Proposed Power Upgrades for SNS

Many technical margins were built into the SNS design and hardware to facilitate a power upgrade from the baseline 1.4-MW level into the 2- to 4- MW range and ultimately to perhaps 5 MW. The upgrade plan builds upon recent progress in SNS development programs to improve superconducting cavity performance in the linear accelerator (linac), to mitigate intensity thresholds in the accumulator ring, and to reduce cavitation damage in the mercury target. Because key elements of the upgrade rely on replicating existing designs, the upgrade is positioned for aggressive deployment, with construction starting in FY 2008 and finishing in FY 2012. The estimated cost of the upgrade is $150 to $173 million U.S. dollars. This upgrade will lead to an immediate improvement in the performance of all installed scattering instruments at the SNS, and in the future will provide beam power that can also be extracted to a potential second target station, widening the suite of SNS instruments and the scope of science that can be studied.

The SNS power upgrade will roughly double the scientific capability of SNS at a cost that is approximately 10% of the initial facility cost. The beam energy will be increased by 30% from 1.0 GeV to 1.3 GeV; and the time-averaged accelerator output beam intensity will be increased by 65% from 1.4 mA to 2.3 mA for 3.0 MW of beam power. There is very little technical, cost, or schedule risk in increasing the SNS accelerator beam energy by 30%. The technical risk lies in the intensity side of the equation; the three main areas of technical risk requiring research and development are in the ion source, carbon stripping foil, and mercury target.

Existing margins in the initial SNS project for higher power operation

• The baseline superconducting linac (SCL) accelerates the beam from 186 MeV to 1.0 GeV, is 157-m long, and contains 23 cryomodules. The linac tunnel was built with another 71 m of length to accommodate nine additional cryomodules.
   
• The cryogenics plant has the additional cooling capacity needed for nine cryomodules. The transfer lines that feed these nine cryomodules are in place.
   
• Superconducting cavity input radio frequency (rf) power couplers have been tested to 750 kW, compared to the 550 kW needed in the initial baseline, and are adequate for many of the cavities in the power upgrade.
   
• Piezoelectric tuners have been installed in all 81 baseline superconducting cavities to reduce the required rf power control margins, thereby allowing the existing rf plant to provide more beam power without major configuration changes.
   
• The superconducting cavity gradients, on average, have been tested to approximately 20% over the baseline specification.
   
• The high-energy beam transport (HEBT)/ring–ring-to-target beam transport (RTBT) tunnel geometry, and in particular the H- bending dipole magnets, can support 1.3-GeV operation.
   
• The ring accelerator physics design is for 2.0 MW at 1.0 GeV.
   
• Most magnets, with the exception of two injection chicane magnets, support 1.3-GeV operation.
   
• Most magnet power supplies support 1.3-GeV operation.
   
• The HEBT design includes energy corrector and spreader rf cavities to minimize downstream ring beam loss.
   
• Beam line hardware devices for active transverse instability dampers are installed in the ring.
   
• Many target systems were constructed for 2 MW.
   
• The target biological shield was constructed for 4 MW.
   
• Electrical, water, and cooling infrastructure has some reserve capacity.

SCIENTIFIC JUSTIFICATION

Because the vast majority of neutron-scattering experiments are intensity limited, even modest improvements in source intensity can lead to scientific measurements that were previously out of reach. An increase in power by a factor of 2 or 3 will enable practical study of smaller samples and real-time studies at shorter time scales. It will also allow a modest increase in resolution on most instruments and will allow more experiments to take advantage of the highest resolution available on each instrument. Since many such experiments are at the scientific frontier, such a power increase will immediately make a significant increase in the scientific productivity of SNS. Another benefit will be an increased volume of research supported because of faster throughput of already feasible experiments. Further increases in scientific capabilities can be expected in the longer term, as instruments optimized to exploit these higher powers come on line. In addition, by providing beam power to support a second target station, a Long-Wavelength Target Station (LWTS), the power upgrade facilitates a significant expansion of capacity and substantial performance gains for long wavelength applications of neutron-scattering techniques. Although the LWTS can be implemented without the power upgrade, its power level, and hence its scientific performance, will be significantly curtailed if its operation were to come at the expense of the High Power Target Station (HPTS).

