US3296442A

US3296442A - Short duration neutron pulse generating system

Info

Publication number: US3296442A
Application number: US231257A
Authority: US
Inventors: Funfer Ewald; Kronast Benedikt
Original assignee: Institut fuer Plasmaphysik GmbH
Current assignee: Institut fuer Plasmaphysik GmbH
Priority date: 1961-10-25
Filing date: 1962-10-17
Publication date: 1967-01-03
Anticipated expiration: 1984-01-03
Also published as: GB1018504A; DE1238586B

Description

Jan. 3,1196? 5. FUNFER ETAL 3,296,442

SHORT DURATION NEUTRON PULSE GENERATING SYSTEM Filed Oct. 17, 1962 .7n venfors:

comes: greater as the distance increases.

United States i Patent 3,296,442 SHORT iDUlRA'IION NEUTRON PULSE GENERATING SYSTEM Ewaldil Fiinfer and Benedikt Kronast, Munich, Germany, assignprs; to Institut fur. Plasmaphysik. Gesellschaft mit beschrankter Haftung, Munich-Garching, Germany Filed Oct. 17,1962, Ser. No. 231,257 Claimsxpriority, applicatizon Germany, Oct. 25, 1961.

17 Claims. c1.2s0--s4.5

s This invention relates to an arrangement for the productionof very short neutron pulses of high intensity and to a method of operating this arrangement. In particular,

l j thelyinvention relates to a tube which delivers neutrons and Iini which.deuterium-ionsare accelerated by means of a high voltage and are caused to hit a deuterium and/ or i tritium ;;target, so that high speed neutrons are formed kvmis. applied. The spacing of the accelerating electrodes .is limited at the. lower end by the minimum distance at which there is a risk of field emission and at the upper end by the tendency toward fiashover as a result of colljision ionization between electrons and ions, which be- The conditions in the tube correspond to the region on the left hand side of the minimumin thePaschen curve (dependence of the ignition voltage. of a gas discharge on the product of pressure and electrode spacing).

Such arrangements are described in: Rev. Sci. Instr. 30/315/59; 31/235/60; 311/241/60; Nucleonics l8/No. 12, pages 69-76 (1960).

. In;addition,xa tube of this type is known, the anode of which consists of a hemispherical metal part which is fused} to a glass tube: At the other end of the glass tube is a target arrangement. and at a short distance in front of which a grid electrode is arranged. This grid electrode is negatively. biased in relation to the target and prevents I the secondary electrons formed at the: target from reaching]; the. anode. Mounted directly on the anode cap is a pulse transformer with an open magnet core which supplies the cap with a high-voltage pulse which ionizes the gasrfilling in the tube. The considerable stray magnetic fields; which increase the probability of ionization in the fregion: of the anode, appear to be essential to the functioning of the tube.. Such a tube is described in Rev. Sci.

. the. gas. filling and an acceleration of the ions produced occurs, yet a low-pressure high-voltage disruptive discharge .does. not. Disruptive discharges are highly undesirable in the known tubes because the discharge then passes over. anywhere between the anode and cathode in an uncontrolled manner, no ions are accelerated towards the .targetn and the neutron yield drops substantially to zero. In general, the disruptive discharge takes place in the form of a surface discharge on the glass envelope In 11 itself, a low-pressure high-voltage disruptive discharge has the desired property that a large number of positive ions are formed without any separate ion source being necessary This advantage is offset, however, by

a very serious disadvantage which hither-to made working with disruptive discharges appear impossible. Here the ionization is not effected mainly by electrons as in disruptive discharges in which the product of pressure and electrode spacing lies above the Paschen minimum, but by ions because the working range lies below the Paschen minimum. The majority of ions are therefore produced in the vicinity of the cathode and the ions which finally impinge on the target cathode have mostly passed through only small differences in potential and therefore have small effective cross-sections for reaction. The yield of the nuclear reactions mentioned at the beginning is actually very greatly dependent on the accelerating voltage in the voltage range which is available in practice and the maximum is only at an ion energy of about 2 m.e.v.

