Professor Mike Poole Free Electron Lasers Liverpool Physics Teachers’ Conference July 2010 1 M W Poole IOP Teachers’ Conference 1 July 2010 Free Electron Lasers M W Poole Ex-Director STFC Accelerator Science and Technology Centre (ASTeC) Daresbury Laboratory M W Poole IOP Teachers’ Conference 1 July 2010 Elements of a Laser All lasers contain a medium in which optical gain can be induced and a source of energy which pumps this medium Pump Optics Gain medium Light Amplification by Stimulated Emission of Radiation M W Poole IOP Teachers’ Conference 1 July 2010 Lasers at 50 - Mature ? Lasers are very bright sources of radiation Lasing media include solids, liquids and gases Extremely high powers and ultra-short pulses Mainly limited to IR-UV output (some exceptions) Tunability severely limited (in general) Power limited by thermal effects M W Poole IOP Teachers’ Conference 1 July 2010 Other Radiation Sources The Sun Fire Laboratory Black Bodies (ie thermal) Gases Radioisotopes X-ray Sets (eg rotating anodes)
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Professor Mike Poole Free Electron Lasers
Liverpool Physics Teachers’ Conference July 2010 1
M W Poole IOP Teachers’ Conference 1 July 2010
Free Electron Lasers
M W Poole Ex-Director
STFC Accelerator Science and Technology Centre (ASTeC)
Daresbury Laboratory
M W Poole IOP Teachers’ Conference 1 July 2010
Elements of a Laser
All lasers contain a medium in which optical gain can be induced and a source of energy which pumps this medium
Pump
Optics
Gain medium
Light Amplification by Stimulated Emission of Radiation
M W Poole IOP Teachers’ Conference 1 July 2010
Lasers at 50 - Mature ?
Lasers are very bright sources of radiation
Lasing media include solids, liquids and gases
Extremely high powers and ultra-short pulses
Mainly limited to IR-UV output (some exceptions)
Tunability severely limited (in general)
Power limited by thermal effects
M W Poole IOP Teachers’ Conference 1 July 2010
Other Radiation Sources
The Sun
Fire
Laboratory Black Bodies (ie thermal)
Gases
Radioisotopes
X-ray Sets (eg rotating anodes)
Professor Mike Poole Free Electron Lasers
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Particle and nuclear physics
Photon sources (alternative to laser ?)
Neutron sources
Medical
Industrial
ADS – reactors, transmutation, …
Applications of Particle Accelerators
M W Poole IOP Teachers’ Conference 1 July 2010 Cockcroft Education Lectures 2009
EM Radiation from Accelerating Charge
Non-relativistic charge source
Can this compete with a laser ?
M W Poole IOP Teachers’ Conference 1 July 2010 Cockcroft Education Lectures 2009
Fundamentals of Radiation Emission
Any charge that is accelerated emits radiation
Properties calculated since 1897 (Larmor)
Lienard and Schott studied relativistic particles on circular trajectories:
P α E4/R2
So this applies to accelerated beams of charged particles in a ring (synchrotron)
SYNCHROTRON RADIATION
=> Severe losses and energy restrictions
M W Poole IOP Teachers’ Conference 1 July 2010 Cockcroft Education Lectures 2009
Relativistic Emission
Relativistic Emission Cone
Electron rest frame
β φ
Laboratory frame
β θ
Transforming between frames
tan θ = γ -1sin φ (1 + β cos φ ) -1
if φ = 900
then: θ = γ -1
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Emission from Bends
First observed 1947 GE Synchrotron
Electron
Cone angle = 1/γ Acceleration
γ = E/m0c2
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High Field Insertion Devices Normal Straight
With Insertion Device
SRS Superconducting Wavelength Shifter 6.0 Tesla
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Wigglers and Undulators
Several successive chicanes
Combined wiggles
M W Poole IOP Teachers’ Conference 1 July 2010 Eindhoven Symposium 28/03/08
Trajectory in Multipole Wiggler Sinusoidal field with period λu and peak value B0:
Electron also has a sinusoidal trajectory.
Electron angle and displacement will be:
Maximum angle is equal to K/γ
M W Poole IOP Teachers’ Conference 1 July 2010 Eindhoven Symposium 28/03/08
K >> 1
The radiation opening angle is typically ±1/γ so there is
little overlap between radiation from different poles
2/γ
K/γ
But what if K ~ 1 ?
