1. Enhanced Self-Amplified Spontaneous Emission
P. Emmaa, W. Fawleyb, Z. Huanga,
S. Reichec, G. Stupakova,
A. Zholentsb
a) SLAC, b) LBNL, c) UCLA

2. ESASE: “nuts and bolts”
Modulation
Acceleration
Bunching
Energy modulation in the wiggler at ~ 4 GeV
20-25 kA
50 fs laser pulse
lL= 2 microns
Peak current, I/I0
z /lL
Only one optical cycle is shown
Electron beam after bunching at optical wavelength
Laser peak power ~ 10 GW
Wiggler with ~ 10 periods

3. Zoom-in on a single spike
Electron beam phase space after bunching
B=Dg/sg
Peak current
Energy spread
z /Dz0
Peak current and energy distribution within one micro-bunch
*) Dz0 should be > slippage ~ 8 MGlx= 240 nm

4. Shaping x-ray pulse Peak power, P/P0 z /lL
The x-ray radiation output from the entire electron bunch
Radiation from electrons interacted with laser dominate, thus
Absolute synchronization to the pump laser source for ultra-fast experiments with x-rays

5. The output x-ray radiation from a single micro-bunch
-200
-100
100
200
Each spike is nearly temporally coherent and Fourier transform limited
Carrier phase for an x-ray wave is random from spike to spike
Pulses less than 100 attoseconds may be possible with 800 nm laser

6. Coherent off-axis radiation in the undulatorsz c bz y z lu
Off-axis radiation field in the undulator catches up the beam
by one l ~ 2psz in one undulator period lu
1D “wake” (E. Saldin et al., NIM A 417, )
For Ipeak = 23 kA, and z = 30 nm,
we get:  / = 0.15% at Lu ~ 30 m

7. Suppression of off-axis coherent radiation due to finite beam size
angle y<<1
2 = 4p/(lu k)
Bunch form factor :
Dg per meter of undulator
z/lL 1D 2D
Dg per meter of undulator
A consideration for a choice of focusing !
z/lL
Energy loss distribution along the spike

8. Coherent synchrotron radiation in the chicane
Does not look bad at all !
A finite horizontal beam extend prevents the micro-bunching until almost the very end of the chicane. ex ey
Slice emittance after chicane at various locations along the e-beam

9. Resistive wall wake field2b
e-beam
lL= nm 2a
a) Transient wake field effect
Single bunch catch-up distance c 2a bz
~30 m

10. Resistive wall wake field cont’d
b) Steady state wake
800 nm
2a=5 mm
We calculate the electron Coulomb field at the wall of the vacuum chamber: .
and compare it with the field produced by continuous electron beam:
The field from a modulated beam is only a factor ~ 1.3 larger

11. A schematic of the LCLS with ESASE
Linac-0
L 6 m
Linac-1
L  9 m
rf -25°
Linac-2
L  330 m
rf  -41°
Linac-3
L  550 m
rf  -10°
BC1
R5639 mm
BC2
R5625 mm
DL2
R560
DL1
R56-6 mm
undulator
L 130 m
…existing linac
new rf
gun X
Laser Heater
SC Wiggler
SLAC linac tunnel
undulator hall
14.1 GeV
4.54 GeV
z  0.02 mm
Wiggler
Laser
Heater
Laser
New elements

12. Beta and dispersion functions in the LCLS
3-m wiggler
existing buncher

13. Peak current, emittance and energy spread after linac
gex
gey

14. Beta and dispersion functions in the DL2 buncher
two 5% quadrupole gradient changes, plus one new ‘tweaker’ quad
E = 14.1 GeV
R56 = 0.19 mm (nom. is 0.11 mm)

15. At undulator entrance: E = 14.1 GeV

16. X-ray radiation
Average power vs z
2200 nm

17. Pump-probe experiment concept
Laser excitation pulse
X-ray probe pulse ∆t
X-ray detector
sample
ion or e- detector
Requires control/measure of Dt with a resolution better than x-ray pulse duration (possibly as small as 100 attoseconds)

18. Pump-probe cont’d
One can hope to get a required synchronization if all sources are linked to a common origin
Master laser source
Near IR pump
controlled delay
XUV pump
Courtesy H. Kapteyn
laser - e-beam energy modulation
X-ray probe pulse

19. Second Harmonic correlator
Pump-probe cont’d
Jitter control by cross-correlation of wiggler radiation with seed laser pulse
Near IR pump
Second Harmonic correlator
one period wiggler
isochronous bend
LCLS
wiggler radiation, ~0.5 GW
x-rays

20. Potential of ESASE Saturation length Saturation length B=Dg/sg
Shorter gain length
Larger emittance
1.2 mm-mrad, Std.
b=28 m
2.4 mm-mrad, B=8
Saturation length
Saturation length
b=14 m
b=7 m
B=Dg/sg
Beta-function, m
Smaller wavelength
1.5Å, Std.
0.75Å, B=8
Saturation length
Beta-function, m

21. Summary
ESASE offers:
1) Short gain length, high peak power, comparable average power.
2) Easy tunability for a duration of x-ray pulse by laser pulse shaping.
3) Nearly temporally coherent and Fourier transform limited radiation within the spike with random carrier phase between spikes. A solitary attosecond x-ray pulse.
4) Absolute synchronization between laser pulse and x-ray pulse.
5) Relaxing emittance requirement.
6) Shorter x-ray wavelengths