2119 LYSO:Ce best spatial resolution 1 0.9 σ y = 1.44 μ m 0.8 beam size in excellent 0.7 agreement with independent intensity / a.u. 0.6 0.5 OTR measurement 0.4 0.3 G. Kube et al., Proc . IBIC’15, 0.2 0.1 Melbourne (Australia), 2015, p.330 0 -40 -30 -20 -10 0 10 20 30 40 y / m 4
“Smoke - ring” shape profile @ XFEL “Smoke -ring “ shaped beam profiles • projected emittances larger than expected ~ 1 - 4 mm.mrad • same origin of large emittance and „smoke - ring“ shaped profiles ? 600 2000 650 700 1500 750 Intensity 800 1000 850 900 500 950 0 1000 600 650 700 750 800 600 650 700 750 800 x [pixel] courtesy: M. Scholz (DESY) Excluded options • COTR contribution suspicious: • Space charge effects from gun might lead to depopulation of bunch center effect of scintillator • CCD saturation effects 5
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 T. F. Silva et al., Proc . PAC‘09 , Vancouver (Canada), 2009, p. 4039 model for saturated beam profiles: 𝐽 𝑦 = 𝐽 𝑛𝑏𝑦 1 − exp − 1 𝜇𝑗 0 𝜏 exp − 𝑦 2𝜏 2 2𝜌 XFEL U. Iriso et al., Proc . DIPAC‘09 , Basel ( Switzerland), 2009, p. 200 2000 YAG:Ce / OTR 1500 measurements at Intensity ALBA 1000 500 0 600 650 700 750 800 R. Ischebeck, FEL2017 Santa Fe (USA), 2017, WEP039 (unpublished) x [pixel] saturation of scintillators in profile monitors 6
Scintillator experience Application of inorganic scintillators in HEP Explanation in terms of energy loss: • creation of el.magn. shower in target Calorimetry → nonlinearity in energy measurents • 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 10 10 particles / bunch 7
Light generation inside scintillator Application of inorganic scintillators in HEP • energy conversion • thermalization • localization • transfer to luminescent centers • radiative relaxation A.N. Vasil‘ev , Proc . SCINT’99, Moscow (Russia), 1999, p.43 Stage responsible for density effects, non- linearity effects, … Quenching high density in ionization track (calorimetry: @ low shower particle energies) effects Auger-like non-radiative recombination of excitation states (e/h pairs, excitons) 8
Transfer to beam profile diagnostics Collisional stopping power Bethe-Bloch Fermi plateau: ~1/ β 2 • saturation polarization of target material by rise of transverse Minimum particle field particle field „Fermi Ionizing plateau “ Particle • transverse field range → Fermi radius 𝑆 𝐺 = ℏ𝑑 ħω p : plasma energy ℏ𝜕 𝑞 R F : radius of ionization track → R F (LSO) ~ 3.85 nm Radiative stopping power (thin targets) LYSO screen thickness @ XFEL → t = 200 μ m no el. magn. shower evolution Bremsstrahlung mean free path length → λ BS = 1.24 mm Ionization track density essentially determined by primary beam particle density → not by secondary particle energies 9
Ionization track density Electron passage through scintillator 2D representation Low charge density beam High charge density beam 10
Beam profile model Starting point: Gaussian beam profile Weight factor for each point of beam profile Birks-type weight factor for scintillator saturation J.B. Birks, Proc. Phys. Soc. A64 (1951) 874 1 d E 𝑥 = with d x ∝ 𝑜 𝑢 3 1 + 𝛽 d E d x Distorted beam profile ( α = 6.4 × 10 -5 ) 4 horizontal central cut x 10 undistorted 9 distorted 8 7 6 intensity / a.u. 5 4 3 2 1 -400 -300 -200 -100 0 100 200 300 400 x / m 11
Model calculations Q b = 0.1 nCb Q b = 1.0 nCb Q b = 0.5 nCb σ x = 100 μ m σ x = 100 μ m σ x = 100 μ m σ y = 50 μ m σ y = 50 μ m σ y = 50 μ m Q b = 0.5 nCb Q b = 0.5 nCb Q b = 0.5 nCb σ x = 90 μ m σ x = 75 μ m σ x = 50 μ m σ y = 50 μ m σ y = 50 μ m σ y = 50 μ m 12
Comparison screen monitor / wire scanner Bunch charge: Q b = 500 pCb 13
Conclusions and outlook XFEL screen monitors: perturbed beam profiles measured emittance values larger than expectet Lu 2(1-x) Y 2x SiO 5 :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 SiO 4 4- moiety 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 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 ongoing investigation at DESY (both theoretical and experimental) 14
YAG / LYSO comparison First test experiments @ XFEL both scintillators mounted in screen station OTRBW.1635.L3 E = 14 GeV, Q b = 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 15
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