A Sub-Picosecond Photon Pulse Facility for SLAC

TL;DR: In this article, it is shown that the 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.

Abstract: 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{sup 25} photons/(sec x mm{sup 2} x mrad{sup 2} x 0.1% bandwidth), and 10{sup 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 and D for the LCLS.

Summary

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.
• This proposal envisions electron bunch compression in three stages, starting with the existing damping ring bunch compressor (Ring-To-Linac section, RTL).
• 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.
• This paper is organized as follows: - Section 1: Layout of the facility and main performance parameters.
• Cost and construction schedule, also known as - Section 7.

1. General layout and projected performance

• A single electron bunch with 2.2×1010 electrons is extracted from the North Damping Ring, as is currently done for injection into the PEP-II storage ring.
• 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 µm).
• 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.

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.
• Probe techniques in use today include optical laser spectroscopy5, electron diffraction6, x-ray diffraction7, and x-ray absorption spectroscopy8.
• For complicated structures, this becomes impossible, and a direct structural measurement through diffraction or core-level spectroscopy is needed.
• 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.
• These are all facilities capable of delivering bunches of the order of 100-200 fsec long.

LCLS FEL11

• 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 SPPS should prove to be a valuable tool for developing ultrafast x-ray techniques.
• And <100 fsec long pulses, important R&D, critical to the LCLS and x-ray FELs in general, could be conducted with a tool that would have no equal in the world in terms of short pulses, peak brightness, and photon flux.
• One method planned for the LCLS uses a transverse RF deflector to ‘streak’ the bunch onto a profile monitor.
• When operated at full bunch compression, the average temporal structure of the SPPS photon pulses will be similar, both in profile and duration, to the 80 fsec (fwhm) long electron bunches.

3. Electron acceleration and compression

• The electron beam parameters are largely governed by the boundary conditions of supplying beam to the SPPS parasitically to PEP II operation.
• The positron and electron bunches for PEP II are extracted from the linac at the 3 GeV and 9 GeV locations respectively into beamlines that by-pass the rest of the linac.
• The linearly correlated energy spread of 1.6% (rms) in the bunch is introduced by accelerating, in the linac sections between the damping ring and the chicane, at an RF phase of −20° from accelerating crest, as shown in Figure 5d.
• It was found, through computer simulation studies, that the problem of wakefields could be alleviated by shaping the initial charge distribution in the bunch in such way as to over compress19 the bunch in the first RTL compressor.
• 19 F.-J. Decker, R. Holtzapple, T. Raubenheimer, “Over-Compression, a Method to Shape the Longitudinal Bunch Distribution for a Reduced Energy Spread” in LINAC94, August 1994.

Parameter Symbol Value Unit

• These parameters have been verified by 6-D particle tracking using the code Elegant21.
• The processes of emittance growth due to incoherent (ISR) and coherent synchrotron radiation (CSR) in the various bends have been computed and found to increase the emittance by 15-20%.
• In the FFTB, the large energy spread requires a new sextupole magnet to be located at the point of largest momentum dispersion.
• The new 10-cm long sextupole magnet is already available from the SLC final focus where it was never used.
• With the new sextupole, the remaining chromatic effects increase the emittances by only 2-3 %.

4. The undulator

• The choice of photon energy range depends on the electron energy and on the undulator characteristics.
• An internally copper plated stainless steel beampipe, with either circular or oblong cross section, will be inserted in the gap when the segments are assembled.
• The structure of the undulator is shown in the figures below:.
• The two figures of merit for the photon beam distribution are εp, the photon emittance, and σp,hutch, the photon beam size in the experimental hutch a distance D = 92 meters from the undulator.

5. The radiation characteristics

• This Section tabulates the main properties of the radiation.
• The simulations described in Section 3 indicate that, at full compression, the energy spread in the electron beam will be ~1.5 % .
• In these units the quantity γ∗θ [rad] represents the angular radius of the far-field distribution for arbitrary values of γ.
• **100% multilayer efficiency assumed Power loading of SPPS optical elements.

6. Take-off optics and experimental area layout

• This choice is suggested by the limited space in the FFTB and by the desire to operationally decouple the experimental area from the electron beam to allow access to the experiment when the linac is being set up or during accelerator studies.
• In the first case the incidence angle would be too large to transmit X-rays and in the second case the mirror array would need to be impracticably long.
• In the present conceptual layout the extracted beam would exit the chamber at an angle of 8.5°.
• Given the cited beam line parameters, multilayer optics will be employed for energies between ~800 eV to ~13 keV, and crystals for energies of ~ 6 keV and higher.
• The section of the beam line between the outside of the FFTB tunnel and the hutch will rest on standard support frames and will be shielded with modular concrete segments to satisfy radiation safety requirements.

