• A. Grudiev, A. Cappelletti, O. Kononenko
  CERN, Geneva

Compensating transient beam loading to maintain a 0.01% relative beam energy spread is a key issue for the CLIC two-beam acceleration technique. The combination of short pulses, narrow bandwidth rf components and the limited number of rf pulse shaping 'knobs' given by the drive beam generation scheme makes meeting this specification challenging. A dedicated model, which takes into account all stages of drive beam generation, including the delay loop and combiner rings, the single-bunch response of the power generation structure (PETS), the RF waveguide network transfer function and dispersive properties of the accelerating structure has been developed. The drive beam phase switching delays, resulting rf pulse shape, loaded and unloaded voltages and finally the energy spread are presented.

• M. Lechartier, D. Besnier, R. Beunard, J.F. Leyge, M. Michel, P. Robillard, P. Toussaint
  GANIL, Caen

Three normal conducting rebunchers are located in the MEBT line of the SPIIRAL2 driver.  The cavity are designed for a beta of 0.04, work at 88 MHz and have to supply beam voltages up to 120 kV in continuous mode or up to 190 kV in  pulsed mode with 50%dutycycle. The  paper describes the  RF measurements and first results

• S.V. Kutsaev, R.O. Bolgov, M. Gusarova, D.S. Kamenshikov, K.I. Nikolskiy, A.Yu. Smirnov, N.P. Sobenin, S.E. Toporkov
  MEPhI, Moscow

This paper presents the results of a research that analyzed the possibility of using a magnetic coupled disk-loaded structure (DLS-M) as an accelerating structure. DLS-M seems to have decent advantages comparing to the classical electrical coupled structure (DLS). The electrodynamics parameters of such a structure at various modes in C-band for a wide range of phase velocities as a function of aperture radii and coupling slot sizes are presented. Both forward and backward travelling wave regimes are considered. The essential parameters are compared to those of classical DLS. The design of an input coupler to the accelerator consisting of this type structure cells is also presented.

• A. Grudiev, W. Wuensch
  CERN, Geneva

The design of the CLIC main linac accelerating structure has been refined based on an improved understanding of the high-gradient limits given by rf breakdown and pulsed surface heating. In addition, compact couplers have been developed and HOM damping loads have been designed. The rf design has also been made consistent with details of the present manufacturing procedure, based on bonded asymmetrical disks, and with requirements coming from integration of the accelerating structure in the two-beam module which includes all subsystems. This completion and refinement of the structure design has been made to produce the self-consistent parameter set required for preparation of the CLIC conceptual design report.

• M.M. Dehler
  PSI, Villigen
• A.E. Candel, L. Lee
  SLAC, Menlo Park, California

Currently an X-band traveling wave accelerator structure is fabricated in a collaboration between CERN, PSI and Sincrotrone Trieste (ST). PSI and ST will use it in their respective FEL projects, CERN will test break down limits and rates for high gradients. A special feature of this structure are two integrated wake field monitors monitoring the beam to structure alignment. The design used an uncoupled model for the fundamental mode, assuming the overall behavior to be the superposition of the individual components. For the wake field monitors, an equivalent circuit was used. This approach has been proven to produce valid structure designs. None the less it cannot approach the quality of a numerical electromagnetic simulation of the full structure, which is ideal for a validation capturing the differences between design models and the real cavity as e.g. internal reflections inside the structure or higher order dispersive terms altering the response of the wake field monitor. Using SLAC's family of massive parallel codes ACE-3P, first results are presented for the fundamental mode and the first transverse mode. They are compared with earlier simulations using simplified models.

• D.C. Plostinar
  STFC/RAL/ASTeC, Chilton, Didcot, Oxon
• R. Enparantza, M. Larrañaga
  Fundación TEKNIKER, Eibar (Gipuzkoa)

The Medium Energy Beam Transport (MEBT) line for the Front End Test Stand (FETS) at Rutherford Appleton Laboratory (RAL) will transport a 60 mA, 2ms, 50 pps H^- beam at 3 MeV. Its main components include a number of quadrupoles, re-bunching cavities and a fast-slow chopping system with dedicated beam dumps, as well as a diagnostics beam line. In this paper we present the design approach for the MEBT re-bunching cavities. A description is given for the proposed geometry and the main design choices are examined. In addition, the latest RF simulations results performed with 2D and 3D electromagnetic codes are presented including optimisation details and manufacturing plans.

