Michael Drury

RF Conditioning and Testing of Fundamental Power Couplers for SNS Superconducting Cavity Production

The Spallation Neutron Source (SNS) makes use of 33 medium beta (0.61) and 48 high beta (0.81) superconducting cavities. Each cavity is equipped with a fundamental power coupler, which should withstand the full klystron power of 550 kW in full reflection for the duration of an RF pulse of 1.3 msec at 60 Hz repetition rate. Before assembly to a superconducting cavity, the vacuum components of the coupler are submitted to acceptance procedures consisting of preliminary quality assessments, cleaning and clean room assembly, vacuum leak checks and baking under vacuum, followed by conditioning and RF high power testing. Similar acceptance procedures (except clean room assembly and baking) were applied for the airside components of the coupler. All 81 fundamental power couplers for SNS superconducting cavity production have been RF power tested at JLAB Newport News and, beginning in April 2004 at SNS Oak Ridge. This paper gives details of coupler processing and RF high power-assessed performances.

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.

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.

Overview of SNS Cryomodule Performance

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 (Q0), microphonics, and response to Lorentz forces. The Qext’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.

A magnetostrictive tuning mechanism for SRF cavities

Energen, Inc. has designed, built and demonstrated a fine tuning mechanism for superconducting radiofrequency (SRF) cavities used in particle accelerators. This tuner is based on giant magnetostrictive materials being developed by Energen Inc. Magnetostrictors elongate when exposed to a small magnetic field. This extension is reversible and repeatable enabling a wide range of applications. The magnetostrictive tuner was specifically designed to meet the requirements of the Thomas Jefferson National Accelerator Facility in Newport News, VA. The tuner consists of a high force linear actuator that elongates the cavity along its axis thereby changing its resonant frequency. It is installed in the dead leg of the existing mechanical tuner. This mechanism has a motion range that provides a tuning range of up to 6400 Hz. Preliminary tests at Jefferson Laboratory demonstrated cavity tuning capability.

Cryogenic testing of the RF input waveguide for the CEBAF upgrade cryomodule

In order to support the planned CEBAF upgrade at the Jefferson Lab a new cryomodule has been designed. A key component of the new cryomodule is the upgraded RF input waveguide, which may couple as much as 10 kW of RF power to superconducting radio frequency (SRF) cavities contained in a bath of superfluid helium. The coupler consists of a straight copper plated stainless steel waveguide as a thermal transition between 2 K and 300 K and one ceramic window at the warm end of the waveguide. The waveguide interior shares a common vacuum with the cavity. Initial testing of the upgraded coupler has been conducted at Jefferson Lab in a representative cryomodule. During testing, data was obtained regarding waveguide temperature profiles as well as coupler arcing and multipacting. Predicted temperature profiles were used to determine the optimum location of the 60 K heat intercept on the waveguide, and were found to be comparable to the actual measured profiles. The coupler was found to be free of multipacting up to 1.7 kW. No arcing occurred during multiple eight-hour runs in heavy field emission with typical radiation levels of 0.5 to 1.0 R/hr outside the cryostat.

Thermal Performance of the SNS Cryomodule

When complete, the Spallation Neutron Source (SNS) will provide a 1 GeV, 2 MW beam for experiments. One portion of the machine’s linac consists of over 80 Superconducting Radio‐Frequency (SRF) 805 MHz cavities housed in a total of 23 cryomodules (CMs) operating at 2.1 K. Minimization of the total heat load is critical to machine performance since the refrigerator capacity is fixed. The total heat load of the cryomodules consists of the fixed static load and the dynamic heat load, which is proportional to the cavity performance. The heat load of the cryomodules is the single largest load to the cooling circuits of the refrigerator. During acceptance testing at Jefferson Lab, a series of measurements have been taken on the prototype and first three production CMs. Calorimetric measurements of the primary heat load and shield heat load are presented and discussed. Temperature measurements taken allow a comparison between actual and predicted thermal performance of two components unique to this cryomodule des...

CEBAF cryomodule testing

The authors describe test results, Qs, gradients, and static and dynamic heat loads, for the CEBAF (Continuous Electron Beam Accelerator Facility) cryomodules along with the electrical and mechanical performance of the cryomodules manufactured to date. The vertical tests have resulted in Qs >10/sup 10/ at 5 MV/m as well as gradient approaching >16 MV/m. To this point, the authors have observed decreases in the Q/sub 0/ in the cryomodule test area followed by a good recovery during commissioning to nearly twice design value. The gradient has not appreciably decreased during any of the testing. The static heat loads have also met or exceeded the design specification. The current installed injector cryomodules exceeds the design of 25 MeV/c.<<ETX>>

CEBAF cryomodules: test results and status

The Continuous Electron Beam Accelerator Facility is under construction in Newport News, Virginia. When completed in 1994, the accelerator will be the largest installation of radio frequency superconductivity. Production of cryomodules, the fundamental cryogenic building blocks of the machine, has started. They consist of four pairs of 1497-MHz, five-cell cavities contained in separate helium vessels and mounted in a cryostat with appropriate end caps for helium supply and return. The first cryomodule has been successfully assembled, tested, and moved to the accelerator. Some of the individual components in this prototype performed below, but close to, the specification. As a whole, the cryomodule performs in a more than acceptable way for operation in the injector. It will provide 20 MeV to the electron beam with a tolerable dissipation of about 75 W at 2 K. A new cryomodule with full production components will replace this prototype.

Impact of remanent magnetic field on the heat load of original CEBAF cryomodule

The heat load of the original cryomodules for the continuous electron beam accelerator facility is ~50% higher than the target value of 100 W at 2.07 K for refurbished cavities operating at an accelerating gradient of 12.5 MV/m. This issue is due to the quality factor of the cavities being ~50% lower in the cryomodule than when tested in a vertical cryostat, even at low RF field. Previous studies were not conclusive about the origin of the additional losses. We present the results of a systematic study of the additional losses in a five-cell cavity from a decommissioned cryomodule after attaching components, which are part of the cryomodule, such as the cold tuner, the He tank, and the cold magnetic shield, prior to cryogenic testing in a vertical cryostat. Flux-gate magnetometers and temperature sensors are used as diagnostic elements. Different cool-down procedures and tests in different residual magnetic fields were investigated during the study. Three flux-gate magnetometers attached to one of the cavities installed in the refurbished cryomodule C50-12 confirmed the hypothesis of high residual magnetic field as a major cause for the increased RF losses.