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A trip to Mars and back takes about 500 days using traditional chemical propulsion systems. Spending so much time in deep space poses serious health risks for astronauts, who would be exposed to lots of radiation and would have to exercise like mad to minimize bone and muscle loss.
Developing a faster propulsion system is thus a chief goal of NASA, which aims to get people to the vicinity of the Red Planet by the mid-2030s. The space agency has funded Pancotti's fusion-rocket team — led by John Slough of the University of Washington — through its NASA Innovative Advanced Concepts program, or NIAC.
The researchers are designing their work around a potential manned Mars mission that would last a total of 210 days — 83 days for the trip out, 30 days on the surface of the Red Planet and 97 days to get back home to Earth.
"We feel we've defined a very good problem, a very good mission, and we're focused on the fusion device to fit this mission," he said.
Because nuclear fusion is an extremely efficient and powerful energy source, this mission could be accomplished in a single launch of the most powerful version of NASA's Space Launch System mega-rocket, which is in development. It would take perhaps nine launches to mount such an effort with chemical propulsion, Pancotti said.
[Q:]So that explains the strangely twisted form of the coils in the Wendelstein 7-X. How did you come up with this?
[A:]The geometric characteristics of the plasma in a conventional stellarator make it very difficult to achieve good plasma confinement. It's like having a limp: you can do as much training as you like, but you're never going to be a 100-metre sprinter. However, our former Director, Jürgen Nührenberg, discovered a hidden symmetry characteristic of plasmas in the 1980s which makes it possible to also confine a plasma without plasma current. The shape of the plasma and the magnetic field [in W7-X design] resulted from this. Using what were very powerful computers at the time, Jürgen Nührenberg calculated how the magnetic coils had to be shaped to generate this field.
University of Maryland physicist Matt Landreman has made an important revision to one of the most common software tools used to design stellarators. The new method is better at balancing tradeoffs between the ideal magnetic field shape and potential coil shapes, resulting in designs with more space between the coils. This extra space allows better access for repairs and more places to install sensors. Landreman's new method is described in a paper published February 13, 2017 in the journal Nuclear Fusion.
The new approach, recently detailed in the journal Nature Physics, uses a fuel composed of three ion species: hydrogen, deuterium, and trace amounts (less than 1 percent) of helium-3. Typically, plasma used for fusion research in the laboratory would be composed of two ion species, deuterium and hydrogen or deuterium and He-3, with deuterium dominating the mixture by up to 95 percent. Researchers focus energy on the minority species, which heats up to much higher energies owing to its smaller fraction of the total density. In the new three-species scheme, all the RF energy is absorbed by just a trace amount of He-3 and the ion energy is boosted even more—to the range of activated fusion products.
it [the camera] has to be fast enough to record plasma phenomena changing in less than one-thousandth of a second
A first analysis in the 2017 experimental campaign indicates that the observed temperature matches with theoretical predictions. Longer discharges of up to 30 seconds became routine by the end of the campaign. The divertor allowed deposition of up to 75 megajoules of heating energy in the plasma, this being more than 18 times as large as the energy limit of the first campaign without divertor.
While by the end of the first campaign pulse lengths of six seconds were being attained, plasmas lasting up to 26 seconds are now being produced. A heating energy of up to 75 megajoules could be fed into the plasma, this being 18 times as much as in the first operation phase without divertor. The heating power could also be increased, this being a prerequisite to high plasma density.
In this way a record value for the fusion product was attained. This product of the ion temperature, plasma density and energy confinement time specifies how close one is getting to the reactor values needed to ignite a plasma. At an ion temperature of about 40 million degrees and a density of 0.8 x 10^20 particles per cubic metre Wendelstein 7-X has attained a fusion product affording a good 6 x 10^26 degrees x second per cubic metre, the world's stellarator record.