Materials and big physics
Sharon Ann Holgate* investigates the developments allowing scientists to go to the next level at some of the most advanced physics facilities ever built.
The surfaces of the reactor walls in the International Thermonuclear Experimental Reactor (ITER) will attempt to prove the feasibility of fusion as a large-scale and carbon-free source of energy. It is under construction in southern France and will be centimetres away from the 150 million °C plasma of deuterium and tritium fuel. To guard against this, 8–10mm thick beryllium armour has been developed for the 440 water-cooled copper alloy and austenitic steel-based modules of the blanket system, which lines approximately 600m2 of the fusion reactor’s plasma chamber.
Beryllium was chosen for the plasma-facing surface because it will not contaminate the plasma and can absorb the several million watts of heat per square metre and deliver it to the cooling water system, according to the Head of ITER’s Internal Components Division, Mario Merola. Meanwhile, the high melting point of tungsten (3,410°C) makes it suitable for the divertor at the bottom of the chamber, which removes impurities from the plasma and replenishes it with fresh fuel.
Both the blanket and divertor materials will also be subject to the chamber’s ultra-high vacuum, which is 10 billion times lower than atmospheric pressure. ‘This has major implications in the fabrication processes of those materials,’ says Merola. ‘Even the tiniest inclusion may result in a micro-path for a microleak, which is unacceptable for plasma operation. To prevent this, some parts, even if made of standard materials like steel, require dedicated manufacturing processes like Electroslag Remelting (ESR) or Vacuum Arc Remelting (VAR). These processes [in which inclusions are removed via carefully controlled re-melting and solidification] ensure high-quality materials with an extremely low level of inclusions.’
A total of 10,000 tonnes of superconducting magnets working at -269°C will generate the magnetic fields that control the plasma, keeping it from touching the chamber walls. The coils are wound from conductors comprised of niobium-tin (Nb3Sn) or niobium-titanium (Nb-Ti) strands mixed with copper, housed within an outer stainless steel conduit. These conductors can cope with the large sideways force – of up to 70 tonnes of load per metre – from the magnetic fields, as well as exceptionally high levels of radiation and voltage. But producing enough high-purity Nb3Sn strands to make the 200,000km of superconducting wire required was challenging.
‘When we started ITER procurement in 2007, the worldwide production of Nb3Sn strands was about 20 tonnes per year,’ explains Neil Mitchell, Director of ITER’s Magnets Division. ‘We had to ramp up to more than 100 tonnes per year while keeping the same performance and quality from 10 different suppliers, all with differing technical capabilities.’
The first plasma stage for ITER, which has 35 member countries, is scheduled for 2025. This will test the performance of the materials and components – including the shielding – before the plasma current is increased enough to assess the viability of the reactor’s design for providing commercial fusion power.
Although the neutrons created during fusion can make materials radioactive, this ‘does not necessarily render a material unsuitable for its function. But the dose needs to be minimised for workers and the environment,’ Michael Loughlin, Nuclear Shielding and Analysis Coordinator for ITER, tells Materials World.
While impurities are controlled in ITER materials to reduce neutron activation, standard shielding materials, including concrete and borated steel, will be used to stop the neutrons and the gamma radiation produced by fusion.
At the European Spallation Source (ESS), which is being built in Lund, Sweden, there are ‘some relatively unique material applications’, says John Haines, ESS Project Manager. The project, which focuses on building and operating the world’s most powerful neutron source, has 15 partner countries and will allow neutron scattering experiments on a more complex physical, biological, and chemical specimen level than ever before.
Within the linear accelerator, niobium superconducting cryomodules cooled to 2K (-271.15°C) will help accelerate the protons used to create the highest ever intensity proton beam. By contrast, the tungsten spallation target, which will emit neutrons isotropically when the near-light speed high-energy (2GeV) protons hit and blow its atoms apart, will operate at up to 700K (426.85°C).
‘Tungsten was chosen because it gives 30 to 40 neutrons for every proton that hits,’ explains Haines, adding that tungsten atoms are displaced by protons and neutrons, making the target brittle. Because of the high power at ESS, standard tungsten spallation targets would only retain structural integrity for a few months. Therefore, the ESS design consists of a 36-sector wheel that will rotate in the proton beam, spreading out the radiation damage and so increasing the target lifetime to several years.
High-purity beryllium reflectors will channel the emitted neutrons into four liquid hydrogen moderators made from a high-strength aluminium alloy.
These moderators slow the neutrons down to an energy level useful for scattering experiments.
‘To achieve the high neutron brightness, we have a unique configuration for the moderator structure. Our partners in Germany at the research centre in Jülich have successfully developed new welding techniques for aluminium alloys under this configuration’, says Haines. These included attaching the invar piping that brings in the liquid hydrogen to the moderator by friction welding, and electron beam welding the moderator structure within a vacuum chamber.
