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| MPPP027 | Suppression of the Longitudinal Coupled-Bunch Instabilities by the RF Phase Modulation in the Pohang Light Source | synchrotron, simulation, storage-ring, damping | 1970 | ||
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In the 2.5 GeV Pohang Light Source, we have investigated the suppression of the longitudinal coupled instabilities (CBI) caused by higher order modes (HOMs) of RF cavities. At higher beam current than 170 mA the 758 MHz or 1300 MHz HOMs occurred and the beam could be unstable. The longitudinal CBI could be suppressed by modulating the phase of an RF accelerating voltage at a frequency of 2 times the synchrotron oscillation frequency and by adjusting the water temperatures of the RF cavities. The longitudinal beam oscillations measured by streak camera in synchro-scan mode were shown. The experiment results were compared with the macro particle tracking simulation.
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| MPPP045 | Two Dimensional Aspects of the Regenerative BBU in Two-Pass Recirculating Accelerators | recirculation, dipole, polarization, insertion | 2872 | ||
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Funding: Work supported by DOE Contract DE-AC05-84ER40150. |
In this paper, I present the formula, describing a threshold of the regenerative multi-pass Beam Breakup (BBU) for a single dipole higher order mode with arbitrary polarization in a two-pass accelerator with a general-form, 4x4 recirculation matrix. To illustrate specifics of the BBU in two dimensions, the formula is used to calculate the threshold for the reflecting and rotating optics of the recirculator that can lead to higher threshold currents. Then, I present a mathematical relation between transfer matrices between cavities of the accelerating structure and recirculation matrices for each cavity, which must be satisfied in order to successfully suppress the BBU by reflection or rotation in several cavities. At the end of the paper, a fast, two-dimensional BBU code developed at the Jefferson Lab is described. |
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| TPPP028 | Simulation of HOM Leakage in the PEP-II Bellows | vacuum, damping, simulation, coupling | 2050 | ||
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Funding: Work supported by U.S. DOE contract, DE-AC02-76SF00515. |
An important factor that limits the PEP-II from operating at higher currents is higher-order-mode (HOM) heating of the bellows. One source of HOM heating is the formation of trapped modes at the bellows as a result of geometry variation in the vacuum chamber, for example, the masking near the central vertex chamber. Another source comes from HOMs generated upstream that leak through the gaps between the bellows fingers. Modeling the fine details of the bellows and the surrounding geometry requires the resolution and accuracy only possible with a large number of mesh points on an unstructured grid. We use the parallel finite element eigensolver Omega3P for trapped mode calculations, and the S-matrix solver S3P for transmission analysis. The damping of the HOMs by the use of absorbers inside the bellows will be investigated. |
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| TPPP030 | Damping Higher Order Modes in the PEP-II B-Factory Vertex Bellows | damping, positron, vacuum, impedance | 2131 | ||
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Funding: Work supported by the U.S. Department of Energy under contract number DE-AC03-76SF00515. |
Higher stored currents and shorter bunch lengths are requirements for increasing luminosity in colliding storage rings. As a result, more HOM power is generated in the IP region. This HOM power propagates to sensitive components causing undesirable heating, thus becoming a limiting issue for the PEP-II B-factory. HOM field penetration through RF shielding fingers has been shown to cause heating in bellows structures. To overcome these limitations, a proposal to incorporate ceramic absorbers within the bellows cavity to damp these modes is presented. Results show that the majority of modes of interest are damped, the effectiveness depending on geometrical considerations. An optimal configuration is presented for the PEP-II B-factory IR bellows component utilizing commercial grade ceramics with consideration for heat transfer requirements. |
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| TPPP033 | Cavity Alignment Using Beam Induced Higher Order Modes Signals in the TTF Linac | dipole, alignment, single-bunch, linac | 2284 | ||
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Funding: DE-AC02-76SF00515. |
Each nine cell superconducting accelerator cavity in the TESLA Test Facility (TTF) at DESY* has two higher order mode (HOM) couplers that efficiently remove the HOM power.** They can also provide useful diagnostic signals. The most interesting modes are in the first 2 cavity dipole passbands. They are easy to identify and their amplitude depends linearly on the beam offset from the cavity axis making them excellent beam position monitors (BPM). By steering the beam through an eight-cavity cryomodule, we can use the HOM signals to estimate internal residual alignment errors and minimize wakefield related beam emittance growth. We built and commissioned a four channel heterodyne receiver and time-domain based waveform recorder system that captures information from each mode in these two bands on each beam pulse. In this paper we present an experimental study of the single-bunch generated HOM signals at the TTF linac including estimates of cavity alignment precision and HOM BPM resolution. *P. Piot, DESY-TESLA-FEL-2002-08. **R. Brinkmann et al. (eds.), DESY-2001-011. |
