GB2107512A - Apparatus for producing a laser-active state in a fast subsonic flow - Google Patents

Abstract

In order to increase the power density of the electric discharge in apparatus producing a laser-active state in a fast subsonic flow comprising a flow channel (1), a first electrode pair having dielectric electrodes (4, 5) disposed opposite one another in the wall region of the flow channel and a second electrode pair, the electrodes (9) and (10) of which are arranged upstream and downstream of the discharge in the flow channel, the electrodes of the first electrode pair being connected to a HF voltage source (21), the electrodes (9, 10) are connected to a second HF voltage source (22) and comprise a plurality of individual dielectric electrodes distributed over the cross section of the flow channel which each comprise a metallic core surrounded on all sides by a dielectric material. <IMAGE>

Description

SPECIFICATION Apparatus for producing a laser-active state in a fast subsonic flow The invention relates to apparatus for producing a laser-active state in a fast subsonic flow, comprising a flow channel, a first electrode pair having dielectric electrodes disposed opposite one another in the wall region of the flow channel and a second electrode pair, one electrode of which is arranged upstream of the discharge in the flow channel and the other electrode of which is arranged downstream of the discharge in the flow channel, the electrodes of the first electrode pair being connected to a HF voltage source.

Electric flow discharges have been used for a long time to excite molecular gases for lasers, particularly for CO2 lasers. The development of CO2 lasers started with longitudinal discharges in glass tubes, the extractable output being limited to approximately 80 W/m on account of the heat dissipation of the plasma towards the wall by diffusion.

An increase in the specific power output is obtained with convective heat dissipation in a fast flow in the longitudinal direction of the discharge tube: with discharge stabilisation by turbulence formation, 500 W/m is attained.

High pressure differences are needed in order to overcome the resultant high flow resistances, which necessitate a complicated pump system for gas circulation.

Lasers having a cross flow, in which fairly high heat dissipation takes place in a flow at right angles to the optical axis, provide fairly high outputs. The electric discharge is superimposed either at right angles or parallel to the flow. Both methods require special measures for discharge stabilisation. The discharge in the direction of flow thus tends to be constricted on account of inhomogeneities, particularly of the temperature and the electron density, which are determined by the electrodes disposed in the flow. In order to suppress the instabilities originating from such places, strong turbulences have to be stimulated, which necessitates the use of measures relating to apparatus (Nighan et al, Appl.

Phys. Lett., 25, 633, 1974).

When the discharges are at right angles to the flow, measures have to be taken which oppose the entrainment of the discharge in the direction of flow.

Such transverse discharges have been subjected to a variety of intensive tests: 1. Self-sustained direct-current discharges In this case, owing to the direct connection between field strength and charge carrier production (extreme dependency of the ionization coefficients on the electric field strength), only a limited power density is obtained in a stable manner. Electrodes with extensive segmentation and complicated cooling with a network of series resistors are therefore necessary in order to stabilise the discharge.

2. Combined discharges The discharge stability is improved, and thus the power density increased, by separating the charge carrier production and vibrational excitation functions. The following are known methods of separate charge carrier production: (a) Pulser-sustainer Short high-voltage pulses of a high frequency are superimposed on a non-self-sustaining direct current discharge of a low electric field strength in order to ionize the plasma. The field strength of the direct current discharge is generally lower than in the case of self-sustaining discharges and adapted to optimum vibrational excitation of the molecular gas. The short high-voltage pulses produce a high electron density in a stable plasma recombining to a predominant extent in time. However, the improvement in the discharge necessitates a complicated power supply.This concept was further developed by additionally UV-preionization superimposed on the so-called PIE-discharge (Nam et al, IEEE J.

Quantum Electronics, QE 15, 44, 1979).

(b) Non-self-sustaining direct current discharge with electron beam ionization This concept enables the best output data to be obtained, although the apparatus is complicated and sensitive. The electron beam is coupled by a thin metal foil into the discharge, the interruption of which leads to serious operating disturbances. The operation is not without problems, as high voltages and shielding against X-rays are necessary.

3. Self-sustaining high-frequency discharges In accordance with the variation of the electric field in time, the high frequency discharge consists of an alternating sequence of short self-sustaining and long non-self-sustaining discharge phases in the case of an electron density which is constant in time. In a plasma intended for recombination, higher specific power densities are used rather than volumetric instabilities. The high frequency discharge provides the possibility of reducing the loss in charge carriers by selecting an appropriate frequency. This is effected by keeping the amplitude of the electron drift motion small with respect to the spacing of the electrodes. The high frequency energy can be coupled in by means of metallic electrodes or capacitively. (European published application 3280, US patent specification 3 748 594; Gavrilyuk et al, Sov, J.Quantum Electron., 9, 326, 1979; Christensen et al, IEEE Quantum Electronics, QE 16, 949, 1980).

