Energy Recovery Linac (ERL) Properties. Physics Dept. & Cornell High Energy Synchrotron Source (CHESS) Ithaca, NY Cornell University

Transcription

1 Energy Recovery Linac (ERL) Properties Sol M. Gruner Physics Dept. & Cornell High Energy Synchrotron Source (CHESS) Cornell University Ithaca, NY

2 Acknowledgements T. Allen (Special thanks to Qun Shen for help w/ slides) S. Belomestnyhk D. Douglas S. Gray A. Kazimirov L. Merminga V. Shemlin D. Smilgies M. Tigner B. Barstow D. Bilderback K. Finkelstein R. Helmke G. A. Krafft H. Padamsee Q. Shen K. Smolenski V. Veshcherevich I. Bazarov J. Brock E. Fontes G. Hoffstaetter M. Liepe J. Rogers C. Sinclair R. Talman J. Welch (now SLAC) Key: Red = Cornell; Blue = Jlab CHESS is supported by the NSF & NIH LEPP is supported by the NSF The Cornell ERL project is supported by Cornell University

3 What do we ideally want from a SR source for coherence applications? 1. High average and high peak Brilliance (phot/s/0.1% bw/mrad 2 /mm 2 ) Flux (phot/s/0.1% bw) 2. Flexible pulse structure. Programmable pulse trains (interval, bunch size) Adjustable pulse lengths down to the femtosecond regime 3. Small x-ray source size of desired shape, e.g., circular. 4. Flexibility of source operation. No fill decay Stability & robustness Easily upgraded (reset the point of diminishing returns)

4 These desirable characteristics depend on fundamental facts of SR Flux I (current) Brilliance I ( ε is emittance) εε Peak Brilliance I ( τ is bunch length) εετ Coherent Flux x y Photon Degeneracy I εε x y x y I εετ x y Thus, are fundamental. I, ε x, ε y, τ

5 Storage Ring Limitations Transverse effects of magnetic focusing, SR & RF equilibrium emittance, ε, after ~10 4 revolutions. Longitudinal effects of phase focusing, stochastic SR mission & energy loss, and RF equilibrium bunch length, τ. Interparticle effects (e.g., Touschek effect) & population of tails of cross-sectional distribution Limit current, I, lifetime. So ε, τ, I are all affected!!

6 Linacs present an alternative Advantages: Injector determines emittances, pulse length, current. Complete flexibility of pulse timing, structure. No fill decay. Disadvantage: You d go broke!! (7 GeV) x (100 ma) = 700 MW!!

7

8 Energy Recovery Linac idea Accelerating bunch Returning bunch

9 Proof of principle First demonstrated at Stanford SCA/FEL [Smith et al., NIM A 259, p. 1-7 (1987)] IR FEL Project at JLAB I ave = 5 ma E max = 48 MeV ε n 7 µm σ l > 330 fs *G. R. Neil, et al., Physical Review Letters, 84, 622 (2000) 10 ma, 210 MeV upgrade is in the works

10 Two key technologies for ERL Superconducting RF cavities (Want Q ~ MV/m) Laser-driven photoinjector (Want ε n ~ 100 ma) First TESLA 5-cell cavity undergoing chemical processing at Cornell s SRF development facility DC photogun at JLAB IR FEL (courtesy of JLAB)

11 ERL Development Efforts ERL development intentions (including low photon energy ERLs) have been announced by Russia U.S. (several labs) Japan England Germany (several labs) The challenge will be to develop hard x-ray, high flux, high brilliance ERLs. Cornell, in collaboration with Jlab, has preformed a study of ERLs and developed a strawman model of what may be expected, using reasonable assumptions of attainable technology. Following properties based on the model (see

12 Preliminary Design Parameters of ERL ERL high-flux ERL high-coherence Energy E G (GeV) Current I (ma) Charge q (nc/bunch) εx (nm-rad) εy (nm-rad) Bunch fwhm τ (ps) * * Machine design # of bunches f (Hz) Undulator L (m) Period λu (cm) # of period N u Horizontal βx (m) Vertical βy (m) Undulator E Accelerating bunch Decelerating bunch Insertion device 1 st harmonic E 1 (kev) * Assuming emittance scales with photocathode area illuminated.

