The Helium used for cooling expands a lot going from liquid to gas during a quench (iirc by a factor of ~1800). The pressure built-up from the phase change has

The Helium used for cooling expands a lot going from liquid to gas during a quench (iirc by a factor of ~1800). The pressure built-up from the phase change has to be relieved *quickly* before the machine turns into a crude pipe bomb. Oh and you can't get enough Oxygen from the air if most of it has been replaced with leaked Helium.