LCLS: Realization of the Dream

Report
LCLS: Realization of the Dream
P. Emma (SLAC)
Oct. 2, 2010
TOWARDS A 5TH GENERATION LIGHT SOURCE
Catalina Island, CA, USA
First Hard X-Ray Free-Electron Laser is Successful
gex,y = 0.4 mm (slice)
Ipk = 3.0 kA
sE/E = 0.01% (slice)
(25 of 33 undulators installed)
LG = 3.3 m
IT WORKS!
The LCLS Proposal… 1992
C. Pellegrini, “A 4 to 0.1 nm FEL based on the SLAC linac”, in
Workshop on 4th Generation Light Sources, M. Cornacchia and
H. Winick, (Eds), pp. 364-375, 1992. SSRL-Report-92/02.
“…one is forced to have high gain, i.e. to use electron beams with large
peak current, and at the same time small emittance and energy spread.
The road to an X-ray FEL requires the development of electron
beams with unprecedented characteristics.”
Will show only one simple equation…
-+
?
=
Claudio’s & UCLA’s Efforts Produce 3 FELs in 5 years
M. Hogan et al., Phys. Rev. Lett., 80, 289–292 (1998).
M. Hogan et al., PRL, 81, 4867–4870 (1998).
SASE
Process
Just FEL
Taking
16Offmm
UCLA
16-mm
LANL/UCLA AFEL 12 mm
IR Detector Signal [mV]
70
60
50
Spontaneous @ 16µm, w ithin žc
16µm w ithin žc
Ginger - normalized to 0.2nC data
Fit
40
30
>105 gain
20
10
0
0.2
0.3
0.4
0.5
0.6
Charge [nC]
(LEUTL at ANL Saturates in Sep. 2000)
VISA
A. Tremaine et al., PRL. 88, 204801 (2002)
840 nm
March 2001
saturation
starts
BNL-LLNL-SLAC-UCLA
VISA at BNL
16 March 2001
Thanks to Herman Winick, Max Cornacchia, and John Galayda
H. Winick
First Design Study
(1992 – 1995)
Report describes final
LCLS quite accurately
Journal of Electron Spectroscopy and Related
Phenomena, Vol. 75 (1995), pp. 1-8.
M. Cornacchia
Design Study Report
(1996 – 1999)
First funding and
collaborations established
SLAC-R-521, Dec. 1998
J. Galayda
Construction Phase
(2001 – present)
Scope expands to user
facility, construction,
commissioning, + user op’s.
First light: April 9, 2009
What’s so Special About “1 nC” Anyway?
C. Pellegrini, X. Ding, J. Rosenzweig, “Output Power Control in an X-Ray FEL”,
PAC-99, New York, NY, March 1999.  1 - 0.1 nC
We had never even considered <1 nC
before this (PE).
P. Emma, “LCLS Accelerator Parameters and Tolerances for Low Charge Operations”,
SLAC-TN-05-042, May, 1999.  0.1 nC
P. Emma et al., “An Optimized Low-charge Configuration of the Linac Coherent Light
Source”, PAC-05, Knoxville, TN, May 2005.  0.2 nC (resistive wakes under control)
LCLS runs mostly at 0.25 nC, much due to Claudio’s 1999 suggestion
J. Rosenzweig, …, C. Pellegrini, … et al., “Generation of ultra-short,
high brightness electron beams for single-spike SASE FEL operation”,
Nucl. Instrum. Methods Phys. Res., Sect. A 593, 39 (2008).  0.001 nC
Y. Ding et al., “Measurements and simulations of ultralow emittance and
ultrashort electron beams in the linac coherent light source”, Phys. Rev.
Lett. 102, 254801 (2009).  0.02 nC
LCLS Low Charge Operation Now Routine (20 pC)
15 Å,
z = 25 m,
2.41011 ph’s,
Ipk = 2.6 kA,
ge  0.4 µm
Y. Ding
1.2 fs
0.14 µm
z = 25 m
20 pC, 135 MeV, 0.6-mm
spot diameter, 400 µm
rms bunch length (5 A)
Sliced OTR screen with
transverse deflector ON
Idea to run with
20 pC was first
suggested by Joe
Frisch, but it was
hastened by
discussions with
Claudio at SLAC in
2008.
How do we get to <10 femtosecond pulses?
