Lecture 8 - Synchrotron radiation - Fermilab

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

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

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

(a) Electric ield Lines (b) Wavefronts

Static charge

Particle moving in a straight line withconstant velocity

E field

B field

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.

Particle moving in a circleat constant speed

Field energy flows to infinity

dQ = q dl

Remember that fields add, we can computeradiation from a charge twice as long

dQ = 2q dl

The wavelength of the radiation doubles

All these radiate

Not quantitatively correct because E is a vector;But we can see that the peak field hits theobserver twice as often

Current loop: No radiation

B field

Field is static

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?

Electrons radiate ~αγ photons per radian of turning

P" =q2

6#$0m02c 3

% 2dp"dt

P|| =q2

6"#0m02c 3

dp||dt

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

cdsdtctcts =!"= #

For relativistic electrons:

= "PSR = "2c re

3 m0c2( ) 3

#2 $ U0 = PSR dtfinite #% energy lost per turn

U0 =1c

PSR dsfinite "# =

2reE04

3 m0c2( )

finite "#

For dipole magnets with constant radius r (iso-magnetic case):

U0 =4" re

3 m0c2( ) 3

3$o% 4

The average radiated power is given by:

PSR =U0

=4" c re

3 m0c2( ) 3

#L where L $ ring circumference

Energy lost per turn by electrons

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)

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 )

Uo,electron (keV ) =e2" 4

3#0$= 88.46 E(GeV )

Uo,proton (keV ) =e2" 4

3#0$= 6.03 E(TeV )

Pproton (kW ) =e" 4

3#0$Ib = 6.03 E(TeV )

4 I(A)$(m)

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

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

Critical frequency

"c =32c#$ 3

% "rev$3

Higher frequencieshave smaller critical

Critical angle3/11

d3Id"d#

16$ 3%0c2#&3c' 2(

1+ ' 2. 2( )2K2 / 32 (/) +

' 2. 2

1+ ' 2. 2K1/ 32 (/)

For frequencies much larger than the critical frequency and angles much largerthan the critical angle the synchrotron radiation emission is negligible

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%

For electrons, the critical energyin practical units is

][][665.0][][218.2][ 23

TBGeVEmGeVEkeVc !!=="

where the critical photon energy is

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 =

hc&# 3!

U0 ~e2" 4

n" # 2$% fine"

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

= "PSR = "2c re

3 m0c2( ) 3

• 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

dUdE E0

PSR E0( )dt $[ ]"DX ,"DY damping in all planes

emittancesandspreadmomentummequilibriupYX

radiuselectronclassicalre !

curvaturetrajectory!"

Synchrotron radiation plays a major rolein electron storage ring dynamics

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 !=="

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

"'= qV0L

sin(#s +$%) & sin#s[ ] and

E$='!"

&!" = angular synchrotron frequency

=++ !"!

ss dtd

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

&!" = angular synchrotron frequency

&+'= ' (

)*+ ) tAe

2/ 4sin

longitudinal damping time

ste !/"

Radiation damping of energy fluctuations

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

C" = 8.9 #10$5meter $GeV $3

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.

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

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

#Ecrit Eo

#Cq$ o

J%&curvEo

" z2 = 2# $E

) * +cREo

e ˙ V

Schematic of radiation cooling

Transverse cooling:

Passage through dipoles

Acceleration in RF cavity

P⊥ lessP|| less

P⊥ remains lessP|| restored

Limited by quantum excitation

• At equilibrium the momentum spread is given by:

=Cq) 0

1 *3 ds+1 *2 ds+

where Cq = 3.84 ,10-13m

casemagneticiso

• For the horizontal emittance at equilibrium:

H s( ) = "T # D 2 + $T D2 + 2%T D # D

" = Cq# 02

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:

# +1" and "X =

with " # coupling factor

Emittance & momentum spreadare set by beam energy & lattice functions

Equilibrium emittance & ΔE

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( )

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

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

Interaction of Photons with Matter

RadiographyRadiography

Diffraction Diffraction

Photoelectric EffectPhotoelectric Effect Compton ScatteringCompton Scattering

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

dx dφdθ

Spectral brightness

Spectral brightness is thatportion of the brightness lyingwithin a relative spectralbandwidth Δω/ω:

How bright is a synchrotron light source?

Remark: the sources are comparedRemark: the sources are comparedat different wavelengths!at different wavelengths!

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.

Energy dependence of SR spectrum

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)

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

1 eV 10 eV 100 eV 1 keY 10 keY

Photon energy

• See smaller features

• Write smaller patterns

• Elemental and chemical sensitivity

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

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

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

Light sources provide three types of SRL'ni\'(.'fhll} of lJubljll/1lI

F Bending magnet radiation

1 » Yl

F Wiggler radiation

e- Y{Nl F Undulator radiation

~ T lim

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

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)

Undulator radiation: What is λrad?

An electron in the lab oscillating at frequency, f, emits dipole radiation of frequency f

What about therelativistic electron?

Power in the central coneof undulator radiation

~ (1 + K2+ "(292) A.x = 212 2

Peen -1tey2] K2 f(K)

EoAu (1 + ~f

(~Al = .1. AJcen N

eBa~ K = 21tmoC

~ y* =11/ 1 + 2-

~ '1 1.00 10..

ALS, 1.9 GeV 400 rnA Au = Bcm N = 55 K = 0.5-4.0

------- L'rtll't'fSi(}o/lJrliJJ;mm

Tuning

200 300 400

Photon energy (eV)

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 =

Courtesy of Patrick Naulleau, LBNL / Kris Rosfjord, UCB and LBNL

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).

X-ray beamlines transport the photonsto the sample

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

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

Coherent emission ==> Free Electron Laser

100 MeV linac

to experiment

Injector High energy linacbuncher

long wiggler

See movie

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

Fundamental FEL physics

Electrons see a potential

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

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 +")

= "J e" ix

DownLoad: FELOde.zip

The simulation will show us the bunching and signal growth

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

FOM 1 from condensed matter studies:Light source brilliance v. photon energy

Duty factor

correctionfor

pulsed linacsERLsERLs

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)

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

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 "

Lecture 8 - Synchrotron radiation - Fermilab

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