Spallation Neutron Source Research Articles

H− Ion Source RF Plasma Testing with 13 MHz and 27 MHz at the Spallation Neutron Source (SNS)1

The baseline RF-driven H− ion source configuration at the Spallation Neutron Source (SNS) facility uses a continuous wave (CW) 600 W 13 MHz RF system to ignite and maintain a low-density plasma inside the ion source vacuum chamber. After the continuous low-density 13 MHz plasma has been established, a pulsed (typical 1 ms pulse width and 60 Hz pulse repetition rate) 80 kW 2 MHz RF system is used to increase the plasma density to produce the pulsed H− ion beam. Incremental upgrades and improvements to the SNS ion source systems have resulted in the ability to reliably operate an H− ion source for SNS neutron production run cycles that can last up to four months. Conditions inside the SNS H− ion source evolve throughout a four-month run cycle due to changes in impurity levels, sputtering, and erosion. As the internal ion source conditions change during the run cycle, there can also be changes in plasma stability and the 13 MHz RF power level required to ignite the plasma. This paper presents the preliminary results of testing performed on the SNS Ion Source Test Stand (ISTS) system where we looked at plasma ignition and plasma stability using 27 MHz RF in place of the baseline 13 MHz RF system.

Design improvements to the SNS ion source and diagnostics

The U.S. Spallation Neutron Source (SNS) is a state-of-the-art neutron scattering facility delivering the world’s most intense pulsed-neutron beams to a wide array of instruments which are used to conduct investigations in many fields of science and engineering. The accelerator system is fed by an RF-driven, multicusp, H− ion source which nominally provides pulsed beam currents of 50-60 mA (1ms, 60Hz). This report provides a discussion of ongoing design improvements to the SNS ion source and Low Energy Beam Transport (LEBT) as well as diagnostic upgrades undertaken since the previous ICIS conference. These improvements include (i) simple mechanical modifications to the source outlet aperture which resulted in dramatically increased extracted beam current and comparable or lower emittance at similar beam currents, (ii) design improvements to the LEBT chopper target which will enable full power beam-dumping during physics studies, (iii) refinement of the SNS Allison emittance scanner that has enabled the first reliable LEBT beam measurements at full beam power (65kV, 50-100mA, 1ms, 60Hz) on the SNS ion source test stand and (iv) the implementation of a thermal imaging camera for the monitoring the LEBT electrode temperatures. (v) The design of an advanced Cs system, capable of more efficient Cs utilization with significantly lower Cs losses from the source is also presented. Mechanical details, computational simulations and experimental results are discussed within the context of these improvements.

Thermal deflection in neutron scattering sample environments at Oak Ridge National Laboratory.

The neutron sources at Oak Ridge National Laboratory use a wide suite of sample environment equipment to deliver extreme conditions for a number of experiments. Much of this instrumentation focuses on extremes of temperature, such as cryostats, closed-cycle refrigerators in both low and high temperature configurations, and radiant heating furnaces. When the temperature is controlled across a large range, thermal deflection effects can notably move the sample and affect its alignment in the beam. Here, we combine these sample environments with neutron imaging and machine vision to determine the motion of a representative sample with respect to the neutron beam. We find vertical sample displacement on the order of 1-2mm and horizontal displacement that varies from near-negligible to 1.2mm. While these deflections are not relevant for some of the beamlines at the first target station at the spallation neutron source and the high flux isotope reactor, they will become critical for upcoming instrumentation at the second target station, as well as any instruments targeting sub-mm samples, as neutron sources and optics evolve to smaller and more focused beams. We discuss mitigation protocols and potential modifications to the environment to minimize the effect of misalignment due to thermal deflection.

Spallation neutron source purifier replacement project for the central helium liquefier

The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) operates the Central Helium Liquefier (CHL) to provide 2K helium refrigeration to the superconducting radio frequency (SCRF) cavities in the linear accelerator (LINAC). To maintain the purity of the helium, the CHL operates a LN2 cooled, dual-bed purifier. This purifier was purchased as a refurbished unit during the construction of SNS about 20 years ago. The current operational issues with the existing purifier will be discussed. The procurement, fabrication status and commissioning plans for this new purifier will also be discussed.

