GB2578749A

GB2578749A - Improved rod pinch diode

Info

Publication number: GB2578749A
Application number: GB1818102.4A
Authority: GB
Inventors: Jones Aled; Martin Phillip
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 2018-11-06
Filing date: 2018-11-06
Publication date: 2020-05-27
Also published as: GB201818102D0

Abstract

A rod pinch diode having an elongated anode 12 axially positioned at the centre of an annular cathode 10, the anode having an adlayer of one or more surface compounds selected to liberate hydrogen when the anode is heated by intense pulses of electrons emitted by the cathode. The surface compounds of the adlayer may comprise water or a liquid organic compound, e.g. an oil, applied as a coating on the anode or adsorbed into the metal surface thereof. The ionisation and fragmentation of the hydrogen rich species results in the generation of a plasma at the anode from which a return current of protons is drawn. The inner face of the cathode may be provided with a coating selected to produce neutrons or gamma rays when impacted by said protons.

Description

Improved Rod Pinch Diode This invention relates to Rod Pinch Diodes, which are used in the formation and propagation of particle beams. Rod Pinch Diodes as described, for example in US 4213073, are known, and generally comprise an annular cathode, with an elongate, sometimes tapered, anode extending through the bore of the cathode. The region between cathode and anode is evacuated.

With application of an intense electrical pulse electrons emitted from the cathode reach the anode, they form local surface heating which causes a plasma to form on the anode surface, from which a return current of positive ions is drawn. Magnetic forces due to these currents perturb the current flow, sweeping the point of electron deposition towards the tip of the anode.

This electron beam can then be used by impacting a suitable target to produce x-rays in the conventional fashion for use in appropriate applications as desired.

The Rod Pinch anode may be made out of a number of electrically conducting materials such as steel, copper, brass and tungsten. For many high energy uses Tantalum is used, although this can be expensive and is often hard to machine.

Such pinch diodes have become widely used as x-ray sources for imaging rapidly moving explosive arrangements. A side effect of this type of diode is that the electron flux, as it propagates along the length of the rod, causes localised surface heating. This heating can cause the rod to deteriorate and destruct after a single operation, so multi pulse uses are not possible in such cases.

Whilst electrons may be useful for x-ray generation it is often beneficial to use the properties of the electron flux in other applications. It is an object of the present invention to create a reusable Rod Pinch Diode that can withstand repeated electron pulses to produce a range of diagnostic particles from the return ion current.

Accordingly, the present invention provides a rod pinch diode comprising an elongated anode axially positioned substantially at the centre of an annular cathode, said cathode having an inner circumference closest to the anode and an outer circumference furthest from the anode wherein the anode comprises an adlayer of one or more surface compounds selected to liberate hydrogen when heated.

Such surface compounds include compounds such as water and hydrocarbons, but other hydrogen rich reagents can be used. These may be applied as a surface coating or adsorbed into the metal surface. As the electron stream heats up the anode, these compounds evaporate and the hydrogen in the contaminants provides protons in the plasma. The contaminant might usefully be water or hydrocarbon based. These protons necessarily flow away from the anode and thus a useful proton flux is created.

This ion flux can be used, for example in the detection of fissile actinides via the emission from the actinides of high energy y-rays. This is achieved by using the proton flux to stimulate the emission of neutrons or gammas from the Cathode surface which, if incident on the fissile material will cause fissions within the actinide to occur.

In order to be useful, the flux must be pulsed. i.e. capable of repetition. This induces a problem in that the rod pinch tends to degrade quickly at the high powers necessary. This is largely due to the rod being unable to cope with the pulse of energy deposited without deformation. Accordingly the rod may be comprised of a plurality of different metal regions to manage heating stresses. For example, the rod might comprise steel at one end and tungsten at the other. A skilled person would understand how best to arrange metals in a composite fashion for the purpose of managing heat absorption and durability to control surface temperature rise and thereby ion generation. In experiments, a combination of an aluminium rod having a tungsten sheath and a stainless steel tip has proved to successfully withstand the pulses and temperature profile whilst providing a useable ion flux.

Efficiency in the device, when used as a proton ion source, can be increased by maximising the emission area from the anode plasma and by controlling the path lengths of the electron and ion flows between cathode and anode. Again a skilled person would understand how to manage the emission areas and flow paths.

A second means of controlling heat absorption is to make the rod bigger. Naturally, dimensions of the components of the diode will be selected to meet the shape, power and duration of the applied voltage pulse, in order to generate sufficient temperature rise to form the ion current, but below deformation thresholds of the metal electrodes. Based on the component material or materials different dimensions might be more efficient.

The device can be configured to produce Gamma rays or neutrons. As the protons will follow the field lines back to the Cathode, the inner surface of the Cathode can be coated with suitable materials to stimulate a P-n or a P-y reaction. Suitable materials include compounds rich in 19florine (for y) or 'lithium for neutrons.

The direction of the neutron beam may be pre-determined by forming the coating at the anode in such a way as to promote their emission in a desired direction, with reference to the field lines and thus the direction of incidence of the protons. This would be readily understood by the person skilled in the art.

Gamma, by contrast, are emitted isotropically, so that the direction of the beam would need to be focused or collimated by shielding. It is clear that one could use a combination of atoms to emit both gamma and neutrons, should this be desired.

