DL/RAL Joint Accelerator Workshop 21 st January 2009 Free Electron Laser Studies David Dunning MaRS ASTeC STFC Daresbury Laboratory

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Free Electron Laser (FEL) Studies What is a free electron laser? And why are we interested? How does a free electron laser work? What is the current state of the art? What are we working on? ALICE oscillator FEL Seeding an FEL with HHG + harmonic jumps Mode-locked FELs including HHG amplification High-gain oscillator FELs New Light Source FELs

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 What is a free electron laser? And why are we interested? Extremely useful output properties: Extremely high brightness(>~10 30 ph/(s mm 2 mrad 2 0.1% B.W.)). High peak powers (>GW’s). High average powers – 10kW at JLAB Very broad wavelength range accessible (THz through to x- ray) and easily tuneable by varying electron energy or undulator parameters. High repetition rate. Short pulses(<100fs). Coherent Synchronisable Accelerator-based photon source that operates through the transference of energy from a relativistic electron beam to a radiation field. Molecular & atomic ‘flash photography’

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 How does an FEL work? Basic components N SS NS N S NS N B field Electron path E field B E z v x y vxvx

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Coherent emission through bunching

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 What is a FEL? NOT a quantum source! EnEn E n-1 e-e- A classical source of tuneable, coherent electromagnetic radiation due to accelerated charge (electrons) vzvz

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 r e-e- u 2nd Harmonic 3rd Harmonic Harmonics of the fundamental are also phase- matched. Resonant wavelength, slippage and harmonics

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Lose energy Gain energy Axial electron velocity r Electrons bunch at resonant radiation wavelength – coherent process Resonant emission – electron bunching

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Types of FEL – low gain and high gain Low-gain FELs use a short undulator and a high-reflectivity optical cavity to increase the radiation intensity over many undulator passes High-gain FELs use a much longer undulator section to reach high intensity in a single pass

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Low Gain – needs cavity feedback

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 ALICE IR-FEL

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Single pass high-gain amplifier Self-amplified spontaneous emission (SASE)

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Some Exciting FELs LCLS ( to 1.5Å !) XFEL ( ~6nm to 1Å !) JLAB (10kW average in IR) SCSS (down to ~1Å ) FLASH

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 FEL studies So we have low-gain oscillator FELs which have a restricted wavelength range and high-gain FELs which have no restriction on wavelength range but random temporal fluctuations in output. Recent research with ASTeC, in collaboration with the University of Strathclyde has been directed towards: Seeding an FEL with HHG (improving temporal coherence in high-gain FELs) Seeding + harmonic jumps (reaching even shorter wavelengths) Mode-locked FELs (trains of ultra-short pulses) HHG amplification with mode-locked FELs (setting train lengths in mode-locked FELs) High-gain oscillator FELs (improved temporal coherence with low-reflectivity mirrors)

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Seeding a high gain amplifier with HHG HHG Proceedings FEL 2006 New Journal of Physics 9, 82 (2007) * B W J McNeil, J A Clarke, D J Dunning, G J Hirst, H L Owen, N R Thompson, B Sheehy and P H Williams,

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Modelocking a Single Pass FEL Borrow modelocking ideas from conventional lasers to synthesise ultrashort pulses. Modelocking in conventional lasers: Cavity produces axial mode spectrum Apply modulation at frequency of axial mode spacing to lock axial modes The mode phases lock and the output pulse consists of a signal with one dominant repeated short pulse In single pass FEL we have no cavity: Produce axial mode spectrum by repeatedly delaying electron bunch by distance s between undulator modules. Radiation output consists of a series of similar time delayed radiation pulses. Lock modes by modulating input electron beam energy at frequency corresponding to mode spacing.

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 SASE Spike FWHM ~ 10fs Mode- Locked Spike FWHM ~ 400 as Mode- Coupled Spike FWHM ~ 1 fs Schematics and simulated output Neil Thompson and Brian McNeil, PRL, 2007

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January D Simulation: Mode locking mechanism Mode-locked SASE - 1D simulation

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Amplification of an HHG seed in mode-locked FEL Brian McNeil, David Dunning, Neil Thompson and Brian Sheehy, Proceedings of FEL08

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Amplified HHG – retaining structure HHG spectrum Drive λ=805.22nm, h =65, σ t =10fs

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January D Simulation: HHG amplification mechanism Amplified HHG – 1D simulation

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Amplification of an HHG seed Comparison of simulations with varying energy modulation amplitude – including case with no modulation.

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January D Simulation: HHG amplification mechanism with energy modulation period and slippage at multiple of pulse spacing Amplified HHG – increasing pulse spacing

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 High gain oscillator FELs Improving temporal coherence in high-gain FELs through the use of a low-reflectivity optical cavity Could be applied for very short wavelength FELs – where suitable seeds are not available. Builds on the 4GLS design of a high gain oscillator FEL operating in the VUV wavelength range.

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 VUV-FEL: Main features Five 2.2m undulator modules. Gain 10,000% 2mm outcoupling hole: outcoupling fraction ~75%

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 High gain oscillators at short wavelengths Very low feedback fractions are required to improve the temporal characteristics for very high gain FELs. There is an optimum feedback fraction for temporal coherence, above and below this the system reverts to SASE-like behaviour.

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Summary Low gain oscillator FELs and high gain SASE FELs are currently in operation. ALICE FEL soon to be commissioned. Schemes for improving the temporal properties of high gain FELs operating at short wavelengths are being studied. New Light Source will have three FELs in its baseline design – next stage is deciding on suitable FEL schemes and optimising designs.

David Dunning, DL/RAL Joint Accelerator Workshop 21 st January 2009 Thanks for listening. And thanks to Neil Thompson and Brian McNeil for the use of slides.