Although a rectangular waveguide coupler has the conceptual advantages of simplicity and capability of withstanding higher power, builders of modern superconducting accelerators are routinely choosing instead a coaxial coupler for its proven performance. Multipacting induced discharge has been found to be an operating mechanism that prevents a rectangular waveguide coupler from reaching its full potential. Earlier calculations predicted the existence of two-sided multipacting in a rectangular waveguide geometry. In the present study, special waveguide sections of CESR type were built and tested. Multipacting characteristics of the waveguide were identified. Two multipacting suppression methods, the slotted waveguide method and the DC magnetic bias method, were experimentally evaluated. The multipacting current is suppressed by a factor of more than 2 by opening a slot on the broad wall. Complete multipacting suppression can be realized by using the DC magnetic bias method.

Suppression of multipacting in rectangular coupler waveguides.

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

Small Scale Refrigeration and Liquefaction Systems.- Progress in Cryocot lers.- Linear Drive Stirling Cooler Technology (Invited).- Cooling of GeZn Detectors by Commercially Available Closed-Cycle Cryogenic Refrigerators.- Magnetic Refrigeration: A Large Cooling Power Cryogenic Refrigeration Technology.- 1 KW Capacity Refrigeration System at 80 K (LINIT-R1).- Cryogenic Instrumentation.- Cryogenic Thermometry - An Overview.- Monitoring Rapidly Changing Temperatures of the Oscillating Working Fluid in a Regenerative Refrigerator.- Performance Characteristics of Silicon Diode Cryogenic Temperature Sensors.- Thermally-Coupled Cryogenic Pressure Sensing.- Cryogenic Cooling of Biological Samples for Electron and Optical Microscopy.- Plenary Lectures.- Technology Commercialization: Opportunities and Challenges (Invited).- Cryogenic Heat Transfer: A Survey of Recent Developments (Invited).- Short and Long Term Storage Systems.- The Universe of Cryogenic Storage - From Helium II to CO2 (Invited).- Cryogenic Liquid Cylinder Development.- Composites for Cryogenics.- Development of 3-D Composites.- Cryogenics in the Medical Field.- Cryobiology - Two Sides of the Same Coin ? (Invited).- Vitrification Capability of Metal Mirror Ultra-Rapid Cooling Apparatus: A Theoretical Evaluation.- Microfractures in Cryopreserved Heart Valves: Valve Submersion in Liquid Nitrogen Revisited.- An Estimation of Thermal Stress Induced by the Freezing Process for Biological Cell Preservation.- Role of Membrane Phospholipids in Nonfreezing Cold Injury.- Some Cryosurgery-Related Heat Transfer Problems.- General Session.- Cryobench - Apparatus for Testing CEBAF Cryogenic Subcomponents.- CEBAF Cryounit Loss of Vacuum Experiment.- Novel Approaches for Attaining High Accelerating Fields in Superconducting Cavities.- Author Index.

Applications of Cryogenic Technology

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 new superconducting RF system consisting of four single-cell cavity modules is an important part of the CESR Luminosity Upgrade. We describe the commissioning of the first three accelerating modules. This includes in situ testing and conditioning, pulsed power and beam processing of RF windows, commissioning of various cryogenic feedback loops, measuring cavity spacing and phasing with beam, and high-current operation.

/pdf/commissioning-of-the-superconducting-rf-cavities-for-the-2dvy7v657f.pdf

Commissioning of the superconducting RF cavities for the CESR Luminosity Upgrade

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

Crab cavity for the B factories

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

The effects and benefits of high peak power RF processing as a means of reducing field emission loading in 3-GHz niobium accelerator cavities are being investigated. The test apparatus includes 3-GHz klystron capable of delivering RF pulses of up to 200-kW peak power with pulse lengths up to 2.5 ms at a repetition rate of approximately 1 Hz. The test apparatus has variable coupling such that the input external Q varies between 10/sup 5/ and 10/sup 10/ without breaking the cavity vacuum. Low-power, continuous-wave (CW) tests before and after HPP show that HPP is effective in removing emissions which are unaffected by low-power RF processing. CW measurements show that field emission reduction is dependent on maximum field reached during HPP. HPP fields of E/sub peak/ = 70-72 MV/m have been attained. These tests showed FE elimination to E/sub peak/ = 40 MV/m, and maximum fields of E/sub peak/=50-55 MV/m. Temperature mapping is now available. A cavity which showed strong FE loading and had extensive temperature mapping is now being investigated in a scanning electron microscope. A nine-cell cavity has been successfully tested, and, through HPP, reached E/sub acc/=15 MV/m, with Q/sub 0/=6.0*10/sup 9/.<<ETX>>

High peak power RF processing studies of 3-GHz niobium cavities

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