Ion laser generator

Abstract: In an ion laser generator, an electromagnet (5) must produce a sufficiently strong magnetic field throughout the length of the bore (4) where plasma must be converged, and the magnetic field intensity at the end surface (7) of the cathode (2) must not exceed a certain limit. If the magnetic field intensity at the cathode (2) is too strong, the plasma is converged on an area (7) of the cathode (2) and destroys the cathode (2) at the area (7). A magnetic shield member (10) is provided at an end surface of the electromagnet (5) facing to the cathode (2) for shielding the cathode surface (7) from the leakage flux.

Description: BACKGROUND OF THE INVENTION This invention relates to an ion laser generator. An ion laser tube comprises a bore, through which plasma is conducted, a cathode for plasma discharge facing to an end of the bore, an anode for plasma discharge facing to the other end of the bore, a pair of Brewster windows for transmitting laser light. And an ion laser generator comprises an ion laser tube, an electromagnet surrounding the ion laser tube, and a pair of light resonating mirrors situated at both ends of the ion laser tube. An electric voltage is impressed between the cathode and the anode for generating the plasma, and an exciting current flows in the electromagnet to generate a magnetic field for converging the plasma around the axis of the bore. When the ion laser tube discharges, the laser light is resonated by the pair of resonating mirrors, and a part of laser light is taken out through one of the light resonating mirrors. For a numerical example, a magnetic field of about 10⁻¹ Tesla (1000 gauss) is required at the bore for a sufficient convergence of the plasma, whereas the magnetic field at the end surface of the cathode must be less than 10⁻² Tesla (100 gauss). When the magnetic field at the end surface of the cathode is stronger than 100 gauss, the plasma is concentrated on an area of the cathode and destroys the cathode at the area. On the other hand, when the magnetic field at the bore is not strong enough, insufficient convergence of plasma makes plasma sputtering strong, and sputtered material will stain the Brewster windows. In a heretofore known ion laser generator, the Electromagnet for converging the plasma must be positioned with a sufficient distance apart from the end surface of the cathode in order to obtain a sufficiently decreased magnetic field intensity at the end surface of the cathode while keeping a sufficiently strong magnetic field at the end surface of the bore. An increase in the distance between the end surface of the cathode and that of the bore decreases the length of the bore in a limited length of an ion laser tube. Since the laser light is generated only in the bore, decrease of the bore length decreases the intensity of the laser output in a limited length of an ion laser tube. This means that the length of an ion laser tube is not effectively used in a heretofore known ion laser generator. US-A-4974228 discloses an ion laser generator in which the axial magnetic field is reduced to zero or at least minimised in the cathode transition region. SUMMARY OF THE INVENTION A primary object of the present invention is to obtain an ion laser generator in which the length of an ion laser tube is effectively used for generating laser light. In order to achieve this object, the end surface of the cathode must be placed as near as possible to that of the bore in an ion laser tube. And, in this reduced distance between the end surface of the cathode and that of the bore, the magnetic field at the end surface of the cathode must be sufficiently weak, while the magnetic field at the end surface of the bore must be sufficiently strong. In this invention, means for example a magnetic shield, are provided to divert at least partially the flux between the end surface of the bore and the end surface of the cathode. A part of magnetic flux at the end surface of the bore enters into this magnetic shield member, and the magnetic field intensity (that is the flux density) at the end surface of the cathode is decreased. Thus the invention provides an ion laser comprising: an ion laser tube having a bore and a pair of discharge electrodes constituting a cathode and an anode, said cathode being spaced from and facing towards an end of said bore, and said anode being positioned facing towards the other end of said bore; an electromagnet for converging plasma generated between said pair of discharge electrodes, around the longitudinal