Particle Beam Diagnostics Prof. Carsten P. Welsch Further Reading . CAS Beam Diagnostics, Dourdan, France (2008) . DITANET Beam Diagnostics Schools in 2009 and 2011 . DITANET Topical Workshops – Transverse Beam Profile Monitoring, – Longitudinal beam profile monitoring, CI, UK – Beam Loss Monitoring, DESY, Germany www.liv.ac.uk/ditanet or CERN indico, search for DITANET Credits Many thanks to R. Fiorito, T. Lefèvre, E. Bravin, A. Jeff, H. Braun, R. Jones, P. Forck and S. Jolly. Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Overview . Lecture 1 („the basics“) – Transverse beam profile – Longitudinal beam profile – Beam position – Beam energy . Lecture 2 („advanced topics“) – Beam emittance – Beam halo – Non-invasive beam profile measurements This school covers wide range of charged particle beams in terms if their time structure, profile, energy and other characteristics. Diagnostics suitable for essentially and particle beam will be presented and specific challenges in a novel accelerators context highlighted. Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 The ideal diagnostics . Precise information about the beam (Sensitivity, time/spatial resolution, accuracy, DR) . Single shot diagnostics – Avoids problems with reproducibility – And timing jitter . Non-interceptive – On-line monitoring of the beam – No risk of damage by the beam itself – Important for high power beams ! Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Scintillator Screen . Default scintillator choice is Lanex - Manufactured by Kodak. - Used in Medical Physics as X-ray phosphor for imaging. - Phosphor grains on reflective backing. - Properties not particularly well documented/studied - Bulk effect – thickness affects signal. Curl- control Backing Polyester 178 microns support Phosphor Layer 50 to 305 microns Clear overcoat 7.6 microns nature.com Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Synchrotron Radiation SR appears when charged particles are accelerated (radially). 2 is Lorentz-factor 1 q c 4 P 2 60 is the bending radius 15 10 10 3 c 10 Critical frequency : c 3 5 2 10 Beam energy Beam curvature 0 I=3.5A, =1.1m 10 40MeV: --> 0.12eV (9.8m) c 80MeV: --> 0.98eV (1.2m) c Flux (ph/s/0.1%BW) -5 Limitations 10 0.1 1 10 100 . Needs many particles Photon energy (eV) . For visible light and e- require E > 150 MeV . Limited to e- and very high energy proton/heavy ions . Radiation only in bending elements, chicanes, undulators, etc. Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Cherenkov Radiation Threshold process: Particles go faster than light > 1/n . n is index of refraction (n>1) . is relativistic factor v/c . c is Cherenkov light emission angle 1 cos c n . l is the length of the Cherenkov radiator The total number of photons is 1 1 1 proportional to the thickness of N 2l 1 cherenkov 2 2 the Cherenkov radiator. a b n Limitations : n 1 . Using transparent material (glass n=1.46) t l c c . Time resolution limited by the length of the radiator Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Optical Transition Radiation (OTR) OTR is generated when a charged particle passes through the interface between two materials with different permittivity. 4x104 max = 1/ Polarization Parallel I (, ) Perpendicular I ( = 0, ) 4 e- 3x10 ve 4 2x10 1x104 Light intensity (arb.unit.) 0 -0.10 -0.05 0.00 0.05 0.10 Angle (rad) 2 # OTR photons/charged particle N ln b ln 2 1 -3 OTR 2 ~ 5 10 in [400-600] nm a Radiation wavelength Beam energy Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 OTR - Properties . Intensity is linear with bunch charge over wide range . Obtain information about beam profile, size, but also beam energy and even emittance (more later) . Source material is critical choice: r 2 dE N 2 tot 2 T (r) 2 e dx 2 c p Recipe: Use a ‘good’ material Low energy: High light yield High energy: Good thermal conductivity and high thermal limit e.g. (C, Be, SiC) limit is ~ 106 nC/cm2 Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 A „typical“ setup . Material sciences . Thermodynamics OTR Light . Electro-Magnetism . Optics . Mechanics . Electronics . Nuclear Physics Camera . … Diagnostics is exciting ! Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Optical Diffraction Radiation (ODR) ODR is generated when a charged particle passes near the edge of a dielectric and the distance to the target h satisfies the condition : Beam Radiation wavelength energy h 2 Limitation Limited # of photons in the visible for low energy particles (E < 1 GeV) and decent impact parameters (100 m). Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 What is Beam Profile? . The beam is made up of many particles which move independently. However the distribution generally stays the same. The distribution of particles plotted against x or y is the (horizontal or vertical) profile. Some instruments measure cross-section, others the profiles. Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 What is Beam Profile? . The distribution of particles often follows a Gaussian curve. Describe profile by a single number σ . Beam size is often defined as 4σ. Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Wire Scanners . A very thin wire is passed through the beam . Correlate number of particles hitting the wire to the position of the wire Profile. Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Wire Scanners . A very thin wire is passed through the beam . When the beam hits the wire there are various effects: – Some particles are lost – X-rays are generated (bremsstrahlung) – Electrons are kicked out of the wire (secondary emission) . Any of these effects can be measured. All are proportional to the number of particles hitting the wire. Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Longitudinal Diagnostics - Overview Optical Methods Bunch Frequency Spectrum . Produce visible light Shorter bunches . Analyze the light pulse using dedicated broader bunch frequency instruments spectrum RF Manipulation Laser-based beam diagnostics RF techniques to Short laser pulses convert time information & into spatial information sampling techniques Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Streak Camera Streak cameras uses a time dependent deflecting electric field to convert time information in spatial information on a CCD. M. Uesaka et al, Nucl. Instr. Meth. A 406 (1998) 371 200 fs time resolution . Using reflective optics . 12.5 nm bandwidth optical filter (800 nm) and Hamamatsu FESCA 200 Limitation - Time Resolution . Initial velocity distribution of photoelectrons - Solution: narrow bandwidth optical filter . Spatial spread of the slit image - Solution: small slit width . Dispersion in the optics Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Streak Camera Examples Observation of 5 MeV electron bunch train using Cherenkov radiation; sweep speed of 250 ps/mm Measure of bunch length = 4.5 ps (1.4 mm) using OTR and OSR Sweep speed of 10 ps/mm = 8.9 ps (2.7 mm) Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 RF Deflecting Cavity eV . Old idea from the 60’s eV0 . RF Deflector ~ relativistic streak tube z y y e z c 6 0° p Beam profile RF off Deflecting Voltage 2 2 eV 2 2 0 sin cos Beam profile RF on y y0 z c p E0 Betatron phase advance Bunch length (cavity-profile monitor) RF deflector sin∆ψ = 1, β small wavelength p Make βc large Beta function at cavity Beam energy and profile monitor P. Emma et al, LCLS note LCLS-TN-00-12, (2000) Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Examples of RF Deflectors CTF3 @ CERN LOLA @ Flash Courtesy: M. Nagl Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 RF Accelerating Structures Electron energy is modulated by the zero-phasing RF accelerating field. Bunch distribution is then deduced from the energy dispersion measured downstream. t E x Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 RF Accelerating Structures CEBAF injector, Newport News 45MeV spectrometer 1st SRF module dipole D. X. Wang et al, Physical Review E57 (1998) 2283 84 fs, 45 MeV beam but low charge beam 2nd SRF module Beam used for zero-phasing profile monitor RF off RF on Limitations . RF non-linearities . Beam loading and wakefields for high charge beam Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Laser Wire Scanner – Compton scattering Thomson/Compton scattering High power laser hsc=2 h h Scanning system - e scsc e- beam 1.0 0.9 0.8 Detection system based on 0 0.7 -24 2 / c 0.6 0 = 6.65 10 cm . The measurement of the 0.5 scattered photons 0.4 . The measurement of degraded 0.3 102 103 104 105 electrons Electron beam energy (MeV) Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Laser Wire Scanner – Compton scattering Energy spectrum of scattered Emission angle of the photons scattered photons Using a 266nm wavelength laser 10 10 GeV electron beam 10-2 10-3 2 h 1 1 0 /dE (arb. unit.) 2 -4 10 mc c -5 500 GeV electron beam Critical angle (rad) 10 0.1 1.5 TeV electron beam -6 Cross section d 10 0 300 600 900 1200 1500 1 10 100 1000 Electron energy (GeV) - rays energy (GeV) At very high energy . The photons steal most of the electron energy (electron recoil becomes extremely important) . The photons are emitted within a very small angle (a few mrad) in the forward direction . Measurement of degraded electrons only feasible at high energies Prof. C.P. Welsch – Particle Beam Diagnostics, CAS, November 2014 Laser Wire Scanner – Compton scattering ALS @ LBNL .