We present an updated design for a proposed source of ultra-fast  synchrotron radiation pulses based on a recirculating superconducting linac,  in particular the incorporation of EUV and soft x-ray production. The project  has been named LUX - Linac-based Ultrafast X-ray facility. The source produces  intense x-ray pulses with duration of 10-100 fs at a 10 kHz repetition rate,  with synchronization of 10 s fs, optimized for the study of ultra-fast  dynamics. The photon range covers the EUV to hard x-ray spectrum by use of  seeded harmonic generation in undulators, and a specialized technique for  ultra-short-pulse photon production in the 1-10 keV range. High-brightness rf  photocathodes produce electron bunches which are optimized either for coherent  emission in free-electron lasers, or to provide a large x/y emittance ration  and small vertical emittance which allows for manipulation to produce  short-pulse hard x-rays. An injector linac accelerates the beam to 120 MeV,  and is followed by four passes through a 600-720 MeV recirculating linac. We  outline the major technical components of the proposed facility

Barletta, W A

Corlett, J

De Santis, S

Doolittle, L

Fawley, W

Heimann, P A

Leone, S

Li, D

Lidia, S M

Parmigiani, F

Pirkl, Werner

Placidi, Massimo

Ratti, A

Rimmer, R A

Robinson, K

Schönlein, R W

Staples, J

Wan, W

Wang, S H

Wells, R P

Wolski, A

Zholents, A

Corlett, J.N

Barletta, W.A.

DeSantis, S.

Doolittle, L.

Fawley, W.M.

Green, M.A.

Heimann, P.

Leone, S.

Lidia, S.

Li, D.

Ratti, A.

Robinson, K.

Schoenlein, R.

Staples, J.

Wan, W.

Wells, R.

Wolski, A.

Zholents, A.

Placidi, M.

Pirkl, W.

Parmigiani, F.

