Tag: Donna Strickland

Using Ultrafast Laser, ‘Mini’ Particle Accelerator Sets New Energy Record

Smaller accelerators will be more affordable. In the hands of university students, they could democratise biological studies. In hospitals and clinics, they could make diagnostics more affordable.

As particle physicists around the world debate whether humankind needs a $22-billion, 100-km long ‘supercollider’, other physicists are working on shrinking these machines to fit on a tabletop.

The idea for tabletop – or, more accurately, wakefield – accelerators was first birthed in the late 1970s, and then realised technologically in the 2000s by American physicist Chandrashekhar Joshi. Where the Large Hadron Collider (LHC) accelerates protons over thousands of kilometres (round and round in a ring), a wakefield accelerator does the same thing over a few centimetres. The only reason it hasn’t entered mainstream use is because it can barely achieve the same energy the LHC does.

The underlying problem here concerns the acceleration gradient.

Opportunities

A particle accelerator is a machine that drives subatomic particles to very high velocities and energies. These machines typically also make sure that the accelerated particles are kept in a tight, narrow beam.

The acceleration gradient here is the amount of energy imparted to a particle over a given distance. A linear accelerator like at the Stanford Linear Accelerator Centre (SLAC) can manage 0.7-0.8 gigavolt per metre (GV/m). So if it had to energise a particle from zero to 10 GeV, it will need to accelerate the particle across 12-14 metres. Circular accelerators have a gradient of about 0.25 GV/m. To compare, a wakefield accelerator can achieve 10-100 GV/m.

However, it can only sustain this over a very short distance, in the order of a few centimetres. This means it can accelerate the particle only to a few GeV. In a world where higher energies are mandatory for research, this is no good. Machines already exist to push particles to 7,000 GeV.

A section of the LHC showing a module of radio-frequency (RF) cavities. RF cavities are used to accelerate charged particles. Credit: CERN

A section of the LHC showing a module of radio-frequency (RF) cavities. RF cavities are used to accelerate charged particles. Credit: CERN

But physics research isn’t everything, and this problem doesn’t render the accelerators useless. To the contrary, opportunities abound.

In fact, lower energies are more important in some contexts. Most medical instruments operate at the sub-GeV level; it’s physics that requires 1 GeV and above. In the mid-2000s, it was estimated that there were around 10,000 accelerators around the world used in diagnostics and biomedical research, out of 18,000 accelerators in all. By 2014, the total had ballooned to 30,000, with over 15,000 used for medical purposes.

A “third generation” medical linear accelerator energises electrons to about 20 MeV, then smashes them against a heavy-metal plate to produce X-rays. These X-rays are used to look inside the human body. Such machines are about 10 feet tall and, according to a 2013 review, cost $4 million (Rs 28.5 crore) apiece and need to be replaced every five or so years. Their size makes them harder to maintain as well as harder to manoeuvre around a person’s body.

Notwithstanding certain caveats (discussed below), a wakefield machine could potentially reduce this cost by a factor of thousand. In turn, they will become more accessible to doctors, medical researchers and – crucially – university students. When used with an X-ray free-electron laser, one physicist speculates, “a vast number of applications, most notably the determination of the 3D structure of biomolecules, could be carried out by a much broader community of researchers, complementing large-scale facilities where beam time is expensive and scarce”.

With a view to surmounting these challenges, scientists have been hard at work.

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Lasers

In 2014, a group at the Lawrence Berkeley National Laboratory (LBNL), California, pushed electrons to 4.25 GeV in a wakefield accelerator – a record at the time. In 2015, scientists from the University of Maryland found that certain subtle modifications to the accelerator could help achieve a higher acceleration gradient with lower power consumption. On the back of this, the LBNL group found a way to connect multiple accelerators back-to-back in 2016. And now, they’ve reported another increase, a new record: electrons energised to 7.8 GeV.

This is a big leap but there are problems. One is that wakefield machines that can rival the LHC’s energy will have to be a few hundreds of metres long, and no longer table-sized. But this is still not bad considering the LHC is 26.7 km long. Moreover, wakefield accelerators that can achieve a few thousandths of a GeV are still very small, the size of a shoebox, much smaller than the room-sized machines that accelerate particles to similar energies the ‘conventional’ way.

The LHC deploys powerful magnetic fields to accelerate charged particles through large distances. The SLAC uses instruments that rapidly alternate the voltage between two points and use microwaves to bump particles forward. A wakefield accelerator has fewer components but is more sophisticated than both.

All wakefield accelerators accelerate electrons by exposing them to an electric field – forcing them to jump from a negatively charged side of it to a positively charged one. And all wakefield accelerators choose plasma as the medium in which to conduct this jump because it transfers energy more efficiently.

