Thyratron gas discharge device with magnetic field for improved ionization

In a thyratron gas discharge device, magnetic material is located coaxially with the anode to produce a magnetic field between the anode and cathode which is substantially parallel to a discharge established between them. This causes electrons emitted from the cathode to have longer path lengths than would otherwise be the case and so the ionization density within the device is increased. This improved this operating characteristics of the thyratron and results in greater utilization of the cathode.

BACKGROUND OF THE INVENTION 
This invention relates to gas discharge devices and particularly, but not 
exclusively, to thyratrons. 
A thyratron generally comprises an anode, a cathode, and an intervening 
grid structure contained within an envelope filled with gas. When it is 
wished to establish conduction through the device, a discharge is produced 
within the thyratron by applying a suitable potential to a control grid. 
SUMMARY OF THE INVENTION 
The present invention seeks to provide improved gas discharge devices. 
According to the invention, there is provided a gas discharge device 
comprising an anode, a cathode and means arranged to produce a magnetic 
field within the device such that charged particles of a discharge have a 
longer path length than they would in the absence of the field whereby the 
amount of ionisation within the device is increased. Charged particles 
which travel parallel to magnetic field lines experience zero force. Those 
which do not move parallel to the field lines experience a force which is 
perpendicular to the direction of travel and the magnetic field lines. 
This results in the particles following a curved path about the field 
lines. Thus electrons emitted from the cathode in a non-parallel direction 
to the magnetic field travel along a helical path as they move towards the 
anode. They therefore have a longer path length when the magnetic field is 
present than would otherwise be the case. This increases the number of 
collisions which occur and hence the ionisation density within the device. 
A gas discharge device in accordance with the invention thus enables 
greater ionisation density to be achieved than would be obtained in a 
conventional device. This may result in an improved rate of voltage fall 
after triggering, a reduction in the triggering energy required and an 
improved cathode life. Also, it has been found that a more uniform 
ionisation in the cathode region is produced, the ionisation extending 
into regions which were previously unused in the absence of a magnetic 
field. 
It is preferred that the magnetic field is arranged to be present during 
switching when a current is passing between the anode and cathode. That 
is, the magnetic field exists during conduction of a pulse through the 
device. Preferably, the magnetic field comprises a component substantially 
parallel to the direction of a discharge within the device. This is 
particularly advantageous as the charged particles which travel in a 
spiral path about the magnetic field component lines tend to be retained 
within the main discharge region. If the magnetic field had only one 
component in a direction inclined to the direction of the discharge, the 
charged particles would tend to be drawn from the discharge region and 
thus ionised particles would be produced in a less effective location. 
Preferably, the means arranged to produce a magnetic field comprises 
magnetic material, which advantageously is samarium cobalt, although an 
electro-magnet could be used. In a preferred embodiment of the device, the 
magnetic material is located at the anode, although it could, for example, 
be located coaxially about the cathode. 
The invention may be particularly advantageously applied where the device 
is a thyratron. At least part of the grid structure may be included in a 
magnetic circuit forming part of the means arranged to produce the 
magnetic field.

DESCRIPTION OF PREFERRED EMBODIMENTS 
With reference to FIG. 1, a thyratron comprises a ceramic envelope 1 within 
which is contained an anode 2, a thermionic cathode 3 and a grid structure 
4 located between them. Hydrogen at a pressure of a few torr is also 
contained within the envelope 1. A cylindrical samarium cobalt magnet 5 is 
located coaxially about the anode stem outside the envelope 1. The part of 
the magnet nearest the cathode is a south pole and the other end a north 
pole. The magnetic field produced within the thyratron by the magnet 5 is 
substantially parallel to the direction normal to the cathode and anode 
surfaces as indicated by the broken lines, which represent magnetic field 
lines. During operation of the thyratron, electrons are emitted from the 
cathode 3. Those which do not travel in a direction parallel to the 
magnetic field follow helical paths about the field lines and are drawn 
towards the grid by the electric field applied to it. Each electron 
travelling along a spiral path has the opportunity to make any more 
ionizing collisions as it moves towards the grid 4 and anode 2 than would 
be the case if it moved in a substantially direct path to the anode 2, 
which would happen if the magnetic field were absent. It has been observed 
tht the region of intense glow usually situated at one side of a cathode 
structure in a conventional thyratron is spread around the cathode fairly 
uniformly in a thyratron in accordance with the invention, indicating 
improved utilisation of the cathode 3. 
With reference to FIG. 2, another thyratron in accordance with the 
invention is similar to that shown in FIG. 1, but includes magnetic 
material 6 located coaxially about the cathode 7 and having pole pieces 8, 
part of the magnetic circuit being formed by the grid structure 9.