Tokamak experiments exceed plasma density limit, offering new approach to nuclear fusion ignition

Good news! This could be a breakthrough! Are we close to reaching a burning plasma or the sun on Earth?

"Researchers working on China's fully superconducting Experimental Advanced Superconducting Tokamak (EAST) have experimentally accessed a theorized "density-free regime" for fusion plasmas, achieving stable operation at densities well beyond conventional limits. ...

By realizing a novel high-density operating scheme on EAST, the team demonstrated that plasma density, long constrained by empirical limits in tokamak operation, can be substantially extended without triggering disruptive instabilities. ...

For deuterium-tritium fusion reactions, plasmas must be heated to an optimal temperature of around 13 keV (150 million kelvin). Under these conditions, thermonuclear power scales with the square of fuel density.

However, in conventional tokamak operation, plasma density has long been restricted by an empirical upper limit. Exceeding this limit often leads to instabilities that disrupt plasma confinement and endanger tokamak operation, posing a major challenge to improving fusion performance.

The recent development of the plasma–wall self organization (PWSO) theory provides a novel perspective on understanding the disruptive density limit.  ...The theory predicts that a new density-free regime could be accessed by achieving a delicate balance between the plasma and the metallic walls of the device, which are dominated by physical sputtering. ..."

"... While long-term international fusion research has indicated that the physical processes triggering the density limit occur at the plasma-wall boundary region, the underlying mechanisms have remained unclear.

In this study, the Chinese research team developed a self-organized plasma-wall interaction theoretical model. Through this model, they identified the critical role of radiation instability induced by boundary impurities in triggering the density limit, thereby elucidating the underlying mechanism.

Building on this theoretical insight, researchers experimentally controlled the plasma to exceed the density limit and successfully guided it into a new "density-free zone."

These results mark the first experimental confirmation of such a zone in tokamaks. ..."

From the abstract:

"High plasma density operation is crucial for a tokamak to achieve energy breakeven and burning plasma. However, there is often an empirical upper limit of electron density in tokamak operation, namely, the Greenwald density limit ng

, above which tokamaks generally disrupt.

Achieving high-density operation above the density limit has been a long-standing challenge in magnetic confinement fusion research.

Here, we report experimental results on the Experimental Advanced Superconducting Tokamak (EAST) achieving line-averaged electron density in the range of (1.3 to 1.65) ng, significantly above the typical EAST operational range of (0.8 to 1.0) ng. This is performed with electron cyclotron resonance heating (ECRH)–assisted ohmic start-up and sufficiently high initial neutral density.

These experiments are shown to operate in the density-free regime first predicted by a recent plasma-wall self-organization theory. These results suggest a promising scheme for substantially increasing the density limit in tokamaks, a critical advancement toward achieving burning plasma."

Tokamak experiments exceed plasma density limit, offering new approach to fusion ignition

China's "artificial sun" Experiment Finds Way to Break Fusion Plasma Density Limit (original news release)

Accessing the density-free regime with ECRH-assisted ohmic start-up on EAST (open access)

With a new molecule-based method, physicists peer inside an atom’s nucleus

Amazing stuff! However, can this approach be transferred to atoms of other elements?

"Physicists at MIT have developed a new way to probe inside an atom’s nucleus, using the atom’s own electrons as “messengers” within a molecule.

In a study appearing today in the journal Science, the physicists precisely measured the energy of electrons whizzing around a radium atom that had been paired with a fluoride atom to make a molecule of radium monofluoride. They used the environments within molecules as a sort of microscopic particle collider, which contained the radium atom’s electrons and encouraged them to briefly penetrate the atom’s nucleus. ... 

The team’s new molecule-based method offers a table-top alternative to directly probe the inside of an atom’s nucleus.

Amazing stuff!

Within molecules of radium monofluoride, the team measured the energies of a radium atom’s electrons as they pinged around inside the molecule. They discerned a slight energy shift and determined that electrons must have briefly penetrated the radium atom’s nucleus and interacted with its contents. As the electrons winged back out, they retained this energy shift, providing a nuclear “message” that could be analyzed to sense the internal structure of the atom’s nucleus.

The team’s method offers a new way to measure the nuclear “magnetic distribution.” In a nucleus, each proton and neutron acts like a small magnet, and they align differently depending on how the nucleus’ protons and neutrons are spread out. The team plans to apply their method to precisely map this property of the radium nucleus for the first time. What they find could help to answer one of the biggest mysteries in cosmology: Why do we see much more matter than antimatter in the universe? ..."