• P. Balleyguier
  CEA, Bruyères-le-Châtel
• P. Bertrand, M. Di Giacomo, G. Fremont, M. Michel
  GANIL, Caen

The high current driver accelerator of the SPIRAL 2 project uses a single-bunch selector to reduce the bunch repetition rate at the experimental target. The device works at almost 1 MHz and handles fast RF pulses of 18 ns with transient times shorter than 6 ns. The first electrode prototype, built in the framework of the Eurisol DS project, was used for thermal and RF tests and didn't show correct delay and matching. The paper describes the studies to improve these two important issues and the results of thermal tests

• D. Noll, L.P. Chau, M. Droba, O. Meusel, H. Podlech, U. Ratzinger
  IAP, Frankfurt am Main

The Frankfurt Neutron Source (FRANZ) currently under construction at IAP (Goethe University of Frankfurt) is designed to produce short neutron pulses at high intensity and repetition rates up to 250 kHz [*]. To achieve a bunch length of one nanosecond despite the high space charge forces, a bunch compressor of the Mobley type [**] using four dipole magnets and two rebunchers has been developed [***] to merge 9 linac bunches into the final focus. The first rebuncher cavity, a λ/4 resonator operating at 87.5 MHz, has to feature nine beam paths due to the multi-trajectory system. Additionally the gaps have to be displaced relatively to each other in a way that all bunches arrive at the correct rf phase. The second rebunching cavity will provide final focusing as well as an energy variation of ±0.2 MeV in front of the target and will be operating at 175 MHz. This paper presents the design of these novel cavities as well as the simulated beam dynamic properties.

* Meusel et al., LINAC 2006
** Mobley, Phys. Rev. 88(2), 360-361 (1951)
*** Chau et al, LINAC 2010

• K. Saito, F. Furuta
  KEK, Ibaraki
• T. Konomi
  Nagoya University, Nagoya

MO sealing developed by Prof. H. Matsumoto in KEK and his collaborator M. Ohotsuka has been successfully applied to SRF cavities. Its leak ratio is smaller than 3·10^-8 Pam3/s or much better in the superfluid Helium, which is the allowed level to successfully measure the cavity performance for more than 3 hours at 2K. Tightening torque is 15Nm and the bolt material is SUS304 (JIS). Titanium is usable as cavity flange material. Copper looks better than pure Aluminium as the gasket material. We have observed an additional residual surface resistance about 5nΩ Zero impedance characteristics of the MO sealing is a remained issue. In this paper we report the results in detail.

• J.M. Maus, A. Schempp
  IAP, Frankfurt am Main
• A. Bechtold
  NTG, Gelnhausen

The IH-type RFQ for the MAFF project at the LMU in Munich was operated at a beam test stand at the IAP in Frankfurt. It is the second IH-RFQ after the HIS at GSI and it has been designed to accelerate rare isotope beams (RIBs) with mass to charge ratios A/q up to 6.3 from 3 keV/u to 300 keV/u at an operating frequency of 10¹.28 MHz with an electrode voltage of 60 kV. Experimental results such as shunt impedance, energy spectrum and transmission will be presented.

• K. Yamada, S. Arai, Y. Chiba, H. Fujisawa, E. Ikezawa, O. Kamigaito, M. Kase, N. Sakamoto, K. Suda, Y. Watanabe
  RIKEN Nishina Center, Wako
• Y. Touchi
  SHI, Tokyo

A new linac injector, which will be exclusively used for the RIKEN RI-Beam Factory, has been constructed to increase the beam intensity of very heavy ions such as xenon and uranium. The injector system consists of a superconducting ECR ion source, RFQ linac, three DTLs, and beam transport system including strong quarupole magnets and beam bunchers. Two DTL resonators were newly designed while existing devices including the RFQ* were modified to the other resonators. Direct coupling scheme was adopted for the rf-sytems of the DTLs, where the design study was successfully perfomed by using the MWS code. This paper focuses on the design procedure of the DTLs and RFQ as well as the results of their low and high power tests.

*H. Fujisawa, Nucl. Instrum. and Methods A345 (1994) 23-42.

• C.E. Reece, H. Tian
  JLAB, Newport News, Virginia

Recent incorporation of analytic electrochemistry into the development of protocols for electropolishing niobium SRF cavities has yielded new insights for optimizing this process for consistent, high-performance results. Use of reference electrodes in the electrolyte, electrochemical impedance spectroscopy (EIS), rotating disk electrodes (RDE), and controlled sample temperatures has greatly clarified the process dynamics over the empirical understanding developed via years of practice. Minimizing rf losses at high operational gradients is very valuable for CW linacs. Jefferson Lab is applying these new insights to the low-loss 7-cell cavity design developed for the CEBAF 12 GeV Upgrade. Together with controlled cleaning and assembly techniques to guard against field-emission causing particulates, the resulting process is yielding consistent cavity performance that exceeds project requirements. Cavity tests show BCS-limited Q well above 30 MV/m. Detailed process data, interpretation, and resulting rf performance data will be presented.

