STM-D-0796Paper1968Designed, not yet built

The Possibility of Producing a Dense Thermonuclear Plasma by an Intense Field Emission Discharge

F. Winterberg

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In one page

Friedwardt Winterberg, writing from the International Centre for Theoretical Physics in Trieste, asks how to light a fusion burn without a fission bomb to start it. Magnetic confinement, he argues, keeps running into instabilities nobody predicted, so he takes the other road: heat a small pellet of liquid or solid tritium and deuterium so fast that it burns before it can fly apart. His arithmetic sets the bar — about fifty million kelvin, a pellet a quarter of a centimetre across, and roughly five and a half megajoules delivered in less than fifty nanoseconds. His driver is a brush of a few hundred needle-sharp field-emission cathodes fired by a Marx-circuit capacitor bank, focusing an electron beam less than a millimetre wide onto the pellet, with a heavy metal backing to hold the target together a moment longer. The beams that existed in 1968 already had the right pulse length and the right spot size, and were short of the required energy by almost a thousand times. This is one of the founding papers of pulsed-power inertial fusion.

Why it matters hereChapter 12 is the chapter about getting nuclei to fuse in something small, and this is the paper that put the whole pulsed-power branch of that programme on paper: no magnetic bottle, no reactor hall, just a capacitor bank, a beam and a pellet. Chapter 9’s subject is the dense, short-lived plasma knot, and Winterberg here writes down what it costs to make one deliberately.

What it claims

01A dense thermonuclear plasma may be produced by irradiating a liquid or solid tritium–deuterium target with electrons or ions from an intense field emission discharge; the discharge energy can come from conventional megajoule capacitor banks using the Marx circuit technique, or from an electrostatic store based on charging a superconducting ring to very high voltage.Abstract
Designed, not yet built
02The ignition conditions are stated as arithmetic rather than aspiration: the fuel must reach about 5 times 10 to the seventh kelvin; the sphere must be at least about 0.25 centimetres in radius for the Lawson criterion to be met against expansion at the plasma sound speed; the expansion time is then about 2.5 nanoseconds; and heating a sphere of that size to that temperature takes about 5.4 megajoules, which must arrive in less than roughly 50 nanoseconds.Section I, conditions (a) to (g), equations 1.2 to 1.11
Settled physics
03The driver is a concave brush of field-emission cathodes — about three hundred tungsten needles with tip radii near a hundredth of a millimetre, fired at ten million volts by a Marx bank — whose electron beam is held to less than a millimetre across by its own self-magnetic field once the space charge is neutralised by a tenuous plasma in front of the target, and whose target sits on a dense high-atomic-number backing that acts as anode and as inertial tamper.Section II, Figure 1 and equations 2.5 to 2.9
Designed, not yet built
04Field-emission discharges of the day already reached about 10 to the twelfth watts for one to ten nanoseconds at several MeV, with beam currents above a hundred kiloamps, and they already had the right pulse duration and the right millimetre focal spot — but their total energy of about ten thousand joules fell short of what ignition needs by almost three orders of magnitude.Section II, the paragraph beginning ‘Experimentally pulsed field emission discharges have been achieved’
Published and peer-reviewed
05Single-particle stopping would defeat the scheme, since ten-MeV electrons range about 30 centimetres in liquid tritium–deuterium, but the counterstream instability deposits the beam energy over a length of order 10 to the minus fourth centimetres, which is short enough for complete absorption; a field above 600 kilogauss from a small theta-pinch coil would curl the electrons inside the target as well, and a field ion emission discharge is offered as the fallback because MeV protons stop within centimetres by ordinary Coulomb scattering.Section II, equations 2.10 and 2.11; Section IV
Designed, not yet built
06Whether a beam can stay stable at the current densities this requires is left open and named as an experimental question, with Budker’s preliminary analysis quoted as the only guide that relativistic streams tend to be more stable than non-relativistic ones.Section II, the paragraph on beam stability citing Budker
What to watch

The way in

https://doi.org/10.1103/PhysRev.174.212The article is closed access under the APS default licence, so no text is reproduced on this page. The full text was read for this sheet from the American Physical Society’s own harvest service at harvest.aps.org (the nine-page article PDF for this DOI), and every claim below is located to a numbered section or equation of that text. The work was supported in part by NASA grant NGR-29-001-016 and was written during a visit to the International Centre for Theoretical Physics of the IAEA in Trieste; Winterberg’s permanent address on the paper is the University of Nevada, Reno.

How to cite it

F. Winterberg (1968) The Possibility of Producing a Dense Thermonuclear Plasma by an Intense Field Emission Discharge. doi:10.1103/PhysRev.174.212

Where it sits in the curriculum

Lattice confinement fusion Plasmoids, charge clusters and the orbs