lecture 8 - synchrotron radiation - fermilab
TRANSCRIPT
US Particle Accelerator School
Fundamentals of Accelerators - 2012Lecture - Day 8
William A. BarlettaDirector, US Particle Accelerator School
Dept. of Physics, MITEconomics Faculty, University of Ljubljana
US Particle Accelerator School
What do we mean by radiation?
Energy is transmitted by the electromagnetic field toinfinity Applies in all inertial frames Carried by an electromagnetic wave
Source of the energy Motion of charges
US Particle Accelerator School
Schematic of electric field
From: T. Shintake, New Real-time Simulation Technique for Synchrotron andUndulator Radiations, Proc. LINAC 2002, Gyeongju, Korea
L'ni\'(.'fhll} of lJubljll/1lI
4 3
1
Q
(a) Electric ield Lines (b) Wavefronts
US Particle Accelerator School
Static charge
US Particle Accelerator School
Particle moving in a straight line withconstant velocity
Q
E field
B field
US Particle Accelerator School
Consider the fields from an electronwith abrupt accelerations
At r = ct, ∃ a transition region from one field to the other. At large r,the field in this layer becomes the radiation field.
US Particle Accelerator School
Particle moving in a circleat constant speed
Field energy flows to infinity
dQ = q dl
US Particle Accelerator School
Remember that fields add, we can computeradiation from a charge twice as long
dQ = 2q dl
The wavelength of the radiation doubles
US Particle Accelerator School
All these radiate
Not quantitatively correct because E is a vector;But we can see that the peak field hits theobserver twice as often
US Particle Accelerator School
Current loop: No radiation
B field
Field is static
US Particle Accelerator School
Charged particles in free space are “surrounded” by virtual photons Appear & disappear & travel with the particles.
Acceleration separates the charge from the photons & “kicks” photonsonto the “mass shell”
Lighter particles have less inertia & radiate photons more efficiently
In the field of the dipoles in a synchrotron, charged particles move on acurved trajectory. Transverse acceleration generates the synchrotron radiation
QED approach: Why do particles radiatewhen accelerated?
!
!
!e
Electrons radiate ~αγ photons per radian of turning
US Particle Accelerator School
!
P" =q2
6#$0m02c 3
% 2dp"dt
&
' (
)
* + 2
!
P|| =q2
6"#0m02c 3
dp||dt
$
% &
'
( ) 2
negligible!
!
P" =c6#$0
q2%&( )4
'2' = curvature radius
Radiated power for transverse acceleration increases dramatically with energy
Limits the maximum energy obtainable with a storage ring
Longitudinal vs. transverse acceleration
US Particle Accelerator School
cdsdtctcts =!"= #
For relativistic electrons:
!
dUdt
= "PSR = "2c re
3 m0c2( ) 3
E 4
#2 $ U0 = PSR dtfinite #% energy lost per turn
!
U0 =1c
PSR dsfinite "# =
2reE04
3 m0c2( )
3
ds"2
finite "#
For dipole magnets with constant radius r (iso-magnetic case):
!
U0 =4" re
3 m0c2( ) 3
E04
#=e2
3$o% 4
#
The average radiated power is given by:
!
PSR =U0
T0
=4" c re
3 m0c2( ) 3
E04
#L where L $ ring circumference
Energy lost per turn by electrons
US Particle Accelerator School
Energy loss to synchrotron radiation(practical units)
Energy Loss per turn (per particle)
Power radiated by a beam of averagecurrent Ib: to be restored by RF system
Power radiated by a beam ofaverage current Ib in a dipole oflength L (energy loss per second)
eTI
N revbtot
!=
!
Pelectron (kW ) =e" 4
3#0$Ib = 88.46 E(GeV )
4 I(A)$(m)
!
Pe (kW ) =e" 4
6#$0%2 LIb =14.08 L(m)I(A)E(GeV )
4
%(m)2
!
Uo,electron (keV ) =e2" 4
3#0$= 88.46 E(GeV )
4
$(m)
!
Uo,proton (keV ) =e2" 4
3#0$= 6.03 E(TeV )
4
$(m)
!
Pproton (kW ) =e" 4
3#0$Ib = 6.03 E(TeV )
4 I(A)$(m)
US Particle Accelerator School
Frequency spectrum
Radiation is emitted in a cone of angle 1/γ Therefore the radiation that sweeps the observer is emitted
by the particle during the retarded time period
Assume that γ and ρ do not change appreciably during Δt.
At the observer
Therefore the observer sees Δω ~ 1/ Δtobs
!
