Magnetic mirror diagrams (fig. 11).

Schematic diagrams showing heating in a magnetic mirror device: (a) plasma injected while field is weak; (b) field intensity increased with radial compression of plasma; (c) longitudinal compression by inward movement of magnetic mirrors.

field to increase continuously during the injection period prevents particles reflected at the other end from coming back and striking the source. The field cannot be made to rise indefinitely, however, and there is a limit to injection currents; therefore, there is a limit to the ion density that can be built up in the device by this process.

EXPERIMENTAL PROGRAM

Three main lines of development are being pursued in the Pyrotron program at Livermore, high-compression experiments, transfer and multiple compression and end-injection of energetic ions.

High compression experiments. In this set of experiments, relatively cold plasma is trapped in a weak magnetic field and subsequently heated by compression. The compression time is made short compared to the times anticipated for the loss of the trapped particles. The main losses to be considered are due to collision leakage through the mirrors and to charge-exchange, which was explained earlier.

These experiments have demonstrated the confinement of plasma for periods of several milliseconds, the radial compression of plasma, and the heating of electrons in the plasma to temperatures of about 10 million degrees. A continuing effort is devoted to developing means of measuring the energy distribution of the positive ions, and of injecting plasma with higher initial temperatures. It is hoped that these efforts will lead to final plasma temperatures of the order of tens of millions of degrees, with sufficient plasma density to produce a substantial yield of thermonuclear neutrons.

Transfer and multiple compression. In this experiment, compression of initially cold plasma occurs in three stages. Plasma is injected into an 18-inch diameter vessel provided with a weak mirror field. The plasma is then compressed radially by increasing the field. Through manipulation of the confining field, the plasma is then transferred to a neighboring smaller (9-inch diameter) vessel, there compressed further, and transferred to a third still smaller (3-inch diameter) vessel, where a high field compression is applied. This transfer sequence moves the plasma several feet from the point of origin and is intended to separate the deuterium ions from heavy ion and neutral atom contaminants.

Plasma diagnostics performed during the operation show that compressions and transfers of the plasma occur substantially as intended. A new method of plasma injection, using a gun which emits a blob of plasma at high velocity has been developed. It is expected to provide a relatively hot plasma at the first state so that, after compression,

very high plasma temperatures and densities can be attained in the small volume.

Figure 12 shows a photograph of the apparatus being used in this type of experiment.

End-injection of energetic ions. In this experiment, relatively high energy (5-10 kev.) deuterium ions are injected from a source just inside one of the magnetic mirrors. The ions are injected in a direction nearly perpendicular to the magnetic lines of force (which are parallel to the axis) and spiral onward into the longitudinal center of the device. The field intensity must increase steadily during the injection period in order to provide positive trapping. At the end of the injection period, the magnetic field continues to rise so that the ions are heated adiabatically by compression to about five times their initial energy, which in the case of the device being used at present should be about 25 kev.

This method of injection insures that ions retained for the full cycle will automatically have energies corresponding to about 300 million degrees. The main problem is to inject and retain enough energetic ions to form a plasma of sufficient density for the production of an interesting thermonuclear yield.

Discussion

Transient mirror devices of the type being used at University of California Radiation Laboratory are very complicated pieces of apparatus and experimentation with them is beset with the usual assortment of difficulties. The devices are particularly sensitive to the effects of impurities and a great deal of effort has gone into finding ways to minimize the influx of contaminants. Post's group has been responsible for studying the excitation radiation problem in some detail. Much stress is laid on careful diagnostic work. In spite of present technological difficulties, the magnetic mirror devices are considered very promising. They make excellent devices for research in plasma physics and incorporate a high degree of flexibility.

The mirror geometry, because of its simplicity and convenience has found applications elsewhere. It is used at Oak Ridge in connection with the study of high energy injection, and also at Los Alamos and at the Naval Research Laboratory for the confinement of plasmas produced by colliding shock waves.

Several of the Geneva exhibits will demonstrate key features of the magnetic mirror approach.

Magnetic mirror device (fig. 12). Mirror device used for transfer and multiple compression experiments at University of California Radiation Laboratory at Livermore.

VI

HIGH ENERGY INJECTION

The problems involved in heating a low temperature plasma up to the extreme temperatures required for thermonuclear reactions are known to be formidable. Great difficulties have been encountered in reaching the relatively modest temperatures achieved thus far. By adding energy to charged particles moving essentially at random within a confining field, attempts are made to bring the plasma up to high temperatures through intermediate regions in which (a) the confinement may be inadequate (in general, the higher the plasma temperature, the better magnetic confinement becomes), and (b) other energy loss processes such as excitation radiation and charge exchange, are particularly detrimental. There is some evidence also that heating processes can encourage the initiation of instability phenomena.

However, in principle, there exists a second method, other than heating a low temperature plasma for obtaining high temperature ions. It consists of first adding energy directly to ions, and then trapping or confining them and allowing their motion to become random. One can easily produce a beam of ions having sufficiently high kinetic energy by means of a conventional particle accelerator. The problem then becomes one of using these energetic ions to construct a hot plasma, in which ion motion will be random rather than unidirectional as in accelerator beams.

The mirror machine program at University of California Radiation Laboratory has made use of moderately energetic ions trapped by increasing the intensity of magnetic fields. However, the use of highly energetic ions is the key principle in the approach to thermonuclear power being investigated by Oak Ridge National Laboratory. It was perhaps a natural path for this laboratory to pursue in view of the extensive experience developed at Oak Ridge over the past 15 years in the electromagnetic separation of isotopes.

Injection Process

The most promising methods of trapping a charged particle in a magnetic field appear to involve either: (a) using a time-varying field, as discussed earlier, or (b) causing the charge-to-mass ratio of the