eScholarship - University of California

Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleA recirculating linac-based facility for ultrafast X-ray sciencePermalinkhttps://escholarship.org/uc/item/56k8g9dsAuthorsCorlett, J.NBarletta, W.A.DeSantis, S.et al.Publication Date2003-05-06eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaA RECIRCULATING LINAC-BASED FACILITY FOR ULTRAFAST X-RAYSCIENCE*J. N. Corlett, W. A. Barletta, S. DeSantis, L. Doolittle, W. M. Fawley, M.A. Green, P. Heimann, S. Leone, S. Lidia,D. Li, A. Ratti, K. Robinson, R. Schoenlein, J. Staples, W. Wan, R. Wells, A.Wolski, and A. Zholents, LBNL,Berkeley, California, USAM. Placidi, W. Pirkl, Geneva, SwitzerlandF. Parmigiani, UCSC, Brecia, Italy                                                * This work supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098Abstract We present an updated design for a proposed source ofultra-fast synchrotron radiation pulses based on arecirculating superconducting linac [1,2], in particular theincorporation of EUV and soft x-ray production. Theproject has been named LUX – Linac-based Ultrafast X-rayfacility. The source produces intense x-ray pulses withduration of 10-100 fs at a 10 kHz repetition rate, withsynchronization of 10’s fs, optimized for the study ofultra-fast dynamics. The photon range covers the EUV tohard x-ray spectrum by use of seeded harmonic generationin undulators, and a specialized technique for ultra-short-pulse photon production in the 1-10 keV range. High-brightness rf photocathodes produce electron buncheswhich are optimized either for coherent emission in free-electron lasers, or to provide a large x/y emittance rationand small vertical emittance which allows formanipulation to produce short-pulse hard x-rays. Aninjector linac accelerates the beam to 120 MeV, and isfollowed by four passes through a 600-720 MeVrecirculating linac. We outline the major technicalcomponents of the proposed facility.1 OVERVIEWA recirculating linac accelerates 2 ps electron bunchesto 2.5-3 GeV, where the bunches radiate in multipleinsertion devices. Intense soft x-rays are produced bycascaded harmonic generation scheme, similar to high-gainharmonic-generation (HGHG) - a laser-seeded process in acascaded series of undulators, resulting in enhancedradiation at selected harmonics of the seed [3]. Thecoherent soft x-rays can be tuned over a range of tens ofeV to 1 keV, and ultrashort seed laser pulses produce pulsedurations of 10-200 fs.  Hard x-rays are produced byspontaneous emission of the electrons in narrow-gap,short-period undulators. By use of a novel bunch tiltingprocess followed by optical compression, hard x-ray pulsedurations of 50-100 fs are obtained over a range of 1-10keV [1,4].  Synchronization of the x-rays with lasers iscritical for experiments, and optical pulses initiate bothcascaded harmonic generation seed lasers and experimentalend station amplifiers for precise timing [1,5]. Thefemtosecond x-rays are produced at a 10 kHz repetitionrate, with variable polarization, and with peak fluxescomparable to third generation light sources. Thus, theFigure 1. Machine layout. The beams generated at the rfphotocathode guns travel through the injector linac, mainlinac, transport arcs, deflecting cavities, and either hard x-ray production section or a cascaded harmonic generationsection, to the beam dump. Machine footprint isapproximately 150x50 m.proposed LUX facility would provide stable, synchronized,tunable, ultrafast x-ray pulses to multiple beamlinesoperating simultaneously over a broad range of x-raywavelengths.2 ACCELERATOR DESIGNElectron pulses are produced at a rate of 10 kHz in high-brightness rf photocathode guns [6]. Two sources are used- one with a conventional circular cross-section beamoptimized for production of high-brightness EUV and softx-ray radiation in the harmonic generation scheme, theother with a flattened cross-section beam for production ofhard x-rays [7]. The beam quality requirements of the rfphotocathode guns are already demonstrated, withnormalized emittance of approximately 3 mm-mrad at 1nC charge, although higher repetition rates have not beenaddressed to date. Conventionally, rf photocathode gunsemploy a simple right-circular geometry or "pillbox"Cascaded harmonicgeneration in FEL’sHard x-ray production in narrow-gapshort-period insertion devicesMagnetic transportsections for each ringSuperconductinglinacsRfphotocathodegunsBeamlineendstationsshaped cavities, and operate over 10-100 Hz pulserepetition rate. The cathode is mounted on the cavity axisand electrons are rapidly accelerated by the rf electric fields,minimizing space-charge effects in the low-energy beam.For high duty cycle operation, thermal limitations preventsuch designs from operating at an electric field sufficientlyhigh to produce good beam emittance. For the LUXfacility we have produced a conceptual design optimizedfor operation at high gradient and high repetition rate, andproducing low-emittance bunches. This designincorporates features that increase cavity surface area toreduce deposited power density and enhance theaccelerating electric field at the cathode (82 W/cm2maximum power density for 64 MV/m at the cathode).Application of a solenoidal magnetic field on thecathode of the flat-beam gun, followed by a speciallyconfigured