MEMO
10 March 2014
To: The Members of the International X-band XFEL Study Group
From: The Uppsala University TIARA CTF3/ CLIC Group
Ref.: Proposal for an X-band XFEL test plan
A design of a generic X-band accelerating structure for XFEL operation has been made at CERN. Structures of this
design have been fabricated and are now being used as bunch energy linearization units at SwissFEL and
ELETTRA. A study of the design of the various components of a generic XFEL X-band linac is currently been made
by the International X-band XFEL Study Group, which has participants from a number of laboratories including
CERN, the Italian ELETTRA Synchrotron Light Laboratory, the Turkish TAC Accelerator Center, the Australian AS
Synchrotron Light Source, the Uppsala University TIARA Laboratory and others. At the CLIC X-band workshop at
CERN in February 2014 speakers from ELETTRA, SwissFEL and other laboratories planning to build compact XFELs
on their sites expressed an interest to continue this design work.
Recently, the question of the effects of the short-range wake-fields on the energy dispersion of the electron
bunches in the generic X-band XFEL accelerating structure has been raised by several labs. We propose that
these effects be tested using an electron beam with a bunch length of order 100-10 fs. The minimal bunch
length that can be realized with the current CALIFES probe beam linac in the CLIC Test Facility at CERN is about 1
ps. A report has previously been written on how this bunch length can be compressed using a dogleg chicane
[1]. Applying the analysis method derived in this report for an energy spread in the CALIFES of 5*10-3 results in
compressed bunch length of ca 300 fs. It is stated in the report that the presented analysis method is “rough” by
the fact that several detrimental factors were omitted in the derivation of the analysis method.
We propose for the consideration of the International X-band XFEL Study Group the following test strategy plan;
1) The analysis method in ref. [1] should be further elaborated by taking the detrimental factors
mentioned in the end of the report into account in the calculation of the compressed bunch length.
2) In order to make a more reliable estimate of the obtainable compressed bunch-length, efforts should be
made to measure the actual energy spread in CALIFES and to, if at all possible, find ways to reduce it
further.
3) An evaluation should be made of the possibility to install, in the combined CLEX-CTF2 hall at CERN,
instrumentation in the CALIFES beam with which the bunch length can be compressed and measured
and the bunch energy dispersion measured upstream and downstream of a test stand with X-band XFEL
structures of the kind already designed and built by CERN.
The instrumentation required to realize the measurements in 3) is;
- an accelerating cavity operated with a relative rf phase such that a maximal dE/dz gradient be
induced in the bunch,
- a dogleg chicane to compress the bunch length,
-
an X-band crab cavity, of the kind already designed and built by CERN for the CLIC development
project, followed by a high resolution OTR screen, to measure the bunch length and
a system, consisting of dipole magnet and, at a sufficiently large distance, a high resolution OTR
screen, positioned upstream, and another such system positioned downstream, of the accelerating
structure, to measure the bunch energy dispersion in the two positions. For this measurement the
structure need not be rf powered.
4) Provided that the result of the studies under 1, 2 and 3 indicate the feasibility of the proposed method
to shorten the CALIFES bunch to the order of 100-10 fs and to measure the bunch energy dispersion
upstream and downstream of the X-band XFEL structures with satisfactory resolution, the International
X-band XFEL Study Group should act for that the proposed set-up with CALIFES be planned and realized
with the aim to carry out the measurements in 2016-2017.
Under the assumption of that the results of these beam tests are satisfactory and lead to that X-band structures
be used for XFELs and that the current application in Sweden for funding of a X-band Test Stand in Uppsala will
be approved, the Uppsala Group will, in agreement with CTF3 and the International X-band XFEL Study Group,
undertake to test, condition and measure the spark rate of X-band XFEL structures for the FEL laboratories
taking part in the International X-band XFEL Study Group.
Tord Ekelof
Head of the Uppsala University FREIA Division
References
1. V. Ziemann, “Short Pulse Capabilities of the Instrumentation Beam line”, Draft of May 6, 2010