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Plans are under way to double the capacity of the Spallation Neutron Source. Although installation of the instruments for the massive target building at ORNL's Spallation Neutron Source has barely reached the half-way point, the promise of discovery is such that managers of the world's most powerful neutron source, while still in its operational infancy, are already planning to double the facility's research capabilities. Three years after the Spallation Neutron Source produced its first neutrons, the instrument hall in the vast target building remains filled with hard-hatted workers and construction equipment, as the suite of state-of-the-art instruments takes shape. The count of operating instruments, some larger than a suburban home, is currently 13 of an ultimate 25. Beamline power, which has already blown past the previous world record, is approaching one megawatt, with an eventual peak of 1.4 megawatts.
With the installation of the instruments in the SNS target building only about half complete, some wonder if it is premature to accelerate plans for the construction of a second target station and power upgrade. The extraordinary complexity of the project, combined with the high demand for neutron analysis beam time, required that planning for a second SNS target begin almost immediately after the first target began producing neutrons in the spring of 2006. The Department of Energy officially endorsed the need for a second SNS target in early 2009 by granting the project "Critical Decision Zero" status. At an estimated cost of $1 billion, the second target station will concentrate on nanoscale and biological sciences with an emphasis on novel materials for energy production, storage and use. Kent Crawford, who led the scientific instrumentation portion of the SNS construction, heads the planning for the second target station. Crawford says the second target station will be very different from the first, designed to serve a burgeoning demand for advanced materials research. "We have three types of moderators on the existing target station. The two that are dedicated to cold neutrons are pretty much completely subscribed. Cold neutron beams are very popular with researchers and seem to be the direction in which much of future science is headed," Crawford says. Cold neutrons, literally chilled to nearly absolute zero with liquid hydrogen, have longer wavelengths that make them ideal for probing slower excitations and material structures at longer distances. Both of those factors come into play for more complex materials, from assemblies of nanoparticles to biological systems. "For the SNS, the ability to examine soft materials and self-assembling nano-materials is going to be a strong asset for the foreseeable future," Crawford says. The first target station's capacity to produce very short neutron pulses makes the SNS ideally suited for studies of so-called "fast neutrons" in the thermal range and "time-of-flight" measurement, which is the length of time required for the neutron to go from the source to the neutron detector. Time of flight is a very important parameter in understanding a material's structure. "A pulsed source is ready-made for time-of-flight measurements. The first target station is optimized for providing high resolution in the timing, but less so for producing high intensities of cold neutrons. The second target station will broaden our capabilities by being optimized for cold neutrons," Crawford says. The thrust toward cold neutron research means that the second target will differ from its predecessor in several important ways. As envisioned, the second target will be exclusively a cold-neutron facility optimized to produce maximum intensity, which Crawford estimates will enable researchers to improve by as much as tenfold their ability to perform certain classes of research. The first target would remain optimized for different experiments. Perhaps even more significant, the process of spalling neutrons in the second target would bypass the accumulator ring. The current target receives neutrons at the rate of 60 pulses per second from the accumulator ring, at a pulse length of 700 nanoseconds. If the beam is not channeled through the ring, however, the pulses are lengthened to about a millisecond in length. As currently planned, one pulse in three will be tapped from the accelerator and sent in long-pulse mode directly to the second target at a rate of 20 pulses per second. The technique would avoid sending protons through the SNS's accumulator ring, which is already operating at world-record levels. The other two short-mode pulses would travel, as they do currently, through the accumulator ring to the first target station. "Not going through the accumulator ring has two advantages," Crawford says. "Running in the long-pulse mode generates more power per pulse, which enables us to optimize our instruments to that higher power. The second advantage involves risk. The ring is probably the SNS's most complicated system and is being pushed to its limits by the performance we are asking. Not running protons through the ring results in less technical risk." Bypassing the accumulator ring offers another significant bonus. When the ring is off-line for scheduled maintenance, the second target will still be capable of operation, thus expanding its accessibility to researchers and enhancing the facility's efficiency.
Also being planned in parallel with the second target is another upgrade, a boost in linear accelerator energy from 1 GeV to 1.3 GeV. In anticipation of the need for future upgrades, SNS designers built the linac with the space needed to expand beamline power up to 3 megawatts. The power upgrade, which will further extend opportunities for instrument optimization, will help ensure that the SNS remains the world's foremost neutron scattering facility for decades to come. One of the key remaining decisions is the composition of the second target. Designers could eventually settle on a second version of the SNS's unique mercury target, the first of its kind. The SNS designers chose mercury for the original target, partly because the element is rich in neutrons and its liquid state allows it to be circulated and cooled. Planners are also considering a target made of tungsten. A little more than a meter in diameter, the tungsten target would rotate at about 30 rpm, slightly slower than an LP record. The rotating solid target would thus distribute heat and radiation damage from the beam. One potential advantage of the tungsten target would be a projected 10-year service life. The current mercury target must be changed more frequently, although the three-year performance of SNS's original mercury target was considerably longer than many expected. Although the idea of a tungsten target is not new, Crawford says the use of a rotating tungsten target would, like the use of a mercury target, be the first of its kind. Major design decisions, including the target selection, could come in the fall of 2009. A precedent exists for twin-target neutron sources. The United Kingdom's ISIS facility has two short-pulse targets, although the power is much lower and the moderators are different. The SNS would be the first long-pulse target, offering researchers a unique analytical tool. If constructed, the proposed European Spallation Source would be a long-pulse system comparable to the SNS's second target. Researchers already are gathering to plan instrumentation geared to the longpulse, cold neutrons the second target will produce. Crawford emphasizes that the experience of designing the SNS taught the value of a long lead time for planning. The $1.4 billion construction project that was finished in 2006 began with planning that started in 1995. "We started developing the second target idea immediately after construction was complete. We currently are looking at a potential completion date of 2019," Crawford says. With that kind of long-range vision, the ORNL team seems intent on maintaining their position among the world's leaders in materials research.
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