Transcript Single Pass SASE Free Electron Lasers A potential 4th

1 km
EECS 213
Final Report
Kevin Bowers
May 18, 1999
Undulator
To Photon
Lines
To B
Factory
Linac 3
Bunch
Compressor 2
Linac 2
Bunch
Compressor 1
Linac 1
Linac 0
RF Gun
To Existing Linac
Single Pass SASE
Free Electron Lasers
A potential 4th generation light source
What is a Single Pass SASE-FEL?
 FEL - Free Electron Laser
• Electrons traveling through a undulator radiate coherently if
bunched in a length much less than the radiation wavelength
• Storage ring technology cannot bunch electrons to get
radiation wavelength less than about 50 nm
 SASE - Self Amplified Spontaneous Emission
• Radiation interacts with beam; modulating electron energy
• Energy modulation causes electrons to bunch up
• Bunched electrons emit more FEL radation, coherently

Single Pass - No optical cavity
• The radiation is generated in one pass of the electron beam
• Conventional lasers use multiple passes of the radiation
through a gain medium; requires optical cavity
• Present FELs (IR to UV) also typically use optical cavities
The SASE Collective Instability
For an electron in an undulator:
x
z
Erad
 1 K2 
Kc  2z 

vz  c 1 
,
v

sin 
2  x
2 


 u 
y
Beam
But, the radiation field changes :
Bund
 
d e
d
e
 ev  Erad 

vE
2 x rad
dt
dt mc
Fields and Electron Trajectory in a typical undulator
(Espace chage, Bbeam current and Brad omitted)
Erad
ne(z,downstream)
z
vx
z
Initial radiation field and z-vx phase space
d/dt
z
dvz/dt
z
Bunching occurs here in beam frame
z
Velocity modulation causes bunching on axis downstream.
Bunching enhances Erad which further enhances bunching
Conditions for SASE
• Beam and Radiation Satisfy a
Resonance Condition
• Monochromatic Beam
• Gain Length shorter than
Rayleigh Range
• Slippage less than initial bunch
length
To Existing Linac
RF Gun
The SLAC LCLS
Stanford Linear Accelerator Linac Coherent Light Source
Linac 0
 A proposed
 Design
Linac 1
Bunch
Compressor 1
1 km
Linac 2
Bunch
Compressor 2
Linac 3
overview
• A RF laser photoinjector shoots a 3 ps (rms) electron
pulse into the last km of the linac (120 Hz rep rate).
• Electrons are accelerated to 14.3 GeV and compressed
up to a peak current of 3,400 A and 67 fs (rms) pulse
• The electrons are passed into a 99.8 m undulator (n =
3328, K = 3.7, B = 1.32 T). Some R&D issues here.
• SASE FEL interaction produces 820 eV to 8200 eV
tunable coherent radiation (1st harmonic). The
instability saturates at 90m through the undulator.
 Brightness
Undulator
To B
Factory
To Photon
Lines
Single Pass SASE-FEL
greatly exceeds 3rd generation
Peak ~ 1033 photons / s  mm2  mrad 2  01%
. BW
Avg ~ 1022 photons / s  mm2  mrad 2  01%
. BW
 Planned
construction 2002-2005 ($100M)
Conclusions

FELs are a Promising Technology for 4th
Generation Light Sources
 Peak
brightness about ten orders of magnitude over
current 3rd generation facilities
 Similar advance from 1st generation X-ray tubes to
2nd and 3rd generation synchrontron sources
 Many possible applications

SASE FEL technology not ready for
commercial applications though
 LCLS
$100M on top of an already existing facility
 LCLS 1 km linear beam line, 100 m undulator