US3903489A

US3903489A - Plasma waveguide switch permitting triggering with both cathode and waveguide grounded

Info

Publication number: US3903489A
Application number: US415894A
Authority: US
Inventors: David C Schubert
Original assignee: US Air Force
Current assignee: US Air Force
Priority date: 1973-11-14
Filing date: 1973-11-14
Publication date: 1975-09-02
Anticipated expiration: 1992-09-02

Abstract

A plasma waveguide switch is provided which permits triggering with both the cathode and waveguide grounded. An anode and a slotted grid are made to act as part of the walls of the waveguide for RF purposes while being isolated from the waveguide with respect to DC and video signals.

Description

United States Patent [1 1 Schubert PLASMA WAVEGUIDE SWITCH PERMI'I'IING TRIGGERING WITH BOTH CATHODE AND WAVEGUIDE GROUNDED [75] Inventor: David C. Schubert, Pasadena, Md.

[73] Assignee: The United States of America as represented by the Secretary of the Air Force, Washington, DC.

[22] Filed: Nov. 14, 1973 [21] Appl. No.: 415,894

[52] U.S. Cl. 333/98 S; 313/205; 315/39 R [51] Int. Cl. 333 13;HO1P H14 [58] Field of Search 333/98 S, 99 MP, 13;

[56] References Cited UNITED STATES PATENTS 3,480,828 11/1969 Goldie 313/13 FOREIGN PATENTS OR APPLICATIONS 65,567 2/1956 France 333/13 [4 1 Sept. 2, 1975 OTHER PUBLICATIONS Goldie, H. S-Band Thyratron Waveguide Switch as a pretrigged Megawatt Duplexer, Microwave Jr. 8-1968, pp. 41-47.

Primary Examiner-Alfred E. Smith Assistant Examiner-Wm. H. Punter Attorney, Agent, or Firm-George Fine; Joseph E. Rusz [5 7] ABSTRACT A plasma waveguide switch is provided which permits triggering with both the cathode and waveguide grounded. An anode and a slotted grid are made to act as part of the walls of the waveguide for RF purposes while being isolated from the waveguide with respect to DC and video signals.

2 Claims, 6 Drawing Figures PLASMA WAVEGUIDE SWITCH PERMI'ITING TRIGGERING WITH BOTI-I'CATHODE AND WAVEGUIDE GROUNDED CROSS REFERENCE TO RELATED APPLICATION This invention is related to the US. patent application filed at an even date herewith entitled, AN INSU- LATING CONE PLASMA ENHANCEMENT CON- FIGURATION FOR A PLASMA WAVEGUIDE SWITCH," by David C. Schubert, now U.S. Pat. No. 3,845,427, and may be utilized in the aforesaid U.S. patent application in the modification thereof.

BACKGROUND OF THE INVENTION Plasma waveguide switches (PWS) for broadband control of RF energy transmission have undergone many years of development. They consist of a section of waveguide with pressure windows and a suitable atmosphere for supporting a glow discharge. Slots or holes in the walls of the guide permit the establishment of an externally initiated discharge with high currents passing from a cathode on one side of the guide to an anode on the other side. A high density plasma may thus be established inside the waveguide by suitable excitation of the external anode and cathode. Radio fre quency waves directed into one end of the waveguide section are reflected back to their source when plasma of sufficient density fills the guide, but they pass through the guide virtually unimpeded when the plasma is absent.

Performance specifications for this type of tube consist of the RF isolation provided, the RF insertion loss in the absence of a discharge, the speed of charge buildup and decay when the discharge is initiated and concluded, and the external circuit requirements. As the designed operating frequency for the PWS increases, it becomes necessary to inject plasma at higher electron density into a smaller section of waveguide. Injection of the required charge density into the waveguide through a charge focusing configuration and entrance slot configuration which does not produce excessive insertion loss when the plasma is absent becomes progressively more difficult as the RF frequency is raised. In practice the measures designed to optimize high frequency isolation and insertion loss produce a tube which requires a high anode voltage and high trigger energy to initiate the discharge. The high anode voltage increases the tendency for a discharge to start spontaneously between the anode and the grid. Prevention of this condition, known as kickout," requires precise control of the gas composition and pressure. In addition, the need to avoid insertion losses at insulated flanges dictates that the waveguide section must be operated at DC and video ground potential. If the cathode is also operated at ground potential, there is no potential drop available to 'drive into the waveguide the plasma generated near the cathode. For this reason, it

is has been impossible in conventional configurations to pulse the PWS with both cathode and waveguide grounded. The circuit which does pulse successfully the PWS when the waveguide is grounded has the disadvantage that the pulse generator is loaded with the inter-turn capacitance of the filament transformer.

Thus any change of configuration which made the PWS easy to fire with a low anode voltage would be de' sirable, and one which did this'with the waveguide and cathode both grounded is the subject matter of this invention. This is accomplished by increasing the penetration of the anode field throgh the perforated waveguide section without introducing unacceptable insertion losses when the charged plasma is absent.

