accelerator physics: synchrotron radiation lecture 2 henrik kjeldsen – isa
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Accelerator Physics:Synchrotron radiation
Lecture 2
Henrik Kjeldsen – ISA
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Synchrotron Radiation (SR)
• Acceleration of charged particles– Emission of EM radiation– In accelerators: Synchrotron radiation
• Our goals– Effect on particle/accelerator– Characterization and use
• Litterature– Chap. 2 + 8 + notes
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General Electric synchrotron accelerator built in 1946, the origin of the discovery of synchrotron radiation. The arrow indicates the
evidence of arcing.
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Emission of Synchrotron Radiation
• Goal– Details (e.g.): Jackson – Classical Electrodynamics– Here: Key physical elements
• Acceleration of charged particles: EM radiation• Lamor: Non-relativistic, total power
• Angular distribution (Hertz dipole)
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Relativistic particles
• Lorenz-invariant form
• Result
c
v
cm
Edtddt
,
1
1,
122
0
2
2
221
d
dE
cd
dp
d
dP
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Linear acceleration
• Using dp/dt = dE/dx:
• Energy gain: dE/dx ≈ 15 MeV/m– Ratio between energy lost and gain:
– = 5 * 10-14 (for v ≈ c)• Negligible
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Circular accelerators
• Perpendicular acceleration:– Energy constant...– dp = pd→ dp/dt = p = pv/R
– E ≈ pc, = E/m0c2
• In praxis: Only SR from electrons
vv
dt
d
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Energy loss per turn
• Max E in praxis: 100 GeV (for electrons)
[m]
[GeV]5.88
2 4
R
E
c
RPdtPE ss
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Angular distribution I
• Similar to Hertz dipole in frame of electron– Relativistic transformation
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cE
p
cE
p
cE
p
p
p
p
P
S
SS
z
y
x
t
/'
'
0
/'
0
'
0
/'
'00
1
'
'tan
0
0 p
p
p
p
z
y
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Spectrum of SR
• Spectrum: Harmonics of frev
• Characteristic/critical frequency
• Divide power in ½
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Spectral Brightness
1E+11
1E+12
1E+13
1E+14
1E+15
1E+16
1E+17
0.001 0.01 0.1 1 10Photon Energy (keV)
Ph
/s*m
m^
2*m
rad
^2
*0.1
BW
Undulator, ASTRID2
Undulator
2T 12 pol wiggler, ASTRID2
Bend, ASTRID1
Bend ASTRID2
ASTRID2
• Horizontal emittance [nm]– ASTRID2:12.1– ASTRID: 140
• Diffraction limit:
4' RR
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Storage rings for SR• SR – unique broad spectrum!• 0th generation: Paracitic use• 1st generation: Dedicated rings for SR• 2nd generation: Smaller beams
– ASTRID?• 3rd generation: Insertion devices (straight sections), small beam
– ASTRID2• 4th generation: FEL
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Insertion devices
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Wigglers and undulators(Insertion devices)
• The magnetic field configuration
• Technical construction
• Equation of motion
• Wigglers vs. Undulators
• Undulator radiation
• The ASTRID undulator
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Coordinate system
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Magnetic field
• Potential:
• Solution:
• Peak field on axis:
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Magnetic field on axis
Constructiona) Electromagnet; b) permanet magnets; c) hybrid magnets
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Insertion devices
• Single period, strong field (2T / 6T)– Wavelength shifters
• Several periods– Multipole wigglers– Undulators
• Requirement– no steering of beam
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Example (ASTRID2):Proposed multi-pole wiggler (MPW)
• B0 = 2.0 T
• = 11.6 cm
• Number of periods = 6
• K = 21.7
• Critical energy = 447 eV
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Summary – multi-pole wiggler(MPW)
• Insertion device in straight section of storage ring
• Shift SR spectrum towards higher energies by larger magnetic fields
• Gain multiplied by number of periods
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Equation of motion
Set Bx = 0, vz = 0
→ coupl. eq.
