ZETA was an experiment devised and built at Harwell in the 1950s to tackle the problem of nuclear fusion.

If successful, it paved the way to the successful exploitation of fusion to produce electrical power.

Sir John Cockroft evidently thought that fusion was taking place during these experiments, and Harwell produced a booklet entitled "Facts about ZETA". These pages provide some excerpts from that brochure. For a brochure intended for popular consumption, it is remarkably detailed in its technical description.

With more than a touch of hubris, there was a section at the end entitled "Questions and Answers on ZETA", which gave some rather confident predictions.

Sadly, it turned out that the neutrons detected were not produced by nuclear fusion, and the result was a considerable letdown for Harwell. More than half a century later, the possibility of nuclear fusion power stations seems just as remote.

The following pages are transcribed from the booklet.

The operation of present atomic power reactors is based on the fission (or speaking) of atoms. The possibility now being explored is the harnessing of power from the fusion (or joining) of atoms which provides the heat for the stars.

Results obtained from the Harwell apparatus ZETA suggest that "thermonuclear neutrons" have been obtained, but further experiments will be necessary before this can be proved conclusively. Temperatures reached in this apparatus have been as high as 5 million degrees Centigrade, higher than the measured surface temperatures of any star.

Many major problems have still be solved before its practical application can seriously be considered and the work must be expected to remain in the research stage for many years yet. If it proves ultimately possible to construct a power station operating on the fusion of deuterium, the oceans of the world will provide a virtually inexhaustible source of fuel.

On 12th August 1957, a large experimental apparatus for studying controlled release of energy from the thermonuclear reactions was started up at the Atomic Energy Research Establishment, Harwell. On 30th August, 1957, this apparatus was first operated under conditions that produced nuclear reactions; neutrons emitted in these reactions were observed when deuterium gas was heated electrically to temperatures in the region from 2 to 5 million degrees Centigrade. The hot gas was isolated from the walls for periods of 2 to 5 thousandths of a second. The heating process was repeated every 10 seconds. The high temperatures achieved, together with the relatively long duration for which the hot gas has been isolated from the tube walls, are the most important experimental results obtained so far. Whilst much longer times (perhaps several seconds) are required for a useful power output that appears to be no fundamental reason why these longer times, together with much higher temperatures, cannot be achieved.

The source of the observed neutrons has not yet been definitely established. There are good reasons to think that they come from thermonuclear reactions, but they could also come from other reactions such as collisions of deuterons with the walls of the vessel, or from bombardment of stationary ions by deuterons accelerated by internal fields produced in some forms of unstable discharge.

In the ZETA apparatus the number of neutrons produced by each pulse of energy as the current was doubled was roughly that which might be expected from a thermonuclear reaction at the measured temperatures. These temperatures have been definitely established.

As the Atomic Energy Authority have stated in their last two annual reports, research has been in progress for some years to investigate the ability of producing energy in a controlled manner from thermonuclear reactions. Over two years ago design began of a large installation for this work and in August 1957 the apparatus, which is called ZETA (for zero-energy thermonuclear assembly) started up, with the results described above stop the reaction being studied in ZETA is that in which deuterons collided with one another and fuse to form heavier nuclei, releasing energy and some neutrons in the process. For fusion to be possible the deuterons must have enough energy to overcome the initial electrical pulses of repulsion between them; this necessitates heating the deuterium gas to temperatures of millions of degrees centigrade. The hot gas must be kept away from the walls of the container otherwise it falls in temperature.

The principle adopted in ZETA is to pass a large electric current through the deuterium gas. This current sets up an electric discharge in the gas which heats it and also produces an intense magnetic field around the column of hot gas. Shis magnetic field causes the discharge to become constricted and hence heated still more. Since it also causes the discharge to wriggle about, this field by itself is not enough to keep the discharge away from the walls. The wriggling has been suppressed by applying an additional steady magnetic field parallel to the axis of the tube.

