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National Ignition Facility’s ignition milestone sparks fresh push for laser fusion

For well over a decade, physicists at the Lawrence Livermore National Laboratory in California have been attempting to do something in the lab that had only ever previously occurred inside the warheads of hydrogen bombs. Their aim has been to use intense pulses of light from the world’s biggest laser – the $3.5bn National Ignition Facility (NIF) – to crush tiny capsules of hydrogen fuel such that the exceptional temperatures and pressures created therein yield energy-producing fusion reactions. Until the end of last year, a series of technical setbacks had prevented them from reaching their goal, known as ignition. But just after 1 a.m. on 5 December a larger-than-usual burst of neutrons in the detectors surrounding the laser’s focus signalled success – the reactions in this case having produced more than 1.5 times the energy they consumed.

The feat created headlines around the world and stimulated the imagination of the public, politicians and fusion experts alike. US energy secretary Jennifer Granholm hailed the “landmark achievement”, while Michael Campbell of the University of Rochester in the US described the result as a “Wright Brothers moment” for fusion research. For Steven Rose of Imperial College London, the announcement removes any lingering doubt that such high fusion energies are attainable. “If you don’t get an energy gain greater than one, people might claim you can never achieve it,” he says.

The result renewed optimism that fusion might finally enable a new source of clean, safe, secure and sustainable energy. Now, governments and especially private companies are looking to exploit the huge potential of fusion energy – with some firms even promising that they will deliver electricity to the grid from pilot power plants by early in the next decade.

Some scientists, however, reckon that such timescales are unrealistic, given the huge technical hurdles that remain on the road to fusion energy. Others contend that a 10–15-year time horizon is feasible, so long as researchers and their funders adopt the right mindset. For Troy Carter at the University of California, Los Angeles, this means ending reliance on large, expensive, centralized facilities such as the football-stadium sized NIF and turning instead to smaller, cheaper projects led by the more risk-tolerant private sector. “We have to change the way we do business,” he says.

Finally on target

Harnessing the energy given off when light nuclei fuse requires the nuclear fuel to be held in the form of a plasma at temperatures of around 100 million kelvin. One way of doing this is to confine the plasma in a magnetic field for fairly long periods of time while heating it with radio waves or particle beams. So far, such “magnetic confinement” has been physicists’ preferred route to fusion energy. This will be utilized in both the world’s priciest public and private reactors: the $20+bn ITER facility under construction in the south of France and a machine built by the company Commonwealth Fusion Systems outside Boston, US, which has so far raised at least $2bn in funding.

Rather than attempting to obtain a steady state, “inertial confinement” reactors operate somewhat like an internal combustion engine – generating energy through a repetitive cycle of explosions that fleetingly create enormous temperatures and pressures. NIF does this by amplifying and focusing 192 laser beams onto a tiny hollow metal cylinder at the centre of which is a peppercorn-sized capsule containing the hydrogen isotopes deuterium and tritium. X-rays generated from the walls of the cylinder blast off the outer surface of the capsule, forcing the rest of it inwards thanks to momentum conservation and causing the deuterium and tritium nuclei within it to fuse – in the process releasing alpha particles (helium nuclei), neutrons and lots of energy.

This process is extremely demanding, requiring exceptionally precise beam focusing and ultra-smooth capsules to ensure the near perfectly symmetrical implosions needed for fusion. Indeed, instabilities in the plasma created by the implosions and defects in the capsules, among other things, meant that the Livermore researchers fell well short of their initial target of ignition (or “breakeven”) by 2012. But through a series of painstaking measurements on successive laser shots they were able to gradually refine their experimental set-up and ultimately fire the historic shot – yielding 3.15 million joules (MJ) of fusion energy after delivering 2.05 MJ of laser energy to the target.

Omar Hurricane, chief scientist of Livermore’s inertial-confinement fusion programme, says that they now plan to “reprioritize” their work to push for higher, reproducible gains by boosting NIF’s laser energy in steps of about 0.2 MJ. They also intend to study the effect of varying the thickness of the nuclear fuel inside the capsules and reducing the size of the cylinder’s laser entrance holes. However, he points out that NIF was never designed to demonstrate practical fusion energy – given that the facility’s main purpose is providing experimental data to support the US’s (no longer tested) stockpile of nuclear weapons. As such, NIF is extremely inefficient – its 2 MJ flash-lamp pumped laser requiring around 400 MJ of electrical energy, which equates to a “wall-plug” efficiency of just 0.5%.

Riccardo Betti of the University of Rochester says that modern lasers pumped by diodes could reach efficiencies as high as 20% but points out that margins required for power plants (including energy lost during conversion of heat to electricity) means that even these devices will need target gains of “at least 50–100” (compared to NIF’s 1.5). They will also have to “fire” several times a second, while NIF only generates a shot about once a day. This high repetition rate would require mass-produced targets costing at most a few tens of cents, compared to the hundreds of thousands of dollars needed for those at NIF (which are made from gold and synthetic diamond).

Entering the market

One company that believes it can commercialize fusion energy despite all the hurdles is California-based firm Longview Fusion Energy Systems. Set up in 2021 by several former Livermore scientists, including ex-NIF director Edward Moses, Longview aims to combine NIF’s target design with diode-pumped solid-state lasers. The company announced its existence on the same day that Livermore reported NIF’s record-breaking shot, saying that it planned to start building a pilot power plant within the next five years.