skew-quadrupole channel located after theinjector linac, allows production of a “flat” beam with x/yemittance ratio 50/1 and small vertical normalizedemittance of 0.4 mm-mrad. This technique has beensuccessfully demonstrated at Fermilab, with propertiesclose to LUX design parameters [8].Following the injector linac, a third-harmonic cavity isused to linearize the correlated energy spread introduced inthe injector linac, and to manipulate the longitudinalphase-space in preparation for bunch compression. Thebeam is then transported to the entrance of therecirculating linear accelerator. In the transport line frominjector linac to main linac, the bunches are compressedfrom 20 ps to 2 ps, with a final energy spread of ±200keV. This transport line is carefully designed tocompensate for the effects of coherent synchrotronradiation at the shorter bunch lengths [9].In the recirculating linac the maximum energy of 3GeV is achieved after four passes through the 720 MeVsuperconducting rf structure, or 2.5 GeV with the mainlinac operating at 600 MeV. Identical cryomodulescontaining multiple accelerating cavities are used for themain linac and the injector linac. The superconductinglinacs have advantages in providing a compact andefficient accelerator, extremely stable rf fields, andinherently small perturbative effects on the beam.Significant advances have been made in superconducting rftechnology in recent years, and the parameters of theproven TESLA superconducting rf systems developed atDESY have been used in LUX design studies [10].Planned upgrades for the CEBAF facility at TJNAF alsomeet the requirements for the LUX linacs [11]. Our designis for an accelerating gradient of up to 20 MV/m in themain linac. The electron bunch repetition interval in LUXis less than the superconducting cavity filling time, andthe linacs are more efficiently operated in cw mode.Engineering modifications to exisiting cryomoduledesigns required to allow significantly increased thermalload in the liquid helium are described in [1].The flexibility of the LUX lattice design allows controland preservation of electron beam transverse andlongitudinal emittances, minimizing the influence ofcollective effects [12]. Longitudinal and transversedynamics have been modeled from the RF gun through theinjector linac and all passes of the main linac. In theinjector, a harmonic cavity will be used to control thelongitudinal phase-space following the injector linac [9].The bunch length and magnet bend angle in the lowestenergy arcs of the machine result in a regime in whichcoherent synchrotron radiation emission could be expected,and the vacuum chamber geometry is designed tominimize this effect by shielding against lower-frequencyradiation. The recirculating ring arcs are achromatic andisochronous to preserve beam quality. Our studiesincluding particle tracking with cavity wakefields,resistive wall impedance, and magnet errors andmisalignments, show only modest emittance growth, withnegligible impact on machine performance. The lattice isdesigned to allow manipulation of the bunch phase spaceon each pass if required, and also to accommodate bunchrates greater than the 10 kHz baseline design.LUX will have the capacity for energy recovery in thelinacs. However, for the baseline beam power of a fewtens of kW, the beam will be taken directly to a shieldeddump after the x-ray production sections.At the exit of the final arc the flat-beam electronbunches receive a time-correlated vertical kick in a dipole-mode RF cavity. This imparts to the electron bunch atransverse momentum that is correlated in amplitude tolongitudinal position within the bunch. The electrons thenradiate x-rays in the downstream chain of undulators anddipole magnets, imprinting this correlation in thegeometrical distribution of the x-ray pulse. The correlatedx-ray pulse is then compressed to 10’s fs duration by useof asymmetrically cut crystal optics. The bunch deflectingtechnique is identical to the “crab-cavity” schemesproposed for several electron-positron colliders. A totaldeflecting voltage of 8.5 MV is required, and we havedeveloped a preliminary design for a 7-cellsuperconducting deflecting cavity [14].  Narrow-gap in-vacuo superconducting undulator designsprovide tunable high-flux sources in the 1–12 keV range.The flux of 10 keV photons from 1 nC bunches at 10 kHzis 6x1010 photons/s/0.1%BW for a 4 mm gap, 14 mmperiod, 2 T peak magnetic field undulator. Similarinsertion devices are currently being prototyped anddesigns are expected to mature in the near future.A laser-seeded cascaded harmonic-generation schemeproduces high-flux, short-pulse photons over an energyrange of tens of eV to 1 keV. In this process the circularcross-section high-brightness electron beam is extractedfrom the recirculating linac, and passed through anundulator where a co-propagating seed laser modulates thecharge distribution over a short length of the bunch. Thescheme has been developed and demonstrated at theBrookhaven DUV FEL facility [3]. The imposedmodulation results in enhanced radiation at specificwavelengths and a selected wavelength is amplified in afollowing undulator, tuned to a higher harmonic of theseed laser. The electron pulse is then delayed in a shortchicane, and the process repeated by modulating a freshportion of the