Abrami A

Battistello L

Bliss N

Bracco R

Chesworth AA

Clarke JA

Couprie ME

Dattoli G

Diviacco B

Eriksson M

Fava C

Fell B

Ferianis M

Gambitta A

Garzella D

Giannessi L

Godnig R

Hill C

Iazzourene F

Loda G

Lonza M

Marl R

Marsi M

Mazzolini F

Mezi L

Mullacrane I D

Nahon L

Nolle D

Nutarelli D

Ottaviani P L

Pangon G

Pangos N

Pasqualetto M

Poole M W

Pradal F

Quick H

Reid R J

Renault E

Roux R

Svandrlik M

Torre A

Tromba G

Trovo M

Vascotto A

Visintini R

Walker RP

Werin S

Wille K

Zangrando D

English

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THE EUROPEAN UV/VUV STORAGE RING FEL PROJECTAT ELETTRA*R.P. Walkera, on behalf of the Project  Team:A. Abramia, L. Battistelloa, N. Blissc, R. Braccoa, A.A. Chesworthc1, J.A. Clarkec, M.E. Couprieb,G. Dattolie, B. Diviaccoa, M. Erikssonf, C. Favaa, B. Fellc, M. Ferianisa, A. Gambittaa, D. Garzellab,L. Giannessie, R. Godniga, C. Hillc, F. Iazzourenea, G. Lodaa, M. Lonzaa, R. Marlc, M. Marsia,F. Mazzolinia, L. Mezie, I.D. Mullacranec, L. Nahonb, D. Nölled2, D. Nutarellib, P.L. Ottavianie,G. Pangona, N. Pangosa, M. Pasqualettoa, M.W. Poolec, F. Pradala, H. Quickd, R.J. Reidc,E. Renaultb, R. Rouxa, M. Svandrlika, A. Torree, G. Trombaa, M. Trovòa, A. Vascottoa,R. Visintinia, S. Werinf, K. Willed, D. ZangrandoaaSincrotrone Trieste, 34012 Trieste, Italy  bCEA-SPAM and LURE, Orsay, FrancecCLRC-Daresbury Laboratory, Warrington, UK  dUniversity of Dortmund, Dortmund, GermanyeENEA-Frascati, Frascati, Italy  fMAX-lab, Lund, Sweden1now at Imperial College, London;  2now at SD&M, Ratingen, Germany                                                * partly funded under EC Contract No. FMGE-CT98-0102AbstractAn overview of the European storage ring FEL projectis presented, including details of initial lasing and futureplans.1  INTRODUCTIONFree-electron lasers (FELs) are sources of tuneable,spatially coherent, monochromatic radiation with highpeak and average power. Of the various types of possibleimplementations the storage ring FEL [1,2] has a longhistory, the ACO FEL being the second FEL in the worldto operate in 1983, in the visible region of the spectrum.Since then 7 further projects have been successful infurther developing the source characteristics and inpushing the lower wavelength limit into the UV and mostrecently into the VUV (194 nm). Storage ring FELs havealso reached a stage of maturity in that user programmesare active at two centres, SuperACO and Duke University.  Storage ring FELs, while not being candidates forachieving the very shortest wavelengths, because of thedifficulties of obtaining mirrors with sufficiently highreflectivity, or electron beams with sufficiently lowemittance and high peak current, nevertheless have severalparticular characteristics that make them suitable forcarrying out a range of experiments:• continuous MHz repetition frequency• synchronization with synchrotron radiation from thesame storage ring, on a 1:1 pulse basis, allowingthe possibility of pump-probe experiments• relatively small wavelength spreadIn addition, if the storage ring in which it is installed is asynchrotron radiation user facility:• the additional cost of the FEL is relatively small• the source is automatically integrated in anenvironment that is likely to lead its best utilisationas an experimental facility.In this case the storage ring FEL can rightly be thoughtof as a relatively inexpensive additional feature, or a moresophisticated insertion device. The price to pay however isa limit to the amount of available commissioning timecompared to a dedicated FEL experiment.The European storage ring FEL project at ELETTRA[3] began officially in May 1998 with partial EC funding.It is as a collaboration between six laboratories interestedin the development of storage ring free-electron lasers,five of which operate