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I was skimming through the ITER Technical Basis and in page 16 of the PDF, under the subheading "Design Requirements", it says that the engineering requirements are:

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I'm confused because these units do not match wikipedia's units for neutron flux and fluence:

  • Neutron flux: $\text{cm}^{-2}\text{s}^{-1}$
  • Neutron fluence: $\text{cm}^{-2}$

Any help on the matter would be greatly appreciated.

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    $\begingroup$ What is" a" in this context? $\endgroup$
    – Triatticus
    Jul 13, 2020 at 0:50
  • $\begingroup$ I don't know that either; Searching through the booklet for 'MWa' doesn't reveal anything interesting either $\endgroup$
    – Krish
    Jul 13, 2020 at 7:32

2 Answers 2

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Fluence is the time integrated flux . Here , replace " a " for annual , by " y " for year . You obtain a more known unity : 0.3 MW-y / ...

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As about first parameter it seems that specification is talking about irradiance flux density. Neutron flux can be extracted from it like that : $$\Phi _ n = \frac {J} {E_n} $$ Where $J$ is irradiance flux density $[\text{W} /\text{m} ^2]$ and $E_n$ is neutron average kinetic energy.

As about second parameter I'm not sure. In one page above in other formula they give that $a$ is plasma minor radius. In such case their "neutron fluence" will have dimensions of $[\text {W} /\text{m} ]$, to which closest interpretation would be spectral flux density usually measured in $[\text{W} /\text{nm} ]$ - radiation power per unit wavelength. But I doubt that variable $a$ has same context as in page above. Probably it does not. Then it's not clear what they had in mind.

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  • $\begingroup$ Paired with @Jean's answer, I think you've hit the nail on the head, One final question before I mark this as accepted: What is $\Phi_n$? A single word answer will suffice, thank you. $\endgroup$
    – Krish
    Jul 14, 2020 at 9:34
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    $\begingroup$ @Krish It's a neutron flux - number of neutrons crossing unit area per unit time span. I. E. definition of it is exactly as in wikipedia page. $\endgroup$
    – Agnius Vasiliauskas
    Jul 14, 2020 at 9:45

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