US3808551A

US3808551A - Secondary foil for apparatus producing a controlled discharge which provides molecular excitation of a gaseous working medium

Info

Publication number: US3808551A
Application number: US00303241A
Authority: US
Inventors: D Ahouse
Original assignee: Avco Corp
Current assignee: Combustion Engineering Inc
Priority date: 1972-11-02
Filing date: 1972-11-02
Publication date: 1974-04-30
Anticipated expiration: 1991-04-30

Abstract

An electron beam (E-beam) laser has a vacuum chamber in which an E-beam terminal generates a directed stream of electrons. A primary foil which seals the chamber is in the path of the stream of electrons and permits passage of the stream of electrons to the exterior of the chamber. A cathode is positioned closely adjacent the primary foil and an anode is spaced from the foil to form an elongated laser cavity between them. The electrodes are maintained at a high electrical potential which, along with the electron beam, molecularly excites a gaseous working medium between the electrodes. A secondary foil is positioned between the cathode and the primary foil. The secondary foil is formed from material transparent to the passage of electrons but which forms a physical barrier between the primary foil and the cathode. This prevents rupturing of the primary foil when the cathode material sputters off upon arcing between the anode and the cathode.

Description

United States Patent Ahouse SECONDARY FOIL FOR APPARATUS Apr. 30, 1974 Primary Examiner-Ronald L. Wibert Assistant ExaminerR. J. Webster Attorney, Agent, or Firm-Charles M. Hogan; Gary M. Gron MEDIUM ABSTRACT [75] Inventor: David R. Ahouse, Andover, Mass.

An electron beam (E-beam) laser has a vacuum cham- 1 Asslgnee: Avco Corporation Clncmnatl, Ohio ber in which an E-beam terminal generates a directed [22] Filed. N0 2 1972 stream of: electrons. A primary foil which seals the chamber 1s m the path of the stream of electrons and 1 1 PP N05 303,241 permits passage of the stream of electrons to the exterior of the chamber. A cathode is positioned closely adjacent the primary foil and an anode is spaced from [52] US. Cl 331/945, 313/74, 330/43 the foil to form an elongated laser cavity between [51] Int. Cl. H015 3/22, HOls 3/02 them. The electrodes are maintained at a high electri- [58] Field of Search 313/74; 331/945; 330/4.3 cal potential which, along with the electron beam, mo-

lecularly excites a gaseous working medium between the electrodes. A secondary foil is positioned between [56] References and the cathode and the primary foil. The secondary foil is UNITED STATES PATENTS formed from material transparent to the passage of 2,004,176 6/1935 Schroter et a1. 313/74 X electrons but which forms a physical barrier between 3,375,387 3/1968 Leiss et al 313/74 X the primary foil and the cathode. This prevents ruptur- 3,440,466 4/1969 C01V111 61 a1. 313/74 X of the primary when the cathode material ut- 3,702.973 11/1972 Daugherty et a1. 313/74 X ters off upon arcing between the anode and the cath ode.

9 Claims, 3 Drawing Figures ANODE SECONDARY FOIL 44 23 90 82 f 22 CATHODE 50 104 |QQ T I06 50 74 76 68 Y/ /Z/ n n k\\ k 3 k 64 V 56 58 72 58 1 \J \40 60 f SECONDARY FOIL FOR APPARATUS PRODUCING A CONTROLLED DISCHARGE WHICH PROVIDES MOLECULAR EXCITATION OF A GASEOUS WORKING MEDIUM BACKGROUND OF THE INVENTION In recent years electron beam generating devices have been used to produce a molecular excitation of a gaseous working medium. This molecular excitation is useful in producing a lasing action within an optical cavity. In addition, it may be used with advantage to provide the desired electrical conductivity of a gaseous working medium in a magnetohydrodynamic device, such as a generator and accelerator. It also may be used with other devices that require or use electrically conductive or ionized gases.

An excellent example of this type of apparatus may be found in copending Pat. application Ser. No. 72,982, entitled Apparatus for and the Method of Producing a Controlled Discharge in a Gaseous Medium, in the name of Jack D. Daugherty et al., of common assignment with the present invention.

