Scheme for generating and transporting THz radiation to the X-ray experimental hall at the European XFEL

Abstract

The design of a THz edge radiation source for the European XFEL is presented.
We consider generation of THz radiation from the spent electron beam downstream
of the SASE2 undulator in the electron beam dump area. In this way, the THz
output must propagate at least for 250 meters through the photon beam tunnel to
the experimental hall to reach the SASE2 X-ray hutches. We propose to use an
open beam waveguide such as an iris guide as transmission line. In order to
efficiently couple radiation into the iris transmission line, generation of the
THz radiation pulse can be performed directly within the iris guide. The line
transporting the THz radiation to the SASE2 X-ray hutches introduces a path
delay of about 20 m. Since THz pump/X-ray probe experiments should be enabled,
we propose to exploit the European XFEL baseline multi-bunch mode of operation,
with 222 ns electron bunch separation, in order to cope with the delay between
THz and X-ray pulses. We present start-to-end simulations for 1 nC bunch
operation-parameters, optimized for THz pump/X-ray probe experiments. Detailed
characterization of the THz and SASE X-ray radiation pulses is performed.
Highly focused THz beams will approach the high field limit of 1 V/atomic size.

Full text

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arXiv:1112.3511v1 [physics.acc-ph] 15 Dec 2011
DEUTSCHES ELEKTRONEN-SYNCHROTRON
Ein Forschungszentrum der Helmholtz-Gemeinschaft
DESY 11-244
December 2011
Scheme for generating and transporting THz
radiation to the X-ray experimental hall at the
European XFEL
Winfried Deckinga, Gianluca Gelonib, Vitali Kocharyana,
Evgeni Saldinaand Igor Zagorodnova
aDeutsches Elektronen-Synchrotron DESY, Hamburg
bEuropean XFEL GmbH, Hamburg
ISSN 0418-9833
NOTKESTRASSE 85 - 22607 HAMBURG

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Scheme for generating and transporting THz
radiation to the X-ray experimental hall at the
European XFEL
Winfried DeckingbGianluca Geloni,a,1Vitali Kocharyanb
Evgeni Saldinband Igor Zagorodnovb
aEuropean XFEL GmbH, Hamburg, Germany
bDeutsches Elektronen-Synchrotron (DESY), Hamburg, Germany
Abstract
The design of a THz edge radiation source for the European XFEL is presented.We
consider generation of THz radiation from the spent electron beam downstream of
the SASE2 undulator in the electron beam dump area. In this way, the THz output
must propagate at least for 250 meters through the photon beam tunnel to the
experimental hall to reach the SASE2 X-ray hutches. We propose to use an open
beam waveguide such as an iris guide as transmission line. In order to efficiently
couple radiation into the iris transmission line, generation of the THz radiation
pulse can be performed directly within the iris guide. The line transporting the
THz radiation to the SASE2 X-ray hutches introduces a path delay of about 20
m. Since THz pump/X-ray probe experiments should be enabled, we propose to
exploit the European XFEL baseline multi-bunch mode of operation, with 222 ns
electron bunch separation, in order to cope with the delay between THz and X-ray
pulses. We present start-to-end simulations for 1 nC bunch operation-parameters,
optimizedforTHzpump/X-rayprobeexperiments.Detailedcharacterizationofthe
THz and SASE X-ray radiation pulses is performed. Highly focused THz beams
will approach the high field limit of 1 V/atomic size.
1 Introduction
The exploitation of a high powercoherent THz source as a part ofthe LCLS-
II user facility has been proposed in the LCLS-II CDR [1]. THz radiation
1Corresponding Author. E-mail address: gianluca.geloni@xfel.eu
Preprint submitted to 16 December 2011