• R.M. Jones, I.R.R. Shinton, P. Zhang
  UMAN, Manchester
• N. Baboi
  DESY, Hamburg
• T. Flisgen, H.-W. Glock, U. van Rienen
  Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock

Transverse modes in the 3.9 GHz cavities designed and fabricated by FNAL are reported on. These modes have the potential to cause significant emittance dilution if they not sufficiently suppressed. Recent experiments, both probe-based and beam-excited, have indicated significant discrepancies between modes predicted in stand-alone 9-cell cavities compared to those in 4-cavity modules. We employ a suite of computer codes and circuit models to analyze these modes, coupled through beam tubes whose cut-off is above that of the first dipole band. We also report on preparations to instrument the higher order mode couplers with electronics suitable for diagnosing both the beam and cavity position, based on modes with sufficient R/Q values.

• J. Norem, M. Kharitonov, J. Klug, M.J. Pellin, Th. Proslier
  ANL, Argonne
• N. Becker, J. Zasadzinski
  IIT, Chicago, Illinois
• G. Ciovati
  JLAB, Newport News, Virginia
• A.V. Gurevich
  NHMFL, Tallahassee, Florida

An effort, centered at Argonne, has started to explore the use of Atomic Layer Deposition (ALD) to study and improve the performance of superconducting rf (SRF) accelerating structures. This effort has a number of parts: a survey the properties of ALD deposited films, a study of loss mechanisms of SRF structures, and a program of coating single cell cavities, to begin to optimize the performance of complete systems. Early results have included improving the performance of individual structures and, identification of magnetic oxides as a loss mechanism in SRF. We describe the program and summarize recent progress.

• O. Heid, T.J.S. Hughes
  Siemens AG, Healthcare Technology and Concepts, Erlangen

An RF accelerator driver concept is introduced, which integrates a distributed solid-state RF power source with the RF resonator. The resulting structure plays a double role as RF combiner and particle accelerating structure [1]. The key enabling technologies are Silicon Carbide RF transistors and a power combiner concept which includes insulating parallel cavities to ensure consistent RF current injection. An experimental direct drive lamda/4 cavity with a power rating of 500kW at 150MHz has been constructed. The Direct Drive RF power source consists of 64 RF modules constructed from Silicon Carbide vJFETs, radial power combiner and isolation cavity. The initial results from the integration of the direct drive RF source are presented. These results demonstrate experimentally for the first time the validity of the direct drive concept and the key characteristics of such a drive.

[1] O. Heid, T Hughes. "Compact Solid State Direct Drive RF LINAC" presented at IPAC 2010, Kyoto, Japan.

• M. Panniello, V.G. Vaccaro
  Naples University Federico II and INFN, Napoli
• M.R. Masullo
  INFN-Napoli, Napoli

The Wire Method for Coupling Impedance evaluations is quite appealing for the possibility to make bench measurements on the Device Under Test (DUT). However, it is not entirely reliable because the stretched wire perturbs the boundary conditions, introducing a TEM wave that has a zero cut off frequency. We expect that, for frequencies smaller than the cutoff one, this behaviour produces an additional power loss which drastically lowers the high Q resonances of DUT. Above cutoff frequency, the impact of the stretched wire is not as dramatic as below cutoff. The Mode Matching Technique will be used to simulate the measurement with the Wire Method. In this way one may get a result which is not affected by the errors intrinsic of experimental measurements. The same method will be used to get, according to its standard definition, the Coupling Impedance of the real structure. The two results will be compared in order to define the frequency ranges in which they agree or disagree. As expected large discrepancies appear below cutoff frequency, while above cutoff, for certain ranges of parameters, an agreement is found.

• Y.W. Kang, R.E. Fuja, T.W. Hardek, S.W. Lee, M.P. McCarthy, M.F. Piller, K.R. Shin, M.P. Stockli, A.V. Vassioutchenko, R.F. Welton
  ORNL, Oak Ridge, Tennessee

The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory is in the process of ramping up the H^- ion beam power to 1.4 MW, its full design power for the neutron production. For robust operation of the neutron facility, work is underway for various improvements on the RF power systems of the ion source. For short and long-term higher beam power operations, an RF-driven H^- ion source employing external antenna with a water-cooled, ceramic aluminum nitride (AlN) plasma chamber has been developed*. The new ion source has been tested to deliver up to 42 mA in the SNS Front End (FE) and unanalyzed beam currents up to ~100mA (60Hz, 1ms) in the ion source test stand. In addition to the external antenna design for improved antenna lifetime, other RF developments for improvement of reliability are running 2 MHz power amplifier system is with isolation transformer, employing full solid-state 2 MHz power amplifier, more precise 2 MHz capacitive impedance matching, and upgrading 13 MHz RF plasma gun system. This paper discusses the engineering solutions with analysis and development of the above RF systems for the new ion source system.

R.F. Welton, N.J. Desai, J. Carmichael, B. Han, Y.W. Kang, S.N. Murray, T. Pennisi, M. Santana, and M.P. Stockli, "The Continued Development of the SNS External Antenna H^- Ion Source," ICIS2009