Physicists in Germany say they have passed an important milestone in the development of laser-driven, plasma-based particle acceleration. Proton pulses with energies as high as 150 MeV were created by Tim Ziegler and colleagues at Helmholtz Centre Dresden–Rossendorf (HZDR). This is about 50% higher than the previous record for the technique, and was achieved by better exploiting the temporal profile of laser pulses.
Conventional particle accelerators use radio-frequency cavities to create the high voltages needed to drive particles to near the speed of light. These facilities tend to big; energy hungry; and often require expensive cryogenic cooling. This limits the number of facilities that can be built and where they can be located. If accelerators could be made smaller and less expensive, it would be a boon for applications as diverse as cancer therapies and materials science.
As a result, there is a growing interest in laser-driven plasma-based accelerators, which have the potential to be far more compact and energy efficient that conventional systems.
These accelerators work by firing intense laser pulses into wafer-thin solid targets. The pulse rips away electrons from the target, leaving behind the positively charged atomic cores. This creates a very large voltage difference over a very small distance – which can be used to accelerate pulses of charged particles such as protons.
While these voltage gradients can be much larger than those in conventional accelerators, significant challenges must be overcome before this technique can be used in practical facilities.
“The adoption of plasma-based proton acceleration has been hampered by the slow progress in increasing ion energy,” Ziegler explains. One challenge is that today’s experiments are done at one of just a few high-power, ultrashort-pulse lasers around the world – including HZDR’s DRACO-PW facility. “Firing only a few shots per day, access and availability at these few facilities is constrained,” adds Ziegler.
One curious aspect of the ultrashort laser pulses from DRACO-PW is that some of the light precedes the main pulse. This means that the full power of the laser is not used to ionize the target. But now, Ziegler’s team has turned this shortcoming into an advantage.
“This preceding laser light modifies our particle source – a thin plastic foil – making it transparent to the main laser pulse,” Ziegler explains. “This allows the light of the main pulse to penetrate deeper into the foil and initiates a complex cascade of plasma acceleration mechanisms at ultra-relativistic intensities.”
The researchers tested this approach at DRACO-PW. When they previously to irradiated a solid foil target, the plasma accelerated protons to energies as high as 80 MeV.
In their latest experiment, they irradiated the target with a pulse energy of 22 J, and used the leading portion of the pulse to control the target’s transparency. This time, they accelerated a beam of protons to 150 MeV – almost doubling their previous record.
This accelerated proton beam had two distinct parts: a broadband component at proton energies lower than 70 MeV; and a high-energy component comprising protons travelling in a narrow and well-defined beam.
“Notably, this high-energy component showed a linear scaling of maximum proton energy with increased laser energy, which is fundamentally different to the square-root scaling of the lower energy component,” Ziegler explains. The experiment also revealed that the degree of transparency in the solid target was strongly connected with its interaction with the laser – providing the team with tight control over the accelerator’s performance.
Ziegler believes the result could pave the way for smarter accelerator systems. “This observed sensitivity to subtle changes in the initial laser-plasma conditions makes this parameter ideal for future studies, which will aim for automated optimization of interaction parameters,” he says.
Now that they have boosted the efficiency of ion acceleration, the researchers are hopeful that laser-driven facilities could be built a fraction of the space and energy requirements of conventional facilities.
This would be particularly transformative in medicine, says Ziegler. “Our breakthrough opens up new possibilities to investigate new radiobiological concepts for precise, gentle tumour treatments, as well as scientific studies in efficient neutron generation and advanced materials analysis.”
The research is described in Nature Physics.
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As the chief executive officer (CEO) of Digital Science – a company that improves the information and software tools for all stakeholders in the research ecosystem – I use a variety of skills every day. Many of these are exactly what most people would expect: managing people, reading financial statements – all the usual CEO activities. Thankfully for all concerned, I don’t programme anymore. It’s more than a decade since my code was in a production environment.
However, perhaps surprisingly to some, I do a lot of data analysis. Digital Science’s core strength is our passion for understanding the research world as a route to offering better tools. For me, that means looking at what research is trending, understanding collaboration patterns, and gaining insight into how the scholarly record is changing. Not only are the data completely fascinating, but they are also the start of so many interesting discussions.
Let’s start with what I like least – which is travel, specifically the jet lag. While I do love spending time in different cultures, meeting people and seeing the beautiful nature and architecture in the places that I’m fortunate to visit, I find the jet lag to be very difficult and I’m constantly worried about my carbon footprint.
Last year I managed to do almost every trip in Europe by train and felt very good about it. But trips to Australia, New Zealand, Japan and the US still managed to make their way into my diary. This is somewhere I’m hoping that hybrid meetings find their feet soon.
