A Sub-Picosecond Photon Pulse Facility for SLAC
Summary (3 min read)
- 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.
- 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.
Did you find this useful? Give us your feedback
Cites methods from "A Sub-Picosecond Photon Pulse Facil..."
...This has recently been confirmed experimentally at the sub-picosecond pulse source at SLAC, where 30 kA peak currents are achieved from 80 fs, 3 nC electron bunches accelerated to 28 GeV ....
Cites background from "A Sub-Picosecond Photon Pulse Facil..."
...However, for ultra-relativistic electron beams (E > 1 GeV), the space charge limitation on charge density is reduced and short bunches containing charge on the order of several nC can be achieved (Cornacchia et al., 2001)....