• J. Wu, P. Emma, L. Hendrickson
  SLAC, Menlo Park, California

The Linac Coherent Light Source (LCLS) will be the world's first x-ray free-electron laser (FEL). To ensure the vitality of FEL lasing, it is critical to preserve the high quality of the electron beam during acceleration and compression. The peak current and final energy are very sensitive to system jitter. To minimize this sensitivity, a longitudinal feedback system on the bunch length and energy is required, together with other diagnostics and feedback systems (e.g., on transverse phase space). In this paper, we describe a simulation framework, which includes a realistic jitter model for the LCLS accelerator system, the RF acceleration, structure wakefield, and second order optics. Simulation results show that to meet the tight requirements set by the FEL, such a longitudinal feedback system is mandatory.

• J. Wu, P. Emma, Z. Huang
  SLAC, Menlo Park, California

The Linac Coherent Light Source (LCLS) will be the world's first x-ray free-electron laser (FEL). To ensure the vitality of FEL lasing, a longitudinal feedback system is required together with other diagnostics. In this paper, we study the possibility of using Coherent Synchrotron Radiation (CSR) from the chicane as the diagnostic tool for bunch length feedback. Calculations show that CSR is a good candidate, even for the non-Gaussian, double-horn longitudinal charge distribution. We further check the feasibility for low and high charge options, and also the possibility for detecting the microbunching.

• C.D. Barnes, F.-J. Decker, P. Emma, M.J. Hogan, R.H. Iverson, P. Krejcik, C.L. O'Connell, R. Siemann, D.R. Walz
  SLAC, Menlo Park, California
• C.E. Clayton, C. Huang, D.K. Johnson, C. Joshi, W. Lu, K.A. Marsh
  UCLA, Los Angeles, California
• S. Deng, T.C. Katsouleas, P. Muggli, E. Oz
  USC, Los Angeles, California

• Z. Huang, D. Dowell, P. Emma, C. Limborg-Deprey, G.V. Stupakov, J. Wu
  SLAC, Menlo Park, California

• 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.

• P. Emma, L.D. Bentson, R.A. Erickson, H. Fieguth, J. Seeman, A. Seryi
  SLAC, Menlo Park, California

With the impending construction of the Linac Coherent Light Source (LCLS) at SLAC, displacing the well-used Final Focus Test Beam (FFTB) area, there is growing interest in developing a new test beam facility, available during LCLS operations and located in the old B-Line tunnel at the end of the linac. The success of the Sub-Picosecond Pulse Source (SPPS) and the desire to preserve this capacity suggests a new beamline with similar or improved electron beam quality, including bunch length compression to 10 microns. Beam availability during LCLS operations requires a new 1.2-km bypass line connecting the 2/3-point of the linac with the B-Line. A second operating mode, with LCLS not in use, involves a trajectory directly from the end of the linac to the B-line. This feature provides the highest beam quality at 30 GeV, and also allows a possible third operational mode by deflecting a few of the very high-brightness 120-Hz, 14-GeV LCLS bunches at low rate (1-10 Hz) into the B-line. Finally, linear collider research can be carried out in a short final focus system at the end of the B-Line, capable of producing a 70-nm rms beam size. We describe a possible design for these systems.

• M.J. Hogan, C.D. Barnes, F.-J. Decker, P. Emma, R.H. Iverson, P. Krejcik, C.L. O'Connell, R. Siemann, D.R. Walz
  SLAC, Menlo Park, California
• C.E. Clayton, C. Huang, D.K. Johnson, C. Joshi, W. Lu, K.A. Marsh, W.B. Mori
  UCLA, Los Angeles, California
• S. Deng, T.C. Katsouleas, P. Muggli, E. Oz
  USC, Los Angeles, California

In the plasma wakefield accelerator, a short relativistic-electron bunch drives a large amplitude plasma wave or wake. In experiment E-164X, we use the 28.5 GeV, ultra-short (?80 femtosecond), high peak-current (?30 kiloamperes) bunch now available at the Stanford Linear Accelerator Center Final Focus Test Beam facility. The head of this bunch fieldionizes a lithium vapor and excites the wake, and the tail samples the accelerating field. The latter is accomplished by setting the plasma density to match the plasma wavelength to the bunch length. After the plasma, the bunch is dispersed in energy by an imaging magnetic-spectrometer. Preliminary analysis shows that gradients in excess of 15 GeV/m are excited over a plasma length of approximately 10 cm, leading to energy gain on the order of of 1.5 GeV, or about an order of magnitude larger than energy gains reported to date. This gradient is also three orders of magnitude larger than that in the three-kilometer long Stanford linear accelerator that produces the incoming beam. These results are obtained in a new regime for beam-driven plasma accelerators in which the electron bunch creates its own plasma. The current status of the experiment as well as future plans will be discussed.

• P.A. McIntosh, R. Akre, R.F. Boyce, P. Emma, S. Hill, E. Rago
  SLAC, Menlo Park, California

The Linac Coherent Light Source (LCLS) at SLAC, when it becomes operational in 2009, will provide its user community with an X-ray source many orders of magnitude brighter than anything available in the world at that time. The electron beam acceleration will be provided by existing and new RF systems capable of maintaining the amplitude and phase stability of each bunch to extremely tight tolerances. RF feedback control of the various RF systems will be fundamental in ensuring the beam arrives at the LCLS undulator at precisely the required energy and phase. This paper details the requirements for RF stability for the various LCLS RF systems and also highlights proposals for how these injector and Linac RF systems can meet these constraints.