SUMMARY OF THE INVENTION A plasma waveguide switch is provided which permits triggering with both the cathode and waveguide grounded. An anode and a slotted grid are made to act as part of the walls of a plasma waveguide switch waveguide for RF purposes while being isolated from the waveguide with respect to DC and video signals. This arrangement greatly eases the problem of pulsing the plasma waveguide switch even to the heretofore impossible extent of pulsing the switch with cathode and waveguide grounded. The isolation of plasma generated in the cathode-gridregion from the anode can be reduced by an order of magnitude if either the anode or the control grid is built into the broad wall of the waveguide. It can be eliminated altogether if boththe anode and the control grid are built into opposite broad walls of the waveguide. The order of magnitude increases of flux directed from the control grid to the anode occur when one or both of these electrodes are built into the broad wall of the waveguide. It is noted that in the conventional grounded waveguide arrangement, two grounded screens (the opposite broad walls, pierced by plasma slots) in cascade capture practically all of the flux lines from the positive anode. Where the anode replaces part of one waveguide there is eliminated the screening effect of one set of slots and webs. An increased, but still small, portion of the flux lines penetrate the waveguide and ends on the control grid. Where the conventional anode is used, but the control grid is built into the waveguide wall, a few anode flux lines reach the control grid thus ensuring that any plasma drawn to the face of the control grid would be exposed to the breakdown producing anode field. With both anode and control grid built into the waveguide walls, virtually the entire field from the anode is available to accelerate electrons which reach the vicinity of the control grid. Thus one or two free electrons near the control grid will normally be all that is needed to initiate the discharge, and starting anode potentials much beyond I00 V are not needed.

DESCRIPTION OF THE DRAWINGS FIG. I shows the prior art conventional plasma wave- 4 guide switch;

FIG. 2 shows a plasma waveguide switch with the anodes as part of the waveguide wall;

FIG. 3 shows a plasma waveguide switch with a slotted cone face as part of the waveguide wall;

FIG. 4 shows a plasma waveguide switch with both a trigger electrode and the anode built into the waveguide walls;

FIG. 5 shows the geometry for providing an RF short to the waveguide wall for anode or trigger electrode while maintaining DC isolation; and

FIG. 6 shows an alternate geometry for DC isolation of the anode or trigger electrode with an RF short circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, there is shown a conventional prior art grounded plasma waveguide switch. Curved at flange 13b is pressure window 17 for the outputting of the RF signal. Pressure windows 16 and 17 also serve as airtight seals for waveguide 13.

Housings

18 and 19 are attached to and integrated with waveguide 13 and also are airtight sealed thereto. They serve as insulating envelopes. In housing 18, there is positioned anode 20 which is connected to anode lead 21. Anode lead 21 is air-tight sealed with housing 18. Housing 19 has disposed therein heater 22 having airtight sealed leads 22a and 221;. Also included in housing 19 is cathode 23 having airtight sealed lead 23a. Focusing conducting cone is also positioned in housing 19 and connected thereto is electrode 11 which is airtight sealed to housing 19. Airtight plasma waveguide switch is comprised of waveguide 13, housings l8 and 19 and the associated elements therein.

The airtight plasma waveguide switch permits a low pressure controlled atmosphere operation therein. The aforesaid atmosphere is suitable for supporting a glow discharge. Slots or holes 140-1412 in the walls of waveguide 13 permit establishment of an externally initiated discharge withhigh currents passing from cathode 23 on one side of waveguide 13 to anode on the other side. A high density plasma is thus established inside waveguide 13 by suitable excitation of anode 20 and cathode 23. Cone 10 is employed to compress the plasma to obtain the required electron density. Radio frequency waves directed into one end of waveguide 13 by way of pressure window 16 are reflected back to their source when plasma of sufficient density fills waveguide 13, but they pass through waveguide 13 virtually unimpeded when plasma is absent.

It is noted that two grounded

screens

24 and 25 are in effect provided by the opposite broad walls of waveguide 13 being pierced by plasma slots l4a-l4h in cascade capture practically all of the flux lines from positive anode 20. The electric lines of force are shown as lines with arrows. Most lines from anode 20 fail to penetrate the two sets of slots 14a-14z1 and 14e-14/1. Arrows are in the direction of acceleration during triggering. c Electrons are decelerated before arrival in waveguide 13 and only weakly accelerated through the waveguide.

Now referring to FIG. 2, there is shown only a portion of the plasma waveguide switch of FIG. I with a modification wherein anode 31 replaces part of the waveguide wall, thus eliminating the screening effect of one set of slots. An increased but still small portion of the flux lines penetrates the waveguide 33 and ends on the control grid. Placing anode 31 as part of the waveguide wall gives enhanced accelerating field inside the waveguide and reduces deceleration of electrons between the opening of focusing, cone 34 and entrance slots 35. Electric lines of force are shown with arrows.