Another desirable characteristic of the disruptive discharge is its short duration. The known pulsated neutron sources all require more or less expensive electronic pulse sources.

It is the object of the invention to provide an arrangement for the production of neutron pulses which arrangement utilizes the advantageous properties of low-pressure high-voltage disruptive discharges but largely avoids their disadvantages. As a result, it is possible to produce a particularly simple neutron source which enables intensive neutron pulses of very short duration to be produced. It is true that with a constant ignition voltage, it is impossible to exert a direct influence on the components of the energy spectrum originating from high-speed ions and neutral particles. In itself, as high an ignition voltage as possible is desirable, but the height of the ignition voltage is limited by technical high-voltage considerations and economic considerations and possibly also by questions of space because the difficulties increase very rapidly when the voltage is raised above about kv.

It has been found, however, that the portion of the energy spectrum originating from the electrons can not only be increased but also shifted to higher energies. In this case, secondary electrons which are formed at the target are utilized to a considerable extent in contrast to the known arrangements in which these secondary electrons are always kept away from the anode by special screening electrodes in front of the target.

An arrangement for the production of neutrons by bombardrnent of a deuterium and/or tritium target with deuterium and/or tritium ions comprising an envelope containing deuterium and/or tritium under reduced pressure, an anode to which a high-voltage pulse can be applied and a target connected to a cathode, is characterised according to the invention in that the envelope geometry is selected in such a manner that the discharge in the form of a low-pressure high-voltage disruptive discharge starts at least substantially centrally on the target and that the anode is of hollow construction, is provided with an aperture at its end facing the target and is shaped in such a manner that the secondary electrons formed at the target and the electrons formed in the discharge in front of the target are accelerated towards the anode by the high voltage prevailing between the anode and cathode and are focused into the interior of the anode, and the ions produced by these electrons in the interior of the anode are withdrawn from the anode by the field reaching through the aperture into the interior of the anode and are accelerated towards the target.

As a result of the fact that the anode is hollow in construction and is provided with an aperture at the end on the target side, the electrons formed at or in front of the target can travel over considerably longer paths and so form a large number of fresh ions substantially at anode potential. By these means, the energy spectrum of the ions impinging on the cathode is given a second lower maximum which lies substantially at the maximum possible energy (which corresponds substantially to the ignition voltage).

The current supply section for the tube according to the invention can be designed in a very simple manner; it only needs to comprise a capacitor which can be charged to a high voltage and can be connected to the tube through a switch, in the simplest case a spark gap. Since, in contrast to the known arrangements, the tube according to the invention is operated with a disruptive discharge, there are considerably fewer restriction with regard to the operating voltage and higher voltage can be used more easily if only because of the simplicity of the arrangement. a

The invention will now be explained in more detail in connection with the drawing which shows examples of embodiments which should not be interpreted to limit the scope of the invention. In the drawing FIGURE 1 shows an arrangement constructed according to the invention for the production of high-intensity short neutron pulses;

FIGURE 2 is a graph of the energy distribution of the ions bombarding the target during a disruptive discharge, and FIGURE 3 shows a further example of an embodiment of a discharge tube according to the invention.

The arrangement illustrated in FIGURE 1 comprises a glass envelope 1 in the form of a hollow cylinder into one of which is introduced a cathode 3, which is curved inwardly in the form of a cup and carries a target 2, and into the other end of which an anode 4 is introduced in a vacuum tight manner. The anode 4 is narrowed like a diaphragm at the end 5 adjacent to the target, as will be described in more detail hereinafter. Inside the anode 4 there may be a reflector rod 6 which is electrically connected to the anode at the end of the tube remote from the target and which carries at its end of the tar-get side a head 7 which is concave towards the front.

The anode 4 is surrounded by a glass sheath 8 which is formed like a diaphragm at its end wall between the anode and cathode.

