Liverpool Physics Teachers Conference July

Transcription

1 Elements of a Laser Pump Optics Ex-Director STFC Accelerator Science and Technology Centre (ASTeC) Daresbury Laboratory Gain medium All lasers contain a medium in which optical gain can be induced and a source of energy which pumps this medium Light Amplification by Stimulated Emission of Radiation Lasers at 50 - Mature? Other Radiation Sources Lasers are very bright sources of radiation The Sun Lasing media include solids, liquids and gases Fire Extremely high powers and ultra-short pulses Laboratory Black Bodies (ie thermal) Mainly limited to IR-UV output (some exceptions) Gases Tunability severely limited (in general) Radioisotopes Power limited by thermal effects X-ray Sets (eg rotating anodes) Liverpool Physics Teachers Conference July

2 Applications of Particle Accelerators EM Radiation from Accelerating Charge Particle and nuclear physics Photon sources (alternative to laser?) Neutron sources Medical Industrial ADS reactors, transmutation, Non-relativistic charge source Can this compete with a laser? Cockcroft Education Lectures 2009 Fundamentals of Radiation Emission Relativistic Emission Any charge that is accelerated emits radiation Electron rest frame Laboratory frame Properties calculated since 1897 (Larmor) Lienard and Schott studied relativistic particles on circular trajectories: P α E 4 /R 2 φ β θ β So this applies to accelerated beams of charged particles in a ring (synchrotron) SYNCHROTRON RADIATION => Severe losses and energy restrictions Transforming between frames tan θ = γ -1 sin φ (1 + β cos φ ) -1 Relativistic Emission Cone if φ = 90 0 then: θ = γ -1 Cockcroft Education Lectures 2009 Cockcroft Education Lectures 2009 Liverpool Physics Teachers Conference July

3 Emission from Bends Synchrotron Radiation Electron Acceleration Cone angle = 1/γ γ = E/m 0 c 2 First observed 1947 GE Synchrotron Pulse Length (electron transit arc - photon transit chord) Typical wavelength For 2 GeV, 1.2 T: R ~ 5.5 m, γ ~ 4000 Wavelength ~ 0.1 nm Cockcroft Education Lectures 2009 Dedicated Usage: Daresbury SRS Applications - Not Time Domain Design studies completed 1975 LIGA Based on an electron storage ring 80 kev 2 GeV World s first dedicated x-ray source Linac First user programme 1981 Major UK success story for 27 years Storage Ring Booster 12 MeV 600 MeV Beamlines FMV Virus Structure (1990) Light Harvesting Complex (photosynthesis) Protein Crystallography Pioneering developments and upgrades August RIP This was the inaugural 2 nd generation solution Materials science Liverpool Physics Teachers Conference July

4 High Field Insertion Devices Normal Straight Wigglers and Undulators Several successive chicanes With Insertion Device SRS Superconducting Wavelength Shifter 6.0 Tesla Combined wiggles Cockcroft Education Lectures 2009 Trajectory in Multipole Wiggler Sinusoidal field with period λ u and peak value B 0: Electron also has a sinusoidal trajectory. Electron angle and displacement will be: Radiation Emission from Wiggler The radiation opening angle is typically ±1/γ so there is little overlap between radiation from different poles K >> 1 K/γ 2/γ Maximum angle is equal to K/γ But what if K ~ 1? Interference effects can occur 2/γ Liverpool Physics Teachers Conference July

5 Interference Condition Undulator Equation Substituting in for the average longitudinal velocity of the electron, β s : d θ λ u Electron For a 3 GeV electron passing through a 50 mm period undulator with K = 3, the wavelength of the first harmonic (n = 1) on axis (θ = 0) is ~ 4 nm Cockcroft PG Education 2009 Emission Line Shape (θ=0) K < 1 Similar behaviour as diffraction grating with N slits Observer sees the electron continuously as it oscillates by less than ~1/γ. The electric field due to this electron is then a pure sinusoidal and so there is only one harmonic. Width ~ 1/nN λ Angular spread of harmonic For λ ~ 1nm and L ~ 5m, σ r ~ 14 µrad Liverpool Physics Teachers Conference July

6 K > 1 (on axis) Insertion Device Technologies Observer sees the electron briefly as it oscillates by more than ~1/γ. The electric field due to this electron is then a series of spikes of alternating polarity. If the observer is on axis the spikes are equally spaced. The Fourier Transform of these spikes only contains odd harmonics (n = 1, 3, 5, ) Electromagnets Superconducting magnets Permanent magnets Typical periods Typical fields ~ mm ~ T Most undulators and MPWs utilise permanent magnets High fields in short periods - no power supplies or cooling Modern powerful materials - SmCo or NdFeB (1T remanence) Arrays constructed new engineering challenges PPM Undulators or MPWs Periodic Magnet Engineering g = gap between the two arrays Daresbury Solutions With 4 blocks per period the field is quite sinusoidal Liverpool Physics Teachers Conference July

7 Third Generation Light Sources Synchrotron Radiation in Space ESRF 6 GeV Grenoble Diamond Light Source: Harwell, Oxon Crab Nebula This generation introduced (almost) laser-like solutions from 1990 s onwards, based on extensive use of undulators Particle-Wave Interaction in Undulator Next Generation Solution: Coherence Electron-wave energy exchange (Lorentz) Transverse modulation Magnet couples TEM field to particle (weakly) Incoherent emission: Synchrotron Radiation Intensity ~ N e Resulting axial velocity modulation can cause bunching electrons light Relative phasing controls energy gain/loss Electron Decelerator Coherent emission: Free-Electron Laser (FEL) Intensity ~ N e 2 Liverpool Physics Teachers Conference July