Interference effects can occur
2/γ
Radiation Emission from Wiggler
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Interference Condition
Electron
d
λu θ
M W Poole IOP Teachers’ Conference 1 July 2010 Cockcroft PG Education 2009
Undulator Equation
Substituting in for the average longitudinal velocity of the electron, βs :
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
M W Poole IOP Teachers’ Conference 1 July 2010 Eindhoven Symposium 28/03/08
Emission Line Shape (θ=0)
Width ~ 1/nNλ
Similar behaviour as diffraction grating with N slits
Angular spread of harmonic
For λ ~ 1nm and L ~ 5m, σr’ ~ 14 µrad
M W Poole IOP Teachers’ Conference 1 July 2010 Eindhoven Symposium 28/03/08
K < 1
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.
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K > 1 (on axis)
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, …)
M W Poole IOP Teachers’ Conference 1 July 2010 Eindhoven Symposium 28/03/08
Insertion Device Technologies
Typical periods ~ 20 - 200 mm
Typical fields ~ 0.1 – 1.0 T
Electromagnets Superconducting magnets
Permanent magnets
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
M W Poole IOP Teachers’ Conference 1 July 2010 Eindhoven Symposium 28/03/08
PPM Undulators or MPWs
g = gap between the two arrays
With 4 blocks per period the field is quite sinusoidal
M W Poole IOP Teachers’ Conference 1 July 2010 Eindhoven Symposium 28/03/08
Periodic Magnet Engineering
Daresbury Solutions
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Third Generation Light Sources
ESRF
Grenoble
6 GeV
Diamond Light Source: Harwell, Oxon
This generation introduced (almost) laser-like solutions from 1990’s onwards, based on extensive use of undulators
M W Poole IOP Teachers’ Conference 1 July 2010
Synchrotron Radiation in Space
Crab Nebula
M W Poole IOP Teachers’ Conference 1 July 2010 Eindhoven Symposium 28/03/08
Particle-Wave Interaction in Undulator
Electron-wave energy exchange (Lorentz)
Transverse modulation
Magnet couples TEM field to particle (weakly)
Resulting axial velocity modulation can cause bunching
Relative phasing controls energy gain/loss
Electron Decelerator
M W Poole IOP Teachers’ Conference 1 July 2010
Incoherent emission:
Synchrotron Radiation
Intensity ~ Ne
Coherent emission:
Free-Electron Laser (FEL)
Intensity ~ Ne2
electrons light
Next Generation Solution: Coherence
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Spontaneous Emission and Gain Curve
Gain α derivative of line
John Madey 1972
M W Poole IOP Teachers’ Conference 1 July 2010 Cockcroft PG Education 2009
Free Electron Laser (FEL) Principle
• relativistic electron beam passes through periodic magnetic field - radiates
• mirror feeds spontaneous emission back onto the beam
• spontaneous emission enhanced by stimulated emission
Oscillator illustrated
M W Poole IOP Teachers’ Conference 1 July 2010 20 March 2002 Liverpool Physics Colloquium M W Poole
The World’s First FEL at Stanford
Pioneering 1970’s studies
Infra-red output
IRFEL User Facilities follow in 1980’s and 1990’s
M W Poole IOP Teachers’ Conference 1 July 2010 20 March 2002 Liverpool Physics Colloquium M W Poole
FEL User Facilities
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FELIX – Dutch IRFEL Facility
UK undulator(s)
New high quality linac
Lased August 1991
UK Agreement 1993 ….
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FELIX Characteristics
M W Poole IOP Teachers’ Conference 1 July 2010 20 March 2002 Liverpool Physics Colloquium M W Poole
FEL Tunability Example
CLIO is French Project
No table top laser can achieve such a tuning range
M W Poole IOP Teachers’ Conference 1 July 2010 20 March 2002 Liverpool Physics Colloquium M W Poole
FEL Output Power Record
Courtesy G Neil – Jefferson Laboratory, Virginia
Energy Recovery LInac
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UV/VUV Experiments
Energy range suggests storage ring (SRFEL)
Small number of active centres
Synergy with 3rd generation light sources
Mirror problems - normal incidence
User doubts - but pump/probe attractive
M W Poole IOP Teachers’ Conference 1 July 2010 20 March 2002 Liverpool Physics Colloquium M W Poole
ELETTRA – Italian National Light Source
Trieste
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High Energy EU Funded FEL Project
M W Poole IOP Teachers’ Conference 1 July 2010 20 March 2002 Liverpool Physics Colloquium M W Poole
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 !
