In India, scientific research in tokamak plasmas has been continuing for more than a decade now. In tokamaks, the plasma is formed by an electrical breakdown in an ultra high vacuum toroidal vessel and a current is inductively driven in the plasma. As the plasma temperature rises the efficiency to heat the plasma drops. To further raise the temperature of the plasma to fusion grade, one has to use auxiliary heating schemes. During experimentation at high temperatures, it is also required to diagnose the plasma with various sophisticated diagnostic tools. Inherent drawback for future uses is the pulsed nature of tokamaks. One of the areas of research, therefore, has been steady state operation of tokamaks.

A steady state superconducting tokamak, SST-1, is in advanced stage of fabrication at the Institute for Plasma Research, Gandhinagar. The objectives of SST-1 include :
lTo study physics of plasma Processes in tokamak under steady-state conditions & contribute to the tokamak physics data base for very long pulse operations.
lLearning new technologies relevant to steady state tokamak operation:
lSuperconducting magnets and associated power supplies and protection system
lLarge scale cryogenic system (Liquid helium and liquid nitrogen)
lHigh Power Radio Frequency Systems
lEnergetic Neutral Particle Beams
lHigh heat flux handling

The machine has a major radius of 1.1 m, minor radius of 0.20 m, a toroidal field of 3.0 Tesla at plasma centre and a plasma current of 220 kA.
 

Controlled thermonuclear fusion is one of the attractive futuristic sources of energy. All over the world, the research in this field of energy has been continuing for the last fifty years.  Research efforts in this area are broadly divided into inertial confinement and magnetically confined plasmas. Among the magnetically confined systems,Tokamaks have been the most successful machines to achieve the technological goals .  In India, the scientific research in tokamak plasmas has been continuing for more than a decade now.  The tokamak Aditya developed at the Institute for Plasma Research, Gandhinagar, Gujarat, is one of the milestones of this endeavour. A steady state superconducting tokamak, SST-1 is in advnced stage of fabrication at the Institute. Present here is the status of this venture.

Superconducting coils for both toroidal field and poloidal field are to be deployed in the SST-1 tokamak. NbTi superconductor at 4.5K is used for the superconducting magnets and maximum field at the conductor is restricted to 5.1 Tesla. An ultra high vacuum compatible vacuum vessel, placed in the bore of the toroidal field coils, houses the plasma facing components. A high vacuum cryostat encloses all the superconducting coils and the vacuum vessel. Liquid nitrogen cooled thermal shield between the vacuum vessel and superconducting coils as well as between cryostat and the superconducting coils reduce the radiation heat load on the superconducting coils .
 

The sketch showing relative positions  of various components.

Normal conductor ohmic transformer system is provided to initiate the plasma and sustain the current for initial period. A pair of vertical field coils is provided for circular plasma equilibrium at the startup stage of the plasma. A set of saddle coils placed inside the vacuum vessel provide fast vertical control of the plasma while poloidal field coils are to be used for shape control. Other subsystems include radiofrequency systems for pre-ionization, current drive and heating, neutral beam injection system for supplementary heating, cryogenic systems at liquid helium and liquid nitrogen temperatures, chilled water system for heat removal from various subsystems. A large number of diagnostics for plasma and machine monitoring will be deployed along with a distributed data acquisition and control system.

The above three dimensional sketch shows the relative positions of various components.

All superconducting coils have been successfully fabricated using a cable-in-conduit conductor (CICC) based on niobium-titanium (NbTi) and copper. The CICC has been fabricated by a Japanese firm under specification and supervision of IPR. In order to test the performance of this CICC under SST-1 operating scenarios, a Model Coil was designed, fabricated and tested at Kurchatov Institute(KI), Russia using the SST-1 CICC. The results obtained from these model coil tests have validated the CICC design parameters as well as its appropriateness as the base conductor for the SST-1 superconducting magnet systems.
The toroidal field coils are encased in a stainless steel casings to take care of forces acting on the coils. The coils and the casings have been manufactured by the Bharat Heavy Electicals Ltd., Bhopal with specifications and supervision from IPR. Such large size superconducting coils have been manufactured for the first time in the country. An insulation system, compatible with low temperature (4.5K) operation of these coils, and the winding technologies have been indigenously developed for these superconducting coils.

The superconducting magnet system, consisting of toroidal field and poloidal field coils, in SST-1 has to be maintained at 4.5 K in presence of steady state heat loads. In addition, the pulsed heat loads during the plasma operation have to be taken care of by the cooling system so as to maintain the magnets in superconducting state.

