Engineering design and integration of in-vessel single turn segmental coil in vacuum vessel of SST-1

SST-1 tokamak is having the error field due to unsymmetrical positioning of Toroidal field coils which push the plasma to inner side from its major radius of 1100 mm. hence it is required to install the In-vessel Coil (PF6) at a location of 1350 mm radius and elevation of 350 mm above and below the mid plane of the toroidal field coils. The In-Vessel coil was decided to make in eight segments for futuristic use, to control the individual localized error field correction by supplying the different current. A single turn, eight segments, copper conductor with 18 mm diameter with GFRP insulation and in housed in SS304 L casing to carry 8000 A current for 10 s was designed and installed in vacuum vessel of SST-1. This paper will present the design drivers, material selection, advantages and constraints of the in-vessel coils, its conceptual and engineering design, CAD models, finite element analysis using ANSYS, its fabrication, quality assurance/control and assembly/integration aspects inside vacuum vessel of SST-1.


Introduction
During the refurbishment of the SST-1, the error field was mapped with the help of the hall probe and mirnov's probe magnetic field mapping methods. Which in turn revels the shifting of plasma null to one side due error in magnetic field generated due to unsymmetrical positioning of the Toroidal field coils. The unsymmetrical positioning was due to error in TF supporting ring design and partially due to uneven tightening of Toroidal field coils. These error field causes plasma to form inboard side of the vacuum vessel from its major radius of 1100 mm. the simulation of the exact positioning of the TF coil and correcting of this field lead to evolution of the design of the in-vessel coil. To counter the effect of the magnetic field error, it was required to install pair of In-vessel Coil (PF6) at a location of 1300 mm radius and elevation of 350 mm above and below the mid plane of the toroidal field coil (figure 1). The In-Vessel coil was decided to make in eight segments (each segment with 3000 mm and one coil of 6900 mm length) for futuristic use, to control the individual localized error field correction by supplying the different current. The design of the in-vessel coil has many critical aspects for considerations like terminals of two successive copper conductor along with stainless steel casing must be come out from 46 mm diameter nozzle, secondly casing should be welded to radial port such to vacuum integrity of vessel remains intact, thirdly copper conductor can able to withstand maximum current of 8000 A for 10 s with maximum temperature rise of 50° C, there must be close tolerance between copper conductor and SS casing to reduce any movement of copper conductor inside the casing for keeping geometrical stability within vessel. Hence The in-vessel coil was made from single solid ETP copper conductor encased inside the prefabricated SS 304 L piped casing in eight segment  figure 2. To maintain the circular shape of the coil copper rod inside the SS casing, very close tolerances are maintained e.g. copper conductor has outer diameter of 18mm and after FRP insulation and Kepton tap wrapping the outer diameter reaches to 19mm ( figure 3). This coil is made from solid copper rod which under forces while charging can vibrate and change the shape. Hence it is required to keep in circular shape by encasing in steel pipe casing with close tolerances.

Design of SS Casing
The Stainless Steel pipe is experiencing internal vacuum pressure of vessel and is open to atmosphere. Though entire casing house the copper conductor along with insulation in tight tolerances, the entire assembly is act as solid rod. Also all pipes has to go through hydraulic testing at 70Psi pressure, there isn't need to design check for bursting pressure. Even though it has been verified by the following

Design of In-vessel coil conductor
Design of copper conductor is mainly done with input rating of 8000 A current carrying capacity in 10 seconds and with maximum differential temperature rise to 50° C with following equations (2.2.1,2.2.2,2.2.3) and result is validated with ansys analysis software and is plotted below.
(a) Resistance of the single copper segment  The above analysis plot shows the rise of the temperature of about 79° C from the ambient temperature while charging to maximum current of 8000 A for 10 s. The cool down time for the invessel coil after maximum current charging condition is also calculated to find out time lapse required between two successive plasma shots as per equation (2.2.4). The below mentioned formula used to calculate the cooling time and also analysis plot at the end validated the calculation as the cool down time is 10° C for 10 min. of cooling by radiation.

Design and Analysis of support structure
The pair of In-vessel coil experiencing the vertical force from each other which is very much negligible, hence design is done from force generated by radial control coils which experience both vertical and radial forces. The support for holding the In-vessel coil and Radial control coil is taken from Outer passive stabilizer support at eight Inter Connecting Ring locations at top and bottom as shown in modelled figure 6 and figure 7.
These supports experiences the Radial force of 30000 N and Vertical force = 1300 N with total 8 nos. of support for holding 8 segments of in-vessel coil (Load data provided by simulation group). A simplified free body force diagram is shown in figure 8 below showing the vertical and radial force acting on single support and the stresses and deflection found which are around 49.14 MPa and 0.58

Fabrication and Installation
The In-vessel coil copper conductor is duly insulated with fiberglass tap and kepton inserted within close tolerances to SS casing. These pair of coil is installed on outboard passive stabilizer support mounted in Inner connecting ring at the radius of 1300mm and elevation of 350mm above and below the mid-plane of Toroidal field coil with ECDS system with vary close tolerances as shown in image. The Stainless steel casing are welded from outside with vessel radial port in order to make the coil vacuum proof. The copper conductor leads comes out form the vessel is brazed with successive terminal to form full circle. In future, these terminals can be separated for individual segment operation for localized field error correction (N mode correction). All the weld joint of the radial control coil is tested at the leak rate of 1x 10 -8 mbarl/s. The final megger test is done between In-vessel coil casing and vacuum vessel to find resistance in order of giga-ohms. The installed in-vessel coil inside SST-1 VV is shown in Figure 11 and 12.

Characterization of the coils
In vessel coil has been tested with giving 200 A, 500 mHz square signal current and checked the magnetic field effect at the plasma radius of 1100 mm with putting hall probe at plasma radius at different location which measured value of average 3.5 Gauss. This measured value comes to vary close to the theoretical value as well with standard 1 st & 2 nd order elliptical equations for determining magnetic fields of circular loop. A measured signal wave form is also shown in below image.