synchrotron radiation facilities.The goals of the project are orientated towards a futureFEL user facility in the UV/VUV, providing tuneableoutput between 350 nm and at least 190 nm. Other thanpursuing development of the FEL itself, other importantissues to be addressed therefore are the compatibility withoperation of other synchrotron radiation beamlines, andthe carrying out of pilot user experiments.The main distinguishing features of the project are –• the integration of the FEL on a low emittance “third-generation” synchrotron radiation facility• the use of a helical undulator (optical klystron) toreduce power loading on the mirrors• the use of sophisticated mirror chambers that allowin-situ switching between different mirrorsIn this first report since first lasing in February 2000,we describe the layout and main parameters of the project,the main hardware items, pre-commissioning studies, firstand subsequent lasing, as well as future plans.2  LAYOUT, PARAMETERS,HARDWARE2.1  LayoutFigure 1 shows the layout of the free-electron laser inELETTRA. The optical cavity length of 32.4 m,synchronized to the 4-bunch mode of operation, is a   93Proceedings of EPAC 2000, Vienna, AustriaFigure 1: Layout of the Free-Electron Laser in the ELETTRA storage ring.convenient choice since it leads to the location of themirror chambers immediately outside the shielding wall.The possibility to install the FEL in straight 1 of the ringwas also very convenient since it allows FEL radiationfrom the upstream end of the cavity to be directed into anexisting diagnostic area, thus allowing the possibility ofmeasuring simultaneously the electron and  FEL radiationtime structures. The downstream mirror chamber isfollowed by a standard switching mirror used on severalbeamlines, thereby permitting a double use of theundulator source: during FEL operation the radiation issent to a special FEL diagnostic area, while during normaloperation the undulator radiation will be able to be sent toa synchrotron radiation beamline, presently underconstruction. This arrangement, and the choice ofbeamline, also allows a convenient third option, namelythe possibility for the beamline to carry out experimentswith the FEL radiation.  2.2  ParametersTable 1 lists the principle parameters. Further details ofthe choice of parameters can be found in [3].  2.3 Undulator/Optical KlystronThe two permanent magnet undulator sections are basedon the APPLE-2 design, and were designed and built atSincrotrone Trieste. The maximum wavelength availablein the circular polarization mode at 1 GeV, at theminimum gap of 19 mm, is 450 nm. There is someflexibility therefore to allow an increase in storage ringenergy in the future, for greater compatibility with theoperation of other SR beamlines; for example up to 290nm could be obtained at 1.25 GeV.The undulator field quality was optimised by blocksorting and by vertical block displacements, to minimiseboth phase errors and field integrals [4]. The final rmsphase errors are < 3o, and the field integrals < 2 Gm and< 3 Gm2, at any gap and phase. Adjustment of the phaseof the radiation emitted in the two undulators is carriedout by a 3-pole electromagnet. For FEL operation thisTable 1: Main Parameters of the ELETTRA FELElectron BeamEnergy 1 GeVNo. of bunches 4Total current up to 100 mANatural rms emittance 1.7 nm radNatural rms energy spread 0.04 %Natural rms bunch length 6.3 psOptical CavityLength 32.4 mDistances from mirrors to IDcentre17.7 m, 14.7 mMirror radius 17.5 mRayleigh length 4.59Stability parameter 0.72Optical KlystronUndulator type permanent magnetAPPLE-2Period length 100 mmNo. of periods 2 x 19Minimum gap 19 mmK max. (circular polarization) 5.9Max. wavelength(circular polarization, 1 GeV)450 nmPhase modulator type electromagneticNd max. (1 GeV, 350 nm) 85also allows an optical klystron