Niobium based superconducting radio frequency(scrf) cavities comprising niobium components joined by laser welding, method and apparatus for manufacturing such cavities

Abstract

Niobium or its alloy based Superconducting Radio Frequency (SCRF) Cavities involving at least one laser beam welded components in the SCRF cavity welded from inside surface of the wall of cavity directed to achieving more than half the thickness to full depth penetration with minimum HAZ, minimizing distortion and shrinkage. The method ensures improved weld quality and surface finish substantially free of any weld defects. Also disclosed is the welding nozzle system and welding rigs adapted to facilitate such laser welding of the Niobium or its alloy based Superconducting Radio Frequency (SCRF) Cavities. The invention is thus directed to enhancing productivity, ensuring consistent quality and reliability, enhanced weld penetration with minimum HAZ, smooth finish of weld joints at possible reduced costs.

We claim: 
     
         1 . Niobium or it's alloys based superconducting radio frequency (SCRF) cavities, comprising of at least one component made of Niobium or it's alloys which are joined by laser welding from inside surface of the wall of the cavity with depth of penetration of welding ranging from greater than half the thickness to full depth of the material being welded. 
     
     
         2 . Niobium or it's alloys based superconducting radio frequency (SCRF) cavities as claimed in  claim 1  comprising iris joints and equator joints in the RE field region formed by laser welding from inside surface of the wall of the cavity with depth of penetration of welding ranging from greater than half the thickness to full depth of the materials being welded. 
     
     
         3 . Niobium or it's alloys based superconducting radio frequency (SCRF) cavities as claimed in anyone of  claim 1  or  2  wherein anyone or more of said equator and/or iris joints comprise full depth laser welding from inside surface of the wall of the cavity. 
     
     
         4 . Niobium or it's alloys based super conducting radio frequency (SCRF) cavities as claimed in anyone of  claims 1  to  3  wherein said thickness of the material of laser weld joint from inside surface of the wall of the cavity is greater than 1 mm. 
     
     
         5 . Niobium or it's alloys based super conducting radio frequency (SCRF) cavities as claimed in anyone of  claims 1  to  4  comprising single and/or multi cell SCRF cavity for charged particle acceleration. 
     
     
         6 . Niobium or it's alloys based super conducting radio frequency (SCRF) cavities as claimed in anyone of  claims 1  to  5  comprising elliptical shaped Niobium or it's alloys based super conducting radio frequency (SCRF) cavities wherein said laser weld joints comprise anyone or more of laser weld dumbbells produced from half cells, end group components welded to end group main body including flanges and higher order mode (HOM) couplers, welds at RF field free regions including stiffening rings welded from outside and RF field region joints including Equator joints and iris joints welded from inside surface of the wall of the said cavity. 
     
     
         7 . A method of producing Niobium based superconducting radio frequency (SCRF) cavities as claimed in anyone of  claims 1  to  6  comprising:
 providing at least one component made of Niobium or it's alloys joined by laser welding from inside surface of the wall of the cavity with depth of welding ranging from greater than half the thickness to full depth of the material being welded following the steps of (i) a first phase of controlled keyhole welding from inside surface of the wall of cavity followed by (ii) a second phase of conduction welding, to achieve very smooth finish of the weld joint, 
 thus minimizing distortion and shrinkage with narrow HAZ and weld surface finish adapted for achieving reliable operation of SCRF cavity. 
 