The magnets will be cooled using forced flow of supercritical helium through the void space in the CICC. Further the magnets have to be energized from power supplies at room temperature. A closed cycle 1 kW class He refrigerator/liquefier, has been deployed for this purpose The system is at present under commissioning tests at IPR. He gas management system, including high pressure and medium pressure storage vessels and recovery system, required for the He refrigerator/liquefier, has been commissioned. This is the biggest liquid helium system in the country at present.

In order to minimize the heat loads on magnets and support system at 4.5 K, liquid nitrogen shields are provided between the cold mass at 4.5 K and warmer surfaces. A liquid nitrogen management system, including liquid nitrogen storage and distribution system, has been commissioned for this purpose. An integrated flow distribution system for distribution of cryogens to magnets and radiation shield has been installed and is in final stages of testing.

SST-1 has two vacuum chambers, (i) Vacuum vessel for plasma production and confinement, and (ii) Cryostat for enclosing all superconducting magnets. Vacuum vessel is a toroidally continuous single wall metallic structure made of SS 304L material. The poloidal cross-section of the vessel is close to ‘D’ shape. Vacuum vessel is designed and fabricated for ultra high vacuum operation. Cryostat is toroidally continuous sixteen sided polygonal vacuum chamber which encloses vacuum vessel and all superconducting magnets. Cryostat is designed and fabricated for high vacuum operation.
 

Full scale prototype of SST-1 cryostat and Vacuum vessel

Commissioning and operational requirements of vacuum vessel and cryostat demand for high dimensional accuracy, special in-situ welding procedures, very high surface finish etc. It was essential to establish all fabrication techniques, manufacturing of appropriate tools and fixtures, detail inspection / testing stages and procedures etc.; before commencing the fabrication of main vacuum vessel and cryostat. For this 45º toroidally continuous full scale Prototype Vacuum vessel and Cryostat has been successfully fabricated and tested for its functional parameters. All the components of SST-1 vacuum vessel and cryostat are at the final stage of completion at M/s BHEL,Tiruchirappali. The SST-1 Vacuum vessel is the largets ultra high vacuum vessel being fabricated in the country.

During normal pumping and baking / wall conditioning, vacuum vessel will be pumped with 10,000 l/s net pumping speed using two turbromolecular pumps and 10,000 l/s net pumping speed for water vapor and condensable vapors using two cryopumps. During plasma discharge vacuum vessel will be pumped with 62,000 l/s net hydrogen pumping speed using 16 nos. of turbomolecular pumps (8 nos. each for top and bottom divertors).

Cryostat will be pumped with 10,000 l/s net pumping speed using two turbomolecular pumps. However, all surfaces at cryogenic temperature (less than 80 K) will provide large pumping speed for all gases except hydrogen and helium.
The Plasma Facing Components of SST-1, comprising divertors & baffles, poloidal limiters and passive stabilizers, are designed to ensure steady state heat removal capability. Particle removal in steady state is also a major concern. Plasma facing components are made of graphite tiles backed by copper alloy plates with cooling channels. One of the important aspects of the fabrication of the plasma facing components is identifying a suitable process to braze the SS tube in the grooves of the copper alloy heat sinks and regaining the mechanical strength after the brazing. This has been done for Cu-Cr-Zr and Cu-Zr alloys. The graphite material has been tested for heat removal capability in a prototype experiment by irrdiating it by CO2 laser at CAT, Indore. Suitability of mechanical joining of the graphite tiles to the heat sink also has been tested using the same high power laser. Fabrication of the plasma facing components is underway.

High Power Radio Frequency Systems
SST-1 will have three different high power radio frequency systems to additionally heat and non-inductively drive plasma current to sustain the plasma in steady state for a duration of up to 1000 sec. Ion Cyclotron Resonance Frequency system would operate in a range between 22 to 91MHz to accommodate various heating schemes at 1.5 Tesla and at 3.0 Tesla operation of SST-1. The same system would also be used for initial breakdown and wall conditioning experiments. Fast wave current drive in the centre of the plasma is also planned at a later stage. A multi-stage 1.5 MW continuous wave radio frequency system is being built to meet these goals. All the system components require active cooling. Lower hybrid current drive system is being prepared at 3.7 GHz. The system is based on two 500 kW, continuous wave Klystrons with four outputs. Power at these arms are further divided successively to sixty four channels which then finally delivers the power to a grill type window positioned at the equatorial plane on a radial port at the low field side of SST-1. Electron Cyclotron Resonance Heating system is based on a 200 kW, continuous wave gyrotron at 82.6 GHz. Beam launching systems have been designed, fabricated and tested for microwave compatibility for radial and top launch. The system would be used for initial break down and heating of the plasma. Localized current drive would also form a part of experimentation.