beam this time with the harmonic radiationproduced in the previous undulator. Using a tunableoptical parametric amplifier as the seed, and variableundulators, allows significant tunability in four stages ofharmonic generation, variable flux up to 1013 photons perpulse, and variable pulse duration depending on the seedlaser parameters [15]. Two chains of cascaded harmonicgeneration are proposed, providing exceptional flexibilityin producing EUV and soft x-ray pulses. Circularpolarization is attainable by use of elliptical undulators,and flux stability of 0.1% or better is obtained in secondsfrom random pulse-pulse flux variations of 10-20% at 10kHz repetition-rate. The use of tapered undulators allowstailoring of flux to individual experiments, to avoid space-charge effects in, for example, photoemission processes.Sophisticated laser systems will be an integral part ofthe LUX facility, providing experimental excitationpulses, stable timing signals, as well as the electronsource through the photocathode laser. Each endstationwill have it’s own dedicated laser system and opticalmanipulation and diagnostics, and optical tables andequipment will be contained within a stable and controlledenvironment. Multiple tuneable lasers covering a range of267-3000 nm and pulse durations of ≤ 50 fs are requiredfor experiment initiation, together with temporal andspatial filtering to optimize performance for specificexperimental applications. Distribution systems usingfibre-optic transmission lines will provide optical seedpulses to each beamline, with feedback based oninterferometric measurements to stabilize the path lengths[5].  Developments in laser technology are expected toresult in significant improvements in the coming years,which will be incorporated into our design with minimalimpact on accelerator systems.Synchronization and timing of the ultra-short x-raypulses to the experimental excitation pulse is critical tostudies of ultra-fast dynamics. For LUX we propose togenerate inherently stable pulses by using seeded laserssystems and bunch manipulation. In the case of EUV andsoft x-ray production, the cascaded harmonic generationseed laser oscillator also drives the sample excitation laser,resulting in timing stability of approximately 20 fs. Forour scheme of hard x-ray production by bunchmanipulation followed by x-ray pulse compression, wefind that the phase jitter of the deflecting cavities withrespect to the experimental laser pulse dominates timingissues. Phase and amplitude feedback of the deflectingcavities is expected to provide x-ray pulse to laser pulsetiming stability of 50 fs or better. To stabilize all timingand rf signals in the facility, we propose to use a phase-locked laser oscillator as the facility master oscillator. TheRF gun, linacs, and deflecting cavities may thus be phase-locked to the experimental excitation lasers, and timingjitter between the optical laser and the x-ray pulse emittedby the beam minimized [5].3 SUMMARYA recirculating linac user facility is proposed to addressthe growing national and international need for ultrafast x-ray scientific research.  The LUX facility is based onexisting accelerator technology, coupled with an array ofadvanced tunable femtosecond lasers, and is capable ofperforming an enormous variety of pump-probe typeexperiments with soft and hard x-rays.  The facility hasbeen specifically designed with a view toward solvingproblems in ultrafast science, and it’s impact will beacross all fields of science, from biology, chemistry, andphysics, to novel areas such as quantum computing,spintronics, and highly nonlinear phenomena.4 REFERENCES[1] J. N. Corlett et al, “Feasibility study for arecirculating linac-based facility for femtoseconddynamics”, LBNL formal report LBNL-51766, December2002.[2] J. N. Corlett, et al, " A Recirculating Linac BasedSynchrotron Light Source for Ultrafast X-ray Science”,Proc. EPAC’02, Paris, France, June 2002.[3] L.-H. Yu et al, “High-Gain Harmonic-Generation Free-Electron Laser”, Science 289 932-934 (2000).[4] A. Zholents et al “Generation of subpicosecond x-raypulses using RF orbit deflection”, NIM A 425 (1999)385-389.[5] J. Corlett et al, "Techniques for Synchronization of X-ray Pulses to the Pump Laser in an Ultrafast X-rayFacility", this conference.[6] J. Staples et al, “The LBNL Femtosource 10 kHzPhotoinjector”, this conference.[7] S. Lidia et al, “An Injector for the Proposed BerkeleyUltrafast X-Ray Light Source”, this conference.[8] D. Edwards et al, "The Flat Beam Experiment at theFNAL Photoinjector", Proc. XXth International LinacConference, Monterey, 2000.[9] A. Zholents et al, “Longitudinal phase-space control inthe Berkeley Femtosecond X-ray light source”, thisconference.[10] TESLA Technical Design Report, DESY 2001-011,March 2001.[11] L. Harwood, C. Reece, “CEBAF at 12 and 25 GeV”,Proc. SRF2001, Tsukuba, Japan, Sept. 2001.[12] S. De Santis et al “Collective effects analysis for theBerkeley femtosource”, this conference[13] R. Wells et al, “Recirculating linac vacuum system”,this conference[14] D. Li et al, “Deflecting rf cavity design for arecirculating linac-based facility for ultrafast x-rayscience”, this conference.[15] W. Fawley et al, “Simulation studies of an XUV/softx-ray harmonic-cascade FEL for the proposed LBNLrecirculating linac”, this conference.

