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The Polywell reactor arranges magnetic fields differently to that of a standard, traditional tokamak. In a tokamak, the fields set up a plasma that has a donut shape. The plasma is free to circulate around the donut but is squeezed to a thin, high pressure stream.

The Polywell concept, instead, tries to create a magnetic box to confine the plasma in place, which reduces the turbulence and solves many control problems. However, a true box is simply not possible. This is because the force applied by a magnetic field depends on the direction of motion of a charged particle, which causes the electrons and ions to travel in a corkscrew motion around magnetic field lines. At each corner of the magnetic field box, the magnetic field lines point outward away from the center of the box, so the plasma can spiral out of the box. The upshot is that the harder you squeeze, the quicker the plasma leaks out, leaving you with a low beta plasma.

To help overcome this, additional high energy electrons are injected into the plasma. The electrons create a large negative potential that draws the ions to the center of the box, slowing their escape. Nevertheless, even with the electrostatic draw slowing ion escape, the magnetic field still wins in the end, because the electrons are also driven to spiral along magnetic field lines.

But researchers quickly realized that if the plasma was dense enough—in other words, if it had a high beta—it would exclude the magnetic field lines, creating a sharp boundary between the plasma and the magnetic field. The sharp boundary acts like a mirror for charged particles, vastly slowing their rate of ion escape. This unfortunately creates a chicken and egg scenario: if you have a high beta plasma, a Polywell design will keep it confined at high beta. But first, you must have a high beta plasma...

Magnetic mirror holds promise for fusion | Ars Technica

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