Unit 11 - Lecture 18 Synchrotron Radiation -...
Transcript of Unit 11 - Lecture 18 Synchrotron Radiation -...
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US Particle Accelerator School
Unit 11 - Lecture 18
Synchrotron Radiation - I
William A. Barletta
Director, United States Particle Accelerator School
Dept. of Physics, MIT
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What do we mean by radiation?
Energy is transmitted by the electromagnetic field to
infinity
Applies in all inertial frames
Carried by an electromagnetic wave
Source of the energy
Motion of charges
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Schematic of electric field
From: T. Shintake, New Real-time Simulation Technique for Synchrotron and
Undulator Radiations, Proc. LINAC 2002, Gyeongju, Korea
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Static charge
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Particle moving in a straight line withconstant velocity
Q
E field
B field
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Consider the fields from an electron withabrupt 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.
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Particle moving in a circle
at constant speed
Field energy flows to infinity
dQ = q dl
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Remember that fields add, we can computeradiation from a charge twice as long
dQ = 2q dl
The wavelength of the radiation doubles
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All these radiate
Not quantitatively correct because E is a vector;
But we can see that the peak field hits the
observer twice as often
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Current loop: No radiation
B field
Field is static
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Question to ponder:What is the field from this situation?
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Charged particles in free space are “surrounded” by virtual photons
Appear & disappear & travel with the particles.
Acceleration separates the charge from the photons and “kicks” the photons onto
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 a curved
trajectory.
Transverse acceleration generates the synchrotron radiation
QED approach: Why do particles radiate
when accelerated?
e
Electrons radiate ~ photons per radian of turning
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P =q2
6 0m02c 3
2 dpdt
2P|| =
q2
6 0m02c 3
dp||
dt
2
negligible!
P =c
6 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. TransverseAcceleration
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c
dsdtctcts ==
For relativistic electrons:
dU
dt= PSR =
2c re
3 m0c2( ) 3
E 4
2 U0 = PSR dtfinite
energy lost per turn
U0 =1
cPSR ds
finite
=2re E0
4
3 m0c2( )
3
ds2
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
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Energy loss via synchrotron radiationemission (practical units)
Energy Loss per turn (per particle)
Power radiated by a beam of average
current Ib: to be restored by RF system
Power radiated by a beam of
average current Ib in a dipole of
length L (energy loss per second)
e
TIN
revb
tot=
Pelectron (kW ) =e 4
3 0
Ib = 88.46E(GeV )4 I(A)
(m)
Pe (kW ) =e 4
6 02 LIb =14.08
L(m)I(A)E(GeV )4
(m)2
Uo,electron (keV ) =e2 4
3 0
= 88.46E(GeV )4
(m)
Uo,proton (keV ) =e2 4
3 0
= 6.03E(TeV )4
(m)
Pproton (kW ) =e 4
3 0
Ib = 6.03E(TeV )4 I(A)
(m)
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Frequency spectrum
Radiation is emitted in a cone of angle 1/
Therefore the radiation that sweeps the observer is emitted
by the particle during the retarded time period
Assume that and do not change appreciably during t.
At the observer
Therefore the observer sees ~ 1/ tobs
tret c
tobs = tret
dtobs
dtret
=1
2 tret
~c 3
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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 =3
2
c 3
rev3
Higher frequencies
have smaller critical
angle
Critical angle3/1
1= c
c
d3I
d d=
e2
16 30c
2
3c 2
2
1+ 2 2( )2
K2 / 32 ( ) +
2 2
1+ 2 2 K1/ 32 ( )
For frequencies much larger than the critical frequency and angles much larger
than the critical angle the synchrotron radiation emission is negligible
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Integrating over all angles yields thespectral density distribution
dI
d=
d3I
d dd =
4
3e2
4 0c C
K5 / 3(x)dx/ C
<< c
3/1
0
2
cc4
e
d
dI >>
cc/
2/1
c0
2
ec4
e
2
3
d
dI
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Frequency distribution of radiation
The integrated spectral density up to the critical frequency contains half of the
total energy radiated, the peak occurs approximately at 0.3 c
50% 50%
3
2
3 c
cc
hh ==
For electrons, the critical energy
in practical units is
][][665.0][
][218.2][
23
TBGeVEm
GeVEkeV
c==
where the critical photon energy is
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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
1
3 c =h c
3=
1
2
hc 3
U0 ~e2 4
n 2 fine
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Comparison of S.R. Characteristics
From: O. Grobner CERN-LHC/VAC VLHC Workshop Sept. 2008
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dU
dt= 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/ 2).
