http://accelconf.web.cern.ch/AccelConf/IPAC2012/papers/tuppp005.pdfInternational audienceLUNEX5 is a new Free Electron Laser (FEL) source project aimed at delivering short and coherent X-ray pulses to probe ultrafast phenomena at the femto-second scale, to investigate extremely low density samples as well as to image individual nm scale objects

B. Pruvost, J.

Benabderrahmane, C.

Bessière, M.

Betinelli, P.

Bielawski, S.

Bouvet, F.

Buteau, A.

C. Denard, J.

Carré, B.

Cassinari, L.

Daillant, J.

Devanz, G.

Dubois, André

E. Couprie, M.

Evain, C.

Eymard, P.

Farvacque, L.

Gagey, B.

Herbeaux, C.

L. Marlats, J.

Labat, M.

Lambert, Guillaume

Lebec, G.

Lestrade, A.

Lifschitz, Agustin

Loulergue, A.

Luong, M.

Lüning, J.

Malka, Victor

Marchand, P.

Miron, C.

Morin, Pascal

Nadji, A.

Polack, F.

Ribeiro, F.

Ricaud, J.-P.

Rousse, Antoine

Roux, R.

Roy, Pascal

Szwaj, C.

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LUNEX5: A FRENCH FEL TEST FACILITY LIGHT SOURCE PROPOSALA. Loulergue, C. Benabderrahmane, M. Bessière, P. Betinelli, F. Bouvet, A. Buteau, L. Cassinari, M. E. Couprie, J. Daillant, J. C. Denard, P. Eymard, B. Gagey, C. Herbeaux, M. Labat, A. Lestrade, P. Marchand, J. L. Marlats, C. Miron, P. Morin, A. Nadji, F. Polack, J. B. Pruvost, F. Ribeiro,J. P. Ricaud, P. Roy, Synchrotron SOLEIL, Saint-Aubin, FranceR. Roux, Laboaratoire de l’Accélérateur Linéaire, Orsay, FranceS. Bielawski, C. Evain, C. Szwaj,  PhLAM/ CERLA, Lille, FranceG. Lambert, A. Lifschitz, V. Malka, A. Rousse, LOA, Palaiseau, FranceA. Dubois, J. Lüning, LCPMR, Paris-VI, FranceG. LeBec, L. Farvacque, ESRF, Grenoble, FranceM. Luong, G. Devanz, CEA-SACM, IRFU, Saclay, FranceB. Carré, CEA-SPAM, IRAMIS, Saclay, FranceAbstractLUNEX5 is a new Free Electron Laser (FEL) source project  aimed  at  delivering  short  and  coherent  X-ray pulses to probe ultrafast phenomena at the femto-second scale, to investigate extremely low density samples as well as  to  image individual  nm scale  objects.  The proposed machine layout is based on a 400 MeV superconducting Conventional Linear Accelerator (CLA) mainly composed of  2  XFEL  type  cryomodules  together  with  a  normal-conducting high brightness photo RF gun. Such a mature and  reliable  technology  is  able  to  deliver  high  quality electron  bunches  up  to  few  kHz  suitable  for  user experiments. Furthermore, the last decade improvements in  synchronization  and  stability  offer  a  fertile  land  to explore  the  different  and  innovative  seeded  FEL operations  aiming  at  producing  higher  coherence  and energetic X-rays for the pilot user benefits. In parallel of the CLA branch, the very promising and highly innovative Laser Wake-Field Accelerator (LWFA) enables to produce very  short  electron bunches  in  the  range of  the  femto-second and high peak current up to few GeV, as a FEL bench test using the same undulator lines. INTRODUCTIONLUNEX5 [1] objectives are manifold. The first one is to build a demonstrator of a fourth generation light source between  40  and  4  nm  with  typically  20  fs  pulses, incorporating  the  new  seeding  concepts  (seeding  with High order  Harmonics Generation (HHG) [2,  3,  4] and Echo Enable  Harmonic  Generation (EEHG) [5]).  These new  concepts  should  enable  further  FEL  sources  cost reduction.  