Spallation Neutron Source reliability improvement projects for the Cryogenic Moderator System

The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) operates the Cryogenic Moderator System (CMS) which provides supercritical hydrogen cooling at 20K to three neutron moderators. A helium refrigerator provides 8 kW of cooling capacity to three parallel hydrogen circuits. Several projects have been completed to improve the operational reliability of the CMS. The status of helium compressor oil removal system, helium oil processor, helium purity monitoring, hydrogen circulator bearing investigation and operational procedure improvement will be presented. Future plans for sustained reliable operation at design capacity will also be presented.

Spallation Neutron Source Hydrogen Relief Analysis for the Cryogenic Moderator System

The Spallation Neutron Source at Oak Ridge National Laboratory operates the Cryogenic Moderator System (CMS) which provides hydrogen cooling at 20K to three neutron moderators. Each hydrogen loop is protected by multiple burst discs and reclosing relief valves. As a result of the Proton Power Upgrade project, the CMS moderator loops will hold more hydrogen and thus require piping modifications. Operational experience has demonstrated that transient pressure increases in these hydrogen loops occasionally result in the rupture of the burst discs. The causes for pressure transients in the system that might lead to hydrogen venting are varied and complex, for example loss of vacuum or cooling. Dynamic simulation and analysis of the parameters considered as the worst-case relief venting scenario were performed and will be presented.

Mechanical and process design of the interface to a 20,000 liter liquid helium dewar

A new 20,000-liter (20 kL) liquid helium dewar is being integrated in to the existing cryogenic system at the Facility for Rare Isotope Beams at Michigan State University (MSU-FRIB). This dewar will mainly be used for the helium inventory management at FRIB (preserving helium inventory during unplanned / planned maintenance of the cryogenic system) and also in sharing the capacity to support the various cryogenic refrigeration systems to increase the operational efficiency and availability. It is also designed to interface to future cryogenic system upgrades (i.e., new 4.5 K refrigerators). This dewar has a single large penetration, or ‘neck’, that all the process lines and instrumentation pass through. On top of the ‘neck’ is a vacuum insulated interface box, or ‘neck can’, which houses all the connections between the dewar and the connecting distribution system. Though not generally common, this design approach has been successfully implemented on 5 to 10 kL liquid helium dewars at the Spallation Neutron Source (SNS), Jefferson Lab (JLab), and also at FRIB. It has been found that this approach greatly simplifies the mechanical design, while allowing for multiple process and instrumentation interfaces and still permitting a low heat in-leak. This paper presents an overview of the mechanical and process design of this neck can.

Design of the cryogenic moderator system for the second target station

The Second Target Station (STS) at Oak Ridge National Laboratory will be a 700 kW pulsed spallation neutron source designed to provide the world’s highest brightness cold neutron beams. In order to produce the required neutron performance, two compact liquid hydrogen moderators are located adjacent to the tungsten spallation target and must be supplied with less than 20 K hydrogen and a para hydrogen fraction of 99.8% or greater. The Cryogenic Moderator System (CMS) will consist of a single hydrogen loop feeding the two moderators in series cooled by a helium refrigerator with a cooling capacity of 2.5 kW at 17 K. The hydrogen loop consists of a hydrogen circulator, hydrogen helium heat exchanger, ortho-para converter, accumulator, transfer lines and heater. The design of the hydrogen loop is based on the CMS design of the First Target Station at the Spallation Neutron Source and some of the component designs may be reused. General hydrogen temperature control is provided by controlling the flowrate of helium to the heat exchanger. The hydrogen loop will have a constant flowrate of 0.5 L/s and remove a nuclear heat load of about 850 W from the two moderators, which is deposited both directly in the hydrogen and the adjacent hydrogen containing structures. Because the nuclear heat load is accelerator driven, the hydrogen system must remain stable when the heat load is removed instantaneously during beam trips. System stability is maintained passively with the accumulator and actively with the heater. Ionizing radiation which interacts with the liquid hydrogen drives backconversion of the hydrogen from parahydrogen to orthohydrogen. The STS moderator performance is very sensitive to small fractions of orthohydrogen requiring an ortho-para converter to maintain the hydrogen supplied to the moderators at near equilibrium parahydrogen concentration. STS CMS is in the early stage of preliminary design and current focus is evaluating component sizing and system stability during beam transients.

