Liquid Cooled Magnet Coil For Particle Acceleration

Abstract

A magnet coil for a particle accelerator is formed of a plurality of turns of a conductor each composed of two yoke-shaped parts of conducting material containing a central metallic tube. The tube extends from each end of the parts. The parts are so positioned that the projecting tubes engage each other, where they are welded together. The inside tube is then tested. Conducting members are then placed around the tube in the gaps between the adjacent yoke-shaped parts and are welded to the conductor members. The whole unit is then embodied in insulation.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid-cooled magnet coil for a particle accelerator comprising turns of a relatively thick conductor provided with a cooling channel and surrounded by insulation, several turns being arranged in the same plane, and a method for manufacturing such a coil.

With coils where the conductor has a very large cross section and such rigidity that special bending equipment is required for shaping, it is known to construct the coil of yoke-shaped conductor parts, each comprising a half turn, the turns being insulated from each other with the help of spacers of pressboard or the like. The connection between the conductor parts is usually effected with the help of brazing since welding is too expensive.

With magnet coils for the particle accelerator of the above-mentioned type it is not only the large cross section of the conductor but also several other requirements which make special demands on the construction if the manufacturing costs are to be kept at a reasonable level. Thus the conductor must be surrounded entirely by insulation since shock voltages of about 30 kv. may arise. It must be possible to make the coils long, for example 5 m., and with small tolerance. Since the conductor is provided with a central cooling channel for liquid cooling the yokes must be joined together so that all risk of leakage-- and consequently destroyed insulation-- is eliminated. During welding it must also be ensured that no molten material penetrates into the cooling channel and causes blockage or throttling, and it is important that the channel opening in one of two yoke ends joined together exactly coincides with the channel opening in the other.

2. The Prior Art

According to a known proposal an accelerator coil having the properties mentioned in the introduction can be made by shaping a number of yokes of different lengths from a conductor with undivided cross section and a central cooling channel, the end surfaces of the yokes being directly welded to each other so that the coil has two welding seams per turn. Since great reliability is required each welding seam is checked immediately after the welding by inserting the spliced part in a tension testing means and subjecting it to high tensile stress. Possible bubbles or particles of welding flux are then brought to light. This testing method means that the conductor, which during the test must be surrounded by clamps along a relatively long stretch, cannot be wrapped with insulation until the test has been carried out-- at least not to any great extent. This means that the coil must be completely or substantially insulated by hand. During static experiments it has been found that welding or silver soldering of very thick-walled copper tubes resistant to high liquid pressure (the directly cooled conductor may be counted as such) cannot be carried out under normal industrial conditions. A relatively high percentage of connections must be discarded due to unsatisfactory sealing. When a coil comprises a great number of such connections-- as in the present case-- the necessary reliability of the coil can only be effected by systematic testing of the splices.

SUMMARY OF THE DISCLOSURE

The above described, known accelerator coils is substantially constructed in the manner usual for particularly large conductor cross sections and a directly cooled conductor is thus used which, as with known directly cooled conductors, only fulfils the function of enclosing the coolant flowing through the conductor. By manufacturing the accelerator coil according to the present invention a considerably cheaper and at least as reliable product is achieved. It seems surprising when it is observed that the conductor of the coil in a construction according to the invention must be welded or silver soldered at at least twice as many points as with the known construction and that, furthermore, a considerable number of welding seams is required for a centrally arranged cooling tube according to the invention and it is unexpected that the insertion of a special lining tube described in the following, in spite of the extra material and assembly costs, should contribute to a cheaper construction.

A magnet coil according to the invention is spliced at at least one relatively short splice stretch per turn and is characterized in that said cooling channel is lined by a metal tube which is joined by means of welding at the central part of said splice section and that along said splice section said conductor is represented by a special conductor part, formed by at least two adjacent parts which are metallically joined at each end to the rest of the conductor.

The invention also comprises a method for manufacturing such a magnet coil and is characterized in that each yoke part is built up of a number of copper bars which together form said conductor provided with a cooling channel, the bars being arranged to surround a tube arranged as a lining in the cooling channel, the tube ends projecting from the ends of the bars, after which said bars are mechanically surrounded along substantially their entire length by a prestressed insulating tape and thus brought into effective heat-conducting contact with the centrally arranged tube, after which tubes belonging to different yoke-shaped parts are joined together by welding the projecting tube ends, and the tightness of the weld is checked by introducing a pressure medium into the tubes, after which the tube parts not surrounded by copper bars are surrounded by joining bars and the joining bars joined to adjacent yoke-shaped copper bars, after which the joining bars and adjacent, uninsulated parts of the yoke-shaped copper bars are furnished with surrounding insulation.