Longview says that it intends to provide 50 MW of electricity to the grid by 2035 at the latest. The company acknowledges that this will not be easy, envisaging a laser efficiency and repetition rate of 18% and 10–20 Hz respectively. In particular, it says that while the necessary diodes already exist, they have “not yet been packaged into an integrated beamline for a fusion-scale laser”. But it remains confident that it can meet its deadline, noting that the laser is within a factor of two of the optics damage threshold needed for the pilot plant.

Not everyone is convinced. Stephen Bodner, previously head of the laser-fusion programme at the US Naval Research Laboratory in Washington DC, maintains that NIF’s “indirect-drive” technology wastes too much energy in generating X-rays (rather than illuminating fuel capsules directly). He is also sceptical of Longview’s claim that it can reduce the target cost to below $0.30 by spreading the considerable engineering and capital expenses over the 500 million targets it says it will need for its pilot plant. “There is no possible way for a fusion target like that used on NIF to ever be improved enough for commercial fusion energy,” he says.

Yet Longview is far from alone in believing that it has the technology at hand to bring fusion energy to the world. A report compiled last year by the Fusion Industry Association trade body lists 33 companies in the US and elsewhere as working on fusion technology – many of which also have aggressive timescales for developing power plants. One such company is First Light, based near Oxford, UK. Rather than using laser pulses to compress fuel capsules, First Light instead launches material projectiles – postage-stamp shaped pieces of metal – at extremely high speeds using the electromagnetic force provided by a huge bank of capacitors all discharging nearly instantaneously. The projectiles strike specially made targets, each of which directs and enhances the impact pressure on a fuel capsule embedded inside.

The company has so far raised some £80m in funding and demonstrated fusion using the largest pulsed-power facility in Europe. The next steps, according to co-founder and chief executive Nicholas Hawker, will be demonstrating ignition with a much bigger machine in around five years’ time and then a pilot plant in “the early- to mid-2030s”. Hawker admits that numerous challenges lie ahead – such as being able to load projectiles one after another and developing suitably robust high-voltage switches – but he is confident that the scheme’s physics is solid. “The fuel capsule is exactly the same as NIF’s so the recent result massively de-risks our system as well.” 

Cash needed

When it comes to physics, Betti reckons that inertial confinement fusion is better placed than magnetic confinement. While NIF has now demonstrated that the former can generate self-sustaining reactions, he argues that instabilities generated close to the ignition threshold mean there are still big uncertainties about whether tokamaks can follow suit. Nevertheless, he says that both forms of fusion must overcome formidable hurdles if they are to yield economically-competitive energy – including the demonstration of high gains from mass-produced targets when it comes to laser fusion. “I find it hard to believe that an energy system can be ready in 10 years,” he says.

NIF scientists did a superb job over the past decade solving some very difficult physics problems. They should be recognized for their great work

Stephen Bodner

Carter is more optimistic. He maintains that pilot plants could be realized in about a decade’s time, so long as private companies lead the charge in their construction while governments support more basic underpinning research such as that on radiation-resistant materials. But he cautions that the necessary funding will be considerable – about $500m extra per year in the case of the US government. If the money is forthcoming, he adds, full-scale commercial plants might then turn on “sooner than 2050”.

As to which technology will end up inside the plants, Bodner insists it will not be based on indirect drive. Most likely, he maintains, it will be inertial confinement based on a different kind of laser system such as argon-fluoride gas lasers. But he acknowledges that scaling up any system brings uncertainties. And he praises NIF scientists for getting fusion research to this point. “They did a superb job over the past decade solving some very difficult physics problems,” he says. “They should be recognized for their great work.”

Ask me anything: Jessica James – ‘I am never happier than with my head down doing maths and coding’

Jessica James

What skills do you use every day in your job?

I am glad I learned not to be afraid of coding when I was doing my PhD – many periods of my life have seen me buried in code and spreadsheets for hours on end. I think that my ability to be completely obsessive is extremely handy here and in other areas of work. My PhD also started my writing career. I have published several books and many articles on physics and finance, and the ability to write for a variety of reader types, from technical to layperson, is very much part of my life. Public speaking is another part of my world; I’ve given talks at many different levels and I love audience feedback. I mourn the growth of remote presenting when all you have is a screen to talk to, though its undoubted convenience means that I can reach a global audience without leaving my home. Leadership and teamwork are likewise so important; I’ve always been able to switch between “team mode” and “manager mode” pretty easily, and many of these skills began to be acquired during my PhD. Finally, science has a strong code of ethics and integrity that has transferred to my finance career.

What do you like best and least about your job?

What is best is the excitement of discovering new things and I have found areas of finance and financial mathematics that other people did not even know existed. I am never happier than with my head down doing some maths or coding, and then finding that the clock has moved on many hours while I was working. I love being in a team and engaging with clever, interesting people. Also having writing and presenting as part of my job is a real pleasure. My least favourite thing is the time pressure – long hours, stress, demands on my time for crazy things. And I resent having to do my travel expenses when there’s a juicy problem to work on.

What do you know today that you wish you knew when you were starting out in your career?

A view on the future of the markets would have been useful. But I think it’s so important to value your own time and set limits and boundaries. We all need a balance in our lives and it’s so easy to get subsumed in work. Apart from that, there are two things I always say to people starting off in my industry: never be embarrassed about saying “I don’t understand” and never be afraid of saying “I made a mistake”. If folk could say these things more easily then a lot of the problems in the finance industry might never have happened. Personally, these days I love to say “I don’t know” – because then there is an opportunity to learn.