With respect to direct current discharge, coupling in involving electrodes entails the further advantage that the polarity change periodically breakes the cathode operation of the respective electrode into periods which are much shorter than the typical growth time of thermal instabilities, which would otherwise originate from the cathode. In comparison to direct current discharges, the HF-discharge can be coupled in by means of simply constructed electrodes with coarse segmentation.

Furthermore, the high-frequency discharge has a positive current-voltage-characteristic, as a result of which the otherwise usual ohmic stabilisation is unnecessary (German patent specification 29 1 7 995; Schock et al, LAS ER + Elecktro-Optik, 2, 76, 1981).

It is also known to superimpose an electric HF-filed, directed transversely to the flow, on a constant electric field extending parallel to the direction of flow and thus to increase the power density of the discharge (Eckbreth et al, Appl. Phys. Lett., 21, 25, 1972). In this case the superimposition of the high-frequency field serves to stabilize the longitudinal direct current discharge. However, as before, this arrangement entails the disadvantage of inhomogeneities limiting the power density on account of the metallic electrodes arranged in the channel cross-section.

What as desired is the development of apparatus of this known type such that homogeneous electric excitation of high power density is possible in a fast-flowing gas.

The present invention provides apparatus of the above-mentioned type in which the electrodes of the second electrode pair are connected to a second HF-voltage source and the electrodes of the second electrode pair comprise a plurality of dielectric individual electrodes having a metallic core surrounded on all sides by a dielectric material.

An electric field whose electric field vector varies as regards magnitude and direction is obtained by connecting the second electrode pair to a HF voltage source.

The stabilization is based essentially on the following effects: (a) The change in direction of the electric field vector suppresses the growth of instabilities determined by direction.

(b) The variation in magnitude of the electric field vector causes the self-sustaining and non-self-sustaining discharge phases to alternate. The plasma is intended to recombine during the non-self-sustaining phase and thus has a stabilising effect.

There is a surrounding field strength vector of a constant magnitude for the special case of vertical superimposition and temporal phase shift of sir/2.

It can also be favourable for the two HFvoltage sources to produce different frequencies without phase coupling.

If the frequencies of the generators are different and there is no phase coupling, a field strength vector with statistical angular velocity and modulation of the magnitude is produced.

In this connection it is pointed out that an experiment is already known in which the discharge is operated with uniformly rotating E field vectors inside a glass tube (Zhilinskii et al, Sov. Phys. Tech. Phys., 23, 1293, 1978).

The high-frequency power is coupled in by two electrode pairs which are arranged outside a glass tube. This tube arrangement limits the power density which can be obtained, as is obvious from the obtained electric power density of 4 W/cm3 which is coupled in.

A limited increase in power by the additional superimposition of a longitudinal flow in the tube would be conceivable in that arrangement. However, the arrangement according to the invention of the second electrode pair in the flow channel and the disintegration of the electrode pair into individual electrodes permits a cross flow having a high gas speed to be superimposed. The advantages of a rotating electric field can thus be combined with those of fast gas exchange in the discharge.

However, the electrode which is disintegrated into individual electrodes and arranged upstream of the discharge region also has an effect in terms of flow, i.e. it homogenizes the gas flow entering the discharge region. Similarly, the electrode which is disintegrated into individual electrodes and arranged downstream has an additional effect, i.e. it restricts the discharge region to the side of the discharge region disposed downstream and thus effectively prevents the discharge from being entrained in the direction of flow.

The use of electrodes covered with dielectric material in the second electrode pair permits a homogeneous current supply at the electrodes, without the otherwise necessary measures of electrode segmentation on one hand and ohmic stabilisation on the other.

Cooling, which is particularly complicated in the case of high-power lasers, is thus unnecessary. Furthermore, direct contact between gas and metal is avoided by using dielectric electrodes, as a result of which the laser has an improved long-term behaviour.

On account of its electrical properties (dielectric strength, dielectric constant), Al2O3 is particularly suitable as a dielectric.

It is particularly advantageous for the individual electrodes to be formed as plates which are arranged at a distance from one another and parallel to the direction of flow. The smoothing effect of the flow when entering the discharge region is particularly marked on account of this arrangement.

in a preferred embodiment the individual electrodes end upstream directly before the electrodes of the first electrode pair and the individual electrodes begin downstream di rectly behind the electrodes of the first electrode pair. A precisely defined discharge region is thus obtained between the electrodes of the first electrode pair.

According to the use of the laser, it can be operated continuously, pulsed, or pulse-superimposed. If, for example, one of the two electrode systems is acted upon by a continuous high-frequency voltage and the other by a pulsed voltage, a stable discharge for continuous laser irradiation with pulse superimposition can be obtained by using the E-vector rotation occurring in this case.