The external shape of the front end 5 of the anode is determined primarily by considerations concerning highvoltage techniques, and the radius of curvature must not be too small lest premature disruptive discharge should occur as a result of field emission. The size of the aperture at the front of the anode is selected in such a manner that, on the occurrence of a disruptive discharge, a tubular plasma of sufficient diameter is formed to cover a sufiiciently large area of the target 2 which is secured to the cathode 3. In addition, the aperture should allow the cathode field to extend to a certain extent into the anode as shown by the equipotential areas indicated by broken lines.

The diameter of the front aperture in the sheath 8 is selected equal to or somewhat smaller than the diameter of the anode aperture. On the one hand, the glass sheath 8 serves the purpose of preventing the discharge from starting at an unwanted point on the outside of the anode and, on the other hand, it acts as a lens electrode because it is negatively charged by the secondary electrons emerging from the target.

In selecting the spacing of the individual parts of the tube, the following conditions should be taken into consideration: The distance between the front end 5 of the anode 4 and the target 2 mounted on the cathode 3 should be as small as possible in order to keep as low as possible the effects of charges in charge of the ions accelerated by the anode to the target. The lower limit is determined by the setting in of field emissions. The distance between the anode 4 and the glass sheath 8 must be small enough to be able to prevent the discharge from starting externally on the cylindrical part of the anode and, on the other hand, should not be too small because otherwise field emission may occur.

The distance between the sheath 8 and the outer wall 1 is determined by similar considerations: if the distance is too great the discharge strikes upwards between the sheath and the envelope of the tube to the upper terminal electrode 11; if the distance is too small, unwanted discharges or disruptive discharges may occur in the space in between, caused by wall charges and field emission. The wall thickness of the sheath must correspond at least to the dielectric strength for the operating voltage.

Finally, these considerations also apply to the cathode 3. The distance between the cylindrical wall of the cathode and the glass envelope ll must be so narrow that the discharge cannot start at the cathode between it and the glass envelope. In order to prevent the discharge from starting in the region of the fusing of the cathode to the glass envelope, the cylindrical part of the cathode must be suificiently long. The fact must also be taken into con sideration that target material is vaporized during the discharges and may accumulate at the diaphragm-like c0nstriction of the glass sheath so that field emission could occur at the glass sheath. In the most unfavourable case, charges may form on the glass walls which are at the full anode or cathode potential.

A tube constructed in practice had the following dimensions although these are not to be interpreted as being restrictive:

Length of the anode 4 About 300 mm. Diameter of the anode About 54 mm. Diameter of the anode aperture About 25 mm. Diameter of the sheath aperture About 25 mm. Wall thickness of the sheath About 5 mm. Length of the cathode (axial) About mm.

Spacing: Millimeters Anode 4-sheath 8 (radial) 3 Anode 4sheath 8 (axial) 45 Sheath S-envelope 1 3-4 Target 2-sheath 8 (axial) 17 Cathode 3-envelope 1 (radial) 10 The diameter of the glass diaphragm substantially determines the diameter of the tubular plasma being formed and hence the thermal loading of the target zone at which the discharge starts. The diameter of the sheath aperture is preferably smaller than that of the target and that of the anode.

During the high-voltage disruptive discharge, electrons, most of which are released from the cathode by the bombarding ions, fly into the hollow anode 4. Because of the length of path extended into the anode and the probability of ion formation is increased and the electrons produce ions substantially at anode potential. However, the ions must be prevented from being produced at such a great distance behind the anode aperture that the field extending from the cathode into the anode is no longer able to withdraw the ions out of the anode and to accelerate them towards the target. In order to ensure this, the depth of the anode cavity is limited. The anode cavity is preferably spherical in shape or has the shape of an ellipsoid of revolution. In the embodiment illustrated in FIGURE 1, this shape is approximately obtained 'by the configuration of the inner wall at the end of the anode at the target side and by the reflector rod 6 which extends coaxially forwardly from the terminal electrode 11 into the anode and which terminates in a concave end at the front. The electrons impinging on the anode and the re flector rod are difiused back and are subjected to multiple reflection in the course of which their probability of ionization is particularly great because of the relatively low energy and the great length of path.