Patient positioning chair paves the way for upright radiotherapy

Upright radiotherapy

Cancer patients typically lie in a supine position (on their back) during radiotherapy. But for some malignancies, including thoracic, pelvic and head-and-neck tumours, upright body positioning may improve treatment delivery and possibly patient outcome. Upright treatments could increase tumour exposure, reduce radiation dose to adjacent healthy tissue and make breath holds easier for some patients.

To perform upright radiotherapy safely, however, patient immobilization is critical. With this in mind, researchers at the Centre Léon Bérard in France evaluated a patient positioning system currently in commercial development by Leo Cancer Care. The team assessed the immobilization accuracy, set-up time and comfort of the system for 16 patients undergoing radiotherapy for pelvic cancer (prostate, bladder, rectal, endometrial and cervix/uterine tumours).

Findings of the pilot study, reported in Technical Innovations & Patient Support in Radiation Oncology, are encouraging. Initial patient set-up took 4 to 6 min when performed by two radiation therapy technologists working together, and subsequent positionings took between 2 and 5 min. Inter-fraction repositioning was achieved with below 1 mm accuracy on average, and intra-fraction motion over 20 min was within 3 mm for more than 90% of patients. Most patients reported that upright positioning was as good as, and in some cases better than, the supine position that they had to maintain during their standard radiation treatment.

The positioning system (known as “the chair”) is designed to place the patient in appropriate postures according to the cancer type being treated. For prostate and pelvic treatments, patients are perched on the chair, supported by the back of the thigh and a knee rest. Patients are seated vertically for head-and-neck treatments, seated leaning slightly backward for lung and liver radiotherapy, and slightly forward for breast radiotherapy.

The chair itself comprises a seat, a backrest with arm support, a shin rest and a heel stop, all of which adjust to different positions and angles. The chair can rotate at a speed of one rotation per minute, and can simultaneously move in the cranio-caudal direction (vertically in this set-up) by up to 70 cm, allowing the generation of helical movement.

The system incorporates an optical guidance and tracking system, comprising up to five high-resolution cameras. In this study, each patient had their own custom-moulded vacuum cushion and a belt was positioned on the upper part of their abdomen.

For the study, participants undergoing conventional radiotherapy had three additional appointments during their scheduled treatment course to test the upright positioning device. Patients were repositioned at the second and third appointments and the researchers verified the repositioning accuracy using the optical image reference system. They note that image registration was performed using the skin surface, with no skin tattoos or landmarks needed. After being accurately positioned, patients underwent a simulated treatment session with several helical movements lasting 20 min.

Principal investigator Vincent Grégoire and his colleague Sophie Boisbouvier calculated the inter-fraction position shifts after manual registration between reference images and images taken during the repositioning. They report that the chair provided accurate repositioning, with average inter-fraction shifts of –0.5, –0.4 and –0.9 mm in the x-, y– and z-directions, respectively.

The researchers also monitored intra-fraction motion during the chair movements, performing positioning checks every 4 min. After 20 min, the mean intra-fraction shifts were 0.0, 0.2 and 0.0 mm in the x-, y– and z-directions, respectively. Only 10% of patients had inter-fraction shifts exceeding 3 mm and intra-fraction motion exceeding 2 mm. The majority of patients reported that that they were more comfortable in the upright versus the supine position. All patients said that they could breathe comfortably when upright.

The study revealed some modifications required for the chair, including redesigning the belt to improve patient comfort and improvements regarding head positioning. The researchers intend to investigate a new backrest that has been optimally designed to position the head and neck. They also plan to conduct similar assessments of patient immobilization for head-and-neck, lung, breast and upper-abdomen tumours.

Grégoire advises that the team plan to compare upright with supine positioning in terms of internal positioning and motion for the tumour types they have investigated. They also will perform in silico dose distribution comparisons between patients in supine and upright positioning, for both photons and protons. They also hope to estimate potential gains in terms of normal tissue complication probability (NTCP) and tumour cure probability (TCP).

The Centre Léon Bérard is a research partner of Leo Cancer Care, which is developing a range of upright radiotherapy products. In addition to the positioning system, these include a vertical diagnostic CT scanner and a 6 MV horizontal-beam linear accelerator to deliver rotational image-guided intensity-modulated radiotherapy. After French government regulatory authorities authorize the importing of the CT scanner, which does not yet have a CE Mark, the team plans to include vertical CT imaging in future research.

Sterile neutrinos fade as STEREO finds no evidence of oscillations

Data from an experiment in France called STEREO suggest that sterile neutrinos cannot explain why the neutrino flux observed from uranium-235 fission is lower than predicted by theory. Instead, the STEREO team believes that the discrepancy arises from theoretical difficulties in modelling the decay process. The result marks the culmination of an 11-year project to test a hypothesis first advanced by a team of scientists that included two members of the STEREO collaboration.

The 1998 discovery of neutrino oscillations by the Super-Kamiokande detector in Japan was one of the most consequential events in 20th century particle physics. This is because it showed that neutrinos must have a tiny but non-zero mass. As a result, neutrinos propagate through space as oscillating quantum superpositions of electron neutrinos and their heavier cousins – muon neutrinos and tau neutrinos. This explained puzzling experiments done in the 1960s in which physicists observing the Sun had detected significantly fewer neutrinos than predicted. What was happening was that many of these solar neutrinos had oscillated into neutrino flavours that the experiments were not designed to detect.

