| Workshop Summary The19th Advanced ICFA Beam Dynamics Workshop on
"The Physics of, and the Science with, X-Ray Free-Electron Lasers" took place in
Arcidosso (Italy) from the 10th to the 15th of September, 2000. The Workshop was sponsored
by the International Committee for Future Accelerators, the US Department of Energy, the
University of California at Los Angeles, the Stanford Linear Accelerator Center, the
Deutsches Elektronen-Synchrotron and the Lawrence Berkeley National Laboratory, together
with local authorities of the Tuscany, Grosseto and Arcidosso areas. The Workshop's
chairmen were M. Cornacchia (SLAC), I. Lindau (SLAC/Lund. Un.) and C. Pellegrini (UCLA).
Seventy-five scientists, of which 50 are involved in the physics and technology of
accelerators, free-electron lasers and x-ray optics, and 25 in the scientific
applications, attended the workshop. There were plenary and parallel sessions and many
lively discussions, during and after the regular workshop schedule.
Arcidosso is a medieval town in southern Tuscany, close to the city of Sienna. The meeting
took place in the historically evocative scenario of an 11th century castle atop a hill
dominating the nearby valley. The castle was restored in 1989, and preserves the
atmosphere and raggedness of medieval times.
There were two invited lectures on Monday, September 11, to open the subjects and two
summary talks in the afternoon of Friday, September 15. All the other presentations were
either informal or in the form of
posters.
The Group on "Physics and Technology of the XFEL" with introductory talks by
Kwang-Je Kim (ANL) and Jamie Rosenzweig (UCLA), was coordinated by Alberto Renieri
(ENEA-Frascati).
The Group on "Science with the XFEL" was coordinated by Mark Sutton (McGill
University) with introductory talks by Andreas Freund (ESRF) and Ingolf Lindau (SLAC/Lund.
Un.).
These notes reflect the summary talks of the coordinators and the impressions and
recollections of the organizers. The American Institute of Physics will publish the
proceedings of the Workshop.
Summary of discussions and conclusions of Group 1: Physics and Technology of the XFEL
The main issues that were discussed by the 50 participants in this group were the
photo-injector, the production of ultra-short pulses, the effects of wake-fields induced
by the electron bunch, the operation at lower charge and emittance, the possibility of
harmonic generation and the diagnostics in the undulator. The following is a short summary
of the discussions and their conclusions.
It is important to measure the {\it electron bunch emittance}, length and energy spread as
a function of charge and not focus exclusively on the standard photo-injector parameters
(1 nC charge, 1 pi mm-mrad emittance). The low charge (about 0.2 nC charge, 0.6 pi mm-mrad
emittance) option appears as feasible as the standard case used in the LCLS design, and offers the clear
advantage of being less vulnerable to the effects of wake-fields. It has not been studied
as much as the standard case and requires more work.
One should follow the progress with the new guns currently under study, like the pulsed
gun being developed at BNL and Eindhoven, and the Van der Wiel plasma gun.
The studies of the feasibility of {\it electron bunch compression} and/or {\it x-ray pulse
slicing and compression} must be continued, given the importance of this option for the
experimental program. In particular one should study
the possibility of bunch compression when operating at low charge, and the effect of
wake-fields in the two-undulator (seeding) scheme.
Much attention was given to the {\it wake fields} in the undulator vacuum pipe.
Comparative estimates were made using different models proposed by A. Agavonov (Levedev
Physical Institute, Moscow), A. Novokatsky (Darmstadt
Un.), L. Palumbo (Rome University) and G. Stupakov (SLAC). The effects have been
calculated for the following situation: 15 GeV, undulator length of 100 m, a pipe radius
of 2.5 mm, 1 nC charge, 230 fs long bunch. The maximum energy changes along the undulator
length, according to different models
and regimes are:
| Agafonov model: |
2×10-6 |
(roughness height: 100 nm, roughness period: 100 micro-m) |
| Novokatsky: |
2×10-3 |
(roughness height: 100 nm, roughness period: 0.1 micro-m) |
| Palumbo: |
3×10-6 |
(roughness height: 500 nm, roughness period: <10 micro-m) |
| Stupakov: |
10-6 |
(roughness height: 500 nm, roughness period: 100 micro-m) |
In addition, the contribution of the resistive wall effect is about 1.5×10-4.
Additional contribution will come from vacuum ports, instrumentation, discontinuities.
Since this energy change is of the order of the FEL parameter, it can have a serious and
deleterious effect on the LCLS performance. The message from the workshop is that one
should be aware of these effects, in particular for the LCLS small gap undulator. Notice
that the minimum undulator gap considered for the TESLA X-ray FEL is 12 mm, compared to
the present 6 mm of the LCLS.
Possible strategies to reduce the undulator wake-fields effects include reducing the bunch
charge, increasing the undulator gap and reducing the undulator length.