There are different kinds of wakefield accelerators. A typical setup involves first creating a plasma. Second, a laser is fired into the plasma, ‘driving’ the particles forward. Because the ions are much heavier than the electrons, they separate out into two groups as they move.

As they push forward, a trailing electric field develops behind them – the so-called wakefield. Accelerators differ on how they transfer the energy in this wakefield to electrons moving inside it.

A supercomputer simulation showing a rendering of plasma waves (blue) excited by a petawatt laser pulse (red) at Berkeley Lab’s BELLA Center as it propagates in a plasma channel. Caption and credit: Carlo Benedetti/Berkeley Lab

The simplest way is to ‘drop’ an electron into this setup and let it speed along the wakefield, becoming energised as a result. But there’s a problem. As the electron accelerates, it moves faster – but as the driving laser moves through the plasma, it slows down.

So eventually, the electron could break out of the accelerating part of the wakefield and bring the experiment to a premature close. This leads to the accelerating gradient problem.

One workaround is to connect multiple wakefield accelerators end-to-end, as the LBNL group did in 2016. But this isn’t so straightforward because successive accelerators achieve lower accelerating gradients.

Another is to use multiple carefully tuned laser pulses. The LBNL group used this method, together with some technological interventions, to improve the acceleration gradient.

First, physicists created the plasma and moved it into a very narrow tube of glass. They applied a voltage across its ends to separate the electrons and ions. Second, they fired an 850-trillion watt laser into the plasma, in pulses each 35 femtoseconds long, to generate the wakefield. Third, they used another laser to heat the plasma such that it was cooler along the glass and hotter towards the centre. Such a density gradient ensured that the plasma could sustain the acceleration over almost 20 cm, and along a tightly focused beam.

The result: 7.8 GeV.

The technological sophistication at work here is best illustrated by the following factoid: it took physicists 13 years to go from accelerating electrons in a wakefield to 1 GeV to nearly 8 GeV. In the pioneering 2006 experiment, also conducted at LBNL, physicists used a 40-trillion watt laser and a 3.3-cm-long tube. When the 4.25 GeV record was set, Georg Korn, technology manager at the Extreme Light Infrastructure Beamlines facility, Prague, wrote:

Laser plasma acceleration has … been driven by technological advances that boosted laser peak and average power, together with stability, repetition rate (pulses emitted per second), and electrical efficiency.

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Awake

A third workaround involves using a different kind of driver altogether – as physicists at CERN did last year. They worked on an experiment called AWAKE, which stands for ‘Advanced proton-driven plasma Wakefield Acceleration Experiment’.

AWAKE's 10-metre-long plasma cell in place in the experiment tunnel. Credit: Maximilien Brice/CERN

AWAKE’s 10-metre-long plasma cell in place in the experiment tunnel. Credit: Maximilien Brice/CERN

The physicists first created a plasma. Then, instead of a high-power laser, they accelerated a bunch of protons in the Super Proton Synchrotron, a smaller particle accelerator at CERN, to 400 GeV and fired them into the plasma as the ‘driver’. Next, they fired a relatively weaker laser focused on the middle of the bunch. This had the effect of breaking up the back half of the proton bunch into a series of ‘microbunches’ while pushing on the front half.

The trailing microbunches then created a wakefield for electrons to surf in. As a result, they were able to achieve an acceleration gradient of “hundreds” of MV/m across 10 cm.

Two studies released by the AWAKE collaboration (this and this) show that the proton-driven technique can work with a lower plasma density than the laser-driven one. It is also easier to conduct because it doesn’t require as much optical, mechanical and electronic coordination between different components of the experiment as a laser accelerator does. But on the flip side, it does require pre-accelerated protons, which may not always be available.

Then, again a laser-driven accelerator requires high-power, short-duration lasers that are just as hard to find. In fact, a fourth workaround has a similar issue. In 2013, physicists from Stanford were able to accelerate electrons at 300 MV/m simply by shining infrared light over a specifically engineered series of ridges. But the technique only worked if a pre-accelerator was used.

So although neither the LBNL nor the AWAKE machines are ready for primetime, they have demonstrated important advancements that can help scientists build better ‘small accelerators’ in the future. One way they have done this is by highlighting the problems we need to focus on depending on who is going to use them: biomedical students or particle physicists.

Thanks to the people working on wakefield acceleration, we also have a clearer picture of the opportunities these machines present, and what we need to do to get there. Now is the time to explore – to find out what is possible and what isn’t. Doing them better comes later.

Do You Speak Differently of the Physics Nobel Prize If a Woman Wins It?

With Donna Strickland having won (part of) the Nobel Prize for physics in 2018, there is a unique opportunity to revive the prize’s social symbolism, to hoist once more the dusty flag of its virtues.

Almost all news reports around the world on the Nobel Prize for physics 2018 are going to flunk the Finkbeiner test today.