In 2011, David Lhuillier at CEA and colleagues in France published a paper suggesting that measurements of the neutrino flux from nuclear reactors collectively showed an anomalously low flux of neutrinos from uranium-235 relative to predictions of theoretical models. Moreover, they showed, this anomaly could be explained by the neutrinos oscillating into “sterile” neutrinos that would not interact via the weak force and therefore not be detected. Sterile neutrinos are hypothetical particles that are invoked by some theoretical extensions of the Standard Model of particle physics, so the idea that they might have been glimpsed in neutrino oscillations was tantalizing. In 2016, Lhuillier and colleagues  installed the STEREO detector at the Institut-Laue-Langevin (ILL) research reactor in Grenoble – with STEREO standing for Search for Sterile Reactor Neutrino Oscillations.

Shorter oscillations

“Before, everyone was seeing a mean deficit [in neutrinos],” explains Lhuillier, “The idea was ‘OK, maybe if we go closer, we will be able to see this first or second oscillation’.” The hypothetical oscillation wavelength was not known, he says, but “for sure, the oscillation was already smeared out after 100 m.” The STEREO detector comprises six separate gadolinium-doped 1.8 m3 hydrocarbon-filled scintillators located less than 20 m from the ILL’s almost fully-enriched, 40 cm-diameter uranium-235 nuclear fission reactor, which effectively behaves as a point source of uranium-235 neutrinos.

When a neutrino strikes a hydrogen atom in one of the liquid scintillators, it stimulates inverse beta decay, converting the electron to a positron and the proton to a neutron and knocking both free. The deceleration of the positron generates prompt gamma rays. Subsequently, the neutron is likely to be captured by a gadolinium nucleus, exciting it to a metastable state. When this subsequently decays, it produces a second, larger gamma ray pulse: “That’s the signal we are looking for,” says Lhuillier; “a small pulse from the positron and then, a few microseconds later, a big pulse from the gadolinium.”

If it’s not a sterile neutrino, the problem has to be on the prediction side

David Lhuillier

Using their six sequential detectors, the researchers confirmed that there was indeed a deficit in the neutrino flux relative to that theoretically predicted. However, this deficit appeared constant in all six detectors, so the researchers concluded it was not explained by the neutrinos oscillating in and of some undetectable state. “If it’s not a sterile neutrino, the problem has to be on the prediction side,” says Lhuillier. “It’s very difficult to predict what neutrinos a reactor can emit because you have something like 800 ways to break a uranium nucleus apart, so you need a huge amount of nuclear data.”

The STEREO team describes its findings in a paper in Nature.

“People were looking forward to this, so in that sense it’s a useful paper,” says theoretical physicist André de Gouvêa of Northwestern University in Illinois. “It does confirm the trend of results we have been getting that suggest that this reactor neutrino anomaly is more closely associated with our mis-modelling of how neutrinos are produced in nuclear decays and less with some exciting new physics phenomenon.” He adds, however, that, “The title of the paper [‘STEREO neutrino spectrum of 235U fission rejects sterile neutrino hypothesis’] is a little bit optimistic…Sterile neutrinos in principle are not ruled out as an explanation for this anomaly. Very heavy sterile neutrinos could only be ruled out by cosmological constraints,” he says, adding “There’s some fascinating results [on that] coming out of the KATRIN experiment”.

Theoretical physicist Patrick Huber of Virginia Tech in the US adds “There has been this reactor antineutrino anomaly since 2011, and I think this closes that chapter”. Huber performed one of the neutrino flux calculations showing the tension with experimental observation and adds “We now know that the reason for the discrepancy was flawed input data, and I think this is important going forward when we consider possible applications of neutrino physics such as nuclear security. Their result is a capstone on a community-wide effort to understand why the calculations of 2011 and the data did not agree, and now they do.  That’s the scientific method at work.”

Seeking cosmic particles using a super-pressure balloon, the physics of babies

This episode of the Physics World Weekly podcast features an interview with Angela Olinto, who is principal investigator of the EUSO-SPB2 mission. EUSO stands for Extreme Universe Space Observatory and SPB refers a super pressure balloon, which will soon be hoisting the experiment to an altitude of 33 km. There it will spend about 100 days detecting neutrinos and ultra-high energy cosmic rays.

Olinto, who is based at the University of Chicago, talks about the challenges of operating a particle-detection system floating high above Earth and what the EUSO-SPB2 collaboration hopes to observe.

Also in this episode, Physics World’s Michael Banks talks about his new book, The Secret Science of Baby, which charts the first 1000 days of human development starting at conception. Banks talks about the physics of three key phenomena related to the creation and development of a child – the swimming of sperm; the operation of the placenta; and the development of speech. These are also described in a feature article by Banks that appears in Physics World.

Defect suppression enables continuous-wave deep-UV lasing at room temperature

Researchers have succeeded in making the first room-temperature continuous-wave deep-UV laser diode, using wide-bandgap semiconductor materials. The device could find applications in novel sterilization systems and for more precise laser processing.

Invented in the 1960s, laser diodes today operate at wavelengths ranging from the infrared to blue-violet, with applications such as optical communications devices and Blu-ray discs. Until now, however, they did not work in the deep-UV part of the electromagnetic spectrum.