"tret #$%c
!
"tobs = "tretdtobsdtret
=1# 2"tret
!
"# ~ c$% 3
US Particle Accelerator School
Critical frequency and critical angle
Properties of the modified Bessel function ==> radiation intensity is negligible for x >> 1
!
" =#$3c% 3
1+ % 2& 2( )3 / 2
>>1
Critical frequency
!
"c =32c#$ 3
% "rev$3
Higher frequencieshave smaller critical
angle
Critical angle3/11
!"
#$%
&=
'
'
() cc
!
d3Id"d#
=e2
16$ 3%0c2#&3c' 2(
) *
+
, -
2
1+ ' 2. 2( )2K2 / 32 (/) +
' 2. 2
1+ ' 2. 2K1/ 32 (/)
0
1 2
3
4 5
For frequencies much larger than the critical frequency and angles much largerthan the critical angle the synchrotron radiation emission is negligible
US Particle Accelerator School
Integrate over all angles ==>Frequency distribution of radiation
The integrated spectral density up to the critical frequency contains half of thetotal energy radiated, the peak occurs approximately at 0.3ωc
50% 50%
3
23
!"
#$c
cch
h ==
For electrons, the critical energyin practical units is
][][665.0][][218.2][ 23
TBGeVEmGeVEkeVc !!=="
#
where the critical photon energy is
US Particle Accelerator School
Number of photons emitted
Since the energy lost per turn is
And average energy per photon is the
The average number of photons emitted per revolution is
!
"# $13"c =
h%c
3=12
hc&# 3!
U0 ~e2" 4
#
!
n" # 2$% fine"
US Particle Accelerator School
Comparison of S.R. Characteristics
From: O. Grobner CERN-LHC/VAC VLHC Workshop Sept. 2008
LEP200 LHC sse HERA VLHe
Beam particle e+ e- p p p p
Circumfe.·ence km 26.7 26.7 82.9 6.45 95
Beam energy TeV 0.1 7 20 0.82 50
Beam current A 0.006 0.54 0.072 0.05 0.125
Critical energy of SR eV 7105 44 284 0.34 3000
SR power (total) kW 1.7104 7.5 8.8 3 10-4 800
Linear power density W/m 882 0.22 0.14 8 10-5 4
Desorbing photons s-l m-1 2.4 1016 1 1017 6.61015 none 3 1016
US Particle Accelerator School
!
dUdt
= "PSR = "2c re
3 m0c2( ) 3
E 4
#2
• Charged particles radiate when accelerated
• Transverse acceleration induces significant radiation (synchrotron radiation)while longitudinal acceleration generates negligible radiation (1/γ2).
!
U0 = PSR dtfinite "# energy lost per turn
!
"D = #1
2T0
dUdE E0
=1
2T0
ddE
PSR E0( )dt $[ ]"DX ,"DY damping in all planes
emittancesandspreadmomentummequilibriupYX
p
!!
"
,0
radiuselectronclassicalre !
curvaturetrajectory!"
Synchrotron radiation plays a major rolein electron storage ring dynamics
US Particle Accelerator School
RF system restores energy loss
Particles change energy according to the phase of the field in the RF cavity
)sin()( teVteVE RFo !=="
For the synchronous particle)sin(00 seVUE !=="
US Particle Accelerator School
Energy loss + dispersion ==>Longitudinal (synchrotron) oscillations
Longitudinal dynamics are described by1) ε, energy deviation, w.r.t the synchronous particle2) τ, time delay w.r.t. the synchronous particle
Linearized equations describe elliptical phase space trajectories
!
" '= #$c
Es
%
!
"'= qV0L
sin(#s +$%) & sin#s[ ] and
!"
#s
c
E$='!"
dtdV
Te0
'=
00
2
ETVec
s
&!" = angular synchrotron frequency
τ
ε
US Particle Accelerator School
02 22
2
=++ !"!
#!
ss dtd
dtd
dEdU
Ts
0
0211
=!
The derivative
is responsible for the damping of thelongitudinal oscillations
)0(0 >dEdU
Combine the two equations for (ε, τ) in a single 2nd order differential equation
00
2
ETVe
s
&!" = angular synchrotron frequency
!!"
#$$%
&+'= ' (
)*+ ) tAe
ss
t s2
2/ 4sin
longitudinal damping time
ste !/"
Radiation damping of energy fluctuations
US Particle Accelerator School
Damping times
The energy damping time ~ the time for beam to radiate itsoriginal energy
Typically
Where Je ≈ 2, Jx ≈ 1, Jy ≈1 and
Note Σ Ji = 4 (partition theorem)!