axis of said bore; and a pair of reflecting mirrors, each mirror facing towards a respective opposite output end of said ion laser tube, the distance between said pair of reflecting mirrors being tuned to a wavelength of laser light generated in said plasma; characterised by means for at least partially diverting the magnetic field generated by the electromagnet in the space between the cathode and the bore so as to reduce the magnetic field strength in the region of the cathode. BRIEF DESCRIPTION OF THE DRAWINGS Further objects, features, and advantages of this invention will become apparent from a consideration of the following description and the accompanying drawings, which are by way of example only. In the drawings the same numerals indicate the same or the corresponding parts. Fig. 1 shows a cross-sectional view of an embodiment of this invention. Fig. 2 is an enlarged view illustrating the vicinity of the cathode in Fig. 1. Figs. 3A and 3B illustrate an effect of this invention. Fig. 4 shows a side view of an embodiment of a magnetic shield member of this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1 and Fig. 2, an ion laser tube 1 is a hermetically sealed tube including a cathode 2, an anode 3, and a bore 4. The cathode 2 emits electrons, and a voltage impressed between the cathode 2 and the anode 3 generates plasma. A plasma converging electromagnet 5 is placed surrounding the ion laser tube 1. The magnetic field of the electromagnet 5 converges the plasma around the axis of the bore 4. A pair of reflecting mirrors 6 are placed facing to both ends of the ion laser tube 1. When a wavelength of a light in the plasma resonates to the distance between the two reflecting mirrors, laser light of the wavelength is generated. In this invention, the end surface 7 of the cathode 2 is positioned nearer to the end surface 9 of the bore 4 than in heretofore known ion laser oscillators. And a magnetic shield member 10 for terminating a part of magnetic flux from the end surface 9 of the bore 4, reduces the magnetic field strength at the end surface 7 of the cathode. For a numerical example, a magnetic field strength at the end surface 7 is made less than 10⁻² Tesla (100 gauss), for this order of a magnetic field strength is proved to be safe to the life span of the cathode 2. A stronger magnetic field at the end surface 7 converges the plasma at a place of the end surface 7 and deteriorates the cathode 2 at the place. An example of the magnetic shield member 10 is shown in Fig. 4 as a ring-shaped member split in two sections across the diameter. The magnetic shield member 10 is made of a ferromagnetic material and has a thickness of, for example, 10mm. The workings of the magnetic shield member 10 is illustrated in Fig. 2 and Figs. 3. A pair of solid lines 8 in Fig. 2 illustrates a range of magnetic flux entering into the end surface 7 when the magnetic shield member 10 is provided. And a pair of dotted lines 12 in Fig. 2 illustrates a range of magnetic flux entering into the end surface 7. Magnetic flux going out from an end surface of the electromagnet 5 comes back to the other end surface of the magnet 5, and when a ferromagnetic member is in the flux path, the flux tends to take a path in the ferromagnetic member. Thus, when the magnetic shield member 10 is provided at an end surface of the electromagnet 5, a part of the magnetic flux going out from the end surface of the electromagnet 5 is gathered by (concentrated by) the magnetic shield member 10, leaving less flux to enter the end surface 7 of the cathode. When the end surface 7 is a circle of radius r c and the magnetic field intensity at the end surface 7 is denoted by B c for flux represented by solid lines 8, and by B c ′ for flux represented by broken lines 12, the total number of flux Φ entering into the end surface 7 is Φ =B c πr c ² for flux represented by solid lines and Φ′=B c ′πr c ² for flux represented by dotted lines 12. When the magnetic field intensity of the flux Φ is B, the radius of the flux is r, the magnetic field intensity of the flux Φ′is B′, and the radius of the flux r′ at the end surface 9 of the bore, Φ=Bπr² and Φ′=B′π(r′)². Thus, B c πr c ²=Bπr²···(1) and B c ′πr c ²=B′π(r′)²···(2). And in general, for a point d between the end surface 7 and the end surface 9, Φ =B d πr d ² and Φ′=B d ′π(r d ′)², where r d (r d ′) is the radius of the flux and B d (B d ′) is the magnetic field intensity at point d. And therefore, B c πr c ²=B d πr d ²···(3) and B c ′πr c ²=B d ′π(r d ′)²···(4). The relation