We have developed a digital process that emulates an analog oscillator and ultimately a self excited loop (SEL) for field control. The SEL, in its analog form, has been used for many years for accelerating cavity field control. In essence the SEL uses the cavity as a resonant circuit - much like a resonant (tank) circuit is used to build an oscillator. An oscillating resonant circuit can be forced to oscillate at different, but close, frequencies to resonance by applying a phase shift in the feedback path. This allows the circuit to be phased-locked to a master reference, which is crucial for multiple cavity accelerators. For phase and amplitude control the SEL must be forced to the master reference frequency, and feedback provided for in both dimensions. The novelty of this design is in the way digital signal processing (DSP) is structured to emulate an analog system. While the digital signal processing elements are not new, to our knowledge this is the first time that the digital SEL concept has been designed and demonstrated. This paper reports on the progress of the design and implementation of the digital SEL for field control of superconducting accelerating cavities.

/pdf/a-digital-self-excited-loop-for-accelerating-cavity-field-2jr0u4481w.pdf

A digital self excited loop for accelerating cavity field control

A Digital Self Excited Loop for Accelerating Cavity Field Control

Thomas Jefferson National Accelerating Facility (Jefferson Lab) has completed production of 24 Superconducting Radio Frequency (SRF) cryomodules for the Spallation Neutron Source (SNS) superconducting linac. This includes one medium-β (0.61) prototype, eleven medium-β and twelve high-β (0.81) production cryomodules. Nine medium-β cryomodules as well as two high-β cryomodules have undergone complete operational performance testing in the Cryomodule Test Facility at Jefferson Lab. The set of tests includes measurements of maximum gradient, unloaded Q (Q 0 ), microphonics, and response to Lorentz forces. The Q ext ’s of the various couplers are measured and the behavior of the higher order mode couplers is examined. The mechanical and piezo tuners are also characterized. The results of these performance tests will be discussed in this paper.

/pdf/overview-of-sns-cryomodule-performance-48022sdcm4.pdf

Overview of SNS Cryomodule Performance

Since the start of the Spallation Neutron Source project, the field of high intensity (in the mA range and above) proton linacs based on superconducting resonators is living a time of great interest. A large variety of possible applications (nuclear waste transmutation, spallation neutron production, energy amplifier, radioactive ion production, neutrino factories, etc.) prompted many studies and proposals of new machines with final energy ranging typically from a few tens of MeV to about 1 GeV; high power requirements and reliability issues introduced new kinds of problems in linac and resonators design. A “zoo” of different superconducting cavity types, often evolved either from the electron or from the heavy ion SRF technology, is growing to cover the many different proposed applications, and triggering development in related high power couplers and fast tuners. During the last few years, high gradient resonators have been successfully prototyped in the full range of interesting beam energy, down to beta=0.1. The trend is to extend the SC sections of high current proton linacs to very low beta, taking advantage of the large acceptance of short cavities in order to increase linac reliability and, in some cases, to allow acceleration of deuteron beams. The construction of new, superconducting high intensity proton linacs is foreseen in the next future.

/pdf/high-intensity-proton-sources-1whijzreua.pdf

High Intensity Proton Sources

The performance of the operational SRF-based electron linacs CEBAF, ELBE, and S-DALINAC continues to evolve positively. These facilities are exploiting opportunities to improve operational capability by both remediation of past limitations and also construction of new capacity using state-of-the-art designs and processes. A project to rework the weakest ten cryomodules in CEBAF was completed and enabled robust operation for physics at 6 GeV. The 12 GeV Upgrade of CEBAF is now underway and involves construction of ten new CW >100 MV cryomodules with 80 new 7-cell low-loss cell shaped fine-grained niobium cavities, all electropolished. The technical challenges associated with the preparation of these cavities will be reviewed and their performance in both individual acceptance testing and cryomodule testing to date will be summarized. The ELBE facility at Helmhotz Zentrum Dresden Rossendorf continues to develop its source with an SRF gun system. The longrunning S-DALINAC at Technische Universitat Darmstadt is operating reliably and is evolving in the direction of reduced energy spread and use of polarized electrons for nuclear physics and astrophysics research.