Ti =4"C#
R$JiEo
3
!
C" = 8.9 #10$5meter $GeV $3
US Particle Accelerator School
Quantum Nature of Synchrotron Radiation
Synchrotron radiation induces damping in all planes. Collapse of beam to a single point is prevented by the quantum
nature of synchrotron radiation
Photons are randomly emitted in quanta of discrete energy Every time a photon is emitted the parent electron “jumps” in
energy and angle
Radiation perturbs excites oscillations in all the planes. Oscillations grow until reaching equilibrium balanced by radiation
damping.
US Particle Accelerator School
Energy fluctuations
Expected ΔEquantum comes from the deviation of <N γ>emitted in one damping time, τE
<N γ> = nγ τE
==> Δ<N γ> = (nγ τE )1/2
The mean energy of each quantum ~ εcrit
==> σε = εcrit(nγ τE )1/2
Note that nγ = Pγ/ εcrit and τE = Eo/ Pγ
US Particle Accelerator School
Therefore, …
The quantum nature of synchrotron radiation emissiongenerates energy fluctuations
where Cq is the Compton wavelength of the electron
Cq = 3.8 x 10-13 m
Bunch length is set by the momentum compaction & Vrf
Using a harmonic rf-cavity can produce shorter bunches
!
"EE
#Ecrit Eo
1/ 2
Eo
#Cq$ o
2
J%&curvEo
~ $&
!
" z2 = 2# $E
E%
& '
(
) * +cREo
e ˙ V
US Particle Accelerator School
Schematic of radiation cooling
Transverse cooling:
Passage through dipoles
Acceleration in RF cavity
P⊥ lessP|| less
P⊥ remains lessP|| restored
Limited by quantum excitation
US Particle Accelerator School
• At equilibrium the momentum spread is given by:
!
" p
p0
#
$ %
&
' (
2
=Cq) 0
2
JS
1 *3 ds+1 *2 ds+
where Cq = 3.84 ,10-13m
casemagneticiso
J
C
p S
qp
!
=""#
$%%&
'
(
)* 20
2
0
• For the horizontal emittance at equilibrium:
!
H s( ) = "T # D 2 + $T D2 + 2%T D # D
!
" = Cq# 02
JX
H $3ds%1 $2ds%
where:where:
• In the vertical plane, when no vertical bend is present, the synchrotron radiationcontribution to the equilibrium vertical emittance is very small
• Vertical emittance is defined by machine imperfections & nonlinearities thatcouple the horizontal & vertical planes:
!
"Y =#
# +1" and "X =
1# +1
"
!
with " # coupling factor
Emittance & momentum spreadare set by beam energy & lattice functions
US Particle Accelerator School
Equilibrium emittance & ΔE
Set
Growth rate due to fluctuations (linear) = exponential damping rate due to radiation
==> equilibrium value of emittance or ΔE ~ γ2 θ3
!
"natural = "1e#2t /$ d + "eq 1# e
#2t /$ d( )
US Particle Accelerator School
Quantum lifetime
At a fixed observation point, transverse particle motionlooks sinusoidal
Tunes are chosen in order to avoid resonances. At a fixed azimuth, turn-after-turn a particle sweeps all possible
positions within the envelope
Photon emission randomly changes the “invariant” a Consequently changes the trajectory envelope as well.
Cumulative photon emission can bring the envelopebeyond acceptance at some azimuth The particle is lost
This mechanism is called the transverse quantum lifetime
!
xT = a "n sin #" nt +$( ) T = x,y
US Particle Accelerator School
Several time scales governparticle dynamics in storage rings
Damping: several ms for electrons, ~ infinity for heavierparticles
Synchrotron oscillations: ~ tens of ms
Revolution period: ~ hundreds of ns to ms
Betatron oscillations: ~ tens of ns
US Particle Accelerator School
Interaction of Photons with Matter
RadiographyRadiography
Diffraction Diffraction
Photoelectric EffectPhotoelectric Effect Compton ScatteringCompton Scattering
US Particle Accelerator School
Brightness of a Light SourceBrightness of a Light Source
Brightness is a principal characteristic of a particle source Density of particle in the 6-D phase space
Same definition applies to photon beams Photons are bosons & the Pauli exclusion principle does not apply Quantum mechanics does not limit achievable photon brightness
FluxFlux = # of photons in given = # of photons in given ΔλΔλ//λλsecsec
BrightnessBrightness = # of photons in given = # of photons in given ΔλΔλ//λλsec, mrad sec, mrad θθ, mrad , mrad ϕϕ, mm, mm22
!== " dSdBrightnessdNd
Flux#
&
dy
dx dφdθ
cdt
US Particle Accelerator School
Spectral brightness
Spectral brightness is thatportion of the brightness lyingwithin a relative spectralbandwidth Δω/ω:
US Particle Accelerator School
How bright is a synchrotron light source?