Specialist beam guides with supermirror surfaces – coated with nickel-titanium multilayers deposited on sodium float glass, superpolished aluminium or copper slabs, or silicon wafers – will reflect some of the neutrons exiting the moderators and funnel them into separate beamlines, enabling simultaneous neutron scattering experiments. A total of 15 beamlines will gradually be brought online after ESS opens for use in 2023.
Furthermore, steel, lead, and concrete will provide protection from radioactive isotopes, high energy neutrons, gamma rays, and X-rays that are by-products of spallation, while boron carbide and cadmium will absorb low-energy neutrons that would otherwise activate surrounding materials and lead to noise in the scattering experiments.
Across the pond
When considering experimental physics facilities outside of Europe, one in the USA comes to mind – the Deep Underground Neutrino Experiment (DUNE), which is due to start full operation in 2026.
DUNE has 31 member countries and will use neutrino detectors to look for proton decay, study supernovae, and seek to explain why the universe is made from matter. The Long-Baseline Neutrino Facility (LBNF), now under construction, will send out a high-intensity neutrino beam from Fermilab in Chicago to the Sanford Underground Research Facility (SURF) in South Dakota, 1,300km away.
The 15m2, 50m-deep neutrino detector at SURF – sited 1.5km underground to shield it from cosmic radioactive particles – will contain 61.6 thousand tonnes of purified liquid argon in a stainless steel vessel at 88K (-185.15°C). Neutrinos entering the detector will interact with the liquid argon, producing charged particles that, in turn, create electrons and photons. To capture the electrons, and help reveal details of the neutrino interactions, a strong electric field is applied across the liquid argon, which causes the electrons to drift towards collecting wires.
The cathode plane used to create the electric field would generally be made from stainless steel. However, the DUNE detector is so big that any shorting of a stainless steel plane would cause an immediate electrical discharge large enough to destroy the detector electronics. ‘We’ve had to develop a resistive cathode plane, which, in the event of a short, would discharge in a safe way over a much longer period of time. Our partners at CERN are currently testing the suitability of planes made out of non-conductive fibreglass laminate, G10, which have been deposited with various partially resistive materials,’ says Eric James, Fermilab scientist and DUNE Technical Coordinator.
‘As there is no room for big phototubes, we are in the process of developing a new type of photon detector with collaborators in Brazil, named the arapuca after a type of bird trap,’ continues James. The detector traps the UV photons created by the neutrino interactions via a wavelength shifting material coated on the outside. This shifts the photon wavelength from the UV into the visible, then reflective materials coated inside keep the photons from escaping and enabling them to be captured on a read-out device.
No ordinary matter
Back in Europe is the FAIR project – the Facility for Antiproton and Ion Research. ‘Materials are a main driver to enhance the performance of our accelerator facility,’ says Jens Stadlmann, an accelerator physicist working on the project.
FAIR, located in Darmstadt, Germany, has 10 partner countries and will use two ring particle accelerators to deliver the highest intensity beams of heavy ions for investigating the sub-atomic structure of matter. The facility is being built alongside the heavy-ion accelerator (GSI) and aims to be operational and accommodating simultaneous experiments by 2025.
Ultimately, FAIR will have a double ring accelerator, with one ring on top of the other, 1.1km in circumference around 17m below ground. This will be fed by beams from the existing accelerator at GSI, which is being upgraded to make it suitable. Heavy ions create several different kinds of particles, including neutrons, when they impact the experimental targets, so FAIR will use standard radiation shielding materials such as concrete and steel.
Ions at such high energies will also desorb gas from any materials they hit during their journey along the beamline. This releases gas molecules into the vacuum chamber that destroy the interacting ion beam by causing the ions to gain or lose charge. To reduce this effect, the high steel vacuum chambers in the ultra-high vacuum GSI accelerator beam tube now have a non-evaporable getter coating developed by CERN. Stadlmann explains, ‘The coating is made from a mixture of aluminium, zirconium, titanium, vanadium, and iron. This creates a surface that absorbs the gas molecules, and helps increase the beam lifetime in the tube from around 4-60 seconds.’
Superconducting magnets at -269°C will guide the ions around the FAIR accelerator, so to handle the large temperature gradient, the chamber surrounding the much warmer cryocatchers, which will absorb unwanted molecules, will be made from copper-plated high steel.
‘Both materials combined give you the strength of steel and the high heat conductance of copper. We would normally see materials created by explosion welding on the sides of modern high-rise buildings,’ says Stadlmann. The cryocatchers themselves are made from a new material developed by the FAIR team comprising very pure copper plated with 1μm of nickel then 1μm of gold. ‘For experiments like this, ordinary materials just don’t do the job,’ says Stadlmann.
* Sharon Ann Holgate is a freelance science writer and broadcaster based in the UK.