Preparing the cryogenic moderator system for higher beam power

The Spallation Neutron Source (SNS) incorporates a 20 K helium refrigeration system to cool three circuits supplying cold hydrogen to neutron moderators. The helium refrigeration system consists of a warm compressor, oil removal, purifier, 20 K cold box, and three helium-hydrogen heat exchangers. The hydrogen loops consist of a circulator, moderator, accumulator with expandable bellows, and the heat exchangers. Historically, the cryogenic system has had periods of instability and has operated with a reduced capacity. While performing the Proton Power Upgrade, the proton beam power on the first target station will increase from 1.4 MW to 2 MW. This will result in a 43% increase in dynamic load to the cryogenic system. The first directive in preparing the system for higher beam power was to achieve consistent operational stability at 1.4 MW beam power. After this was achieved, the next task was to increase the system capacity. This paper will describe the initiatives taken to produce consistent high-capacity performance.

Recent advancements with the H− injector performance for the Spallation Neutron Source operation and upgrade

The Spallation Neutron Source (SNS) located at the Oak Ridge National Laboratory is an accelerator-based, pulsed neutron facility utilized for a broad range of scientific research and industrial applications. In the past 5 years, the facility has reliably been operated at its baseline design beam power of 1.4 MW, and it is currently undertaking an upgrade to double its power to 2.8 MW. A 65-keV H− injector, which comprises an rf-driven H− ion source and an electrostatic low energy beam transport, delivers the required high-current, time-structured H− beam to the accelerator. The H− injector can reliably provide 50-60 mA beam current at 6% duty-factor (1 ms, 60 Hz) for a 3-4 month service cycle. To ensure sufficient operational margins for the SNS routine operations and ongoing upgrade requirements, the injector system has been continuously improved on a R&D test stand. This paper presents the operational status and recent advancements with the injector system performance including the enhanced ion source beam output capability up to ∼110 mA, the improved ion source plasma ignition reliability using a 27.12-MHz RF system, and the upgraded LEBT beam chopper system etc.

Dynamic systems modeling of the spallation neutron source cryogenic moderator system to optimize transient control and prepare for power upgrades

Through support of the US Department of Energy’s Office of Basic Energy Sciences, Oak Ridge National Laboratory has begun applying machine learning methods to improve accelerator and target performance of the Spallation Neutron Source (SNS). These methods are being applied to the control optimization and power upgrade of the SNS Cryogenic Moderator System (CMS). A numerical model of the CMS has been developed to study these optimizations and system modifications using EcosimPro. This paper compares steady-state and transient numerical results with experimental data. Control optimization studies focused on dampening mass flow, temperature, and pressure fluctuations during sudden losses of accelerator beam power. This analysis was conducted by adjusting five proportional-integral-derivative controllers connected to four flow control valves and one heater. Future efforts include power uprate studies focused on increasing the CMS cooling capacity. The current accelerator power is 1.4 MW; the first target station is being upgraded to 2.0 MW as part of the Proton Power Upgrade effort. The CMS cooling capacity is sufficient for 2.0 MW operation.

RF-driven H− sources of long life-time and new LEBT developed at CSNS

An RF-driven ion source has been put into commissioning on the China Spallation Neutron Source (CSNS) accelerator since September 2021. In the last two run cycles, the ion source has operated for 310 and 323 days respectively, with an availability of almost 100%. To fully meet the requirements of the CSNS project Phase-II (CSNS-II), the beam intensity from the linac should be enhanced to above 40mA, and the transverse emittance should be minimized to suppress beam loss during acceleration and transportation. A new test bench consisting of an ion source and a LEBT has been constructed to carry out these optimization and research. The featured functions of the LEBT are associated with proton elimination and electrostatic beam chopping. This report covers the operation status and development of the RF-driven H− source and the new LEBT.

Performance and painting scheme considerations for the CSNS injection

The accelerator complex of China Spallation Neutron Source (CSNS) consists of an 80 MeV H- Linac and a 1.6 GeV proton rapid cycling synchrotron (RCS). The beam injection is one of the most important issues for the CSNS accelerator complex. In this paper, the injection methods have been comprehensively studied, including phase space painting and H- stripping. By using the design scheme of the anti-correlated painting, the beam power has successfully reached 50 kW. However, some difficulties have been found in the higher power beam commissioning. In order to solve these key problems, flexibility in the CSNS design has been exploited to implement the correlated painting by using the rising current curve of the pulse power supply. The effectiveness of the new method has been verified in the simulation and beam commissioning. By using the new method, the beam power on the target has successfully risen to the design value. Secondly, according to the CSNS beam commissioning experience, based on the present design of the injection system, a new idea is proposed to perform both correlated and anti-correlated painting in the CSNS. By adopting an additional vertical shift bump generated by four additional alternating current (AC) magnets in the injection region, the local orbit can be manipulated to meet the requirement of correlated painting. The new method can not only perform the correlated painting, but also optimize the anti-correlated painting. The simulation study shows that the new method works well, and both correlated and anti-correlated painting methods are well performed.

