Description of the Polywell

The polywell is an approach for a nuclear fusion reactor. It is composed of six current carrying rings positioned in the shape of a cube to create opposing magnetic fields for confining electrons (Fig 1). The opposed fields cancel causing a null point to form at the center. Electrons emitted into the polywell oscillate around the magnetic field lines, and as they reach the strongest magnetic fields at the rings, some encounter a magnetic mirror and reflect back into the polywell (Fig 1).

Strong magnetic fields confine enough electrons to create a sufficient voltage drop, heating ions to fusion conditions. Tests have shown a dense plasma rejects the outside magnetic field leading to a better trap – a phenomena known as cusp confinement (Fig 2). For more on this concept visit Polywell Fusion In Nine Steps by Matthew Moynihan.

Our Polywell: Materials and Methods

A rigorous approach to redesign the vacuum and circuit systems addressed issues preventing their successful attainment from 2015-2017. Objectives, functions, and constraints were morphologically analyzed to ensure optimized performance and minimized cost. Over 100 hours were spent redesigning the vacuum and electrical systems before new parts were purchased. Vacuum and circuit schematics are shown below (Fig 3, 4).

A vacuum chamber was assembled including three electrical feedthroughs, a 6-inch viewport, shutoff valves, and two pressure gauges (Fig 5, 6). The chamber held roughing pressures overnight, indicating a negligible leak rate. Pressures of 0.14 mTorr were achieved with two roughing pumps and an oil diffusion pump. Differential pumping enabled pump down times under 40 minutes.

Two circuits were employed to output 0-200A at 0-3V continuously and 1000A at 450V for less than one second. A step-up transformer and diode bridge charged a 14.4mF 450V capacitor bank. Capacitors were safely discharged by a triggering circuit and a silicon-controlled rectifier (SCR) (Fig 9, 10). Together, these circuits enable the polywell to operate at various magnetic pressures. Validation testing for the high current circuit demonstrated wire vaporization (Fig 7). An independent 24A, 12V DC supply powered electron emitting filaments mounted in the chamber (Fig 8).

Previous Projects: The Farnsworth-Hirsch Fusion Reactor

A Farnsworth-Hirsch demo reactor was built using a vacuum chamber and 6.5kV neon-sign transformer supply. The video below shows a demo reactor constructed from a borosilicate glass chamber. The purple glow results from ionization of nitrogen and oxygen under a partial vacuum. For more information on high voltage breakdown as a function of pressure see Paschen's Law. Farnsworth-Hirsch reactor design is explained in detail on