New retractable effusion cell

New retractable effusion cell to be used to protect the cell charge from oxidation when the growth chamber is vented.

The retractable effusion cell allows re-filling an effusion cell without venting the growth chamber. The retractable cell can also be used to protect the effusion cell charge from oxidation when the growth chamber is vented. The cell is separately pumped and comes with a bakeout heater jacket.