In a radiation-controlled lab in Japan, a compact device no bigger than a refrigerator is helping scientists solve one of nuclear fusion's most persistent problems.
For decades, scientists have pursued nuclear fusion as the holy grail of clean energy. Yet even as we approach the dawn of the ITER era, one critical challenge persists: finding materials that can withstand fusion's extreme conditions.
The Compact Divertor Plasma Simulator (CDPS), developed at Tohoku University's International Research Center for Nuclear Materials Science, represents a breakthrough in addressing this very challenge. This ingenious machine allows researchers to study how fusion reactor materials behave when bombarded by plasma—especially after they've been damaged by neutron radiation 1 .
In a fusion reactor, the divertor serves as the exhaust system—a component that manages extreme heat and particle fluxes while removing helium ash and other impurities from the plasma. It's one of the most brutally stressed components in any fusion device, facing heat loads comparable to the surface of the sun.
Divertors face temperatures approaching 10 million °C and particle fluxes of up to 10²⁴ m⁻²s⁻¹
The interaction between the plasma and the wall materials determines both the lifetime of reactor components and the safety of the entire facility 3 . When fast neutrons from fusion reactions damage the wall material, they create microscopic defects that act as traps for radioactive tritium fuel. This not only depletes valuable fuel but also creates serious safety concerns .
"Both the ITER experiment currently under construction and DEMO, the first demonstration reactor, will bring about particular challenges," notes Forschungszentrum Jülich, highlighting the need for advanced plasma-wall interaction studies 3 .
Traditional plasma research often required massive, expensive facilities. The CDPS revolutionizes this field through its elegant compact design while maintaining impressive performance metrics:
With deuterium and/or helium plasmas
Exceeding 10¹⁸ m⁻³
Approximately 10²² m⁻²s⁻¹ 1
What truly sets the CDPS apart is its location within a radiation-controlled area, allowing scientists to safely test materials that have been previously irradiated with neutrons 1 . This capability bridges a critical gap in fusion materials research.
| Component | Function | Research Importance |
|---|---|---|
| Sample-Carrier System | Transfers samples between plasma exposure and analysis without air exposure | Prevents oxidation, minimizes delay before analysis 1 |
| Temperature Control System | Adjusts cooling air flow to sample holder | Maintains sample temperature within 5°C during plasma exposure 1 |
| Infrared Heater | Heats samples for Thermal Desorption Spectroscopy (TDS) | Measures how much hydrogen fuel is trapped in materials 1 |
| Neutron-Irradiated Materials | Test samples damaged by neutron radiation in fission reactors | Simulates actual fusion reactor damage patterns 3 |
In a crucial experiment, researchers used the CDPS to expose both pristine and neutron-irradiated tungsten samples to deuterium plasma 1 . Tungsten is ITER's chosen material for the most heat-stressed regions, prized for its high melting point and historically low fuel retention properties .
Researchers prepared an ITER-like tungsten sample that had been irradiated with neutrons in a fission reactor to 0.06 dpa (displacements per atom), simulating years of damage in a fusion reactor 1 .
The sample was exposed to steady-state deuterium plasma inside the CDPS, with temperature precisely controlled throughout the process 1 .
Using the specialized sample-carrier system, the plasma-exposed sample was transferred directly to an infrared heater for TDS analysis without air exposure 1 .
Scientists gradually heated the sample while measuring the deuterium released at different temperatures, creating a "thermal desorption spectrum" that reveals how much fuel was trapped and how strongly it was bound 1 .
The results were striking. The neutron-irradiated tungsten retained significantly more deuterium than pristine tungsten, with the TDS spectrum broadening toward higher temperatures 1 . This indicated that the deuterium was being trapped more strongly in the radiation-damaged material.
| Material Property | Pristine Tungsten | Neutron-Irradiated Tungsten (0.06 dpa) |
|---|---|---|
| Deuterium Retention | Lower | Significantly increased 1 |
| TDS Spectrum | Normal | Broadened toward high temperatures 1 |
| Trapping Strength | Standard | Enhanced, with deuterium bound more tightly 1 |
The relationship between plasma exposure time and deuterium retention followed a clear mathematical pattern: total deuterium retention was proportional to the square root of exposure time 7 . This provides valuable predictive capability for estimating tritium buildup in future reactors.
Interactive chart showing the relationship between exposure time and deuterium retention would appear here
The CDPS represents part of a global research initiative to solve the plasma-wall interaction challenge. Similar research is underway at:
A linear plasma device investigating hydrogen retention in tungsten and characterizing low-activation steels 3
Studying plasma detachment and instabilities around recombination fronts 7
A new device under construction that will combine plasma exposure with transient thermal loads on radioactive samples 3
Planning to expose various neutron-irradiated materials, including advanced ceramics, to tokamak plasmas 8
The CDPS and similar devices provide something previously unavailable to fusion scientists: the ability to study synergistic effects of neutron damage and plasma exposure. In actual fusion reactors, these effects don't occur in isolation—they interact in complex ways that can accelerate material degradation 3 .
Understanding these interactions is crucial for designing future fusion power plants where strict limits on tritium retention must be maintained for safety reasons . The knowledge gained from the CDPS experiments directly informs the design of DEMO, the planned demonstration reactor that will follow ITER.
Understanding tritium retention is critical for maintaining a closed fuel cycle
As research continues, each discovery brings us closer to solving the materials challenge that has long hindered practical fusion energy. The CDPS exemplifies how targeted, compact research tools can provide crucial insights that massive, billion-dollar facilities cannot—proving that sometimes, the most powerful solutions come in small packages.
The journey to viable fusion energy continues, with each experiment lighting the way forward—one plasma-particle interaction at a time.
References will be listed here in the final publication.