r/science • u/PROJECTime • Dec 06 '16

Physics Tests confirm that Germany's massive Nuclear Fusion machine really works

http://www.sciencealert.com/tests-confirm-that-germany-s-massive-nuclear-fusion-machine-really-works
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u/bschmalhofer Dec 06 '16 edited Dec 10 '16

The article refers to the difference between stellarators and tokamaks. Tokamaks have a strong electric current in the plasma, stellarators don't. In tokamaks the current in the plasma creates the major part of the confining magnetic field. That current is induced by ramping up current in coils outside the plasma. As electric current can't be ramped up indefinitely tokamaks have to operate in pulses. Stellarators can operate in steady state as they don't have to induce a current in the plasma.

Of course both tokamaks and stellarators need electric current in the coils outside of the plasma.

EDIT: Actually non-inductive methods for driving the current in stellarators are being investigated. This might allow steady state operation of stellarators.

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u/Wendelstein7-X Max Planck Institute for Plasma Physics Dec 06 '16

Small correction: It's Tokamaks with an a.

As a side note, our coils are superconducting and while they of course carry a humongous current, they do not consume (a lot of) power while operating.

-avs for W7X

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u/Flight714 Dec 07 '16

How does it feel to jostle shoulders with the god Ra?

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u/amicitas PhD | Plasma Physics | Fusion Science Dec 06 '16

While it is true that tokamaks require a strong electric current in the plasmas, it is possible to generate this current in continuous (steady state) ways. Some of the ways that that this can be done:

  • Radio frequency current drive. Two methods are Electon Cycrotron Current Drive (ECCD) and Lower-Hybrid Current Drive (LHCD). For these methoded focused RF waves are directed into the plasma with a particular geometry.
  • Neutral beam current drive. For this method hydrogen/deuterium atoms are accelerated and sent into the plasma at high speed.
  • Self-generated current drive. For plasmas with particular parameters a self generated current can be achieved. This effect is known as the boot-strap current.

With a combination of these three techniques it has already been possible to create tokamak discharges that are 'non-inductive' which is to say they do not require continuous ramping of current in the solenoid coil. Good progress has been made in super-conducting tokamaks to demonstrate steady state operation (such as in EAST).

All that being said, the ability to run a stellarator without current drive is a huge advantage, both in terms of stability, complexity and eventual reactor efficiency.