GB1589298A

GB1589298A - Thyratron circuits

Info

Publication number: GB1589298A
Application number: GB4523676A
Authority: GB
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1976-10-30
Filing date: 1976-10-30
Publication date: 1981-05-13
Also published as: DE2659049A1

Description

(54) IMPROVEMENTS IN OR RELATING TO THYRATRON CIRCUITS (71) We, ENGLISH ELECTRIC VALVE COMPANY LIMITED, of 106, Waterhouse Lane, Chelmsford, Essex CMl 2QU, do hereby de clare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particu early described in and by the following state ment: This invention relates to thyratron circuits.

It is often required to obtain rapid switch ing and rate of rise of current with a thyra tron. A paper entitled "Grounded Grid Thyra tons" in pages 41 to 45 of the IEEE Confer ence Record of the 1976 Twelfth Modulator Symposium describes a technique for utilising specially designed low inductance thyratrons with the control grid connected as the earth return of the load circuit and with triggering applied to the cathode. Such an arrangement is schematically represented in Figure 1 of the drawing accompanying the provisional specification.

Referring to Figure 1, the thyratron 1 is shown with its anode 2, cathode 3 and single grid electrode 4. Grid electrode 4 is earthed at 5. The anode circuit of the thyratron 1 con sists of an inductor 6 which is connected to the junction between a resistor 7 and a capaci tor 8. The other end of resistor 7 is connected to a point 9 of positive HT, whilst the other side of capacitor 8 is connected to earth via a further resistor 10.

Triggering pulses are applied via input ter minals T to the cathode 3 of the thyratron 1, via a d.c. blocking capacitor 11. A leak resistor 12 extends between the cathode 3 and grid electrode 4 of the thyratron.

In practice, with an arrangement as just described a metallic arc is generated between the anode 2 and the grid electrode 4, which whilst resulting in extremely rapid switching and rate of rise of current also results in a relatively short life for the thyratron. The short life is due to erosion of the electrodes by the metallic arc.

The present invention seeks to provide an improved thyratron arrangement in which the self inductance is relatively low, whilst at the same time the tendency to arc is reduced.

According to this invention a thyratron arrangement comprises a thyratron having at least a cathode an anode and a grid electrode connected to earth and wherein means are provided for directing towards said cathode electron current which would otherwise flow via said grid electrode to said anode.

The invention is applicable both to single ended thyratrons and double ended thyratrons such as are the subject of our U.K.

Patent Number 1 334 527.

Where said thyratron includes for the or each cathode more than one grid electrode (for example two or three), the final grid electrode of which is connected to said cathode, the circuit behaves as though the electron emitting electrode were said final grid electrode.

In one embodiment of the invention in which for one cathode, two grid electrodes are provided, the final grid electrode (that is to say the one remote from the cathode) is earthed, means are provided for applying triggering signals to an intermediate grid electrode between said final grid electrode and said cathode and an inductive core is provided in the region of the anode of said thyratron which core is inductively coupled to a connection to the cathode of said thyratron.

In another embodiment of the invention, in which for one cathode, two grid electrodes are provided, the final grid electrode (that is to say the one remote from the cathode) is earthed, means are provided for applying triggering signals to an intermediate grid electrode between said final grid electrode and said cathode and the cathode/grid space of said thyratron is surrounded by a substantially re-entrant metallic canister which is conductively connected both to said final grid electrode and the cathode of said thyratron.

Normally, a bias resistor is connected between said final and said intermediate grid electrode.

The invention is further described with reference to Figures 2 and 3 of the drawing accompanying the provisional specification which schematically illustrates two different embodiments of the present invention.

In Figures 2 and 3 like references are used for like parts in Figure 1.

Referring to Figure 2, it will be seen that the anode load circuit of the thyratron arrange ment, that is to say inductor 6, resistors 7 and 10 and capacitor 8 is identical to that of Figure 1. The thyratron, however, is one in which between the cathode 3 and the anode 2, two control grid electrodes are provided, the first or intermediate grid electrode being referenced 13 and the second or final grid electrode, remote from the cathode 3, being referenced 14.

The final grid electrode 14 is connected to earth and the terminals T are connected to apply triggering signals via the d.c. blocking capacitor 1] to the first or intermediate grid electrode 13. Provided to surround the anode region of the thyratron 1 is an annular iron or ferrite core 15 to which is inductively coupled to a lead 16 connected to earth at one end and to the cathode 3 of the thyratron at the other.

