Historical research guide · 1091 · 2001
Neutron production rate and plasma characteristics of spherically convergent beam fusion
Kunihito Yamauchi · Yasushi Takeuchi · Yutaka Ogino · Masato Watanabe · Akitoshi Okino · Yoshitaka Sunaga · Eiki Hotta
In one minute
An inertial-electrostatic confinement device is the simplest fusion machine anyone can build: a wire cage inside a vacuum chamber, held tens of kilovolts negative, so that deuterium ions fall inward through the gaps and meet in the middle. Yamauchi, Takeuchi, Ogino, Watanabe, Okino, Sunaga and Hotta at the Tokyo Institute of Technology built one and measured it properly. Running deuterium at a few millitorr, 40 kilovolts and 2 milliamps, it produced ten thousand neutrons a second, steadily, from a table-top. They then asked the harder question — where are the fusions actually happening — and answered it two ways. The neutron count rises with current a little faster than in a straight line but well short of squared, which says most reactions are fast ions striking slow background gas rather than beam meeting beam. And both a probe pushed into the plasma and a particle simulation show the same thing: a second potential hill, a virtual anode, standing in the very centre.
Connection
Chapter 12 is the ledger of fusion routes, and this is the cheapest working one: real deuterium-deuterium neutrons out of a wire cage and a power supply, characterised well enough to say which collisions produced them. Chapter 9 gets the virtual anode — a plasma building its own potential structure in mid-air and holding it — measured and simulated in the same paper.
From the source
The device is a 45 cm diameter, 31 cm high stainless-steel cylindrical chamber holding a 30 cm spherical mesh anode and, inside it, an open spherical grid cathode 5 cm across made of 2.0 mm stainless-steel wire, run on hydrogen or deuterium at a steady 1 to 15 millitorr.Abstract; Section 2, Fig. 2 and Fig. 3
With deuterium, a steady-state neutron production of ten thousand per second was measured at a discharge of 40 kilovolts and 2 milliamps, using a helium-3 counter mounted 25 cm above the device centre and converting the dose rate on the assumption that emission is spherically symmetric.Abstract; Section 3.3, Figs. 7 and 8
Both the electrostatic-probe measurement and the particle simulation confirm that a virtual anode forms in the central part inside the grid cathode — a second potential structure, built by the space charge of the converging ions themselves, that traps and re-focuses them.Abstract; Section 3.2, Fig. 6 and Section 4.2, Fig. 10
Neutron production rises with cathode voltage along the curve expected from the deuterium-deuterium reactivity if every reaction were a beam ion striking background gas with kinetic energy equal to the applied cathode voltage; the measurement follows that curve fairly closely.Section 3.3, Fig. 7
Neutron production scales as roughly the 1.3 power of discharge current, which sits between the linear scaling of beam-background reactions and the quadratic scaling of beam-beam reactions. Fitting the rate as a linear term plus a quadratic term and plotting the ratio of the two against cathode voltage shows the beam-background share growing as the voltage rises, and shows that below about 4 milliamps in this device beam-background reactions dominate: beam-beam collisions have the larger cross-section but happen only in a very small volume near the centre.Section 3.3, Eq. 1 with Figs. 8 and 9
The particle simulation is one-dimensional in position and two-dimensional in velocity, using ten thousand superparticles each for ions and electrons started from a uniform density with a Maxwellian spread, at 0.026 eV for ions and 2.0 eV for electrons; spherical symmetry lets the self-generated magnetic field cancel and be neglected. The measured Paschen curve also shows deuterium breaking down at roughly twice the voltage of hydrogen at the same pressure-times-distance.Section 4.1, Table 1 and Section 3.1, Fig. 5
The way in
https://doi.org/10.1002/eej.1How to cite it
Kunihito Yamauchi, Yasushi Takeuchi, Yutaka Ogino, Masato Watanabe, Akitoshi Okino, Yoshitaka Sunaga, Eiki Hotta (2001) Neutron production rate and plasma characteristics of spherically convergent beam fusion doi:10.1002/eej.1