Remark: the sources are comparedRemark: the sources are comparedat different wavelengths!at different wavelengths!
US Particle Accelerator School
Angular distribution of SR
1-94 '589M
Casen: i -1 , ... 751tAS
When the electron velocity approaches the velocity of light, the emission pattern is folded sharply
forward.
US Particle Accelerator School
Energy dependence of SR spectrum
~ 10
13
r-~~~~~~~~~~::::~~~~~~~~~~~~~ : t Bending Magnet -0 c o .c ~ a 10 12
-I '"0 o "-E lOll
I~ CI)
(f) z o
SPECTRAL DISTRIBUTION OF SYNCHROTRON RADIATION FROM SPEAR (p = 12.7 m)
b I I~IO~~~~~~~~~~~~~,~~~ __ ~~~~~~~~~ 0... 0.001 0.01 0.1 10 100
PHOTON ENERGY O<eV)
US Particle Accelerator School
Spectrum available using SRL'ni\'(.'fhll} of lJubljll/1lI
Wavelength
100 nm 10 nm 1 nm 0.1 nm = 1A I
CuKa: VUV 2ao
UV Hard X-rays
CUK
1 eV 10 eV 100 eV 1 keY 10 keY
Photon energy
• See smaller features
• Write smaller patterns
• Elemental and chemical sensitivity
US Particle Accelerator School
Two ways to produce radiationfrom highly relativistic electrons
Synchrotron radiation
• 1010 brighter than the most powerful (compact) laboratory source
• An x-ray "light bulb" in that it radiates all "colors" (wavelengths, photons energies)
Undulator radiation
• Lasers exist for the IR, visible, UV, VUV, and EUV
• Undulator radiation is quasimonochromatic and highly directional, approximating many of the desired properties of an x-ray laser
US Particle Accelerator School
Relativistic electrons radiatein a narrow cone
Dipole radiation
Frame of reference moving with electrons
Lorentz k'x transformation
k' = 2m,,-'
Laboratory frame of reference
kz = 2-yk'z{Relativistic Doppler shift)
e _ kx _ k~ = tane' __ 1 - kz - 2ykz 2y - 2y
US Particle Accelerator School
Third generation light sourceshave long straight sections & bright e-beams
Modern Synchrotron Radiation Faci lity
• Many straight sections containing periodic magnetic structures
• Tightly controlled electron beam
• Many straight sections for undulators and wigglers
• Brighter radiation for spatially resolved studies (smaller beam more suitable for microscopies)
L'lliWfSil} of lJllbljlllw
• Interesting coherence properties at very short wavelengths
US Particle Accelerator School
Light sources provide three types of SRL'ni\'(.'fhll} of lJubljll/1lI
F Bending magnet radiation
lim
1 » Yl
/ ~
F Wiggler radiation
lim
1
e- Y{Nl F Undulator radiation
~ T lim
US Particle Accelerator School
Bend magnet radiation
Advantages: Broad spectral range Least expensive Most accessible
• Many beamlines
Disadvantages: Limit coverage of hard X-rays Not as bright at undulator
radiation
US Particle Accelerator School
For brighter X-rays add the radiationfrom many small bends
Magnetic undulator (N periods)
Relativistic electron beam" Ee = ymc2
1 e ~ -cen y-IN
[11'A.] _ 1 Teen -N
. _ photon flux Bnghtness - (/1A) (Lill)
photon flux Spectral Brightness = (/1A) (~n) (~JJA)
US Particle Accelerator School
Undulator radiation: What is λrad?
An electron in the lab oscillating at frequency, f, emits dipole radiation of frequency f
f
What about therelativistic electron?
US Particle Accelerator School
Power in the central coneof undulator radiation
~ (1 + K2+ "(292) A.x = 212 2
Peen -1tey2] K2 f(K)
2
EoAu (1 + ~f
(~Al = .1. AJcen N
eBa~ K = 21tmoC
~ y* =11/ 1 + 2-
2.00
1.50
~ '1 1.00 10..
