Fusion startup Zap Energy has reached two milestones that could nudge it ahead in the race to offer low-cost, carbon-free energy — a $160 million Series C round and a successful test of a prototype fusion reactor that could pave the way to a commercial version.

Fusion power has become an unlikely investor favorite as carbon emissions continue to rise and the effects of climate change become more apparent. We have been trying to harness the power of the sun to produce energy for many years, but after tens of billions of dollars and decades of research, fusion remains just out of reach.

Still, clever new approaches to contain scorching hot plasma — which burns at more than 100 million degrees Celsius — have brought fusion power tantalizingly close to reality. Investors are flocking to the field, hopeful that advances wrought by continued research and increasingly sophisticated computer simulations will finally help fusion pull away from its long string of failures.

Zap Energy’s oversubscribed Series C was led by Lowercarbon Capital. New investors include Breakthrough Energy Ventures, Shell Ventures, DCVC and Valor Equity Partners. Existing investors Addition, Energy Impact Partners and Chevron Technology Ventures also contributed to the round. The startup’s core technology was spun out of research performed at the University of Washington and Lawrence Livermore National Laboratory.

Generally speaking, fusion power generates electricity by fusing hydrogen isotopes (either deuterium or tritium) into helium. The process releases neutrons, which are then captured to generate heat and spin a turbine. Atomic nuclei don’t like to fuse, so to coax them close enough for fusion to happen, nuclear scientists use extreme pressure and heat, creating a fourth state of matter known as plasma.