From the editor's summary and the abstract:

"Editor’s summary

Precision molecular spectroscopy is increasingly being used to probe symmetry violations relevant to fundamental physics studies. Of particular interest are molecules containing heavy radioactive nuclei, such as the pear-shaped radium isotope 225Ra. Wilkins et al. performed laser spectroscopy measurements of the hyperfine structure of the radium monofluoride molecule, which is especially challenging given the molecule’s short lifetime. In combination with calculations, the researchers were able to test models of magnetization distribution inside the radium nucleus. Their findings may lead to improved tests of fundamental symmetries. ...

Abstract

Precise experimental control and interrogation of molecules and calculations of their structure are enriching the investigation of nuclear and particle physics phenomena. Molecules containing heavy, octupole-deformed nuclei, such as radium, are of particular interest.

Here, we report precision laser spectroscopy measurements and theoretical calculations of the structure of the radioactive radium monofluoride molecule 225Ra19F.

Our results reveal fine details of the short-range electron-nucleus interaction, indicating the high sensitivity of this molecule to the distribution of magnetization, within the radium nucleus.

These results provide a stringent test of the description of the electronic wave function inside the nuclear volume, highlighting the suitability of these molecules for investigating subatomic phenomena."

With a new molecule-based method, physicists peer inside an atom’s nucleus | MIT News | Massachusetts Institute of Technology "An alternative to massive particle colliders, the approach could reveal insights into the universe’s starting ingredients."

Observation of the distribution of nuclear magnetization in a molecule (no public access)

Observation of the distribution of nuclear magnetization in a molecule (open access)

Lead-208's doubly magic nucleus defies expectations with surprising shape properties with possibly far reaching implications

Amazing stuff!

"In a surprising turn of events, an international team of scientists has found that lead-208 (208Pb), the heaviest known "doubly magic" nucleus, exhibits unexpected shape characteristics that current nuclear models fail to predict accurately. ...

Doubly magic refers to nuclei that have complete shells of both protons (82) and neutrons (126), a configuration that should theoretically favor a perfectly spherical shape. ...

The researchers conclusively demonstrated that both excited states they studied exhibit large, negative spectroscopic quadrupole moments, indicating the nucleus prefers an elongated (prolate) rather than flattened shape.

When compared to predictions from three different theoretical approaches—the nuclear shell model, density functional theory, and Hartree-Fock calculations—none could reproduce the sign and magnitude of the observed deformation. ...

lead-208 plays a crucial role in understanding how heavy elements are formed in cosmic events like neutron star mergers, i.e., the r-process. Therefore, the impact extends from nuclear studies to stellar evolution. ..."

"... Lead-208 is exceptionally stable due to being a "doubly magic" nucleus – and is the heaviest that we know of. However, a new study published in Physical Review Letters used a high-precision experimental probe to examine its shape and found that rather than being perfectly spherical, the nucleus of lead-208 is slightly elongated, resembling a rugby ball (prolate spheroid). ..."

From the abstract:

"Lead-208 is the heaviest known doubly magic nucleus and its structure is therefore of special interest. Despite this magicity, which acts to provide a strong restorative force toward sphericity, it is known to exhibit both strong octupole correlations and some of the strongest quadrupole collectivity observed in doubly magic systems.

In this Letter, we employ state-of-the-art experimental equipment to conclusively demonstrate, through four Coulomb-excitation measurements, the presence of a large, negative, spectroscopic quadrupole moment for both the vibrational octupole 3−1 and quadrupole 2+1 state, indicative of a preference for prolate deformation of the states. The observed quadrupole moment is discussed in the context of the expected splitting of the 3−⊗3− two-phonon states, due to the coupling of the quadrupole and octupole motion. These results are compared with theoretical values from three different methods, which are unable to reproduce both the sign and magnitude of this deformation. Thus, in spite of its well-studied nature,  208 Pb remains a puzzle for our understanding of nuclear structure."

Lead-208's doubly magic nucleus defies expectations with surprising shape properties

Breakthrough study challenges long-held beliefs about the shape of atomic nuclei (original news release) "An international research collaboration led by the University of Surrey’s Nuclear Physics Group has overturned the long-standing belief that the atomic nucleus of lead-208 (²⁰⁸Pb) is perfectly spherical. The discovery challenges fundamental assumptions about nuclear structure and has far-reaching implications for our understanding of how the heaviest elements are formed in the universe."