The reflector rod 6 not only serves for the back diffusion of the part of the electrons which are focussed into the hollow anode but also co-acts with the hollow anode to form a high-frequency tank circuit which is excited to natural oscillation by the voltage variations between the anode, and cathode and by is:higher, than the disruptive voltage of the tube.

odeat the target; 1 not involve the spatial; restrictions which are caused by the inwardly curved oathode 3 of FIGURE 1.

interaction with the injected electrons: The high-frequency alternating field developing in the vicinity of the anode aperture lengthens the electron paths and accelerates slow electrons.

In addition, an annular electrode 13, which is electrically connected. to the cathode 3, may be provided externallyi on the glass envelope 1. This electrode produces anelectrical field extending radially in the region of the front: end of the anode and so contributes to the prevention of the development of surface discharges between the fusion points :of the anode and the cathode. The annular electrodefl 13 may be connected to the cathode 3 through aninductancez: 12 so that an oscillatory circuit is formed which; circuit is preferably. tuned to the frequency of the tank circuit formed-by=the anode 4 and reflector rod 6.

The tannular. electrode 13. and the mentioned high-frequency oscillating circuits also lead to a reduction in the ignition pressure which is highly desirable because of the associated .reduction in changes in charge phenomena.

By. this means the ignition pressure could be reduced for example, from 0.14 torr to 7X torr of the deuterium filling with an operatingvoltage of 150 kv.

i which: can be charged to a high voltage, for example 150 kv. or more by means of a known arrangement which is not illustrated, is connected to the anode and cathode through: a high-voltage switch, in the simplest case a spark gap 10. The ignition voltage of the spark gap 10 In the simplest case, the capacitor 9 may consist of a plurality of. lengths of high-voltage cable connected in parallel. The inductance of the supply conductors should be kept as low as possible if short neutron pulses are required.

FIGUREZ shows the energy spectrum during a disruptive, discharge. The number N of the ions is plotted in arbitrary units as ordinates against the energy E with which; the ions impingeon'the target. The maximum possible; energy (ignoring any superimposed highfrequency oscillations) corresponds to the ignition voltage Z. 1. The curve shown in broken lines indicates the energy distribution for a disruptive discharge in a known tube while the full lcurve shows the energy distribution of the ions, in a tube according to theinvention. It will be seen that in the vicinity of themaximum energy Z, the distribution curve forms a maximum whichis responsible for the neutrons produced in the tube. FIGURElB shows another example of an embodiment of a tube in which the target2' is concave in construction and is mounted on the inner wall of the metallic cathode 3 which forms :a part of the tube envelope. The cathode cap is fused to a glass cylinder 1 further up in known manner. Otherwise. the parts of the tube, only part of which is illustrated, correspond to those in FIGURE 1; corresponding reference numerals are provided with, a prime. i

i In operation, the disruptive discharge takes place over the longestlpossible path, that is to say between the front end 5' of; the anode andthe concave target 2', corresponding to the left-hand region. of the Paschen curve. The secondary electrons formed are focussed by the concave targetlinto the anode aperture. A flashover between the cathode and the :upperterminal electrode of the anode,

which is not illustrated,,is prevented by the short distance between th anode. and cathode and between the anode and the glass envelope respectively. The distances should not, however, be so short that there is a risk of field emission.

The substances to be irradiated with neutrons can be arranged round: the tube or immediately; outside the cath- The tube illustrated in FIGURE 3 does Naturally, a concave reflector rod may also be used in FIGURE 2 instead of the plane reflector rod illustrated. The interior of the anode may be lined with titanium or another material which absorbs deuterium and then acts as a deuterium store. Deuterium ions are then released pulse-wise as a result of the heating caused by the disreuptive discharge. The arrangement in FIGURE 1 may likewise comprise a target which is concave in respect of the anode in order to focus the secondary electrons into the anode aperture.

In order to increase the probability of ionization in the anode cavity still further, magnetic fields may be provided in a manner known per se; thus it is possible, for example, to construct the stem of the reflector rod 6 in the form of a permanent magnet or to construct the head of the reflector rod from a permanent-magnet material with a sufliciently high Curie point.