In ZETA the discharge chamber is a ring-shaped tube or torus of 1 metre bore and 3 metres mean diameter, containing deuterium gas at low pressure. The tube is linked (i.e. encircled over part of its length) by the iron core of a large pulse transformer. A current pulse of electricity is passed into the primary winding of the transformer from a bank of capacitors capable of storing 500,000 joules of energy. This pulse in turn induces a very large unidirectional pulse of current in the gas, which forms a short-circuited secondary for the transformer. Peak currents up to 200,000 amperes have been passed through the ionised gas for periods up to 5 milliseconds. The current pulse is repeated every 10 seconds. Emission of neutrons throughout the current pulse is observed regularly in routine operation of ZETA with deuterium; there are up to 3 million neutrons emitted per pulse.

The temperature of gas discharges may be determined from measurements of the light emitted from the gas atoms but measurements of this kind in these experiments present problems because, at the temperature of the discharge, the hot deuterium atoms are completely stripped of their electrons and therefore do not emit a line spectrum. One method of solving this problem is to mix the deuterium a small quantity of some heavier gas, such as oxygen or nitrogen, the atoms of which are not stripped of all their electrons under these conditions and to study the spectral lines emitted by this impurity; the random motion of the high-energy impurity atoms which make many collisions with the deuterium atoms and so reach the same energy causes the spectral lines to broaden, owing to the Doppler effect, and the amount of broadening is a measure of the ion energy. Many measurements by this method have indicated temperatures in the region of 2 to 5 million degrees Centigrade. Whilst temperatures in this range are required to explain the observed rate of neutron production on the basis of a thermonuclear process, electric fields in the gas arising from instabilities, can also accelerate deuterium ions and lead to nuclear reactions. Such a process was described by academician Kurchatov in his lecture at Harwell in 1956. Therefore it is not altogether certain that the observed neutrons come from a thermonuclear reaction. Experiments are continuing to study the details of the neutron-producing processes.

In order to obtain a net energy from the reaction it would be necessary to heat deuterium gas to temperatures in the region of 300 million degrees centigrade, and to maintain it at this temperature for long enough for the nuclear energy released to exceed the energy needed to heat the fuel and that lost by radiation. The high temperatures achieved in ZETA, and the relatively long duration for which the hot gas has been isolated from the tube walls are the most important experimental results obtained so far. Whilst a much longer time (perhaps several seconds) is required for a useful power output that appears to be no fundamental reason why these longer times, together with much higher temperatures, cannot be achieved. There are, however, any major problems still to be solved before its application can be seriously considered and the work must be expected to remain in the research stage for many years yet.

Questions and Answers on ZETA

Q   What did ZETA cost?

A   ZETA cost about £300,000.

Q   How many people have worked on the ZETA Project?

A   About 50 professional scientists and engineers had been directly engaged.

Q   What are the next steps in this field of research?

A   The current circulating in ZETA is to be increased by the provision of more condensers [modern usage 'capacitors' ]; it is hoped this will increase the temperature to 25 million degrees Centigrade. Even at this temperature the amount of energy produced will be small compared with the amount of energy put in.

To break even (i.e. to produce an amount of energy equal to the energy put in) it will be necessary to obtain temperatures of over 300 million degrees, or above 40 million degrees, in deuterium and tritium ('super heavy' hydrogen). While experiments continue with ZETA, Harwell will design and build its successor – ZETA II – which will aim at achieving the break-even point. This should take about four years. Stage III would be work leading to the construction of a prototype of a practical and economic thermonuclear power station. Stage IV will be commercial application.

Q   If it proves ultimately possible to build fusion reactors which will generate electricity economic, what will be in the long-term significance of this development?

A   ZETA's long-term significance is that, if commercial fusion reactors can be built, a virtually inexhaustible source of fuel will be available in atomic development through out the world.

This makes it possible to contemplate a continued increase in standards of living which would otherwise – 200 or 300 years from now – have to level out or even slip back. It should, however, be borne in mind that practical application for electricity development are not likely to be obtained 10, 20 or even 50 years.

It is not necessarily true that "fusion electricity" will be cheap. The fuel will be in abundant supply any: but it must extract from water, and no one, at this stage, can assess what the capital costs of plant producing power on a commercial scale would be. The present researchers are unlikely to have any effect on the UK nuclear power programme or on UK uranium requirements for at least 20 years. Development of fusion reactors does not necessarily mean (even after 20 years) uranium will no longer be used in fission reactors (of the Calder Hall type). It may well be economic to plan for a combination of the two.

