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The present type fusion reactors use the magnetic confinement technique for the production of energy. Moreover only 48% for the energy is only absorbed and converted into useful energy . Why we are not able to convert 100% fusion reaction power into electricity and why a controlled fusion reaction is not possible.

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  • $\begingroup$ This is a very broad question that really requires an answer outside the scope of this forum... where does your information come from (in particular the "48%" number)? $\endgroup$ – Floris Sep 28 '14 at 6:00
  • $\begingroup$ for ITER see the answer here : physics.stackexchange.com/q/70209 $\endgroup$ – anna v Sep 28 '14 at 7:02
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I wonder if your number 48% comes from the typical Carnot efficiency of a heat engine - see for example a detailed description at http://www.visionofearth.org/industry/fusion/how-do-we-turn-nuclear-fusion-energy-into-electricity/

When you want to use heat to create electricity, you typically convert the heat into motion (for example by rotating a turbine, that is, by heating a substance like water to make steam, and extracting work as the steam expands). Thermodynamics says that as a substance cools down, you can only extract a certain amount of work from it that depends on the input and output temperatures -

$$\eta = 1 - \frac{T_{low}}{T_{high}}$$

This means that you must be able to make the steam very hot in order to extract the most work from it. For a magnetically confined fusion reactor, it is hard to let the working substance get very hot - remember there are lots of constraints on the materials and temperatures you can have (especially in the next generation of superconducting reactors which have large sections at cryogenic temperatures - you need to keep heat away from those).

Some combined cycle machines that burn gas at a very high temperature and end up heating steam with the lower temperature gas to extract the maximum heat reach efficiencies up to 61% - but you can't get that hot by capturing neutrons in a fusion reactor.

The link I quoted in the first paragraph claims that direct conversion techniques may be able to push the limit of efficiency up. Note that the method as described would not work in a torus (currently the preferred method for magnetic confinement) as it requires hot plasma to be channeled off through an expander; as far as I can figure out that only works with a magnetic mirror arrangement (as shown in the article linked - I reproduce just one figure here):

enter image description here

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  • $\begingroup$ I agree, the 48% probably comes from the direct conversion experiments. Specifically those done on the TMX magnetic mirror machine [1]. I tout this number often - but it is based on a thin data set. This finding - at best - just says that direct conversion is very promising. In real fusion power plants the efficiency could be very different. 1. Barr, William, and R. Moir. "Experimental Results from a Beam Direct Converter at 100 KeV." Journal of Fusion Energy 2.2 (1982): 131-43. Print. $\endgroup$ – The Polywell Guy Mar 16 '15 at 17:02

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