SLAC-PUB-8950
A Sub-Picosecond Photon Pulse Facility for SLAC
Work supported by Department of Energy contract DE–AC03–76SF00515.
M. Cornacchia et al.
Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309
August 2001
LCLS-TN-01-7
1
A Sub-Picosecond Photon Pulse Facility for SLAC*
M. Cornacchia
a
, J. Arthur
a
, L. Bentson
b
, R. Carr
a
, P. Emma
b
, J. Galayda
a, b
, P. Krejcik
b
,
I. Lindau
a
, J. Safranek
a
, J. Schmerge
a
, J. Stohr
a
, R. Tatchyn
a
, A. Wootton
c
Introduction
It is possible to generate very bright sub-picosecond pulses of spontaneous x-ray radiation utilizing the
electron beam from the SLAC linear accelerator and an undulator. The present injection-damping ring
system used to inject into the PEP-II B-Factory can be used for this purpose, without any modification to
the linear accelerator except for a sequence of 4 bending magnets to compress the electron bunch. With a
charge of 3.4 nC per bunch accelerated to 28 GeV and a 10 m long undulator it is quite feasible to
generate pulses of x-rays of 8.3 kV energy (in a spectrum extending to over 1 MeV), 80 fsec long (full-
width-half-maximum), with a peak brightness of the order of 10
25
photons/(sec×mm
2
×mrad
2
×0.1%
bandwidth), and 10
8
photons per pulse in a 0.1% bandwidth.
This facility could be built and operated ahead of the LCLS schedule and would provide both a powerful
tool for research in its own right, as well as a way to conduct critical accelerator and x-ray optics R&D for
the LCLS.
This proposal envisions electron bunch compression in three stages, starting with the existing damping
ring bunch compressor (Ring-To-Linac section, RTL). A second stage of compression is added in the
form of a simple magnetic chicane installed at the 9-GeV location in the linac. A third compression stage
is the existing Final Focus Test Beam (FFTB) beamline where a bunch length of 80 fsec (full-width-half-
maximum, fwhm) at 28 GeV is produced. A new 10-m long undulator located here produces intense
spontaneous x-ray radiation, which is transported to an experimental hutch, placed outside the FFTB area.
The FFTB hall, in terms of its size and infrastructure (mechanical and temperature stability), is very
suitable for setting up such a source.
This paper is organized as follows:
- Section 1: Layout of the facility and main performance parameters.
- Section 2: Scientific opportunities and LCLS-directed R&D.
- Section 3: Physics and engineering aspects of electron bunch compression.
- Section 4: The undulator and its electron optics.
- Section 5: The characteristics of the spontaneous radiation emitted by the undulator.
- Section 6: Take-off x-ray optics and experimental area.
- Section 7: Cost and construction schedule.
The concept of using the SLAC linac to generate radiation from an undulator has evolved from
several earlier proposals
1,2,3,4
a
Stanford Synchrotron Radiation Laboratory, a division of Stanford Linear Accelerator Center
b
Technical Division, Stanford Linear Accelerator Center
c
Lawrence Livermore National Laboratory
1
P.H. Fuoss, NIM A264, 497 (1988).
2
J. Seeman, R. Holtzapple, “An ‘NLC-Style’ Short Bunch Length Compressor in the SLAC Linac”, SLAC-PUB-6201 (1993).
3
P. Emma, J. Frisch, “A Proposal for Femtosecond X-ray Generation in the SLC Collider Arcs”, SLAC-PUB-8308 (1999).
4
P. Krejcik, “Parameters for a 30 GeV Undulator Test Facility in the FFTB/LCLS”, SLAC-PUB-8806 (2001).
SLAC-PUB-8950
LCLS-TN-01-7
August 2001
2
1. General layout and projected performance
The general layout of the facility is shown in Figure 1.
new
new
chicane
chicane
in linac at 9 GeV
in linac at 9 GeV
Damping
Damping
Rings
Rings
e
e
-
-
1.2 mm
1.2 mm
50
50
µ
µ
m
m
12
12
µ
µ
m
m
6 mm
6 mm
SLAC Linac
SLAC Linac
1.2 GeV
1.2 GeV
28 GeV
28 GeV
FFTB
FFTB
RTL
RTL
e
e
+
+
Figure 1. Layout of the SPPS in the SLAC accelerator complex. The root-mean-square bunch length (rms) is
shown at various stages.
