• A. Neumann, M. Abo-Bakr, J. Knobloch
  BESSY GmbH, Berlin

Funding: Work partially funded by the European Commission in the Sixth Framework Program, contract no. 011935 EUROFEL-DS5, BMBF and Land Berlin.
Most concepts for next generation light sources base on linear accelerators (linac) due to their excellent beam properties. In case of high electron energies and extreme average currents Energy Recovery Linacs (ERL) are mandatory. In this paper we investigate the rf field stability in a generic superconducting, cw operated ERL. By using rf control cavity simulations and longitudinal beam dynamics the influence of rf field stability on the energy recovery process is analyzed. Since the ERL aims for a small net beam loading cavities are operated at a high loaded quality factor. Therefore they are operated at a low bandwidth and are very susceptible to microphonics detuning. We considered the field stability under the influence of limited rf power, mechanical cavity detuning, varying beamloading, synchronization deviations and varying bunch parameters at injection into the linac. The resulting temporal and energy jitter at the linac end will be transformed in the return arc and leads to rf phase deviations on the return path. Implications of varying beam loading on the ERL performance are examined.

• A.E. Dabrowski, S. Bettoni, H.-H. Braun, E. Bravin, R. Corsini, S. Döbert, C. Dutriat, T. Lefèvre, M. Olvegård, P.K. Skowronski, F. Tecker
  CERN, Geneva

In the CLIC Test Facility 3 (CTF3), the strong coupling between the beam and the accelerating cavities (full beam loading) induces transient effects such that the head of the pulse is accelerated almost twice as much as the steady-state part of the pulse. The beam optics in the machine is tailored for the steady-state and not for the higher energy electrons, which are gradually lost. This can lead to inefficiency and contributes to the activation of the machine. A beam loading compensation scheme has been proposed to minimize this effect. By delaying appropriately the arrival time of rf pulse in accelerating cavities with respect to the beam, the transient energy can be brought close (to within a few percent) of the steady-state one. This paper presents the measurements done on CTF3 using time resolved energy measurements.

• G. Burt, P.K. Ambattu, R.G. Carter, A.C. Dexter, M.I. Tahir
  Cockcroft Institute, Lancaster University, Lancaster
• C. Adolphsen, Z. Li, A. Seryi, L. Xiao
  SLAC, Menlo Park, California
• C.D. Beard, D.M. Dykes, P. Goudket, A. Kalinin, L. Ma, P.A. McIntosh
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
• L. Bellantoni, B. Chase, M. Church, T.N. Khabiboulline
  Fermilab, Batavia
• R.M. Jones
  UMAN, Manchester
• A. Latina, D. Schulte
  CERN, Geneva

Crab cavities have been proposed for a wide number of accelerators and interest in crab cavities has recently increased after the successful operation of a pair of crab cavities in KEK-B. In particular crab cavities are required for both the ILC and CLIC linear colliders for bunch alignment. Consideration of bunch structure and size constraints favours a 3.9 GHz superconducting, multi-cell cavity as the ILC solution, whilst bunch structure and beam-loading considerations suggest an X-band copper travelling wave structure for CLIC. These two cavity solutions are very different in design but share complex design issues. Phase stabilisation, beam loading, wakefields and mode damping are special issues for these crab cavities. Requirements and potential design solutions will be discussed for both colliders.

• P.K. Ambattu, G. Burt, R.G. Carter, A.C. Dexter
  Cockcroft Institute, Lancaster University, Lancaster
• R.M. Jones
  UMAN, Manchester
• P.A. McIntosh
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire

The CLIC linear collider will require a crab cavity to align bunches prior to collision. Consideration of the bunch structure leads us to favour the use of X-band copper cavities. Due to the large variation of train to train beam loading, it is necessary to minimise the consequences of beam loading. One solution is to use a travelling wave structure with a large group velocity allowing rapid propagation of amplitude errors from the system. Such a design makes this structure significantly different from previous travelling wave deflecting structures. This paper will look at the implications of this on other cavity parameters and the optimization of the cavity geometry.

• Z. Fang, S. Anami, S. Michizono, S. Yamaguchi
  KEK, Ibaraki
• T. Kobayashi
  JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
• H. Suzuki
  JAEA, Ibaraki-ken

At the J-PARC 181 MeV proton linac, the rf sources consist of 4 solid-state amplifiers and 20 klystrons with operation frequency of 324 MHz. The rf fields of each rf source are controlled by a digital feedback system installed in a compact PCI (cPCI). A very good stability of the accelerating fields has been successfully achieved about ±0.2% in amplitude and ±0.2 degree in phase, much better than the requirements of ±1% in amplitude and ±1 degree in phase. Besides, the tuning of each accelerator cavity including 3 DTL and 15 SDTL is also controlled by this LLRF system through a cavity tuner. We pre-defined the cavity resonance states with the tuner adjusted to obtain a flat phase during the cavity field decay. The cavity auto-tuning is well controlled to keep the phase of rf fields within ±1 degree. Furthermore, from the amplitude waveform during the cavity field decay, the Q-value of each cavity is calculated in real-time and displayed in the PLC TP of the LLRF control system.

