The world’s biggest particle accelerator, the Large Hadron Collider (LHC), sits in a circular tunnel about one hundred meters beneath the Swiss French border near Geneva. It is huge—some 17 kilometers in circumference—and capable of accelerating sub-atomic particles to energies of 10^12 electronVolts (Tera eV or TeV), the highest ever achieved.
Constructed in the 1990s and switched on in the noughties, the LHC is getting old and physicists now want to smash particles together at even higher energies to see if anything new emerges from the wreckage. The problem is that these higher energies generally require longer tunnels to house bigger, more energy hungry accelerators, which are all difficult and expensive to build.
So physicists are looking for cheaper, smaller machines that can achieve higher energy in a tiny space and at a fraction of the cost.
Now Bifeng Lei at the University of Liverpool in the UK, and colleagues, say they have worked out in principle how to achieve 10^15 electronVolts (Peta eV or PeV) in a tabletop-sized device. Their machine could pave the way for a new generation of compact accelerators that could help study the behavior of matter under ultra-high electric fields, relevant to both particle physics and astrophysics.
“This work represents a promising avenue for the development of ultra-compact, high-energy particle accelerators,” they say.
Particle accelerators like the LHC work by repeatedly propelling charged particles through electromagnetic fields, gradually increasing their energy with each pass. The acceleration of charged particles takes place inside cavities filled with powerful electromagnetic waves of radio frequency. In effect, the particles accelerate by surfing on these waves.
However, powerful radio-frequency waves are hard to generate, requiring expensive superconducting cavities cooled to the temperature of liquid helium.
Another way to accelerate particles is inside a plasma. The trick here is carve a path through the plasma using a laser or electron beam and then allow charged particles to “surf” on the resultant wake.
So-called wakefield accelerators are more compact and energy efficient. But their accelerating power is limited by the density of the plasma, which is typically a gaseous substance with fewer than 10^18 particles per cubic centimeter. That’s significantly lower than the density of free electrons in a metal which can be as high as 10^24 per cubic centimeter.
It’s easy to imagine then that metals must be excellent particles accelerators. However, physicists do not yet have x-ray lasers powerful enough to carve a path through such high-density plasmas in metals, so they have yet to be exploited.
The breakthrough that Lei and co have made is to work out how to exploit similar plasma densities in an entirely different material – an array of carbon nanotubes.
In theory, the walls of carbon nanotubes house a sea of degenerate electrons that have a similar density to metals. But they also have a hollow, vacuum-filled center that the electrons can move into, if nudged powerfully enough.
So the materials they investigate consist of an array of carbon nanotubes with an inner hole, like a packet of dry spaghetti with some strands removed from the center to create a pathway throughout.
The team then simulate the effect of beaming a pulse of electrons through this passageway, using the surrounding carbon nanotubes as waveguides. The beam interacts with the electrons in the walls of the nanotubes, forcing them outwards as it passes and then back to their original position afterwards.
This sets up a powerful electric field inside the carbon nanotube that follows the electron beam as it moves. It is this electric field that can accelerate other charged particles. This wakefield acceleration mechanism, already explored in plasma-based accelerators, takes on a new and highly efficient form within the confined geometry of nanotubes.
In simulations, the researchers show that this setup can generate acceleration gradients in the range of hundreds of TeV per meter—orders of magnitude greater than conventional RF accelerators, like those in the LHC. “In principle, electrons can be accelerated to PeV energies over distances of several meters,” say Lie and co.
The team map out how currently available facilities at CERN and other particle physics laboratories could be used to test the idea in practice.
However, there are some potential limitations. One is that if the field inside the nanotubes is too great, the electrons can be blown out altogether and so do not return to their original positions and do not set up an accelerating field. So careful calibration will be needed to prevent such blowouts.
Another is that researchers must create a highly compact and dense electron pulse to pass through the carbon nanotube passageway to begin with. Pulses of this density may soon be possible with state-of-the-art equipment at the world’s leading particle physics labs.
If these experiments are successful, carbon nanotube accelerators could revolutionize several fields. Powerful compact accelerators could allow new particle physics experiments without requiring kilometer-scale infrastructure.
Miniaturized accelerators could advance radiation therapy for cancer treatment, providing high-energy electron or ion beams with unprecedented precision. The same kind of device could also be used for advanced materials processing, non-destructive testing, and security scanning or even as a novel propulsion technology for spacecraft.
The hugely powerful electric fields inside these machines could also allow physicists to reproduce the conditions inside certain astrophysical phenomena.
Lie and co are optimistic about their potential. “Carbon nanotube-based solid-state plasma accelerator offers transformative potential for advancing the development of ultra-compact particle accelerators, opening new avenues for various advanced applications.”
Ref: Carbon Nanotube-Based 100s TeV/m-Level Particle Accelerators Driven by High-density Electron Beams : arxiv.org/abs/2502.08498v1
