• S. Pei, C. Adolphsen
  SLAC, Menlo Park, California
• J. Carwardine
  ANL, Argonne
• N.J. Walker
  DESY, Hamburg

The FLASH L-band (1.3 GHz) superconducting accelerator facility at DESY has a Low Level RF (LLRF) system that is similar to that envisioned for ILC. This system has extensive monitoring capability and was used to gather performance data relevant to ILC. Recently, waveform data were recorded with both beam on and off for three, 8-cavity cryomodules to evaluate the input RF and cavity gradient stability and study the RF overhead required to achieve constant gradient during the 800μs pulses. In this paper, we present the recent experimental results and discuss the pulse-to-pulse input RF and cavity gradient stability for both beams on and off cases. In addition, a model of the gradient variation observed in the beam off case will be described.

• F. Zimmermann, O.S. Brüning, E. Ciapala, F. Haug, J.A. Osborne, D. Schulte, Y. Sun, R. Tomás
  CERN, Geneva
• C. Adolphsen
  SLAC, Menlo Park, California
• R. Calaga, V. Litvinenko
  BNL, Upton, Long Island, New York
• S. Chattopadhyay
  Cockcroft Institute, Warrington, Cheshire
• J.B. Dainton, M. Klein
  The University of Liverpool, Liverpool
• A.L. Eide
  LPNHE, Paris

We consider three different scenarios for the recirculating electron linear accelerator (RLA) of a linac-ring type electron-proton collider based on the LHC (LHeC): i) a basic version consisting of a 60 GeV pulsed, 1.5 km long linac, ii) a higher luminosity configuration with a 60 GeV 4 km long cw energy-recovery linac (ERL), and iii) a high energy option using a 140 GeV pulsed linac of 4 km active length. This paper describes the footprint, optics of linac and return arcs, emittance growth from chromaticity and synchrotron radiation, a set of parameters, and the performance reach for the three scenarios.

• F. Zhou, C. Adolphsen, Y.T. Ding, Z. Li, A.E. Vlieks
  SLAC, Menlo Park, California

SLAC is studying the feasibility of using an X-band RF photocathode gun to produce low emittance bunches for applications such as an MeV gamma source (in collaboration with LLNL) and an injector for a compact FEL. Systematic beam dynamics study are being done for a 5.5 cell X-band gun followed by several 53 cm long high-gradient X-band accelerator structures. A fully 3D program, ImpactT*, is used to track particles taking into account space charge forces, short-range longitudinal and transverse wakefields and the 3D rf fields in the structures, including the quadrupole component of the couplers. The effect of misalignments of the various elements (drive-laser, gun, solenoid and accelerator structures) are being evaluated. This paper presents these results and estimates of the expected bunch emittance versus bunch charge and cathode gradient.

*Ji Qiang, LBNL-62326, January 25, 2007.

• T. Higo, Y. Higashi, S. Matsumoto, K. Yokoyama
  KEK, Ibaraki
• C. Adolphsen, V.A. Dolgashev, A. Jensen, L. Laurent, S.G. Tantawi, F. Wang, J.W. Wang
  SLAC, Menlo Park, California
• S. Döbert, A. Grudiev, G. Riddone, W. Wuensch, R. Zennaro
  CERN, Geneva

A CERN-SLAC-KEK collaboration on high gradient X-band accelerator structure development for CLIC has been ongoing for three years. The major outcome has been the demonstration of stable 100 MV/m gradient operation of a number of CLIC prototype structures. These structures were fabricated basically using the technology developed from 1994 to 2004 for the GLC/NLC linear collider initiative. One goal has been to refine the essential parameters and fabrication procedures needed to realize such high gradient routinely. Another goal has been to develop structures with stronger dipole mode damping than those for GLC/NLC. The latter requires that surface temperature rise during the pulses be higher, which may increase the breakdown rate. Structures with heavy damping will be tested in late 2009/early 2010, and this paper will present these results together with some of the earlier results from non-damped structures and structures built with a quadrant geometry.

