Fluidised Tungsten Targets
Our team at High Power Targets have been developing an experimental new target technology to cope with the highest accelerator beam powers.
As accelerator technology continues to develop, there is a greater demand for targets that can accommodate higher beam powers and more focused beam pulses, as they could allow rare events to be investigated. However, these conditions cause a range of target issues including high thermal stress, high radiation damage, and high residual heat when the beam is switched off. Stationary solid targets cannot survive under these conditions, so a rotating target wheel or a flowing metal target is typically used instead. However, rotating solid targets are still vulnerable to breaking and liquid metal targets are prone to cavitation damage (damage due to the collapse of low-pressure air bubbles in the liquid), so even these existing solutions have potential long-term reliability issues.
Therefore, the HPTG has proposed fluidised tungsten as an alternative solution which could mitigate many of these problems, providing a reliable target technology for the most demanding high-power applications. A long-running programme of research and development has been committed to exploring this new technology. Initial testing was focused on the feasibility of fluidised tungsten targets: showing that tungsten powder can be handled using available technology; and understanding how the powder behaves when shot with a proton beam.
The fluidised powder handling rig at Rutherford Appleton Laboratory, showing key components of the cycle: 1. Pressurised powder hopper, 2. Ejection of dense phase powder, 3. Suction lift of lean phase powder, 4. Gravity chute refills powder hopper.
More recent developments have taken these results and extended them: investigating erosion due to tungsten powder; optimising geometry for a high-yield target; and using these ideas to propose a working fluidised tungsten target that could be used in future accelerators, like the proposed Muon Collider. The research into this technology also has relevance to industrial applications – from catalyst transport to food production – and the team has published several peer-reviewed papers on this research.
Initial Testing
Initial Powder Handling Rig
Fluidised powders are routinely handled in industry, for example moving flour around a cake factory or coal dust around a power plant. However, tungsten is many times denser than any material that had previously been fluidised, so a custom test rig was built at Rutherford Appleton Laboratory. The rig has been developed over many years, and has successfully demonstrated the key powder handling processes required for a real target facility; the powder can be ejected in a dense phase suitable for a target, lifted in a lean phase, and then dropped back to the starting point under gravity. Managing erosion due to the powder is challenging, but methods have been considered to measure and reduce this. Since undertaking this work, other organisations have investigated similar technologies for processing dense materials, such as nuclear fuels.
Initial In-Beam Testing
At the HiRadMat facility at CERN, the team was able to investigate the response of powdered tungsten to a proton beam. They designed and built an experiment that allowed a proton beam to pass through an open-topped trough of tungsten powder while keeping the powder safely contained. The effect on the powder was filmed using a high-speed camera and the force on the trough was measured using a laser Doppler vibrometer. The beam caused the powder to be ejected from the trough. However, the powder is ejected much less violently than liquid mercury under the same conditions, implying that a powder target should be easier to contain than a liquid mercury target.
The observed disruption of the tungsten powder could be explained by a coulombic eruption of the tungsten particles. The high-energy beam pulse leaves a pattern of charge distributed in the poorly conducting powder sample, which creates an electric field that consequently results in a force acting on the individual charged particles. A combination of FLUKA and multi-phase CFD modelling has been used to calculate the charge deposited on the tungsten powder, the electric field and the resulting acceleration showing that this is a plausible mechanism for causing the observed eruptions.
Experiment setup for tungsten (W) powder in-beam testing at CERN
Developing Target Concept
After their initial success, our HPTG continued this research by considering the design of a potential target, using fluidised tungsten. To develop this concept, they performed materials research to ensure the target’s longevity, analysing factors such as heat deposition and erosion. The final proposed concept is still under development.
Optimising geometry for particle yield
To investigate the particle yield of tungsten powder, simulations were used to determine the maximum theoretical yield from a fluidised tungsten target, by optimising its geometry.
Since the target would be exposed to high-power beams, in a small space, it would be subject to rapid heating. This poses a major problem as the target would likely be surrounded by superconducting magnets (to focus the beam). These magnets would need to remain cool as, if they got too hot, it could lead to quenching – when the magnet leaves its superconducting state, heats up rapidly and expands, potentially damaging nearby components. To tackle this, the team analysed how target diameters and volume fraction (a measure of powder density) affected the heat deposition. It was found that the higher the target diameter and lower the volume fraction, the greater the allowable power on the target. These results will be taken into consideration when putting together a final design, to ensure that the target can safely handle high powers without overheating.
Understanding the behaviour of tungsten powder was vital for the concept development. First, the team investigated how tungsten powder shape affects factors like maximum bulk density, friction, and cohesion. Two types of tungsten powder were investigated: powder with spherical granules and powder with mixed-size angular granules (as used in the initial rig). Using custom setups, the team tested the differences in the behaviour of the two types of powder.
The tests found that spherical powder could flow through pipes with significantly smaller diameters without clogging (estimated to be 15mm compared to 50mm for the angular powder). In addition, the spherical powder had a higher bulk density – it could pack more efficiently into the same space. Since the tests were able to determine the internal friction and cohesion of the powder, the team will also be able to predict the required pressure needed to stabilise flow in the system.
Subsequently, the team investigated how the tungsten powder erodes different materials. The eroding power of both spherical and angular tungsten granules on a range of materials (Inconel 718, Ti-6Al-4V and stainless steel 316) was tested. These tests were performed using a custom rig that could fire tungsten powder grains at the materials.
Images of material surface after erosion by different tungsten powders, taken using a scanning electron microscope.
Surface inspection and precise weighing was then used to measure the effect of the powder on the samples. The investigation found that larger particles caused more erosion, across all materials and powder shapes. In addition, spherical particles had a significantly lower erosion rate than angular particles, for all materials. Inconel 718 had the lowest volume wear rate, making it a stand-out material for any future targets.
These results will help to inform a future target and rig design. Potential features of this rig could include walls made from stainless steel, lined with Inconel 718 using laser cladding. This would be cheaper than pure alloy, and make the system easier to repair. The optimised target geometry will also be taken into consideration, alongside engineering limits of the system.
Possible Future Applications
Muon Collider
The fluidised tungsten target was developed as part of the conceptual design stage of the proposed Muon Collider. This system would require the highest power on target ever proposed for a short pulse facility (4MW) and among the shortest, most intense pulses ever proposed (2ns). Therefore, a fluidised tungsten target has been considered for this experiment, as it could be better suited to operate in this harsh environment. To support this, FLUKA simulations have been performed to investigate theoretical ideal target geometries, which, alongside further testing, would help inform the development of a potential tungsten target.
Other Industries
In addition to target technologies, the technologies and materials research involved in this project have potential applications in other industries. Fluidised tungsten itself has a range of industrial applications. Since it is extremely dense, it can be used to test nuclear fuel rigs, without exposure to harmful radiation. Similarly, methods developed to convey fluidised tungsten could be adapted for the transport of similarly hard materials, like nickel catalysts. The erosion research carried out has demonstrated the abrasive properties of tungsten powder. This makes it an extremely good candidate for other research into wear, like research into erosion-resistant coatings used in petrochemical industries. The various properties of fluidised tungsten make it an ideal candidate for testing different materials. This research could lead to the development of systems with longer lifetimes, leading to less waste and cost in these industries.
Publications
Fluidised Powder Handling:
In-beam testing:
Investigating material properties:
Written by Jessica Yu.