Max and Claudio form a “brain-storming” series of meetings in
2002 with a goal of <10 fs…
Many ideas emerge – some fall by the wayside, and 1-2 look good
Transverse RF cavities in the undulator
More electron compression
Chirped FEL + X-ray compression1
Chirped FEL + monochromator2
Slotted foil…
1. C. Pellegrini, “High Power Femtosecond Pulses from an X-ray SASE-FEL”, NIM A 445 (2000), 124-127.
2. C. B. Schroeder, C. Pellegrini, et al., “Chirped-beam Two-stage Sase-FEL For High Power Femtosecond Xray Pulse Generation”, PAC-01, Chicago, IL, 2001.
Pulse Length Easily Adjusted (500-60 fs)*
X-Ray Pulse Energy
vs. Pulse Length
(2.5 – 3.8 mJ)
Peak FEL Power
vs. Pulse Length
(5-40 GW)
1.7 keV, 250 pC, 23 of 33 undulators inserted
e- bunch length is quickly adjustable (<1 min)
from 60 to 500 fs (hard x-rays: 60 to 100 fs)
* for soft x-rays (0.5-2 keV)
Energy
DEFW/E ≈ 1.0%
Dt ≈ time
200 fsec
x-ray pulse
Self-seeding may
soon be added to
LCLS (Geloni et al)
allowing Claudio’s
scheme to be tested
Energy
Two-stage Undulator for Shorter Pulses
10-4
Dt ≈ time
40 fsec
Si monochromator
(T = 40%)
time
Mitigates
e- energy
jitter and
undulator
wakes
time
UCLA
e43 m
30 m
SASE gain (Psat/103)
C. Schroeder, et al.
52 m
SASE Saturation (25 GW)
Also a DESY scheme which emphasizes line-width reduction (B. Faatz)
Slotted Foil To Produce Narrow or Double X-Ray Pulses
Idea grew out of meetings with
Max and Claudio in 2002-2003:
Thanks to Clive Field, Mark Petree, et al.
PRL 92, 074801 (2004).
FEL power is proportional to slot width (short pulses?)
FEL X-ray Pulse Energy (mJ)
Scan only the
single-slot section
OTR screen in BC2
(1 ft up-beam of foil)
How is an FEL Like a Refrigerator?
Great work was certainly done at SLAC to build this machine,
…and we are all, understandably, quite proud of this effort.
However, as I get more time to appreciate the novelty,
complexity, and history of this amazing machine, I also come
to appreciate another side…
The performance of this machine was theoretically predicted
so accurately that the damn thing worked like a brand new
refrigerator just out of the box?
Yes, we did a good job, but don’t forget that we were provided
with a clear and realistic recipe from Claudio and many others
who led the way.
I stand in amazement at the FEL theorists who brought
us this far, and did it mostly with pencil and paper!
A Crazy Idea?
From Herman Winick:
“…the great majority of scientists thought that it was a
crazy idea and that it would never work.”
So let’s look at just a few of the accelerator and FEL
physics challenges that stood in the way to
see how crazy it really was…
CSR Can Ruin Bend-Plane Emittance in Bunch Compressors!
Use new skew-quad
diagnostic to see timeresolved CSR effects…
Actual Measurements
250 pC
skew quad
OTR12
skew quad streaks beam on OTR12
Energy loss 
BPM X Reading after BC1 (mm)
-25.0°
-22.0°
-28.0°
-27.0°
-25.5°
-26.0°
-23.0°
-24.0°
-26.5°
bunch length
Skew quad in BC1
streaks beam vertically
on OTR screen
Energy-loss
induced
steering
after BC1
Electron beam needs heating or it will m-bunch in the linac!
Heater OFF
bunch
length
m-bunching on
dump screen
in overcompression
Heater ON
Must have a 5-mm straight trajectory over a gain length!
Get additional e-/photon slippage (phase
error) with imperfect trajectory
1Å
e- and photons
phase matched
e- vs. photon
phase error
<5 mm
5m
Trajectory straightness requirements are frighteningly tight !
Producing a sufficiently straight undulator trajectory
requires an empirical beam-based alignment method
Must Preserve 1-Å Micro-Bunching Over Long Undulator!
Now let’s draw this more accurately, choosing a 1-mm period…
2 km!
1 Å (0.0001 mm)
Aspect ratio?
30 mm
And we must preserve
this over a 130-m long
1 mm undulator!
FEL m-bunching
?
e-
In Closing…
So was it a crazy idea … ?
Yes, I must agree…
…completely bonkers!
But thanks Claudio, for such a wonderfully
crazy idea !

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