English

A recirculating linac-based facility for ultrafast X-ray science

CERN Document Server

A Recirculating Linac-Based Facility for Ultrafast X-Ray Science

We present an updated design for a proposed source of ultra-fast synchrotron radiation pulses based on a recirculating superconducting linac, in particular the incorporation of EUV and soft x-ray production. The project has been named LUX - Linac-based Ultrafast X-ray facility. The source produces intense x-ray pulses with duration of 10-100 fs at a 10 kHz repetition rate, with synchronization of 10 s fs, optimized for the study of ultra-fast dynamics. The photon range covers the EUV to hard x-ray spectrum by use of seeded harmonic generation in undulators, and a specialized technique for ultra-short-pulse photon production in the 1-10 keV range. High-brightness rf photocathodes produce electron bunches which are optimized either for coherent emission in free-electron lasers, or to provide a large x/y emittance ration and small vertical emittance which allows for manipulation to produce short-pulse hard x-rays. An injector linac accelerates the beam to 120 MeV, and is followed by four passes through a 600-720 MeV recirculating linac. We outline the major technical components of the proposed facility

Corlett, J. N.

Barletta, W. A.

Fawley, W. M.

Green, M. A.

UNT Digital Library

A RECIRCULATING LINAC-BASED FACILITY FOR ULTRAFAST X-RAYSCIENCE*J. N. Corlett, W. A. Barletta, S. DeSantis, L. Doolittle, W. M. Fawley, M.A. Green, P. Heimann, S. Leone, S. Lidia,D. Li, A. Ratti, K. Robinson, R. Schoenlein, J. Staples, W. Wan, R. Wells, A.Wolski, and A. Zholents, LBNL,Berkeley, California, USAM. Placidi, W. Pirkl, Geneva, SwitzerlandF. Parmigiani, UCSC, Brecia, Italy                                                * This work supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098Abstract We present an updated design for a proposed source ofultra-fast synchrotron radiation pulses based on arecirculating superconducting linac [1,2], in particular theincorporation of EUV and soft x-ray production. Theproject has been named LUX – Linac-based Ultrafast X-rayfacility. The source produces intense x-ray pulses withduration of 10-100 fs at a 10 kHz repetition rate, withsynchronization of 10’s fs, optimized for the study ofultra-fast dynamics. The photon range covers the EUV tohard x-ray spectrum by use of seeded harmonic generationin undulators, and a specialized technique for ultra-short-pulse photon production in the 1-10 keV range. High-brightness rf photocathodes produce electron buncheswhich are optimized either for coherent emission in free-electron lasers, or to provide a large x/y emittance rationand small vertical emittance which allows formanipulation to produce short-pulse hard x-rays. Aninjector linac accelerates the beam to 120 MeV, and isfollowed by four passes through a 600-720 MeVrecirculating linac. We outline the major technicalcomponents of the proposed facility.1 OVERVIEWA recirculating linac accelerates 2 ps electron bunchesto 2.5-3 GeV, where the bunches radiate in multipleinsertion devices. Intense soft x-rays are produced bycascaded harmonic generation scheme, similar to high-gainharmonic-generation (HGHG) - a laser-seeded process in acascaded series of undulators, resulting in enhancedradiation at selected harmonics of the seed [3]. Thecoherent soft x-rays can be tuned over a range of tens ofeV to 1 keV, and ultrashort seed laser pulses produce pulsedurations of 10-200 fs.  Hard x-rays are produced byspontaneous emission of the electrons in narrow-gap,short-period undulators. By use of a novel bunch tiltingprocess followed by optical compression, hard x-ray pulsedurations of 50-100 fs are obtained over a range of 1-10keV [1,4].  