The tubes described render it possible to produce very short neutron pulses of high intensity with a minimum expenditure; for example, the duration may be between about 10- seconds and 10- seconds and the neutron peak source about 10 neutron sec." in D(dn)He reactions and about 3X10 neutron sec.- in T(dn)He reactions. Since no complicated electronic devices are necessary for operation, the operating voltage can be raised to the extent permitted by economic considerations, the high-voltage sources available and insulation problems. What we claim is:

1. A device for the production of neutron pulses by bombardment of a target with ions, comprising, in combination:

(a) an envelope;

(b) a free gas in said envelope under reduced pressure and adapted for a gas discharge;

(c) a target in said envelope;

(d) a hollow anode in said envelope spaced from said target and having an aperture facing said target;

(e) a cathode spaced from said anode and on which said target is mounted;

(f) means for applying a high voltage pulse between said anode and said cathode for producing a lowpressure high-voltage disruptive discharge between said anode and said cathode; and

(g) means for focussing into the interior of the anode the electrons which are formed at the target and in the region of the discharge in front of the target.

2. A device as defined in claim 1, wherein said means for applying a high voltage pulse between said anode and said cathode includes a capacitor which is periodically charged to a high voltage and then discharged, and a high voltage switch means for connecting said capacitor, which is charged to a voltage which is higher than the breakdown voltage between said anode and said cathode, directly across said anode and said cathode.

'3. A device for the production of neutron pulses by bombardment of a target with ions, comprising, in combination:

(a) an envelope;

(b) a gas in said envelope under reduced pressure and forming an ion source;

(c) a target in said envelope;

(d) a hollow anode in said envelope spaced from said target and having an aperture facing said target and to which a high voltage pulse can be applied; (e) a cathode spaced from said anode and on which said target is mounted;

(f) means for producing a low-pressure high-voltage disruptive discharge in said envelope; and

(g) means for focussing into the interior of the anode the electrons which are formed at the target and. in the region of the discharge in front of the target, said focussing means including the target being concave with respect to the anode so that secondary electrons formed on the target are focussed into the aperture in said anode.

4. A device for the production of neutron pulses by bombardment of a target with ions, comprising, in combination:

(a) an envelope;

(b) a gas in said envelope under reduced pressure and forming an ion source;

(c) a target in said envelope;

(d) a hollow anode in said envelope spaced from said target and having an aperture facing said target and to which a high voltage pulse can be applied;

(e) a cathode spaced from said anode and on which said target is mounted;

(g) means for focussing into the interior of the anode the electrons which are formed at the target and in the region of the discharge in front of the target, said focussing means including a glass sheath surrounding said anode, spaced from said envelope and having an aperture adjacent to and coaxial with said anode aperture.

5. A device as defined in claim 4 wherein said glass sheath surrounds the end of the anode having the aperture as a diaphragm and the glass sheath aperture is at most equal to the anode aperture.

6. An arrangement as claimed in claim 1, characterised in that the interior of the anode is at least approximately an ellipsoid of revolution in shape and that its depth is such that the ions produced in the interior are accelerated towards the cathode by the voltage set up between the anode and cathode.

7. An arrangement as claimed in claim 1, characterised in that the anode is a hollow cylinder and is narrowed like a diaphragm at the front end (5) adjacent to the target.

8. A device for the production of neutron pulses by bombardment of a target with ions, comprising, in combination:

(a) an envelope;

(b) a gas in said envelope under reduced pressure and forming an ion source;

() a target in said envelope;

(e) a cathode spaced from said anode and on which said target is mounted;

(f) means for producing a low-pressure high-voltage disruptive discharge in said envelope;

(g) means for focussing into the interior of the anode the electrons which are formed at the target and in the region of the discharge in front of the target, said anode being a hollow cylinder and narrowed like a diaphragm at the front end adjacent to the target; and

(h) a reflector rod arranged substantially coaxially in the interior of the anode cylinder.

9. A device as defined in claim 8 wherein said envelope is glass and further comprising an annular electrode mounted externally on the glass envelope in the region of the anode aperture and electrically connected to said cathode.

