Use much lower burnup fraction

[jonmaddock]

The vacuum pumping during flat-top and the burnup are both affected by tauratio, the ratio of He and pellet particle confinement times. This should also be consistent with the helium enrichment in the divertor zone. Altogether it's a can of worms, but an important one.

The vacuum pumping in STEP is being studied by Antulio Tarazona.

Related issues: #305, #1746.

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In GitLab by @mkovari on May 15, 2015, 08:32
The DT fuelling rate is very small, because of the unrealistically high value of burn-up fraction. We should try to use a more sensible value, at least an order of magnitude smaller.

 Fuelling rate (nucleus-pairs/s)                                          (qfuel)                   2.940E+21
 Fuel burn-up rate (reactions/s)                                          (rndfuel)                 6.009E+20
 Burn-up fraction                                                         (burnup)                      0.204

I have implemented this simple modification:
tauratio /1.0/ : ratio of He and pellet particle confinement times
The default is 1, giving full backwards compatibility.

The burnup is reduced by the same ratio.

    !  burnup = fuel ion-pairs burned/m3 / initial fuel ion-pairs/m3;
    !  fuel ion-pairs burned/m3 = alpha particles/m3 (for both D-T and D-He3 reactions)
    !  initial fuel ion-pairs/m3 = burnt fuel ion-pairs/m3 + unburnt fuel-ion pairs/m3
    !  Remember that unburnt fuel-ion pairs/m3 = 0.5 * unburnt fuel-ions/m3

    burnup = dnalp / (dnalp + 0.5D0*deni) / tauratio

NOTE. If tauratio is bigger than 1, this may increase the number of vacuum pumps required, which will affect the availability. If the vacuum pumping system size is governed by the requirements for helium ash removal, then there should be no effect.

[jonmaddock]

In GitLab by @mkovari on May 18, 2015, 15:15

Commit 015c19c0

OUT.DAT now has:

 Fuelling :
 
 Ratio of He and pellet particle confinement times                        (tauratio)                5.000E+00
 Fuelling rate (nucleus-pairs/s)                                          (qfuel)                   1.311E+22
 Fuel burn-up rate (reactions/s)                                          (rndfuel)                 5.342E+20
 Burn-up fraction                                                         (burnup)                      0.041

The number of vacuum pumps is now very large. This may indicate a serious problem with pumping away the gas.

[jonmaddock]

In GitLab by @mkovari on Jun 4, 2018, 10:39

A more detailed model of burn-up would be valuable. See, for example,

https://www.sciencedirect.com/science/article/pii/S0920379618304927