It is well known in the related field that the superconducting radio frequency (SCRF) cavities are used to accelerate particle beams in high-energy particle accelerators. These are in operation for last 30-35 years. SCRF cavities have the advantage of operating at high gradients, negligible AC power demand and favorable beam dynamics. The present day technology for manufacturing of these SCRF cavities is highly complex and expensive. Most of the SCRF cavities are presently made from bulk niobium material involving a fabrication process, which is exorbitantly costly and complex.
It is also known in the art that during the manufacturing of such cavities, niobium components are joined by electron beam welding (EBW) process. However, the existing electron beam welding (EBW) process is applied to weld both types of joints of SCRF cavities: those which are exposed to RF field and those that fall outside RF field region; these have been termed as joints in RF field region and joints in R.F. field free region respectively in the embodiments. Conventional EBW process used for fabrication of the SCRF cavities suffers from following drawbacks:                a) It is an expensive process as it has to be performed in vacuum and the Capital cost of the EBW machine is very high.        b) Chemical etching required before every electron beam welding operation is an expensive and hazardous operation, which, needs to be minimized, if not eliminated.        c) In order to weld intricate joints, complicated manipulation of electron beam or parts to be welded is required. Hence intricate joints are difficult to make with this process.        
Joints made in SCRF cavities with EB welding, cause significant amount of mechanical distortion and shrinkage.
It has also been experienced in the art that to weld Nb components for SCRF cavity having high thickness (1 mm or above), the energy deposited per unit time is high in case of EBW and as a consequence the possibility of distortion or shrinkage is high. Moreover, weld joints located in the RF field region of the SCRF cavity require extremely smooth surface finish, since presence of any sharp points on weld bead can cause field emission. This is difficult to achieve with existing high energy EBW weld process consistently, which causes detrimental effects on performance of SCRF cavities so fabricated. The existing EBW weld process does not also provide on-line, means for inspection of weld fit up, progress and finished weld bead. The electron beam welding process as being applied today makes it necessary that weld operations are carried out in many settings and hence vacuum has to be broken many times which means more time and cost. Also the existing weld systems and methods applied in the SCRF cavity fabrication do not suggest any method for removal of evaporating plasma material due to electron beam material interaction from weld surface and a method for smoothening for the intricate weld joints in precise weld set-up for the SCRF cavity. The existing EBW welding process applied for fabrication of the Niobium components of the SCRF cavity is thus expensive, low in productivity and consistency in weld quality for Nb components.
U.S. Pat. No. 5,239,157 discloses a superconducting accelerating tube which is constructed in a manner such that a plurality of components, formed of a superconducting material and individually having peripheral end portions adapted to be butted to one another, are butted to one another at the peripheral end portions, and the butting portions butted to one another are welded together. In the superconducting accelerating tube according to the said '157, the butting portions are welded by means of a laser beam, and the laser beam is applied to the butting portions such that the components are laser-welded to one another. Preferably, the accelerating tube is designed so that at least only the respective inner surfaces of the butting portions are laser-welded, and the depth of welding is not greater than half the thickness of the superconducting material and not smaller than 150 μm.
Importantly the said U.S. Pat. No. '157 suggested that it was preferred that the weld thickness should not be more than half the thickness of the sheet. Particularly, it was stated in the said prior art that if the depth of welding was greater than half the thickness of the superconducting material, the dimensional accuracy of the manufactured superconducting accelerating tube was lowered by a contraction of the weld portion. It was further mentioned under the said prior art that if the thickness of the superconducting material was smaller than 0.1 mm, the strength of the resulting superconducting accelerating tube was too low and the wall of the tube would be too thin for satisfactory laser welding and if the thickness of the superconducting material was greater than 1 mm on the other hand, the thermal conductivity was low and inevitably therefore the cooling efficiency obtained during the use of the superconducting accelerating tube was low.
Thus it would be clearly apparent from the above teachings flowing from the US prior art that under said art the thickness of material “t” for weld was restricted in the region of 0.1≦t≦1 mm and moreover the weld had to have a limitation of not exceeding beyond half the said thickness. However, usually the cavity wall thickness for the purposes of SCRF cavities have over the years been in the range of about 2 mm and more and in the present day elliptical type SCRF cavities are generally made of Nb with thickness of typically 3 mm. Hence the said prior art on laser welding could not be applied in the industry which is manufacturing superconducting cavities.
It would also be clearly apparent from the above state-of-the-art that while the present day requirement of Nb based SCRF cavity fabrication requires a wall thickness of 2 mm and above, the laser welding of less than half the depth and up to 1 mm wall thickness proposed in the U.S. Pat. No. 5,239,157 was not sufficient to provide for weld joints with desired strength and other characteristics for Nb based SCRF cavities. Moreover, the laser weld of SCRF cavities proposed in the U.S. Pat. No. '157 could never have provided for proper weld joints involving the standard thickness of Nb of 2 mm or more used for fabricating SCRF cavities. In particular attempting to achieve greater depth of welding beyond the less than half depth of even less than 1 mm thickness attempted and proposed in the US involving higher power lasers would have invariably resulted in serious failure of the weld joints in terms of high HAZ, large weld distortion and shrinkage.
There has, therefore, been a continuing requirement in the related field to advance the technology of manufacturing/fabricating SCRF accelerator cavities involving laser welding in order to achieve higher reliability and productivity and simultaneously bring down the manufacturing time and costs, to make the process and equipments affordable as well as ensuring reliable weld performance for fabrication of SCRF cavities. Involving Niobium and its alloy based components with possibility of achieving welding from inside surface of the wall of cavity to more than half the thickness to even full depth of thickness which was not possible here to before. Moreover, for such weld penetration using laser beam it has been desirable to meet some of the critical requirements like good surface finish, strength and freedom from risks of field emission so that consistent performance from the RF joints can be obtained while in service for charged particle acceleration in these cavities. The other requirement in the art over the years had been of meeting high penetration with minimized HAZ to ensure reliable weld quality with high surface finish free of presence of any sharp points in weld surface to avoid field emission, which is not attainable by the existing EBW process consistently. Above all, the intricacies and weld finish coupled with strength, smoothness of weld bead that is required in particular for the RF field region of the SCRF cavity, needed to be met by proper selection of process and equipments, designing of the weld process and optimization of parameters thereof to achieve economy, attaining user-friendly provisions for such laser welding of SCRF cavity joints have been the challenging constraints to adopting laser weld technique for such fabrication of Nb based SCRF cavities.