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| TPPT037 | A Coaxial Subharmonic Cavity Design for Direct Injection at the Advanced Photon Source | booster, linac, electron, injection | 2497 | ||
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Funding: Work supported by U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. W-31-109-ENG-38. |
Coaxial subharmonic cavity designs are being investigated at the Advanced Photon Source to improve injector reliability by injecting beam directly from the linac to the booster in storage ring top-up mode. The subharmonic system must operate jointly with the present 352-MHz booster to accelerate the beam to 7 GeV with minimal beam degradation. Design considerations must be made to ensure that bunch purity is maintained and that a large percentage of the linac macropulse is captured. An analysis of rf cavity designs using electromagnetic simulation software has been conducted at 58 MHz and 117 MHz. The final design evaluates the total power loss, field uniformity, and peak surface fields to achieve the required gap voltage. |
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| TPPT070 | Development of the Superconducting 3.9 GHz Accelerating Cavity at Fermilab | simulation, damping, vacuum, resonance | 3825 | ||
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Funding: U.S. Department of Energy. |
A superconducting third harmonic accelerating cavity (3.9 GHz) was proposed to improve beam quality in the TTF-like photoinjector. Fermilab has developed, built and tested several prototypes, including two copper 9-cell cavities, and niobium 3-cell and 9-cell cavities. The helium vessel and frequency tuner for the 9-cell cavity was built and tested as well. In cold tests, we achieved a peak surface magnetic field of ~120mT, well above the 70mT specification. The accelerating gradient was limited by thermal breakdown. Studies of the higher order modes in the cavity revealed that the existing cavity design with two HOM couplers will provide sufficient damping of these modes. In this paper we discuss the cavity design, results of the studies and plans for further development. |
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| TPPT073 | Testing of the New Tuner Design for the CEBAF 12 GeV Upgrade SRF Cavities | vacuum, SNS, radiation, coupling | 3910 | ||
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Funding: This manuscript has been authorized by SURA, Inc. under Contract No. DE-AC05-84ER-40150 with the U.S. Department of Energy. |
The new tuner design for the 12 GeV Upgrade SRF cavities consists of a coarse mechanical tuner and a fine piezoelectric tuner. The mechanism provides a 30:1 mechanical advantage, is pre-loaded at room temperature and tunes the cavities in tension only. All of the components are located in the insulating vacuum space and attached to the helium vessel, including the motor, harmonic drive and piezoelectric actuators. The requirements and detailed design are presented. Measurements of range and resolution of the coarse tuner are presented and discussed. |
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| TPPT082 | High Thermal Conductivity Cryogenic RF Feedthroughs for Higher Order Mode Couplers | damping, pick-up, simulation, SNS | 4108 | ||
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Funding: This manuscript has been authorized by SURA, Inc. under Contract No. DE-AC05-84ER-40150 with the U.S. Department of Energy. |
The use of higher-order-mode (HOM) pickup probes in the presence of significant fundamental rf fields can present a thermal challenge for cw or high average power SRF cavity applications. The electric field probes on the HOM-damping couplers on the JLab "High Gradient" and "Low Loss" seven-cell cavities for the CEBAF upgrade are exposed to approximately 10% of the peak magnetic field in the cavity. To avoid significant dissipative losses, these probes must remain superconducting during operation. Typical cryogenic rf feedthroughs provide a poor thermal conduction path for the probes, and provide inadequate stabilization. We have developed solutions that meet the requirements, providing a direct thermal path from the niobium probe, thorough single-crystal sapphire, to bulk copper which can be thermally stationed (or heat sunk). Designs, electromagnetic and thermal analyses, and performance data will be presented. |
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| TPPT094 | Design of the CW Cornell ERL Injector Cryomodule | emittance, linac, damping, alignment | 4290 | ||
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Funding: This work is supported by Cornell University. |