0.50
ALS, 1.9 GeV 400 rnA Au = Bcm N = 55 K = 0.5-4.0
100
------- L'rtll't'fSi(}o/lJrliJJ;mm
Tuning
200 300 400
Photon energy (eV)
US Particle Accelerator School
Spatial coherence of undulator radiation
Undulator A = 13.4 nm
1 e = -can y*,fN
A = 2.5 nm
1 ~mD pinhole 25 mm wide CCO at410 mm
A d· e =
231:
Courtesy of Patrick Naulleau, LBNL / Kris Rosfjord, UCB and LBNL
L'ni\'(.'fhll} of lJubljll/1lI
US Particle Accelerator School
Characteristics of wiggler radiation
For K >> 1, radiation appears in highharmonics, & at large horizontalangles θ = ±K/γ One tends to use larger collection
angles, which tends to spectrallymerge nearby harmonics.
Continuum at high photon energies,similar bend magnet radiation,
• Increased by 2N (the number ofmagnet pole pieces).
US Particle Accelerator School
X-ray beamlines transport the photonsto the sample
Photon flux
Photon flux
Grating or crystal
" \
"-- "V'" J
Monochromator
\ \
Curved focusing
mirror (glancing inddence refledion)
Observe at sample:
• AbsOIption spectra • Photoelectron spectra • Diffraction • •
Focusing lens (pair of curved mirrors,
zone plate lens, etc.)
Sample
L'ni\'(.'fhll} of lJubljll/1lI
US Particle Accelerator School
To get brighter beamswe need another great invention
The Free Electron Laser (John Madey, Stanford, 1976)
Physics basis: Bunched electrons radiate coherently
Madey’s discovery: the bunching can be self-induced!
START MIDDLE END
US Particle Accelerator School
Coherent emission ==> Free Electron Laser
100 MeV linac
to experiment
Dump
Injector High energy linacbuncher
long wiggler
See movie
US Particle Accelerator School
Laser interaction in the wigglermanipulates electron beam in longitudinal phase
Electron trajectory throughwiggler with two periods
In resonance the electrons always“run uphill” against the E field
Energy lost from the electrons augments the electromagnetic field
US Particle Accelerator School
Fundamental FEL physics
Electrons see a potential
where
and ϕ is the phase between the electrons and the laser field
Imagine an electron part way up the potential well butfalling toward the potential minimum at θ = 0 Energy radiated by the electron increases the laser field &
consequently lowers the minimum further. Electrons moving up the potential well decrease the laser field
!
V (x) ~ A 1" cos x +#( )( )
!
A"Bw#wElaser
US Particle Accelerator School
The equations of motion
The electrons move according to the pendulum equation
The field varies as
where x = (kw-k) z - ωt
!
d2xdt 2
= A sin(x +")
!
dAdt
= "J e" ix
DownLoad: FELOde.zip
The simulation will show us the bunching and signal growth
US Particle Accelerator School
Basic Free Electron Laser PhysicsResonance condition:
Slip one optical period per wiggler period
FEL bunches beam on an optical wavelength atALL harmonicsBonifacio et al. NIM A293, Aug. 1990
Gain-bandwidth & efficiency ~ ρGain induces ΔE ~ ρ
1) Emittance constraintMatch beam phase area to diffraction limitedoptical beam
2) Energy spread conditionKeep electrons from debunching
3) Gain must be faster than diffraction
US Particle Accelerator School
FOM 1 from condensed matter studies:Light source brilliance v. photon energy
Duty factor
correctionfor
pulsed linacsERLsERLs
US Particle Accelerator School
Near term: x-rays from betatron motionand Thomson scattering
laser pulse
plasma
radiationchannel
Betatron oscillations:
λx=λu/2γ2λu
+
-
-
Radiation pulse duration = bunch duration
Betatron: aβ=π(2γ)1/2rβ/λp
Thomson scattering: a0 = e/mc2A
Strength parameter
Esarey et al., Phys. Rev E (2002)Rousse et al., Phys. Rev. Lett. (2004)
US Particle Accelerator School
Potential Thompson sourcefrom all optical accelerator
Schoenlein et al., Science 1996Leemans et al., PRL 1996
Gas jet
e fs - laser
pulse ~
femtosecond electron beam 50 MeV (Y=98)
4.5 nC/pulse
15 fs
x
femtosecond las&( (pulse undulator "
I 800 nm, 50 f$, 100 mJ
femtosecond x-ray Pulses < 20 fs 10 - 30 keV > 105 photons Ipulse ~8 - 10 mrad
• ( r '
Experimental I
sample "I "
L'ni\'(.'fhll} of lJubljll/1lI