As a second objective,  this  radiation will  be used by the pilot user experiments. In addition, as a test platform to develop the fifth generation FEL, the CLA is replaced by a LWFA [6] using the same undulator line as sketched in figure 1.CLA BRANCHSuper Conducting L-bandThe 400 MeV CLA in its baseline version delivers one electron bunch at 50 Hz. The superconducting RF, based on the XFEL technology [7] (two 1.3 GHz cryomodules (CM) and a third harmonic one), is more attractive than the  warm option,  as  it  achieves better  performance,  for comparable  investment  and  operational  costs  in  pulsed operation. The long RF pulse (1.4 ms with 500 µs flat-top) at 50 Hz opens the door to operate, as a further upgrade, with macro-pulses containing up to a hundred of bunches. Such a pulsed operation with a duty cycle around 10 % allows for the use of 2 CM, operated at 24 MV/m, with a total cryogenic heat load below 120 W at 2 K, which is suited to the capability of the first phase cryogenic station. At  low  beam  current,  the  optimum  coupling  depends mainly  on  the  level  of  peak  detuning  induced  by  the microphonics  (~15 Hz).  A  trade-off  between  cryogenic load and RF power with a reasonable duty cycle around 10 % requires lowering QL down to ~107, at the expense of a 50 % increase in peak RF power. The use of one RF transmitter system for each cavity will allow achieving the highest  stability  in  phase  and  amplitude,  which  is mandatory for the FEL performance. Although the Solid State Amplifiers (SSA) technology [8] is not yet currently used in such application (12 kW CW and up to 16 kW peak  at  1.3  GHz),  its  extreme  modularity  and  the elimination of HV equipment bring significant advantages as compared to the vacuum tubes, while the cost remains Figure 1: LUNEX5 accelerator layout.Proceedings of IPAC2012, New Orleans, Louisiana, USA TUPPP00502 Synchrotron Light Sources and FELsA06 Free Electron LasersISBN 978-3-95450-115-11611 Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)comparable.  Moreover,  the  SSA  uses  switching  mode power supplies with switching frequencies far beyond the cavity bandwidth, which results in a strong reduction of the phase noise. A last upgrade is to operate in CW mode with  an  additional  cryomodule  in  order  to  reduce  the gradient  from  24  to  16.5  MV/m  hence  keeping  the cryogenic load at a reasonable level.The GunThe  normal  conducting  1.3  GHz  RF  photo-injector developed  by  the  PITZ  collaboration  [9]  at  DESY-Zeuthen, since 2000, is regarded as a good candidate. It delivers 1 nC bunch charge with a low emittance suitable for FEL operation as required by LUNEX5. In the latest version,  the  total  transverse  normalized  emittance  has been measured down to 1 .mm.mrad  for 1 nC with an  accelerating  voltage  of  60  MV/m  and  a  flat  top  laser profile of 20 ps with a rise time of about 2 ps. The gun will be powered by a 10 MW Multi Beam Klystron. The HV will be pulsed by a commercial solid state modulator.Beam Dynamics SimulationsBeam dynamics simulations have shown that a flat top  profile (can beer like) is desirable to optimize the beam emittance compensation [10] and consequently the FEL output power. According to simulations performed with ASTRA  [11]  code  along  the  RF-gun  and  the  first cryomodule  up  to  about  200 MeV, the  total  transverse emittances are of the order of 0.9 . mm.mrad for 1 nC  bunch  and  about  50  A  peak  current.   A  magnetic compressor,  located  just  downstream  of  the  first cryomodule, further increases the peak current. To contain the  emittance  degradation,  we  opt  for  a  S-chicane compressor with rather low magnetic field. Upstream to the compressor, a third harmonic cavity with about 16.5 MV/m in opposite phase linearises the longitudinal phase space  profile.  CSRtrack  [12]  simulation  through  the compressor exhibits an horizontal  emittance increase up to  1.2  . mm.mrad,  mainly  induced  by  the  Coherent  Synchrotron Radiation (CSR) effect. The  peak current is increased  from  50  to  500  A.  The  final  section accommodates the second cryomodule, a long free section for  an  optional  additional  one,  an  achromatic  dogleg Figure  2:   Longitudinal  chirped  phase  space  and  slice parameters  along the  bunch in  the  undulator  section  at 400 MeV, (1nC).dedicated  to  collimation and finally  a matching  section for  the  downstream  4  undulalor  sections.  