A team led by Nobel laureate Hiroshi Amano at Nagoya University’s Institute of Materials and Systems for Sustainability (IMaSS) began developing deep-UV laser diodes back in 2017, thanks to a collaboration with Asahi Kasei, the company that made the first 2-inch aluminium nitride substrates. These materials are ideal for growing aluminium gallium nitride (AlGaN) films for UV-light-emitting devices.

The first devices that the team made required input powers of 5.2 W, which was too high for continuous-wave lasing because it heated up the diode too quickly and stopped it from lasing.

In their new work, Amano and colleagues overcame this problem. By improving the design of the device structure, they could suppress the heat generated during operation. In particular, they eliminated the crystal defects that occur at the laser stripe in the AlGaN and deteriorate the paths through which current propagates. They achieved this by tailoring the side walls of the laser stripe such that current was able to efficiently flow to the active region of the laser diode. In this way, they could reduce the required operating power for 274 nm laser diodes to just 1.1 W at room temperature.

“Compared to conventional deep-ultraviolet lasers, our laser is more compact and can achieve higher efficiency,” says Amano. “The device could be employed in practical applications in healthcare, including virus detection. More broadly, it could be used to detect particulates, in gas analysis and high-definition laser processing.”

“Its application to sterilization technology could be ground-breaking,” adds team member Zhang Ziyi. “Unlike the current LED sterilization methods, which are time-inefficient, lasers can disinfect large areas in a short time and over long distances.”

The Nagoya team now plans to improve the operating characteristics of their laser diode for practical use. “We also hope to realize a shorter-wavelength laser diode,” Amano tells Physics World.

The study is detailed in Applied Physics Letters.

Einstein as you’ve never seen him before

In November 1922, just over a century ago, Albert Einstein received the Nobel Prize for Physics. That much is clear. What you might not know is that the prize was officially awarded for the year 1921. The delay was due, in part, to a controversy in the physics community over Einstein’s general theory of relativity and, to a lesser extent, antisemitism among certain physicists – including the Nobel laureate Philipp Lenard, who would later support the Nazis.

Indeed, relativity went unmentioned in the citation from the Royal Swedish Academy of Sciences. It referred instead to Einstein’s “services to theoretical physics and in particular…his discovery of the law of the photoelectric effect”, which was his key contribution to quantum theory. Not that Einstein was concerned: when he gave his Nobel-prize lecture in 1923, he ignored the citation and talked about relativity, not quantum theory.

Published to mark the Nobel prize’s centenary, the book is a lavishly produced and highly appealing collection of photographs, most of which are signed by Einstein

I mention the Nobel-prize episode because it is characteristic of Einstein’s unique work and personality. As he remarked in 1930: “To punish me for my contempt for authority, fate made me an authority myself.” I was therefore delighted to see a handwritten version of this immortal aphorism, with his signature, appearing in solitary splendour on the final page of Einstein: the Man and His Mind by Gary Berger and Michael DiRuggiero.

Published to mark the prize’s centenary, the book is a lavishly produced and highly appealing collection of photographs, most of which are signed by Einstein and others. Arranged chronologically, they are complemented by images of his letters, scientific manuscripts and publications. These are all rounded off with quotations from Einstein and a lively commentary and captions by Berger and DiRuggiero.

The earliest known signed photograph comes from 1896, taken to commemorate Einstein’s graduation from high school in Switzerland. Even as a teenager, his frizzy hair shows hints of its legendary adult unruliness. The book ends with what is believed to be the last photograph he signed, at home in Princeton not long before his death, aged 76, in 1955.

A physician, rather than a physicist, Berger is a retired surgeon who has written 170 medical articles and a dozen books about reproductive medicine. He first became fascinated with Einstein about 20 years ago while working in Chapel Hill, North Carolina, where he started collecting portraits of the great man, some signed by him.

With the encouragement of DiRuggiero, owner of the Manhattan Rare Book Company, Berger then began to acquire documents relating to Einstein’s scientific life. Eventually he invited DiRuggiero to help curate this material. Located in Chapel Hill as the Berger Collection, it is now probably the largest archive of Einstein imagery in private hands.

“Not being a physicist, I could appreciate his pictures, if not the complex mathematics in his writings,” writes Berger in his preface. “The photos gave me the feeling of a personal connection to Albert Einstein – the real, living man – almost as if I knew him.” It is a view shared by DiRuggiero, who describes in the epilogue how he would often find himself staring at a photo of Einstein and wondering what makes his image so powerful.

“Whether he is trying to solve a difficult scientific problem, full of despair contemplating the fate of the world, or lightheartedly playing with children, Einstein seems to be communicating his emotions directly to us,” DiRuggiero writes. “Somehow, in looking at these photographs we feel we know him, that if he walked into the room right now we could talk to him and understand each other. This is extraordinary, considering we are contemplating someone who explored realms of thought inaccessible to nearly all of us.”

Albert Einstein with his children

The book contains a foreword by the physicist Hanoch Gutfreund, academic head of the Albert Einstein Archives at the Hebrew University of Jerusalem, who feels the photos from the second half of Einstein’s life “evoke an image of a friendly non-conformist”. He thanks Berger and DiRuggiero for generously donating all royalties from the book to the extraordinary Jerusalem collection. It deserves to succeed.

