• P. Emma, Z. Huang, C. Limborg-Deprey, J. Wu
  SLAC, Menlo Park, California
• W.M. Fawley, M.S. Zolotorev
  LBNL, Berkeley, California
• S. Reiche
  UCLA, Los Angeles, California

The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser (FEL) project based on the SLAC linac. The nominal parameter set is founded on a 1-nC bunch charge and normalized emittance of about 1 micron. The most challenging issues, such as emittance generation, wakefields, and coherent synchrotron radiation (CSR), are associated with the high bunch charge. In the LCLS in particular, with its strong linac wakefields, the bunch compression process produces sharp temporal horns at the head and tail of the bunch with degraded local emittance, effectively wasting much of the charge. The sharp horns intensify CSR in the bends and further drive a strong resistive-wall wakefield in the long FEL undulator. Although these issues are not insurmountable, they suggest a lower bunch charge may be more suitable. This study uses a 0.2-nC bunch charge and 0.85-micron emittance with only 30 A of peak current in the injector, producing the same FEL saturation length. The resulting performance is more stable, has negligible resistive-wall wakefield, greatly reduced CSR effects, and no transverse wakefield emittance dilution in the linac, with no change to the baseline engineering design.

• S. Reiche
  UCLA, Los Angeles, California
• K.L.F. Bane, P. Emma, Z. Huang, H.-D. Nuhn, G.V. Stupakov
  SLAC, Menlo Park, California
• W.M. Fawley
  LBNL, Berkeley, California

The Linac Coherent Light Source (LCLS) Free-Electron Laser will operate in the wavelength range of 1.5 to 15 Angstroms. Energy loss due to wakefields within the long undulator can degrade the FEL process by detuning the resonant FEL frequency. The wakefields arise from the vacuum chamber wall resistivity, its surface roughness, and abrupt changes in its aperture. For LCLS parameters, the resistive component is the most critical and depends upon the chamber material (e.g. Cu) and its radius. To study the expected performance in the presence of these wakefields, we make a series of "start-to-end" simulations with tracking codes PARMELA and ELEGANT and time-dependent FEL simulation codes Genesis 1.3 and Ginger. We discuss the impact of the wakefield on output energy, spectral bandwidth, and temporal envelope of the output FEL pulse, as well as the benefits of a partial compensation obtained with a slight z dependent taper in the undulator field. We compare these results to those obtained by decreasing the bunch charge or increasing the vacuum chamber radius. We also compare our results to those predicted in concurrent analytical work.

• B.A. Prichard, R.J. Briggs
  SAIC, Alamo, California
• J. Barraza, M. Kang, K. Nielsen
  LANL, Los Alamos, New Mexico
• F.M. Bieniosek, K. Chow, W.M. Fawley, E. Henestroza, L. R. Reginato, W. Waldron
  LBNL, Berkeley, California
• T.E. Genoni, T.P. Hughes
  ATK-MR, Albuquerque, New Mexico

DARHT employs two perpendicular electron Linear Induction Accelerators to produce intense, bremsstrahlung x-ray pulses for flash radiography. The second axis, DARHT II, features an 18 MeV, 2-kA, 2-microsecond accelerator. DARHT II accelerator cells have undergone a series of test and modeling efforts to fully understand their sub par performance. These R&D efforts have led to a better understanding of Linear Induction Accelerator physics for the unique DARHT II design. Specific improvements have been identified and tested. Improvements in the cell oil region, the cell vacuum region, and the PFNs have been implemented in the prototype units that have doubled the cell’s performance. A series of prototype acceptance test are underway on a number of prototype units to demonstrate that the required cell lifetime is met at the improved performance levels. Early acceptance tests indicate that the lifetime requirements are being exceeded. The shortcomings of the previous design are summarized. The improvements to the original design, their resultant improvement in performance, and various test results are included. The final acceptance test results will also be included.

• C. Ekdahl, E.O. Abeyta, R. Bartsch, L. Caudill, K.-C.D. Chan, D. Dalmas, S. Eversole, R.J. Gallegos, J. Harrison, M. Holzscheiter, E. Jacquez, J. Johnson, B.T. McCuistian, N. Montoya, S. Nath, K. Nielsen, D. Oro, L. Rodriguez, P. Rodriguez, L.J. Rowton, M. Sanchez, R. Scarpetti, M. Schauer, D. Simmons, H.V. Smith, J. Studebaker, G. Sullivan, C. Swinney, R. Temple
  LANL, Los Alamos, New Mexico
• H. Bender, W. Broste, C. Carlson, G. Durtschi, D. Frayer, D. Johnson, K. Jones, A. Meidinger, K.J. Moy, R. Sturgess, A. Tipton, C.-Y. Tom
  Bechtel Nevada, Los Alamos, New Mexico
• R.J. Briggs
  SAIC, Alamo, California
• Y.-J. Chen, T.L. Houck
  LLNL, Livermore, California
• S. Eylon, W.M. Fawley, E. Henestroza, S. Yu
  LBNL, Berkeley, California
• T.P. Hughes, C. Mostrom, Y. Tang
  ATK-MR, Albuquerque, New Mexico
• M.E. Schulze
  GA, San Diego, California

When completed, the DARHT-II linear induction accelerator (LIA) will produce a 2-kA, 18-MeV electron beam with more than 1500-ns current/energy "flat-top." In initial tests DARHT-II has already accelerated beams with current pulse lengths from 500-ns to 1200-ns full-width at half maximum (FWHM) with more than1.2-kA, 12.5-MeV peak current and energy. Experiments are now underway with a ~2000-ns pulse length, but reduced current and energy. These pulse lengths are all significantly longer than any other multi-MeV LIA, and they define a novel regime for high-current beam dynamics, especially with regard to beam stability. Although the initial tests demonstrated absence of BBU, the pulse lengths were too short to test the predicted protection against ion-hose instability. The present experiments are designed to resolve these and other beam-dynamics issues with a ~2000-ns pulse length beam.