M. Wiseman

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.

Improved prototype cryomodule for the CEBAF 12 GeV upgrade

In order to provide a higher performance building block cryomodule for the CEBAF 12 GeV upgrade, modifications have been made to the design of the Upgrade Cryomodule. The prototype cryomodule will be completed in 2004 and be installed for operation in CEBAF. Design changes enable the use of higher gradient cavities to achieve greater than 100 MV per cryomodule while not exceeding the budgeted cryogenic load of 300 W during steady-state operation. They also include refinements based on experience gained during the construction of the first generation upgraded cryomodules as well as the prototype cryomodule for the Spallation Neutron Source. Two cavity designs will be used in the prototype, one optimized for E/sub peak//E/sub acc/ ratio, and the other optimized for minimum cryogenic load. The input waveguides, thermal shield and piping have been redesigned to accommodate the higher expected heat loads. The vacuum connections consist of niobium-titanium flanges, aluminum-magnesium seals and stainless steel clamps to provide reliable UHV sealing. The cavity tuner features one cold motor and two piezoelectric actuators to provide coarse and fine tuning respectively.

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.

Spallation neutron source cryomodule heat loads and thermal design

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 minimum of 23 cryomodules operating at a saturation temperature of 2.1 K. Minimization of the total heat load is critical to machine performance and for efficient operation of the system. The total heat load of the cryomodules consists of the fixed static load and the dynamic load, which is proportional to the cavity performance. The helium refrigerator supports mainly the cryomodule loads and to a lesser extent the distribution system loads. The estimated heat loads and calculated thermal performance are discussed along with two unique features of this design: the helium heat exchanger housed in the cryomodule return end can and the helium gas cooled fundamental power coupler.

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.

Loss of Cavity Vacuum Experiment at CEBAF

Two experiments have been conducted at the Continuous Electron Beam Accelerator Facility to simulate the loss of cavity vacuum in cryomodules full of liquid helium. Each CEBAF cryomodule contains eight superconducting radio frequency cavities and 1300 liquid liters of 2 K superfluid helium. The experiments were done on a quarter cryomodule with two SRF cavities and 314 L of liquid helium. One experiment was done with 2 K superfluid helium and one with 4 K normal helium. Testing was done to verify the sizing of the cryomodule pressure reliefs and to test the efficiency of the beamline vacuum interlocks used to protect adjacent cryomodules in the CEBAF Linacs. In both experiments the pressure was limited to the vessel design rating of 412 kPa, and the contamination downstream of the cryomodule was limited by the vacuum interlocks. The maximum sustained heat flux was 20 kW/m2 with peak heat fluxes of 35.0 kW/m2 and 28.4 kW/m2 for the He II and He I experiments, respectively. The SRF cavities showed no frequency or passband shifts after each test. The experimental setup and results are discussed.

Thermal Performance of the Cebaf Superconducting Linac Cryomodule

When complete, the Continuous Electron Beam Accelerator Facility (CEBAF) will be centered on a 4 GeV recirculating linac. Each of the two linacs contains 160 superconducting radio frequency (SRF) 1497 MHz niobium cavities1 in 20 cryomodules operating between 2 and 2.3 K. Minimization of the total heat load is critical to machine performance, since the refrigeration capacity is fixed. The total heat load of the cryomodule consists of the static load (fixed heat leak) and the dynamic load (proportional to the cavity performance Qo, or quality factor). The heat load of the cryomodules is the single largest load to both the primary and secondary cooling circuits of the refrigerator. The optimization of the thermal performance of the cryomodule considers recent test data of multilayer insulation (MLI) systems developed for the SSC, in addition to the effect of the dynamic heat load on the design of the cryostat. The design of the cryomodule and the measured thermal performance of the installed north and south linac cryomodules are discussed. The performance to date is shown to meet the design heat loads for the accelerator.

FMEA on the Superconducting Torus for the Jefferson Lab 12 GeV Accelerator Upgrade

As part of the Jefferson Lab 12 GeV accelerator upgrade project, Hall B requires two conduction cooled superconducting magnets. One is a magnet system consisting of six superconducting trapezoidal racetrack-type coils assembled in a toroidal configuration and the second is an actively shielded solenoidal magnet system consisting of five coils. Both magnets are to be wound with Superconducting Super Collider-36 NbTi strand Rutherford cable soldered into a copper channel. This paper describes This paper describes a failure modes and effects analysis (FMEA) that was done on these magnets to identify their various failure modes, which were assessed in terms of their Risk Priority Numbers (RPN). Mitigating actions were identified that would reduce the RPNs to acceptable values.

Torus CLAS12-Superconducting Magnet Quench Analysis

The JLAB Torus magnet system consists of six superconducting trapezoidal racetrack-type coils assembled in a toroidal configuration. These coils are wound with SSC-36 Nb-Ti superconductor and have the peak magnetic field of 3.6 T. The first coil manufacturing based on the JLAB design began at FNAL. The large magnet system dimensions (8 m diameter and 14 MJ of stored energy) dictate the need for quench protection. Each coil is placed in an aluminum case mounted inside a cryostat and cooled by 4.6 K supercritical helium gas flowing through a copper tube attached to the coil ID. The large coil dimensions and small cryostat thickness drove the design to challenging technical solutions, suggesting that Lorentz forces due to transport currents and eddy currents during quench and various failure scenarios are analyzed. The paper covers the magnet system quench analysis using the OPERA3d Quench code.

CEBAF Cryounit Loss of Vacuum Experiment

In order to test the sizing of the safety relief devices provided for the Continuous Electron Beam Accelerator Facility’s 42 1/4 Cryomodules, several loss of vacuum experiments have been planned with a modified 1/4 Cryomodule called the Cryobench. The first of these experiments, presented here, is a controlled spoiling of the insulating vacuum to air. The most credible vacuum failure was simulated with in a 3.2 mm diameter hole being opened to the vacuum space yielding an air flow of 0.0033 kg/s and corresponding peak heat fluxes of 749 W/m2 just before the rupture disc opened. The maximum calculated helium mass flows of.025 kg/s indicate that the process relief valve provided on each cryomodule is adequate to handle this type of vacuum failure. The experimental configuration and recorded data are presented and discussed.