Synchronization of the x-rays with lasers iscritical for experiments, and optical pulses initiate bothcascaded harmonic generation seed lasers and experimentalend station amplifiers for precise timing [1,5]. Thefemtosecond x-rays are produced at a 10 kHz repetitionrate, with variable polarization, and with peak fluxescomparable to third generation light sources. Thus, theFigure 1. Machine layout. The beams generated at the rfphotocathode guns travel through the injector linac, mainlinac, transport arcs, deflecting cavities, and either hard x-ray production section or a cascaded harmonic generationsection, to the beam dump. Machine footprint isapproximately 150x50 m.proposed LUX facility would provide stable, synchronized,tunable, ultrafast x-ray pulses to multiple beamlinesoperating simultaneously over a broad range of x-raywavelengths.2 ACCELERATOR DESIGNElectron pulses are produced at a rate of 10 kHz in high-brightness rf photocathode guns [6]. Two sources are used- one with a conventional circular cross-section beamoptimized for production of high-brightness EUV and softx-ray radiation in the harmonic generation scheme, theother with a flattened cross-section beam for production ofhard x-rays [7]. The beam quality requirements of the rfphotocathode guns are already demonstrated, withnormalized emittance of approximately 3 mm-mrad at 1nC charge, although higher repetition rates have not beenaddressed to date. Conventionally, rf photocathode gunsemploy a simple right-circular geometry or "pillbox"Cascaded harmonicgeneration in FEL’sHard x-ray production in narrow-gapshort-period insertion devicesMagnetic transportsections for each ringSuperconductinglinacsRfphotocathodegunsBeamlineendstationsshaped cavities, and operate over 10-100 Hz pulserepetition rate. The cathode is mounted on the cavity axisand electrons are rapidly accelerated by the rf electric fields,minimizing space-charge effects in the low-energy beam.For high duty cycle operation, thermal limitations preventsuch designs from operating at an electric field sufficientlyhigh to produce good beam emittance. For the LUXfacility we have produced a conceptual design optimizedfor operation at high gradient and high repetition rate, andproducing low-emittance bunches. This designincorporates features that increase cavity surface area toreduce deposited power density and enhance theaccelerating electric field at the cathode (82 W/cm2maximum power density for 64 MV/m at the cathode).Application of a solenoidal magnetic field on thecathode of the flat-beam gun, followed by a speciallyconfigured skew-quadrupole channel located after theinjector linac, allows production of a “flat” beam with x/yemittance ratio 50/1 and small vertical normalizedemittance of 0.4 mm-mrad. This technique has beensuccessfully demonstrated at Fermilab, with propertiesclose to LUX design parameters [8].Following the injector linac, a third-harmonic cavity isused to linearize the correlated energy spread introduced inthe injector linac, and to manipulate the longitudinalphase-space in preparation for bunch compression. Thebeam is then transported to the entrance of therecirculating linear accelerator. In the transport line frominjector linac to main linac, the bunches are compressedfrom 20 ps to 2 ps, with a final energy spread of ±200keV. This transport line is carefully designed tocompensate for the effects of coherent synchrotronradiation at the shorter bunch lengths [9].In the recirculating linac the maximum energy of 3GeV is achieved after four passes through the 720 MeVsuperconducting rf structure, or 2.5 GeV with the mainlinac operating at 600 MeV. Identical cryomodulescontaining multiple accelerating cavities are used for themain linac and the injector linac. The superconductinglinacs have advantages in providing a compact andefficient accelerator, extremely stable rf fields, andinherently small perturbative effects on the beam.Significant advances have been made in superconducting rftechnology in recent years, and the parameters of theproven TESLA superconducting rf systems developed atDESY have been used in LUX design studies [10].Planned upgrades for the CEBAF facility at TJNAF alsomeet the requirements for the LUX linacs [11]. Our designis for an accelerating gradient of up to 20 MV/m in themain linac. The electron bunch repetition interval in LUXis less than the superconducting cavity filling time, andthe linacs are more efficiently operated in cw mode.Engineering modifications to exisiting cryomoduledesigns required to allow significantly increased thermalload in the liquid helium are described in [1].The flexibility of the LUX lattice design allows controland preservation of electron beam transverse andlongitudinal emittances, minimizing the influence ofcollective effects [12]. Longitudinal and