As for what I like best about my job – that’s easy. Not only do I work with the most talented, kindest and most passionate team, but we also serve those who are the positive agents of change in our world.
Like many people who started off working toward a research career, I defined my success very narrowly – specifically, in terms of being successful in a classically defined research setting. However, the skills that I learned as a researcher are all generally applicable and helpful skills in any walk of life.
They include having an entrepreneurial spirit, a willingness to try to solve a problem, the capacity to work hard and focus on that problem, and not give up when you don’t find a solution with the first approach that you take. Success looks different for everyone and the problems that we contribute to solving, in any context, have the capacity to make people’s lives better.
So, sometimes it’s not good to “buy in” to what we’re so often taught success should look like.
The post Ask me anything: Daniel Hook – ‘The skills I learned as a researcher are applicable and helpful in any walk of life’ appeared first on Physics World.
Mathematical physicists in the Netherlands and Germany have proposed a new “Cavendish-like” gravitation experiment that could offer an alternative means of determining whether gravity is a classical or quantum phenomenon. If built, the experiment might bring us closer to understanding whether the theory of gravity can be reconciled with quantum-mechanical descriptions of the other fundamental forces – a long sought-after goal in physics.
Gravity is one of the four known fundamental forces in nature. It is different from the others – the electromagnetic force and the weak and strong nuclear forces – because it describes a curvature in space-time rather than interactions between objects. This may be why we still do not understand whether it is classical (as Albert Einstein described it in his general theory of relativity) or governed by the laws of quantum mechanics and therefore unable to be fully described by a local classical field.
Many experiments that aim to resolve this long-standing mystery rely on creating quantum entanglement between two macroscopic objects placed a certain distance from each other. Entanglement is a phenomenon whereby the information contained in an ensemble of particles is encoded in correlations among them, and it is an essential feature of quantum mechanics – one that clearly distinguishes the quantum from the classical world.
The hypothesis, therefore, is that if massive, distant objects (known as delocalized states) can be entangled, then gravity must be quantum.
The problem is that it is extremely difficult to make large objects behave as quantum particles. In fact, the bigger they get, the more likely they are to lose their quantum-ness and resort to behaving like classical objects.
Ludovico Lami of the University of Amsterdam, together with Martin Plenio and Julen Pedernales of the University of Ulm, have now thought up a new experiment that would reveal gravity’s quantum nature without having to generate entanglement. Their proposal – which is so far only a thought experiment – involves studying the correlations between two torsion pendula placed close to each other as they rotate back and forth with respect to each other, acting as massive harmonic oscillators (see figure).
This set-up is very similar to the one that Henry Cavendish employed in 1797 to measure the strength of the gravitational force, but its purpose is different. The idea, the team say, would be to uncover correlations generated by the whole gravity-driven dynamical process and show that they are not reproducible if one assumes the type of dynamics implied by a local, classical version of gravity. “In quantum information, we call this type of dynamics an ‘LOCC’ (from ‘local operations and classical communication’),” Lami says.
In their work, Lami continues, he and his colleagues “design and prove mathematically some ‘LOCC inequalities’ whose violation, if certified by an experiment, can falsify all LOCC models. It turns out that you can use them to rule out LOCC models also in cases where no entanglement is physically generated.”
The researchers, who detail their study in Physical Review X, say they decided to look into this problem because traditional experiments have well-known bottlenecks that are difficult to overcome. Most notably, they require the preparation of large delocalized states.
The new experiment, Lami says, is an alternative way of realizing experiments that can definitively indicate whether gravity is ultimately fully classical, as Einstein taught us, or somehow non-classical – and hence most likely quantum. “While we don’t claim that our method is completely and utterly better than the others, it is quite different and, depending on the experimental platform, may prove easier to practically set up,” he tells Physics World.
Lami, Plenio and Pedernales are now working to bring their analyses closer to real-world experiments by taking into account other interactions besides gravity. While doing so will complicate the picture and make their analyses more involved, they recognize that it will eventually be necessary for building a “bulletproof” experiment.
Plenio adds that the approach they are taking could also reveal other finer details about the nature of gravity. “In our work we describe how to decide whether gravity can be mimicked by local operations and classical communications or not,” he says. “There might be other models, however – for example, where gravity follows dynamics that do not obey LOCC, but still do not have to create entanglement either. This type of dynamics is called ‘separability preserving’. In principle we can also solve our equations for these.”
The post ‘Cavendish-like’ experiment could reveal gravity’s quantum nature appeared first on Physics World.
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