The benchtop fusion machines

Why the library suggests it

Both bench architectures are drawn here beside the large machines, each as a cutaway through its own principle and captioned with the figure its own paper reports.

The machine gallery

Six ways to hold fuel together long enough for nuclei to fuse, drawn as cutaways. Choose an architecture and the panel says how it confines the fuel and what that family has actually measured — each figure the one its own paper reports, and nothing added to it.

Architecture

The sheared-flow Z pinchThe dense plasma focusInertial electrostatic confinementThe tokamakThe stellaratorLaser inertial drive

Selected architecture: The sheared-flow Z pinch

How it holds the fuel. A large current runs straight down a column of gas and the column's own magnetic field squeezes it. Plasma moving faster on the axis than at the edge — a sheared axial flow — keeps the kink from growing.

What it has measured. On FuZE the plasma stays quiet for about sixteen microseconds and fusion neutrons come out steadily for five of them, at a pinch current near 200 kiloamps and a pinch radius of about three millimetres. The yield rises with the square of the deuterium fraction and vanishes with no deuterium, which is the signature of thermal fusion rather than a stray accelerated beam.

Quiet interval:

1.6E-5 s

Steady neutron production:

5E-6 s

Pinch current:

2E5 A

Pinch radius:

3E-3 m

Neutron yield per pulse:

1.25E5

A peak ion energy is not a temperature, and a neutron yield is not an energy balance. Each figure below is labelled with the quantity its own paper reports.

Read the sheet behind these figures

A Tokyo Institute of Technology device running deuterium at 40 kilovolts and 2 milliamps produced ten thousand neutrons a second steadily from a table-top, and the authors then did the harder work of establishing where the fusions happen — finding a virtual anode standing in the centre, in a probe measurement and a simulation that agree (Neutron production rate and plasma characteristics of spherically convergent beam fusion, 2001). The star mode, the virtual anode and the nested virtual cathode of such a device are modelled particle by particle against probe data in Kinetic characteristics of ions in an inertial electrostatic confinement device (2020), and the Defense Intelligence Agency's own survey of the architecture closes with a concrete proposal — twelve radio-frequency ion guns aimed at one core to demonstrate breakeven on hydrogen-boron fuel (DIRD Inertial Electrostatic Confinement Fusion, 2010). On the plasma-focus side, a Chilean group showed that a machine storing 32 to 98 joules still pinches, because it is fast rather than large — a 160 nanofarad bank, 38 nanohenries, current rising in about 150 nanoseconds (Pinch evidence in a fast and small plasma focus of only tens of joules, 2004). A few hundred joules with a 300-nanosecond rise gave about 1.06 million neutrons a shot (Neutron emission from a fast plasma focus of 400 Joules, 2003), and interferometry inside a tens-of-joules machine confirmed that the dynamics, pinch conditions and electron densities of the hall-sized machines are all present (Dynamics and Density Measurements in a Small Plasma Focus of Tens-of-Joules-Emitting Neutrons, 2011).

The experiment or build

Build the inertial electrostatic device first: the parts list is a vacuum chamber, a high-voltage supply, a wire cage and a neutron counter, and the whole thing has been sold commercially for ore inspection. Then build the fast, small plasma focus, which is a pulsed-power exercise — low inductance is the entire design problem. The settling measurement in both cases is the deuterium-deuterium neutron yield per shot or per second, with the yield scaling correctly against current and vanishing when the deuterium is removed, and with the counter calibrated against a known source. That last clause is what separates a fusion machine from a machine that sprays ions at a wall.