Deformation and Collectivity in Doubly Magic 208 Pb (open access)

MIT: Visualizing the Nucleus

Recommendable!

U.S. back in race to make unknown, superheavy elements

Good news!

"... From 1936 through 1976, LBNL [Lawrence Berkeley National Laboratory] used its atom smashers to discover 16 elements, from element 43 (technetium) to 106 (seaborgium). But in the decades that followed the axis of superheavy research shifted to facilities in Germany, Japan, and Russia, which collectively discovered the last 12 elements. ..."

"Key Takeaways

• Scientists at Berkeley Lab have successfully made known element 116 (livermorium) using a titanium beam for the first time, a breakthrough that enables the lab to try to make new element 120.
• If discovered, element 120 would be the heaviest atom ever created and fall on the 8th row of the periodic table.
• Element 120 is near the theorized “island of stability,” where superheavy elements could be long-lived – enabling scientists to better study them.

...
The recipe for making superheavy elements is simple in theory. You smash together two lighter elements that, combined, have the number of protons you want in your final atom. It’s basic math: 1+2=3.
In practice, of course, it’s incredibly difficult. It can take trillions of interactions before two atoms fuse successfully, and there are limitations on what elements can reasonably be turned into a particle beam or target. ..."

U.S. back in race to forge unknown, superheavy elements | Science | AAAS

A New Way to Make Element 116 Opens the Door to Heavier Atoms Researchers at Berkeley Lab’s 88-Inch Cyclotron successfully made superheavy element 116 using a beam of titanium-50. That milestone sets the team up to attempt making the heaviest element yet: 120.

From the archives: Robert Oppenheimer in 1965 on if the bomb was necessary

Very recommendable! This interview was two years before his death! You will see an emotional scientist in doubt. He pointed out how the enemies of the Cold War changed their behavior to avoid a nuclear Armageddon. And how would China avoid a misunderstanding.

Can protons decay?

Very recommendable! Estimated lifetime of a single proton 10^34 years. Expected lifetime of any of 10^34 protons?

How Does The Nucleus Hold Together?

Very recommendable! Mind boggling!

Abrupt changes in atomic nucleus properties at magic number

Amazing stuff! Science is not without its moments! 😄 Will new nuclear theories be needed?

"... to measure how the nuclear electromagnetic properties of indium isotopes evolve when an extreme number of neutrons are added to the nucleus. These nuclei do not exist in nature, and once created, their lifetimes [are extremely short] ... 
When measuring a [exotic nuclei] with a certain “magic” number of neutrons — 82 — the magnetic field of the nucleus exhibited a drastic change, and the properties of these very complex nuclei appear to be governed by just one of the protons of the nucleus. ...
The motion of protons and neutrons orbiting inside the atomic nucleus generates a magnetic field, effectively turning the nucleus into a femtometre-scale (one-quadrillionth of a meter) magnet. Understanding how nuclear electromagnetism emerges from the underlying fundamental forces of nature is one of the major open problems of nuclear physics. ..."

"In spite of the high-density and strongly correlated nature of the atomic nucleus, experimental and theoretical evidence suggests that around particular ‘magic’ numbers of nucleons, nuclear properties are governed by a single unpaired nucleon. A microscopic understanding of the extent of this behaviour and its evolution in neutron-rich nuclei remains an open question in nuclear physics. The indium isotopes are considered a textbook example of this phenomenon, in which the constancy of their electromagnetic properties indicated that a single unpaired proton hole can provide the identity of a complex many-nucleon system. Here we present precision laser spectroscopy measurements performed to investigate the validity of this simple single-particle picture. Observation of an abrupt change in the dipole moment at N = 82 ... To investigate the microscopic origin of these observations, our work provides a combined effort with developments in two complementary nuclear many-body methods: ab initio valence-space in-medium similarity renormalization group and density functional theory (DFT). We find that the inclusion of time-symmetry-breaking mean fields is essential for a correct description of nuclear magnetic properties, which were previously poorly constrained. These experimental and theoretical findings are key to understanding how seemingly simple single-particle phenomena naturally emerge from complex interactions among protons and neutrons."

A new spin on nuclear magnetic moments | MIT News | Massachusetts Institute of Technology New results from researchers at MIT reveal an unexpected feature of atomic nuclei when a “magic” number of neutrons is reached.

Nuclear moments of indium isotopes reveal abrupt change at magic number 82