The Cornell ERL Prototype injector will accelerate bunches from an electron source to an energy of several MeV, while preserving the ultra-low emittance of the beam. The injector linac will be based on superconducting RF technology with five 2-cell RF cavities operated in cw mode. The beam tubes on one side of the cavities have been enlarged to propagate Higher-Order-Mode power from the cavities to broadband RF ring-absorbers located at 80 K between the cavities. The axial symmetry of these ferrite based absorbers, together with two symmetrically placed input couplers per cavity, avoids transverse on-axis fields, which would cause emittance growth. Each cavity is surrounded by a LHe vessel and equipped with a frequency tuner. The cryomodule provides the support and alignment for the cavity string, the LN cooling of the ferrite loads, and the 2K LHe cryogenic system for the high cw heat load of the cavities. In this paper we give an overview of the ERL injector cryomodule design. |
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| WPAT017 | Commissioning of the New RF System with the HOM Damped RF Cavity | storage-ring, vacuum, impedance, damping | 1555 | ||
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A new 178 MHz RF system has been commissioned at Duke Storage Ring. It consists of a 140 kW tetrode transmitter, a high order modes (HOM) damped RF cavity and the necessary frequency and voltage control electronics. The cavity walls are made of copper-on-stainless steel bimetal (8 mm Cu, 7 mm SS). The cavity has a larger beam pipe opening (700 mm in diameter) in the down-stream side, which allows the HOM propagating out of the cavity and being absorbed by the ceramic loads. The design details and the commissioning results are presented in this paper.
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| WPAT031 | Design and Operation of a High Power L-Band Multiple Beam Klystron | klystron, electron, cathode, focusing | 2170 | ||
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A 1.3 GHz, 10 MW, higher-order-mode multiple beam klystron (MBK) has been developed for the TESLA program. The relative advantages of such a device are many-fold. Multiple beams generate higher beam currents and thereby require much lower operating voltages which allows for the use of smaller, less expensive modulators. A lower perveance per cathode can also be used which leads to higher operating efficiencies. Higher-order-mode cavities allow for the use of much larger cathodes which leads to lower cathode current density loadings and subsequently longer cathode lifetimes. This requires that the cathodes be located far off the geometric axis of the device. The compromise is an increase in the complexity of the magnetic focusing circuit required to transport the off-axis electron beams. Such a device has been successfully built and tested. Excellent beam transmission has been achieved (99.5% DC and 98% at RF saturation). A peak power of 10 MW with 150 kW of average power and 60% efficiency, 49 dB gain have also been measured. The achieved low cathode loading of 2.1 A/cm2 corresponds to an expected cathode life of over 140,000 operational hours.
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| RPPE003 | Operational Experience of Cooling Water Systems for Accelerator Components at PLS | linac, storage-ring, power-supply, klystron | 850 | ||
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Funding: Work supported by MOST and POSCO in Republic of Korea. |
The cooling water system has been utilized for absorbing heat generated by a multitude of electromagnetic power delivering networks at PLS. The separate cooling water distribution systems for the storage ring, beam transport line and linear accelerator have been operated with a different operating temperature of supplying water. All water used for heat removal from the accelerator components are deionised and filtered to provide with over 2 MO-cm specific resistance. The operating pressures and flows of input water are also controlled with flow balancing scheme at a specified range. The operating temperature of components in the accelerator is sustained as tight as below ±0.1 deg C to minimize the influence of temperature fluctuation on the beam energy and stability. Although the PLS cooling systems were initially installed with a high degree of flexibility to allow for easy maintenance, a number of system improvements have been employed to enhance operational reliability and to incorporate the newly developed operating interfaces such as EPICS accelerator control systems. The important design and operational features of PLS cooling water systems are presented as well as lessons learned from around 10-years normal operation. |
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| RPPT014 | Design and Measurements of an X-Band Accelerating Cavity for SPARC | coupling, resonance, linac, scattering | 1407 | ||
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The paper presents the design of an X-band accelerating section for linearizing the longitudinal phase space in the Frascati Linac Coherent Light Source (SPARC). The structure, operating on the pi standing wave mode, is a 9 cells structure feeded by a central waveguide coupler and has been designed to obtain a 5 MV accelerating voltage. The 2D profile has been obtained using the e.m. codes SUPERFISH and OSCARD2D while the coupler has been designed using HFSS. Bead-pull measurement made on a copper prototype are illustrated and compared with the numerical results. Mechanical details of the realized prototype and RF properties of the structure as a function of the assembly characteristics are also discussed.
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