# Difference between neutron flux and fluence?

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: 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.

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

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.
• 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. Jul 14, 2020 at 9:34