transversedynamics have been modeled from the RF gun through theinjector linac and all passes of the main linac. In theinjector, a harmonic cavity will be used to control thelongitudinal phase-space following the injector linac [9].The bunch length and magnet bend angle in the lowestenergy arcs of the machine result in a regime in whichcoherent synchrotron radiation emission could be expected,and the vacuum chamber geometry is designed tominimize this effect by shielding against lower-frequencyradiation. The recirculating ring arcs are achromatic andisochronous to preserve beam quality. Our studiesincluding particle tracking with cavity wakefields,resistive wall impedance, and magnet errors andmisalignments, show only modest emittance growth, withnegligible impact on machine performance. The lattice isdesigned to allow manipulation of the bunch phase spaceon each pass if required, and also to accommodate bunchrates greater than the 10 kHz baseline design.LUX will have the capacity for energy recovery in thelinacs. However, for the baseline beam power of a fewtens of kW, the beam will be taken directly to a shieldeddump after the x-ray production sections.At the exit of the final arc the flat-beam electronbunches receive a time-correlated vertical kick in a dipole-mode RF cavity. This imparts to the electron bunch atransverse momentum that is correlated in amplitude tolongitudinal position within the bunch. The electrons thenradiate x-rays in the downstream chain of undulators anddipole magnets, imprinting this correlation in thegeometrical distribution of the x-ray pulse. The correlatedx-ray pulse is then compressed to 10’s fs duration by useof asymmetrically cut crystal optics. The bunch deflectingtechnique is identical to the “crab-cavity” schemesproposed for several electron-positron colliders. A totaldeflecting voltage of 8.5 MV is required, and we havedeveloped a preliminary design for a 7-cellsuperconducting deflecting cavity [14].  Narrow-gap in-vacuo superconducting undulator designsprovide tunable high-flux sources in the 1–12 keV range.The flux of 10 keV photons from 1 nC bunches at 10 kHzis 6x1010 photons/s/0.1%BW for a 4 mm gap, 14 mmperiod, 2 T peak magnetic field undulator. Similarinsertion devices are currently being prototyped anddesigns are expected to mature in the near future.A laser-seeded cascaded harmonic-generation schemeproduces high-flux, short-pulse photons over an energyrange of tens of eV to 1 keV. In this process the circularcross-section high-brightness electron beam is extractedfrom the recirculating linac, and passed through anundulator where a co-propagating seed laser modulates thecharge distribution over a short length of the bunch. Thescheme has been developed and demonstrated at theBrookhaven DUV FEL facility [3]. The imposedmodulation results in enhanced radiation at specificwavelengths and a selected wavelength is amplified in afollowing undulator, tuned to a higher harmonic of theseed laser. The electron pulse is then delayed in a shortchicane, and the process repeated by modulating a freshportion of the beam this time with the harmonic radiationproduced in the previous undulator. Using a tunableoptical parametric amplifier as the seed, and variableundulators, allows significant tunability in four stages ofharmonic generation, variable flux up to 1013 photons perpulse, and variable pulse duration depending on the seedlaser parameters [15]. Two chains of cascaded harmonicgeneration are proposed, providing exceptional flexibilityin producing EUV and soft x-ray pulses. Circularpolarization is attainable by use of elliptical undulators,and flux stability of 0.1% or better is obtained in secondsfrom random pulse-pulse flux variations of 10-20% at 10kHz repetition-rate. The use of tapered undulators allowstailoring of flux to individual experiments, to avoid space-charge effects in, for example, photoemission processes.Sophisticated laser systems will be an integral part ofthe LUX facility, providing experimental excitationpulses, stable timing signals, as well as the electronsource through the photocathode laser. Each endstationwill have it’s own dedicated laser system and opticalmanipulation and diagnostics, and optical tables andequipment will be contained within a stable and controlledenvironment. Multiple tuneable lasers covering a range of267-3000 nm and pulse durations of ≤ 50 fs are requiredfor experiment initiation, together with temporal andspatial filtering to optimize performance for specificexperimental applications. Distribution systems usingfibre-optic transmission lines will provide optical seedpulses to each beamline, with feedback based oninterferometric measurements to stabilize the path lengths[5].  