First Lasing of the ALICE Free Electron Laser David Dunning On behalf of the ALICE FEL team (Jim Clarke, Neil Thompson, Mark Surman and Andy Smith), and all the ALICE team

Introduction On October 23 rd 2010, the ALICE infra-red free electron laser (FEL) at Daresbury Laboratory was successfully operated for the first time. This is the first FEL of its type to operate in the UK, and the first FEL operating with an energy recovery linac accelerator in Europe. What it is Motivation How it was done Results so far Future prospects

Why this particular free electron laser? A step towards developing a cutting- edge FEL facility for the UK. The UK doesn’t have an FEL facility, but has aspirations to develop one: The ALICE FEL has allowed the team proposing advanced FELs for these facilities to develop the skills to design, build and operate such a machine. The ALICE FEL is an infra-red oscillator FEL - it’s not the first of its kind, but it puts us in a position to seriously consider advanced concepts in a future machine. In addition, a free electron laser is kind of the ultimate test for our prototype accelerator facility. Why free electron lasers? Light sources have served as a powerful tool for scientific experiments for decades. A free electron laser is an accelerator- based light source with an exceptional combination of properties : –Applicable over a very wide wavelength range (THz to x-ray demonstrated so far) –Easily tuneable –High repetition rate –Short pulse –High brightness/peak power –… FEL facilities increasingly significant: –FLASH, LCLS Enabling new science Motivation for the ALICE FEL

How an Oscillator FEL works ELECTRONS Electron bunches arrive with a fixed repetition rate The undulator is a periodic magnetic field which causes the electrons to oscillate transversely and emit synchrotron radiation. The transverse component of the electron velocity allows the exchange of energy between the electrons and the stored radiation field An optical cavity stores the emitted radiation. The cavity length is set such that the cavity roundtrip time matches the repetition time of the electron bunches A hole in one cavity mirror outcouples a small fraction of the stored radiation The interaction causes the electrons to bunch at the radiation wavelength, and so emit coherently UNDULATOR UPSTREAM MIRROR DOWNSTREAM MIRROR Key components: electron beam, undulator and optical cavity

ALICE FEL layout UNDULATOR UPSTREAM MIRROR DOWNSTREAM MIRROR ELECTRON PATH Key components: electron beam, undulator, optical cavity + diagnostics…

Electron Beam UPSTREAM MIRROR UNDULATOR DOWNSTREAM MIRROR ELECTRON BEAM AT FEL Energy27.5MeV Bunch Charge60-80pC FWHM Bunch Length ~1ps Normalised Emittance ~12 mm-mrad Energy Spread~0.5% rms Repetition Rate16.25MHz Macropulse Duration ≤100µs Macropulse Rep. Rate 10Hz BUNCH COMPRESSOR ELECTRON PATH

UNDULATOR On loan from JLAB where previously used on IR-DEMO FEL Now converted to variable gap PARAMETERS TypeHybrid planar Period27mm No of Periods40 Minimum gap12mm Maximum K (rms)1.0 Undulator

Optical Cavity OPTICAL CAVITY Mirror cavities on kind loan from CLIO. Previously used on Super-ACO FEL PARAMETERS TypeNear Concentric Resonator Length9.2234m Mirror ROC4.85m Mirror Diameter38mm Mirror TypeCu/Au OutcouplingHole Rayleigh Length1.05m Upstream Mirror MotionPitch, Yaw Downstream Mirror MotionPitch, Yaw, Trans. UPSTREAM MIRROR DOWNSTREAM MIRROR

Diagnostics for IR radiation LASER POWER METER FEL BEAMLINE TO DIAGNOSTICS ROOM SPACE FOR DIRECT MCT DETECTOR MCT (Mercury Cadmium Telluride) DETECTOR on Exit Port 1 SPECTROMETER Based upon a Czerny Turner monochromator PYRO-DETECTOR on Exit Port 2 DOWNSTREAM ALIGNMENT HeNe

FEL Commissioning Strategy In order for the FEL to operate we needed to co-align: Undulator axis Optical cavity axis Electron beam axis

