Nuclear Fusion Simulations at Exascale - Nu-FuSe

目前世界范围内的能源消耗在20世纪增长了20倍，而且增长速度没有显示出饱和的迹象。在目前的15太瓦负荷中，80-90%来自化石燃料，但随着石油峰值即将到来，煤炭供应有限，能源生产的巨大变化至关重要。可再生能源将对未来的能源生产做出贡献，但它们通常与季节和地理需求无关——而核裂变引发了重大的环境和政治担忧。另一方面，核聚变有望以低污染的方式可持续地产生很大一部分世界能源需求。然而，设计这样一个反应堆的科学和工程挑战是巨大的，预计在2050年之前不会有商业发电厂。因此，随着煤炭和石油的衰落，如果想要核聚变发挥作用，现在就需要取得真正的进展。核聚变是恒星的能量来源。在陆地条件下，氢的两种同位素氘和氚最容易发生聚变，这是磁聚变反应堆的关键反应。这个过程发生在大约10^ 8k的温度下，产生一个氦核、一个中子和几个MeV的能量。在这样的温度下，氢被完全电离，因此可能被限制在一个被称为托卡马克的甜甜圈状磁场中。为了提高效率，等离子体必须在足够高的压力和能量限制时间下运行，同时保持燃料的纯度。前两个量受到不稳定性的限制，其动力学只能通过经过验证的大规模计算才能完全理解。同时，必须仔细控制等离子体与壁的相互作用，以保持等离子体的纯净，并选择最大限度延长其使用寿命的壁材料。关键等离子体物理和材料科学过程的百亿亿次计算将有助于设计、运行和解释ITER和DEMO等昂贵的大规模实验，并显著加快聚变发电厂的实现。

Current worldwide energy consumption has risen 20-fold during the 20th century and the growth rate shows no sign of saturation. Of the current 15 TW load 80-90% is derived from fossil fuels, but with peak oil imminent and coal supplies limited, a sea change in energy production is vital. Renewables will make a contribution to future energy production, but they are generally unrelated to seasonal and geographical demands – while nuclear fission raises significant environmental and political worries. Nuclear fusion, on the other hand, promises a low pollution route to generate a large fraction of the world’s energy needs sustainably. However, the scientific and engineering challenges in designing such a reactor are formidable and commercial power plants are not expected before 2050.Real progress therefore needs to be made now if fusion is to be relevant as coal and oil decline. Fusion is the energy source of the stars. Under terrestrial conditions it is the two hydrogen isotopes, deuterium and tritium which can be made to fuse most readily, and this is the key reaction in a magnetic fusion reactor. This takes place at about 10^8 K and produces a helium nucleus, a neutron, and several MeV of energy. At such temperatures the hydrogen is fully ionized and hence may be confined in a doughnut-shaped magnetic field known as a tokamak. For efficiency, the plasma must operate at a sufficiently high pressure and energy confinement time, while preserving purity of the fuel. The first two quantities are limited by instabilities whose dynamics can only be fully understood through validated, large-scale computation. At the same time, the plasma-wall interaction has to be carefully controlled to keep the plasma pure, and the wall materials chosen to maximize their lifetime. Exascale computations of key plasma physics and materials science processes will help to design, run, and interpret expensive large-scale experiments like ITER and DEMO, and speed up the realization of fusion power plants significantly.