• P.A. McIntosh, R. Akre, J. Brooks, P. Emma, E. Rago
  SLAC, Menlo Park, California

A single X-band (11.424 GHz) accelerating structure is to be incorporated in the LCLS Linac design to linearize the energy-time correlation (or gradient) across each bunch, features which originate in the preceding accelerating structures (L0 and L1). This harmonic RF system will operate near the negative RF crest to decelerate the beam, reducing these non-linear components of the correlation, providing a more efficient compression in the downstream bunch compressor chicanes (BC1 and BC2). These non-linear correlation components, if allowed to grow, would lead to Coherent Synchrotron Radiation (CSR) instabilities in the chicanes, effectively destroying the coherence of the photon radiation in the main LCLS undulator. The many years devoted at SLAC in the development of X-band RF components for the NLC/JLC linear collider project, has enabled the technical and financial realization of such an RF system for LCLS. This paper details the requirements for the X-band system and the proposed scheme planned for achieving those requirements.

• D. Wang, W. Graves, D. Wang, T. Zwart
  MIT, Middleton, Massachusetts
• P. Emma, J. Wu
  SLAC, Menlo Park, California
• G. Huang
  LBNL, Berkeley, California

The next generation of x-ray FEL requires very high quality electron beams for producing unprecedented x-ray radiations. In proposed x-ray FEL facilities, especially those that use multi-stage high gain high harmonic (HGHG) principle to obtain coherence in both transverse and longitudinal dimensions, the arrival timing of electron bunches must be very precise to ensure the seed laser overlap the desired sections of the electron bunch. A scheme is proposed to achieve 10s fs level of arrival timing control level.

• C. Limborg-Deprey, P. Emma
  SLAC, Menlo Park, California

The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser (FEL) project based on the SLAC linac. The LCLS Photoinjector beamline has been designed to deliver 10 ps long electron bunches of 1nC with a normalized transverse emittance of less than 1 mm.mrad for 80% of the slices constituting the core of the bunch at 135 MeV. Tolerances and regulation requirements are tight for this tuning. The main contribution to emittance is the "cathode emittance which counts for 0.72 mm.mrad for the nominal tuning. As the "cathode emittance" scales linearly with laser spot radius, the emittance will be dramatically reduced for smaller radius, but this is only possible at lower charge. In particular, for a 0.2nC, we believe we can achieve an emittance closer to 0.4 mm.mrad. This working point will be easier to tune and the beam quality should be much easier to maintain than for the nominal one. In this paper, we also discuss how emittance could be further reduced by using the appropriate laser pulse shaping.

• 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.

• P. Emma, Z. Huang, J. Wu
  SLAC, Menlo Park, California

• P. Emma, K.L.F. Bane
  SLAC, Menlo Park, California

Many linear accelerators, such as linac-based light sources and linear colliders, apply longitudinal phase space manipulations in their design, including electron bunch compression and wakefield-induced energy spread control. Several computer codes handle such issues, but most require detailed information on the transverse focusing lattice. In fact, in most linear accelerators, the transverse distributions do not significantly affect the longitudinal, and can be ignored initially. This allows the use of a fast 2D code to study longitudinal aspects without time-consuming considerations of the transverse focusing. LiTrack is based on a 15-year old code (same name) originally written by one of us (KB), which is now a MATLAB-based code with additional features, such as a graphical user interface and output plotting. The single-bunch tracking includes RF acceleration, bunch compression to 3rd order, geometric and resistive wakefields, aperture limits, synchrotron radiation, and flexible output plotting. The code was used to design both the LCLS and the SPPS projects at SLAC and typically runs in <1 minute. We describe the features, show some examples, and provide access to the code.

• L. Serafini, F. Alessandria, A. Bacci, S. Cialdi, C. De Martinis, D. Giove, M. Mauri, M. Rome, L. Serafini
  INFN-Milano, Milano
• D. Alesini, M. Bellaveglia, S. Bertolucci, M.E. Biagini, R. Boni, M. Boscolo, M. Castellano, A. Clozza, G. Di Pirro, A. Drago, A. Esposito, M. Ferrario, L. Ficcadenti, D. Filippetto, V. Fusco, A. Gallo, G. Gatti, A. Ghigo, S. Guiducci, M. Incurvati, C. Ligi, F. Marcellini, M.  Migliorati, A. Mostacci, L. Palumbo, L. Pellegrino, M.A. Preger, R. Ricci, C. Sanelli, M. Serio, F. Sgamma, B. Spataro, A. Stecchi, A. Stella, F. Tazzioli, C. Vaccarezza, M. Vescovi, C. Vicario
  INFN/LNF, Frascati (Roma)
• I. Boscolo, C. Maroli, V. Petrillo
  Universita' degli Studi di Milano, MILANO
• F. Broggi
  INFN/LASA, Segrate (MI)
• L. Catani, E. Chiadroni, A. Cianchi, E. Gabrielli, S. Tazzari
  INFN-Roma II, Roma
• F. Ciocci, G. Dattoli, A. Dipace, A. Doria, F. Flora, G.P. Gallerano, L. Giannessi, E. Giovenale, G. Messina, P.L. Ottaviani, S. Pagnutti, G. Parisi, L. Picardi, M. Quattromini, A. Renieri, G. Ronci, C. Ronsivalle, M. Rosetti, E. Sabia, M. Sassi, A. Torre, A. Zucchini
  ENEA C.R. Frascati, Frascati (Roma)
• D. Dowell, P. Emma, C. Limborg-Deprey, D.T. Palmer
  SLAC, Menlo Park, California
• D. Levi, M. Mattioli, G. Medici, P. Musumeci, D. Pelliccia
  Università di Roma I La Sapienza, Roma
• M. Nisoli, S. Stagira, S. de Silvestri
  Politecnico/Milano, Milano
• M. Petrarca
  INFN-Roma, Roma
• J.B. Rosenzweig
  UCLA, Los Angeles, California