FEL Commissioning Strategy OPTICAL TARGET UNDULATOR ARRAYS DOWNSTREAM FEL MIRROR REFERENCE AXIS LASER TRACKER 1. Undulator Arrays and two Optical Targets surveyed onto Reference Axis with Laser Tracker UNDULATOR UPSTREAM MIRROR DOWNSTREAM MIRROR ALIGNMENT WEDGES OPTICAL TARGET DOWNSTREAM FEL MIRROR 2. Alignment Wedges and Downstream Mirror aligned optically using Theodolite CCD VIEWER CAMERAS 3. Downstream Mirror aligned using Upstream alignment laser (HeNe) ALIGNMENT MIRROR HeNe CCD VIEWER CAMERAS 4. Upstream Mirror aligned using Downstream alignment laser (HeNe) ALIGNMENT MIRROR HeNe CCD VIEWER CAMERAS ELECTRONS 5. Electron Beam steered to Alignment Wedges CCD VIEWER CAMERAS POWER METER MCT DETECTOR SPECTROMETER 6. Cavity length scanned looking for enhancement of spontaneous emission, then LASING. CCD VIEWER CAMERAS

ALICE IR-FEL Commissioning 2009 –November: FEL Undulator installation begins –January: FEL Undulator and Cavity Mirrors installed and aligned: all hardware in place to start commissioning. –Throughout 2010: FEL programme proceeded in parallel with installation & commissioning of EMMA, plus THz programme. One shift per day for commissioning. Of available beam-time, FEL programme got 12%. –February: First observations of undulator spontaneous emission. Stored in cavity immediately, indicating transverse pre- alignment reasonable. Scanned cavity length but no enhancement (limited to 40pC bunch charge, design was for 80pC). –May/June: Spectrometer installed and tested. First spectra of undulator spontaneous emission. Analysis of spontaneous emission used to optimise electron beam steering and focussing Also indicated pre-alignment was acceptable. 17 th November th February th June 2010

27 th July 2010 ALICE IR-FEL Commissioning 2010 (continued) –June: Strong coherent emission with dependence on cavity length Indication of correct cavity length. BUT NO LASING! Gain not exceeding losses. –Identified need to: Reduce losses Increase gain Get reliable measure of cavity length –July: Changed outcoupling mirror from 1.5mm radius to 0.75mm to reduce losses Re-gained cavity enhancement but still no lasing Installed encoder to get cavity length measurement Mirror radius of curvature tested, and matched spec. EO measurements indicated correct bunch compression. –17 th October: installed a Burst Generator to reduce laser repetition rate by a factor of 5, from 81.25MHz to 16.25MHz. This enabled us to increase the bunch charge to >60pC and hence increase the gain… 17 th October th June 2010

23 rd October 2010: ALICE FEL First Lasing First Lasing Data: 23/10/10 Simulation (FELO code)

Highest recorded average power = 32 mW –Macropulse length = 100 μs, repetition rate = 0.1 s. –Micropulse repetition rate = 16 MHz, micropulse spacing = μs, i.e pulses per macropulse, pulses per second. –Energy per pulse = 32mW/16260 = 2 μJ per pulse. –Average power within a macropulse = 32 W –Transmission efficiency of vacuum window=0.7 – so power from FEL ~1.4x higher. The FEL pulse duration has been inferred from the spectral width to be ~1 ps The peak power is therefore ~3 MW. Results

Further Progress FEL Spectra at Different Gaps The first demonstration of wavelength tuning of the ALICE FEL has been carried out. By increasing the undulator gap the wavelength was continuously tuned from 8.0 μm to 5.7 μm, with lasing maintained throughout the range.

Further Progress The FEL radiation has been transported to a diagnostics room, and the first measurements of the transverse profile have been made.

Improved electron beam set-ups with reduced energy spread and jitter. Improve transport of FEL beam to diagnostics room, then full output characterisation. Reduced Mirror ROC to improve gain, plus selection of outcoupling hole sizes to optimise output power. Plan to run and characterise at two different energies –27.5MeV (5-8µm) –22.5MeV (7-12µm) Simulation results ALICE FEL Future Plans

Summary Further work on characterising the FEL performance and output is continuing, with lasing now achieved routinely. The successful operation of the free electron laser is a significant achievement for the team, and provides invaluable experience for future FEL facility proposals. Thanks to everyone who contributed to the project, and thanks to you all for listening!

ALICE FEL Systems Schematic (OPTICAL TARGET) POWER METER MCT DETECTOR SPECTROMETER INFRA-RED ELECTRONS ALIGNMENT MIRROR HeNe ALIGNMENT WEDGES CCD VIEWER CAMERAS ALIGNMENT MIRROR HeNe ELECTRONS ALICE FEL Layout UNDULATOR UPSTREAM MIRROR DOWNSTREAM MIRROR