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pulses can be generated by the spent electron beam downstream of the X-
ray undulator. In this way, intrinsic synchronization with the X-ray pulses
can be achieved. A first, natural application of this kind of photon beams is
for the pump-probe experiments. Through the combination of THz pump
and X-ray probe, XFELs would offer unique opportunities for studies of
ultrafast surface chemistry and catalysis [1]. Also, the LCLS team started
an R&D project on THz radiation production from the spent electron beam
downstream of the LCLSbaseline undulator [2]-[4]. In that case, THz pulses
aregeneratedbyinsertingathinBerylliumfoilintotheelectronbeam.Inthis
paper we describe a scheme for integrating such kind of radiation source at
the European XFEL facility [5].
We begin our considerations with the generation of THz radiation from the
spent electron beam downstream of the SASE2 undulator in the electron
beam-dump area. We then move to consider the transport of THz radiation
pulses from the XFEL beam dump-area to the experimental hall. This con-
stitutes a challenge, because the THz output must propagate at least 250
meters in the photon beam tunnel and in the experimental hall to reach the
SASE2 X-ray hutches. Since THz beams are prone to significant diffraction,
a suitable beamtransport system must be provided to guide the beamalong
large distances maintaining it, at the same time, within a reasonable size.
Moreover, the THz beamline should be designed to obtain a large transmis-
sionefficiencyfortheradiationoverawidewavelengthrange.Transmission
of the THz beam can only be accomplished with quasi-optical techniques.
In this paper, similarly as in [6], which focused on the LCLS baseline, we
propose to use an open beam waveguide such as an iris guide, that is made
of periodically spaced metallic screens with holes, for transporting the THz
beam at the European XFEL. The eigenmodes of the iris guide have been
calculated numerically for the first time by Fox and Li [7] and later obtained
analytically by Vainstein [8, 9]. In [6] we already presented a complete iris
guide theory. In particular, the requirements on the accuracy of the iris
alignment were studied. In order to efficiently couple radiation into the
transmission line, it is desirable to match the spatial pattern of the source
radiation to the mode of the transmission line. To this end, it is advisable
to generate radiation from the spent electron beam directly in the iris line
with the same parameters used in transmission line. In this way, the source
generates THz radiation pulses with a transverse mode that automatically
matches the mode of the transmission line. The theory described in [6]
supports this choice of THz source.
InthepresentworkwepresentaconceptualdesignforaTHzedgeradiation
source at the European XFEL. It includes an 80 m-long electron beam vac-
uumchamberequippedwithanirisline,anda250m-longtransmission line
with the same parameters. The transmission line, which develops through
the XTD7 distribution tunnel and field 5 of the experimental hall, includes
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at least ten 90-degrees turns with plane mirrors at 45 degrees as functional
components. It is possible to match incident and outgoing radiation with-
out extra losses in these irregularities. The transport line introduces a path
delay of about 20 m between THz and X-ray pulses generated from the
same electron bunch. Since THz pump/X-ray probe experiments should be
enabled, in order to cope with this delay we propose to exploit the unique
bunch structure foreseen as baseline mode of operation at the European
XFEL, with 222 ns electron bunch separation, together with an additional
delay line in experimental hall2.
2Principles of THz radiation generation
The availability of any THz source at the European XFEL facility should
be complemented with the availability of a suitable THz beam transport
system, which must guide the beam for distances in the 300 meters range.
The THz beam can only be transmitted with quasi-optical techniques. In
particular, the idea of providing a periodic phase correction for the free-
space beam, in order to compensate for its divergence, is very natural. In
the 1960s numerous attempts of designing various quasi-optical transmis-
sion lines were reported. In particular, it was proposed to use open beam
waveguides such as lens guides, mirror guides, and iris guides [7, 8],[14]-
[19]. The competition among different proposals ended with the victory of
mirror guides, still in use today for example for plasma heating [18, 19].
Focusing of the THz beam can only be provided with reflective optics since
lenses made of any material would reflect and absorb all radiation at long
distances. Fig. 1 (a) shows the optical arrangement of the transport system
based on the use of a mirror guide. Each focusing unit is composed by two
matched copper mirrors. The mirrors are separated in such a way that the
incidentangleissufficientlysmallto minimizeastigmatism. Existing mirror
guides are characterized by a maximal length of about 40-60 m [20, 21]. As
discussed above, atEuropean XFEL facility scientists needa beam transport
system working for significantly longer distances, and the advantages of a
mirror-guide based transport system are not evident.
In this paper we propose, as an alternative to the mirror line solution, to
use an iris line, Fig. 1 (b). Iris lines are characterized by a low attenuation
of the fundamental mode, self-filtering of high order modes, wide opera-
2Thetemporalresolutionofpump-probeexperimentsusingTHzandX-raypulses
should be limited by the duration of the THz pulse (about 300 fs) rather than by
the jitter between the electron bunches (about 50 fs).
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Fig. 1. Optical arrangement of the transport system based on the use of (a) a mirror
guide and (b) an iris guide
tional wavelength range and mechanical integrity of the structure. The first
investigation of the influence of diffraction effects on the formation of the
field eigenmodes in an iris line was carried out by Fox and Li using physical
opticstechniques[7].Averydifferentandmathematicallysolid approach to
the same problem was introduced by Vainstein [8, 9]. His studies are based
on direct solution of Maxwell equations. An analysis of diffraction and re-
flection from the iris edges allowed Vainstein to derive for the first time
analytic expressions for field distribution and mode losses in iris guides. An
infinite, discrete set of eigenfunctions with corresponding complex eigen-
values can be found. This set of modes comprises leaky modes which are
similar albeit not identical to modes of microwave waveguide with resis-
tive walls. For instance, one can find that iris line modes, in contrast with
microwave waveguide leaky modes, are independent of the polarization of
the radiation.
Consider an axially symmetric iris guide with inner radius a and distance
between the two screens b, and call?E(? r,z,ω) the temporal Fourier trans-
form of the transverse electric field of the radiation in the guide. Since the
transverse size of the radiation is much larger than the reduced wavelength
Ż = c/ω, the field envelope??E =?Eexp[−iωz/c] turns out to be a slowly
length, and it obeys the paraxial Maxwell equation. Given the symmetry
of the problem we can introduce polar coordinates (r,φ), and seek a solu-
tion for the slowly varying field amplitude?E(r,φ,z) of given polarization
varying function of the longitudinal coordinate z with respect to the wave-
component in the form
?E = unj(r)exp[−inφ − i∆kzz] ,
(1)
5