New FAQ - What are the first-wall material limits?

[KitemanSA]

Here is a new FAQ added by a new contributer (way to go TallDave!)
Any comments or improvements?

Q: What are the first-wall material limits?

A: The first wall limits for current materials are about 1 MW/m^2 (for context, ITER is looking at loads of 50MW/m^2 on the divertors).

This limit (1 MW/m^2) is roughly the load we would expect to reach on the Magrid casings at 100MW in a 1.5m radius D-D machine such as that envisioned by Bussard.

For p-B11 reactors, the first wall limits do not apply because the fusion products are charged and will be funneled out of the cusps by the magnetic fields after about 1000 circuits.

[WizWom]

The exotic materials in a polywell will be in the magrid casings; and those are cooled by Cryogenic Liquid in order to keep the magnets operating in superconducting mode.

A polywell will be able to keep these materials cool even with very high radiant energy flux by increasing flow rate.

From the design as proposed, I'd say the vacuum chamber walls will be at least a meter from the fusing plasma, and its radiation load will be significantly lower.

[D Tibbets]

Here is a new FAQ added by a new contributer (way to go TallDave!)
Any comments or improvements?
...
For p-B11 reactors, the first wall limits do not apply because the fusion products are charged and will be funneled out of the cusps by the magnetic fields after about 1000 circuits.

I wouldn't say first wall limits do not apply. I'd say
The first wall limits are much less as the coils would not need to deal with any neutron bombardment. The bremsstrulung radiation and other radiations deliver significanly less energy to the surfaces of the magrid.

Of course if bremsstrulung radiation cannot be controlled and it's energy is above the fusion energy, the picture changes. Of course, in that case the issue is moot as the reactor would not work. Actually, if the bremsstrulung radiation in P-B11 exceeds ~ 50% of the fusion output, it would be delivering more energy to the magrids than the neutrons from D-D fusion. The charged fusion products from D-D fusion would avoid the magrid just like the alpha particles from P-B11 fusion. Then, if the magrid is somewhat transparent to the x-rays (at least the lateral insulating and cooling areas- where the superconducting windings are not exposed), the total heat load from x-rays would be less.

Considering, the heat loads with D-D fusion, do the calculations account for the ~ 50% of the energy that is in the form of charged particles that do not heat the magrids?

Dan Tibbets

[KitemanSA]

Could someone develop a short discussion of what is meant by the "first wall" limit? Might help the flow.

[D Tibbets]

Could someone develop a short discussion of what is meant by the "first wall" limit? Might help the flow.

First wall could be considered as the material surface closest to the reacting plasma volume. In the Polywell, this is a moving target as the magrids intercept only a portion of the radiant energy, perhaps 20-30% of the total. The magrids also have an inner facing surface and an exterior facing surface. This probably modifies the heating load per unit area and the cooling considerations. In a working Polywell practically few charged particles reach the magnets through cross field transport so they would not heat the magrid much. The vacuum vessel wall will intercept any radiant energy that is not shadowed by the magrid. That energy (minus the 20-30% intercepted by the magrid) would be ~ 1/4th the intensity on the magrid if a convient assumption that the vessel walls are twice as far away as the grids.
All of the charged particles escaping the Wiffleball would hit the vessel wall. This would be ~ 50% of the energy with D-D and ~ 100% (divided by Bremsstrulung contribution not intercepted by the grids). If there is direct energy conversion of some or almost all of this charged particle energy the thermal load on the walls would be minimized. With D-D fuel, the neutrons would heat the grids and the wall depending on the distance and the transparency of the structures. The per unit dose to the grids would be four times as great per unit of area with the above assumption.
Then add the nubs and/ or standoffs, the electron guns, the ion guns, any diagnostic equipment, conversion grids etc, and the answers are complex.

A short answer for D-D would be some value for the magrids, say 1 MW/ M^2, and less than 1/4th that for the vacuum vessel wall. For P-B11 the grid would be ~ much less (assuming high fusion output vs bremsstrulung- like a Q of 20). The vacuum vessel wall loads would also be ~ 5-6 times less because of the kinetic energy harvesting direct conversion grids (including the harvesting grids with the wall)

If a D-D Polywell reactor has a radius of 1 meter for the magrid and 2 meters for the vacuum vessel wall, and the reactor is producing ~ 90 MW of fusion power plus ~ 10 MW of input power, the load on the wall would be ~ 100MW distributed over ~ 96 M^2 or ~ 1.05 MW / M^2. Subtracting the 10% ( ~ 1/2 of the intercept ratio (which is eg 20% of the neutrons and 0% of the ion fusion power, plus some small percentage of the input power, for a net interception of ~ 10% of the total that otherwise hits the vacuum vessel walls. So the vacuum vessel thermal wall loads would be less than 1 MW / M^3. The magrid coils would be ~ 2 MW/M2 on the inward facing surfaces ( ~4 MW because it is twice as close divided by two because the ~ 50 % of the power that is in the form of charged particles which do not hit it).

[EDIT] To add even more complexity. In a D-D reactor without direct conversion, the charged particles that carry ~ 50% of the fusion power exit the magrids at cusps, and depending on how much divergence of these beams occurs, the local heating per unit area of the vacuum vessel walls aligned with the cusps could be substantially more, up to ~ 1.5 times as much (?). Also, have to consider differential heating of the vacuum vessel wall by neutrons where the walls are not shadowed by the magrid.

Dan Tibbets

[nferguso]

Could you clear up a misunderstanding I have? I understand from previous discussions that the charged fusion products (regardless of fuel) would be at such a high energy (MeV's) that the magnetic forces and magnet housing charge would not deflect them significantly. Those headed toward the inner wall components would hit those components, and those headed between would miss.

[DeltaV]

The alpha particles are less of a problem than originally thought for p-B11 fusion:

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