Srf challenges for improving operational electron linacs

The Thomas Jefferson National Accelerator Facility has recently completed the C50 cryomodule refurbishment project. The goal of this project was to enable robust 6 GeV, 5 pass operation of the Continuous Electron Beam Accelerator Facility (CEBAF). The scope of the project included removal, refurbishment and reinstallation of ten CEBAF cryomodules at a rate of three per year. The refurbishment process included reprocessing of SRF cavities to eliminate field emission and to increase the nominal gradient from the original 5 MV/m to 12.5 MV/m. New 'dogleg' couplers were installed between the cavity and helium vessel flanges to intercept secondary electrons that produce arcing in the fundamental Power Coupler (FPC). Other changes included new ceramic RF windows for the air to vacuum interface of the FPC and improvements to the mechanical tuner. Damaged or worn components were replaced as well. All ten of the refurbished cryomodules are now installed in CEBAF and are currently operational. This paper will summarize the performance of the cryomodules.

/pdf/summary-report-for-the-c50-cryomodule-project-14260776i0.pdf

Summary Report for the C50 Cryomodule Project

The CEBAF Accelerator at Jefferson Lab is presently a 6 GeV five pass electron accelerator consisting of two superconducting linacs joined by independent magnetic transport arcs. Energy will be upgraded to 12 GeV with the addition of 10 new high gradient cryomodules (17+ MV/m). The higher gradients pose significant challenges beyond what the present analog low level RF (LLRF) control systems can handle reliably; therefore, a new LLRF control system is needed. A prototype system has been developed incorporating a large FPGA and using digital down and up conversion to minimize the need for analog components. The new system is more flexible and less susceptible to drifts and component nonlinearities. Because resonance control is critical to reach high gradients quickly, the new cryomodules will include a piezoelectric tuner (PZT) for each cavity, and the LLRF controls must incorporate both feedback and feed-forward methods to achieve optimal resonance control performance. This paper discusses development of the new RF system and system performance as measured during recent tests on a prototype cryomodule for phase and amplitude stability and resonance control under Lorentz detuning.

High gradient operation with the cebaf upgrade rf control system

The Thomas Jefferson National Accelerator Facility is currently engaged in the 12 GeV Upgrade Project. The goal of the project is a doubling of the available beam energy of CEBAF from 6 GeV to 12 GeV. This increase in beam energy will be due primarily to the construction and installation of ten "C100" cryomodules in the CEBAF linacs. The C100 cryomodules are designed to deliver an average 108 MV each from a string of eight seven-cell, electropolished superconducting RF cavities operating at an average accelerating gradient of 19.2 MV/m. The new cryomodules fit in the same available linac space as the original CEBAF 20 MV cryomodules. Cryomodule production started in September 2010. Initial acceptance testing started in June 2011. The first two C100 cryomodules were installed and tested from August 2011 through October 2011, and successfully operated during the last period of the CEBAF 6 GeV era, which ended in May 2012. This paper will present the results of acceptance testing and commissioning of the C100 style cryomodules to date.

Performance of First C100 Cryomodules for the CEBAF 12 GeV Upgrade Project

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-beta (0.61) prototype, 11 medium-beta production, and 12 high-beta (0.81) production cryomodules. Each of the medium-beta cryomodules is scheduled to undergo complete operational performance testing at Jefferson Laboratory before shipment to ORNL. To date, the prototype and three production models of the medium beta cryomodule have been tested. The performance results of the tested cryomodules will be discussed.

/pdf/sns-medium-beta-cryomodule-performance-3ksy5u3lea.pdf

SNS Medium Beta Cryomodule Performance

The cryomodule test facility (CMTF) was originally implemented in the late eighties for testing of a small fraction of the cryomodules during the production run for the Continuous Electron Beam Accelerator Facility [1]. The original system was built using a dedicated wiring scheme and a pair of 2 kW, 1497 MHz RF sources. This dedicated system made it difficult to test cryomodules and other RF structures of non-standard configuration. Additionally, due to a previously installed cyclotron, there were static magnetic fields in excess of 6 Gauss within the test cave, which limited the capability of the facility when measuring the quality factor of superconducting cavities. Testing of the Spallation Neutron Source cryomodules as well as future upgrades to the CEBAF accelerator necessitated that the facility be reconfigured to be flexible both with respect to RF source power and cryomodule wiring configuration. This paper will describe the implementation of a generalized wiring scheme t hat is easily adapted to different cryomodule configurations. It will also describe the capabilities of the LabView based low level RF controls and the related data acquisition systems currently being used to test cryomodules and related hardware. The high power RF source capabilities will be described. Themore » magnetic shielding put in place in order to reduce the ambient magnetic file to levels below 50 mGauss will also be described.« less

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