Cross sections of neutral-current neutrino scattering on 98,100Mo isotopes

Coherent elastic neutrino–nucleus scattering (CEνNS) is a neutral-current low-energy electro-weak reaction-channel detected recently by the COHERENT experiment at the Oak Ridge National Laboratory (ORNL), USA, in the Spallation Neutron Source facility. The extremely weak signal on the CsI detector of the first experiment and on the liquid Ar of the repeated COHERENT experiment is the energy-recoil due to the neutrino–nucleus interaction, where the nucleus is elastically scattered as a whole while simultaneously the neutrino goes out. Today, several promising nuclear detectors are on the way to be employed in designed and ongoing experiments. In our present work, we provide predictions for incoherent scattering cross sections of low-energy neutrinos on 98,100Mo isotopes obtained with the deformed shell model employed previously for similar predictions in other electroweak processes. We mention that, Mo detector medium has been used previously in the MOON and NEMO double beta decay experiments.

Correlation between microstructure and hardening of 9Cr ferritic/martensitic steels irradiated in spallation neutron source

Significant hardening can be caused in ferritic/martensitic (FM) steels because of intensive displacement damage and transmutation helium/hydrogen when irradiated with high-energy protons and spallation neutrons. It is important but difficult to correlate microstructure and corresponding hardening of the FM steels irradiated in the Swiss spallation neutron source (SINQ). Based on the results of the China low activation martensitic (CLAM) steel and comparison with other 9Cr FM steels irradiated in SINQ up to 20.8 dpa/328 °C, total hardening of 9Cr FM steels was considered to be made up of helium bubbles, defect clusters/loops and α′ phase. Barrier strength of helium bubbles and clusters/loops in the CLAM specimens were obtained and investigated. Results show that the hardening of 9Cr FM specimens irradiated in SINQ increased with dose up to 20.8 dpa. After annealing at 600 °C, the hardening of FM specimens approximately showed a linear relationship with dose. Annealing made the clusters/loops in FM steels almost completely disappear under all irradiation conditions. In contrast, annealing changed the bubble size slightly and changed the bubble density to some degree. Barrier strength of helium bubbles in the CLAM specimens approximately increased linearly with size below 1.79 nm. Barrier strength of clusters/loops in the CLAM specimens also increased from 0.23 to 0.48 when their size increased from 3.5 nm to 19.1 nm. The obtained barrier strength of helium bubbles and loops in the CLAM steel is useful to explain the composition of irradiation hardening of other FM specimens irradiated in SINQ.

High pressure neutron diffraction study of magnetic ordering in erbium

We have studied magnetic ordering in polycrystalline erbium at high pressures up to 32 GPa and low temperatures down to 10 K using neutron diffraction techniques at the Spallation Neutron Source at Oak Ridge National Laboratory, USA. For the hexagonal close-packed (hcp) phase, strong nuclear and magnetic satellite intensities permit a simultaneous refinement of the nuclear and magnetic structures. At 1 GPa of applied pressure, a modulation vector q=γc∗ with γ≈2/7 for the c-axis modulated and cycloidal phases is consistent with prior single-crystal studies at low pressures. At 6.7 GPa in the hcp phase, we find γ≈0.31, indicating a reduction in the period of the magnetic structure with respect to the crystal lattice. The magnetic ordering temperature at 6.7GPa is slightly above 60 K. At 32 GPa in the double hexagonal close-packed phase, the magnetic scattering constrains the magnetic ordering temperature to 25±5 K. Our neutron diffraction study demonstrates that the magnetic ordering persists in the high-pressure double hexagonal close-packed phase of erbiumto the highest pressure of 32 GPa.

