Anyone who’s helped a buddy move a fridge knows lifting the thing is easy. Getting it through a doorway without gouging the frame, the floor or your own knuckles is where the swearing starts.<\/p>\n
So spare a thought for the engineers at the UK Atomic Energy Authority, who just did a version of that with about 26,500 pounds of stainless steel. On November 3, UKAEA announced<\/a> that its engineers had maneuvered a vessel it calls the double beam box into position at MAST Upgrade, the agency’s flagship fusion machine at its Culham Campus in Oxfordshire. In American terms, that’s a little over 13 tons of steel swinging on a crane inside a fusion lab, which is a pretty nerve-racking thing to do with a crane.<\/p>\n The box won’t make any heat itself. It’s a housing. UKAEA describes it as a double-decker vessel built to hold two neutral beam injectors, and once those are fitted, the beams heating the machine’s plasma go from 4.4 megawatts of power to 8.8.<\/p>\n So what’s a neutral beam, and why does it need a box this heavy?<\/p>\n MAST Upgrade is a tokamak. That’s a machine that holds a cloud of superheated hydrogen, called a plasma, inside a cage of magnetic fields so it never touches the walls. UKAEA bills it as the world’s largest operational spherical tokamak, and “spherical” just means the usual donut shape has been squeezed until it’s closer to a ball with a narrow hole through the middle.<\/p>\n The magnets are where it gets tricky. They steer anything with an electric charge, which is great for keeping the plasma in place. It’s less great when you want to push extra energy in from outside, because charged particles fired at the plasma get steered away too.<\/p>\n A neutral beam injector gets around that with a pretty clever trick. It speeds up positively charged particles, then hands each one back an electron just before it leaves, so what comes out is a beam of fast atoms with no charge at all. The magnets basically ignore them. Once those atoms are inside the plasma, collisions knock the electron off again, the magnetic field traps them, and they pass their energy on by bumping into everything around them.<\/p>\n If you want the kitchen version, it’s like warming a bath by throwing in pebbles that are moving absurdly fast. UKAEA says that’s how its injectors get the plasma past 54 million degrees Fahrenheit today. We’ve covered a California machine that uses eight of these beams to build its whole magnetic trap<\/a>, so the technology isn’t exotic. It’s just heavy.<\/p>\n With two more injectors on top of the ones MAST Upgrade already has, total beam power climbs from 4.4 megawatts to 8.8. UKAEA expects the plasma to get hotter too, going from about 54 million degrees to about 108 million. Keep the word “expected” in mind there. The new injectors haven’t been installed, so nobody’s run a single plasma with them.<\/p>\n Finished, with its lid on, the vessel will weigh around 33,000 pounds, or roughly 16.5 tons, according to UKAEA. The agency’s own comparison is two and a half African elephants, which is a very British way to describe a steel tank, and I’m not going to argue with it.<\/p>\n Inside, there’s about 700 cubic feet of space, or roughly 130 bathtubs by UKAEA’s count. The whole thing is built from 316L stainless steel, the same grade that goes into offshore structures and medical implants. UKAEA says it picked that steel because it’s strong, it doesn’t corrode and it works well in a vacuum.<\/p>\n The vacuum matters a lot here. Any stray gas floating around in the box can put the charge back on those fast atoms before they reach the plasma, and then the magnets bat them away like everything else. Some materials also slowly leak gas that’s trapped inside them, which you really don’t want in a place that has to stay almost perfectly empty.<\/p>\n That’s why one of the next jobs is fitting a cryopump inside the box. A cryopump is basically a surface kept so cold that stray gas freezes onto it and stays put, and UKAEA says this one will control the pressure inside the vessel.<\/p>\n I couldn’t find the box’s actual dimensions in anything UKAEA has published, just its weight and its volume, so I can’t tell you how tall that double-decker stack stands.<\/p>\n Because the machines that come after this one will run hotter, and somebody has to find out what gives first. Andrew Thornton, head of MAST Upgrade operations at UKAEA, said the extra heating means “we will explore plasma conditions we simply couldn’t reach before.”<\/p>\n He pointed to two things in particular. One is whether plasma at higher pressure stays stable. The other is how the machine’s exhaust system copes with hotter, higher-performance plasma. Thornton called both crucial questions for STEP, the UK’s planned prototype fusion power plant, and for the plants that follow it.<\/p>\n I’d argue the exhaust question is the more interesting one. In a tokamak, the exhaust is a set of surfaces where the heat and particles leaking out of the plasma get steered on purpose, so they don’t land on the rest of the wall. MAST Upgrade has spent years on that problem, and in its last round of experiments it learned to track its plasma by the glow coming off those surfaces<\/a>, along with several ways to stop the plasma’s edge from throwing bursts at the walls. All of that happened at the heating power the machine has now. Doubling the beams is a pretty direct way to find out whether those tricks still hold up.<\/p>\nSo how do you heat something that’s already 54 million degrees?<\/h2>\n
The box is mostly empty for now<\/h2>\n
Why would anyone want a hotter plasma?<\/h2>\n