WO1999044264A1

WO1999044264A1 - Gas laser with magnetic field inducing loops to stir the gas

Info

Publication number: WO1999044264A1
Application number: PCT/IL1998/000092
Authority: WO
Inventors: Victor Isaevich Adamovski
Original assignee: Motors Engineering A.K. Ltd.
Priority date: 1998-02-25
Filing date: 1998-02-25
Publication date: 1999-09-02

Abstract

The plasma chamber (12) of a gas laser is encircled with loops of conducting wire (32, 34, 36). Pulses of electrical current in the loops create time-varying magnetic fields that stir the plasma and keep its temperature distribution uniform.

Description

GAS LASER WITH MAGNETIC FIELD INDUCING LOOPS TO STIR THE GAS

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to lasers and, more particularly, to a gas laser of improved efficiency.

Figure 1 is a partial schematic cross-section of a typical gas laser 10. A hermetically sealed cylindrical chamber 12 contains a gas 14 such as carbon dioxide under low pressure. The longitudinal axis of chamber 12 is represented by a dashed line 16. Ends 18 of chamber 12 are transparent to the stimulated radiation emitted by gas 14. Two mirrors (not shown) are provided beyond ends 18. These two mirrors define between them a laser cavity.

Gas laser 10 also includes means for exciting gas 14 energetically, thereby inducing the stimulated emission of radiation by gas 14. Two such means are shown in Figure 1. One is a pair of electrodes 20, within chamber 12, between which an arc discharge is created to heat gas 14. The other is a solenoid 22, outside of chamber 12 and concentric with axis 16, by means of which an RF electromagnetic field is created within chamber 12, typically at frequencies between 8 KHz and 20 KHz..

High energy gas lasers typically have efficiencies on the order of 5%. For example, a typical high energy carbon dioxide laser is driven by 190 kilowatts of power but emits coherent radiation having a power of only 9 kilowatts. The reason for this is that the excitation means heat gas 14 non-uniformly. For example, electrodes 20 create a plasma within chamber 12 that is much hotter near axis 16 than near the walls of chamber 12. Solenoid 22 creates a plasma within chamber 12 whose region of maximum temperature is an annular region concentric with axis 16 but closer to the walls of chamber 12 than to axis 16. The radiative transitions of the molecules of gas 14 that are the source of the emitted radiation occur predominantly in parts of the plasma that are cooler than the maximum temperature. In the case of a plasma created by electrodes 20, the active part of the plasma is near the walls of chamber 12. In the case of a plasma created by solenoid 22, the active part of the plasma is concentrated around axis 16. As a result, only about 20% to 30% of the volume of gas 14 participates in the emission of radiation.

Another source of inefficiency in laser 10 is radiational losses. These losses also are associated with the fact that part of the plasma in chamber 12 is hotter than it needs to be. By the Stefan-Boltzmann law, the rate of loss of energy from a hot body by radiation is proportional to the fourth power of the temperature of that body. Thus, even if the part of the plasma in chamber 12 that is too hot to support stimulated emission is only slightly too hot, that part of the plasma is a much stronger source of radiational losses than the active part of the plasma.

There is thus a widely recognized need for, and it would be highly advantageous to have, a gas laser in which most of the plasma is in a temperature range that supports stimulated emission of radiation.

SUMMARY OF THE INVENTION

According to the present invention there is provided an improved laser of the type in which stimulated emission of radiation is induced in a gas contained in a chamber having an axis, during which induction the gas is at least partially ionized, the improvement comprising electromagnetic means for stirring the ionized gas.

According to the present invention there is provided a method for increasing the efficiency of a gas laser in which stimulated emission of radiation is induced in a gas contained in a chamber having an axis, the gas thereby becoming at least partially ionized, the method comprising the step of electromagnetically stirring the ionized gas.

The present invention is based on the fact that a plasma, being an ionized gas, interacts strongly with a magnetic field. According to the present invention, a time- varying electromagnetic field is imposed on the plasma to stir and mix it, thereby creating a uniform temperature distribution in the plasma. Preferably, this is accomplished by encircling chamber 12 with loops of electrically conductive wire that are tilted with respect to axis 16, and pulsing electrical current through the loops.

