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X-Ray Free-Electron Laser Amplifiers and Oscillators for Materials and Fundamental Research Kwang-Je Kim ANL and U. of Chicago ICABU Meeting November 12, 2013 DaeJeon, Korea Relativity and Synchrotron Radiation Electron velocity v c; 𝜸 = 𝟏/ 𝟏 − 𝒗 𝟐 𝒄 g ~ 2000× electron energy [GeV] – APS; Ee=7 GeV, g~ 14,000!! Emission within an angle ~1/ g from the electron motion – about 70 mrad for APS About a ≅1/137 photons per 1/g trajectory bending KJK ICABU Nov 12 2013 2 More efficient x-ray production with an “undulator” Compact undulator with permanent magnets KJK ICABU Nov 12 2013 3 Radiation by one electron in Nu period undulator The e- emits EM wave in the forward direction due to its x-acceleration. Consider the wave fronts from successive undulator periods: lu K/g e E-field direction l1 Nul1 The e- is slower since (1) c > v c(1-1/2g2), and (2) its trajectory is curved. Thus, the EM wave slips ahead of the e- in one undulator period by a distance l1=wavelength: l1=lu(1+K2/2)/2g2 , e1[keV]=12.4/l1[Å] After travelling Nu periods of the undulator, an Nu-cycle wave-train is formed: Dzrad = Nul1 , Dw/w =1/Nu KJK ICABU Nov 12 2013 4 An Nu-cycle wave-train c Length of the train Dz=lNU 𝟐 Spectral intensity 𝒅𝑾(𝝎)/𝒅𝝎~ 𝑬(𝝎) ~ (sin x/x)2 x=pNU(w- w1)/w, peaked around w1 with relative bandwidth Dw/w ~ 1/NU Dz ×Dw/w ~ l1 ( equivalent to pure state in QM) KJK ICABU Nov 12 2013 5 Undulator radiation from electrons randomly distributed in a “bunch” Normally Dzel >> Nul1 “Chaotic light” Each wavetrain has random phase intensity ∝ 𝑵𝒆 Spectral property is the same as that of a single electron Dw/w=1/Nu Temporal phase space area Wz ~(Dw/w) Dzel KJK ICABU Nov 12 2013 6 Transverse coherence The phase space area Wx= (DxDf) of incoherent e-beam can be divided into smaller and smaller area With coherent beam the phase space area Wx cannot be divided to area smaller than DxDf =l/2 Undulators are placed in the straight sections of low emittance, high current electron storage rings “the third generation light source” KJK ICABU Nov 12 2013 7 Amplification in the presence of e-beam When the EM wavelength satisfies the undulator condition, an electron sees the same EM field in the successive period sustained energy exchange A0 A1 A2 A3 l1 An e- arriving at A0 loses energy to the field (ev E <0). Similarly the eat distance nl1, n=1,2,… also loses energy. However, those at l1(1/2 +n) away gain energy. The electron beam develops energy modulation (period length l1). Higher energy electrons are faster density modulation develops Coherent EM of wavelength l1 is generated “Free electron laser” KJK ICABU Nov 12 2013 8 SASE: Initial undulator radiation is amplified to intense, quasi-coherent radiation Saturation Exponential Gain Regime Transverse mode z = 25 m z = 37.5 m Undulator Regime z = 50 m z = 90 m Electron Bunch Micro-Bunching KJK ICABU Nov 12 2013 9 SASE: Microbunching in each coherent region # of coherent regions= nlc # of electrons in one coherent region = Nlc =Ne/nlc Radiation intensity = (Nlc)2nlc= (Ne)2/nlc =Ne Nlc X-ray FELs are driven by linear accelerators KJK ICABU Nov 12 2013 10 An FEL for x-rays requires high e-beam qualities not achievable from storage rings photo-cathode gun & a linear acc BNL type LCLS S-band RF Photocathode KEK/JAERI DC gun LBNL 180 MHz RF Photocathode 11 Hard X-Ray FELs in Operation & Under Construction LCLS-I, II 2009, 2018 14.5 GeV, 120 Hz NC XFEL 2015 17.5 GeV, 3000 x 10 Hz SC KJK ICABU Nov 12 2013 SACLA 2011 8.5 GeV, 60 Hz NC PAL XFEL 2015 10 GeV, 100 Hz NC SWISS