GB2334616A - Laser electrodes with internal pipes for coolant fluid - Google Patents

Abstract

The invention relates to a device for dissipating waste heat from lasers, such as radio frequency excited CO 2 slab lasers, which are equipped with electrodes (1) for coupling radio frequency energy and wherein the electrode material is in thermal contact with a flowing cooling medium for the purpose of dissipating the heat. A cooling medium pipe (5) is provided in the inside of the electrode material, whereby the thermal and mechanical loadings which occur during the operation do not result in any change in the shape of the electrode (1). Pump channels (7,8) are provided which guarantee a high serviceable life of the laser in the sealed-off operation.

Description

DESCRIPTION DEVICE FOR DISSIPATING WASTE HEAT The invention relates to a device for dissipating waste heat from lasers, such as from RF-excited Cm2 slab lasers which are equipped with electrodes to couple RF energy, the material of the said electrodes being in thermal contact with a flowing cooling medium for the purpose of dissipating the heat.

It is known to equip for example RF-excited Cm2 slab lasers with cooling arrangements which have the task of dissipating waste heat from the laser-active region. The efficiency of such lasers is approx. 10%, from which it follows that for the output levels which are currently typical for such lasers, a waste heat of approx. 0.5 to 30 kW must be dissipated. This heat is present in the discharge gap which is formed between the slab electrodes.

The slab electrodes serve on the one hand to couple the radiofrequency energy and therefore must be made from a material which is capable of high electrical conductivity whilst simultaneously being free of magnetism and which also dissipates efficiently the heat which arises from the loading with high currents.

Moreover, however, the electrodes must form an optical waveguide which is as loss-free as possible and which for this purpose have waveguide surfaces which face the discharge gap.

The waveguide surfaces must have a high optical quality for the laser radiation and also maintain this under operating conditions, i.e. they themselves must not deform under extreme thermal loading.

Especially in the case of modern CO2 lasers which for reasons of compactness and user-friendliness are formed as tightly sealed sealed-off systems, the heat cannot be dissipated to the outside by gas flows, but rather must diffuse to cooled sections of the discharge region.

In addition to this, the electrodes are subjected to mechanical loadings since such lasers are used in various and furthermore different mounting positions, such as for example on robot arms. As a consequence, the electrodes are required to have a sufficiently high stability and rigidity so that as freely-carried elements they do not deform to an inadmissible extent under operating conditions.

A radiofrequency excited diffusion-cooled slab laser is known from EP 0 585 482 Al, in which the two electrodes consist in each case of a special steel hollow profile comprising a rectangular cross-section and a thin copper plate soldered on the hollow profile. Milled into the copper plate are cooling channels which are partly enclosed by the material of the copper plate and where the copper plate lies against the special steel hollow profile these cooling channels are covered by a special steel surface. These cooling channels serve to supply and return the cooling medium in different embodiments.

Since the copper plates are soldered on one side to a surface of the special steel hollow profile and the heat must be dissipated to the side of the electrode which is equipped with the copper plate, it is possible owing to the poor heat conducting properties of the special steel wall for the temperatures on the cooled and non-cooled side of the electrode to be considerably different which, given the relatively thin wall of the special steel hollow profile, can result in the electrode surface deforming and as a consequence the function of the waveguide can be impaired. This danger is particularly great if, at the point in time of switching on the cooling circuit, the temperature of the cooling medium is substantially below the temperature of the electrode.

A CO2 1aser is known from US patent 5,237,580 in which the slab electrodes are manufactured from aluminium and comprise on their rearside, i.e. on the side remote from the waveguide surface, open channels in which are placed the copper pipes through which the cooling medium flows. A disadvantage of this known laser is that channels formed in this manner considerably impair the rigidity of the electrode in the transverse direction with respect to the laser radiation, which under operating conditions can lead to the electrodes deforming in the transverse direction.

One object of the present invention is to develop further the known devices for dissipating waste heat from lasers in such a manner that the heat can be dissipated to a great extent without impairing the functionability of the electrodes.

In accordance with the invention, pipes are provided as cooling medium conduits extending within the electrodes. In so doing, the pipe cross-section is completely encompassed by the electrode material, and the pipe outer surface, as seen in the pipe cross-section, is at least in sections directly contacted by the electrode material.

By arranging the cooling medium conduits in the inside of the electrode material it is achieved that the recesses, for example the channels, necessary to accommodate the pipes in the electrode material do not cause any change in shape of the electrode during the thermal and also mechanical loading occurring during the operation, since the mechanical rigidity of the electrode in the transverse direction is considerably higher than when the channels are milled out on one side in accordance with the prior art. Moreover, the arrangement of the pipes in the neutral fibre of the electrode material prevents the electrode deforming in the longitudinal direction under thermal loading.