s
z
s
x
B
B
v
v
ee
0
0BvF
constant c and )( 0Set svsxss
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Undulator/wiggler parameter: K
• K – undulator/wiggler parameter– K < 1: Undulator
• w < 1/
– K > 1: Wiggler• w > 1/
• Equation of motion: s(t)
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Undulator radiation I• Coherent superposition of radiation produced from each periode• Electron motion in lab frame:
• Radiation in co-moving frame (c*):
• Radiation in lab:
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Undulator radiation II
• If not K << 1: Harmonics of w
2
02
2
2, 21
2
K
nu
nw
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0 50 100 150 2000.0
2.0x1014
4.0x1014
6.0x1014
8.0x1014
1.0x1015
Ph
oto
n fl
ux
Photon energy (eV)
K = 2.3 (25 mm gap) Integrated flux
2.02 mrad2
1.02 mrad2
0.52 mrad2
0.252 mrad2
Undulator radiation III
-0.0010-0.0005
0.00000.0005
0.0010
0.00
0.05
0.10
0.15
0.20
0.25
-0.0006-0.0004
-0.00020.0000
0.00020.0004
0.0006
-0.0010-0.0005
0.00000.0005
0.0010
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
-0.0006-0.0004
-0.00020.0000
0.00020.0004
0.0006
-0.0010-0.0005
0.00000.0005
0.0010
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
-0.0006-0.0004
-0.00020.0000
0.00020.0004
0.0006
-0.0010-0.0005
0.00000.0005
0.0010
0.0
0.1
0.2
0.3
0.4
-0.0006-0.0004
-0.00020.0000
0.00020.0004
0.0006
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Insertion devices: Summary
• Wiggler (K > 1, > 1/)– Broad broom of radiation– Broad spectrum– Stronger mag. field: Wavelength shifter (higher
energies!)– Several periods: Intensity increase
• Undulator (K < 1, < 1/)– Narrow cone of radiation: Very high brightness
• Brightness ~ N2
– Peaked spectrum (adjustable)• Harmonics if not K<<1
– Ideal source!
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Use of SR
• Advantage: broad, intense spectrum!
• Examples of use:– Photoionization/absorption
• e.g. h + C+ → C++ + e-
– X-ray diffraction– X-ray microscopy– ...
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Optical systems for SR I
• Purpose– Select wavelength: E/DE ~ 1000 – 10000– Focus: Spot size of 0.1∙0.1 mm2
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Optical systems for SR II
• Photon energy: few eV’s to 10’s of keV– Conventional optics cannot be used
• Always absorption
– UV, VUV, XUV (ASTRID/ASTRID2)• Optical systems based on mirrors
– X-rays• Crystal monochromators based on diffraction
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Mirrors & Gratings
• Curved mirrors for focusing
• Gratings for selection of wavelength
• r and r’ – distances to object and image
• Normally ~ 80 – 90º– Reflectivity!
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sRrr
)cos(2
'
11
Mirrors: Geometry of surface: Plane, spherical, toriodal, ellipsoidal, hypobolic, ...
• Plane: No focusing (r’ = -r)• Spherical: simplest, but not perfect...
– Tangential/meridian– Saggital
• Toriodal: Rt ≠ Rs• Parabola: Perfect focusing of parallel beam• Ellipse: Perfect focusing of point source
)cos(
2
'
11
tRrr
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Focusing by mirrors: Example
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Gratings
• kN = sin()+sin()– NB: < 0– N < 2500 lines/mm
• Optimization– Max eff. for k = (-)1– Min eff. for k = 2, 3
• Typical max. eff. ≈ 0.2
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Design of ‘beamlines’
• Analytically– 1st order: Matrix formalism– Higher orders: Taylor expansion
• Optical Path Function Theory (OPFT)– Optical path is stationary
• Only one element
• Numerically– Raytracing (Shadow)
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Useful equations• Bending radius
• Critical energy
• Total power radiated by ring
• Total power radiated by wiggler
• Undulator/wiggler parameter
• Undulator radiation
• Grating equation
• Focusing by curved mirror (targentical=meridian / saggital)
sRrr
)cos(2
'
11
)cos(
2
'
11
mRrr
2
02
2
2, 21
2
K
nu
nw eVnm 1240 E