A single electron bunch with 2.2×10
10
electrons is extracted from the North Damping Ring, as is
currently done for injection into the PEP-II storage ring. With reference to Figure 1, the electron
bunch compression will be done in three stages, starting with the existing Ring-To-Linac (RTL)
compressor at the exit of the damping rings, where the bunch length is compressed from 20 psec (6
mm) to 4 psec (1.2 mm) - rms values. A second stage of compression requires the addition of a new
magnetic chicane to the linac at the 9 GeV location, which compresses the bunch down to 160 fsec
rms (50
µ
m). The third stage of compression occurs in the bends of the present FFTB beamline,
which produce a 35 fsec rms bunch length (12
µ
m) at 28 GeV (80 fsec fwhm). A 10 m long
undulator in the FFTB generates spontaneous radiation at the undulator fundamental photon energy
of 8.3 keV and extending to over 1 MeV. The radiation is transported to an experimental area placed
outside the FFTB. Table 1 lists the main characteristics of the spontaneous radiation and of the
electron beam at the undulator.
Table 1. Main SPPS radiation (8.3 keV) and electron beam (28 GeV) parameters
Peak photon brightness
9.1×10
24
photons/(sec×mm
2
×mra
d
2
×0.1% bw)
Pulse length
80
fsec, fwhm
Average photon
brightness
2.2×10
13
photons/(sec×mm
2
×mra
d
2
×0.1% bw)
Average flux
3.1×10
9
ph/(sec×0.1% bw),
integrated over all angles
3
Photons/pulse
1.0×10
8
In a 0.1% bw, integrated
over all angles
Bunch repetition rate
30
Hz
Charge per bunch
3.4
nC
Energy
28
GeV
Horizontal e
-
emittance
(rms)
9.1x10
-10
m-rad
Vertical e
-
emittance
(rms)
1.8x10
-10
m-rad
2. Scientific opportunities and LCLS R&D
The most exciting feature of the SPPS will be its combination of synchrotron radiation brightness
with sub-picosecond pulse length. This combination will allow the powerful techniques of x-ray
diffraction and x-ray absorption spectroscopy to be used to study fast dynamics at the atomic level.
Sub-psec dynamics are typically studied in pump-probe fashion, using a femtosecond optical laser as
pump. Probe techniques in use today include optical laser spectroscopy
5
, electron diffraction
6
, x-ray
diffraction
7
, and x-ray absorption spectroscopy
8
.
Of these techniques, laser spectroscopy has been by far the most useful, because of the ready
availability of tunable, intense, fsec lasers. However, due to its long wavelength, a laser cannot
directly give structural information on an atomic scale. Atomic structure must be deduced indirectly
from spectroscopy of the outer electron levels. For complicated structures, this becomes impossible,
and a direct structural measurement through diffraction or core-level spectroscopy is needed.
Electron and x-ray diffraction can both determine structures, but these methods are difficult to use in
the sub-psec range. Sources for electron diffraction are intense, but the time resolution is limited by
the difficulty in making sub-psec low-energy electron pulses. Specialized x-ray sources such as laser-
induced plasmas can be very fast, but their low brightness is a severe limitation on their utility.
In Table 2 the SPPS performance is compared with some existing and proposed sub-psec x-ray facilities:
the Ultrafast X-Ray Facility at the Advanced Light Source, the proposed re-circulating linac light source
5
Femtochemistry and Femtobiology: Ultrafast Reaction Dynamics at Atomic Scale Resolution, V. Sandstroem, ed., World
Scientific, Singapore 1997.