Preferably, the pipes and the flow direction of the cooling medium extend in parallel with the laser radiation direction and with respect to the direction of the discharge gap which is formed between the two electrodes. In this manner, the heat to be dissipated is absorbed by the cooling medium over the longest possible routes on all sides and as a consequence the electrode is also prevented from deforming by virtue of the fact that excessively high differences in temperature within the electrode material are avoided.

In one embodiment of the invention both the channels in the electrode material and also the pipes have a circular cross-section, the inner diameter of the channels corresponding largely to the outer diameter of the pipes, so that the pipe outer surface and the channel inner surface are in contact over the entire periphery and it is thus possible for the cooling medium to absorb the heat efficiently. The heat transfer can be further improved by virtue of the fact that the pipe outer surface is pressed under pressure into the channel inner surface.

The latter is for example easy to achieve, in that initially pipe-shaped channels are provided in the electrode material and subsequently the pipe is inserted into these channels, the outer diameter of the pipe corresponding approximately to the inner diameter of the pipe-shaped channels. In a next step the cross-section of the inserted pipe is widened which can be achieved as desired mechanically by pressing in a mandril or hydraulically by forcing through the pipe a liquid under such a high pressure as to result in the widening of the pipe cross-section, causing the pipe outer wall to contact the channel inner surface.

However, even in the case of high contacting pressures it is possibly unavoidable that gas bubbles remain between the pipe outer surface and the channel inner surface. If, for example, smooth pipes having a circular cross-section are provided in channels which likewise have a circular cross-section, it is not possible under certain circumstances in the case of typical electrode lengths of 50 to 100 cm to remove the trapped gasses in a justifiable period of time during the manufacturing process. If it is left until the laser is operating for these bubbles to diffuse out slowly, then the serviceable life of the laser is shortened by premature reduction in output.

For this reason a particularly preferred embodiment of the invention is provided between the outer surface of the pipe and the inner surface of the channel with at least one hollow chamber extending as a so-called pump channel in the flow direction of the cooling medium.

This hollow chamber can in accordance with the invention be formed either by drawing inwards the periphery of the pipe on one or a plurality of sites towards the pipe inside or by virtue of forming one or a plurality of grooves in the channels.

In the regions of these sites or grooves the pipe outer surface does not directly contact the channel inner surface. Therefore, it is not necessary for any trapped gases to diffuse over the entire length of the electrode through micro channels but rather merely over a short distance in the peripheral direction around the pipe periphery, until they arrive at the hollow chamber or the pump channel, by means of which they can then rapidly escape in the longitudinal direction.

In this manner it is advantageously possible to remove the trapped gases during the manufacturing process and thus right from the beginning guarantee a long serviceable life of the laser.

In this connection it is further provided in accordance with the invention to use a pipe designed as a swirl-promoter pipe. These pipes comprise on their outer surface indentations which extend in the form of spiral twists over the entire length of the pipe. The substantial advantage of this arrangement resides in that the swirling action effectively transmits the heat into the cooling medium.

In a further extremely preferential embodiment of the invention the electrodes are provided with means for clamping the pipe which extends in the inside of the electrodes. The means for clamping can, for example, be material sections of the electrode material which are formed so as to be deformable in a plastic manner.

This produces in an advantageous manner the possibility during the manufacturing process of mechanically attaching the pipes after insertion into the inside of the electrodes and as a consequence also of improving the heat transfer from the electrode material into the pipe material.

Preferably, the electrode material should be aluminium and the pipe material copper.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows an electrode with an inserted pipe for the cooling medium, Figure 2 shows a variant a) of the embodiment and a variant b) of the embodiment for the pipe and channel cross-sections, and Figure 3 show a variant a) of the embodiment and a variant b) of the embodiment for the clamping arrangement of the pipes in the inside of the electrode.

Figure 1 illustrates a slab electrode 1 in a shortened length with two channels 3 and 4 each of a circular cross-section being worked into its cross-sections. The channels 3, 4 extend in parallel with each other and in the longitudinal direction with respect to the slab electrode 1.

Inserted into the channels 3, 4 is a cooling medium pipe 5 which extends, commencing at one end of the slab electrode 1, almost over its entire length through the electrode material to the opposite end, where it is curved almost in a semi-circular manner and extends back again through the electrode material. The two pipe ends exiting at the same end of the slab electrode 1 are curved at right angles and serve as desired to connect the supply and return lines for a cooling medium.