FEL 2017 5.8 GeV, 100 Hz NC 12 Various R&D programs are in progress to enhance the performance of high-gain XFEL SASE is temporally incoherent fluctuation in spectrum and intensity Coherent soft x-rays (l< 1 nm) via seeding – Laser HHG, Cascaded HGHG, EEHG, self-seeding Self-seeding for hard x-rays Other spectrum enhancing schemes – iSASE, pSASE, two color generation LCLS-II will incorporate CW capability by a super-conducting linac KJK ICABU Nov 12 2013 13 Free Electron Laser Oscillator A low-gain device with high Q optical cavity Optical pulse formed over many electron passes Difficult for x-rays – Electron beam qualities – High-reflectivity normal incidence mirror KJK ICABU Nov 12 2013 14 X-Ray FEL Oscillator (XFEL-O) An FEL oscillator is feasible in hard x-ray region by using Bragg mirrors – R. Collela and A. Luccio, 1983; KJK, Y. Shvyd’ko, and S. Reiche, 2008 Tuning is possible with a four mirror configuration – R. M.J.Cotterill, (1968) KJK & Y. Shvyd’ko (2009) Ultra-high spectral resolution ( meV) with storage ring like stability 15 KJK ICABU Nov 12 2013 15 Example Parameters Electron beam: – Energy 6 GeV, Bunch charge ~ 25-50 pC low intensity, Bunch length (rms) 1 (0.1 ps) Peak current 20 (100) A, Normalized rms emittance 0.2 (0.3) mm-mr, rms energy spread ~ 210-4 , Constant bunch rep rate @ ~1 MHz Undulator: – Lu= 60 (30) m, lu ~2.0 cm, K=1.0 – 1.5 Optical cavity: – 2- or 4- diamond crystals and focusing mirrors – Total round trip reflectivity > 85 (50) % XFELO output: – 5 keV w 25 keV – Bandwidth: Dw/w ~ 1 (5) 10-7 ; rms pulse length = 500 (80) fs – # photons/pulse ~ 1109 – Rep rate ~ a few MHz(limited by crystal heat load and damage) KJK ICABU Nov 12 2013 8 16 Diamond is the best material. The tolerance on optical element placement (10 nr), and R. & fig. errors for focusing mirrors appear feasible. Null feedback on HRM to 50 nr High heat diffusivity at < 100K Yamauch, JTEC, R~ 99%, fig error< 1 mr KJK ICABU Nov 12 2013 17 Damage issue of diamond crystals for XFELO cavity Power density on XFELO crystal – 1 kW/mm2 Power density for APS HHL crystal Power density of focused beam for ESRF experiment in 1994 KJK ICABU Nov 12 2013 18 XFELO applications High resolution spectroscopy – Inelastic x-ray scattering Mössbauer spectroscopy – 103/pulse, 109/sec Moessbauer gs (14.4 keV, 5 neV BW) X-ray photoemission spectroscopy – Bulk-sensitive Fermi surface study with HX-TR-AR PES X-ray imaging with nm resolution – Smaller focal spot with the absence of chromatic aberration picosecond time resolution A second user WS was held at POSTECH in Feb 2013 KJK ICABU Nov 12 2013 19 Nuclear-resonance-stabilized XFELO(B.W. Adams and K.-J. Kim, to be published) The XFEL-O output pulses are copies of the same circulating intra-cavity pulse By stabilizing cavity RT time to less than 0.01l/c, the spectrum of XFELO output becomes a comb The extreme-stabilized XFEL-O will establish an x-ray-based length standard and have applications in fundamental physics such as x-ray Ramsey interferometer to probe quantum gravity, etc. KJK ICABU Nov 12 2013 20 KJK ICABU Nov 12 2013 21 KJK ICABU Nov 12 2013 22 PossibleAccelerator system Injector for XFELO is available from ERL research The 17GeV pulsed Euro XFEL can be operated 7GeV CW A 2-loop, 3 pass system using 25 CEBAF C-100 cryomodule for 2.3 GeV acceleration can fit CEBAF tunnel for multi-XFELO operation KJK ICABU Nov 12 2013 23 Legend of evolving bright & coherent xray sources Brightness= invariant measure= # of Photons/phase space volume Phase space volume = Wx Wy Wz KJK ICABU Nov 12 2013 24