Fundamental power couplers (FPC's) for superconducting cavities must meet very strict requirements to perform at high power levels (hundreds of kilowatts) and in a variety of conditions (CS, pulsed, travelling wave, standing wave) without adversely affecting the performance of the cavities they are powering. Producing good coupler designs and achieving operational performances in accelerator environments are challenging tasks that have traditionally involved large resources from many laboratories. The designs involve state-of-the-art activities in RF, cryogenic and mechanical engineering, materials science, vacuum technology, and electromagnetic field modeling. Handling, assembly and conditioning procedures have been developed to achieve ever-increasing power levels and more reliable operation. In this paper, the technical issues associated with the design, construction, assembly, processing, and operation of FPC's will be reviewed, together with the progress in FPC activities in several laboratories during the past few years.

/pdf/fundamental-power-couplers-for-superconducting-cavities-te6exwb2x2.pdf

Fundamental Power Couplers for Superconducting Cavities

Single-surface multipactor on metal surfaces is studied in the case when the microwave electric field is superimposed on a dc electric and a permanent magnetic field. Based on a simple analysis of the electron motion, it is predicted that considerable reduction in the multipactor threshold is possible when the electron cyclotron frequency is equal to the microwave field frequency and the permanent magnetic field has the proper orientation with respect to the metal surface. The prediction is confirmed by numerical simulations, and some experimental indications of the reduced multipactor threshold are also presented.

https://publications.lib.chalmers.se/records/fulltext/168990/local_168990.pdf

Reduction of the Multipactor Threshold Due to Electron Cyclotron Resonance

Fundamental power couplers for RF particle accelerators are the most important components that interface with the accelerating structures. In the case of superconducting cavities, the main function of providing appropriate power to the cavity’s fields and to the beams is dwarfed by the requirements the couplers must meet to blend harmoniously into the vacuum, cryogenic, and electromagnetic environment. Recently, a lot of progress has been made in the areas critical to the successful design, construction and operation of fundamental power couplers. Simulations are now routinely performed that allow the prediction of electromagnetic, multipacting, thermal, and mechanical properties of couplers. From these studies, better designs have been conceived which can minimize potential problems ahead of construction. Judicious use of materials and the implementation of clean practices and of careful conditioning have gradually increased the power levels at which couplers can safely operate. Machine operation at hundreds of kilowatts has been achieved in CW at KEK and Cornell, and in a pulse mode at the TESLA Test Facility (TTF). Test stand operations in CW have been achieved at the megawatt level (Accelerator for the Production of Tritium) and in pulse mode at a peak power of 2 MW (Spallation Neutron Source, TTF version II). The recent progress indicates that understanding of fundamental power coupler behavior is rapidly increasing and that optimal designs are being developed which will allow routine attainment of the megawatt power levels necessary for high-beam-power machines under construction and under study.

/pdf/state-of-the-art-power-couplers-for-superconducting-rf-38vyjh3ydd.pdf

State of the art power couplers for superconducting rf cavities

A number of high power fundamental input couplers for superconducting cavities operating in CW mode have been developed in recent years around the world. Some couplers have already been in service for many years delivering hundreds of kilowatts of RF power to beams. In this paper we will summarize experience accumulated in different laboratories and review design options and technical issues associated with R&D, testing and operation of the CW high power couplers. Several future projects will be discussed to highlight new requirements and design challenges.