The  second cryomodule  is  running  on  crest  to  get  the  maximum acceleration and to reach a final energy of 400 MeV. Simulations performed along this section shows that the main total and slice parameters remain unchanged except the  relative  energy  spread  that  is  naturally  decreased below 0.02 % in the core region (Figure 2).  Optimizing the CLA working point with respect to the FEL process requires further investigations.LWFA BRANCHThe SourceAs a test platform to develop the fifth generation FEL, the CLA is replaced by a Laser Wake-Fields Accelerator (LWFA)  using  the  same  undulator  line  as  sketched  in Figure 1.  Short electron beams (1 or 2 ps rms) of about 20  pC  charge  and  few  kA  peak  current  [13]  with  an energy ranging from 0.4 to 1 GeV are obtained over a few cm acceleration. The present best energy spread, of about 1%, is still large, but reduction towards 0.1% is expected in a near future. Just like the acceleration, the transverse phase space is also very strongly focused, producing beam size  of  µm and  divergence  of  mrad  with  a  normalized emittance in the order of 1 . mm.mrad. Transfer LineTogether,  the  large  energy  spread  and  large  beam divergence  make  the  matching,  emittances  and  peak current  preservation  from  LWFA to  the  undulator  line very concerning. A way to contain these degradations is to  place  a  first  set  of  three  quadrupoles  as  close  as possible  from  the  source  with  the  drawbacks  of  large gradient. The use of small gap permanent magnet is then an issue [14]. A possible beam line with this first triplet, located at only 5 cm from the source and separated by 5 cm with a maximum gradient of 130 T/m (over 100 mm bore length) at 400 MeV, allows to contain the emittances growth  and  bunch lengthening.  Based  on  an  optimistic 0.1% energy spread, the emittances dilution induced by higher order chromatic effect is contained to about 20%. The bunch lengthening induced by the large divergence is also contained to about 100%, from 2 fs to 4 ps. Including the 20 pC space charge effect, the emittances are further increased up to 50% in both planes while the bunch length is not affected any more. All these effects mainly happen in the first matching triplet of quadrupoles. As for the CLA case,  the figure 3 plots the main slice relevant  parameters  at  the  undulator  line  entrance.  The peak current  is  still  higher  than 2 kA. A specific  slice emittance profile exhibiting a decrease from the tail to the front  is  shown.  This  correlation  emerges from particles having  a  large  divergence  (large  emittance)  which  are slowed down and drift from the head toward the tail of the bunch.TUPPP005 Proceedings of IPAC2012, New Orleans, Louisiana, USAISBN 978-3-95450-115-11612Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Synchrotron Light Sources and FELsA06 Free Electron LasersFigure  3:  Longitudinal  chirped  phase  space  and  slice parameters  along the  bunch in  the  undulator  section  at 400 MeV, (20 pC).FEL LINESingle pass Free Electron Lasers are high quality light sources, which can be operated over a very large spectral range:  from visible  to  hard  X-rays.  They provide  very high brightness and ultra-short pulses essential  to many user applications. The LUNEX5 facility aims both at the optimization  of  compact  FEL  configurations  for  short wavelength generation and at the delivery of high quality photon beams at  the targeted wavelength of  20 and 12 nm. The proposed FEL line to be implemented is of high flexibility,  i.e.  allowing for the test  and optimization of various  FEL  configurations:  amplifier,  cascade,  Echo Enable Harmonic Generation (EEHG), etc... The FEL line will  consist  in  two modulator  sections,  each  one being followed  by  a  magnetic  chicane,  and  in  four  radiator sections.  