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From two different places the old tokamak confinement method has gotten a new lease on life. Well, its life is set by the ITER effort paid for by governments across the planet with tens of billions of dollars. It might make one cringe or celebrate, depending on one’s view towards the basic tokamak concept to confine plasma to yield fusion.
The bigger news maker comes out of Italy at the RFX-mod Reversed Field Pinch (RFP) fusion device located in Padova, Italy.
To explain, if you twist up a string or rubber band at some point it starts to kink into knots in a disorganized way. The image is akin to what’s going on inside a tokamak with the whirling plasma inside the tokamak’s donut. The phenomenon concerns scientists exploring fusion power, who use powerful magnetic fields inside tokamaks and stellarators to confine plasma during their experiments.
A discovery made in the RFP fusion device demonstrated that when increasing the current above 1.5 million amperes, the helix becomes very smooth, the trapping of the plasma in the magnetic fields improves and the plasma gets hotter. Moreover, the helical state appears to be the preferred one at high current and is spontaneously chosen, or self-organized, by the plasma.
Past RFP experiments suffered from poor stability, which reduced confinement performance due to magnetic turbulence (those knots in the string). But as a result of increased order in the self-organized helical state, magnetic confinement improves and a broad zone of the plasma becomes hot. This renews the fusion prospects of the RFP device, a magnetic confinement configuration germane to the tokamak, which uses a magnetic torus for plasma confinement, because a current flows in the plasma doughnut, and to the stellarators, which uses a magnetic helix, because the plasma takes on a helical shape.
Now here’s the kicker, most of the RPF’s magnetic field is produced by the current flowing in the plasma. (They’re not saying how.) This eliminates many of the costs and technical difficulties associated with producing strong magnetic fields using high-tech superconducting coils, such as those used in pure tokamak and stellarator fusion containment designs. The helical shape of the plasma comes with an additional bonus: the current lines are also helical. This greatly increases the length of the electrical circuit with respect to the tokamak. This could make reaching thermonuclear temperatures possible with only the electric power dissipated in the plasma. In principle, no additional heating is necessary, an additional positive economic and technical feature.
The RPF’s issue is to stay relevant for fusion as its been studied for decades already. The Italian team is going to try to increase the density and improve the shape of the helix. The hope is progress in magnetic control; a further increase of plasma current, and use of modern wall-conditioning techniques might accomplish this. We’ll see.
Meanwhile at the DIII-D National Fusion Facility in San Diego CA, scientists are confirming long-standing theoretical predictions that performance, efficiency and reliability are simultaneously obtained in tokamaks. The facility is run by General Atomics and the experiments designed to test these predictions have successfully demonstrated the interaction of these conditions.
These new findings were presented at the American Physical Society — Division of Plasma Physics 51st annual meeting.
The GA team using a laboratory tokamak, where power performance is determined by the temperature and density achieved by plasma, an ionized gas formed when hydrogen isotopes are heated to temperatures of over 10 million degrees Celsius. A key element of recent experiments is the confirmation of theoretical predictions that one can rely on the walls of the tokamak chamber to improve plasma stability at high pressure. Once plasma becomes sufficiently hot and dense, fusion occurs, producing large quantities of high-energy helium ions (known as alpha particles). For optimal efficiency, this self-generated heat must be well contained within the tokamak’s “magnetic bottle.” Computer models have predicted that the heat loss from the tokamak due to turbulence is quite sensitive to the exact details of the magnetic field configurations.
The news is the GA team recently found that turbulence is minimized in the same configuration necessary for achieving the highest pressures. Therefore, performance and efficiency can be synergistic. It just seems so conveniently odd.
Those knots in the string apply here as well, turbulent eddies in the plasma can also affect plasma heating by high-energy helium nuclei formed by the fusion of hydrogen atoms. Recent theoretical work suggests that these energetic particles not only “feel” turbulence differently, but can also stir up large eddies of their own. While these fine-scale turbulent eddies are predicted to cause negligibly small transport of energetic alpha particles, the new large eddies can increase this transport substantially. As the alpha particles cool, their transport becomes similar to the background level. Note this is theoretical work, not experimental results.
But the recent work has shown that tokamak plasmas can be induced to exhibit the following relationships: higher pressure => more self-generated electrical currents that help control the plasma => less reliance on external controls => longer pulse (including potentially steady-state) operation => higher reliability.
The news release made Science Daily, ending with a little promotional commentary, “After decades of effort to improve the behavior and output of fusion plasmas, scientists are discovering that nature may actually be so kind as to simultaneously allow high performance (lots of electricity!), optimal efficiency (affordable!), and high reliability (the electrical outlet will always work!) in the design of future power plants.”
All three of these, the RPF Pinch, Tokamaks and Stellarators are old ideas. Maybe they have all hit on something, in the short version, “turn up the power” so that they can find an economical means to exist. The history isn’t encouraging to date. Untold billions of dollars have already gone into these efforts with little to show but decades more to go and more tens of billions of dollars. It would be gratifying if these three or Bussard fusion, Lerner fusion or the other ideas would finally get to power production and on to net power.
Outside of the old three, the innovators have made great progress for mere fractions of a penny in comparable investment.
One wonders, those knots in the string image might be better understood and exploited than theoretically determined to be a problem in search of solutions. A little creative innovation might go a long way in this part of the fusion field.
There are surely lots of fusion ideas working. Which one will finally work?
Post written by: New Energy and Fuel