When most people picture a particle accelerator, they imagine an enormous underground ring like the Large Hadron Collider, where beams of particles smash together at nearly the speed of light. However, accelerators come in many shapes and sizes, and they are not just for physics experiments. They power X-ray lasers that let us watch chemical reactions unfold, create beams for cancer therapy, and generate radiation for inspecting delicate materials. At the heart of all of these machines lies a deceptively small but crucial step: the production of electrons. Without a reliable stream of electrons, the whole accelerator cannot function.

This is where the electron injector comes in. An injector is like the starter pistol of a race – it provides the first burst of electrons that then get handed off to the rest of the accelerator for further acceleration. The quality of the initial ‘shot’ is critical, because if the electron beam is unstable or messy, it doesn’t matter how advanced the rest of the machine is – the results will be compromised.

For decades, researchers have built injectors using a combination of conventional and superconducting materials. In connection with the construction of the European X-ray Free Electron Laser in Germany, an international team carrying out this project proposed the use of a fully superconducting, radio-frequency electron injector. Dr. Lorkiewicz’s group is currently implementing this concept in cooperation with other research centres. Superconducting machines are prized because they waste almost no energy as heat, making them more efficient and capable of sustaining long, powerful pulses. A fully superconducting injector would be the ideal complement – a perfectly matched beginning for a perfectly matched accelerator.

The difficulty, however, is that superconductors are not all alike. The workhorse of accelerator cavities is niobium. When cooled to a few degrees above absolute zero, niobium carries current without resistance and can withstand the powerful radio-frequency fields used to accelerate particles.

However, niobium is not very good at releasing electrons when hit with light, which is exactly what is needed for the photocathodes at the injector’s heart. For that, another material must be brought in: lead. Lead, also a superconductor at low temperatures, is much more efficient at spitting out electrons when struck by a laser pulse. This makes it an excellent candidate for a photocathode material.

Dr Lorkiewicz’s concept requires a hybrid: a niobium base for superconductivity and a lead coating for electron emission. However, niobium and lead are not natural friends. They do not form alloys, they do not mix, and when a thin film of lead is deposited onto niobium, it tends to flake, crack, or peel away. The latter happens especially during the mandatory rinsing of the injector in a high-pressure stream of ultra-pure water. 

Over the years, researchers tested several approaches to overcome this challenge. Some added a thin titanium ‘buffer layer’ to help the lead stick, like using double-sided tape. Others tried different deposition methods – electroplating, pulsed laser deposition, cathodic arc deposition. Each method had some success but also came with serious drawbacks. Sometimes the lead adhered but didn’t emit electrons efficiently, while other times the coatings cracked under stress.

The breakthrough came when Dr Lorkiewicz and his colleagues decided to tackle the problem from a different angle. Instead of trying to find a better glue, they asked: what if we could prepare the niobium so that lead naturally wanted to bond with it? Their solution was ion implantation. Before depositing the lead coating, they fired energetic lead ions into the surface of the niobium. By driving lead atoms beneath the surface of the niobium, they created a ‘transition zone’ where the two metals intermingled. This meant that when the main lead layer was deposited, it wasn’t sitting on a hostile surface – it was sitting on a blended layer already rich in lead.

The team tested this approach with two different coating techniques: magnetron sputtering and cathodic arc deposition. Magnetron sputtering involves knocking atoms off a lead target with energetic ions so that they slowly build up on the substrate. Cathodic arc deposition, in turn, generates a plasma stream of lead atoms, metallic microdroplets and ions. The energy of the latter impacting the substrate surface is higher and more precisely controlled than with sputtering.

For magnetron sputtering, pre-implanting the niobium with lead ions doubled or even tripled the adhesion strength of the lead films. In scratch tests, where a diamond tip is dragged across the coated surface with increasing force, the implanted samples withstood critical loads far higher than untreated samples before the lead layer peeled off. For cathodic arc coatings, however, the story was different: implantation actually reduced adhesion, showing that the success of the technique depends strongly on the deposition method. This revealed just how subtle the physics of thin films can be – the same preparation that strengthens one type of coating can weaken another.

To probe what was happening at the microscopic level, the researchers used nanoindentation, which involves pressing a tiny probe into the material to measure hardness and elasticity. They discovered that the strongest adhesion corresponded to a smooth gradient in mechanical properties across the interface. Instead of an abrupt cliff from soft lead to hard niobium, there was a gentle slope where the properties changed gradually. This gradient acted like a cushion, spreading out stress and preventing cracks.

This research makes the vision of a fully superconducting injector achievable. With robust, well-adhered lead photocathodes, superconducting injectors could become practical reality. In turn, this would pave the way for brighter, more reliable beams in X-ray free-electron lasers and superconducting linacs. Scientists could watch chemical reactions as they unfold, probe biological molecules in unprecedented detail, and design new materials with atomic precision.