In operation, as the thyratron is triggered, the core 15 transforms any anode current directly (and in one to one ratio) to the initially earthed cathode circuit, thus the cathode current very nearly equals the anode current and this should supply sufficient electrons to ensure conduction without any need for the earthed grid to supply electrons. This in turn makes arcing between grid and anode most unlikely to occur and, as far as the load circuit is concerned the circuit behaves as though the electron current originated from the earthed grid.

Once the cathode/grid space has become ionised, following triggering, current flows into the grid/anode space and initiates the main discharge. The core 15 with its single turn "winding" 16, now behaves as current transformer (with the anode current as the primary) and this causes the cathode current to be equal (or very nearly so), to that flowing to the anode. Initially this tends to drive the cathode negative with respect to the earthed final grid electrode 14, which in turn tends to ensure that the triggering signal is reinforced.

Referring to Figure 3, again the anode circuit of the thyratron is identical to that of Figure 1. In this example, however, a thyratron having two grid electrodes 13 and 14, like those of Figures 2, is employed with the final grid electrode 14 conductively connected to the cathode 3 in accordance with the invention in our co-pending application number 12415/76 (Serial No. 1517847). Triggering signals are applied via d.c. blocking capacitor 11 to the intermediate grid electrode 13.

The conductive connection between the final grid electrode 14 and the cathode 3 is in the form of a metallic canister 17, which is substantially re-entrant and completely sur rounds the cathode/grid space of the thyratron 1, except where passage is required for the con nection to grid electrode 13.

In this case, as with the arrangement shown in Figure 2, the cathode and its adjacent grid electrodes are constrained to supply all of the anode current required, but the effect is as though only the grid/anode gap were involved.

The connection of the final grid electrode 14 to earth and the enclosure of the cathode/grid space by the canister 17, minimises the inductances of the cathode/grid space by field cancellation and leaves mainly the inductance of the anode/grid space in series with the load circuit.

Because the grid/cathode space is triggered (and in fact, pre-ionises in the usual fashion prior to triggering) and because the effective impedance between the final grid electrode 14 and the cathode 3 is minimal, electrons are free to flow from the cathode through the grid electrodes and so to the anode 2. Although, unlike the arrangement shown in Figure 2, no transformer action is involved, after triggering anode current can at all times be supplied from the cathode and again the risk of a metallic arc forming is greatly reduced.

In both the embodiment of Figure 2 and the embodiment of Figure 3, the current flow ing to the triggered intermediate grid electrode 13 may be regarded as minimal.

WHAT WE CLAIM IS: 1. A thyratron arrangement comprising a thyratron having at least a cathode, an anode and a grid electrode connected to earth and wherein means are provided for directing towards said cathode electron current which would otherwise flow via said grid electrode to said anode.

2. An arrangement as claimed in Claim 1 and wherein said thyratron is a double-ended thyratron.

3. A thyratron arrangement as claimed in Claim 1 or 2 and wherein for one cathode, two grid electrodes are provided, the final grid electrode is earthed, means are provided for applying triggering signals to an intermediate grid electrode between said final grid electrode and said cathode and an inductive core is provided in the region of the anode of said thyratron which core is inductively coupled to a connection to the cathode of said thyratron.

4. A thyratron arrangement as claimed in Claim 1 or 2 and wherein for one cathode, two grid electrodes are provided, the final grid electrode is earthed, means are provided for applying triggering signals to an intermediate grid electrode between said final grid electrode and said cathode and the cathode/grid space of said thyratron is surrounded by a substantially re-entrant metallic canister which is conductively connected both to said final grid electrode and the cathode of said thyratron.

5. A thyratron arrangement as claimed in any of the above claims and wherein a bias resistor is connected between said final and said intermediate grid electrode.

6. A thyratron arrangement substantially as herein described with reference to Figure 2 of the drawing accompanying the provisional specififations.

7. A thyratron arrangement substantially as herein described with reference to Figure 3

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

ment, that is to say inductor 6, resistors 7 and 10 and capacitor 8 is identical to that of Figure 1. The thyratron, however, is one in which between the cathode 3 and the anode 2, two control grid electrodes are provided, the first or intermediate grid electrode being referenced 13 and the second or final grid electrode, remote from the cathode 3, being referenced 14.