represented by equation(3) is illustrated by solid lines in Figs.3A and 3B, and the relation represented by equation(4) is illustrated by dotted lines in Figs.3A and 3B. In Figs.3A and 3B, the X axis denotes a position between the two end surfaces, and the Y axis of Fig.3A denotes the radius of the flux and that of Fig.3B denotes the magnetic field intensity of the flux Φ and Φ′. As seen in Fig.3B, the magnetic field intensity at the end surface 7 is substantially reduced by an effect of the magnetic shield member 10, while the magnetic field intensity at the end surface 9 of the bore 4 is rather intensified by the magnetic shield member 10. From equations (1) and (2), we obtain, (r/r′)²=(B c /B c ′)(B′/B)···(5). For a numerical example of a magnetic shield member of 10mm thickness, (B c /B c ′)=0.87 and (B′/B)=0.997. Calculated by equation(5), (r/r′)=0.93. For another example of a magnetic shield member of 30mm thickness, (B c /B c ′)=0.55 and (B′/B)=0.995. Calculated by equation(5), (r/r′)=0.74. As shown in Fig. 2, the radius r of the flux Φ is made smaller than that of the bore 4. The diameter of the bore 4 is denoted by 11 in Fig. 2. In the embodiment shown in Fig.2, the end surface of the electromagnet 5 is placed between the end surface 7 of the cathode and the end surface 9 of the bore, and a magnetic shield member 10 is placed abutted on the end surface of the electromagnet 5. But the position of the magnetic shield member 10 is not limited by this embodiment. A magnetic shield member placed in a flux path outside the electromagnet 5 can absorb a part of magnetic flux and can reduce an amount of leakage flux entering in the end surface 7 of the cathode. And more than one magnetic shield member may be inserted for multiple shielding. In another embodiment of this invention, a magnetic shield member 10 may be provided at either end of the electromagnet 5. In this way, the distance between the end surface 7 of the cathode 2 and the end surface 9 of the bore 4 is effectively decreased in an ion laser generator of this invention. This decreased distance increases the effective length of the bore in a limited length of an ion laser tube to the effect of increased output of the laser light.

Claims: An ion laser generator comprising: an ion laser tube (1) having a bore (4) and a pair of discharge electrodes constituting a cathode (2) and an anode (3), said cathode being spaced from and facing towards an end of said bore, and said anode being positioned facing towards the other end of said bore; an electromagnet (5) for converging plasma generated between said pair of discharge electrodes, around the longitudinal axis of said bore; and a pair of reflecting mirrors (6), each mirror facing towards a respective opposite output end of said ion laser tube, the distance between said pair of reflecting mirrors being tuned to a wavelength of laser light generated in said plasma; characterised by means (10) for at least partially diverting the magnetic field generated by the electromagnet in the space between the cathode and the bore so as to reduce the magnetic field strength in the region of the cathode. An ion laser generator as claimed in Claim 1, wherein the diverting means comprises means (10) for increasing the divergence of the magnetic field away from the axis of the bore. An ion laser generator as claimed in Claim 2, wherein the diverting means comprises a flux-concentrator (10) disposed externally of the electromagnet. An ion laser generator according to Claim 3, wherein said flux concentrator (10) comprises a ferromagnetic member placed adjacent to an end surface of said electromagnet, said end surface being an end surface which is nearer to said cathode. An ion laser generator according to Claim 3, wherein said flux concentrator (10) comprises two ferromagnetic members, each said member being adjacent a respective end surface of said electromagnet. An ion laser generator according to Claim 3, wherein said flux concentrator comprises a ring-shaped disk split across the diameter of said disk. An ion laser generator according to any preceding claim, wherein the magnetic field strength at the operative end of said cathode (2) is less than 10⁻² Tesla (100 gauss). An ion laser generator according to any preceding claim, wherein total flux entering said bore (4) is contained within a cross-sectional area which is less than the cross-sectional area of the bore.

Inventors: BABA KENJI C O NEC CORPORATION, BABA, KENJI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: H01S3
IPC codes: H01S3