Jefferson Lab, Newport News, Virginia
For an ongoing high current cryomodule project, a total of 5 higher order mode (HOM) absorbers are required per cavity. The load is designed to absorb RF heat induced by HOMs in a 748.5MHz cavity. Each load is targeted at a 4 kW dissipation capability. Embedded cooling channels are employed to remove the heat generated in ceramic tiles and by surface losses on the waveguide walls. A sequentially coupled RF-thermal-structural analysis was developed in ANSYS to optimize the HOM load design. Frequency dependent dielectric material properties measured from samples and RF power spectrum calculated by the beam-cavity interaction codes were considered. The coupled field analysis capability of ANSYS avoided mapping of results between separate RF and thermal/structural simulation codes. For verification purposes, RF results obtained from ANSYS were compared to those from MAFIA, HFSS, and Microwave Studio. Good agreement was reached and this confirms that multiple-field coupled analysis is a desirable choice in analysis of HOM loads. Similar analysis could be performed on other particle accelerator components where distributed RF heating and surface current induced losses are inevitable.
SLAC, Menlo Park, California
CERN, Geneva
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
Cockcroft Institute, Lancaster University, Lancaster
UMAN, Manchester
Cockcroft Institute, Warrington, Cheshire
In order to verify detailed impedance simulations, the modes in an aluminium model of the ILC crab cavity were investigated using a bead-pulling technique as well as a stretched-wire frequency domain measurement. The combination of these techniques allow for a comprehensive study of the modes of interest. For the wire measurement, a transverse alignment system was fabricated and rf components were carefully designed to minimize any potential impedance mismatches. The measurements are compared with direct simulations of the stretched-wire experiments using numerical electromagnetic field codes. High impedance modes of particular relevance to the ILC crab cavity are identified and characterized
Fermilab, Batavia, Illinois
Fast readout of strategically placed low heat capacity thermometry can provide valuable information of Superconducting RF (SRF) cavity performance. Such a system has proven very effective for the development and testing of new cavity designs. Recently, several RTDs were installed in key regions of interest on a new 9 cell 3.9 GHz SRF cavity with integrated HOM design at FNAL. A data acquisition system was developed to read out these sensors with enough time and temperature resolution to measure temperature changes on the cavity due to heat generated from multipacting or quenching within power pulses. The design and performance of this fast thermometry system will be discussed along with results from tests of the 9 cell 3.9GHz SRF cavity.
MIT/PSFC, Cambridge, Massachusetts
A photonic band gap (PBG) higher-order-mode (HOM) coupler is proposed as an Alternative Configuration Design (ACD) for the HOM coupler for the International Linear Collider (ILC). The PBG HOM coupler uses a two-dimensional triangular PBG structure with good axial symmetry. Simulation studies of a PBG HOM coupler show that it maintains the operating mode at 1.3 GHz with . While a PBG HOM coupler provides superior damping for all the higher order modes in principle, detailed studies of the effectiveness of HOM damping are being carried out, and results will be discussed.
Texas A&M University, College Station, Texas
A polyhedral superconducting cavity is being developed for possible use in linac colliders. In side view it has the contour of a Tesla-type multi-cell string. The surfaces of the cavity are formed by bonding flat foils to solid copper wedge-shaped segments, so that the end view is a polyhedron of such segments. Several features of this structure make it interesting for linac colliders: the cavity segments are totally open for cleaning, polishing, and inspection until the final assembly step; narrow slot gaps at the boundaries between segments strongly suppress all deflecting modes without penalty to the accelerating mode; the solid copper substrate accommodates cooling channels and eliminates the need for an immersion cryostat; and the open geometry makes it possible to utilize advanced superconductors (e.g. multi-layer Nb/Nb3Sn, YBCO, MgB2) on the cavity surface, opening the possibility of higher gradients.
LANL, Los Alamos, New Mexico
We propose to use the photonic band gap (PBG) structures for the construction of a traveling-wave tube (TWT) at W-band. Interest in millimeter-waves has increased in recent years due to applications in environmental monitoring and remote sensing. The development of wide-band mm-wave TWT amplifiers is underway at Los Alamos National Laboratory. A TWT would present a wide bandwidth source for remote mm-wave spectroscopy. PBG TWT structures have great potential for very large bandwidth and linear dispersion. In addition, being cheap to fabricate, the PBG structures enhance the commercial transferability of the W-band TWT technology. We employ an omniguide which is a one-dimensional version of the PBG structure representing a periodic system of concentric dielectric tubes as a slow-wave structure. A silica omniguide was designed to support a TM01-like mode with a phase velocity matching the one of a 120keV electron beam. The structure was fabricated, cold-tested and installed at our laboratory for the hot test.
AMAC, Newport News, Virginia
MIT, Cambridge, Massachusetts
There is an increasing demand for High beam Current, high Radio-Frequency (RF) power S-band cavities in existing and new accelerator projects to produce a very brilliant, broadband, teraherz coherent synchrotron radiation source (CRS). To achieve this goal, the RF cavities must be upgraded to a gap voltage of 1.5 MV in the limited space available in the machine with a high gradient superconducting cavity. At the present time there are no cavities and accessories designed to support the high beam currents of up to 100 mA and at the same time provide a high gap voltage at such a high S-band frequency. AMAC proposed a High Current, Large Aperture, Low HOM, Single Crystal Nb 2.85GHz Superconducting Cavity with high RF Power Coupler and HOM absorber device. Comprehensive simulation and optimization to determine the SRF cavity parameters to meet the requirements, provided two alternate designs for the RF input couplers, performed a detailed Higher Order Modes (HOM) analysis, and proposed an HOM absorber concept to dampen the modes exited in the cavity due to the high beam current and high bunch intensity.
