• H. Hahn, S. Bellavia, I. Ben-Zvi, E. M. Choi
  BNL, Upton, Long Island, New York

Funding: Work supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy.

A 56 MHz Superconducting RF cavity is developed for the luminosity enhancement of the Relativistic Heavy Ion Collider (RHIC). The 56 MHz SRF cavity enables to adiabatically rebucket the beam from the 28 MHz accelerating cavities, which with shorter bunch lengths will enhance the luminosity significantly. The 56 MHz SRF cavity fundamental mode must be damped during injection and acceleration by a fundamental mode damper (FD), which is physically withdrawn at store for operation. The cavity frequency changes from the withdrawing motion but is kept below the beam frequency at store by a judicious axial placement of the FD. Physics studies by numerical simulations, tests of the FD in the prototype cavity, and the challenging engineering issues are here addressed. In addition, higher-order mode (HOM) dampers are necessary for the stable operation of the 56 MHz SRF cavity. The HOM’s are identified and the external Q factors are obtained from tests of the prototype cavity and are compared to simulations with the CST MWS program. The HOM damper blocks the fundamental mode by a 5 element high pass filter. The HOM stability criteria of the cavity are satisfied with four HOM dampers.

• M.L. Neubauer, R. Sah
  Muons, Inc, Batavia
• E.P. Chojnacki, M. Liepe
  CLASSE, Ithaca, New York
• H. Padamsee
  Cornell University, Ithaca, New York

Funding: Supported in part by USDOE Contract. DE-AC05-84-ER-40150

Superconducting HOM-damped (higher-order-mode-damped) RF systems are needed for present and future storage ring and linac applications. Superconducting RF (SRF) systems typically contain unwanted frequencies or higher order modes (HOM) that must be absorbed by ferrite and other lossy ceramic-like materials that are brazed to substrates mechanically attached to the drift tubes adjacent to the SRF cavity. These HOM loads must be thermally and mechanically robust and must have the required broadband microwave loss characteristics, but the ferrites and their attachments are weak under tensile stresses and thermal stresses and tend to crack. A HOM absorber with improved materials and design will be developed for high-gradient 750 MHz superconducting cavity systems. RF system designs will be numerically modeled to determine the optimum ferrite load required to meet the broadband loss specifications. Several techniques for attaching ferrites to the metal substrates will be studied, including full compression rings and nearly-stress-free ferrite assemblies. Prototype structures will be fabricated and tested for mechanical strength.

• Y. Okada
  NETS, Fuchu-shi
• S. Fukuda, H. Katagiri, T. Matsumoto, S. Michizono, T. Miura, Y. Yano
  KEK, Ibaraki

Intermediate-frequency conversion technique has been widely used for rf signal detection. However, this technique has disadvantages such as temperature dependence higher order modes of downconverters. One of our recent attractive developments is the high-speed data acquisition system that combines commercial FPGA board ML555 and fast ADC (ADS5474 14bit, maximum 400MS/s and bandwidth of 1.4 GHz). Direct measurements of 1.3 GHz rf signals are carried out with 270 MHz sampling. The direct sampling method can eliminate a down-converter and avoid calibration of non-linearity of the down-converter. These results are analyzed and compared with conventional measurement system.

• J.C. Smith, L. Keller, S.A. Lundgren, T.W. Markiewicz
  SLAC, Menlo Park, California
• L. Lari
  EPFL, Lausanne

Funding: Work supported in part by the U.S. Department of Energy contract DE-AC02-76SF00515

The Phase II upgrade to the LHC collimation system calls for complementing the 30 high robust Phase I graphite collimators with 30 high Z Phase II collimators. One option is to use metallic rotatable collimators and this design will be discussed here. The Phase II collimators must be robust in various operating conditions and accident scenarios. Design issues include: 1) Collimator jaw deflection due to heating and sagita must be small when operated in the steady state condition, 2) Collimator jaws must withstand transitory periods of high beam impaction with no permanent damage, 3) Jaws must recover from accident scenario where up to 7 full intensity beam pulses impact on the jaw surface and 4) The beam impedance contribution due to the collimators must be small to minimize coherent beam instabilities. The current design will be presented.

• G.R. Werner, C.A. Bauer, J.R. Cary, T. Munsat
  CIPS, Boulder, Colorado

Funding: This work is supported by the U.S. Department of Energy grant DE-FG02-04ER41317.

The RF properties of photonic crystals (PhCs) can be exploited to avoid the parasitic higher order modes (HOMs) that degrade beam quality in accelerator cavities and reduce efficiency and power in RF generators. Computer simulations show that long-range wake fields are significantly reduced in accelerator structures based on dielectric PhC cavities, which can be designed to trap only those modes within a narrow frequency range. A 2D PhC structure can be used to create a 3D accelerator cavity by using metal end-plates to confine the fields in the third dimension; however, even when the 2D photonic structure allows only a single mode, the 3D structure may trap HOMs, such as guided modes in the dielectric rods, that increase wake fields. For a 3D cavity based on a triangular lattice of dielectric rods, the rod positions can be optimized (breaking the lattice symmetry) to reduce radiation leakage using a fixed number of rods; moreover, the optimized structure has reduced wake fields. Using computer simulation, wake fields in pillbox, PhC, and optimized photonic cavities are calculated; a design for a klystron using the optimized photonic cavity structure is presented.

