The Potential of Fluidised Powder Target Technology in High Power Accelerator Facilities

This paper describes the potential of fluidised powdered material for use as a particle production target in high power particle accelerator based facilities. In such facilities a multi-MW proton beam is required to interact with a dense target material in order to produce subatomic particles, e.g. neutrons for a neutron source or pions for a so-called conventional neutrino beam, a neutrino factory or a muon collider. Experience indicates that thermal transport, shock wave and radiation damage will limit the efficiency and reliability of facilities utilising solid targets at around 1 MW beam power. Consequently liquid mercury has been adopted as the target technology for the latest neutron facilities SNS and J-SNS at ORNL and Tokai respectively, and is the baseline for a neutrino factory and muon collider. However mercury introduces new technical challenges. This paper discusses how a fluidised powder target may combine many of the advantages of a liquid metal with those of a solid, and describes an experimental programme at RAL that is currently underway to implement this technology. MOTIVATION FOR A FLOWING POWDER TARGET A new generation of accelerator facilities requires target systems to dissipate powers in the MW range, with pulsed energy depositions of 100s of J/g and beyond. It is widely expected that difficulties of radiation damage, shock wave damage, thermal transport and the need to avoid unacceptable compromises to physics performance will preclude the use of solid targets in such facilities. Consequently liquid metal, namely mercury, has been adopted as the target technology for the latest neutron facilities SNS and J-SNS [1] at ORNL and Tokai respectively. An open mercury jet is the baseline for a neutrino factory and muon collider [2] where pulsed beam induced filamentation of the mercury jet [3] has been demonstrated to be controlled by a solenoidal magnetic field [4]. However implementation of a high velocity mercury jet in an accelerator facility presents considerable technical challenges and alternatives seem worth consideration. In a neutrino Superbeam, i.e. a conventional neutrino facility beyond 1 MW beam power, a free liquid metal jet is not generally viable since it would need to be directed within the bore of a magnetic horn. Unlike for a solenoid, there is no magnetic field within the bore of a horn to damp out the high velocity jets generated by the pulsed proton beam. It is in this context that a research programme has been initiated at the Rutherford Appleton Laboratory in the UK to explore the potential of fluidised powder targets [5]. The motivation is to investigate whether such a technology can combine some of the advantages of a solid target with those of a liquid while avoiding some of the disadvantages of either. The attractions of a static granular target have been outlined before. A helium cooled granular target was investigated for a neutrino factory [6], however the static design had inherent cooling limits. A free-falling powder target has been subject to preliminary investigation [7] however was not considered worth further study. The attractions foreseen for a flowing powder target include: • Intrinsic resilience of individual grains to beam induced shock wave damage compared with macroscopic solid targets. In essence, a granular material is already broken and can only be subdivided into ever smaller grains [6]. • Pulsed beam induced stress waves are contained within each separate grain of material, and cannot generate splashing or jets in the bulk powder as can occur with liquid metals. • The heat conduction path is very short in a powdered material compared with a lump solid. A flowing powder has excellent heat transfer characteristics both within the powder material itself and with container walls, raising the possibility that a flowing powder may be able to remove heat loads generated by secondary particle interactions with pipe walls. • As for a liquid, a flowing powder can be pumped away from the interaction region and cooled externally using heat exchangers. • Possibility for a flowing refractory metal powder (melting point ~3000°C) to withstand multiple beam pulses (ΔT ~100°C) interacting with the same material before cooling. • No cavitation can occur in a powdered solid material within a carrier gas, since this is a phenomenon associated with liquids only. Proceedings of PAC09, Vancouver, BC, Canada WE1GRC04 Accelerator Technology Subsystems T19 Collimation and Targetry