7. Schedule and cost

• The facility would be located in the Final Focus Test Beam (FFTB), in the area eventually occupied by the LCLS undulator.
• The advantage of starting the research in the FFTB is twofold: it is the most cost and time effective way to achieve sub-picosecond pulses and it allows to carry out accelerator studies that are important for the LCLS R&D, while providing a powerful source of spontaneous radiation for experiments.
• This cost (in thousand of dollars) is divided amongst the various components as follows: Compressor and other magnets, electron beam instrumentation 658 Undulator 1,551 X-ray optics 397 Vacuum equipment 564 Controls, cabling, MPS/PPS, power supplies 548 Experimental hall, hutch, laser clean room 664 Laser for pump-probe experiments 691 Accelerator and undulator physics design, project management 1,032 Contigency (30%) 1,832.

Indirect costs 1,710

• The staff needed during the construction phase (17 months) is as follows.
• The numbers represent the Full Time Equivalent over the 17 months construction period.
• Engineering, design and inspection 1.6 Labor and technical support 7.2 Accelerator physics and undulator magnetic design 3.0 X-ray optics physics and beamline design 1.0 Project Management 1.6.
• The cost of running the facility has yet to be estimated, but it will take advantage of the fact that the linear accelerator (where most of the power consumption is) will be available because the rf is normally left on in between PEP-II fills.
• This work is supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences (contract number DE-AC03-76SF00515), also known as *Acknowledgements.

Figures

Figure 12. Undulator spectrum through the 120th harmonic integrated over the emittance-defined angular aperture of the fundamental.

Table 4. Parameters of an undulator optimized for 8.3 keV photon energy

Figure 2. Wavelength chirp and selection with a multi-layer

Table 6. Some of the electron bunch parameters and the main properties of the spontaneous radiation are listed. The parameters of an alternative option of producing ultra-short pulses via time slicing with multi-layers (discussed in Section 2-i) are also listed in Table 6 and are indicated with a *.

Figure 10. Radiation far-field target geometry in normalized angle space.

Figure 8 Vertical photon beam parameters vs. vertical electron β-function. σp,hutch is for αe-= βe-/D.

Figure 7 Horizontal photon beam parameters vs. horizontal electron β-function. σp,hutch is for αe-= βe-/D

Figure 14. Design, construction and commissioning stages.

Figure 4. Layout of the magnetic chicane placed at the 9 GeV location of the linac.

Figure 3. Schematic of ideal bunch compression where the bunch is given a correlated energy spread from head to tail followed by transport through a dispersive section whose path length varies with energy. The vertical axis represents the energy deviation, the horizontal axis the distance along the bunch.

Figure 6. On the left: magnetic structure, with the larger blocks representing the NdFeB magnets and the vanadium permendur poles in between. The end termination is half pole + half magnet. On the right: The mechanical structure end view, with the strong-back frame surrounding the magnetic structure. The screws allow gauge blocks to be placed as spacers.

Table 1. Main SPPS radiation (8.3 keV) and electron beam (28 GeV) parameters

Figure 1. Layout of the SPPS in the SLAC accelerator complex. The root-mean-square bunch length (rms) is shown at various stages.

Figure 11. Full angle-integrated undulator spectrum through the 11th harmonic. The photon energy spread induced by the electron beam energy spread is 3% (rms).

Figure 5. Compression process at various stages. The energy distribution is shown on the left, the phase space distribution is in the center, and the longitudinal distribution is on the right column: a) At damping ring extraction, b) after RTL acceleration, c) after RTL bends, d) at 9 GeV prior to compressor chicane, e) after compressor chicane, f) after linac at FFTB entrance, and g) after FFTB bends. Bunch head toward z < 0.

Table 3. Main electron beam parameters.

Table 7. Normal incidence peak energy loading for various materials at a distance of 34 m from the undulator end (location of the first optical element).

Figure 13. Layout of the x-ray optics transport system and experimental hutch. The magnet downstream of the undulator vertically deflects the electron beam onto the beam dump. The x-ray beam is deflected at the point indicated by “D” and taken to the experimental hutch outside the FFTB tunnel.

Table 2. Radiation characteristics of the SPPS and other ultra-fast x-ray facilities.

Table 5. Electron and photon beam envelope parameters

Figure 9. Optics of the electron transport system to the undulator: β-functions (top curves) and dispersion. The horizontal axis is the distance in meters. QTEST denotes the quadrupole to be added to the line prior to the undulator.

Full text

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.