VACUUM SECTION


1. Introduction:
Vacuum team pursues various upgrade programs of the next-generation vacuum system for K-130 and Superconducting Cyclotron, beam transport line, SCC injection line to improve the accelerator performance and operation efficiency. Our main aim is to have operation of vacuum system for achieving a good quality vacuum in all vacuum spaces used for acceleration and beam transport. The high vacuum is a basic requirement for acceleration of charged particles in an accelerator to avoid high voltage R.F. breakdown as well as to prevent particles losses due to molecular collisions with residual gas. High vacuum production is major challenge to achieve especially in large acceleration chamber of Variable Energy Cyclotron (VEC) as well as in complex system of Superconducting Cyclotron (SCC) of this centre. Theoretical consideration and practical experience show that a pressure in range of 5 x 10-6 Torr – 1 X 10-5 Torr for VEC and ~1 X 10-7 Torr for SCC provide reliable operation and prevent RF breakdown. Vacuum group is involved in activities of the following vacuum systems.

2. K-130 cyclotron vacuum systems:
2.1. Cyclotron vacuum systemThe vacuum systems for K-130 cyclotron consist of RF chamber, acceleration chamber and beam transport line. The cyclotron vacuum chamber comprises a stainless steel acceleration chamber i.e. dee tank and RF chamber i.e. resonator tank made out of mild steel and copper cladded steel. The combined volume of resonator and dee tank is nearly 23 m3.

The resonator tank has two 889 mm openings at the bottom to which a pair of assemblies comprise of 889 mm oil fractionating diffusion pumps having speed of 42000 l/s, refrigerated chevron baffle and electro-pneumatically operated high vacuum gate valve is attached. For the evacuation of the entire system from the atmospheric to few microns before main diffusion pumps are valved, a powerful roughing system comprising roots and rotary mechanical pumps has been installed. Apart from above combination of pumping system, a 30 cm oil diffusion pump of 2000 l/s pumping speed with integrated gate valve and baffle has been also installed on the Dee Tank close to the accelerating region to provide extra pumping. The average pressure 2-3 x 10-6 mbar without beam and 4-5 x 10-6 mbar with beam is achieved.

2.2. Beam line vacuum system:The beam transport line has three channels for experiments. Its combined length is around 58 meter. The beam line connects dee chamber at extraction port in the level of beam line experiments. The beam transport line is made of Aluminum tube and stainless steel dipoles and diagnostic chambers. The entire beam line is pumped down with 19 pumping stations. Each pumping station is combination of oil diffusion pump, rotary pump, gate valve and gauges. The average pressure ~ 5 x 10-6 mbar is achieved regularly inline.

3. SCC vacuum system:Superconducting Cyclotron needs very high vacuum in major vacuum spaces like cryostat outer vacuum chamber (OVC), acceleration chamber i.e. beam chamber, liner chamber, beam line and injection line. Considering the varying requirement of vacuum level in spaces according to necessities and their operational constraints, these spaces are evacuated with independent vacuum system.

3.1. Cryostat Outer Vacuum chamber or the Cryostat (OVC) Vacuum SystemCryostat has an annular space between the inner liquid helium container (the stainless steel bobbin) and the outermost container (known as coil tank) also called cryostat outer vacuum chamber (OVC). The pressure in the vacuum space is less than 10-5 mbar during operating state.

Two pumping system, a Turbo molecular – scroll pump combination, are used alternatively / simultaneously to maintain vacuum level less than < 1.0 x 10-5 mabr. Considering the influence of magnetic field on vacuum gauges and turbo pump, pumping units are placed about 3600 mm below the median plane.

3.2. Beam chamber vacuum system:Beam chamber i.e. the space between upper liner, lower liner and coil tank, is also known as acceleration chamber where charge particles are accelerated in cyclic orbits. This chamber is evacuated with three turbo molecular pumps and each backed by scroll pump. The net pumping speed for all three pumps together is about 150 liter/sec. For evacuation of beam chamber from atmosphere to ~ 5.0 x 10-2 mbar, there is one main roughing port which is connected to the backing pump used for the turbo pump through an electro-pneumatic isolation valve. Subsequent pumping is carried out with three turbo molecular pumps to achieve pressure in the acceleration chamber below 10-6 mbar. At last, three cryopanels are operated to achieve pressure in range of 10-8 mbar. Each cryopanel is made of OFE C10100 copper and the liquid helium cooled panel is surrounded by liquid nitrogen cooled chevron baffles. The three Cryopanels together provide a pumping speed of 6000 liter / sec. for air.

3.3. Liner vacuum systemThe space between the upper liner and the upper magnet iron elements within the cryostat inside diameter forms the upper liner vacuum space. Similarly, the space between the lower liner and the lower magnet iron elements within the cryostat inside diameter forms the lower liner vacuum space. Epoxy impregnated trim coil assemblies on the magnet pole tips lie within the liner 'vacuum' spaces. A low quality vacuum is adequate in the liner vacuum space.

A set of two oil sealed rotary pumps with interlocked valves, one of them as stand-by, assures the availability of pumping. There are three vacuum ports available for each of the two vacuum spaces. Two ports each of the two spaces are connected to a common header.

3.3. Beam transport line vacuum systemBeam line connects the cyclotron through extraction port to transport energetic particle beams to the designated destinations in the caves for experiments. There are a variety of intricate elements for beam diagnostics, beam viewing, bending and focusing along the lines having path length of about 20 meter.

Distributed pumping has been provided between any two major elements / on a diagnostic element along the beam line. Also, at least one pumping station is used between two isolation valves. Initial evacuation from atmosphere to ~ 5.0 x 10-2 mbar is done using backing pump. Subsequent pumping is done directly through turbo pump port to attain pressure around ~ 1.0 x 10-7 mbar. All the demountable ports are CF flanges with OFE copper gaskets.

3.4 SCC injection line vacuum systemInjection line is used to transport low energy beam from ECR ion source to center region of superconducting cyclotron. This is around 18 meter long and comprises of stainless steel tubes, diagnostic and dipole chambers. Injection line is evacuated by three turbo molecular pumps backed by scroll pumps and three cryopumps installed along the injection line. The average pressure ~ 3.0 x 10-7 mbar is maintained along the line to transport low energy beam.


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SRIMANTA
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S.S.
Nandy
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Karmakar
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