ORNL, Oak Ridge, Tennessee
The Superconducting Linac at SNS has been operating with beam for almost two years. As the first operational pulsed superconducting linac, many of the aspect of its performance were unknown and unpredictable. A lot of experience has been gathered during the commissioning of its components, during the beam turn on and during operation at increasingly higher beam power. Some cryomodules have been cold for well over two years and have been extensively tested. The operation has been consistently conducted at 4.4 K and 10 and 15 pulses per second, with some cryomodules tested at 30 and 60 pps and some tests performed at 2 K. Careful balance between safe operational limits and the study of conditions, parameters and components that create physical limits has been achieved. This paper presents the experience and the performance of the superconducting cavities and of the associated systems with and without beam.
KEK, Ibaraki
DESY, Hamburg
Fermilab, Batavia, Illinois
SLAC, Menlo Park, California
CEA, Gif-sur-Yvette
It is well known that an electron beam excites Higher Order Modes (HOMs) as it passes through an accelerating cavity~[panofsky68]. The properties of the excited signal depend not only on the cavity geometry, but on the charge and trajectory of the beam. It is, therefore, possible to use these signals as a monitor of the beam's position. Electronics were installed on all forty cavities present in the FLASH~[flashref] linac in DESY. These electronics filter out a mode known to have a strong dependence on the beam's position, and mix this down to a frequency suitable for digitisation. An analysis technique based on Singular Value Decomposition (SVD) was developed to calculate the beam's trajectory from the output of the electronics. The entire system has been integrated into the FLASH control system.
ANL, Argonne, Illinois
Illinois Institute of Technology, Chicago, Illinois
Metamaterials (MTM) are artificial periodic structures made of small elements and designed to obtain specific electromagnetic properties. As long as the periodicity and the size of the elements are much smaller than the wavelength of interest, an artificial structure can be described by a permittivity and permeability, just like natural materials. Metamaterials can be customized to have the permittivity and permeability desired for a particular application. Waveguides loaded with metamaterials are of interest because the metamaterials can change the dispersion relation of the waveguide significantly. Slow backward waves, for example, can be produced in a LHM-loaded waveguide without corrugations. In this paper we present theoretical studies and computer modeling of waveguides loaded with 2D anisotropic metamaterials, including the dispersion relation for a MTM-loaded waveguide. The dispersion relation of a MTM-loaded waveguide has several interesting frequency bands which are described. It is shown theoretically that dipole mode suppression may be possible. Therefore, metamaterials can be used to suppress wakefields in accelerating structures.
MIT/PSFC, Cambridge, Massachusetts
LANL, Los Alamos, New Mexico
Results are reported on experimental wakefield measurements made on a 6 cell, 17 GHz metallic PBG accelerator structure. Wakefields were observed using a variety of detectors and methods. The PBG structure is open, containing no outer wall, and radiation has been observed through a window in the surrounding vacuum vessel. The input and output ports have also been used with windows to observe radiation coupling out of the ports. Estimations of radiation are made using HFSS and an EFIE code. Measurements have been made using video diode detectors, wavemeters, heterodyne receivers, and a bolometer. Plans are discussed for future experiments with injected power and longer structures.
MIT/PSFC, Cambridge, Massachusetts
High gradient structure design entails optimization of the gradient, while minimizing surface electric fields (associated with breakdown) and surface magnetic fields (associated with pulsed heating). Design studies are reported comparing metallic and dielectric PBG structures and standard disk-loaded waveguide. Operation in a higher order mode is considered. A variety of codes; HFSS, CST MWS, and Superfish have been used to compare and refine designs. Final design work is in preparation for a structure to be cold tested, tuned, and then processed to high gradient operation at the MIT HRC 17 GHz accelerator facility.
SLAC, Menlo Park, California
DESY, Hamburg
Fermilab, Batavia, Illinois
UMAN, Manchester
Higher Order Modes (HOMs) excited by the passage of the beam through an accelerating cavity depend on the properties of both the cavity and the beam. It is possible, therefore, to draw conclusions on the inner geometry of the cavities based on observations of the properties of the HOM spectrum. A data acquisition system based on two 20 GS/s, 6 GHz scopes has been set up at the FLASH facility, DESY, in order to measure a significant fraction of the HOM spectrum predicted to be generated by the TESLA cavities used for the acceleration of its beam. The HOMs from a particular cavity at FLASH were measured under a range of known beam conditions. The dipole modes have been identified in the data. 3D simulations of different manufacturing errors have been made, and it has been shown that these simulations can predict the measured modes.
UMAN, Manchester
SLAC, Menlo Park, California
The effects of the round edge beam hole on the frequency and wake field are studied using variational method, which allows for rounded iris disk hole without any approximation in shape treatment. The frequency and wake field of accelerating mode and dipole mode are studied for different edge radius cases, including the flat edge shape that is often used to approximately represent the actual structure geometry. The edge hole shape has weak effect on the frequency, but much effect on the wake field. Our study shows that the amounts of wake fields are not precise enough with the assumption of the flat edge beam hole instead of round edge.