mode in order to enhanceFEL gain, with maximum phase delay (Nd) equivalent to85 wavelengths (at 350 nm, 1 GeV).2.4  Mirror ChambersTo make the most of the limited beam time availablefor FEL commissioning, as well as to provide  a widerwavelength range in the operational phase, novel UHVmirror chambers have been designed and constructed atDaresbury Laboratory containing 3 remotelyinterchangeable mirrors [5]. A transfer arm and load-lockarrangement also permits mirrors to be withdrawn andProceedings of EPAC 2000, Vienna, Austria94replaced without breaking the vacuum of the mainchamber.A major concern was to avoid as much as possiblethermal distortion of the mirrors due to the synchrotronradiation and laser power. The mirrors are fitted intoOFHC copper holders with loose surface clips to holdthem in place. A thermal interface of indium/galliumeutectic between mirror and holder can be used to improvethermal conductivity. The base of the mirror holders andthe mating surface of the support plate, which is watercooled, are polished to improve heat conduction.  Each of the two identical units contains 8 motionsystems: X, Y, and Z position is adjusted using externalstepping motors; mirror change is effected using an in-vacuum motor; pitch and yaw movement is carried out byin-vacuum combined motor (coarse)+ piezo (fine) units.The mirror chambers were delivered to Trieste on January17th, one month was then needed for re-assembly,installation, alignment and bake-out.2.5  MirrorsGood quality mirrors with high reflectivity and lowabsorption are essential for the operation of a FELoscillator. In order to ensure a rapid start-up of the laser aninitial wavelength of 350 nm was chosen, in order toprofit from the experience available in characterisingmirrors for this wavelength at LURE. A number of silicaand sapphire substrates were prepared and characterised forradius of curvature, roughness, transmission, absorptionand total losses. The best substrates were subsequentlycoated with oxide multilayers for high reflectivity around350 nm and a nominal transmission of about 0.1 %.  To proceed to shorter wavelengths other mirror typesare also being explored, based on both oxide and alsofluoride multilayer coatings. In addition, tests are alsobeing made with silicon substrates, attractive because ofits higher thermal conductivity, using holes for couplingout the radiation.  3  PRE-LASING COMMISSIONING3.1  Bunch LengthFigure 2 shows the result of several measurements ofthe bunch length as a function of bunch current, using astreak camera. Measurements after installation of the newaluminium vacuum vessels in August/September 1999are consistent with previous data, showing no increasetherefore in the effective longitudinal impedance seen bythe short bunches of about 0.15 Ω.3.2  Undulator OperationThe intrinsic poor field homogeneity of the APPLEdesign, combined with the relatively high magnetic fieldand long period length, gives rise to significant focusingeffects in the ring, particularly at 1 GeV [6]. At thesettings for 350 nm operation with circular polarization51 01 52 02 53 03 50στ (ps)5 1 0 1 5 2 0 2 5I b (mA)12/12/99, 4 b.11/12/99, 4 b.28/11/99, 4 b.23/10/99, 4 b.13/10/99, 1 b.13/04/99, 1 b.13&14/04/99, 4 b.Figure 2: Bunch length as a function of current perbunch, in single and 4-bunch modes.