13 Expected Brilliance LCLS SASE Sp8 25m APS 4.8m APS 2.4m LCLS spont. ERL 25m 0.015nm 10mA ESRF U35 Sp8 5m 0.15nm 100mA CHESS 49p wiggler nm 100mA 4.7ps Sp8 25m ESRF U35 CHESS 49-pole G/A-wiggler τ=153ps, f=17.6mhz (9x5) ERL 25m 0.015nm 10mA 0.3ps Sp8 5m APS 2.4m 0.15nm 100mA 0.3ps Average Brilliance (ph/s/0.1%/mm 2 /mr 2 ) Peak Brilliance (ph/s/0.1%/mm 2 /mr 2 ) CHESS 24p wiggler CHESS 24-pole F-wiggler Photon Energy (kev) Photon Energy (kev)

14 Spatial (Transverse) Coherence 2σ 2σ θ l = θ 2σ = λ/2 2σ' => 2θ 2σ ~ λ θ σ' => X-ray beam is spatially coherent if phase-space area 2πσ σ < λ/2 Diffraction limited source: 2πσ'σ = λ/2 or ε = λ/4π Almost diffraction limited: 2πσ'σ ~ λ or ε ~ λ/2π

15 Transverse Coherence from Undulator d θ L Example: APS, L =2.4m, λ =1.5Å σ r' = 13.1 µrad d y = 2.35x21µm, σ y' = 6.9 µrad θ = 1.5 µrad, Θ = 2.35x14.8 µrad => p c (vertical) = 4.3% d x = 2.35x350µm, σ x' = 23.1 µrad θ = µrad, Θ = 2.35x26.6 µrad => p c (horizontal) = 0.15% => p c (overall) = 0.006% ERL: p c ~ 20% (45% in x or y) θ = λ/2d Θ = 2.35 σ r ' = σ 2 ' r λ 2 L A portion, θ/θ in each direction, of undulator radiation is spatially coherent within central cone + σ ' Coherent fraction p c : depends only on total emittances p c = F F c n = ( λ /2) F n 2 B = (4 π λ 2 2 ) ε x ε y 2

16 ERL Transversely Coherent Flux Time-averaged coherent flux comparable to LCLS XFEL Coherent fraction ~100x greater than 3rd SR sources Peak coherent flux (coherent flux per pulse) ~1000x greater than 3rd SR sources, if brilliance can be preserved with short pulses.

17 ERL Transverse Coherence 1 Brilliance goes as α ( εxεy ), so the emittance product needs to be small. ESRF emittance (4nm x 0.01nm) i.e., α = 25 nm -2 Diffraction 8keV ERL emittance (0.015nm), i.e., α =4400 nm -2 Diffraction limited source: 2πσ'σ = λ/2 or ε = λ/4π Almost diffraction limited: 2πσ'σ ~ λ or ε ~ λ/2π Cornell ERL: : diffraction-limited source E < 6.6 kev almost diffraction-limited to 13 kev

18 ERL: Source Size and Pulse Length ERL / 10 ma ESRF ma ε x = ε y = 0.2 / 0.02 nm mrad ε x = 4 nm mrad ε y = 0.01 nm mrad ERL (no compression) ESRF ERL (w/ compression). ~ 100 fsec pulses Normalized flux time

19 ERL Science parameters in new regimes

20 Source Size & Divergence

21 Brilliance vs. Source Size parameters in new regimes

22 Storage Ring Beam pwr dribbled in Low bunch charge Multi-pass Energy stored in beam High rep rate Many simul. beamlines Full x-ray pulse to ~ 30ps Little coherence Flat beams Expts like storage ring ESRF Basic Comparison of ERL, Storage Rings, XFEL ERL XFEL Beam pwr in one pass Beam pwr in one pass Low/high bunch charge High bunch charge Single/few-passes Single-pass Energy stored in linac Energy not stored High rep rate Low rep rate Many simul. Beamlines One beamline/bunch Full x-ray pulse to ~ 100 fs Full x-ray pulse to ~100 fs Mostly transverse coherent Fully coherent Round or flat beams Round beams Expts like storage ring Expts unlike storage ring m

23 THREE REASONS TO DEVELOP ERL TECHNOLOGY 1. ERLs can do essentially everything now possible at the most advanced 3 rd gen SR sources, thus meeting growth in demand for SR in a different way than for XFELs. XFELs are more likely to spur growth in wholly new areas. 2. ERLs additionally enable SR experiments not now possible due to high ERL brilliance, coherence, short pulses and flexible bunch structure. These include new regimes of Microbeam diffraction and fluorescence High pressure diffraction and spectroscopy Femtosecond x-ray studies of solids, molecules and proteins Coherent imaging and microscopy Photon correlation spectroscopy Nuclear resonant scattering Inelastic x-ray scattering Normal diffraction, x-ray metrology, and x-ray interferometry Polarized x-ray beam studies, resonant scattering and circular magnetic dichrosim studies 3. The inherent limits of ERLs are not yet known. In particular, injector improvements may be expected, providing a relatively low cost upgrade pathway. World-wide ERL plans are progressing.

24 END

25 X-ray Imaging is Increasingly Used Not all SR x-ray imaging applications require coherence, but many of the newest and most exciting ones do. These are constrained by coherent flux available from storage rings INSPEC Search

26 Storage Ring Summary Question: What determines bunch properties in a storage ring? Answer: The dynamical equilibrium of the beam in the machine lattice. Equilibrium dynamics determine Minimum emittances, ε Minimum bunch length, τ Bunch size & fill decay, I i.e., all fundamental factors!! But The equilibration times are very long (thousands of revolutions around the ring)