The Daugherty et al., application describes an E- beam generator which in one form may be briefly described for purposes of the present invention as a vacuum chamber in which a high voltage anode discharges a directed stream of electrons which are accelerated toward a cathode in the chamber. A foil in the chamber wall adjacent the cathode provides a physical barrier to maintain the vacuum in the chamber but is transparent to the passage of electrons to permit the stream to pass from the chamber. A cathode is positioned closely adjacent the foil outside the chamber and an anode is spaced from the cathode to form a lasing cavity outside the vacuum chamber that may be at atmospheric pressure. A high voltage potential is applied across the exterior anode and cathode and this potential, plus the electron beam discharge, molecularly excites a working gas flowing between the anode and cathode to produce a population inversion and production of a laser beam.

Frequently the exterior anode and cathode, or the anode within the chamber, are pulsed to produce a pulsed laser beam output. One of the problems that exists with a device of this type is that arcing may occur between the exterior anode and cathode. When this happens material on the cathode is sometimes melted and sputtered off. Since the foil is closely adjacent to the cathode, there is a high likelihood that it may be pierced by the sputtering material. When the foil has a hole in it it is incapable of maintaining the vacuum within the chamber that is necessary for proper operation. The foil may also rupture in time through fatigue failure, which is caused by acoustic pressure waves generated by the pulsing of the exterior anode and cathode. These acoustic pressure waves strike the foil each time the device is triggered. Since the foil already supports a substantial pressure differential, it has a high degree of stress imposed by the acoustic waves.

SUMMARY OF THE INVENTION The invention is an improvement on a device of the above general type not limited solely to a laser sustaining apparatus but to apparatus for producing a controlled discharge to molecularly excite a working gas medium which incorporates a secondary foil positioned between the primary foil and the region in which the working gas is molecularly excited. The secondary foil is formed from material which is transparent to the passage of electrons but at the same time forms a physical barrier for the primary foil to prevent rupturing of the primary foil due to sputtering and other problems.

DESCRIPTION OF THE INVENTION The above and other related features of the present invention will be apparent from a reading of the following description of the disclosure shown in the accompanying drawings and the novelty thereof pointed out in the appended claims.

In the drawings:

FIG. 1 is a highly schematic illustration of a laser which embodies the present invention;

FIG. 2 is a cross-sectional view of a portion of the apparatus shown in FIG. 1; and

FIG. 3 is a cross-sectional view of FIG. 2 taken on line 33 of FIG. 2.

Referring to FIG. 1 there is shown schematically a laser indicated by reference character 10. While the invention will be described in connection with this laser, it should be noted that it is equally applicable to other devices, as discussed above. The laser 10 comprises an outer housing 12 having a chamber 14 forming a laser cavity. Housing 12 is supplied with gas from a gas inlet 16 which passes through laser cavity 14 to a gas outlet 18. While FIG. 1 suggests that gas flow is from right to left, in point of fact it is to be noted that flow is preferrably in the direction normal to the plane of the paper as best shown in FIG. 2. This gas forms a working medium for the laser beam and may consist of gaseous mixture of carbon dioxide, nitrogen and helium. An elongated anode 20 is provided along one side of the chamber and an elongated cathode 22 (suitably grounded) is provided adjacent the opposite wall of the housing to form the laser cavity 14 between them. The anode 20 is supplied with a substantial electrical potential from a suitably grounded power supply 24 via line 26. The gas in the lasing cavity 14 is molecularly excited by a directed stream of electrons from an electron beam assembly comprising a chamber 30. Chamber 30 is maintained at a very low pressure by a vacuum pump 32 connected to a suitable conduit 34 leading from the chamber 30. An elongated high voltage E-beam anode 36 is positioned within chamber 30 and supplied with electrical potential by suitably grounded power and control system 38 via line 39. The E-beam anode 36 may be maintained at a high voltage so that it emits a directed stream of electrodes toward an elongated cathode 42 suitably grounded so that it attracts the electrons. Cathode 42 is formed from a screenlike material so that a substantial portion of the electrons which have been directed at it pass through the cathode 42. The directed stream of electrons also pass through a primary foil 40 mounted in their path. The primary foil 40 is formed from material that physically seals chamber 30 but which permits the passage of the directed stream of electrons with a minimum attenuation.

Many different materials can be used for this, such as I the power supply 24 provides a pulsed potential across the sustainer anode 20 and cathode 22 and the power and control system 38 for the E-beam anode 36 produces a series of pulses coincident with the sustainer pulses.

When the power supply 38 is energized a combination of the action of the anode 20 and cathode 22 and the directed stream of electrons causes an inversion in the gas within chamber 14 to produce a lasing action.

