KitemanSA wrote: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