It’s not the first book of this kind, though, with a similar, large-format title having been published by Ze’ev Rosenkranz and Barbara Wolff of the Einstein Archives in 2007. Entitled Albert Einstein: the Persistent Illusion of Transience, that book was both wonderfully illustrated and authoritative. Einstein: the Man and His Mind, however, is stronger in its illustrations than in its text.

Astonishingly, it mentions neither Cambridge nor Oxford universities. Cambridge was the scientific home not only of Isaac Newton (an inspiration for the young Einstein) but also of Arthur Eddington, who led the team that provided the astronomical proof of general relativity in 1919 and who was friendly with Einstein. Oxford, meanwhile, was the scientific home of the physicist Frederick Lindemann, who hosted Einstein in the city in 1931, 1932 and 1933, on the final occasion as a refugee from Nazism.

In fact, the book contains no reference to Britain’s vital role in rescuing Einstein from likely assassination by Nazi agents in Belgium in 1933. Nor is there any mention of Abraham Flexner, who founded the Institute for Advanced Study in Princeton, where Einstein settled in 1933. Other omissions include Einstein’s first wife, Mileva Marić (mother of his two children); philosopher Bertrand Russell (who organized the crucial Russell–Einstein Manifesto against nuclear weapons in 1955); and Mahatma Gandhi (whom Einstein the pacifist called “the greatest political genius of our time”).

Another person not mentioned in the book is Wolfgang Amadeus Mozart, whom Einstein (a violinist himself) regarded as his favourite composer. “Mozart’s music is so pure and beautiful,” Einstein once said, “that I see it as a reflection of the inner beauty of the universe.” It is a shame that Mozart is missing because Einstein also sought that beauty – not through music, but through mathematics, reasoning and pure thought.

  • 2022 Damiani 209pp £60.00hb

THETIS phantom detects image distortions to support MR-based treatment planning

The THETIS 3D MR Distortion Phantom helps medical physicists to quantify, as well as track over time, potential distortions that can arise in MR images used for radiotherapy treatment planning. Developed by laser and radiotherapy QA specialist LAP, the THETIS phantom enables the multidisciplinary care team to deploy MRI systems safely in a radiotherapy context and, in so doing, maximize clinical effectiveness through the precision targeting of diseased tissue.

In the treatment suite, MRI delivers clinical upsides along multiple coordinates, not least its superior soft-tissue contrast (versus CT) and the ability to visualize a matrix of functional information – including diffusion processes, blood volume and oxygenation, and localized metabolic activity within tumour sites. Equally compelling is the fact that MRI interrogates the patient using non-ionizing radio waves – a major plus when treating children and in cases where serial imaging scans are needed to track tumour response through multiple radiation fractions.

“Because of those upsides, the adoption of MRI and MR-Linac systems in radiation therapy has grown massively in recent years,” explains Torsten Hartmann, director of product management for healthcare at LAP. “We developed THETIS to enable the radiation oncology team to generate MR images of the highest geometrical accuracy – detecting possible distortions of the MR images reliably and quickly.”

Such potential distortions have their origins in tiny perturbations to the uniformity of the MRI scanner’s magnetic field and the field gradients used to image the patient. By extension, when the MRI scanner’s imaging sequences are not optimized, problems can occur downstream and introduce errors in the patient’s treatment plan. “THETIS makes it easy to determine where distortions are affecting the image and whether the scanner’s magnetic field has changed over a period of time,” Hartmann adds.

Enabling independent QA

In terms of specifics, the THETIS phantom exploits a square grid of embedded silicon markers, each of which provides a strong, localized MR signal (and with 258 signal sources per measurement plate). The phantom – which is aligned to the isocentre or the image centre of the MRI scanner using LAP’s MRI laser systems and its integrated levelling aids – can detect residual image distortions from gradient nonlinearities or main-magnet inhomogeneities to ensure they are within acceptable limits. In this way, the silicon markers help the medical physicist to visualize the loss of geometric fidelity with distance from the magnet isocentre, preventing a potentially inaccurate view of organs located in the outer areas of the MR image.

Torsten Hartmann

“Of course, all MRI system vendors provide methods and algorithms for distortion correction,” notes Hartmann. “While that’s as it should be, a large-field phantom like THETIS underpins those all-important independent QA checks to ensure safe deployment of MRI systems in the radiotherapy setting.” Those QA checks begin with the commissioning of a new MRI scanner and characterization of the machine’s baseline imaging performance versus manufacturer specifications. Equally important, THETIS offers streamlined workflows when it comes to systematic QA of MR image distortions over the lifetime of the MRI scanner – ensuring, for example, the geometric fidelity of MR images after major hardware and software upgrades to the imaging system.

“Clinical teams need a granular view of how such upgrades affect MR image quality,” adds Hartmann. “At the same time, THETIS supports the regular QA monitoring of MR image quality – for example, as part of the monthly or quarterly checks of image distortion and how it changes over time.”

Collaborative innovation

Operationally, the clinical and commercial release of the THETIS phantom is the outcome of an R&D collaboration between the LAP product development team and the MRI technology division at Siemens Healthineers, Germany. The latter is increasingly providing dedicated MRI systems into the radiotherapy clinic and, as such, wants to offer a reliable and affordable image distortion phantom tailor-made for its MRI equipment portfolio.