The final grid electrode 14 is connected to earth and the terminals T are connected to apply triggering signals via the d.c. blocking capacitor 1] to the first or intermediate grid electrode 13. Provided to surround the anode region of the thyratron 1 is an annular iron or ferrite core 15 to which is inductively coupled to a lead 16 connected to earth at one end and to the cathode 3 of the thyratron at the other.

In operation, as the thyratron is triggered, the core 15 transforms any anode current directly (and in one to one ratio) to the initially earthed cathode circuit, thus the cathode current very nearly equals the anode current and this should supply sufficient electrons to ensure conduction without any need for the earthed grid to supply electrons. This in turn makes arcing between grid and anode most unlikely to occur and, as far as the load circuit is concerned the circuit behaves as though the electron current originated from the earthed grid.

Once the cathode/grid space has become ionised, following triggering, current flows into the grid/anode space and initiates the main discharge. The core 15 with its single turn "winding" 16, now behaves as current transformer (with the anode current as the primary) and this causes the cathode current to be equal (or very nearly so), to that flowing to the anode. Initially this tends to drive the cathode negative with respect to the earthed final grid electrode 14, which in turn tends to ensure that the triggering signal is reinforced.

Referring to Figure 3, again the anode circuit of the thyratron is identical to that of Figure 1. In this example, however, a thyratron having two grid electrodes 13 and 14, like those of Figures 2, is employed with the final grid electrode 14 conductively connected to the cathode 3 in accordance with the invention in our co-pending application number

12415/76 (Serial No. 1517847). Triggering signals are applied via d.c. blocking capacitor 11 to the intermediate grid electrode 13.

The conductive connection between the final grid electrode 14 and the cathode 3 is in the form of a metallic canister 17, which is substantially re-entrant and completely sur rounds the cathode/grid space of the thyratron 1, except where passage is required for the con nection to grid electrode 13.

In this case, as with the arrangement shown in Figure 2, the cathode and its adjacent grid electrodes are constrained to supply all of the anode current required, but the effect is as though only the grid/anode gap were involved.

The connection of the final grid electrode 14 to earth and the enclosure of the cathode/grid space by the canister 17, minimises the inductances of the cathode/grid space by field cancellation and leaves mainly the inductance of the anode/grid space in series with the load circuit.

Because the grid/cathode space is triggered (and in fact, pre-ionises in the usual fashion prior to triggering) and because the effective impedance between the final grid electrode 14 and the cathode 3 is minimal, electrons are free to flow from the cathode through the grid electrodes and so to the anode 2. Although, unlike the arrangement shown in Figure 2, no transformer action is involved, after triggering anode current can at all times be supplied from the cathode and again the risk of a metallic arc forming is greatly reduced.

In both the embodiment of Figure 2 and the embodiment of Figure 3, the current flow ing to the triggered intermediate grid electrode 13 may be regarded as minimal.

WHAT WE CLAIM IS: 1. A thyratron arrangement comprising a thyratron having at least a cathode, an anode and a grid electrode connected to earth and wherein means are provided for directing towards said cathode electron current which would otherwise flow via said grid electrode to said anode.
2. An arrangement as claimed in Claim 1 and wherein said thyratron is a double-ended thyratron.
3. A thyratron arrangement as claimed in Claim 1 or 2 and wherein for one cathode, two grid electrodes are provided, the final grid electrode is earthed, means are provided for applying triggering signals to an intermediate grid electrode between said final grid electrode and said cathode and an inductive core is provided in the region of the anode of said thyratron which core is inductively coupled to a connection to the cathode of said thyratron.
4. A thyratron arrangement as claimed in Claim 1 or 2 and wherein for one cathode, two grid electrodes are provided, the final grid electrode is earthed, means are provided for applying triggering signals to an intermediate grid electrode between said final grid electrode and said cathode and the cathode/grid space of said thyratron is surrounded by a substantially re-entrant metallic canister which is conductively connected both to said final grid electrode and the cathode of said thyratron.
5. A thyratron arrangement as claimed in any of the above claims and wherein a bias resistor is connected between said final and said intermediate grid electrode.
6. A thyratron arrangement substantially as herein described with reference to Figure 2 of the drawing accompanying the provisional specififations.
7. A thyratron arrangement substantially as herein described with reference to Figure 3

of the drawing accompanying the provisional specification.