The list of recommendations from the workshop on the {\it surface roughness} problem
include the enhancement of the analytical models to predict realistic surface roughness
conditions and of the numerical simulations to
model realistic randomly distributed surfaces roughness. Experiments should be performed
to measure the effect under controlled conditions of surface roughness.
The possibility of operation at a charge different and lower than 1 nC should be
studied in all its implications. Different modes of controlling the bunch charge and
emittance should also be investigated.
Once the wake-fields and the injector operation at different charges are understood, the
system should be re-optimized, including considerations of various types of undulators,
planar or helical, and with a gap chosen to
minimize the wake-fields to an acceptable level.
The sub-group on undulator diagnostics reviewed the issues related to the
electron and photon beams. The centroid of the electron beam can be measured to mm
resolution with rf Beam Position Monitors (BPMs) or Optical
Transition Radiators (OTR). One of the issues is whether the latter can survive the
intense electron beam and how the surface quality of the OTR might affect the emitted
light. Both questions should be soon be answered
by experiments. On the measurements of the beam profile, there was consensus that
saturation makes the scintillators not usable, while OTRs might be useful. It is also
important to measure the longitudinal characteristics (bunch length and momentum spread)
and the time-resolved slice measurements of emittance and momentum spread. A very
promising
technique for measuring very short bunch lengths uses an rf deflector that rotates the
beam onto a screen. It was suggested that it might be possible to measure photon pulses
down to 10 fs using grating Michelson
interferometers.
One of the outstanding questions concerning the measurements of the x-ray beam is whether
one can separate the spontaneous from the FEL radiation and whether the diagnostics can
survive the x-ray and electron fluxes. It
was recommended to estimate the damage mechanism with ionization as the dominant
mechanism.
Crystalline materials directly impacted by the electron beam may see the space charge
field and be subject to damage. It was suggested that an experiment be done at SLAC using
the FFTB beam to create a high field
gradient on a crystal similar to that that would occur in the LCLS.
It is very important to have a diagnostic system capable of measuring low charge beams in
the linac and undulator.
More detailed studies of the survivability of the detectors and the information they
provide are needed.
Some other noteworthy discussions included the following:
1. The wake fields in the undulator could have a strong effect on the harmonics; we need
more experimental and simulation work to investigate this possibility.
2. The same wake-fields could limit the possibility of reducing theline width or the pulse
length.
3. The X-ray FEL must be optimized including collective effects.
4. A proof of principle of a seeded scheme using High Gain HarmonicGeneration has been
done at Brookhaven; the studies of an X-ray FEL usingthis approach should be continued.
Summary of discussions and conclusions of Group 2: Science with the XFEL
About 25 people attended sessions to discuss the possible scientific applications of a
x-ray FEL. Because of the recent focus on the first experiments with the proposed Linac
Coherent Light Source at Stanford, the discussions were mainly focussed on these
proposals. The extension of the characteristics beyond the initial stage and the further
developments of
the source were also part of the program.
Six scientific areas were discussed: Atomic Physics, Warm Dense Matter, Femtosecond
Chemistry, Imaging/Holography, Bio-molecular Structures and X-Ray Fluctuations
Spectroscopy.
New phenomena can be studied in atomic physics. Hollow atoms, where inner core
electrons have been removed with outer valence electrons still in place, appear
especially interesting. Non-linear x-ray interactions are of interest, i.e. parametric
down-conversion, two-photon absorption and two-photon mixing. Even with an unfocussed
LCLS-type beam it is possible to achieve saturation for photo-ionization. With a focussed
beam the Compton scattering will saturate.
Warm dense matter (WDM) is a new form of matter, between highly ionized plasma
and condensed matter. Though WDM is of great importance in many fields, i.e. laser plasma
production, inertial fusion and astrophysics, its basic properties are still basically
unknown. With a x-ray FEL beam, WDM can both be created and probed.
In femtosecond chemistry it is of great interest to study bond changes on the
time scale characteristic for breaking and forming bonds. This would involve pump-probe
experiments where the system is excited with a conventional laser and the structure
changes are probed dynamically with the x-ray FEL beam.
The workshop addressed the possibility of imaging and holography of non-crystalline
samples and small nano-structures. Bio-fragments and bio-molecules are also an
extension of this work. The radiation damage and
the amount of structural information that can be extracted before the molecules fly apart
are key issues. For small structures, great advances have been made in computer modeling.
It would be desirable to extend the models to bulk samples.
X-ray intensity fluctuation spectroscopy is already being pioneered at third
generation light sources and its extension to x-ray FELs, in terms of the time-scales and
length-scales, were discussed, together with the possibility of studying a broad range of
materials.
There were intense discussions trying to define the most important radiation
characteristics. This will of course in many cases depend on the specific experiments, but
in general terms the following order was established, in de-creasing order of importance:
1. Beam position stability
2. Beam focusing
3. Synchronization for pump-probe
4. Shorter pulses
5. Smoother pulses
6. Reduced pulse to pulse intensity fluctuations
M. Cornacchia, I. Lindau, C. Pellegrini
|