8 Spontaneous Emission and Gain Curve Free Electron Laser (FEL) Principle Oscillator illustrated Gain α derivative of line John Madey 1972 relativistic electron beam passes through periodic magnetic field - radiates mirror feeds spontaneous emission back onto the beam spontaneous emission enhanced by stimulated emission Cockcroft PG Education 2009 The World s First FEL at Stanford FEL User Facilities Pioneering 1970 s studies Infra-red output IRFEL User Facilities follow in 1980 s and 1990 s Liverpool Physics Teachers Conference July

9 FELIX Dutch IRFEL Facility FELIX Characteristics New high quality linac UK undulator(s) Lased August 1991 UK Agreement FEL Tunability Example CLIO is French Project FEL Output Power Record Energy Recovery LInac No table top laser can achieve such a tuning range Courtesy G Neil Jefferson Laboratory, Virginia Liverpool Physics Teachers Conference July

10 UV/VUV Experiments ELETTRA Italian National Light Source Energy range suggests storage ring (SRFEL) Small number of active centres Trieste Synergy with 3 rd generation light sources Mirror problems - normal incidence User doubts - but pump/probe attractive 20 March 2002 Liverpool Physics Colloquium High Energy EU Funded FEL Project World Record May 1st '98 Start of EC contract Feb. 29th '00 First lasing (350 nm) Feb. 6th '01 Lasing at 190 nm - WORLD RECORD! Liverpool Physics Teachers Conference July

11 Very Limited Tuning Range FEL Oscillators - Summary Infra-red FEL operated 1977 few sources before 1990 Tunable and high power User facilities highly successful (eg FELIX in Nieuwegein) Short wavelength limited by mirrors (EUFELE <200nm) Wavelength Multilayer mirror type Lasing nm Ta 2 O 5 /SiO 2 yes nm HfO 2 / SiO 2 yes nm Al 2 O 3 / SiO 2 yes nm Al 2 O 3 / SiO 2 yes 186 nm LaF 3 /MgF 2 no Mainly electron linacs but storage ring versions tried 4th Generation Sources need to employ alternative FELs Cockcroft PG Education 2009 FELs for XUV and X-rays? High Gain FEL - Single Pass Major electron accelerators (GeV +) Remove mirrors - new regime (demo 1985) Integrated USA R&D programme No Mirrors! Self Amplified Spontaneous Emission (SASE) European and Japanese projects Enormous challenges - high brightness beams Particle Physics technologies - Linear Collider synergy electrons start emitting incoherent radiation radiation from the tail of the bunch interacts with electrons nearer the front, causing the electrons to bunch on the scale of the radiation wavelength due to the bunching, the electrons emit more coherently more radiation more bunching more radiation an instability! radiation power grows exponentially Need for very high peak currents ~ ka 20 March 2002 Liverpool Physics Colloquium Cockcroft PG Education 2009 Liverpool Physics Teachers Conference July

12 FLASH: a High Gain FEL Light Source First TTF-FEL (FLASH) Lasing (2000) Converted from TESLA Test Facility (TTF) at DESY, Hamburg bunch compressor Early configuration before upgrades J. Rossbach/DESY Nov J. Rossbach/DESY Nov Cockcroft PG Education 2009 New World Record - LCLS 2009 June 2010 achieved 4.5 nm at 1.2 GeV Europe s Answer: XFEL in 2015 Linac Coherent Light Source 20 GeV 1.5 Å 0.1 nm Use of 1/3 SLAC Linac 14 GeV UK unable to join! 1 G euro project courtesy of P. Emma, SLAC Liverpool Physics Teachers Conference July

13 UK Developments Harmonic Generation UK has been a world leader in advanced radiation sources R&D concentrated at Daresbury Laboratory Demonstration High Gain Harmonic Generation Experiments Dispersive L = 0.3m FEL upgrades - harmonics and seeding ALICE and Design Studies Proposed National Source (4 th Generation) Seed 10.6µm P ~ 0.7MW Modulator Period = 8cm Field = 0.16T L = 76cm Radiator Period = 3.3cm Field = 0.4T L = 2m Exponential growth in gain reaches saturation HGHG 5.3µm P ~ 35MW Courtesy: L H Yu, BNL, USA SASE Issues Seeding Alternative UK Developments ALICE at Daresbury SASE SASE spectra are very noisy (in time and frequency) seeded Chirped beam compression ~100 fs FEL included Green machine: energy recovery FLASH, LCLS, XFEL are all SASE machines Seeding improves beam quality enormously ALICE = Accelerators and Lasers in Combined Experiments New vacuum challenges: photocathodes and superconducting RF systems Liverpool Physics Teachers Conference July

14 View of ALICE Proposed New Light Source (NLS) for the UK experimental stations gas filters photoinjector 3 rd harmonic cavity laser heater BC1 BC2 BC3 IR/THz undulators diagnostics accelerating modules spreader collimation FELs Superconducting linac 2.2 GeV Electron gun 3 FELs 50 ev 1 kev IR undulators synchronised to FELs Active STFC/DLS design team produced CDR for May 2010 Cockcroft PG Education 2009 The Best Show on Earth? Conclusions Compared to 3 rd generation storage ring based light sources, FELs can have: 10 4 higher average brightness shorter pulse length 10 8 higher peak brightness narrower bandwidth full transverse coherence full longitudinal coherence (only if seeded ) NLS The Free Electron Laser is a revolutionary source It is the 4 th Generation development of electron beam driven synchrotron radiation sources It is challenging (and expensive)! The electron is not really free Is it a laser? (YES!!) Liverpool Physics Teachers Conference July