(gap = 22.2 mm, phase = 30 mm) each undulator sectiongives rise to a tune change of about ∆Qx = - 0.04, ∆Qy =+ 0.03. To allow the undulators to be set without losingthe beam a temporary procedure had to be developedinvolving closing them one at a time, with anintermediate tune correction. Experiments have shownthat a local tune correction (i.e. using two pairs ofquadrupoles on either side of the undulator) gives asignificantly better beam lifetime than a global correction(using quadrupoles located in all straight sections).Studies are underway to explore other matching schemesas well as to prepare a program for dynamic tunecorrection during undulator closure [6].The undulators also cause a variation in closed orbit,sufficiently small however that all of the present FELwork could be carried out without use of any of thecorrection coils, which will be implemented in the future.3.3  Spectral  MeasurementsVarious measurements of undulator radiation spectrawere made, using a visible/UV monochromator, in orderto verify correct functioning of the undulator, to determinethe spontaneous emission axis, and also to allow adetermination of the electron beam energy spread. Figure3 shows an example of a spectrum taken with maximumsetting of the phase modulator (Nd=85), under FELoperating conditions.The optical klystron spectrum can be written in thefollowing form:I I f N NOK und d= + + ( )( )[ ]2 1 2cos ( )π λ λ∆where Iund  is the spectrum of one undulator and themodulation rate f is given by:f f N No d= − +( )( ) exp /8 2 2π σ γγThus, fitting spectra taken with various values of Nd  toobtain f, and plotting − ln( )f  versus 8 2 2π ( )N Nd+allows one to deduce fo  (the modulation rate for zeroenergy spread) and σ γγ /  for any given beam current.95Proceedings of EPAC 2000, Vienna, Austria0.80.60.40.20.0Intensity (a. u.)380370360350 λ (nm)Figure 3: Measured optical klystron spectrum with Nd=85and 2.8 mA/bunch.Five spectra were taken for each of 8 beam currents.The residual modulation rate fo  was high, 0.91-0.99,indicating a small effect due to transverse emittance.Figure 4 shows the resulting values of energy spread,plotted as a ratio to the natural value (0.04 %), togetherwith the results of a simultaneous measurement of bunchlength, again compared to the natural value (6.28 ps). Thetrend is clearly the same, but the increase in energy spreadis consistently less than that of the bunch length. Furthermeasurements are required to verify this result.11.522.533.54Ratio4.550 2 4 6 8 10I b (mA)στ/ (στ)0(σγ/γ) /(σγ/ γ)0Figure 4. Ratio of bunch length (upper) and energy spread(lower) with respect to nominal zero-current values as afunction of current per bunch.4  LASING AT 350 nm4.1  First LasingThe first FEL shifts began on February 18th-19th. Afteraligning the front mirror 350 nm radiation was detected atthe back mirror, reflected off the front one. During asubsequent shift on February 28th/29th the back mirror wasaligned and multiple reflections of the radiation wereobserved using a photomultiplier; to facilitate this thestorage ring was operated in single bunch mode. Furtherempirical adjustments were then made to the mirror anglesto optimise the shape of the image seen through theupstream mirror. In 4-bunch mode the streak camera wasthen used to adjust the cavity length, by minimising thetemporal width of the trapped radiation pulse. At thispoint with 17 mA total current first lasing was observed.4.2  Lasing StudiesOne of the most important parameters which affectsFEL operation is the cavity length. To achieve therequired measurement precision this is usually carried outby the equivalent method of varying the ring r.f.frequency. In the present case 100 Hz corresponds to alength change of 6.5 µm. Figure 5 shows the variation ofmeasured output power as a function of r.f. frequencydetuning. The width, which increases as expected withbeam current, is fairly typical for storage ring FELs. Sofar a maximum extracted power of 20 mW has beenrecorded, with 30 mA total beam current, in approximateagreement with expectations for the given mirrortransmission etc.14121086420Power (mW)-800 -600 -400 -200 0 200 400∆f (Hz)Figure 5: Power output as a function of cavity detuning atdifferent (total) current levels: 19 mA (upper),10.8 mA (middle), 6.6 mA (lower).The detuning affects also the spectral linewidth andlaser