/pdf/review-of-high-power-cw-couplers-for-superconducting-ygotey3ckr.pdf

Review of high power cw couplers for superconducting cavities

As we have reported at the last SRF Workshop two years ago, staged transition of the Cornell Electron Storage Ring (CESR) RF System from normal conducting to superconducting was completed in September 1999 with installation of the fourth superconducting B-cell cavity [1]. Since that time the last cavity has been commissioned and RF system has operated stably providing CESR beams with up to 1.13 MW of RF power to support total beam current up to 800 mA. CESR is a symmetric energy ee collider operating in a bunch train mode with CLEO detector studying B meson decays at thte center of mass energy near the Υ(4S) resonance (≈5.3 GeV) [2]. At the same time CESR provides beams for experiments with synchrotron radiation (SR) at the Cornell High-Energy Synchrotron Source (CHESS). CHESS facility operates in a parasitic mode simultaneously with high-energy physics experiments at CLEO. The latest running period lasted nine months ending in June 2001. The emphasis was put on a steady running of the machine aiming to reach integrated luminosity of at least 10 fb. This goal was achieved by delivering of just over 11 fb. The typical total beam current at the start of the fill was 750 mA and typical RF power delivered to beam by one cavity was 280 kW. Beginning in the fall 2001 CESR will run at Υ resonances below Υ(4S), and during machine studies periods at J/Ψ resonances [3]. Eventually, after some hardware modification and upgrade, CESR will be able to operate over the energy range of 1.5 to 5.6 GeV with expected luminosity of 3×10 cms at 1.9 GeV. CLEO and CHESS will then operate in a time-sharing mode of operation. The CESR upgrade includes modification of the RF. Two more superconducting cavities will be added to provide higher total RF voltage for bunch shortening.

Performance of the cesr superconducting rf system and future plans

The CESR luminosity upgrade plan calls for shortening bunch length to 1 cm. Such bunch length can be achieved by installing two more superconducting cavities to increase total RF voltage. The RF system upgrade necessary to accommodate this change is discussed.

/pdf/superconducting-rf-system-upgrade-for-short-bunch-operation-4mnlisl61g.pdf

Superconducting RF system upgrade for short bunch operation of CESR

The superconducting cavities for the CESR III upgrade at Cornell utilize rectangular waveguide iris coupling. Waveguide vacuum windows manufactured by Thomson Tubes have been tested off-line at 500 MHz up to 500 kWCW traveling wave, 130 kWCW full reflection, and on-line attached to an SRF cavity with beam. On and off-line window processing characteristics are presented. This experience has yielded insight regarding window processing in traveling wave and full reflection modes coupled with effects of atmospheric exposure. Designs of future windows targeted for 1 MWCW operation are also presented.

/pdf/tests-and-designs-of-high-power-waveguide-vacuum-windows-at-4asdvzbet7.pdf

Tests and designs of high-power waveguide vacuum windows at cornell

After the successful CESR beam test of August 1994 the continued development of a superconducting RF system for the CESR luminosity upgrade is in progress at the Laboratory of Nuclear Studies, Cornell University. The system description as well as recent results are presented.

https://epaper.kek.jp/pac97/papers/pdf/3p021.pdf

Development of superconducting RF for CESR

We present experimental results obtained from standalone waveguide sections designed to test multipacting in CESR type rectangular waveguide couplers. We found that multipacting started at about 30 kW when the RF power was initially applied. Multipacting below 180 kW can be eliminated by RF processing. However, above 180 kW it persists even after an extended processing period. The effect of multipacting is found to be dependent on the RF pulse length. In short pulse (2 ms range) mode it is essentially harmless; but in longer pulse (20 ms range) or CW mode it induces gaseous discharge and hence trips power transmission. The concept of a slotted waveguide was tested and proven beneficial. The complete multipacting suppression is realized with the magnetic bias technique, for which a 10 Gauss bias field suffices.

/pdf/experimental-studies-of-electron-multipacting-in-cesr-type-52qm0b4jol.pdf

Experimental studies of electron multipacting in CESR type rectangular waveguide couplers.

Upon signing a technology transfer agreement with Cornell University, ACCEL began producing turn-key 500 MHz superconducting cavity systems. Five such cryomodules have been delivered and commissioned to date. Four of them are installed in accelerators for operation (two in CESR and one each in Canadian Light Source and Taiwan Light Source) and one serves as an off-line spare at CLS. One more cryomodule is scheduled for testing in early 2005. It will be a spare unit for TLS. Three cryomodules for DIAMOND Light Source are being fabricated at ACCEL. The commissioning results and operational experience with the cryomodules in CESR, CLS and TLS are presented.

/pdf/commissioning-and-operations-results-of-the-industry-arrzif29vb.pdf

Commissioning and Operations Results of the Industry-Produced CESR-Type SRF Cryomodules

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