Fusion for Energy

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With the commissioning of large machines like the ITER tokamak, controlled fusion is poised to make huge step forward towards mastering the energy of the stars for a civil usage. The steady international progress regarding the achieved fusion performance gain – the ratio of the generated fusion power over the injected power – relies on our ability to understand and predict the turbulent transport and confinement properties of the plasma. Core transport studies in tokamak plasmas have now reached maturity with the development of state-of-the-art first-principle-based codes, using the 5-dimensional gyrokinetic description. However, despite their numerous successes to date, their predictive capabilities are still constrained with respect to the energy content in particular in optimized discharges. Challenging this gap requires pushing gyrokinetic modelling towards the edge region of the container vessel, and as far as possible addressing edge and core transport on an equal footing, which makes nonlinear simulations mandatory. In EoCoE-II the goal for the fusion team is to bridge the gap between gyrokinetic core transport modelling and edge plasma physics for reliable predictions of fusion performance, which will require a number of numerical and physics bottlenecks to be overcome. The objective is to develop a new numerical tool to address the core edge issue, which will consist of refactoring and rewriting the flagship gyrokinetic code Gysela (its new name will be GyselaX), targeting the disruptive use of billions of computing cores expected in exascale-class supercomputers. This will be complemented by the upgrade of companion codes Tokam3X/Soledge2D, and GENE, to provide critical inputs both regarding numerical developments and physics issues.

Gysela (http://gyseladoc.gforge.inria.fr/) is a 5D full-f (regarding Vlasov equations) and flux-driven gyrokinetic Fortran parallel code that solves Vlasov (ions and electrons) and Poisson (electric potential) equations to simulate plasma turbulence and transport in Tokamak devices. GyselaX developments include the replacement of the current Gysela code will be initiated to consider the whole tokamak from the core to the edge with advanced geometric constraints would require so deep refactoring of the existing F90 code base that PIs decided to rewrite it in C++, with a special effort on IO and a scalable solver for their Poisson like equation.

The periphery of the confined plasma in tokamaks is characterized by plasma-wall interactions which govern many of its properties. This region, called the SOL (Scrape-Off Layer) is not a mere boundary condition for the core plasma: it is suspected – and partly observed in certain regimes – to impact fusion performance, and it controls the way power is conducted to and deposited on the target plates of the divertor (the element specifically designed to handle the large power out fluxes and particle exhaust). 

Immersed boundary conditions – using an adapted version of the penalization technique already operational in the companion fluid code Tokam3X – have been implemented and successfully tested in the version of GYSELA with adiabatic electrons. Both Vlasov and Poisson equations have been modified: mask functions have been implemented to account for the presence of a limiter – a perfect sink for the plasma – and for the expected electron response in the SOL. Preliminary tests especially show the buildup of a well of the radial electric field in the vicinity of the core-SOL boundary, as experimentally observed in experiments. This is very good news, since such a sheared electric field, if large enough, can lead to improved confinement regimes in tokamak plasmas, the so-called H-mode.”

All the EoCoE-I and EoCoE-II publications are available here (OpenAIRE).