R&D Towards a Laser Based Beam Size Monitor for PETRA and the FLC Thorsten Kamps, Royal Holloway, FLC Group Nanobeam Workshop, Lausanne, September 2002
The next 30 minutes Motivation for the Project History and Status of Laserwire Laserwire at PETRA Environment Detector Simulations, Design and Calibration Laser Scanning, Transport and Focusing Installation − Tunnel, IR and Procedure Conclusion and Discussion
Motivation Maximise Luminosity performance of LC L = n b N 2 e f rep × H D 4 πσ ⋆ x σ ⋆ y Development of a standard diagnostic tool for LCTF and LC operation e+ Control of the transverse beam size and emittance in BDS and at IP IP e− CLIC NLC/JLC TESLA FINAL σ x [ µ m] 3.4 to 15 7 to 50 20 to 150 FOCUS BDS σ y [ µ m] 0.35 to 2.6 1 to 5 1 to 25 BDS BDS BDS x [nm] 196 335 535 σ ⋆ IP y [nm] 4.5 4.5 5 σ ⋆ LINEAR with features ACCELERATOR full reconstruction with error smaller than 10% fast (intra−train) scanning non−destructive for the electron beam INJECTION Optical scattering structures
SLC/SLD Laserwire Succesful test of laserwire principle for LC Vertex Endcap Active Region Complex installation inside SLD Detector CDC Inner Cylinder detector during shutdown Laser� e+ / e- IP Measure electron spot sizes small IP e- as 2.1 x 0.6 µm at the IP +� +� 20 50 Laser spot size 380 nm with Rayleigh Laser� to� Transport Wire� range of 5 µm Scanner 700 1.14+/- 1.8·10-3 µ m σ s = UHV Window 600 and Seal Stainless Steel� light at a repetition rate of 40Hz. 500 Optics Housing Arbitrary Units 400 pulse length. We chose YLF as the lasing material because of e- Beam 300 200 Fused� Input Laser� Silica� Absorber Beam from� 100 Imaging� Optics Bench Fiber� Bundle 0 Diagnostic� Reimaging� for measuring beam size, before the entrance to the transport System -100 -20 -15 -10 -5 0 5 10 15 20 Micrometers 5 cm
Laserwire Interest Group Elevate existing designs of laser based beam size monitors Standard diagnostic tool for LC and LCTF operation RHUL Involvement Collaborations SLAC and KEK on existing laserwire experiments DESY on the development of a fully operational laserwire for PETRA CERN on subsystems for a laserwire for CTF2 RAL on laser systems, optics, and diagnostics RHUL activities Design and test of a laser focusing and fast scanning system Photon calorimeter studies (also with regards to TESLA detector) Experiments Test of laser optics at a laserwire at CTF2 Full laserwire system test according to TESLA specs at PETRA
Laserwire for PETRA HERA Positron Electron Tandem Ring Accelerator U = 6.3 km PETRA Beam Energy E [GeV] 4.5 to 12 Beam Current I [mA] 1.55 to 1.77 7.5 to 8.5 · 10 10 Particles/Bunch Horizontal Beam Size [ µ m] 300 to 100 σ x Vertical Beam Size [ µ m] 30 to 10 σ y Currently used as injector for HERA ring HERA PETRA Upgrade to synchrotron light source U = 2.3 km Vertical beam size comparable e − DETECTOR to TESLA BDS beam γ Interest in beam size monitor for operation DIPOLE of the light source QUAD Hardware installation easy with existing access pipe and laser hut
Laser System Q−switch Nd:YAG Laser from CERN LEP polarimeter Output parameter: Wavelength λ [nm] 1064 532 Energy E [mJ] 250 90 Pulselength ∆ t [ns] 11-12 8-9 Reprate f rep [Hz] 30 30 Beam Divergence (full) θ L [mrad] 0.7 Beam Diameter d L [mm] 7 Rayleigh Range z R [m] 10 5 Laser beam emittance of fundamental mode d o · θ o = 4 π M 2 = d L θ L λ = 1 . 354 · 10 − 6 mrad = 3 . 62 d o θ o Laser currently at workshop for complete overhaul (i.e. new crystal) Not diffraction limited No clean longitudinal mode (mode−beating)
Laser Focusing VIEWPORT Requirements DIAGNOSTICS BEAM MIRROR RMS spot size at interaction smaller than vertical electron beam size, for PETRA 10 to 30 µm BEAM CHAMBER LENS Rayleigh Range larger than horizontal SYSTEM electron beam size, for PETRA 100 y ELECTRON BEAM to 300 µm ω o θ INPUT LASER PLANE BEAM Resistant against high power laser beam x σ y Beam stay clear distance of 100 mm SCANNING MIRROR due to vacuum chamber construction 2x R FOCUS f = 125 mm First order, spot size determined by 0.06 5 ω o ≃ M 2 λ f # = M 2 λ f GAUSSIAN BEAM RAYLEIGH RANGE [mm] MEASURED BEAM FOCUS SPOT SIZE [mm] 4 πω in 0.04 Solution 3 2 Commercially available laser objective 0.02 with 125 mm focal length 1 0 0 1:1 to 1:2 imaging of laser output 2 4 6 8 2 4 6 8 for transport from hut to interaction chamber INPUT BEAM RADIUS [mm] MEASURED INPUT BEAM [mm]
