John Mammosser

Performance experience with the CEBAF SRF cavities

The full complement of 169 pairs of niobium superconducting cavities has been installed in the CEBAF accelerator. This paper surveys the performance characteristics of these cavities in vertical tests, commissioning in the tunnel, and operational experience to date. Although installed performance exceeds specifications, and 3.2 GeV beam has been delivered on target, present systems do not consistently preserve the high performance obtained in vertical dewar tests as operational capability. The principal sources of these limitations are discussed.

An RF input coupler system for the CEBAF energy upgrade cryomodule

Long term plans for CEBAF at Jefferson Lab call for achieving 12 GeV in the middle of the next decade and 24 GeV after 2010. Thus an upgraded cryomodule to more than double the present voltage is under development. A new waveguide coupler system has been designed and prototypes are currently being developed. This coupler, unlike the original, has a nominal Q/sub ext/ of 2.1/spl times/10/sup 7/, reduced sensitivity of Q/sub ext/ to mechanical deformation, reduced field asymmetry within the beam envelope, freedom from window arcing with a single window at 300 K, and is capable of transmitting 6 kW CW both traveling wave and in full reflection.

Cryomodule development for the CEBAF upgrade

Long term plans for CEBAF at Jefferson Lab call for achieving 12 GeV in the middle of the next decade and 24 GeV after 2010. In support of those plans, an Upgrade Cryomodule capable of providing more than three times the voltage of the original CEBAF cryomodule specification within the same length is under development. Development activities have been focused on critical areas thought to have maximum impact on the overall design. These have included the cavity structure, RF power coupling, cavity suspension, alignment, cavity tuning, and beamline interface. It has been found that all design and development areas are tightly coupled and can not be developed independently. Substantial progress has been made toward an integrated design for the Jefferson Lab Upgraded Cryomodule.

SNS cryomodule performance

Thomas Jefferson National Accelerating Facility, Jefferson Lab, is producing 24 Superconducting Radio Frequency (SRF) cryomodules for the Spallation Neutron Source (SNS) cold linac. This includes one medium-/spl beta/ (0.61) prototype, 11 medium-/spl beta/ production, and 12 high beta (0.81) production cryomodules. After testing, the medium-/spl beta/ prototype cryomodule was shipped to Oak Ridge National Laboratory (ORNL) and acceptance check out has been completed. All production orders for cavities and cryomodule components are being received at this time and the medium-/spl beta/ cryomodule production run has started. Each of the medium-/spl beta/ cryomodules is scheduled to undergo complete operational performance testing at Jefferson Laboratory before shipment to ORNL. The performance results of cryomodules to date will be discussed.

Residual Resistance Data From Cavity Production Projects at Jefferson Lab

A fundamental limitation towards achieving high quality factors in superconducting radio-frequency cavities is the so-called residual resistance. Understanding and controlling the residual resistance has important implications towards improving the efficiency and reduce the operating cost of continuous wave superconducting linear accelerators. In this contribution we will report on the residual resistance values obtained from measurements of the quality factor of a large set of cavities, with resonant frequency between 805 MHz and 1.5 GHz, all of them processed and tested at Jefferson Lab. Surface treatments included both buffered chemical polishing and electropolishing. The results indicate an approximate value of the residual resistance of about 7-10 nΩ.

Superconducting cavity development for the CEBAF upgrade

Long-term plans for CEBAF at Jefferson Lab call for achieving 12 GeV in the middle of the next decade and 24 GeV after 2010. In support of these plans, an Upgrade Cryomodule, capable of providing more than twice the operating voltage of the existing CEBAF modules within the same length, is being developed. In particular, this requires the development of superconducting cavities capable of consistently operating at gradients above 12 MV/m and Q/spl sim/10/sup 10/. We have engaged in a complete review of all the processes and procedures involved in the fabrication and assembly of cavities, and are modifying our chemical processing, cleaning, and assembly facilities. While we have retained the cell shape of existing CEBAF cavities, the new superconducting structure will be substantially different in several respects, such as the higher-order-modes damping and the fundamental power coupling systems. Design features and experimental results are presented.

Status of the production electropolishing system at JLAB

Jefferson Lab has installed, and is in the process of commissioning, a production electropolish system, sized for 805 MHz SNS cavities. This paper describes the basic system design, plans for studying the effectiveness of polishing SNS high-/spl beta/ cavities and early results from cavity tests.

Design of the CEBAF energy upgrade cryomodule cold mass

The Jefferson Laboratorys CEBAF Energy Upgrade to 12 GeV will be accomplished by employing new cryomodules with an increase in effective length of 40 percent while maintaining the same nominal cryomodule slot length within the linac. This is being achieved by changing from an eight five-cell cavity structure to an eight seven-cell cavity structure and removing all bellows between the cavities along the beamline. This approach has presented significant challenges with respect to the design as it pertains to the cooldown, support, alignment and tuning criteria. The solutions to these design challenges will be presented with an emphasis on cavity operational criteria along with the integrated support components to include higher order mode couplers, fundamental power couplers and tuning accommodations for a one-hertz resolution.

New window design options for CEBAF energy upgrade

As the Jefferson Laboratory upgrades the existing CEBAF electron accelerator to operate at higher energies, the fundamental power coupler windows will be required to operate with lower RF dissipation and increased immunity to radiation from cavity field emission. New designs and modifications to existing designs which can achieve these goals are described.

HOM damping performance of JLab SL21 cryomodule

The SL21 cryomodule is a first 1.5 GHz, 12.5 MV/m, 7-cell cavity, 8-cavity string superconducting accelerator module produced at JLab. The first two passbands of TE111 and TM110 high order modes (HOM) have to be damped to avoid beam breakup problems at 460 /spl mu/A current. External Qs of the HOM couplers and waveguide were measured on a copper model, on cold niobium cavities in vertical tests and finally in the cryomodule without beam. This paper presents all HOM data and general measurement techniques. A threshold current predicted by MATBBU code based on these data and the 6 GeV machine beam optics is 10 mA.