U0 = PSR dtfinite
energy lost per turn
D =1
2T0
dU
dE E0
=1
2T0
d
dE PSR E0( )dt [ ]
DX , DY damping in all planes
emittancesandspreadmomentummequilibriup
YX
p
,0
radiuselectronclassicalre
curvaturetrajectory
Synchrotron radiation plays a major rolein electron storage ring dynamics
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RF system restores energy loss
Particles change energy according to the phase of the field in the RF cavity
)sin()( teVteVERFo
==
For the synchronous particle
)sin(00 seVUE ==
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Energy loss + dispersion lead tolongitudinal oscillations
Longitudinal dynamics are described by
1) , energy deviation, w.r.t the synchronous particle
2) , time delay w.r.t. the synchronous particle
Linearized equations describe elliptical phase space trajectories
'= c
Es
'=qV0
Lsin( s + ) sin s[ ] and
s
c
E='
dt
dV
T
e
0
'=
00
2
ET
Vec
s
&
= angular synchrotron frequency
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Radiation damping of energy fluctuations
0
0
0
)()(
T
EUteV
T
E=
dE
dU
Tdt
dV
T
e
T
dE
dUU
dt
dVeU
dt
d
o
0
00
000
1)0()0(
=
++
=
But U0 depends on energy E ==> Rate of change of the energy will be given
s
c
Edt
d=
Say that the energy loss per turn due to synchrotron radiation loss is U0
The synchronous phase is such that )sin(00 seVU =
For E << E and << T0 we can expand
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Energy damping
02 2
2
2
=++s
sdt
d
dt
d
dE
dU
Ts
0
02
11=
The derivative
is responsible for the damping of the
longitudinal oscillations
)0(0 >dE
dU
Combine the two equations for ( , ) in a single 2nd order differential equation
00
2
ET
Ve
s
&
= angular synchrotron frequency
+= tAe
s
s
ts
2
2/ 4sin
longitudinal damping time
st
e/
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By performing the calculation one obtains:
dU
dt= PSR =
2c re
3 m0c2( ) 3
E 4
2 D =1
2T0
dU
dE E0
=1
2T0
d
dEPSR E0( )dt[ ]
( )DET
U
D+= 2
200
0
Where D depends on the lattice parameters.
For the iso-magnetic separate function case:D= C
L
2 (<< 1)
Analogously, for the transverse plane:
( )DET
U
X= 1
200
0
and00
0
2 ET
U
Y=
Damping Coefficients
Damping time ~ time required to replace all the original energy
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Damping times
The energy damping time ~ the time for beam to radiate its
original energy
Typically
Where Je 2, Jx 1, Jy 1 and
Note Ji = 4 (partition theorem)
Ti =4
C
R
JiEo3
C = 8.9 10 5 meter GeV 3
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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.
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Energy fluctuations
Expected Equantum comes from the deviation of <N >
emitted in one damping time, E
<N > = n E
==> <N > = (n E )1/2
The mean energy of each quantum ~ crit
==> = crit(n E )1/2
Note that n = P / crit and E = Eo/ P
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Therefore, …
The quantum nature of synchrotron radiation emission
generates 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
E
E
Ecrit Eo
1/ 2
Eo
Cq o2
J curvEo
~
z2
= 2E
E
cREo
e ˙ V
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Schematic of radiation cooling
Transverse cooling:
Passage through dipoles
Acceleration in RF cavity
P lessP|| less
P remains lessP|| restored
Limited by quantum excitation
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• At equilibrium the momentum spread is given by:
p
p0
2
=Cq 0
2
JS
1 3 ds
1 2 dswhere Cq = 3.84 10 13 m
casemagneticiso
J
C
p S
qp=
2
0
2
0
• For the horizontal emittance at equilibrium:
H s( ) = T D 2 + T D2 + 2 T D D = Cq02
JX
H 3ds
1 2dswhere:where:
• In the vertical plane, when no vertical bend is present, the synchrotron radiation
contribution to the equilibrium emittance is very small
• Vertical emittance is defined by machine imperfections & nonlinearities that
couple the horizontal & vertical planes:
Y =+1
and X =1
+1with coupling factor
Emittance and Momentum Spread
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Equilibrium emittance & E
Set
Growth rate due to fluctuations (linear) = exponential damping rate due to radiation
==> equilibrium value of emittance or E
natural = 1e2t / d + eq 1 e 2t / d( )
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Quantum lifetime
At a fixed observation point, transverse particle motion
looks 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 envelope
beyond acceptance at some azimuth
The particle is lost.
This mechanism is called the transverse quantum lifetime
xT = a n sinnt +( ) T = x,y
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Quantum lifetime was first estimated byBruck and Sands
Q varies very strongly with the ratio
between acceptance & rms size.
Values for this ratio > 6
are usually required.
QL DLexp EA
2 2 E2( )
Longitudinal quantum lifetime
EA2
2 E2
JL E0
C h E1
2e ˆ V RF
U0
where T2
= T T + DTE
E0
2
T = x,y
DTtransverse damping time
For an iso-magnetic ring:
( )
lifetimequantumTransverse
yxTAA
TT
T
TDQ TT
,2exp22
2
2
=
E1 1.08 108 eV
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US Particle Accelerator School
Several time scales governparticle dynamics in storage rings
Damping: several ms for electrons, ~ infinity for heavier
particles
Synchrotron oscillations: ~ tens of ms
Revolution period: ~ hundreds of ns to ms
Betatron oscillations: ~ tens of ns