     
     
         8 . A method as claimed in  claim 7  wherein said step of carrying out keyhole welding comprises depositing major energy in keyhole welding phase comprising:
 j) providing only requisite amount of energy and controlling the rate of heating by keeping pulse frequency low, and 
 ii) depositing energy with variation in time domain 
 
     
     
         9 . A method as claimed in anyone of  claim 7  or  8  comprising carrying out laser welding of the iris from inside surface of the wall of the iris joint between two half cells to form dumbbells of SCRF cavity comprising:
 providing the half-cells held together in an iris welding rig comprising a vacuum vessel; 
 carrying out in the first phase (i) keyhole welding from inside surface of the wall of RF field region involving a nozzle having three concentric cylindrical tubes the laser beam passing through the innermost tube, the outermost and innermost tube supplying gas at high velocity and the middle tube connected to suction port, selecting the welding parameters such that pulse profile is varied in time phase to obtain full depth penetration with minimum distortion; followed by 
 (ii) in the second phase carrying out the laser welding involving inclined nozzles brought at the level of joint wherein the beams are inclined and the weld plume rises perpendicular to the surface, 
 the evaporated material formed because of laser material interaction being sucked out through a material suction arrangement in form of an enclosure. 
 
     
     
         10 . A method as claimed in anyone of  claims 7  to  9  comprising carrying out laser welding of the equator joint of single cell cavity or dumbbells constituting the multicell SCRF cavity from inside surface of the wall of cavity comprising:
 providing the dumbbells in insert and carrying the same inside an equator welding rig comprising a vacuum vessel with devices and/or attachments for carrying out the welding from inside surface of the wall of the cavity; 
 welding the equator joints one after the other performing a two step welding operation comprising: 
 (i) in the first phase carrying out keyhole welding with a laser beam inclined at an angle to the vertical in the plane of welding, the energy being varied in time phase for better penetration and followed by (ii) in the second phase carrying out conduction welding involving dual beam in order to obtain a very smooth finish after keyhole-welding operation is over, the evaporated material from laser material interaction rising up perpendicular to the surface of the joint and being collected by an enclosure. 
 
     
     
         11 . A method as claimed in  claim 10  wherein the welding methodology comprises of the sequence, viewing butting edges—keyhole welding—viewing—repairing—viewing—dual beam conduction welding/smoothening—viewing the entire region with boroscope—laser cleaning—final smoothening with defocused laser—final inspection with boroscope. 
     
     
         12 . A method as claimed in anyone of  claims 7  to  11  wherein said keyhole welding is carried out involving a welding nozzle comprising three concentric tube type enclosure adapted to move up and down, the outermost and inner most tube enclosure adapted to provide high velocity inert gas to flow out whereas the middle enclosure is adapted to suck out the evaporated material due to laser material interaction and wherein said conduction welding is carried out involving the dual beam whereby the evaporated material as a result of the laser material interaction rises up perpendicular to the surface of the joint and is collected by a separate enclosure adapted to move up and down. 
     
     
         13 . A method as claimed in anyone of  claims 7  to  12  wherein the end group component joints, which are in RF field free region, are welded from outside surface of the wall of cavity involving keyhole welding only and the ones, which are in RF field region, are welded by a combination of conduction and keyhole welding 
     
     
         14 . A method of producing Niobium based superconducting radio frequency (SCRF) cavities as claimed in anyone of  claims 7  to  13  wherein Nd:YAG or any other solid state laser is used for laser welding. 
     
     
         15 . A method of producing Niobium based super conducting radio frequency (SCRF) cavities as claimed in anyone of  claims 7  to  14  comprising carrying out of said laser welding for maximizing depth of penetration and minimizing heat affected zone (HAZ), involving temporal variation in energy within the pulse and selective repetition rate of the said laser pulse. 
     
     
         16 . A method of producing Niobium based super conducting radio frequency (SCRF) cavities as claimed in anyone of  claims 7  to  15  wherein the temporal profile of the pulse is tailored for Niobium or its alloys preferably by varying the energy within the pulse in time domain in a near trapezoidal shape such that initial preheating of surface results in higher penetration and the tapering off the energy near the end is done in such a way to avoid micro cracks/porosity/other defects, whereby the overall pulse shape ensures high penetration depth and low heat affected zone. 
     