FIG. 3 shows a portion of the plasma waveguide switch with conventional anode 41, but control grid is built into the waveguide wall. Again, a few anode flux lines can reach the control grid, thus ensuring that any plasma drawn to the face of the control grid would be exposed to the breakdown producing anode field. The face of focusing cone 44 is made part of the waveguide wall which brings electrons to the brink of weak anodes accelerating field inside waveguide 43 as indicated by the line of force. The lines of force are those lines with arrows. Cone 44 has slotted face 440 which becomes part of the waveguide wall and serves as the control grid.

Now referring in detail to FIG. 4, there is shown a portion of the waveguide switch whichnis a modification of FIG. 1 and is the preferred embodiment of this invent-ion in which both anode 51 and control grid 55 are built into the waveguide walls. The anode is as shown in FIG. 2 and the control grid is as shown in FIG. 3. Virtually the entire field from anode 51 is available to accelerate electrons which reach the vicinity of control grid 55. Thus one or two free elect ons near control grid 55 will normally be all that isneeded to initiate a discharge, and starting potentials much beyond V are not needed. The reduced anode potential practically eliminates the choice of a kickout discharge between anode 51 and waveguide 53.

It is noted that with both anode 51 and the face of focusing cone 54 built into the waveguide wall, electrons are delivered to the full anode field directly from the cone mouth without deceleration. The combination of electron accelerations permits triggering with very low anode voltages. The electric lines of force are shown as lines with arrows.

To provide DC and video isolation between anode 51 and/or control grid 55 and the remainder of waveguide section 53 without permitting RF leakage or an effective discontinuity in the waveguide may be accomplished by letting the face of the grid or anode be round and arranging the insulating gap as a radial or coaxial waveguide one quarter wavelength long at the RF micfrequency with open circuit termination. Two arrangements are shown in FIGS. 5 and 6 for a single electrode. The open circuit at the end of the insulating guide appears to the RF as a short circuit binding the electrodes to adjacent section of waveguide. The attainment of a good short circuit is not critical, since the electric field, and hence the axial current, has a node at the main waveguide wall. Either arrangement of FIG. 5 or that of FIG. 6 may be used for the anode and independently either arrangement may be used for the trigger grid.

FIG. 5 specifically shows an arrangement for providing an RF short to the waveguide wall for the anode or trigger electrode while maintaining DC isolation. The space between parallel surfaces of the waveguide wall and electrode skirt act as a radial waveguide 62 with open circuit at the end, thus reflecting an effective short circuit at gap 61. The trigger electrode is a figure of revolution.

FIG. 6 is an alternate arrangement for DC isolation of the anode or trigger electrode with an RF short circuit. An annular space between electrode 63 and coaxial waveguide extension 64 acts as separation in a coaxial transmission line open circuited at end away from the main waveguide.

What is claimed is:

l. A plasma waveguide switch permitting triggering with both cathode and waveguide grounded being comprised of a section of waveguide having first and second waveguide walls, said first waveguide wall being opposite to said second waveguide wall, said section of waveguide having only first and second ends, first and second pressure windows airtight sealed to said first and second ends for entrance and exit of RF energy, re spectively, an anode positioned in the central region of said first waveguide wall being part thereof and DC isolated therefrom, a first insulating envelope airtight sealed to said first waveguide wall and fully encompassing said anode, a conducting cone having an apex and base with a slotted face at said apex thereof, said slotted face positioned in the central portion of said second waveguide wall being part thereof and simultaneously DC isolated therefrom, said slotted face operating as a said cone.

Claims

1. A plasma waveguide switch permitting triggering with both cathode and waveguide grounded being comprised of a section of waveguide having first and second waveguide walls, said first waveguide wall being opposite to said second waveguide wall, said section of waveguide having only first and second ends, first and second pressure windows airtight sealed to said first and second ends for entrance and exit of RF energy, respectively, an anode positioned in the central region of said first waveguide wall being part thereof and DC isolated therefrom, a first insulating envelope airtight sealed to said first waveguide wall and fully encompassing said anode, a conducting cone having an apex and base with a slotted face at said apex thereof, said slotted face positioned in the central portion of said second waveguide wall being part thereof and simultaneously DC isolated therefrom, said slotted face operating as a control grid, a second insulating envelope airtight sealed to said second of said waveguide walls and fully encompassing said control grid, a cathode positioned in said second insulating envelope with said base of said cone adjacent thereto, said section of waveguide, said first and second envelopes being airtight sealed to each other forming a housing for said plasma waveguide switch, and a low pressure controlled atmosphere in said housing for supporting a glow discharge therein.

2. A plasma waveguide switch as described in claim 1 further including a triggering electrode connected to said cone.