With the recent progress in surface cleaning, the performance of superconducting RF cavities is mostly limited by a quench. It is important to understand the nature of the quench origin. In a common SRF cavity design the RF magnetic field is concentrated near the equatorial weld of the cavity. This weld has long been the major suspect in forming a surface defect, either as an impurity or in an increased surface roughness, that eventually gives rise to a quench. We used surface mounted thermometers to obtain a temperature map of the cavity in the quench region. A high temperature, temporal, and spatial resolution of the thermometry system allows us to pinpoint the quench origin with an accuracy of a few millimeters. We found that the hot-spot precursor forms in the weld heat-affected area rather than in the melted zone. The high resolution optical inspection found surface defects in exactly the same locations as the temperature mapping system. We will describe the measurement techniques and discuss possible scenarios of formation of these defects.

Quench-Limited SRF Cavities: Failure at the Heat-Affected Zone

Fermilab is involved in an effort to assemble 3.9 GHz superconducting RF cavities into a four cavity cryomodule for use at the DESY TTF/FLASH facility as a third harmonic structure. The design gradient of the cavities is 14 MV/m. This effort involves design, fabrication, intermediate testing, assembly, and eventual delivery of the cryomodule. We report on all facets of this enterprise from design through future plans. Included will be test results of single 9-cell cavities, lessons learned, and current status.

Status of 3.9 GHz superconducting RF cavity technology at Fermilab

Over the last few years Fermilab has developed the superconducting third harmonic section for the FLASH (TTF/DESY) upgrade. Initial vertical tests of 9-cell Nb cavities didn't reach the designed accelerating gradient. The main factor of gradient limitation was multipacting in the HOM coupler. In this paper we present the results of vertical tests accompanied with 3D analysis simulations of multipacting. Also we discuss the RF design of the new HOM couplers. The goal of the new design is to eliminate multipacting and to increase the second resonance frequency of the HOM coupler. Increasing this frequency reduces electric and magnetic fields, resulting in decreased thermal load on the antenna. Two cavities with modified HOM couplers have achieved operating gradients of 23 MV/m.

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New hom coupler design for 3.9 GHZ superconducting cavities at FNAL

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 resistance temperature detectors (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 the fast thermometry system will be discussed along with results from tests of the 9 cell 3.9 GHz SRF cavity.

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Fast thermometry for superconducting RF cavity testing

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Status of 3.9 GHz superconducting rf cavity technology at Fermilab

The FLASH user facility providing free electron laser radiation is built based on the TTF project at DESY. Fermilab has the responsibility for the design and processing of a third harmonic, 3.9 GHz, superconducting cavity which is powered via a coaxial power coupler. Six power couplers have been manufactured at CPI after successful design of the power coupler including RF simulation, multipacting calculation, and thermal analysis. The power couplers are being tested and processed with high pulsed power in an elaborate test stand at Fermilab now. This paper presents the RF design and processing work of the power coupler.

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RF design and processing of a power coupler for third harmonic superconducting cavities

The 3rd harmonic 3.9 GHz accelerating cavity was proposed to improve the beam performance of the FLASH (TTF/DESY) facility [1]. In the frame of a collaborative agreement, Fermilab will provide DESY with a cryomodule containing a string of four cavities. In addition, a second cryomodule with one cavity will be fabricated for installation in the Fermilab photo-injector, which will be upgraded for the ILC accelerator test facility. The first 9-cell Nb cavities were tested in a vertical setup and they didn't reach the designed accelerating gradient [2]. The main problem was a multipactor in the HOM couplers, which lead to overheating and quenching of the HOM couplers. New HOM couplers with improved design are integrated in the next 9-cell cavities. In this paper we present all results of the vertical tests.

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3.9 GHZ superconducting accelerating 9-cell cavity vertical test results

As part of the research and development towards the International Linear Collider (ILC), several test facilities have been developed at Fermilab This paper presents the latest Low Level RF (LLRF) results obtained with Capture Cavity II (CCII) at the ILC Test Accelerator (ILCTA) test facility The main focus will be on controls and RF operations using the SIMCON based LLRF system developed in DESY Details about hardware upgrades and future work will be discussed

Capture cavity II results at FNAL

As part of the research and development towards the International Linear Collider (ILC), several test facilities have been developed at Fermilab. This paper presents the latest Low Level RF (LLRF) results obtained with Capture Cavity II (CCII) at the ILC Test Accelerator (ILCTA) test facility. The main focus will be on controls and RF operations using the SIMCON based LLRF system developed in DESY. Details about hardware upgrades and future work will be discussed.

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E. Harms

Papers

Fast thermometry for superconducting RF cavity testing

RF design and processing of a power coupler for third harmonic superconducting cavities

3.9 GHZ superconducting accelerating 9-cell cavity vertical test results

Capture cavity II results at FNAL

Capture cavity II results at FNAL