According to a further development of the invention the coil conductor between the splice also sections consists of a number of adjacent parts which are held together and pressed against said tube by means of a layer of strongly prestressed insulating tape surrounding the conductor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following with reference to the accompanying drawings where FIGS. 1 and 2 show a coil according to the invention at a certain stage in the manufacture. FIG. 2 shows a cross section through the coil along the line B-B in FIG. 1 and FIG. 1 shows a section along the line A-A in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The coil is wound from a conductor 1 formed by two bars 1a and 1b and having a central channel 2 which is lined with a tube of stainless steel 3. The coil has three turns lying in the same plane. The conductor is surrounded by an insulation layer 4. Each turn consists of two yoke-shaped parts 5 which are bent to the desired shape before assembly and substantially provided with a surrounding insulation which is mechanically applied with the help of equipment for the purpose before the parts are joined and in such a way that the bars 1a and 1b are arranged with effective heat-conducting contact in relation to the surrounded tube 3, for example by wrapping them with a strongly prestressed insulating tape.

When the yoke-shaped parts 5 are being manufactured the tube 3 is given greater length that the surrounding, yoke-shaped conductor so that the tube ends project at each end of the yoke. When the coil is assembled such tube ends are joined by welding seams 6 and this is suitably carried out with an automatic argon welding machine specially developed for welding stainless steel tubes.

When all the tube connections 6 are welded the production stage shown in the drawings has been reached and the yoke-shaped tubes 3 together form a spiral the end points of which are designated 7 and 8. The tightness of the tube spiral is tested by means of a method known per se in which a gas is introduced under pressure, after which a detector for minute concentrations of the gas in question is placed in the vicinity of the joints 6. Any unsatisfactory joints are rewelded. After effecting a tube spiral in this way which fulfils the demands for tightness and strength which are made on the cooling system, the parts of the tube not surrounded by the conductor are surrounded by two joining bars 9a and 9b which suitably have the same cross section as the bars 1a and 1b and which are fitted into the corresponding splice section 11 between the end surfaces 10 of the yoke-shaped conductor which are facing each other. Brazing is then carried out so that the required electrical contact is effected between the yoke-shaped conductors and the joining bars. During the welding the yoke ends are surrounded by water-cooled clamps so that the insulation is not damaged. Finally the joining bars and yoke ends are provided with insulation and the whole coil is then embedded in epoxy resin.

As already mentioned, it is extremely difficult in conventional coil constructions to prefabricate the yoke parts for a large accelerator coil in such a way that the prescribed coil dimensions are attained with the required accuracy. Adjustments in shape are difficult to effect on a yoke part since this is provided with insulation and particularly after one yoke end has been welded to the rest of the coil parts and the yoke cannot be returned to the bending machine.

With a coil construction according to the invention, however, adjustments can easily be carried out by shortening and/or bending the tube parts projecting from the yoke-shaped conductors. As mentioned previously, it has been found that when joining thick-walled copper tubes intended for liquid under pressure it is impossible to achieve repetitively perfect connections under normal factory conditions.

When accelerator equipment is concerned there are great demands for operational safety since a breakdown may destroy a complicated and expensive series of tests. When the tightness of the cooling channel, which with the conventional construction of a directly cooled coil is dependent on the various connections is unsatisfactory, it is necessary for inferior joints to be discarded, which means that a relatively great number of connections can only be accepted after repeated splicing. When the relatively thin-walled tubes of stainless steel as in accordance with the invention is to be joined, however, statistics show that welding gives a considerably higher degree of reliability than that which can be achieved with welding or brazing the coil conductor.

Also with respect to the testing method it is more advantageous if the joints of the liquid-containing system are in the form of welded tubes of stainless steel instead of in the form of brazed connections between thick-walled conductor yokes since faults which only result in leakage after some time in operation (for example layers of welding flux or slay) in the latter case form a relatively large percentage of the total number of faults.

Claims

We claim:

1. Liquid-cooled magnet coil for a particle accelerator comprising a plurality of turns of a conductor provided with a cooling channel and surrounded by insulation, several turns being arranged in the same plane, the conductor being spliced at at least one relatively short section per turn, in which said cooling channel is lined by a metal tube which is joined by means of welding at the central part of said splice section and that along said splice section said conductor is constituted by a conductor part, formed by at least two adjacent conductive parts surrounding the metal tube which are joined at each end to the rest of the conductor.

2. Liquid-cooled magnet coil for a particle accelerator according to claim 1, in which the magnet coil has two splice sections per turn and the conductor is also formed between the splice sections from two adjacent parts which are held together and pressed against said tube by means of a layer of strongly prestressed insulating tape surrounding the conductor.

3. 6 Method of manufacturing a liquid-cooled magnet coil for a particle accelerator which magnet coil comprises turns of a conductor provided with a cooling channel and surrounded by insulation, several turns being arranged in the same plane, each turn substantially comprising two yoke-shaped parts joined to each other, in which each yoke part is built up of a number of copper bars which together form said conductor provided with a cooling channel, arranging the bars to surround a tube arranged as a lining in the cooling channel, the tube ends projecting from the ends of the bars, after which said bars are surrounded along substantially their entire length by a prestressed insulating tape and thus brought into effective heat-conducting contact with the centrally arranged tube, after which tubes belonging to different yoke-shaped parts are joined together by welding the projecting tube ends, and the tightness of the weld is checked by introducing a pressure medium into the tubes, after which the projecting tube parts not surrounded by copper bars are surrounded by joining bars and the joining bars joined to adjacent yoke-shaped copper bars, after which the joining bars and adjacent, uninsulated parts of the yoke-shaped copper bars are furnished with surrounding insulation.