     
         17 . A method as claimed in anyone of  claims 7  to  16  wherein the laser beam is incident at an angle to the surface such that the major evaporated plasma materials from Laser-material interactions is ejected along a small solid angle and is sucked out through evacuation nozzle. 
     
     
         18 . A method as claimed in anyone of  claims 7  to  17  comprising carrying out the said keyhole welding and conduction welding involving inclined laser beam with selective beam inclination for achieving desired penetration with good surface finish wherein for said keyhole welding the angle of inclination of the beam from the vertical is in the range of 15 to 45° preferably 30° to vertical and for conduction welding the beam inclination from the vertical is in the range of 30 to 60° preferably 45° to vertical. 
     
     
         19 . A method as claimed in anyone of  claims 7  to  18  comprising carrying out the keyhole welding joints comprising welding by a laser beam striking at an inclination/normal to the surface being welded by a special welding nozzle with three concentric openings wherein gas is purged from the innermost and outermost openings while evaporated materials from laser-material interaction is sucked out by the middle opening adapted as an evacuation chamber. 
     
     
         20 . A method as claimed in anyone of  claims 7  to  19  comprising the step of smoothening of surface of joints involving low intensity defocused pulse laser beam adapted to penetrate only up to the modulations on the surface. 
     
     
         21 . A method as claimed in anyone of  claims 7  to  20  comprising cleaning the weld region and surroundings involving short pulses of nano-second duration preferably laser pulses of low intensity preferably in the region of 1-100 nano seconds and energy density in the range of about 1-5 J/cm 2 . The important aspect in this case being the sucking out of evaporating material due to laser material interaction. 
     
     
         22 . A method as claimed in anyone of  claims 7  to  21  wherein the wall thickness of the material being welded is more than 1 mm and the welding is done by means of laser welding from inside surface or outside surface of the wall of cavity having depth of welding ranging from greater than half the thickness to full depth of the material being welded. 
     
     
         23 . A keyhole welding nozzle for use in laser welding of Niobium based superconducting radio frequency (SCRF) cavities and the like comprising:
 three concentric tubular members defining a nozzle head at the front and a hollow cylindrical shaft in the center accommodating a lens assembly for welding and viewing of the weld region;   three concentric tube like enclosure assembly having a central region enclosure accommodating a lens assembly and necessary optics to focus the laser beam for welding and for viewing the welding zone along with a provision for high velocity gas for purging;   the second enclosure after the central region is adapted to be wide enough for sucking out the evaporated material due to laser material interaction and the outermost third enclosure is adapted to purge high velocity gas to minimize spread of evaporated material;   the outermost and innermost tube enclosure of nozzle head provided for high velocity inert gas to flow out whereas the middle enclosure is adapted for sucking out the evaporated material due to laser material interaction.   
     
     
         24 . A conduction welding nozzle system for use in laser welding of Niobium based superconducting radio frequency (SCRF) cavities and the like comprising:
 nozzles meant for delivery of two laser beams with lens assembly adapted such that the beams are inclined preferably about 45° With respect to the vertical and having means for sucking out the weld plume rising perpendicular to the surface through an enclosure adapted to be moved up and down telescopically.   
     
     
         25 . A welding rig which is the type I welding rig such as for carrying out welding of elliptical type SCRF cavities and the like comprising:
 a vacuum vessel with motor driven attachments for holding and manipulating the half cells, aligned to form dumbbells of SCRF cavity for welding of cavity joints;   nozzle alignment mechanism;   keyhole nozzle means and conduction welding nozzle system selectively disposed and adapted to sequentially carry out the keyhole and conduction welding from inside surface of the wall of the dumbbell within said cylindrical vacuum chamber.   
     
     
         26 . A welding rig as claimed in  claim 25  comprising laser welding nozzle selectively disposed for carrying out said laser welding of cavity joints from inside surface of the wall of cavity for joints in RF field region and from outside surface for the for joints in the RF field free regions. 
     
