Progress in Better Understanding Tokomak Fusion Physics

A team at the DIII-D National Fusion Facility led by a William & Mary physicist has made a significant advancement in physics understanding that represents a key step toward practical fusion energy.

The work, published in an article in the journal Nuclear Fusion, helps better explain the relationship among three variables – plasma turbulence, the transport of electrons through the plasma and electron density in the core. Because these factors are key elements of the fusion reaction, this understanding could significantly improve the ability to predict performance and efficiency of fusion plasmas, a necessary step toward achieving commercial fusion power plants.

They confirmed low collisionality improves electron density peaking through the formation of an internal barrier to particle movement through the plasma, which in turn altered the plasma turbulence. Previous work had suggested the effect might be due to plasma heating by neutral beam injection, but the experiments show that it was linked to particle transport and turbulence.

“We’ve known for some time that there is a relationship between core electron density, electron-ion collisions and particle movement in the plasma,” said William & Mary’s Saskia Mordijck, who led the multi-institutional research team at DIII-D. “Unfortunately, until now research has not been able to untangle that relationship from the other components that affect electron density patterns.”

DIII-D, which General Atomics operates as a national user facility for the Department of Energy’s Office of Science, is the largest magnetic fusion research facility in the country. It hosts researchers from more than 100 institutions across the globe, including 40 universities. The heart of the facility is a tokamak that uses powerful electromagnets to produce a doughnut-shaped magnetic vessel containment for confining a fusion plasma. In DIII-D, plasma temperatures more than 10 times hotter than the Sun are routinely achieved. At such extremely high temperatures, hydrogen isotopes can fuse together and release energy.

In a tokamak, fusion power is determined by temperature, plasma density and confinement time. Fusion gain, expressed as the symbol Q, is the ratio of fusion power to the input power required to maintain the reaction and is thus a key indicator of the device’s efficiency. At Q = 1, the breakeven point has been reached, but because of heat losses, self-sustaining plasmas are not reached until about Q = 5. Current systems have achieved extrapolated values of Q = 1.2. The ITER experiment under construction in France is expected to achieve Q = 10, but commercial fusion power plants will likely need to achieve even higher Q values to be economical.

Nuclear Fusion Journal – Collisionality driven turbulent particle transport changes in DIII-D H-mode plasmas

The results of the experimental dimensionless scan in this paper confirm that there is an increase in density peaking towards lower collisionality and that this can be partly linked to a shift in the turbulence regime from ITG towards TEM. However at the lowest collisionality, the changes in turbulence and transport are much more pronounced than expected from direct collisionality effect on the turbulence. In this paper, the collisionality is varied by a factor 5, while keeping other variables fixed. Additionally, a 3  Hz gas puff modulation is applied to modulate the electron density profile and extract the perturbed transport coefficients using two diagnostics. The transport analysis shows that the increase in density peaking at low ν * is linked to an increase in the inward particle pinch and not an increase in core fueling. These observations are not only in agreement with prior modeling scans of how turbulence changes as a function of collisionality and its impact upon the particle fluxes, but also with the multi-machine database (Fable E. et al 2010 Plasma Phys. Control. Fusion 52 015007) (Angioni C. et al 2003 Phys. Rev. Lett. 90 205003). The changes in turbulence across the collisionality scan were captured at large scale by the BES and at smaller scale by the DBS. A comparison with gradient-driven GENE simulations showed similar trends at both scales. Moreover, the changes observed in overall transport are in agreement with gradient-driven TGLF particle flux simulations. This indicates that TGLF/GENE when given the gradients as input, are able to reproduce the experimentally observed turbulence changes.

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