The Shield

The only reason the surface of the Earth is habitable is a magnet. Our planet's field turns the solar wind aside thousands of kilometers out. The particles it can't turn away get funneled down the field lines toward the poles, where they light up the upper air. That's the aurora.

Now the ship has a small coil of its own. Watch the stream bend around it.

A moving charge in a magnetic field feels a force at right angles to both, so it curves and never speeds up or slows down. Heavier, faster particles curve less. That's the whole trick, and the whole problem.

Every one of the 900 protons on the stage is being integrated live with the Boris method, published in 1970 and still inside most plasma simulations. It rotates each particle's velocity exactly, so no proton gains or loses energy from rounding error, however long you watch.

In the early 1900s the Norwegian mathematician Carl Størmer spent years calculating, by hand, which paths a charged particle could take around a dipole. He found a region the particles can't reach from outside. Its size grows with the square root of the magnet's strength and shrinks with the square root of the particle's momentum.

The amber ring is that length for this coil. Storm protons now, a hundred million electron-volts each.

To push the circle out past a ship-sized crew volume against storm protons takes around ten million ampere-turns on a ten-meter coil. In copper, holding that field burns gigawatts, every second, forever.

Superconductors fix the power bill and add a new one: tonnes of cryogenic plumbing, and a coil that can "quench", suddenly turn resistive and dump all its stored energy as heat. Engineering studies, including the European SR2S project in the 2010s, kept running into that wall.

Wind the coil from Neodanium and it runs at room temperature with no power draw and no cryostat. Crank it to thirty million ampere-turns and the storm goes around you.

Notice what still gets through when you switch to cosmic rays. A billion electron-volts is a lot of momentum. Even a perfect magnet thins that rain instead of stopping it. Fleet crews still wear dosimeters. They just stopped dying of them.