The SNS instrument designs being developed have been quantitatively benchmarked at the 1-MW performance level. In many instances with the new performance level offered by a megawatt-class spallation source, the instrumentation approaches count rates that will support single-pulse experiments. At 1 MW however, single-pulse measurements will still be limited to a subset of possible materials—that is, those with simple structures or favorable cross sections. By increasing the power level by a factor of 2, the range of materials that can be measured increases dramatically. Some examples of new areas of science that could be addressed include the following:

• Engineering Materials—The increase in power will make feasible dynamic experiments at the engineering diffractometer that can only be dreamt of today. Single-pulse diffraction will become possible for engineering materials such as steel, aluminum, and nickel-based superalloys. Because neutrons of different wavelengths are scattered by the sample at different times, continuous monitoring of the dynamics of a process would be possible. Examples of studies where information on this time scale is important include change of stress state during cyclic fatigue, development of recrystallization texture, decomposition kinetics in bulk metallic glass, and phase stability of metallic clusters. Another area that will benefit from the increase of target power is spatial mapping experiments. Spatial resolutions of 0.1 mm are required for studies of surface engineered materials and coatings; such measurements are difficult to achieve at a 1-MW flux.
   
• Powder Diffraction—Higher power will greatly increase the performance of the powder diffractometer in stroboscopic (crystal structure as a function of applied alternating stimulus, e.g., ferroelectrics under high-frequency ac voltages) and nuclear density distribution measurements (e.g., determination of hydrogen conduction pathways in fuel cell materials). Similarly, in a high-flux scenario, maximum entropy methods could be used to deconvolute the instrument and pulse shape functions from the measured diffraction data, possibly doubling the resolution of the powder diffractometer. This higher resolution is important in the study of subtle structural phase transitions and in separating Bragg peaks in complex, low-symmetry structures, which is a requirement for ab initio powder methods when single crystals are not obtainable. Flux increases will also allow phase diagram determinations to be made extensively and rapidly.
   
• Reflectometry—Higher flux will also allow time-dependent reflectometry studies on thin films at high temporal resolution. Examples include diffusion experiments; parametric studies in which temperature, magnetic/electric fields, chemical environment, and/or pressure are changed; chemical kinetics; solid state reactions; phase transitions; and chemical reactions in general. In many cases, useful data sets could be produced on a pulse-by-pulse basis. We are close enough in flux at 1.4 MW to almost bridge the gap between reflectivity and Bragg diffraction along the specular diffraction rod. A factor of 2 increase in intensity will enable continuous measurement of layered structures from 3 to 10,000 Ǻ in one scan on one instrument. This becomes important for in situ studies of the layer-by-layer growth of multilayers or the study of dynamic processes at surfaces and interfaces. Furthermore, an increase in intensity will break the threshold required for performing inelastic scattering experiments on thin films and surfaces. Such a capability, which does not exist at current neutron sources because of flux limitations, will open up a completely new domain of measurements with applications in membrane function, catalysis, and relaxation processes in magnetic films, for example.
   
• Dynamics—Inelastic experiments are generally flux limited. Two examples where current flux levels make experiments marginal at best are studies of protein dynamics and of thin layers of adsorbed polymers. To study the dynamics (side chain motions as well as global diffusion) of proteins in solutions that mimic biological environments, sample concentrations must be kept low because of protein clustering at higher concentrations, which makes the scattering particularly weak. A factor of 3 will extend the range of proteins that can be studied in this way. Another area of study that will benefit is the dynamics of thin films of adsorbed polymers (1 to several radii of gyration thick). Current study is limited to either intense sample preparation efforts (many iterations of coating individual silicon wafers) or study of relatively thick films. An additional factor of 3 in performance for the inelastic instruments will increase the viability of taking such measurements because less sample material will be required.
   
• Biomaterials—Neutrons have a key role to play in the post-genomic era where the structure-function relationship of biological molecules has shifted more and more to the center stage. Even with SNS (which represents a significant improvement over the current state of the art) or the High Flux Isotope Reactor, many neutron-scattering experiments with biological systems are not practical because of flux limitation. Most potential biological samples are available only in small amounts, behave well only in low concentrations, and have low contrast. Higher flux will enable a number of biological neutron-scattering experiments that are not possible at current flux levels. Moreover, the applicability of neutrons to structural problems where the role of hydrogen and loosely bound water is important is expected to expand considerably.

For more information, see the (PDF format) report
Conceptual Design Report for the Spallation Neutron Source Power Upgrade MIE Project