Mirrors

44 and 46 at opposite ends of anode 20 and cathode 22 form a regenerative laser cavity between them so that a coherent laser beam is generated within cavity 14. Laser mirror 46 is partially transmissive so that a portion of the beam which strikes it passes out of the housing in the form of a directed laser beam. Alternately, as is well known in the art, the

mirrors

44 and 46 may be omitted and an appropriate laser beam passed through the laser cavity if the laser is to operate as an amplifier.

With such an arrangement as noted above, arcing may occur between the anode 20 and the cathode 22. When this occurs the material forming the cathode melts at that point and sputters. The material that is sputtered frequently may strike the primary foil 40 thereby piercing it. However, in accordance with the present invention a secondary foil is provided to protect the primary foil.

The secondary foil is shown in particular detail in H65. 2 and 3 which show the portion of the laser which contains the secondary foil. The anode is secured to an upper support 48 in between the gas inlet 16 and gas exhaust 18. The cathode 22 is elongated in form and spaced from the anode to provide the laser cavity 14 with the mirror 44 at one end. The cathode 22 comprises a series of rods 23 secured in bores 50 in support blocks 52 by a series of set screws 54. The blocks 52 are mounted to a wall 56 by screws 58. The primary foil 40 is sealingly mounted in the chamber 30 by sandwiching it between a spacer block 60 and a wall 62 of chamber 30 by screws 64 which extend through opening 68 in spacer 60 and a threaded bore 70 in wall 62. Screws 66 hold plate 56 to spacer block 60.

The primary foil 40 forms one wall of the chamber 30 and an elongated slot 72 is formed in plate 56 generally in line with the anode 20 and the cathode 22 and the anode 36 (not shown). A pair of secondary

foil support flanges

74 and 76 are secured to plate 56 on opposite sides of slot 72 by

screws

87 and 80, respectively. A secondary foil assembly 82 is positioned over the

edges

84 and 86 of the

flanges

74 and 76.

The foil 82 comprises a backing screen 88 positioned over

edges

84 and 86 to provide support and a relatively thin foil 90 is positioned over the screen 88 and has a first side edge 92 sandwiched between a flange element 96 and flange 74 by screws 98. A second side edge 94 of the foil 90 is sandwiched between a flange 100 and flange 76 by screws 102. Flanges 96 and 100 have

integral platform sections

104 and 106, respectively, that form guides for the gas flowing through the laser cavity 14 and also provide some support for the cathode rods 23.

The secondary foil 90 is formed from a material that permits the free passage of the directed stream of electrons but which provides a physical barrier between the cathode 22 and the primary foil 40. A material that has been found particularly suitable for this purpose is a polyimide plastic film, such as Capton, which is an electrically nonconductive material available from Dupont Inc. of Wilmington, Delaware. Other materials, such as aluminum and titanium, maybe incorporated for this foil when it is desirable to have a greater heat transfer capability. The thickness of the foil 90 is maintained as thin as possible to minimize attenuation of the electron beam but thick enough to have sufiicient thickness to provide a physical barrier between the cathode 22 and the primary foil 40. As shown, the foil 90 is formed from Capton material having a thickness of 0.003 inch which requires a screen 88 to provide physical support. This screen is a 75 percent open area screen of a heavy gauge. If material such as aluminum or titanium is used its thickness may be on the order of 0.001 inch.

During operation of the laser 10 with the secondary foil in place, any arcing between the anode and cathode which causes sputtering impinges on the foil 90 since it protects the primary foil 40. The material may pierce holes in the foil 90 but this does not impair its operation, since the pressure on both sides of the foil 90 is essentially that of the laser cavity 14. It has been found that a substantial portion of the secondary foil 90 can be burnt away without serious harm to the primary foil 40. The secondary foil 90 also acts as a barrier to dampen acoustic pressure waves that occur during generation of the pulsed laser beam. While burning of the secondary foil 90 reduces somewhat its effectiveness as a barrier to the pressure waves, it still materially reduces their impact on the primary foil 40.

The end result of the use of the secondary foil in the laser 10 is that the primary foil 40, which must be put in place using very precise techniques because of the vacuum seal it must maintain, will last indefinitely. In the past, however, primary foils had to be replaced on a fairly regular basis. The secondary foil essentially acts as a sacrifice foil and can be easily replaced by removal of its supporting flanges when it is convenient to do so. This enables a highly efiicient utilization of a pulsed laser because it eliminates what was a fairly common and involved maintenance procedure.

While the invention has been shown in connection with a pulsed E-beam sustained laser, it should be apparent that it may be employed in other devices as pointed out above without departing from the spirit and scope of the present invention.

