Vacuum System

September 6, 2013 at 11:50 pm

The vacuum line in the Fusor is used to evacuate the vacuum chamber to a high vacuum.  This then allows the deuterium atoms to move around easier.  Because of this, fusion is much more common, as the deuterium atoms do not hit into any oxygen, nitrogen, argon or other elements that are common in a normal airspace.

The vacuum system for the Fusor will be composed of two different types of pumps along with tubing and up to six valves, I may also include two optional pieces – A cold trap and/or a foreline trap.

The pumps I will be using will be a rotary vane mechanical pump along with either a turbo-molecular or diffusion pump. The mechanical pump is used  to bring the vacuum in the chamber to a sufficient level for the diffusion or turbo-molecular pump to start working effectively. Turbo-molecular and diffusion pumps are known as high-vacuum pumps.

The Rotary vane pump will be connected to a cross junction, with a valve on each end. A ball valve will be on one end, this will be unconnected to anything  – it’s used to return the line to atmospheric pressure. Two more ball valves will be connected and a gate valve. This junction is followed by either a turbo-molecular pump or a diffusion pump, these are then either connected to a cold trap or straight to a tee junction which will be connected to the chamber and another ball valve.

Vacuum Equipment – Technical Specs

 

A technical diagram of my proposed vacuum system

Technical diagram of my proposed vacuum system

 

Alternative diagram of my proposed vacuum system

Alternative diagram of my proposed vacuum system

 

 

Mechanical pump (approximate cost €100-700): The mechanical pump is essentially the starter and backing vacuum pump in the Fusor.  The mechanical pump brings the pressure in the chamber low enough for a diffusion or turbo-molecular pump to work effectively.  The mechanical pump needs to be a two-stage mechanical rotary vane pump, they are also of the direct-drive nature or the belt-drive nature as is more commonly found in labs. A roughing pump need only have an ultimate vacuum high enough to allow a diffusion or turbo-molecular pump to begin work. A mechanical pump that can reach a vacuum of 5×10-3 torr (6.3×10-3 mbar) and pump at a speed of 0.113 CMM (cubic metres per minute – 4 CFM) is usable.  For example, an Edwards RV5 can reach an ultimate vacuum by itself of 1.5×10-3  torr (2×10-3mbar) and can pump at a speed of 0.116 CMM (4.1 CFM).

 

Ball valves (approximate cost €30-60): Ball valves are used in the vacuum line to immediately return the lines to atmospheric pressure. The ball valves are always closed until the fusion run is finished, and all pumps are off, they are then immediately opened. Any ball valves used should be made of grade 306 stainless steel or better.

 

Gate valve (approximate cost €30-100): Two gate valves are required in the vacuum line to isolate the mechanical pump’s inlet from the diffusion or turbo-molecular pump’s outlet and to isolate the diffusion or turbo-molecular pump’s inlet from the vacuum chamber.  Like the ball valves these should be made of at least grade 306 stainless steel.

 

Diffusion/Turbo-molecular pump (approximate cost €100-900): The high-vacuum pump in the Fusor is used in tandem with the mechanical pump.  It is always either a two-stage diffusion pump or turbo-molecular pump. The diffusion or turbo-molecular pump needs to be able to reach a vacuum of 1×10-5  torr (1.3×10-5  mbar) or better. If using a diffusion pump,  a two-stage, air cooled pump that has a body between 50 and 150 mm in diameter is required. A 150 mm pump can be used but after that there is no need for the extra power the pump and only consumes additional oil. A water-cooled pump can be used but a air-cooled pump is preferable. If a turbo-pump is used it still need only reach an ultimate vacuum of 1×10-5  torr, that is all that the turbo-pump need do as long as it is supplied with its controller, power supply and relevant cabling.

 

Tubing (approximate cost €10-90): Rubber or flexible metal tubing is also required, tubing that is 25 mm wide and vacuum tight is usable. Around four metres of tubing is needed.

 

Oil trap or micro-maze (optional extra approximate cost €25-150): Although this is an optional part of the vacuum line, it is a great help if used.  It is placed in between the diffusion or turbo-molecular pump and mechanical pump, it can make a mechanical pump’s ultimate vacuum a full order of magnitude higher, thus helping the main pump more when it starts up. An oil trap or micro-maze is used to absorb any fluids or oils that may escape from the mechanical or diffusion pump. Any oil trap or micro-maze can be used, there are also a lot of similar products that do much the same thing which can also be used, they’re commonly referred to as ‘fore-line traps.’

 

Gate valve (optional extra approximate cost €15-50): An additional gate valve is required if an oil trap, micro- maze or other fore-line trap is used.  Like the other valves, it must be made of grade 306 or higher stainless steel.

 

Cold trap (optional extra approximate cost €20-100): This is another optional piece of the vacuum line. It is placed just before the last gate valve leading into the vacuum chamber. A cold trap nearly always utilises liquid nitrogen, dry ice, or a small refrigeration system to freeze or condense any excess vapours coming through the vacuum line helping to maintain the vacuum.  Like with the oil trap or micro-maze, any type of cold trap can be used.

 

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