“We moved quickly from prototyping and evaluation into product development and construction – just six months in all before we entered beta-testing,” explains Hartmann. Geographical proximity certainly helped to streamline the product innovation cycle, with LAP’s Nuremberg manufacturing facility just 20 km or so from the Siemens Healthineers MRI technology hub in Erlangen. “Key to successful delivery was being able to jointly test, iterate and optimize the THETIS phantom with our colleagues in the MRI R&D team at Siemens Healthineers,” Hartmann adds.

Worth noting, though, that the commercial phantom is vendor-agnostic, being optimized on Siemens Healthineers’ MRI systems, but also compatible with a range of open-bore scanners from other manufacturers. In the radiotherapy clinic, meanwhile, the phantom is equally versatile. According to Hartmann, THETIS is compatible with emerging MR-only treatment planning workflows as well as the established standard-of-care in which a fused CT-MRI dataset provides the MR information needed to outline the tumour volume and organs at risk, while the CT is used for dose calculation.

Further reading

Carri K Glide-Hurst et al. 2021 AAPM Task Group 284 report. Magnetic resonance imaging simulation in radiotherapy: considerations for clinical implementation, optimization, and quality assurance Med. Phys. 48 (7) e636

THETIS in brief

The THETIS 3D MR Distortion Phantom is designed for QA of MR images in radiation therapy and diagnostic settings. Key features include:

  • Modularity: expansion stages allow optimal adaptation to different system and workflow requirements.
  • MR-safety: the phantom contains no ferromagnetic components and, as such, is ideally suited to the MRI environment.
  • Easy handling: integrated levelling aids streamline alignment, while intuitive handling of the phantom simplifies 3D examination of the entire MRI space.
  • Figures of merit: 10 plates (maximum expansion stage); three extension modules; 3 T MRI-tested; 258 signal sources per measurement plate.

Telescope with large-aperture metalens images the Moon

Telescope made with a metalens

An important step towards the practical use of optical metasurfaces has been taken by researchers in the US. The team used a common semiconductor manufacturing process to produce a large aperture, flat metalens. Its optical performance was demonstrated by using it as the objective lens in a simple telescope that was aimed at the Moon. The telescope achieved superior resolving power and produced clear images of the surface of the Moon.

Telescopes have been used to peer out into the universe for more than 400 years. In the early 1600s, Galileo Galilei used a telescope to observe the moons of Jupiter and last year the James Webb Space Telescope began taking spectacular images of the cosmos.

The telescopes used today by professional astronomers tend to be large and bulky, which often puts limits on how and where they can be used. The size of these instruments is a result of their large apertures and often-complicated multi-element optical systems that are necessary to eliminate aberrations and to provide the desired high performance.

Engineered nanostructures

Optical metasurfaces offer a potential way to make telescopes and other optical systems smaller and simpler. These are engineered nanostructures that can be thought of as a series of artificial optical antennas (see figure). These antennas can manipulate light, changing, for example, its amplitude, phase, and polarization.

These metasurfaces can be engineered to focus light, thereby creating metalenses that can offer significant advantages over conventional optics. For example, the flat surfaces of metalenses are free of spherical aberrations and metalenses are ultrathin and low in weight when compared to conventional optics.

However, the production of metalenses is still in its infancy. Current fabrication methods are based on scanning systems such as electron-beam (e-beam) lithography and focused ion beam (FIB) techniques. These are slow, expensive, and restrict the size of metalenses to just a few millimetres. This makes large-volume production almost impossible and means that metalenses are currently expensive and too small for the large-aperture applications such as telescopes.

A meta-telescope

Now, researchers at Pennsylvania State University and the NASA-Goddard Space Flight Center have come up with a much better way of making metalenses. Their process can be scaled up for large-scale production and can be used to create metalenses with large aperture sizes that are suitable for telescope applications.

The team used deep-ultraviolet (DUV) lithography, which is a technique commonly used in the semiconductor industry. Their process involved patterning the top of a four-inch silica wafer. Their 80-mm-diameter meta-lens was divided into 16 parts that were combined by exposing the same patterns on different quadrants of the wafer. Pattern stitching and wafer rotation eliminated the need for an expensive single large mask that exposes the entire surface.

Intensity profile

The performance of the metalens was characterized by measuring the intensity profile of focused laser beams over a broad wavelength range spanning 1200–1600 nm. The tests showed that the metalens can tightly focus light close to the diffraction limit over the entire range, despite being designed to operate at 1450 nm. However, diffractive dispersion did vary the focal length throughout the wavelength range – a detrimental effect called chromatic aberration.

The resolving power of the metalens was tested by using it as an objective lens inside a telescope. The team used the telescope to successfully image various features of the Moon’s surface with a minimum resolving feature size of approximately 80 km. This is the best reported resolving power for this type of metalens so far.

Next-generation systems

Lead researcher Xingjie Ni at Pennsylvania State University believes that metasurfaces can be a game changer in optics, because their unprecedented capability for light manipulation makes them powerful candidates for next-generation optical systems. This, he says, is why his team is dedicated to advancing the capabilities of scalable, fabrication-friendly metasurfaces.

“We plan to improve our design techniques to achieve fabrication-imperfection-tolerant nanostructures. This will allow us to use high-volume manufacturing technology such as photolithography to make large scale metalenses working in the visible range and incorporate more complex nanoantenna designs, for example, freeform shaped nanoantennas, to compensate for chromatic aberration,” he tells Physics World.

