Historical research guide · 0176 · 1966
Electric discharge device for producing interactions between nuclei
Philo T. Farnsworth · International Telephone and Telegraph Corporation
In one minute
Philo Farnsworth, the man who invented electronic television, spent his last working years on fusion, and this is the patent that came out of it. He filed it in January 1962 and the patent office granted it in June 1966. The machine is a sphere inside a sphere: an outer cathode fires electrons inward through an almost transparent inner anode, and where those electrons crowd together at the middle they build a virtual cathode — a well of negative potential standing in open space, held there by charge alone, with no physical electrode at the centre at all. Deuterium and tritium ions fall into that well, oscillate straight through it, and fuse. Farnsworth gives numbers: 100 kilovolts, 1,500 amperes of oscillating space current, a reacting core about a millimetre across, and a modulated grid that keeps re-timing the ions so they do not lose energy. Fusion by the shape of an electric field, not by magnets.
Connection
This is the ancestor of every inertial-electrostatic fusion machine that followed, and the earliest full statement of the idea chapter 12 rests on — that fusion is a question of putting particles at the right energy in the right place, not of building a bigger magnet. Its nested virtual electrodes, shells of charge holding their own shape in open space, are also an early patented description of the self-organising plasma structures chapter 9 studies.
From the source
A converging spherical electron current produces a virtual cathode: a real potential minimum standing in free space at the centre of the anode, with no physical electrode there. Farnsworth calculates that 1,500 amperes of oscillating space current at an anode potential of 100 kilovolts establishes it, and that a 99 percent transparent anode draws only 15 amperes to sustain that.Electric field generation, columns 4-6; equations 1-3
Positive ions born anywhere in the anodic space fall toward that minimum and oscillate radially through it, and ions above about 30,000 electron volts make up over 95 percent of the core density; the heavy-ion concentration can have a radius as small as one millimetre.Ion generation, columns 7-9
The tritium-deuterium reaction is the one to build for: it releases 17.6 MeV and its cross-section peaks near 5 × 10⁻²⁴ cm² at 100 keV projectile energy, about 100 times larger than the competing deuterium-deuterium and helium-3 reactions at the same energy.Nuclear reaction, columns 9-11; reactions 4 to 7
Sine-wave modulation of the control grid, at a period slightly longer than the ion transit time and in the region of 10⁸ cycles per second, restores the energy an ion loses to scattering each time it crosses the centre, so the oscillation is maintained rather than damped.Ion generation, column 10; FIGS. 6a-6c
At full drive the space charge organises itself into a multiplicity of nested virtual electrodes of alternating polarity — an innermost sheath deep enough to trap and hold target ions, an outer sheath that accelerates projectile ions through it at fusion energies.Claims 15, 24 and 26
The energy comes out as particles, not heat: the 3.5 MeV alpha lands on the electrodes and the 14.2 MeV neutron is caught in a water jacket, and thermalised neutrons on lithium-6 breed the tritium the machine burns.Power generation, columns 11-12
The way in
https://patents.google.com/patent/US3258402A/enHow to cite it
Philo T. Farnsworth, International Telephone and Telegraph Corporation (1966) Electric discharge device for producing interactions between nuclei US3258402A