Fast Scanning Fast scanning system required, enabling beam profile scans within on bunch train Total scan range larger than the 337 ns PETRA PAUSE TRAIN WITH 2820 BUNCHES beamsize under measurement in 5 Hz 100 ps TESLA order to accomodate jitter or drifts 1 ps Scan resolution better than the electron beam size to measure CURRENT Scan pattern matching the bunch timing of linac/storage ring 98 ns 950 µs 200 ms TIME Mode quality preserving (diffraction σ y TESLA σ y = 20 − 30 µm = 1 − 25 µm PETRA limit) and resistant against high power of pulsed laser system Piezo driven platform with mirror Operation in discrete or continuous mode PIEZO STACK (up to 5 kHz) possible M R High damage threshold and diffraction O Y F D T A O MIRROR L limited performance B P Tests with this platform on beam quality and SIGNAL GENERATOR scanning speed in RHUL lab
Signal and Backgrounds � − y 2 � Photon Electron Scattering of moving electrons P L σ C λ 1 N C = N b exp √ c 2 h 2 σ 2 on high energy photons of laser beam 2 πσ s s Background sources P = 2 MW Synchrotron Radiation and Cosmic Rays Beam Energy [GeV] Elastic and Inelastic Gas Scattering 4.5 7 12 500/50 115/689 257/664 685/619 σ x /σ y [ µ m ] Simulation of all relevant processes using 300/30 185/1111 416/1070 1056/998 the Geant4 package with tool kits 100/10 415/2485 930/2393 2362/2231 Aiming at full simulation with realistic setup E tot [ GeV ] / N γ BACKGROUND at 4.5 GeV COMPTON SPECTRUM SIGNAL + BACKGROUND 20 5 130.000 Bunches 10 4.5 GeV 18 7 GeV 25 NUMBER OF PHOTONS [a.u.] 16 4 12 GeV 10 NUMBER OF PHOTONS NUMBER OF PHOTONS [a.u.] 14 20 3 10 12 15 10 2 10 8 10 10 6 5 4 1 2 0 0 1 2 3 4 5 6 7 0 0.5 1.0 1.5 2.0 2.5 60 70 80 90 100 110 PHOTON ENERGY [GeV] PHOTON ENERGY [GeV] PHOTON ENERGY [GeV]
DETECTOR DEPOSIT Detector Simulation 110 RELATIVE ENERGY DEPOSIT 100 Full simulation required of all relevant processes 90 80 (Compton and background) in order to specify LENGTH 230 mm 70 detector design and location 150 mm 60 70 mm 50 Requirements for material include short decay 40 PbWO CRYSTAL time (avoid pile up) and small radiation length and 30 20 γ 350 MeV Moliere radius (compactness) LENGTH WIDTH 10 0 50 100 150 200 CRYSTAL WIDTH [mm] Frameweork Geant4 with model of accelerator environment and parameter set for PETRA DETECTOR RESOLUTION 0.16 12 GeV ENERGY RESOLUTION 0.14 7 GeV Cuboid shaped detector crystal made of PbWO4 4.5GeV 3 by 3 matrix of 18x18x150 mm sized crystals 0.12 0.1 Energy resolution of better than 5% achievable 0.08 0.06 0.04 200 400 600 800 1000 DIPOLE MAGNET LASER NUMBER OF PHOTONS SIGNAL AND DETECTOR BACKGROUND FAN VACUUM WITH B−FIELD Al WALL DETECTOR CLOSE EXPERIMENTAL SETUP BEAM PIPE SIMULATION SETUP
Detector Calibration SINGLE CRYSTAL Detector studies with testbeam from DESY II 0.12 FRACTIONAL ENERGY RESOLUTION DESY II supplies beamline with electrons with 0.10 energy between 450 MeV and 6 GeV 0.08 Energy and energy width of particles are well known 0.06 Ten detector crystals made from PbWO4 were used attached to single PMT 0.04 Individual tests of all crystals 0.02 Combination of nine in detector matrix 0 1 2 3 4 5 � p 1 BEAM ENERGY [GeV] � 2 � 2 � 2 � p 2 � σ E Resolution R 2 = + p 2 = + 3 √ E E CRYSTAL MATRIX E p1: stochastic contributions 0.18 FRACTIONAL ENERGY RESOLUTION p2: noise (electronics, pile up, radioactivity) 0.16 p3: constant (inhomogenity, non−linearities) 0.14 0.12 High intrinsic fractional resolution 0.10 Full matrix less good due to variations 0.08 of individual crystals 0.06 Well within specs for PETRA experiment 0.04 0.02 Facilitate high individual resolution 0 1 2 3 4 5 by using seperate PMT for each crystal BEAM ENERGY [GeV]
Installation − Hut and Tunnel Room in hut and tunnel area readily available Some work in hut necessary before installation of laser Concrete base, water cooling, power Girder for optical table under beam pipe Construction of interaction chamber under way Mounted to optical table Consists four viewport windows WELL HUT SIBERIA PETRA TUNNEL ACCESS PIPE PETRA BEAMPIPE SHIELDING LASER TABLE TABLE
Installation − Interaction Region CCD ELECTRON BEAM PM FOCUS FPD CCD STEERING MIRROR PICKUP CEILING INERACTION CHAMBER BPM FEEDBACK LOOPS CCD STEERING SCANNER MIRROR
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