Beamline design for multipurpose muon beams at CSNS EMuS

A new muon beam facility, called the Experimental Muon Source (EMuS), was proposed for construction at the China Spallation Neutron Source (CSNS). The design of the complex muon beamlines for the EMuS baseline scheme, which is based on superconducting solenoids, superferric dipoles and room-temperature magnets, is presented herein. Various muon beams, including surface muons, decay muons and low energy muons, have been developed for multipurpose applications. The optics design and simulation results of the trunk beamline and branch beamlines are presented. With a proton beam power of 25 kW at a standalone target station that consists of a conical graphite target and high-field superconducting solenoids, the muon beam intensity in the trunk beamline varies from 107/s for surface muons to 1010/s for high-momentum decay muons. And at the endstations, these values vary from 105/s for surface muons to 108/s for decay muons.

Tune optimization for alleviating space charge effects and suppressing beam instability at the RCS of CSNS

The design betatron tune of the Rapid Cycling Synchrotron (RCS) of China Spallation Neutron Source (CSNS) is (4.86, 4.80), which allows for incoherent tune shifts to avoid serious systematic betatron resonances. When the operational bare tune was set at the design value, serious beam instability in the horizontal plane and beam loss induced by half-integer resonance in the vertical plane under space charge detuning were observed. Simulations and experiments have shown that space charge-induced beam loss reduces as the tunes move up and away from half-integer resonance lines. However, experimental observations have shown that instability growth rates increase rapidly as the tune approaches integer from below. The tune requirements for reducing the beam loss caused by space charge effects and suppressing beam instability are different at the RCS of CSNS. The tunes over the whole acceleration process are optimized based on space charge effects and beam instability. The optimized tune pattern was able to well control the beam loss induced by space charge and beam instability. The beam power of CSNS achieved the design value of 100 kW with small beam loss.

Feasibility study of CSNS as an ATLAS ITk sensor QA irradiation site

The current ATLAS Inner Detector will undergo a complete upgrade in order to meet the requirements of the High Luminosity Large Hadron Collider (HL-LHC). The new Inner Tracker (ITk) will be made completely of silicon sensors fabricated by Hamamatsu Photonics K.K. (HPK). Quality Assurance (QA) is focused on providing confidence that quality requirements will be fulfilled in production such as irradiation tolerance and testing specification for the mini sensors. The Associated Proton beam Experiment Platform (APEP) beam line at the China Spallation Neutron Source (CSNS) was commissioned in 2022. This study focused on verifying the feasibility of CSNS as an ITk sensor QA irradiation site. A low-temperature Peltier cold box has been developed to keep the irradiation sensor samples at −15 °C. Several fluence points have been studied from 5.1×1014 neq/cm2 to 1.6×1015 neq/cm2, measured with the activation of the aluminium foils. The post-irradiation measurements (IV, CV, and CCE) are done under cold temperatures (−8 °C for IV and CV, −20 °C for CCE), after annealing for 80 min at 60 °C. Test results show CSNS could be a suitable proton irradiation site for the ATLAS ITk sensor project, after formal site qualification.

The multi-slit very small angle neutron scattering instrument at the China Spallation Neutron Source.

A multi-slit very small angle neutron scattering (MS-VSANS) instrument has been finally accepted at the China Spallation Neutron Source (CSNS). It is the first spallation neutron source based VSANS instrument. MS-VSANS has a good signal-to-noise ratio and can cover a wide scattering vector magnitude range from 0.00028 to 1.4 Å-1. In its primary flight path, a combined curved multichannel beam bender and sections of rotary exchange drums are installed to minimize the background downstream of the instrument. An exchangeable multi-slit beam focusing system is integrated into the primary flight path, enabling access to a minimum scattering vector magnitude of 0.00028 Å-1. MS-VSANS has three modes, namely conventional SANS, polarizing SANS and VSANS modes. In the SANS mode, three motorized high-efficiency 3He tube detectors inside the detector tank cover scattering angles from 0.12 to 35° simultaneously. In the polarizing SANS mode, a double-V cavity provides highly polarized neutrons and a high-efficiency 3He polarization analyser allows full polarization analysis. In the VSANS mode, an innovative high-resolution gas electron multiplier detector covers scattering angles from 0.016 to 0.447°. The absolute scattering intensities of a selection of standard samples are obtained using the direct-beam technique; the effectiveness of this method is verified by testing the standard samples and comparing the results with those from a benchmark instrument. The MS-VSANS instrument is designed to be flexible and versatile and all the design goals have been achieved.