Developments in laser technology are expected toresult in significant improvements in the coming years,which will be incorporated into our design with minimalimpact on accelerator systems.Synchronization and timing of the ultra-short x-raypulses to the experimental excitation pulse is critical tostudies of ultra-fast dynamics. For LUX we propose togenerate inherently stable pulses by using seeded laserssystems and bunch manipulation. In the case of EUV andsoft x-ray production, the cascaded harmonic generationseed laser oscillator also drives the sample excitation laser,resulting in timing stability of approximately 20 fs. Forour scheme of hard x-ray production by bunchmanipulation followed by x-ray pulse compression, wefind that the phase jitter of the deflecting cavities withrespect to the experimental laser pulse dominates timingissues. Phase and amplitude feedback of the deflectingcavities is expected to provide x-ray pulse to laser pulsetiming stability of 50 fs or better. To stabilize all timingand rf signals in the facility, we propose to use a phase-locked laser oscillator as the facility master oscillator. TheRF gun, linacs, and deflecting cavities may thus be phase-locked to the experimental excitation lasers, and timingjitter between the optical laser and the x-ray pulse emittedby the beam minimized [5].3 SUMMARYA recirculating linac user facility is proposed to addressthe growing national and international need for ultrafast x-ray scientific research.  The LUX facility is based onexisting accelerator technology, coupled with an array ofadvanced tunable femtosecond lasers, and is capable ofperforming an enormous variety of pump-probe typeexperiments with soft and hard x-rays.  The facility hasbeen specifically designed with a view toward solvingproblems in ultrafast science, and it’s impact will beacross all fields of science, from biology, chemistry, andphysics, to novel areas such as quantum computing,spintronics, and highly nonlinear phenomena.4 REFERENCES[1] J. N. Corlett et al, “Feasibility study for arecirculating linac-based facility for femtoseconddynamics”, LBNL formal report LBNL-51766, December2002.[2] J. N. Corlett, et al, " A Recirculating Linac BasedSynchrotron Light Source for Ultrafast X-ray Science”,Proc. EPAC’02, Paris, France, June 2002.[3] L.-H. Yu et al, “High-Gain Harmonic-Generation Free-Electron Laser”, Science 289 932-934 (2000).[4] A. Zholents et al “Generation of subpicosecond x-raypulses using RF orbit deflection”, NIM A 425 (1999)385-389.[5] J. Corlett et al, "Techniques for Synchronization of X-ray Pulses to the Pump Laser in an Ultrafast X-rayFacility", this conference.[6] J. Staples et al, “The LBNL Femtosource 10 kHzPhotoinjector”, this conference.[7] S. Lidia et al, “An Injector for the Proposed BerkeleyUltrafast X-Ray Light Source”, this conference.[8] D. Edwards et al, "The Flat Beam Experiment at theFNAL Photoinjector", Proc. XXth International LinacConference, Monterey, 2000.[9] A. Zholents et al, “Longitudinal phase-space control inthe Berkeley Femtosecond X-ray light source”, thisconference.[10] TESLA Technical Design Report, DESY 2001-011,March 2001.[11] L. Harwood, C. Reece, “CEBAF at 12 and 25 GeV”,Proc. SRF2001, Tsukuba, Japan, Sept. 2001.[12] S. De Santis et al “Collective effects analysis for theBerkeley femtosource”, this conference[13] R. Wells et al, “Recirculating linac vacuum system”,this conference[14] D. Li et al, “Deflecting rf cavity design for arecirculating linac-based facility for ultrafast x-rayscience”, this conference.[15] W. Fawley et al, “Simulation studies of an XUV/softx-ray harmonic-cascade FEL for the proposed LBNLrecirculating linac”, this conference.

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A recirculating linac-based facility for ultrafast X-ray science

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