Studying the behaviour of plasma – a state of matter beyond the familiar three: solid, liquid, and gas – is crucial for our understanding of planets, stars, and the possibility of generating unlimited energy on Earth through the process of nuclear fusion. Specialist equipment is needed to produce extreme kinds of plasma in the laboratory but, once created, they last for less than a billionth of a second. How do you study something so fleeting? To address this problem, a team of researchers from Spain have proposed a design for a simple new device so that proton beams may be used to study faster and faster processes. 

The Fast and Furious Lives of Extreme Plasmas

How do you study something that only lasts for trillionths of a second? This is a question physicists working with warm dense matter (WDM) have to tackle. WDM is a chimerical kind of matter somewhere in between solid and plasma. Solids are familiar – they’re dense and tightly packed. Conversely, a plasma is an energised state of matter where the temperature is high enough to rip apart atoms. Negatively charged electrons are disassociated from their positively charged nuclei to form a charged soup of particles.

While solids are dense and cold, plasmas are highly energetic. WDM is a combination of these two behaviours; you get it when matter is too energetic to behave like a solid, but too dense to behave like a plasma. You’ll expect to find WDM in extreme environments, where gravity crushes matter into something dense, but temperatures are too high to have a solid, such as small stars or the centres of gas giants, like Jupiter.

We also encounter WDM in some of our attempts to replicate nuclear fusion. Fusion is what goes on inside our Sun. Light hydrogen nuclei are fused together into heavier helium ones, releasing the huge quantities of energy needed to keep the star burning bright. For a long time, scientists have hoped to harness fusion to provide clean, sustainable energy on Earth. One method they use to trigger fusion reactions is called Inertial Confinement Fusion (ICF). A stationary fuel pellet is zapped with high-power lasers, causing it to implode, and it becomes a dense plasma experiencing similar conditions to those found within the sun. This “Laser Fusion” approach is recognized as a major pathway to fusion energy, with recent milestones at the US National Ignition Facility (NIF), where repeated demonstrations of energy gain (Q > 1) have been achieved. Despite this progress, key challenges remain, particularly the measurement of ion stopping power in plasma, which governs how fusion energy is transported during the burning phase. Advances might be made if we could learn more about the rapid processes going on within such extreme states of matter.

The Need for Ultra-Fast Proton Probes

If we wish to study processes less than a billionth of a second long, we need probes that also operate exceedingly quickly. And it’s not just high-energy physics research that would benefit from this capability; many biological processes within our cells and tissues occur on similar timescales. Recent developments have enabled the construction of intense petawatt lasers which can be used to produce a proton source. In a process called Target Normal Sheath Acceleration (TNSA), a high-power laser hits a thin foil, blasting away electrons so that a layer of protons are left behind. Their net positive charge creates an electric field, which accelerates the protons that are deposited at the rear side of the target. This produces pulsed ion beams which can be fired into materials to learn more about their properties and inner structure.

It’s an effective way to generate bunches of protons, but it’s not perfectly suited for the study of processes that last just a few picoseconds. That’s because the beam produced is a little messy. The protons emitted aren’t all travelling in the same direction and can end up with slightly different energies. Since energy is related to speed, this means that some of the protons will take longer to arrive at the target than others and the duration of the pulse grows larger. If the pulse duration is too large, then it can’t be used to study picosecond-long processes. It would be like trying to investigate the workings of a busy airport, but turning up at night when everything has shut down. To address these issues, Professor Luca Volpe and his team of researchers in Spain have proposed a design for a new magnetic selector. This device would use magnetic effects to “select” protons with the same speeds as they pass through, helping to tidy up the pulse so that it has minimal spread in both space and time.

Using Magnetic Fields to Select the Right Protons

Prof Volpe and his colleagues identified two main issues that were resulting in longer pulse durations: the range of angles and the range of energies at which protons are emitted. To make sure protons arrived at the target at the same time, they needed a device that would first filter out protons travelling in the wrong direction, and then those with the wrong energy. They call this an isochronous magnetic selector – isochronous meaning “same time” – and it’s made up of three different stages.

The first stage is the collimator. It’s the first gate that blocks off some of the emitted protons. It’s made up of two thin slits placed close to the source – one after the other. In order to pass through both gaps, protons need to be travelling within a small range of angles. Protons approaching the slits at wider angles won’t make it to the next stage. This results in a narrow beam on the other side of the collimator, with protons all travelling in almost exactly the same direction. By making sure protons are all taking the same path, time differences due to spreads in space are eliminated.

Once the protons pass through the collimator, they enter the magnetic transporter. This chamber, on the inside of the device, contains an approximately uniform magnetic field. When charged particles like protons move through a magnetic field, they experience a force that pulls them around in a circle, like how gravity keeps the moon in orbit. The clever bit is that the magnetic field affects protons with different energies differently. Slower protons, with less energy, are affected more by the magnetic field and they’re pulled around in a tighter circle. On the other hand, protons with higher energies are less affected and move in a larger circle. This transforms the spread of protons in energy, and hence in time, into a spread of protons in space. This means that the final stage, the selector itself, is just another small hole through which only protons bent by the right amount pass.

Simulations and Significant Results

To test the effectiveness of their design, Prof Volpe and his team used a Monte Carlo simulation. The method takes its name from the luxury resort in Monaco famous for its casinos, since it relies on randomness: scientists run a model thousands of times with random variations and then study the overall pattern of outcomes. In this case, the team simulated the trajectories of tens of thousands of protons. The initial properties of each proton are generated at random, then they’re tracked as they travel through the device.

They considered sources producing protons with energies around 500keV – a typical threshold used for probing WDM – and the simulations yielded pleasing results. They found that their selector design was able to reduce the duration of the proton pulse to around 70 picoseconds. That’s more than 5 times better than the 400 picoseconds achieved using prior methods. The team also demonstrated that with higher beam energies, this pulse duration could be reduced even further, potentially producing pulses lasting less than a picosecond. This opens the door to the possibility of ultra-fast proton imaging, which could be game-changing for biology and medicine.

The model is still being refined, but the team hopes that their simple design already represents a great step forward – one that can produce ultra-fast, compact proton bursts using existing sources. One of the main disadvantages in selecting protons with magnetic fields is that many protons are lost over the selection process. Most of the protons generated from the source are simply wasted because they start off on wide trajectories or have too large or too small an energy. Fortunately, this isn’t a major problem since many modern detectors are sensitive enough so that the proton loss doesn’t matter. But, looking towards the future, the team suggest that improvements in laser technology could help to avoid too many protons being lost at the original collimator, or simply boost the total number of protons.