PSI Scientific Report 2008
PSI-XFEL  9
or semiconductor surface by means of a laser beam. The
higher energies will be arranged at the tail of the bunch and
cathode is placed on the axis of a 2½ cell, 3 GHz accelerating
particles with lower energies at the head of the bunch. Due
cavity, which immediately accelerates the electron bunch after
to the nonlinearity of the 3 GHz accelerating field, the energy
extraction from the cathode. Solenoid and quadrupole mag-
chirp is slightly too large in the head of the bunch and too
nets in the subsequent structure focus the beam, to minimize
small in the tail. Therefore an X-band (12 GHz) cavity is intro-
the emittance at the exit of the gun complex.
duced before the bunch compressor to compensate for these
For the second gun, several options are possible. The decision
deviations.
on which will be based on the success of ongoing R&D work.
The bunch compressor (BC1) consists of a sequence of four
It will either be a newly-developed photo-electron gun or an
bending magnets, which create an orbit bump around the
alternative gun based on field emission arrays, where electrons
straight motion path in the linac. Since particles with higher
are extracted from a surface by means of high electric field
energies are subject to a smaller deflection in the magnets,
gradients (~ 5 GV/m). Such high gradients can be easily
their orbit lengths are shortened. They are consequently
achieved if the field is applied to micro- or nano-structured
moved from the tail towards the centre of the bunch. Simi-
surfaces where the field is strongly enhanced around tips with
larly, the lower-energy particles at the head of the bunch ex-
small apex radii. In order to mitigate space charge effects, the
perience larger deflections that result in a lengthening of the
energy of the beam is rapidly increased by passing the beam
orbit and a transition towards the bunch centre. The net effect
through a high-voltage and high-gradient diode configuration,
after BC1 is that the length of the bunch is reduced from 10 ps
before entering the first RF accelerating structure. A newly-
(for the 200 pC mode) to 450 fs.
developed high voltage pulser is currently being tested and
The subsequent Linac 2 (with the same cell structure as
further developed. Different surface materials are being
Linac 1) raises the energy to 2.1 GeV. At this point, the second
explored, to discover those which can sustain high surface
magnetic bunch compressor (BC2) is introduced, which re-
gradients without breakdown. Since this concept relies on a
duces the bunch duration to 30 fs, with a corresponding in-
longer initial pulse (40 ps fwhm with 5.5 A peak current), a
crease of the peak current to 2.7 kA. For the succeeding Linac 3,
higher compression is required to reach a sufficiently high
the transverse beam dimensions are already considerably
peak current at the entrance to the undulator. This compres-
smaller, due to the increased beam energy, permitting the
sion starts in the first accelerating cavity, which is fed by two
distance between the focusing quadrupoles to be increased.
frequencies (1.5 GHz and 4.5 GHz). In this way, the longitudi-
One cell here is constructed from four two-metre-long accel-
nal energy distribution in the beam can be suitably shaped to
erating sections between two adjacent quadrupoles, and has
reach a very effective velocity compression. In the low relativ-
a total length of 19 m.
istic regime, particles with different energies still have a
After Linac 3, the electron beam is extracted for the longer-
notable difference in velocities. If they are arranged properly
wavelength FEL lines Athos and Porthos. The nominal energy
in energy along the bunch, they move towards the bunch
at this point is 3.4 GeV, but will be reduced to 2.1 GeV for
centre, and the length is reduced.
Athos by not powering Linac 3. It remains to be verified by
After the gun complex, the bunch can be directed into a diag-
simulations whether the focusing lattice can remain un-
nostic line for complete characterization. A more conven-
changed, since the quadrupole strengths are matched to a
tional accelerating structure follows the gun and comprises
higher energy, otherwise a second extraction point after Linac 2
four S-band structures of 4 m length, surrounded by focusing
will need to be inserted.
solenoids. The maximum accelerating gradient is 20 MV/m.
Only for the 1 Ă…ngstrom wavelength of Aramis is an addi-
In the test setup for this injector presently under construction,
tional boost to 5.8 GeV required, provided by Linac 4, which
a bunch compressor will be placed at the end (250 MeV) for
uses the same cell structure as Linac 3.
test purposes. In the final layout, an additional accelerating
The electron beam quality is now sufficient for the lasing
section will be added (Linac 1) in front of the bunch compres-
process as the beam enters the undulators. The emittance is
sor, boosting the energy to 450 MeV. The higher energy will
reduced by adiabatic damping, and the bunch is longitudi-
alleviate the risk of emittance dilution due to space charge
nally compressed.
effects in the bunch compressor. Linac 1 comprises two FODO
In principle, an electron transversally accelerated in a mag-
cells, each of 10 m length, with two accelerating structures of
netic field emits a broad spectrum of radiation. However, in
2 m length between adjacent quadrupoles. One cell will pro-
an undulator the only wavelengths not to be eliminated by
vide an energy increase of 120 MeV on crest, corresponding
interference effects are those for which the electron beam lags
to an accelerating gradient of 30 MV/m.
behind the photon beam by one wavelength (or an odd integer
During the acceleration process prior to the bunch compressor,
multiple). Due to the long undulator structure, the intensity
an energy chirp will be introduced in the beam. Particles with
of the radiation steadily increases and becomes sufficiently