Figure 1 further illustrates that the pipe 5 is designed as a swirl-promoter pipe which comprises on its outer wall indentations 6 which extend in a spiral-like manner; this is again further mentioned in detail in the text.

The slab electrode 1 is manufactured from aluminium whereas copper is used for the pipe material. The outer diameter of the pipe 5 and the inner diameter of the channel 3, 4 correspond with each other such that the pipe 5 sits tightly in the channels 3, 4, as a result of which the outer surface of the pipe 5 lies against the inner surfaces of the channels 3, 4 producing a thermal contact which guarantees an optimum heat transfer from the electrode material to the pipe material.

Figure 2 illustrates in the variant a) of the embodiment the cross-section 2 of the slab electrode 1, in which a swirl-promoter pipe is pressed into the channels 3, 4 in accordance with Figure 1. The twisting provides the pipe 5 on its periphery, as is evident in the pipe cross-section, with four indentations which extend in a spiral manner over the entire length of the pipe. As a consequence four pump channels 7 are formed between the outer surface of the pipe 5 and the inner surface of the channels 3, 4, by means of which trapped air or other gases can be dissipated to the outside.

Owing to the geometric design of the cross-section 2 and the pipe 5 inserted in accordance with the invention, the slab electrode 1 does not deform under the thermal loading of the laser operation either in the transverse direction nor in the longitudinal direction with respect to the laser radiation.

A further variant b) of the embodiment in accordance with Figure 2 provides a pipe 5 having a smooth outer wall but in contrast comprises grooves which are worked in the inner surface of the channels 3, 4 and which form the pump channels 8 and in this manner are likewise suitable for the purpose of dissipating trapped gases from inside the electrodes to the outside.

Figure 3 illustrates in the variant a) of the embodiment a possibility of clamping the pipe 5 in the inside of the slab electrode 1. Material sections 9 of the electrode material are designed as means for clamping in such a manner that by exerting a pressing-in pressure vertically on these sections 9 the channels 3, 4 are squeezed in the pressure direction as a result of which the pipe 5 comes into firm contact with the inside of the channels 3, 4.

In the variant b) of the embodiment as shown in Figure 3, sections 10 of the electrode material are designed as a means for clamping the pipe 5, but wherein, however, in contrast to the variant of the embodiment as shown in Figure 3 (a), the direction of the contacting pressure can be achieved through orifices worked laterally in the electrode material. This also ensures that the pipe 5 is locked within the channels 3 and 4 and thus the prerequisite for an optimum heat transfer from the electrode material into the pipe material is achieved.

Claims (10)

1. CLAIMS 1. Device for dissipating waste heat from slab lasers which are equipped with electrodes for coupling radio frequency energy and in which the electrode material for the purpose of dissipating the heat is in thermal contact with a flowing cooling medium, wherein, for the purpose of guiding the cooling medium, pipes are provided which extend inside the electrodes, the pipe cross-section being fully encompassed by electrode material and the pipe outer surface directly contacting the electrode material at least in some parts.
2. 2. Device according to claim 1, wherein the pipes are inserted into pipe-shaped channels in the electrode material which extend parallel with the laser radiation direction or with the direction of the discharge gap.
3. 3. Device according to claim I or 2, wherein like the pipe, the channels comprise a circular cross-section, wherein the inner diameter of the channels corresponds to the outer diameter of the pipes and as a consequence the pipe outer surface contacts the channel inner surface.
4. 4. Device according to any of the preceding claims, wherein at least one hollow chamber extending in the flow direction of the cooling medium is provided between the outer surfaces of the pipes and the inner surfaces of the channels.
5. 5. Device according to claim 4, wherein the hollow chamber is formed by drawing in the pipe periphery towards the pipe inside or by grooves worked into the inner surfaces of the channels, and wherein, in the regions where the pipe periphery has been drawn in or the grooves, there is no direct contact between the pipe outer surface and the channel inner surface.
6. 6. Device according to claim 5, wherein the pipes are formed as swirl-promoter pipes which comprise on their outer surface indentations extending in the form of spiral-shaped twists.
7. 7. Device according to any of the aforementioned claims, wherein the electrodes are equipped with means for clamping the pipes extending in the inside of the electrodes.
8. 8. Device according to claim 6, wherein as means for clamping the pipes, regions of the electrode material are designed such that they can deform in a plastic manner.
9. 9. Device according to any of the aforementioned claims, wherein aluminium is used as the electrode material and copper pipe is used to guide the cooling medium conduit.
10. 10. Device substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.