An
audience of around 190 scientists and research council representatives enjoyed
presentations from four international speakers (Professors Zhi-Xun Shen ( Stanford
University , USA ), Kioshi Ueda ( Tohoku University , Japan ), Anders Nilsson
(Stanford Synchrotron Radiation Laboratory, USA ), Majed Chergui (EPFL, Lausanne
Switzerland )who gave their visions of the type of science to be enabled by
4GLS. In addition to the presentations,the community were actively engaged in
presenting their views in a series of informal discussion group sessions that
encompassed broad science areas, technical requirements and possible additional
capabilities. Developments in the layout of the 4GLS facility were presented
and feedback is currently being gathered to inform the design studies.
ERLP Sub-System Assembly and Test Activities
Assembly and testing is currently underway of sub systems for the ERLP in the DL Assembly Hall at the rear of the Stores building. The facility consists of an ISO 5 clean room, insertion device construction room, magnet test room and large capacity assembly hall for the range of collaborative projects involving technical build and measurement work at DL. Almost all of the capacity at present is taken up by ERLP to ensure that the project milestones are met.
As shown in the photograph, support girder systems, magnets, diagnostic
devices and vacuum chambers are being assembled into modules prior to installation
into the Accelerator Hall in the Tower building. DL has an excellent track record
of building and operating UHV vacuum systems but new procedures have been necessary
to ensure that the very high particulate standards required for the ERLP are
achieved. All of the vacuum chambers are being subjected to a particle cleansing
process in the clean room that involves the chamber internal surfaces being
subjected to repeated purges of filtered nitrogen, while the chamber external
walls are tapped with a rubber mallet to coach out the particles until a stringent
ISO 5 standard has been achieved.
Major assembly of modules started in June 05 and will continue through to the end of December 05. Transportation of modules from the Assembly Hall to the Accelerator Hall in the Tower building is scheduled from early October 05 to the end of January 06.
ERLP Accelerator Hall Activities
Work in the Accelerator Hall and associated areas continues apace. In
addition to a variety of work on the infrastructure required for ERLP, installation
of the ERLP cryogenic system has been taking place over the summer months and
all works required to enable the commissioning of the 4K liquefier are complete
or nearing completion. The work so far includes installation of the compression
system, the 4K cold box, the 2K pumping system, the liquid nitrogen bulk storage
tank, the control system and most of the interconnecting ambient pipe work,
cabling and utilities. The cryogenic helium transfer lines have arrived on site,
the 4K dewar and 2K cold box are due to be delivered imminently. Engineers from
Linde Kryotechnik and Daresbury Laboratory are scheduled to start 4K commissioning
on 03 October 2005 .
4GLS Technical Design Work Steams Ahead
Work is progressing on the detailed design of 4GLS. In their regular meetings the accelerator physics group are focussing on aspects defining the footprint of the machine, key parameters such as the threshold for beam break-up and minimising the disruptive effects of space charge on the low energy beams. This month will see the first engineering layouts based on the accelerator physics specification. Recruitment in accelerator physics, RF, cryogenics and laser diagnostics is underway - new faces will join the 4GLS team in the near future.
Work towards the design of the 4GLS FEL systems is evolving from the previous broad-brush investigation of various options to more detailed study of the most likely final-design contenders. The overarching aim is to provide robust designs that will meet the wide range of user requirements.
The XUV FEL option currently receiving considerable design attention is an HHG seeded FEL amplifier that incorporates an extra 0-200 MeV linac module immediately before the FEL undulator. The main ERL arc can then operate at a fixed beam energy whilst the XUV FEL line has both undulator gap tuning and 200 MeV electron energy tuning, allowing it to operate across the full 10-100 eV photon energy range. Figures 1 and 2 show the results of full 3D simulations for the 100 eV photons. It demonstrates a peak saturated intensity of ~ 3 GW with near Fourier transform limited pulses of RMS width ~ 35 fs in an FEL interaction length of ~ 20 m. An option for a tunable spontaneous source driven by the exhaust electrons from the XUV FEL is also included, with guaranteed synchronism with the coherent FEL output.
 Figure 1: Plot of the average radiation power as a function of distance through the XUV FEL. |
 Figure 2: Radiation pulse power at saturation (z=19 m) as a function of s=ct. The wider electron beam current of peak 1.5 kA is also shown. |
The current VUV FEL preferred design option is a high average power regenerative amplifier system - effectively a low-Q cavity. 3D simulations have been carried out on the HPCX 1024 processor super-computer at Daresbury Laboratory. These have shown ~100 fs RMS pulses of 10 eV photons at ~220 MW peak power. The peak power can be increased by a factor ~ 3 with cavity detuning at the expense of introducing more structure to the pulse profile. The feedback provided by the cavity improves significantly the spectral bandwidth of the radiation. With operation at a maximum repetition rate of 1.3 GHz the average power is estimated at an impressive 60 kW.
4GLS Collaborations
4GLS international collaborations continue to expand. An Anglo-Italian meeting to cement collaborations between 4GLS and the FERMI@ELETTRA project was held at Daresbury on 6th-7th July 2005. The purpose of the meeting was to identify areas of mutual interest and a number of technical proposals for joint work are now under development. Brian Sheehy, who has DUV-FEL experience from Brookhaven National Laboratory in the USA , is now part of the Team and will be working on FEL seeding lasers. Though based in the USA , he is in frequent contact and will visit the Laboratory again in late September. A one month working visit focussing on low-medium energy space charge effects has also been organised for Sergei Miginsky (from the BINP, Russia ).
Jefferson Laboratory’s accelerator experts led by Lia Merminga, who heads CASA (the Center for Advanced Studies of Accelerators, are boosting the team’s efforts on complex electron beam dynamics issues. Several other collaborations are under negotiation on diagnostics, FELs, FEL optics and photoinjectors.