Experiments have shown that a supersized rod pinch diode with different metal regions as described can support many pulses. In use, it would be advantageous to have some method of replenishing the proton rich surface coating of the anode, for example by spraying a suitable oil onto it after a certain number of pulses.

The set up can produce repeated pulses of 6-7MeV y rays or 60keV neutrons.

The invention will now be described with reference to the following drawings Figure 1 shows a general set up of a rod pinch diode Figure 2 shows a schematic of the pulsed diode in operation Figure 3 shows a cross section of the Anode with metal regions Figure 1 shows an annular cathode (10) and a rod anode (12). The anode (12) is a long rod which is located substantially at the centre of the annulus and projects through axially. The diode operates in a vacuum, in this case of 10-5 Torr (although the vacuum should be below 10-4 Torr). The outer surface of the cathode is treated with oil so as to resist the emission of electrons from its surface.

Figures 2 a,b,c and d show what happens, schematically, when a pulse is applied to the system. In 2a, as the pulse is applied, electrons (22) from the cathode (10) are accelerated through the vacuum onto the anode (12) , and deposit their kinetic energy on the anode surface. This results in heating of the anode surface. Hydrogen rich species on the anode surface (24) such as hydrocarbons or water are fragmented and ionised by the electron beam creating a proton rich plasma at the anode surface. Hydrogen Ions, (26) having the highest charge to mass ratio, accelerate towards the cathode, thereby shielding other species from the electric field.

As the ion current is drawn from the anode, the resultant magnetic field directs the electron beam towards the tip of the rod, sweeping the energy disposition area along the rod surface.

The protons are directed to the inner surface of the annular cathode. Here they impact and deposit their energy into a chosen coating which may lead to gamma or neutron production, depending on the material used to coat the cathode (28) . As gamma rays are emitted isotropically, there will have to be collimation to produce a desired beam. Neutrons can be emitted in a predetermined direction from the impact of the Hydrogen ions, this direction is controlled by ensuring the field lines are appropriately determined by the geometry of the systems and the pulse strength.

In figure 3, the construction of the diode is shown.

In the prior art, one shot diodes have a wide diameter of about 32 mm in cathode bore of 40mm.

the length is about 50 mm The anode is constructed of regions of metals-for example, the sheath can be made of electrically conductive materials which are capable of absorbing energy from multiple MeV electrons, such as tantalum or tungsten. This ensures that the sheath region will heat rapidly during a pulse.

The tip should be manufactured from relatively strong electrically conductive materials which are chosen to resist structural damage caused by intense surface electron bombardment.

The rod beneath the sheath and behind the tip should be constructed of electrically conductive materials of sufficient mechanical strength to support the other anode structures.

In the example shown, the sheath (32) is made of tungsten, with a stainless steel tip (34) and an aluminium rod (36) to support the sheath and the tip.

A means, not shown, of replenishing the coating water, oil or another hydrocarbon is also provided.

The dimensions of the device might be varied to maximise efficiency based on the amplitude of the anticipated electrical impulse used to power it. For example for a pulse of 2-10 MV, variations in geometry might be as follows.

* AK Gap (31) ± 30mm * Rod Diameter (33) ± 10mm * Tip shape (35) ± 10mm * Target Location + 30mm * Rod Protrusion (37) ± 50mm * Rod-Sheath Junction (38) ± 10mm

Claims

Claims 1. A rod pinch diode comprising an elongated anode axially positioned substantially at the centre of an annular cathode, said cathode having an inner face closest to the anode and an outer face furthest from the anode wherein the anode comprises an adlayer of one or more surface compounds selected to liberate hydrogen when heated.
2. A rod pinch diode as claimed in claim 1 in which the anode comprises a first electrically conductive material and a second electrically conductive material, the first electrically conductive material forming a sleeve around at least a portion of the second electrically conductive material wherein the first electrically conductive material heats more rapidly upon impact of an electron beam than the second electrically conductive material.
3. A rod pinch diode as claimed in claim 1 or claim 2 in which the inner face of the cathode comprises a coating selected to emit neutrons when impacted by protons
4. A rod pinch diode as claimed in claim 3 in which the coating on the inner face of the cathode comprises 'lithium atoms
S. A rod pinch diode as claimed in claim 4 in which the coating on the inner face of the cathode is shaped in such a way so that when in operation, the direction of emission of neutrons is predetermined.
6. A rod pinch diode as claimed in claim 1 or claim 2 in which inner face of the cathode comprises a coating configured to emit gamma rays when impacted by protons
7. A rod pinch diode as claimed in claim 3 in which the coating on the inner face of the cathode comprises'flourine atoms.
8. A rod pinch diode as claimed in claim 1 or claim 2 in which the coating on the inner face of the cathode comprises both l'flourine and 'lithium atoms
9. A rod pinch diode as claimed in claim 7 or claim 8 in which the apparatus further comprises a means of collimating gamma rays
10. A rod pinch diode as claimed in claim any of the preceding claims in which the adlayer comprises a liquid organic compound
11. A rod pinch diode as claimed in claim 10 in which the liquid organic compound is an oil.
12. A rod pinch diode as claimed in any of claims 1 to claim 9 in which the adlayer comprises water.
13. A rod pinch diode as claimed in any preceding claim in which the apparatus is provided with a means of replenishing the hydrogen rich adlayer
14. A rod pinch diode as claimed in any preceding claim in which the outer layer of the cathode comprises a coating of oil.