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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)
Mainly electron linacs but storage ring versions tried
4th Generation Sources need to employ alternative FELs
M W Poole IOP Teachers’ Conference 1 July 2010 20 March 2002 Liverpool Physics Colloquium
FELs for XUV and X-rays ?
Major electron accelerators (GeV +)
Remove mirrors - new regime (demo 1985)
Integrated USA R&D programme
European and Japanese projects
Enormous challenges - high brightness beams
Particle Physics technologies - Linear Collider synergy
M W Poole IOP Teachers’ Conference 1 July 2010 Cockcroft PG Education 2009
High Gain FEL - Single Pass
No Mirrors !
Need for very high peak currents ~ kA
• 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
Self Amplified Spontaneous Emission (SASE)
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FLASH: a High Gain FEL Light Source
J. Rossbach/DESY Nov. 2001
Converted from TESLA Test Facility (TTF) at DESY, Hamburg
bunch compressor
Early configuration before upgrades
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First TTF-FEL (FLASH) Lasing (2000)
J. Rossbach/DESY Nov. 2001 June 2010 achieved 4.5 nm at 1.2 GeV
M W Poole IOP Teachers’ Conference 1 July 2010
courtesy of P. Emma, SLAC
1.5 Å
New World Record - LCLS 2009
Use of 1/3 SLAC Linac 14 GeV
Linac Coherent Light Source
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Europe’s Answer: XFEL in 2015
1 G euro project
20 GeV 0.1 nm
UK unable to join !
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UK Developments
UK has been a world leader in advanced radiation sources
R&D concentrated at Daresbury Laboratory
FEL upgrades - harmonics and seeding
ALICE and Design Studies
Proposed National Source (4th Generation)
M W Poole IOP Teachers’ Conference 1 July 2010 20 March 2002 Liverpool Physics Colloquium M W Poole
Demonstration High Gain Harmonic Generation Experiments
Modulator Period = 8cm Field = 0.16T
L = 76cm
Radiator Period = 3.3cm
Field = 0.4T L = 2m
Dispersive L = 0.3m
Seed 10.6µm
P ~ 0.7MW
HGHG 5.3µm
P ~ 35MW
Exponential growth in gain reaches saturation
Courtesy: L H Yu, BNL, USA
Harmonic Generation
M W Poole IOP Teachers’ Conference 1 July 2010
SASE Issues – Seeding Alternative
seeded
SASE
FLASH, LCLS, XFEL are all SASE machines
Seeding improves beam quality enormously
SASE spectra are very noisy (in time and frequency)
M W Poole IOP Teachers’ Conference 1 July 2010
UK Developments – ALICE at Daresbury
Chirped beam compression ~100 fs
FEL included
‘Green’ machine: energy recovery
ALICE = Accelerators and Lasers in Combined Experiments
New vacuum challenges: photocathodes and superconducting RF systems
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View of ALICE
M W Poole IOP Teachers’ Conference 1 July 2010 Cockcroft PG Education 2009
photoinjector 3rd harmonic cavity
BC1 BC2 BC3 laser heater
accelerating modules
collimation
diagnostics
spreader
FELs
IR/THz undulators
gas filters experimental stations
Electron gun
Superconducting linac 2.2 GeV
3 FELs 50 eV–1 keV
IR undulators synchronised to FELs
Proposed New Light Source (NLS) for the UK
Active STFC/DLS design team produced CDR for May 2010
M W Poole IOP Teachers’ Conference 1 July 2010
Compared to 3rd generation storage ring based light sources, FELs can have:
• 104 higher average brightness
• 103-104 shorter pulse length
• 108 higher peak brightness
• narrower bandwidth
• full transverse coherence
• full longitudinal coherence (only if seeded )
NLS
The Best Show on Earth ?
M W Poole IOP Teachers’ Conference 1 July 2010
Conclusions
The Free Electron Laser is a revolutionary source
It is the 4th Generation development of electron beam driven synchrotron radiation sources
