Globus-M2 Tokamak advances small-scale nuclear fusion

“Unlike large pure fusion reactors, compact spherical tokamaks may be used to produce high-energy fast neutrons economically, which can breed fuel for traditional nuclear reactors or burn their waste, producing energy at the same time,” says Nikolai Bakharev, a plasma physicist at the Ioffe Institute in Russia. The Globus-M2 spherical tokamak that Bakharev his team are working on may be an important first step towards this goal.

Tokamaks confine plasma – a hot, ionized gas – using magnetic fields arranged in a doughnut-like configuration called a torus. As the particles in the plasma become energized and heat up, they reach a point at which they fuse together, releasing significant amounts of energy. While all tokamaks arrange magnetic fields around a central cavity, spherical tokamaks have an extremely narrow ‘doughnut hole’ in their centre, so that the outside edge of the plasma appears as a sphere. This smaller footprint makes them more cost-effective than conventional tokamaks.

The original Globus-M spherical tokamak facility began operation in 1999, however, the compact profile of the machine posed a significant technical challenge. There was little space to install sufficient magnetic coils to generate enough energy to power a fusion reaction.

In an attempt to bring experimental conditions closer to those needed for a compact spherical tokamak-based fusion neutron source that could be used as a driver in a hybrid fusion-fission reactor, the Ioffe team completely redesigned the facility with the first full scale Globus-M2 experimental campaign beginning in 2019.

The campaign focused on three areas for improvement: energy confinement; fast ion confinement and lower hybrid current drive (LHCD); and higher outputs of both the magnetic field and the plasma current.

The team generated an additional electric current that pulses within the plasma confined in the tokamak, using lower hybrid waves, making the tokamak able to handle longer pulses of energy. “The upgraded Globus-M2 shows a more reliable LHCD, and this was the first time a spherical tokamak achieved this with the toroidal wave slowing down,” he said.

Compared with the Globus-M, the -M2 showed longer energy confinement time by around 270%, reduced fast ion losses, and generated a toroidal magnetic field that reached 0.83 Tesla – improving on Globus-M’s 0.5 Tesla. It also significantly increased both the plasma current generated – to 0.4 MA, and the plasma total stored energy – more than 7kJ, compared to its predecessor.

The Globus-M and now -M2 are primarily for scientific research and technology demonstration. They show the potential for spherical tokamaks to generate hybrid fusion-fission nuclear power.

“No other tokamak has this size, aspect ratio and plasma current, so the data from the Globus-M2 tokamak is critical to inform future science based on understanding how these devices operate in the real world,” says Bakharev.