Questions and Answers on ZETA

Q   What did ZETA cost?

A   ZETA cost about £300,000.

Q   How many people have worked on the ZETA Project?

A   About 50 professional scientists and engineers had been directly engaged.

Q   What are the next steps in this field of research?

A   The current circulating in ZETA is to be increased by the provision of more condensers [modern usage 'capacitors' ]; it is hoped this will increase the temperature to 25 million degrees Centigrade. Even at this temperature the amount of energy produced will be small compared with the amount of energy put in.

To break even (i.e. to produce an amount of energy equal to the energy put in) it will be necessary to obtain temperatures of over 300 million degrees, or above 40 million degrees, in deuterium and tritium ('super heavy' hydrogen). While experiments continue with ZETA, Harwell will design and build its successor – ZETA II – which will aim at achieving the break-even point. This should take about four years. Stage III would be work leading to the construction of a prototype of a practical and economic thermonuclear power station. Stage IV will be commercial application.

Q   If it proves ultimately possible to build fusion reactors which will generate electricity economic, what will be in the long-term significance of this development?

A   ZETA's long-term significance is that, if commercial fusion reactors can be built, a virtually inexhaustible source of fuel will be available in atomic development through out the world.

This makes it possible to contemplate a continued increase in standards of living which would otherwise – 200 or 300 years from now – have to level out or even slip back. It should, however, be borne in mind that practical application for electricity development are not likely to be obtained 10, 20 or even 50 years.

It is not necessarily true that "fusion electricity" will be cheap. The fuel will be in abundant supply any: but it must extract from water, and no one, at this stage, can assess what the capital costs of plant producing power on a commercial scale would be. The present researchers are unlikely to have any effect on the UK nuclear power programme or on UK uranium requirements for at least 20 years. Development of fusion reactors does not necessarily mean (even after 20 years) uranium will no longer be used in fission reactors (of the Calder Hall type). It may well be economic to plan for a combination of the two.

Questions and Answers on ZETA

Q   What did ZETA cost?

A   ZETA cost about £300,000.

Q   How many people have worked on the ZETA Project?

A   About 50 professional scientists and engineers had been directly engaged.

Q   What are the next steps in this field of research?

A   The current circulating in ZETA is to be increased by the provision of more condensers [modern usage 'capacitors' ]; it is hoped this will increase the temperature to 25 million degrees Centigrade. Even at this temperature the amount of energy produced will be small compared with the amount of energy put in.

To break even (i.e. to produce an amount of energy equal to the energy put in) it will be necessary to obtain temperatures of over 300 million degrees, or above 40 million degrees, in deuterium and tritium ('super heavy' hydrogen). While experiments continue with ZETA, Harwell will design and build its successor – ZETA II – which will aim at achieving the break-even point. This should take about four years. Stage III would be work leading to the construction of a prototype of a practical and economic thermonuclear power station. Stage IV will be commercial application.

Q   If it proves ultimately possible to build fusion reactors which will generate electricity economic, what will be in the long-term significance of this development?

A   ZETA's long-term significance is that, if commercial fusion reactors can be built, a virtually inexhaustible source of fuel will be available in atomic development through out the world.

This makes it possible to contemplate a continued increase in standards of living which would otherwise – 200 or 300 years from now – have to level out or even slip back. It should, however, be borne in mind that practical application for electricity development are not likely to be obtained 10, 20 or even 50 years.

It is not necessarily true that "fusion electricity" will be cheap. The fuel will be in abundant supply any: but it must extract from water, and no one, at this stage, can assess what the capital costs of plant producing power on a commercial scale would be. The present researchers are unlikely to have any effect on the UK nuclear power programme or on UK uranium requirements for at least 20 years. Development of fusion reactors does not necessarily mean (even after 20 years) uranium will no longer be used in fission reactors (of the Calder Hall type). It may well be economic to plan for a combination of the two.

Picture of ZETA from the inside front cover.

The 'Questions and Answers' page.