• T. Kobayashi
  JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
• S. Anami, Z. Fang, S. Michizono, S. Yamaguchi
  KEK, Ibaraki
• E. Chishiro, H. Suzuki
  JAEA, Ibaraki-ken

For the J-PARC linac low level rf system, in order to compensate beam-loading change by pulses in the operation of 25 Hz repetition, a function that switches the feed-forward control parameters in every pulse were installed into the digital accelerating-field control system. The linac provides a 50 mA peak current proton beam to a 3 GeV rapid-cycling synchrotron (RCS). Then the RCS distributes the 3-GeV beam into a following 50 GeV synchrotron (main ring, MR) and the Materials and Life Science Facility (MLF), which is one of the experimental facilities in the J-PARC. The 500-us long macro pulses from the ion source of the linac should be chopped into medium pulses for injection into the RCS. The duty (width or repetition) of the medium pulse depends on which facility the RCS provides the beam to the MR or MLF. Therefore the beam loading compensation needs to be corrected for the change of the medium pulse duty in the 25 Hz operation.

• J. Branlard, B. Chase, S. Nagaitsev, O.A. Nezhevenko, J. Reid
  Fermilab, Batavia

Funding: FRA
In this paper we present a model for the rf power distribution to multiple super-conductive cavities from a single klystron. The goal of this model is to find a distribution scheme in which the cavities are operated as close to their quench limit as possible. The approach presented in this work consists of setting all cavities to the same QL value by adjusting the power coupler, and optimizing the power (Pk) distribution individually to each cavity to maximize the vector sum voltage. The proposed approach yields an operating gradient very close to the theoretical limit and offers a great operational benefit as the gradient stability is conserved for any beam current.

C. Nantista, K.L.F. Bane, C. Adolphsen, RF Distribution Optimization in
the Main Linacs of the ILC. Proceedings of PAC07, Albuquerque,
New Mexico, USA.

• G.I. Cancelo, A. Vignoni
  Fermilab, Batavia

Pulsed Linac accelerators are being designed powering a string of cavities from one klystron. A typical low level rf control loop controls the amplitude and the phase of the klystron's rf power; however, the loop cannot dynamically control individual cavity amplitude and phases. The problem is further complicated by the need to obtain the maximum possible acceleration from the rf unit. Proton Linacs (HINS, ProjectX) add extra complexity. A rf unit may need cavities operating at different synchronous phases. Particles travel cavities at increasing velocities, which implies different beam loading conditions. For pulsed proton Linacs amplitude and phase stability are crucial for beam stability. The usual steady state approach determines optimality conditions for minimum generator power as a function of rf parameters. This approach does not provide constant amplitude and phases when the beam is on. In this paper we propose a novel theory using the cavity transient response. The transient response allows setting flat cavity gradients (A and phi) for each cavity in the unit. The optimized rf parameters for the transient response are the cavity coupling parameter and cavity tuning angle.

• G.I. Cancelo, K.R. Treptow, A. Vignoni, T.J. Zmuda
  Fermilab, Batavia
• C. Armiento
  University of Pisa and INFN, Pisa

A multi cavity real time rf simulator and PID control has been implemented on a Xilinx Virtex-4 FPGA. The rf simulator simulates an entire rf unit with up to 4 cavities connected to a single simulated klystron. Each cavity is allowed to have its own set of parameters, set point gradients, synchronous phases, and beam loadings. The simulator is built based on an interdependent electrical and mechanical model of a cavity. The electrical model is a 1st order differential equation in the complex phase space. The mechanical model is a 2nd order differential equation of the Lorentz force detuning on the cavities. Other spurious effects as microphonics and noises can be added using an external source or a memory table. The simulator has been optimized for size and utilizes only one Xilinx DSP block per cavity. A typical Virtex-4 has of the order of 100 DSP blocks. The simulator bandwidth is 1MHz which is plenty for niobium type superconducting cavities which have a loaded Q of about 3 million and a half bandwidth of about 250 Hz. The Real Time simulator is currently running on hardware comprised by an ESECON LLRF controller* and a Linux based VME processor.

*ESECON, 14 channel LLRF controller, Low Level Radio Frequency Workshop (LLRF07), Knoxville, Tennessee, October 22-25, 2007, presentation 031.