Wendelstein 7-X: hot nuclear fusion reactor now working on second round of testing

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.

www.space.com...

[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.

Phys.org, Feb. 2016 - Plasma physicist discusses the Wendelstein 7-X stellarator .

Wikipedia entry: Stellarator.

They have even done the original fully optimized magnetic configuration a simpler upgrade since the time of that interview.

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.

Phys.org - Physicists improve method for designing fusion experiments.

The creation of the magnetic field continues to improve even before W7-X completes its goal of a 30 minute run. Even as the designs are upgraded the fuel mixture is also being investigated.

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.

phys.org - Physicists explore a new recipe for heating plasma.

The article ends with a call out of W7-X as a target of their research.

a reply to: penroc3

From the March 2015 newsletter (all newsletters can be found at the IPP site and downloaded, PDFs)...

it [the camera] has to be fast enough to record plasma phenomena changing in less than one-thousandth of a second

They also have store all that footage so hard disk space is a consideration. They have a loss-y algorithm that can decide on its own what to keep (they also tag things they know they want to keep). They are using in-line programmable field gate arrays (PFGA) so they can update the system's code without having to take apart the whole reactor. The Hungarian team did a great job on the camera system!



RE: Nuclear fusion power output

ITER.org - The Tao of Q.

That does a good job explaining power input to power output ration (Q). At Q = 1 they have what is termed "break even" where power in equates power out. No reactor has reached that point yet. ITER is designed with a Q = 10, where 50 MW in will result in 500 MW heat out.

Update: New issue of the Wendelstein Newsletter (Jan. 15, 2018)

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.

The newly installed divertors are passively cooled cooled as of now (they plan on active water cooling after the next campaign later this year, 2018). These are the carbon tiles that were installed. When plasma whips around W7-X, they have to navigate turns which allow heat to leak out at those turns. These tiles take that heat load allowing the operators to fine tune the magnetic fields to lift the heat flow (called, "flux" in the newsletter). By doing that, they ran up to 30 seconds (which was their original target during this run) at the higher energy heating (the "18x as large" quote). Any other places the plasma might have touched the wall are collected and pulled out of the reactor here as well.

They also installed a pellet injector that shoots frozen hydrogen pellets into the plasma before it turns into fuel ("ablates and ionizes" in the article). The injector was designed for a tokomak but with a little change here and there, they put it on W7-X (it came from PPPL, IIRC).

There are a few more things in the article but the overall conclusion was this run was a success!

Next run starts in July 2018.

a reply to: TEOTWAWKIAIFF

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.

phys.org, June 25, 2018 Wendelstein 7-X achieves world record for fusion product.

The world record is for a stellarator design. Max Plank also published their first round results in Nature. The above story is a slightly more Americanization of the story linked below. W7-X is in summer break with the second half of this run to start in July.

There you go! Half way through the second run, 40 million ion plasma temps, 26 second run, and they beat the previous heat/density plasma product (previous record was just over 5, the W7-X nearly hit 7, and for a longer time period too!)

From reading the article, the fully optimized stellarator is hitting all the right marks!

IPP newsletter - Wendelstein 7-X achieves world record.