Din Ping Tsai at the City University of Hong Kong was not involved in the research and he thinks that this work expands the working scenarios of metalenses and will inspire research on metalenses with large apertures. He says that DUV lithography could be used to achieve the high throughput manufacturing of low cost metalenses with reasonable resolution. This would bring the components into commercialization and make them part of our daily life in the coming years.

Tsai believes that the chromatic aberration in the Penn State metalens limits its use to monochromatic applications. He also points out that the design of large-area broadband achromatic meta-lens is still a big challenge and is in strong demand. In addition, he believes that a large mask is the preferred way to make metalenses in order to avoid stitching errors and to simplify the fabrication process.

The research is described in ACS Nano Letters.

MRI guidance reduces side effects of prostate cancer radiotherapy

Stereotactic body radiotherapy (SBRT) is an established treatment for prostate cancer. It involves delivering large daily doses of precisely targeted radiation in five or fewer fractions, traditionally using either planar X-ray or cone-beam CT images to guide the radiation delivery.

The prostate is a highly mobile target and it’s essential to account for its motion during irradiation to maximize treatment effectiveness. This is typically achieved by creating a planning target volume (PTV) that includes a margin around the prostate to ensure adequate target dosing. However, the high-dose regions of the PTV often overlap portions of the bladder, rectum and other nearby structures, which can cause side effects such as urinary, bowel and sexual dysfunction.

The recent introduction of MRI-guided linacs could help minimize the risk of such toxicities. MRI-linacs offer high soft-tissue contrast and the ability to track intra-fraction prostate motion directly (rather than relying on fiducial markers) and control the beam in real time during treatment. These advantages should enable the use of significantly smaller margins around the prostate. To date, however, the theoretical advantages of MRI-guided radiotherapy for prostate SBRT have not been demonstrated in a randomized clinical trial.

The MIRAGE (MRI-guided stereotactic body radiotherapy for prostate cancer) trial aims to address this shortfall and determine whether MRI-guided radiotherapy offers an evident benefit for patients. The phase III randomized clinical trial, led by Amar Kishan and Michael Steinberg at the University of California, Los Angeles (UCLA), enrolled men receiving SBRT for localized prostate cancer. Between May 2020 and October 2021, the trial randomized 156 patients to receive SBRT with either CT guidance (77 patients) or MRI guidance using the MRIdian system (79 patients).

Patients were treated with 40 Gy in five fractions, using planning margins of 4 mm in the CT arm and 2 mm in the MRI arm. The researchers note that this 2 mm margin is narrower than used in any previous large study. They hoped to show that this aggressive margin reduction could reduce toxic effects following SBRT.

“MRI guidance offers several advantages over standard CT guidance, most notably the ability to dramatically reduce planning margins, providing more focused treatment with less injury to nearby normal tissues and organs,” says Kishan in a press statement. “MRI technology is more costly than CT, both in terms of upfront equipment expenses and longer treatment times, which is one reason our study set out to determine if MRI-guided technology offers tangible benefits for patients.”

Improved outcomes

The results of the trial, described in JAMA Oncology, revealed that MRI guidance led to fewer toxicities and better quality-of-life, as judged by both patients and the doctors treating them.

In 154 patients available for follow-up, the incidence of acute grade 2 or greater genitourinary (GU) toxic effects was significantly lower following MRI- than CT-guided SBRT: 24.4% in the MRI group versus 43.4% in the CT group. Patients in the MRI group also had fewer acute grade 2 or greater gastrointestinal toxic effects: 0.0% versus 10.5%, respectively. In a multivariate analysis accounting for all candidate variables, the MRI-guided arm remained associated with a 60% reduction in odds of grade 2 or greater GU toxicity.

After 100 patients reached 90 or more days post-treatment, the researchers conducted an interim analysis. At this time, the incidence of acute grade 2 or greater GU toxic effects was significantly reduced in men receiving MRI-guided SBRT compared with those receiving CT-guided SBRT (24 of 51 versus 11 of 49). They re-estimated the required sample size as 154 patients and, as 156 patients had already been treated, closed the trial for further accrual.

“This is the first large scale SBRT trial to use a dose of 40 Gy to the PTV, which we felt was an appropriate dose given the anticipated risk level of the cohort we would be treating. Because dose is associated closely with toxicity, we knew beforehand that the estimates of toxicity we used to power the trial might be underestimates,” Kishan tells Physics World. “Thus, we stipulated an interim analysis should occur after 100 patients were eligible for analysis in order for us to formally re-evaluate the power considerations for the trial.”

One unique aspect of the study was its inclusion of patient-reported outcomes. Significantly fewer patients receiving MRI-guided SBRT experienced large increases in urinary symptoms. Similarly, far more patients experienced a clinically notable decrease in bowel-related quality-of-life with CT guidance.

The researchers note that longer term follow-up is necessary to determine whether these benefits persist, whether differences in late urinary or bowel toxic effects occur and to evaluate differences in sexual outcomes. They plan to continue to monitor toxicity outcomes and perform an analysis of 2-year patient-reported outcomes.

“MRI-guided radiation has apparent theoretical benefits in this treatment scenario, and it was important to conduct a rigorous comparison,” says Steinberg. “Given the significance of the outcomes realized, we’ve evolved our prostate cancer treatment approach at UCLA to preferentially utilize MRI-guided SBRT.”

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