• S.V. Kuzikov, A.A. Vikharev
  IAP/RAS, Nizhny Novgorod
• J.L. Hirshfield
  Yale University, Physics Department, New Haven, CT
• S. Kazakov
  Omega-P, Inc., New Haven, Connecticut

Funding: Sponsored in part by US Department of Energy, Office of High Energy Physics.

A multi-mode cavity design for a two-beam accelerator aimed to achieve an accelerating gradient exceeding 150 MeV/m is reported. The cavity has a square cross section which allows excitation in several equidistantly-spaced eigen modes by a bunched drive beam in such a way that the RF fields reach peak values only during time intervals that can be much shorter than for excitation of a single mode, thus exposing the cavity surfaces to strong fields for shorter times. This feature is expected to raise the breakdown and pulse heating thresholds. In order to measure an increase in breakdown threshold surface electric field due to this reduction of exposure time during each RF period, a high-power experiment is planned. Preliminary calculations show that such a study in which comparison of breakdown threshold would be made of a conventional single-mode cavity with a multi-mode cavity can in principle be carried out using the drive beam of the CTF-3 test stand at CERN.

• I.R.R. Shinton, R.M. Jones, N. Juntong
  UMAN, Manchester

The main linacs of the ILC consist of nine-cell cavities based on the TESLA design. In order to facilitate reaching higher gradients we have re-designed the cavity shape. This leads to a reduction, comparable to several current designs, in both the ratio of the surface electric field to the accelerating field (Es/Ea) and the magnetic field to the accelerating field (Bs/Ea). The bandwidth of the accelerating mode is also optimized. This new shape, which we refer to as the New Low Surface Field (NLSF) design, bears comparison with the Ichiro, Re-entrant and LSF designs.

• R.J. Barlow, A.M. Toader
  UMAN, Manchester

A technique has been developed which enables the calculation of resistive particle wake effects. The technique can simply be calculated to any order, and is easy and quick to evaluate. No assumptions are made about the range of the interaction, but this is especially useful for short range effects. We show how the exact evaluation compares with various common approximations for some simple cases, and implement the technique in the Merlin and PLACET simulation programs. The extension from cylindrical to rectangular apertures is highlighted.

• C.A. Bauer, J.R. Cary, G.R. Werner
  CIPS, Boulder, Colorado

Funding: This work is supported by the U.S. Department of Energy grant DE-FG02-04ER41317.

Through computer simulation, a 2D photonic crystal (PhC) cavity formed from a truncated triangular lattice of dielectric rods is optimized to confine a single accelerating mode efficiently. Photonic crystals have the ability to reflect radiation within only certain frequency ranges, called bandgaps; the bandgaps are determined by the geometry and material of the PhC and so are tunable. For truncated PhCs, reflection is incomplete. Therefore, the confinement of bandgap frequencies to a cavity within a truncated PhC is weakened by the severity of the truncation. For a cavity made of 18 dielectric rods in a truncated triangular lattice arrangement, the desired accelerating cavity mode is weakly confined. Adjusting the positions and sizes of the dielectric rods away from the best lattice configuration within an optimization procedure gives unintuitive structures, ultimately increasing the confinement of the accelerating mode by a factor of 100. Confinement of higher-order modes is also dramatically reduced by the optimization. Similar increases in confinement of the fundamental accelerating mode are found for a 24-rod structure.

• R. Wanzenberg
  DESY, Hamburg
• E. Métral
  CERN, Geneva

Wakefields and trapped Higher Order Modes in the CMS experimental chamber at the LHC are investigated using a geometrical model which closely reflects the presently installed vacuum chamber. The basic rf-parameters of the higher order modes (HOMs) including the frequency, loss parameter,and the Q-value are provided. To cover also transient effects the short range wakefields and the total loss parameter has been calculated, too. Most numerical calculations are performed with the computer code MAFIA. The calculations of the Modes is complemented with an analysis of the multi-bunch instabilities due to the longitudinal and dipole modes in the CMS vacuum chamber.

• Y. Morozumi
  KEK, Ibaraki

The International Linear Collider will employ tens of thousands of superconducting 9-cell accelerating structures for its main linacs. Damping of higher order modes is crucial to beam stability. Study of higher order modes, however, tends to focus on trapped modes in a single 9-cell structure model alone both in simulation and measurement. Propagating modes above cut-off frequencies are left untouched because of difficulty of a realistic model of multiple 9-cell structures. We have simulated a full spectrum of higher order modes in a long string of 9-cell structures.

• N. Hayashizaki, T. Hattori
  RLNR, Tokyo

An Interdigital-H (IH) linac has been used for ion acceleration in low beta range. It can realize a resonant cavity of a convenient size at low frequencies and higher shunt impedance at low energy range. These characteristics are advantageous especially for heavy ion acceleration. Since the shunt impedance of the IH linac reduces according to the increasing of beam energy, the linacs operated by the TM010 mode such as an Alvarez type and a coupling cavity type are adopted for medium and high energy range. However, we propose the new IH linac using the TE11n mode, the higher order mode IH (HOM-IH) linac. By using the higher order mode, the resonance frequency is higher than that of the IH linac. This property is suitable for middle and high beta linacs, and a proton linac as well. The design of the cavity structure and the possibility are presented.