• P.H. Williams, D.J. Dunning, N. Thompson
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
• D. Angal-Kalinin, J.K. Jones, P.H. Williams
  Cockcroft Institute, Warrington, Cheshire
• R. Bartolini, I.P.S. Martin
  JAI, Oxford
• J. Rowland
  Diamond, Oxfordshire

We present progress in the design of the recirculating linac option for the UK New Light Source. Improvements in all accelerator sections have been made such that the output meets the required specifications to drive the seeded NLS FELs. Full start-to-end simulations and tolerance studies are presented together with a comparison to the baseline, single pass linac design.

• D.J. Dunning, N. Thompson
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
• R. Bartolini
  JAI, Oxford
• H. Geng, Z. Huang
  SLAC, Menlo Park, California
• B.W.J. McNeil
  USTRAT/SUPA, Glasgow

Optimisation studies of an HHG-seeded harmonic cascade FEL design for the UK's proposed New Light Source (NLS) facility are presented. Three separate FELs are planned to meet the requirements for continuous coverage of the photon energy range 50-1000 eV with variable polarisation, 20 fs pulse widths and good temporal coherence. The design uses an HHG seed source tuneable from 50-100 eV to provide direct FEL seeding in this range, and one or two stage harmonic cascades to reach the higher photon energies. Studies have been carried out to optimise a harmonic cascade FEL operating at 1 keV; topics investigated include modulator configuration, seed power level and effects of the HHG seed structure. FEL simulations using realistic electron beam distributions are presented and tolerance to increased emittance has been considered.

• N. Thompson, D.J. Dunning
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
• B.W.J. McNeil
  USTRAT/SUPA, Glasgow
• N. Thompson
  Cockcroft Institute, Warrington, Cheshire

A scheme for the generation of attosecond pulse trains in FEL amplifiers was recently proposed*. The method uses repeated equal temporal delays between the electron bunch and co-propagating radiation to generate a modal structure in the radiation field. The modes may be phase-locked via an energy modulation in the electron beam. As a consequence of the radiation /electron delays, the relative radiation /electron slippage during the interaction is increased and leads to a longer cooperation length with the effect of improving the temporal coherence. In this paper we present simulations demonstrating this effect. In particular, we show that the average spacing between the temporal spikes in a SASE FEL is increased in proportion to the increase in the cooperation length. It may therefore be possible to operate a SASE FEL in single-spike mode with longer, higher charge, electron bunches than previously thought possible.

* Physical Review Letters 100, (203901) 2008.

• I.P.S. Martin
  Diamond, Oxfordshire
• R. Bartolini, I.P.S. Martin
  JAI, Oxford
• D.J. Dunning, N. Thompson
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire

The New Light Source project aims to construct a suite of seeded free-electron lasers driven by a 2.25GeV cw super conducting linac. As part of the upgrade path, a number of options are being considered for generating ultra short (<1fs) soft x-ray pulses, with low-charge 'single-spike' operation and bunch slicing like approaches of particular interest, including as a possible extension to echo-enhanced harmonic generation. In this paper we present the status of this work, including recent results from fully start to end simulations.

• H. Geng, Y.T. Ding, Z. Huang
  SLAC, Menlo Park, California
• R. Bartolini
  Diamond, Oxfordshire
• D.J. Dunning, N. Thompson
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire

The crossed-planar undulator is a promising scheme for full polarization control in an x-ray FEL*. For SASE FELs, it has been shown a maximum degree of circular polarization of about 80% is achievable**. In this paper, we study the effectiveness of a cross undulator for a seeded x-ray FEL. The degree of circular polarization for both the fundamental and the harmonic radiation are considered.

* K.-J. Kim, Nucl. Instrum. Methods A445, 329 (2000).
** Y. Ding, Z. Huang, Phys. Rev. ST-AB 11, 030702 (2008).

• Y.M. Saveliev, R. Bate, R.K. Buckley, S.R. Buckley, J.A. Clarke, P.A. Corlett, D.J. Dunning, A.R. Goulden, S.F. Hill, F. Jackson, S.P. Jamison, J.K. Jones, L.B. Jones, S. Leonard, P.A. McIntosh, J.W. McKenzie, K.J. Middleman, B.L. Militsyn, A.J. Moss, B.D. Muratori, J.F. Orrett, S.M. Pattalwar, P.J. Phillips, D.J. Scott, E.A. Seddon, B.J.A. Shepherd, S.L. Smith, N. Thompson, A.E. Wheelhouse, P.H. Williams
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
• P. Harrison, D.J. Holder, G.M. Holder, A.L. Schofield, P. Weightman, R.L. Williams
  The University of Liverpool, Liverpool
• D. Laundy
  STFC/DL, Daresbury, Warrington, Cheshire
• T. Powers
  JLAB, Newport News, Virginia
• G. Priebe, M. Surman
  STFC/DL/SRD, Daresbury, Warrington, Cheshire

Progress made in ALICE (Accelerators and Lasers In Combined Experiments) commissioning and a summary of the latest experimental results are presented in this paper. After an extensive work on beam loading effects in SC RF linac (booster) and linac cavities conditioning, ALICE can now operate in full energy recovery mode at the bunch charge of 40pC, the beam energy of 30MeV and train lengths of up to 100us. This improved operation of the machine resulted in generation of coherently enhanced broadband THz radiation with the energy of several tens of uJ per pulse and in successful demonstration of the Compton Backscattering x-ray source experiment. The next steps in the ALICE scientific programme are commissioning of the IR FEL and start of the research on the first non-scaling FFAG accelerator EMMA. Results from both projects will be also reported.

• R. Bartolini, C. Christou, J.H. Han, I.P.S. Martin, J. Rowland
  Diamond, Oxfordshire
• D. Angal-Kalinin, D.J. Dunning, F. Jackson, B.D. Muratori, N. Thompson, P.H. Williams
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire

We present the progress with the design of the 2.25 GeV superconducting linac for the NLS project. We discuss the performance achieved, the optimisation strategies, the relevance of microbunching instability and the analysis of the effect of various jitter sources