6
H. Ihee, et al., Science 291, 458 (2001).
7
R. W. Schoenlein, et al., Science 287, 2237 (2000).
8
C. P. J. Barty, et al., in Time-Resolved Diffraction, J. Helliwell and P.M. Rentzepis, eds., Oxford Univ. Press, New York
1998, p. 44.
4
(ERL) at LBNL, and the projected LCLS Free-Electron Laser. These are all facilities capable of
delivering bunches of the order of 100-200 fsec long. In this table the SPPS undulator is optimized
for 1.5 Å radiation. As it will be discussed in Section 6, undulators covering different wavelength
ranges are also possible. The final choice of undulator remains to be made, and will be based largely
on the requirements of the experiments.
Table 2. Radiation characteristics of the SPPS and other ultra-fast x-ray facilities.
Facilities Peak
brightness*
Pulse length
(fwhm, fsec)
Average
brightness *
Average
flux
(ph/s, 0.1%-bw)
Photons/pulse
0.1%-bw
Rep.
rate
(Hz)
SLAC SPPS
9.1×10
24
80 2.2×10
13
3.1×10
9
1.0×10
8
30
ALS Ultrafast
Fac.(undulator)
9
6
×10
19
100 6×10
10
3×10
6
300 1×10
4
LBNL ERL
10
1.0×10
23
100 1×10
14
2×10
10
2.0×10
6
1×10
4
LCLS FEL
11
1.5×10
33
230 4.2×10
22
2×10
14
1.7×10
12
120
* photons/sec/mm
2
/mrad
2
/0.1%-bandwidth
The peak brightness of SPPS will exceed that of any existing hard x-ray source by several orders of
magnitude, and its pulse length of <100 fsec will allow it to explore the same time correlations as the
fastest existing or proposed sources. The brightness of SPPS will greatly expand the practical range of
sub-psec x-ray diffraction experiments. Where current experiments are limited to high-reflectivity
perfect crystals, the SPPS would allow the use of weaker reflections and powder diffraction. This
would allow studies of a wide variety of phase transitions and chemical reactions in solids. While
SPPS could not support diffraction from very weakly-scattering samples such as gases and biological
crystals, core-level x-ray spectroscopy (NEXAFS and EXAFS) could be used to provide some atomic
structural information. For most experiments it will be necessary to accumulate data over many
pulses, using reversible reactions or reactions in which the reactants can be replenished. The 30Hz
repetition rate of SPPS is particularly favorable for this kind of stroboscopic experiment, since it
matches well with the repetition rates of fsec pump lasers.
Table 2 lists the expected properties of the LCLS x-ray FEL source, which should be constructed at
SLAC in a few years. This source and the other linac-based x-ray facilities that follow will soon
revolutionize the field of fast time-resolved x-ray science
12
. However, there is much to be learned
about performing sub-psec x-ray experiments properly. The SPPS should prove to be a valuable tool
for developing ultrafast x-ray techniques. Areas that require R&D include:
Synchronization. The synchronization of an optical pump laser with an x-ray source at the sub-
psec level will be challenging. Even measuring the time correlation of two such disparate
pulses with precision below 100 fsec will require exploiting and developing novel nonlinear
x-ray effects.
9
Schoenlein and others, “Generation of femtosecond x-ray pulses via laser-electron beam interaction”, Appl. Phys. B 71,
1-10 (2000), Table 1
10
A. Zholents, “On the possibility of a femtosecond x-ray pulse source vased on a recirculator linac”, CBP Tech Note-210,
Nov. 14, 2000.
11
LCLS Design Study Report, SLAC-R-521 (1998).
12
LCLS: The First Experiments, G. Shenoy and J. Stohr, eds., SSRL 2000.
