The Polywell: A Spherically Convergent Ion Focus Concept

Abstract

Abstract The Polywell spherically convergent ion focus concept for controlled thermonuclear fusion is described. The device magnetically confines electrons by a quasi sphericalcusp magnetic field, forming a potential well. Ions are electrostatically confined by this well, converg ing to a dense focus in the center of the spherical poten tial, where the fusion rate is large because of the high local density of transient energetic ions. Power balance and critical physics issues are outlined, along with cur rent experimental and theoretical work. The potential of the device for D3He operation is described by the derivation of scaling laws for energy gain.

I. Introduction

The Polywell concept, a magnetic version of a spherically convergent ion focus SCIF device, was inventedand proposed by Bussard as a significant variation ofearlier studies on electrostatic confinement. The idea of this device is to inject highenergy electrons intoa quasisphericalcusp magnetic field; the electrons,confined by the cusplike magnetic field, create a potential well of sucient depth to accelerate ions from lowenergy at the periphery to fusion energies within a focusat the center of the sphere. Injection of electrons keepsthe system electrically nonneutral, so that the potentialwell, which accelerates the ions, is maintained at a constant value sucient to confine the ions within the device, returning them again and again at high velocity tothe central focus. Essential to the success of the schemeis that the cusp field needs to confine the electrons sufficiently long that the power required to maintain thecloud of energetic electrons is less than the fusion powerproduced by the convergent ion beams. Equally essentialis that the ions maintain their nonthermal velocity distribution, with primarily radial flow, long enough to produce fusion in the dense focus at the center of thesphere. A schematic of the concept is shown in Figure 1.

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Fusion schemes that use electrostatic fields to accelerateand project ions onto a reacting region have a naturaladvantage for use with advanced fuels such as D3Hecompared with systems that confine a thermal plasma,because the need to heat the plasma to the increasinglyhigh temperatures required for burning advanced fuels isreplaced by a simple increase in the electrostatic accelerating voltage. Although we do not specifically emphasize a particular fuel in this paper, the Polywell SCIFconcept falls into the general category of electrostaticacceleration devices and has the inherent applicability
of that type of device for use with advanced fuels such asD3He. The scaling guides in Table 1 indicate the path tothat application.

The power balance in the Polywell device includes thefollowing:

1. Energy is lost by energetic electrons that leavethe system; this loss depends on the confinement time of electrons by the cusps.
2. Energy is lost in the magnetic coils that confinethe electrons; this loss depends on the strengthof the magnetic field needed to confine theelectrons.
3. Energy is produced by fusion in the dense center; this gain depends on the depth of the potential well and the degree of spherical focus in the ion flow in the device, which in turn deter 100mines the density of ions in the center of the sphere.

This description of the elements of power balance in the Polywell SCIF device highlights the physics behavior,which will determine whether or not the scheme is successful as a fusion reactor.

Implied in the above description is the fact that the plasma in this device is required to be far from thermal equilibrium and that at least the ion velocity distribu tion is far from Maxwellian. The electron orbits in the magnetic field are small compared with the size of the device, as they must be for magnetic confinement of electrons. The ion orbits, by contrast, are comparable to the size of the device, consistent with the idea that electrostatic eects dominate the ion orbits. The orbits of fusion products will be much larger than the size of thedevice since they are more energetic than the fuel ions,whose orbits are comparable to the size of the device.This makes ideas such as direct conversion of chargedparticle energy to electricity an appealing possibility.Finally, the magnetic geometry is magnetohydrodynamically MHD stable by the nature of the cusp fields.

The nonMaxwellian nature of the ion distribution is instrong contrast with the requirements of more typicalmagnetic confinement fusion schemes,9 ,10 which rely onmagnetic fields to confine thermal plasmas of the energyand density required for fusion, with these parametersrelatively constant over the device. In contrast, thePolywell device produces fusion in the dense core, whereions are not, in fact, confined but are passing through onorbits that intersect the core on each of a large numberof passes. The magnetic confinement in the system is ofa much lower density cloud of electrons.

In the next sections of this paper, we describe the physics of this concept in somewhat more detail. The readeris referred to Reference 3 for a definitive description ofthis novel fusion idea.

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