Transcript Slides

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 ~ 210-4 , 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 ~ 1109
– 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