The arrangement of the electrode system according to the invention, which partly assumes the function of influencing the flow and limiting the discharge, permits the use of very high gas speeds without the discharge being entrained in the direction of flow. Three measures are thus simultaneously combined for the purpose of discharge stabilisation: (a) high flow speed, (b) variation in the magnitude of the electric field strength, (c) change of direction of the field strength vector.

The advantages of the apparatus according to the invention can be summarized by the following characteristic features: (1) Owing to the high output density which can be obtained, the laser can have a compact structure with simple electrodes, as there is no need for segmentation and preionization, which are otherwise usual.

(2) There is no lossy ohmic stabilisation.

(3) Electrode cooling is not necessary.

(4) There is no metallic contact with the plasma in the discharge zone, as a result of which the long-term behaviour of the laser is improved.

(5) The electrode arrangement permits pulse-superimposed continuous laser operation.

As a result, particularly of points 1 to 3, the technical complexity of the laser system is low, although its efficiency is high.

The invention will be described further, by way of example, with reference to the accompanying drawing, whose sole Figure shows a schematic longitudinal sectional view of apparatus according to the invention for producing a laser-active state.

Electrodes 4 and 5, which together form a first electrode pair, are embedded in a channel 1, which preferably has a rectangular cross-section, in opposite side walls 2 and 3.

The plane electrodes 4, 5 are flush with the side walls of the channel and each consist of a dielectric 6 with a small loss angle which is provided at the rear side with a metal covering 7. The resonator of the laser is disposed in the space 8 between the two electrodes 4 and 5.

Further electrodes 9 and 10, which together form a second electrode pair, are disposed in front of and behind the space 8.

Both electrodes are formed by individual electrodes 11 and 1 2 which, in the illustrated embodiment, are in the form of plates which are distributed at a mutual spacing parallel to the direction of flow over the cross section of the channel 1. The individual electrodes 11 or 1 2 each consist of a metallic core 1 3 or 14, respectively, which is surrounded by a dielectric material 1 5 or 16, respectively. The electrodes of the first electrode pair and the electrodes of the second electrode pair are connected to high-frequency generators 21 and 22, respectively, by inductors 1 7 and 1 8 and 1 9 and 20, respectively.

The inductors compensate for the capacitive reactance of the electrodes, so that the generators are only loaded by the resistance of the discharge.

The gas in which the laser-active state is to be produced flows in the direction of the arrow A through the arrangement, the gas flow being smoothed and homogenized by the individual electrodes 11 at the upstream end of the space 8. The gas flows through the space 8 and, owing to the electric field which alters as regards direction and possibly strength, a discharge is produced in the gas flow which spreads out homogeneously transversely to the direction of flow over the entire space 8 and is defined at the upstream side of the space 8 by the individual electrodes 1 2. The essential factor is that the discharge starts over a large area on the plate surface, so that, in contrast to conventional direct current longitudinal discharges, the plasma is not constricted in the discharge region.

Extraordinarily high power densities can be obtained with the above-described arrangement. For example, a homogneous discharge of a high power density 35 kW/I and thus a CO2 laser with an overall efficiency of 20% was obtained in first tests with an arrangement of this type in continuous operation. The discharge volume was 0.35 1. Ceramic plates (Al202) having a thickness of 2.5 mm were used as electrodes. The generator frequency was 6 MHz.

Claims (7)

1. Apparatus for producing a laser-active state in a fast subsonic flow, comprising a flow channel, a first electrode pair having dielectric electrodes disposed opposite one another in the wall region of the flow channel, and a second electrode pair, one electrode of which is arranged upstream of the discharge in the flow channel and the other electrode of which is arranged downstream of the discharge in the flow channel, the electrodes of the first electrode pair being connected to a first HF voltage source, the electrodes of the second electrode pair being connected to a second HF voltage source, and the electrodes of the second electrode pair comprising a plurality of individual dielectric electrodes which are distributed over the cross-section of the flow channel and which each comprise a metallic core surrounded on all sides by a dielectric material.

2. Apparatus as claimed in claim 1, in which the individual electrodes are in the form of plates which are arranged at a distance from one another parallel to the direction of flow.

3. Apparatus as claimed in claim 1 or 2, in which the electrodes of the first electrode pair each consist of a dielectric plate, on the rear side of which a metallic plate or a metallic layer is applied.

4. Apparatus as claimed in any preceding claim, in which the individual electrodes end upstream directly before the electrodes of the first electrode pair and the individual electrodes begin downstream directly behind the electrodes of the first electrode pair.

5. Apparatus as claimed in any preceding claim, in which the second HF voltage source produces an HF voltage of the same frequency as the first, both HF voltages having a fixed phase relationship with a phase shift.

6. Apparatus as claimed in any of claims 1 to 4, in which the two HF voltage sources produce different frequencies without phase coupling.

7. Apparatus for producing a laser-active state in a fast subsonic flow, substantially as described with reference to, and as shown in, the accompanying drawing.