System Bits: Nov. 27

Silent, lightweight aircraft powered by ionic wind
Instead of propellers or turbines, MIT researchers have built and flown the first-ever aircraft with no moving parts that is powered by an “ionic wind” — a silent but mighty flow of ions that is produced aboard the plane, and that generates enough thrust to propel the plane over a sustained, steady flight.

A general blueprint for an MIT plane propelled by ionic wind. The system may be used to propel small drones and even lightweight aircraft, as an alternative to fossil fuel propulsion. 

Source: MIT Electric Aircraft Initiative

The team emphasized that unlike turbine-powered planes, the aircraft does not depend on fossil fuels to fly. And unlike propeller-driven drones, the new design is completely silent.

Steven Barrett, associate professor of aeronautics and astronautics at MIT said, “This is the first-ever sustained flight of a plane with no moving parts in the propulsion system. This has potentially opened new and unexplored possibilities for aircraft which are quieter, mechanically simpler, and do not emit combustion emissions.”

In the near-term, Barrett expects such ion wind propulsion systems could be used to fly less noisy drones. Further out, ion propulsion could be paired with more conventional combustion systems to create more fuel-efficient, hybrid passenger planes and other large aircraft.

Perhaps not surprisingly, the inspiration for the team’s ion plane comes partly from the movie and television series, “Star Trek,” which Barrett watched avidly as a kid. He said he was particularly drawn to the futuristic shuttlecrafts that effortlessly skimmed through the air, with seemingly no moving parts and hardly any noise or exhaust. “This made me think, in the long-term future, planes shouldn’t have propellers and turbines. They should be more like the shuttles in ‘Star Trek,’ that have just a blue glow and silently glide.”

So, about nine years ago, Barrett started looking for ways to design a propulsion system for planes with no moving parts, eventually coming upon “ionic wind,” also known as electroaerodynamic thrust — a physical principle that was first identified in the 1920s and describes a wind, or thrust, that can be produced when a current is passed between a thin and a thick electrode. If enough voltage is applied, the air in between the electrodes can produce enough thrust to propel a small aircraft.

For years, electroaerodynamic thrust has mostly been a hobbyist’s project, and designs have for the most part been limited to small, desktop “lifters” tethered to large voltage supplies that create just enough wind for a small craft to hover briefly in the air. It was largely assumed that it would be impossible to produce enough ionic wind to propel a larger aircraft over a sustained flight.

“It was a sleepless night in a hotel when I was jet-lagged, and I was thinking about this and started searching for ways it could be done,” he recalls. “I did some back-of-the-envelope calculations and found that, yes, it might become a viable propulsion system,” Barrett says. “And it turned out it needed many years of work to get from that to a first test flight.”

For further details and to watch a video of the aircraft in flight, click here.

The team is now working to increase the efficiency of the design, to produce more ionic wind with less voltage. The researchers are also hoping to increase the design’s thrust density — the amount of thrust generated per unit area. Currently, flying the team’s lightweight plane requires a large area of electrodes, which essentially makes up the plane’s propulsion system. Ideally, Barrett would like to design an aircraft with no visible propulsion system or separate controls surfaces such as rudders and elevators.

AI outperforms radiologists screening X-rays for certain diseases
According to a new study led by Stanford University researchers, a new artificial intelligence algorithm can reliably screen chest X-rays for more than a dozen types of disease in less time than it takes to read this sentence.

The CheXNeXt algorithm, trained to detect 14 different pathologies, is believed to be the first to simultaneously evaluate X-rays for a multitude of possible maladies and return results that are consistent with the readings of radiologists, the study says.

Specifically, for 10 diseases, the algorithm performed just as well as radiologists; for three, it underperformed compared with radiologists; and for one, the algorithm outdid the experts.

Matthew Lungren, MD, MPH, assistant professor of radiology said, “Usually, we see AI algorithms that can detect a brain hemorrhage or a wrist fracture — a very narrow scope for single-use cases, but here we’re talking about 14 different pathologies analyzed simultaneously, and it’s all through one algorithm.”

The goal is to eventually leverage these algorithms to reliably and quickly scan a wide range of image-based medical exams for signs of disease without the backup of professional radiologists. And while that may sound disconcerting, the technology could eventually serve as high-quality digital “consultations” to resource-deprived regions of the world that wouldn’t otherwise have access to a radiologist’s expertise.

Likewise, there’s an important role for AI in fully developed health care systems too, Lungren added. Algorithms like CheXNeXt could one day expedite care, empowering primary care doctors to make informed decisions about X-ray diagnostics faster, without having to wait for a radiologist.

Researchers from Duke University and from the University of Colorado also contributed to the study.

Quantum building block
Demonstrating the presence of collective spin excitations, which are intrinsic features of all quantum objects, a team of U.S. and Korean physicists has found the first evidence of a 2D material that can become a magnetic topological insulator even when it is not placed in a magnetic field.

Rice University’s Pengcheng Dai, co-author of a study about the material said, “Many different quantum and relativistic properties of moving electrons are known in graphene, and people have been interested, ‘Can we see these in magnetic materials that have similar structures?’”

Dai, whose team included scientists from Rice, Korea University, Oak Ridge National Laboratory (ORNL) and the National Institute of Standards and Technology, said the chromium triiodide (CrI3) used in the new study “is just like the honeycomb of graphene, but it is a magnetic honeycomb.”

Chromium triiodide produced in a high-temperature furnace at Rice University. In neutron-scattering experiments, the material behaved like a magnetic topological insulator.
Source: Rice University

In experiments at ORNL’s Spallation Neutron Source, CrI3 samples were bombarded with neutrons, and a spectroscopic analysis taken during the tests revealed the presence of collective spin excitations called magnons. Spin, an intrinsic feature of all quantum objects, is a central player in magnetism, and the magnons represent a specific kind of collective behavior by electrons on the chromium atoms.

“The structure of this magnon, how the magnetic wave moves around in this material, is quite similar to how electron waves are moving around in graphene,” said Dai, professor of physics and astronomy and a member of Rice’s Center for Quantum Materials (RCQM).

The team explained that both graphene and CrI3 contain Dirac points, which only exist in the electronic band structures of some two-dimensional materials. Named for Paul Dirac, who helped reconcile quantum mechanics with general relativity in the 1920s, Dirac points are features where electrons move at relativistic speeds and behave as if they have zero mass. Dirac’s work played a critical role in physicists’ understanding of both electron spin and electron behavior in 2D topological insulators, bizarre materials that attracted the 2016 Nobel Prize in Physics.

Electrons cannot flow through topological insulators, but can zip around their one-dimensional edges on “edge-mode” superhighways. The materials draw their name from a branch of mathematics known as topology, which 2016 Nobelist Duncan Haldane used to explain edge-mode conduction in a seminal 1988 paper that featured a 2D honeycomb model with a structure remarkably similar to graphene and CrI3.

“The Dirac point is where electrons move just like photons, with zero effective mass, and if they move along the topological edges, there will be no resistance,” said study co-author Jae-Ho Chung, a visiting professor at Rice and professor of physics at Korea University in Seoul, South Korea. “That’s the important point for dissipationless spintronic applications.”

Spintronics is a growing movement within the solid-state electronics community to create spin-based technologies for computation, communicate and information storage and more. Topological insulators with magnon edge states would have an advantage over those with electronic edge states because the magnetic versions would produce no heat, Chung added.

The evidence for topological spin excitations in the CrI3 is particularly intriguing because it is the first time such evidence has been seen without the application of an external magnetic field.