The Spallation Neutron Source (SNS) in Oak Ridge, Tennessee, has completed its construction phase and has begun the transition to operations. The construction project was declared complete near the end of June--within its $1.4B budget, on time, with greater capability than initially promised, and with the best safety record of any Department of Energy (DOE) project (greater than 4M man-hours of construction without a lost-workday accident). The project’s challenge over the next couple of years will be to rapidly ramp up the run-hours, reliability, and availability to outside users. By October 2008, SNS will be operating at 1 MW with ten operating instruments and a gaggle of outside users. There is a lot of work ahead to maintain the ambitious schedule needed to meet this goal.
The SNS incorporates major components designed and built by different national labs into a single facility--an ion source/front end from Berkeley, a “warm” linac from Los Alamos, a superconducting linac from Jefferson Lab, an accumulator ring from Brookhaven, a mercury target from Oak Ridge, and instruments designed by multiple teams and coordinated by Argonne. When fully operational, the linac will produce an H- beam at 1 GeV (and 1.4 mA) in 60 pulses (1 msec each) per second. The electrons are stripped off the H- as the beam enters the accumulator ring. In the ring, the 1,060 minipulses that together make up the 1-msec macropulse are accumulated, resulting in a compressed 0.7-microsec-long pulse. This compressed proton pulse, with the force of about a 50-caliber machine-gun bullet, is extracted from the ring, defocused to about 150 cm2, and directed to the mercury target. The “spallation,” or shattering, of the mercury nucleus by this pulse produces at least 1016 neutrons per pulse, along with an array of by-products similar to that produced by a fission reactor (without, of course, uranium or transuranics). The neutrons are repeatedly reflected through four moderators surrounding the target (three liquid hydrogen and one water) producing thermal and subthermal neutrons for use by the various experiments. The expected neutron flux is up to 107 or 108 neutrons/cm2-sec at the instruments, depending on the instrument distance and choppers. Three instruments (of an eventual 24) are currently approved to accept neutrons from the target. These are the backscattering spectrometer (beamline 2), the magnetism reflectometer (beamline 4A), and the liquids reflectometer (beamline 4B). All three of these instruments are currently working on timing, alignment, analysis, and other start-up housekeeping details, with use limited at this time to in-house staff. These instruments have performed initial experiments that show they work as designed, but optimum performance and resolution will only be achieved by fine-tuning during this commissioning period.
From the public website for the SNS project, there are additional links which provide further details about the facility and instruments. Approximately half of the beam time is currently dedicated to beam tuning and loss minimization, with the other half of the time used for benefit of the experiments by running a stable beam. Since first beam-on-target on April 28, SNS has run two operating cycles and is completing its second maintenance period. The third operating cycle of the full machine will run through the months of October and November. The highest operating power to date has been slightly less than 10 kW. In the next cycle we expect to approach 60 kW for a brief demonstration, with extended periods of 30 kW operation.