The Finkbeiner test cautions against writing about women scientists because they are women instead of writing about women scientists because of their scientific work. But now that Donna Strickland has been awarded a part of the physics Nobel, it is impossible to ignore her gender. Strickland’s work is commendable, yes, but she has become the first woman to ‘attain’ this distinction in 55 years. However, in this celebratory narrative, let us not get carried away by Strickland’s achievement – that of a woman winning the physics Nobel Prize – because it is not an achievement. What we are looking at is actually a terrible failure that has only now begun to rectify itself: that of the Nobel Committee having ignored female candidates for the prize for half a century.

The Nobel Prizes make up a curious institution. It consists of a bunch of Swedish scientists fulfilling the wishes of a long-gone Swedish inventor by choosing up to three people at a time for an award according to rules that have not been changed in the last century – even though the enterprise in which these people are engaged has changed enormously in the same period. It represents, somehow, an aspiration for research institutions as well as governments alike. While the prizes have been lauded for giving the sciences widespread press coverage for at least a few days every year, they have also been censured for imposing impractical restrictions on how many people can win the prize at a time and for the opaque decision-making process, among other things. The prizes also carry prestige enough to tempt their winners to think of themselves as institutions. And many give in to that temptation.

As an award, and like many awards, these prizes are conceptually flawed. However, the Nobel Committee responsible for selecting laureates has exacerbated the issue by ignoring women laureates for the physics prize for 55 years. It may have been the case that women did not constitute a sizeable fraction of the scientific workforce in the last century, and that prizes rewarding achievements in that century as a result cannot have many women to pick from. This consideration automatically renders its male laureates – especially white male laureates from the US and Europe, the overwhelming fraction – as being bearers of an extreme privilege apart from also being men who undertook commendable scientific work. At the same time, women who were indeed candidates by virtue of their pioneering research were actively overlooked.

As a result of all of these issues, it has been difficult to take the Nobel Prizes seriously – at least as seriously as it often suggests it should be, as seriously as if to render the criticism of it to be trivial.

Also read: Why a Test Used to Spot Gender Bias in Science Reports Can Be Myopic

However, now that a woman has won the prize, should the tone change? Should we take the prize more seriously by virtue of this singular development and, for the time being, set aside criticisms?

The answer is yes. The reason is simple, if also somewhat contra-verse. To quote Ed Yong: “Perhaps none of this would matter if the Nobels weren’t such a massive deal.” The Nobel Prizes have captured the public imagination like no other prizes ever have. Even the Wolf Prize and the Lasker Awards, whose laureates often go on to win Nobel Prizes, don’t hold the kind of sway the Nobels do in the public sphere. When you attempt to convince non-academics that the Nobel Prizes should not be taken too seriously, it is impossible to begin without the hope that your audience does in fact take the Nobel Prizes seriously so that they pay attention to what you are saying.

An immediate consequence of this reality is that Nobel prizewinners become role models and beacons of inspiration to people around the world. This aspect does not come to the fore if the prizes are being awarded year after year to old, white men working in developed societies because boys and men already have many more inspirations to pick from apart from that year’s laureates. This is an essential feature of a patriarchal world: that its non-male members have few people of their own biological, leave alone social, kin to look up to whereas its male members have an abundance to pick from. Did you know, for example, that Strickland did not have her own Wikipedia page until after today’s announcement?

In this vein, the Finkbeiner test was unavoidable. It was devised by Christine Aschwanden, and named for Ann Finkbeiner, to alert journalists to the fact that scientists should not be covered in the press because of their gender – at a time when journalists often seize upon women scientists to write about because they are women. It was a test, rather a concept, and one of many at that, that signalled the dawn of a new age: in which it was no longer as remarkable as it was in the 20th century for women to be scientists and excellent scientists at that. But for its commendable aspiration, the Finkbeiner test is flawed because it does not make room to celebrate non-male achievers because the achievers are in fact not men; it flat-out ignores social conditions. To ask that we celebrate Strickland for her science alone would elide the Nobel Prizes’ travesty, and the latter is not something we should forget.

Just as well, because physics Nobels were awarded to 72 men between Marie Curie and Maria Goeppert-Mayer and to almost 127 men between Goeppert-Mayer and Donna Strickland. For women, the physics Nobel Prize was, and arguably still is, a men’s prize, an inheritance of their privilege from a previous era. But with Strickland having won (part of) the Nobel Prize for physics in 2018, there is a unique opportunity to revive the prize’s social symbolism, to hoist once more – and hopefully not for the last time – the dusty flag of its virtues. To brag that Strickland is “only the third woman” to win the Nobel Prize for physics is to completely miss the point, but at the same time, because the “Nobels are such a massive deal”, it is good that we have one more woman laureate to whom young scientists can look up.