Crucial transmission line components of all the three systems have been tested for high power long duration operation on respective dummy loads. Notable are the high power components that have been developed for continuous wave operation. Some of these are water cooled transmission line components for MHz range operation, direction couplers, water dummy loads, transformers at 3.7 GHz and other passive high power continuous wave components. The systems are being erected and some of the subsystems have been successfully commissioned. Radio frequency systems will be integrated to the machine after the machine shell has been tested for ultra high vacuum compatibility.

Auxiliary Heating System
A Neutral Beam Injection with peak power of 0.8 MW with variable beam energy in range of 10-80 keV will be used as additional auxiliary heating system. The engineering designs have been completed and a number of proto-types for various critical components are under development to establish the fabrication methodology. Quantified results have been obtained in many of the prototyping activities. Notable among them is the successful performance demonstration of the country’s first indigenously designed, engineered and fabricated cryocondensation pump for a pumping efficiency of 105 l/s for deuterium at 4.2 K liquid helium temperature yielding a specific pumping speed of ~ 7 l/s/cm2. Results from other prototypes have been equally encouraging. These include successful testing of electroforming of OFHC copper on a similar base; development, manufacture and tests of 80kV compact post insulator; dissimilar material jointing between the heat Cu-Cr-Zr and SS 316 l by explosive bonding and vacuum brazing for the fabrication of heat transfer elements.

Similar achievements have been registered in the larger systems that include the design, fabrication and installation of the country’s largest rectangular (~ 20 m3) vacuum vessel; design, development and testing of 26 units of 160 V / 100 A discharge power supplies with fast turn On and turn Off AC-DC converters; development of VXI based data acquisition system; development of 16 channel multiplexer cards for the 700 channels of data acquisition; fabrication and testing of a computer controlled movement system for the neutral beam power dump.

Plasma Diagnostics
A large number of plasma diagnostics will be deployed on SST-1. These are at various stages of design, fabrication and testing.

Main Machine Support
The main machine support comprises 16 columns, supporting the base frame of the cryostat and the cold mass, which are firmly grouted to ground. The cryostat support frames interface with the central columns which additionally provide support for central solenoid of Ohmic transformer. The cold mass support is provided on eight columns with liquid nitrogen cooled intercept, kept in vacuum and supported on the main columns at the base. A ring with cantilever beams rests on the cold mass support columns. The toroidal field coils freely rest on these beams. The toroidal field casings are nosed in the inner leg and form a rigid vault. Outer inter coil structures between the toroidal field coils provide the rigidity against the turning moments on outer side. The poloidal field coils are supported on the toroidal field coil assembly.

Power Supplies
The power for the different subsystems of SST-1 will be derived from a 132kV line. The 132 kV to 11 kV sub-station has been upgraded to cater to the entire needs of SST-1. The DC power supplies and protection systems for magnets have been designed and are under procurement.

Component Assembly
SST-1 tokamak has a large number of components to be assembled at site to build various systems like machine support structure, plasma chamber, cryostat, magnet system, first wall (plasma facing components) and other auxiliary systems. In this device the required assembly tolerances are in the order of several tenth of a millimeter. The tight installation tolerances, definite assembly sequences and process restrictions govern the efficacy of the assembly procedures. SST-1 assembly demands definite sequence to be followed to ensure sequential testing of each system, accurate positioning of the components in the radial, toroidal, poloidal and vertical direction to meet the tolerances and magnetic axis determination and alignment of the plasma facing components.

To assure compliance with assembly requirement and to minimize the subsequent corrective operations, the assembly plan has been defined. Comprehensive survey of the Tokamak hall and fixation of reference target plates using electronic coordinate determination system (ECDS) has been completed. A machined template, defining the position of the foundation bolts precisely, has been fabricated and used for the preparation of foundation & grouting of the bolts. The support structure has been erected and further assembly of the machine is in progress.
 

Conclusion
In conclusion, most of the components of SST-1,namely, the cryostat and the vacuum vessel have been successfully prototyped and tested. Various other subsystems such as different magnetic field coils, plasma facing components have been, fabricated and are in the process of erection and commissioning at site. Different components of the auxiliary heating and current drive systems have also been fabricated and tested. The systems would be integrated to the machine after the machine shell commissioning is over.