     
         27 . A welding rig as claimed in anyone of  claims 25  to  26  wherein said welding nozzles are adapted to carry out inclined welding along the line of the weld joint but in the plane of periphery to be welded in the first phase by keyhole welding and followed by second phase conduction welding from inside surface of the wall of the cavity. 
     
     
         28 . A welding rig as claimed in anyone of  claims 25  to  27  wherein said welding nozzles are adapted to carry out keyhole welding from outside for joints in the RF field region like stiffening rings and some end group components. 
     
     
         29 . A welding rig which is the type H welding rig such as for carrying out welding of elliptical type SCRF cavities and the like comprising:
 a vacuum vessel with different devices/attachments, to carry out laser welding of joints of Niobium components of SCRF cavity, specifically for the equator joints of SCRF cavity adapted to house a rig insert adapted to assemble together welded half cells forming the dumbbells;   encoders and means for precise positioning of the nozzles from outside;   means preferably boroscopes provided to inspect the surrounding regions after welding is over.   said rig insert comprising two circular flanges held together by three tie rods spaced at 120° adapted to assemble together welded half cells forming the dumbbells and facilitate laser welding such as of equator joints of Niobium components of SCRF cavity;   strain gauges assembled in this insert adapted to monitor the distortions online during welding.   
     
     
         30 . A welding rig as claimed in  claim 29  wherein
 said different devices/attachments comprises of a vacuum vessel, a tie rod mechanism to hold dumbbells system, provision for evacuation of vessel and purging with inert gas and seals for the different protrusions in the vacuum environment; 
 said means for precise positioning of the nozzles from outside comprises of a hollow cylindrical shaft capable of rotating about it's own axis carrying laser heads and also enclosures which move up and down to collect evaporated material from laser material interaction, said rig also including a mechanism involving two motors, ball screws and encoders which remain outside vacuum environment and provide precision rotary and axial movement to nozzles which are read by the encoders, operatively connected pipelines in the rig adapted to take out evaporated material from the welding region to outside the vacuum vessel. 
 
     
     
         31 . A rig system for carrying out welding of elliptical type superconducting cavity both in the RF field region and RE field free regions comprising:
 a set of at least two welding rigs comprising (i) a welding rig assembly as claimed in anyone of  claims 25  to  28  adapted for welding of half cells to form dumbbells and end group components; (ii) a welding rig assembly adapted for carrying out welding of equator joints from inside surface of the wall of the SCRF cavity as claimed in anyone of  claims 29  and  30 .   
     
     
         32 . A system for carrying out welding of elliptical type superconducting cavity components as claimed in  claim 31  wherein each of the said rigs comprise weld accessories including a nozzle assembly with optical fiber connection adapted to facilitate delivery of controlled laser pulses of high and low energy for keyhole welding, low energy defocused laser pulses for weld smoothening and cleaning by nano second duration laser pulses, means for viewing of butting edges and welding of joints intermittently, means to inspect defective cavity and rectify regions having problems, means for weld smoothening and cleaning by removal of evaporated plasma materials from Laser-material interaction and means for evacuating the vessel and purging it by helium or any other suitable inert gas. 
     
     
         33 . A system as claimed in anyone of  claims 31  to  32  wherein in said welding rig assembly the optical fiber is provided to carry four types of pulses comprising (a) high energy pulses having variation in time domain along with high pulse overlap in space domain preferably adapted for carrying out keyhole welding in the RF field and field free region (b) low energy pulses for conduction welding (c) pulses of lower energy adapted to function as defocused beam and (d) applying low energy pulses of nano second duration to those areas of the cavity where some surface defects are seen for laser cleaning. 
     
     
         34 . Niobium based superconducting radio frequency (SCRF) cavities, system for carrying out laser welding of Niobium based superconducting radio frequency (SCRF) cavities and manner of manufacture/welding of Niobium based superconducting radio frequency (SCRF) cavities involving said system substantially as herein described and illustrated with reference to the accompanying figures.