micropulse length as shown in fig. 6. Comparingwith fig. 5 it can be seen that minimum spectral andpulse widths are obtained at the detuning corresponding tomaximum power. Figure 7 shows the minimum profiles.The spectral width corresponds to 4.2 10-4.3025201510-600 -400 -200 0 200 4001.00.80.60.40.2detuning (Hz)∆λ(nm)∆τ(ps)Figure 6: Pulse length (upper, left scale) and spectralwidth (lower, right scale) as a function of cavity detuning,measured with 10.8 mA.The various measurements of detuning curve, thresholdlasing current (1.3 mA, total) as well as the increase inthe electron bunch length caused by lasing (71 % at 10Proceedings of EPAC 2000, Vienna, Austria96mA) all provide  estimates of the gain, which aresummarised in Figure 8. The results, given theuncertainty in the actual cavity losses, can be seen to beconsistent with theoretical predictions.300250200150100500Intensity (a. u.)356.5356.0355.5355.0354.5λ (nm)6050403020100Intensity (a. u.)260240220200180τ (ps)Figure 7: Minimum linewidth (0.15 nm FWHM) andpulse length (8.9 ps FWHM) at 350 nm.14121086420Gain (%)302520151050I tot (mA) Detuning maximum Threshold of laser Bunch  lengthening Theoretical GainFigure 8: Results of various gain estimations (points)compared to the theoretical value (curve); fixed Nd=56.5  LASING AT 220 nmNew multilayer mirrors designed for short wavelengthwere installed on May 27th. After determining empiricallythe undulator gap required for the new wavelength, andafter the usual alignment process, lasing was firstobtained at 220.5 nm with a total current of 10 mA.Changing the undulator gap allowed lasing over the range217.9-224.1 nm. A typical measured spectrum is shownin fig. 9. The width (FWHM) is 0.05 nm (∆λ/λ =2.2 10–4). A maximum power of 10 mW at 27 mA wasobtained, with a detuning curve width of 1.1 kHz FWHM.The lasing threshold was at about 3 mA.     1.00.80.60.40.20.0I (a. u.)224.2224.0223.82 23.62 23.4223.2223.0 λ  (nm)Figure 9: Laser spectrum at 223.6 nm.6  FUTURE PLANSA relatively short amount of time has been availablesince first lasing and so many topics have yet to bestudied in detail including the macrotemporal structure,wavelength tuning range, effect of phase modulatorvariation, as well as lasing stability, reproducibility andthe effects of mirror degradation etc. Another main aim isalso to proceed to shorter wavelength, below 200 nm. Inaddition, the possibility of creating linear polarization, byhaving an opposite direction of circular polarization in thetwo undulators, is of interest for future experiments.Other than FEL performance issues, mirrors are beingprepared to cover the range 280-220 nm for initial singlebeam experiments, using the new beamline which sharesthe same undulator source. Consideration will also begiven to the possibility of carrying out two beam “pump-probe” experiments next year.ACKNOWLEDGEMENTSThe strong support of C. Rizzuto (President,Sincrotrone Trieste) and D. Norman (Director ofSynchrotron Radiation, Daresbury Laboratory) isgratefully acknowledged.REFERENCES[1] M.E. Couprie, Proc. EPAC’96, p. 100.[2] M.W. Poole, Synchrotron Radiation News, Vol. 13,No. 1, 2000, p. 4.[3] R.P. Walker et al., Proc. 20th Int. FEL Conference,Williamsburg, VA, Aug. 1998, Nucl. Instr. Meth.Phys. Res. A429 (1999) 179.[4] B. Diviacco et al., “Development of EllipticalUndulators for ELETTRA”, this Conference.[5] M. W. Poole et al., “An Advanced UV OpticalCavity for the European FEL Project”, Proc. 21st Int.FEL Conference, Hamburg, Aug. 1999, Nucl. Instr.Meth. Phys. Res. A445 (2000) p. 448.[6] L. Tosi et al., “Commissioning of EllipticalUndulators in ELETTRA”, this Conference.97Proceedings of EPAC 2000, Vienna, Austria

The European UV/VUV storage ring FEL project at ELETTRA

The European UV/VUV storage ring FEL project at ELETTRA

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