• T.S. Chu, S.G. Anderson, C.P.J. Barty, D.J. Gibson, F.V. Hartemann, R.A. Marsh, C. Siders
  LLNL, Livermore, California
• C. Adolphsen, E.N. Jongewaard, T.O. Raubenheimer, S.G. Tantawi, A.E. Vlieks, J.W. Wang
  SLAC, Menlo Park, California

A Mono-Energetic Gamma-Ray Compton scattering light source is being developed at LLNL. The electron beam for the interaction will be generated by a X-band RF gun and LINAC at the frequency of 11.424 GHz. High power RF in excess of 500 MW is needed to accelerate the electrons to energy of 250 MeV or greater. Two high power klystrons, each capable of generating 50 MW, 1.5 msec pulses, will be the main RF sources for the system. These klystrons will be powered by state of the art solid-state high voltage modulators. A RF pulse compressor, similar to the SLED II pulse compressor, will compress the klystron output pulse with a power gain factor of five. For compactness consideration, we are looking at a folded RF line. The goal is to obtain 500 MW at output of the compressor. The compressed pulse will then be distributed to the RF gun and to six traveling wave accelerator sections. Phase shifter and amplitude control are located at the RF gun input and additional control points along the LINAC to allow for parameter control during operation. This high power RF system is being designed and constructed. In this paper, we will present the design, layout, and status of this RF system.

• R.A. Marsh, S.G. Anderson, C.P.J. Barty, T.S. Chu, C.A. Ebbers, D.J. Gibson, F.V. Hartemann
  LLNL, Livermore, California
• C. Adolphsen, E.N. Jongewaard, T.O. Raubenheimer, S.G. Tantawi, A.E. Vlieks, J.W. Wang
  SLAC, Menlo Park, California

In support of Compton scattering gamma-ray source efforts at LLNL, a multi-bunch test stand is being developed to investigate accelerator optimization for future upgrades. This test stand will enable work to explore the science and technology paths required to boost the current 10 Hz mono-energetic gamma-ray (MEGa-Ray) technology to an effective repetition rate exceeding 1 kHz, potentially increasing the average gamma-ray brightness by two orders of magnitude. Multiple bunches must be of exceedingly high quality to produce narrow-bandwidth gamma-rays. Modeling efforts will be presented, along with plans for a multi-bunch test stand at LLNL. The test stand will consist of a 5.5 cell X-band rf photoinjector, single accelerator section, and beam diagnostics. The photoinjector will be a high gradient standing wave structure, featuring a dual feed racetrack coupler. The accelerator will increase the electron energy so that the emittance can be measured using quadrupole scanning techniques. Multi-bunch diagnostics will be developed so that the beam quality can be measured and compared with theory. Design will be presented with modeling simulations, and layout plans.

• C.D. Nantista, C. Adolphsen, F. Wang
  SLAC, Menlo Park, California

The phenomenon of rf breakdown presents a technological limitation in the application of high-gradient particle acceleration in normal conducting rf structures. Attempts to understand the onset of this phenomenon and to study its limits with different materials, cell shapes, and pulse widths has been driven in recent years by linear collider development. One question of interest is the role magnetic field plays relative to electric field. A design is presented for a single, non-accelerating, rf cavity resonant in two modes, which, driven independently, allow the rf magnetic field to be increased on the region of highest electric field without affecting the latter. The design allows for the reuse of the cavity with different samples in the high-field region. Available high-power data will also be presented.

• C. Adolphsen
  SLAC, Menlo Park, California

The Linac Group at SLAC is actively pursuing a broad range of R&D to improve the reliability and reduce the cost of the L-band (1.3 GHz) rf system proposed for the ILC linacs. Current activities include the long-term evaluation of a 120 kV Marx Modulator driving a 10 MW Multi-Beam Klystron, design of a second-generation Marx Modulator, testing of a sheet-beam gun and beam transport system for a klystron, construction of an rf distribution system with remotely-adjustable power tap-offs, and development of a system to combine the power from many klystrons in low-loss circular waveguide where it would be tapped-off periodically to power groups of cavities. This paper surveys progress during the past few years.

• Y. Sun, A.L. Eide, F. Zimmermann
  CERN, Geneva
• C. Adolphsen
  SLAC, Menlo Park, California

In this paper, we present a lattice design for the Large Hadron Electron Collider (LHeC) recirculating Linac. The recirculating Linac consists of one roughly 3km long linac hosting superconducting RF (SRF) accelerating cavities, two arcs and one transfer line for the recirculation. Electron beam will have two passes in the SRF linac to get a maximum energy of 140 GeV, or have four passes with a maximum energy of 60 GeV (two for acceleration and two for deceleration) in the Energy Recovery Linac (ERL) option.

• Y. Sun, F. Zimmermann
  CERN, Geneva
• C. Adolphsen
  SLAC, Menlo Park, California

In this paper, we estimate the emittance growth in the LHeC recirculating Linac, the lattice design of which is presented in another paper of IPAC10 proceedings. The possible sources for emittance growth included here are: energy spread from RF acceleration in the SRF (superconducting RF) linac plus large chromatic effects from the lattice, synchrotron radiation (SR) fluctuations in the recirculating arcs. 6-D multi-particle tracking is launched to calculate the emittance from the statistical point of view. The simulation results are also compared with a theoretical estimation.