Fusion has long been considered the Holy Grail for sustainable power generation, and has promised to deliver clean, limitless, safe energy to the world. However, until now no fusion device has yet operated in breakeven conditions. That is, producing more fusion power than the external heating used to maintain the plasma at the needed temperature. Looking ahead at the design and construction of fusion power plants, scientists first need to understand the physics of burning plasmas in such reactor-relevant conditions. To this end, the Massachusetts Institute of Technology has partnered with the private company Commonwealth Fusion Systems to design, build and operate SPARC, a medium-size tokamak-type fusion device capable of sustaining plasmas that produce at least twice as much power than introduced in the plasma (Q> 2, in fusion jargon).

Historically, fusion devices have been designed based on empirical data from past experiments, looking back at decades of experimental plasma physics research and tens of existing machines to estimate the performance of proposed designs. However, thanks to world-wide efforts in developing and validating plasma physics theory and simulations, physics-based models of the SPARC tokamak are also being used during machine design and the planning of its research program. The combination of this physics knowledge along with empirical estimates are very beneficial to ensure the success of the SPARC program. Motivated by this, the processes that govern the performance of the SPARC tokamak have been modeled in state-of-the-art plasma physics simulations.

Burning plasmas can be seen as a soup of charged particles, electrons and ions, confined by strong magnetic fields inside a vacuum chamber. Fusion occurs when the temperature and density of the plasma are high enough. In SPARC, charged particles will be confined by very high magnetic fields, thanks to new high-temperature superconductor technology, and the external heating power to the plasma will be delivered by electromagnetic waves. The confinement of energy and particles in tokamaks like SPARC is determined predominantly by instabilities in the plasma at a wide range of spatial scales, while the heating requires the efficient propagation and absorption of electromagnetic waves in the hot plasma. Both the wave heating and the confinement of energy and particles have been modelled in plasma physics simulations of SPARC. These expensive computer simulations are capable of modeling fusion reactions and also account for the plasma self-heating by the reaction products, providing a comprehensive view of the complicated processes and synergies that take place in the core of the SPARC tokamak. This allows the evaluation of fusion performance from a theoretical point of view that can be compared to the empirical estimates.

Both empirical predictions with the conservative assumptions used for ITER and physics-based simulations predict that SPARC will readily generate ten times the energy that is required to heat up the plasma (Q ≈ 10), significantly above its mission objective (Q> 2). This provides ample margin and confidence that the SPARC program will be successful in reaching its scientific goals and will allow the thorough study of burning plasma regimes in compact, high-field fusion devices. This will be a crucial step that, along with the validation of present plasma physics models and simulations, will pave the way towards the design and optimization of affordable fusion power plants.

Read the Introduction to the SPARC special issue: ‘Status of the SPARC Physics Basis’ by Martin Greenwald

All seven articles in the Journal of Plasma Physics Special Issue: Status of the SPARC Physics Basis are open access and can be found at www.cambridge.org/plasma/sparc