Vacuum | The art and science of cleanliness in a fusion reactor

Just as a few dust particles in a semiconductor fabrication plant can limit manufacturing yield, dust and other impurities inside a tokamak can limit fusion output. The ITER vacuum team is instituting a range of cleanliness protocols and processes at such an unprecedented scale that they are likely to inform the choices made for future reactors. Creating and maintaining the plasma in a fusion reactor requires not only an ultra-high vacuum, but also ultra-clean surfaces. Contamination present on vacuum-facing surfaces can be released through a process known as outgassing. When vacuum facing surfaces are heated—either intentionally (for cleaning) or unintentionally (when the plasma touches walls)—the outgassing process accelerates. The challenge is to keep vacuum facing components so clean that the impurities released through unintentional outgassing do not significantly reduce fusion output. The right materials and the right processes The first step in achieving the required level of cleanliness is choosing the right materials—an exercise that began more than a decade ago at ITER during the design phase of vacuum-important components. "Each material has its own characteristics in vacuum," says Liam Worth, Group Leader for Vacuum Transverse Activities. "Vacuum-compatible materials that are suitable for semiconductor fabrication are not necessarily suitable for fusion—and vice-versa. We look for materials with very low outgassing rates, very low vapor pressures and compatibility with machine operations." "In order to characterize materials and ensure compatibility with ITER operations we run outgassing tests almost continuously on the materials. We also do dry wipe and wet wipe tests on large components to check for gross contamination—we simply rub the surface with a lint-free cloth and see what comes off. To the untrained eye, the cloth may seem relatively clean. But even the slightest speck of gray represents hundreds of millions of particles." Selecting the right materials is important, but so is controlling the processes that those materials have been subjected too. All installed surfaces facing the vacuum and any constituent parts must be kept free of contaminants, such as oils, greases, and packing residues. Vacuum-important components must be kept clean during the transport, unpacking and assembly phases. "We work to a philosophy of continual cleanliness," says Robert Pearce, Section Leader of the Vacuum Delivery & Installation Section. "For instance, our Assembly Hall is a clean room, so that when we assemble vacuum components, we assemble them in a clean environment. Then we put controls on the actions that people take on those components. It's not super high-tech stuff. It's more about procedures and routines, including requiring people to wear coveralls, gloves, and clean overshoes." A different focus for cleanliness after assembly Once components are assembled, a number of techniques will be used to ensure the cleanliness of the vacuum vessel—a set of processes known as conditioning. One technique is simply baking: heating the walls of the vacuum vessel at 200 °C for several days will significantly reduce moisture and hydrocarbons on the surfaces. "In addition to heating we pump the vacuum vessel to remove the outgassing species," says Pearce. "If you don't pump, the dirt comes off one surface, and then sticks to any cooler surface it might hit. As outgassing rate is strongly dependent on temperature, when we drop the temperature of the surfaces the outgassing rate also drops." "We will also likely use a technique called glow discharge cleaning, which involves bombarding the walls with cold plasma. Beyond glow discharge cleaning, we can also use a higher energy tokamak plasma for cleaning. Normally when you run, you try to confine particles so they stay away from the walls. But if you want to clean the wall, you can actually run plasma along the surface." "Another technique is to create radiofrequency-excited discharges to clean the walls," says Pearce. "These discharges desorb impurities that can then be pumped out." From experimental reactors to commercial fusion One can never be too obsessive when it comes to keeping a tokamak clean. Two incidents at one of ITER's predecessors highlight how a seemingly small mistake can have a huge impact. The first case involved a small piece of cable left inside a vacuum chamber, which prevented the reactor from operating for several weeks. In the second case, a piece of tape was left on a wall in the vacuum vessel. It took months to identify the issue and to get the reactor operating again at full performance. "When you find the piece of tape, the problem is obvious," says Worth. "But when you have a vacuum problem, especially one that is associated with contamination, you have various pieces of evidence from different sources—for example, a pump doesn't work or you see a bit of titanium where it shouldn't be. It's like a detective story. You know you will find the answer eventually. Sometimes you get it right away; other times it might take months to find." "One of the most sensitive measurements of contamination is the plasma itself," says Worth. "Different diagnostic instruments look at the radiation from the plasma for example. But we also have our own vacuum instrumentation where we can look at what we call the residual gas, gas that is present in the vacuum vessel after pumping and conditioning. We can do this in between pulses to determine the cleanliness of the vacuum." Ultimately, the goal of the vacuum team is to learn what it takes to achieve the required level of cleanliness and to drive cleaning processes towards industrial application. Much has already been gained from the experiences at JET and other fusion reactors. But because ITER is a different machine (it is pumped differently and it lends itself to different cleaning techniques), there is still more to learn. "What we learn now can positively impact DEMO and commercial reactors," says Pearce. "Sustainable fusion will come faster if we learn and apply the lessons from the past and present, including simple things like cleanliness."