10. An arrangement as claimed in claim 8, characterised in that the reflector rod terminates in a concave disc.

11. An arrangement as claimed in claim 7 characterised in that the inner wall merging into the aperture in the front end of the anode is shaped in such a manner that it approximates a part of a spherical surface.

12. An arrangement as claimed in claim 1, characterised in that the cathode is in the form of a cup (3) which extends into the interior of the envelope (1) and the end of which carries the target (2).

13. An arrangement as claimed in claim 9, characterised in that an inductance (11) is inserted in the connection between the annular electrode (13) and the cath- 8 ode (3) and that an oscillatory circuit which is thus formed is preferably tuned to the frequency of a tank circuit formed by the anode cylinder (4) and the reflector rod (6).

14. An arrangement as claimed in claim 1, characterised in that the cathode (1) forms a portion of the envelope and surrounds the end (5) of the anode (4) at the target side; that the end of the cathode on the anode side is fused to one end of an insulating tube (1') to the other end of which the anode (4') is fused; that the target (2') mounted on the inner wall of the cathode is concave in construction and that the radial distance between the anode and the cathode and the configuration of the end (5) of the anode on the target side are so small that the disruptive discharge takes place between the front end of the anode and the target.

15. A device for the production of neutron pulses by bombardment of a target with ions, comprising, in combination:

(a) an envelope;

(b) a gas in said envelope under reduced pressure and forming an ion source and being one selected from the group consisting of deuterium and tritium;

(c) a target in said envelope and being one selected from the group consisting of deuterium and tritium;

(d) a hollow cylindrical anode in said envelope spaced from said target and having an aperture facing said target and to which a high voltage pulse can be applied;

(e) a cathode spaced from and opposite to the aperture in said anode and on which said target is mounted;

(f) means including a capacitor for periodically producing a low-pressure high-voltage disruptive discharge in said envelope; and

(g) means for focussing into the interior of the anode the electrons which are formed at the target and in the region of the discharge in front of the target, and including a glass sheath surrounding said anode and having an opening adjacent to and coaxial with said anode opening.

16. A device for the production of neutron pulses by bombardment of a target with ions, comprising, in combination:

(a) an envelope;

(e) a cathode spaced from said anode and on which said target is mounted;

(f) means including a capacitor for producing a lowpressure high-voltage disruptive discharge in said envelope; 'and (g) said target being concave with respect to said anode for focussing into the interior of the anode the electrons which are formed at the target and in the region of the discharge in front of the target.

17. A device as defined in claim 1 wherein the free gas is deuterium or tritium.

References Cited by the Examiner UNITED STATES PATENTS 9/1959 Graves et al. 313-61 7/1964 Carr 250-845

Claims

1. A DEVICE FOR THE PRODUCTION OF NEUTRON PULSES BY BOMBARDMENT OF A TARGET WITH IONS, COMPRISING, IN COMBINATION: (A) AN ENVELOPE; (B) A FREE GAS IN SAID ENVELOPE UNDER REDUCED PRESSURE AND ADAPTED FOR A GAS DISCHARGE; (C) A TARGET IN SAID ENVELOPE; (D) A HOLLOW ANODE IN SAID ENVELOPE SPACED FROM SAID TARGET AND HAVING AN APERTURE FACING SAID TARGET; (E) A CATHODE SPACED FROM SAID ANODE AND ON WHICH SAID TARGET IS MOUNTED; (F) MEANS FOR APPLYING A HIGH VOLTAGE PULSE BETWEEN SAID ANODE AND SAID CATHODE FOR PRODUCING A LOWPRESSURE HIGH-VOLTAGE DISRUPTIVE DISCHARGE BETWEEN SAID ANODE AND SAID CATHODE; AND (G) MEANS FOR FOCUSSING INTO THE INTERIOR OF THE ANODE THE ELECTRONS WHICH ARE FORMED AT THE TARGET AND IN THE REGION OF THE DISCHARGE IN FRONT OF THE TARGET.