Accordingly, what is claimed as novel and desired to be secured by Letters Patent of the United States is:

1. An apparatus for producing a controlled discharge for providing molecular excitation of a gaseous working medium comprising:

a vacuum chamber;

means positioned within said vacuum chamber for generating a directed stream of electrons;

a primary foil positioned in one wall of said chamber, said foil sealingly maintaining the pressure in said chamber at a subatmospheric level and permitting passage of the directed stream of electrons from the vacuum chamber and across a defined region;

an anode and cathode positioned outside of said chamber adjacent said foil;

means for maintaining said anode and cathode at a substantial differential electrical potential for providing an electric field across said region traversed by the directed stream of electrons, whereby molecular excitation of a gaseous working medium and population inversion may be maintained in the space between said anode and cathode;

means for stimulating emission of radiation from said gaseous working medium; and

barrier means including a secondary foil assembly positioned between said region and the primary foil and outside of said vacuum chamber and having substantially equal pressure on opposite sides thereof, said secondary foil being formed from material transparent to the passage of electrons and forming a physical barrier between the primary foil and the cathode for preventing rupturing of said primary foil.

2. Apparatus as in claim 1 wherein:

said anode is positioned in the path of the directed stream of electrons;

said cathode is spaced from said anode and positioned between said anode and said primary foil; and

said secondary foil is positioned between the cathode and said primary foil.

3. Apparatus as in claim 1 wherein said secondary foil assembly is relatively thin for minimizing attenuation of the electrons passing therethrough.

4. Apparatus as in claim 1 wherein said secondary foil assembly comprises electrically nonconductive material.

5. Apparatus as in claim 1 wherein said secondary foil assembly comprises electrically conductive material.

6. Apparatus as in claim 1 wherein said secondary 5 foil assembly comprises a relatively thin foil and a supporting screen positioned between said foil and the primary foil.

7. Apparatus as in claim 6 wherein said thin foil comprises a material having a thickness of approximately 10 0.001 inch.

8. Apparatus as in claim 1 wherein said electrodes are elongated and have their longitudinal axes parallel to form an elongated laser cavity in the space therebetween;

said high voltage terminal is elongated and generally parallel to the longitudinal axis of said laser cavity;

posed mirrors positioned at opposite ends of said laser cavity to form a regenerative laser cavity.

Claims

1. An apparatus for producing a controlled discharge for providing molecular excitation of a gaseous working medium comprising: a vacuum chamber; means positioned within said vacuum chamber for generating a directed stream of electrons; a primary foil positioned in one wall of said chamber, said foil sealingly maintaining the pressure in said chamber at a subatmospheric level and permitting passage of the directed stream of electrons from the vacuum chamber and across a defined region; an anode and cathode positioned outside of said chamber adjacent said foil; means for maintaining said anode and cathode at a substantial differential electrical potential for providing an electric field across said region traversed by the directed stream of electrons, whereby molecular excitation of a gaseous working medium and population inversion may be maintained in the space between said anode and cathode; means for stimulating emission of radiation from said gaseous working medium; and barrier means including a secondary foil assembly positioned between said region and the primary foil and outside of said vacuum chamber and having substantially equal pressure on opposite sides thereof, said secondary foil being formed from material transparent to the passage of electrons and forming a physical barrier between the primary foil and the cathode for preventing rupturing of said primary foil.

2. Apparatus as in claim 1 wherein: said anode is positioned in the path of the directed stream of electrons; said cathode is spaced from said anode and positioned between said anode and said primary foil; and said secondary foil is positioned between the cathode and said primary foil.

6. Apparatus as in claim 1 wherein said secondary foil assembly comprises a relatively thin foil and a supporting screen positioned between said foil and the primary foil.

7. Apparatus as in claim 6 wherein said thin foil comprises a material having a thickness of approximately 0.001 inch.

8. Apparatus as in claim 1 wherein said electrodes are elongated and have their longitudinal axes parallel to form an elongated laser cavity in the space therebetween; said high voltage terminal is elongated and generally parallel to the longitudinal axis of said laser cavity; said primary foil is elongated and extends along substantially the entire length of the laser cavity and the high voltage terminal; and said secondary foil is elongated and extends along substantially the entire length of said laser cavity between said lasing cavity and said primary foil.

9. Apparatus as in claim 8 further comprising opposed mirrors positioned at opposite ends of said laser cavity to form a regenerative laser cavity.