BRIEF DESCRTP TION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 (prior art) is a partial schematic cross section through a gas laser;

FIG. 2 is a partial perspective view of a gas laser according to the present invention;

FIG. 3 is a partial perspective view of another gas laser according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a gas laser whose plasma has a relatively uniform operating temperature distribution. Specifically, the plasma is stirred and mixed electromagnetically.

The principles and operation of a gas laser according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring now to the drawings, Figure 2 is a perspective exterior view of chamber 12 equipped with three planar circular loops 32, 34 and 36 of electrically conductive wire that encircle chamber 12. The planes of loops 32, 34 and 36 are tilted at an acute angle with respect to axis 16. Loops 32, 34 and 36 are distributed around chamber 12 at uniform azimuthal spacings of 120°. Pulses of electrical current are provided to loops 32, 34 and 36, preferably at a frequency of between about 50 Hz and about 1000 Hz, to create a helical magnetic field within chamber 12 that keeps the plasma inside chamber 12 uniformly mixed.

For the purpose of the present invention, the axis of the chamber is defined by the means used to create the plasma that is mixed by the time-varying electromagnetic field of the present invention. For example, in Figure 1, if gas 14 is heated by an arc discharge between electrodes 20, then axis 16 is defined as running between electrodes 20. If gas 14 is excited by an RF field created by solenoid 22, then axis 16 is the axis of solenoid 22. Figure 3 is a partial perspective view of a high-power carbon dioxide laser 40. Two RF transmitters 42 define between them a chamber 44 through which carbon dioxide gas is caused to flow, from an inlet 50 to an outlet 52. RF transmitters 42, by defining between them chamber 44, also define between them axis 46. According to the present invention, RF transmitters 42 and chamber 44 defined therebetween are encircled by three planar loops of electrically conductive wire, tilted at an acute angle with respect to axis 46. For clarity, only one of the loops, designated by reference numeral 60, is shown in Figure 3.

In addition to increasing the efficiency of gas lasers by smoothing the temperature profiles of their plasmas, the present invention has the further advantage of reducing the power required to excite the plasma. For example, using electrodes 20 to excite gas 14 in the laser of Figure 1 by a pulsed arc discharge in the absence of coils 32, 34 and 36, each pulse must be sufficiently energetic to heat gas 14 to its centrally peaked operating temperature profile. If coils 32, 34 and 36 are used to stir the plasma electromagnetically, then pairs of lower energy pulses may be used to heat gas 14, with the first pulse heating gas 14 part of the way to its flat operating temperature profile, and the second pulse completing the heating.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims

WHAT IS CLAIMED IS:

1. An improved laser of the type in which stimulated emission of radiation is induced in a gas contained in a chamber having an axis, during which induction the gas is at least partially ionized, the improvement comprising electromagnetic means for stirring the ionized gas.

2. The gas laser of claim 1, wherein said electromagnetic means includes at least one substantially planar, electrically conductive loop outside the chamber, encircling the chamber, and positioned at an acute angle to the axis.

3. The gas laser of claim 2, wherein said electromagnetic means includes at least three substantially planar, electrically conductive loops outside the chamber and encircling the chamber, each of said loops positioned at said acute angle to the axis, said loops being distributed azimuthally uniformly around the chamber.

4. A method for increasing the efficiency of a gas laser in which stimulated emission of radiation is induced in a gas contained in a chamber having an axis, the gas thereby becoming at least partially ionized, the method comprising the step of electromagnetically stirring the ionized gas.

5. The method of claim 4, wherein said stirring is effected by the steps of:

(a) providing at least one substantially planar, electrically conductive loop outside the chamber, encircling the chamber and positioned at an acute angle to the axis; and

(b) causing a time-varying electrical current to flow in said at least one loop, thereby creating a time-varying magnetic field within the chamber.

6. The method of claim 5, wherein said time-varying electrical current is pulsed.

7. The method of claim 6, wherein said pulses are provided at a frequency between about 50 Hz and about 1000 Hz.

8. The method of claim 5, wherein at least three of said loops are provided, said loops being distributed azimuthally uniformly around the chamber.