The  modulator  sections  are  short  InVac  U30 undulators  with 10 periods and the  radiator  section are long InVac U15 undulators with 200 periods.  The FEL tunability simulated with GENESIS [15] in steady state mode with an amplifier configuration over the four U15 sections and 10 kW seed is presented in Figure 4. In the case of the CLA-FEL, the tunability is achieved  changing both the harmonics seeded and the undulator gap.Figure 4 : Spectral tunability calculation of the LUNEX5 FEL with the CLA (red, green, blue) operated at 400 MeV and LWFA (black) branch with 0.1% (A) and 0.5% (B) energy spread.The  seed  wavelength  and  the  undulator  resonant wavelength can be tuned from 15 to 40 nm. In the case of the LWFA-FEL, the tunability is achieved changing the electron beam energy via the laser parameters.SYNCHRONIZATIONHigh-precision  synchronization  and  stabilization techniques are crucial for fourth and five generation light sources and seeded x-ray free electron lasers. The general principle is to use distributed optical fiber links, and to monitor  their  variations  of  optical  length  in  real  time. Two solutions  based  on  CW [16]  or  pulsed  [17]  laser have  already  been  developed  for  other  FEL  facilities. They  both  give  extremely  good  results  with  reported jitters  below 10 fs.  Parts  of  the  pulsed  option  systems being commercially available it will be  considered as the current option for LUNEX5.CONCLUSIONLUNEX5 is aiming for ultra  short  FEL pulses quest, production  and  use  for  creating  a  unique  center  of exchange of ideas and works, for setting a bridge between different scientific and technical domains, for providing a coupled  CLA-LWFA  based  test facility  for  FEL  for complementary use, for searching of scientific excellence in  setting  a  new  collaborating  project  in  the  Saclay Plateau area. It is still very flexible, aiming at advancing on  the  different  R&D  subjects.  It  will  pave  the  path towards  a  next  generation  of  light  sources  (4GLS+, 5GLS) with its vision of science. REFERENCES[1] ME. Couprie et al. LUNEX5 PROJECT, Proc. FLS 2012.[2] High-Gain Harmonic-Generation Free-Electron Laser.  L.H. Yu et al., Science 289, 932, (2000).[3] Lambert, et al., Injection of harmonics generated in gas in a Free Electron Laser providing intense and coherent extreme-UV light. G.  Nature Physics 4, 296-30, (2008)[4] T.  Togashi  et  al.  Extreme  ultraviolet  free  electron  laser seeded  with  high-order   harmonic  of  Ti:sapphire  laser,, Optics Express, 1, 2011, 317-324[5] G. Stupakov, Using the Beam-Echo Effect for Generation of Short-Wavelength Radiation.  Phys. Rev. Lett., 102, 074801 (2009) [6] K. Nakajima, Towards a table-top free-electron laser, Nature Physics, 4 (2008) 92[7] TESLA Technology Collaboration, http://tesla-new.desy.de/[8] P. Marchand, Proc. IPAC 2001, San Sebastian, Spain.[9] DESY- PITZ Collaboration, http://pitz.desy.de/collaboration[10] E. Carlsten, New Photoelectric Injector Design for the Los Alamos  National  Laboratory  XUV  FEL  Accelerator,   B. Nucl. Instr. Meth. A 285, (1989), 313[11] K. Floettman, ASTRA code, https://www.desy.de/mpyflo[12] M. Dohlus and T. Limberg, “CSRtrack”, http://www.desy.de/xfel-beam/csrtrack/[13] V. Malka et al., Principle and applications of compact laser-plasma electron accelerator.  Nature Physics 7, 219  (2011)[14] M. Fuchs et al. Laser-Driven Undulator Source, , FLS Workshop, Stanford, 04.03.2010.[15] S. Reiche, GENESIS code.[16] J. M. Byrd et al, Optics Letters Vol.34,  (Oct. 15,2009). [17] J. Kim et al., Proc. of the FEL conference (2006).Proceedings of IPAC2012, New Orleans, Louisiana, USA TUPPP00502 Synchrotron Light Sources and FELsA06 Free Electron LasersISBN 978-3-95450-115-11613 Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)

LUNEX5: A French FEL Test Facility Light Source Proposal

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