Tag: tabletop accelerators

On tabletop accelerators

Tabletop accelerators are an exciting new field of research in which physicists use devices the size of a shoe box, or something just a bit bigger, to accelerate electrons to high energies. The ‘conventional way’ to do this has been to use machines that are as big as small buildings, but are often bigger as well. The world’s biggest machine, the Large Hadron Collider (LHC), uses thousands of magnets, copious amounts of electric current, sophisticated control systems and kilometres of beam pipes to accelerate protons from 0.09 TeV – their rest energy – to 7 TeV. Tabletop accelerators can’t push electrons to such high energies, required to probe exotic quantum phenomena, but they can attain energies that are useful in medical applications (including scanners and radiation therapy).

They do this by skipping the methods that ‘conventional’ accelerators use, and instead take advantage of decades of progress in theoretical physics, computer simulations and fabrication. For example, some years ago, there was a group at Stanford University that had developed an accelerator that could sit on your fingertip. It consisted of narrow channels etched on glass, and a tuned infrared laser shined over these ‘mountains’ and ‘valleys’. When an electron passed over a mountain, it would get pushed more than it would slow down over a valley. This way, the group reported an acceleration gradient – amount of acceleration per unit distance – of 300 MV/m. This means the electrons will gain 300 MeV of energy for every meter travelled. This was comparable to some of the best, but gigantic, electron accelerators.

Another type of tabletop accelerators uses a clump of electrons or a laser fired into a plasma, setting off a ripple of energy that the trailing electrons, from the plasma, can ‘ride’ and be accelerated on. (This is a grossly simplified version; a longer explanation is available here.) In 2016, physicists in California proved that it would be possible to join two such accelerators end to end and accelerate the electrons more – although not twice as more, since there is a cost associated with the plasma’s properties.

The biggest hurdle between tabletop accelerators and the market is also something that makes the label of ‘tabletop’ meaningless. Today, just the part of the device where electrons accelerate can fit on a tabletop. The rest of the machine is still too big. For example, the team behind the 2016 study realised that they’d need as many of their shoebox-sized devices as to span 100 m to accelerate electrons to 0.1 TeV. In early 2020, the Stanford group improved their fingertip-sized accelerator to make it more robust and scalable – but such that the device’s acceleration gradient dropped 10x and it required pre-accelerated electrons to work. The machines required for the latter are as big as rooms.

More recently, Physics World published an article on July 12 headlined ‘Table-top laser delivers intense extreme-ultraviolet light’. In the fifth paragraph, however, we find that this table needs to be around 2 m long. Is this an acceptable size for a table? I don’t want to discriminate against bigger tables but I thought ‘tabletop accelerator’ meant something like my study table (pictured above). This new device’s performance reportedly “exceeds the performance of existing, far bulkier XUV sources”, that “simulations done by the team suggest that further improvements could boost [its output] intensity by a factor of 1000,” and that it shrinks something that used to be 10 m wide to a fifth of its size. These are all good, but if by ‘tabletop’ we’re to include banquet-hall tables as well, the future is already here.

Exploring what it means to be big

Reading a Nature report titled ‘Step aside CERN: There’s a cheaper way to break open physics‘ (January 10, 2018) brought to mind something G. Rajasekaran, former head of the Institute of Mathematical Sciences, Chennai, told me once: that the future – as the Nature report also touts – belongs to tabletop particle accelerators.

Rajaji (as he is known) said he believed so because of the simple realisation that particle accelerators could only get so big before they’d have to get much, much bigger to tell us anything more. On the other hand, tabletop setups based on laser wakefield acceleration, which could accelerate electrons to higher energies across just a few centimetres, would allow us to perform slightly different experiments such that their outcomes will guide future research.

The question of size is an interesting one (and almost personal: I’m 6’4” tall and somewhat heavy, which means I’ve to start by moving away from seeming intimidating in almost all new relationships). For most of history, humans’ ideas of better included something becoming bigger. From what I can see – which isn’t really much – the impetus for this is founded in five things:

1. The laws of classical physics: They are, and were, multiplicative. To do more or to do better (which for a long time meant doing more), the laws had to be summoned in larger magnitudes and in more locations. This has been true from the machines of industrialisation to scientific instruments to various modes of construction and transportation. Some laws also foster inverse relationships that straightforwardly encourage devices to be bigger to be better.

2. Capitalism, rather commerce in general: Notwithstanding social necessities, bigger often implied better the same way a sphere of volume 4 units has a smaller surface area than four spheres of volume 1 unit each. So if your expenditure is pegged to the surface area – and it often is – then it’s better to pack 400 people on one airplane instead of flying four airplanes with 100 people in each.

3. Sense of self: A sense of our own size and place in the universe, as seemingly diminutive creatures living their lives out under the perennial gaze of the vast heavens. From such a point of view, a show of power and authority would obviously have meant transcending the limitations of our dimensions and demonstrating to others that we’re capable of devising ‘ultrastructures’ that magnify our will, to take us places we only thought the gods could go and achieve simultaneity of effect only the gods could achieve. (And, of course, for heads of state to swing longer dicks at each other.)

4. Politics: Engineers building a tabletop detector and engineers building a detector weighing 50,000 tonnes will obviously run into different kinds of obstacles. Moreover, big things are easier to stake claims over, to discuss, dispute or dislodge. It affects more people even before it has produced its first results.

5. Natural advantages: An example that comes immediately to mind is social networks – not Facebook or Twitter but the offline ones that define cultures and civilisations. Such networks afford people an extra degree of adaptability and improve chances of survival by allowing people to access resources (including information/knowledge) that originated elsewhere. This can be as simple as a barter system where people exchange food for gold, or as complex as a bashful Tamilian staving off alienation in California by relying on the support of the Tamil community there.

(The inevitable sixth impetus is tradition. For example, its equation with growth has given bigness pride of place in business culture, so much so that many managers I’ve met wanted to set up bigger media houses even when it might have been more appropriate to go smaller.)

Against this backdrop of impetuses working together, Ed Yong’s I Contain Multitudes – a book about how our biological experience of reality is mediated by microbes – becomes a saga of reconciliation with a world much smaller, not bigger, yet more consequential. To me, that’s an idea as unintuitive as, say, being able to engineer materials with fantastical properties by sporadically introducing contaminants into their atomic lattice. It’s the sort of smallness whose individual parts amount to very close to nothing, whose sum amounts to something, but the human experience of which is simply monumental.

And when we find that such smallness is able to move mountains, so to speak, it disrupts our conception of what it means to be big. This is as true of microbes as it is of quantum mechanics, as true of elementary particles as it is of nano-electromechanical systems. This is one of the more understated revolutions that happened in the 20th century: the decoupling of bigger and better, a sort of virtualisation of betterment that separated it from additive scale and led to the proliferation of ‘trons’.

I like to imagine what gave us tabletop accelerators also gave us containerised software and a pan-industrial trend towards personalisation – although this would be philosophy, not history, because it’s a trend we compose in hindsight. But in the same vein, both hardware (to run software) and accelerators first became big, riding on the back of the classical and additive laws of physics, then hit some sort of technological upper limit (imposed by finite funds and logistical limitations) and then bounced back down when humankind developed tools to manipulate nature at the mesoscopic scale.

Of course, some would also argue that tabletop particle accelerators wouldn’t be possible, or deemed necessary, if the city-sized ones didn’t exist first, that it was the failure of the big ones that drove the development of the small ones. And they would argue right. But as I said, that’d be history; it’s the philosophy that seems more interesting here.