The Big Quench Rancho Feb 2009

[bernadettelkay]

This magnet was being taken out of service, it was 15 years old and we have replaced the building and the magnet with state of the art new equipment and setting.

[JustinRLynn]

Oh man, so much helium gone and it's only getting more expensive... *sad face*

[ssyreeni]

Is it really so that the quench button just vents the coolant, letting the magnet internally dissipate the energy stored in the field/recirculating current? That sounds sort of...risky. I mean, if you start with the current still circulating, instead of dumping it into a heatsink in an orderly fashion, how do you guarantee that there won't be resistive hotspots, shockwaves in the surrounding helium, and so on?

[ChumpusRex]

@ssyreeni Usually, the quench button activates a small resistive heater coil in the magnet, causing a region of the coil to become resistive.

Most medical MRI magnets don't contain enough energy due to the relatively low field strength (1-3 T) to require active protection circuits on the coils. Very high strength scientific magnets typically install diodes between the coil windings, and heat-spreaders to ensure that the whole coil heats and quenches evenly.

[mhilla02]

ZOMG!!!!

[dragonxxx53]

Jesus, why is this happening?

[infernoacid]

@dragonxxx53

Jesus predates MRI scanners. You should ask a doctor or a scientist.

[Renatodonadio]

Imagine after that the engineer getting out and saying "Annunzio vobis gaudium magnum habemus papam!" :-D

[bernadettelkay]

The system is built with a "quench button".The primary use for the button is to render the unit non-magnetic in case of an emergency (object projectile into the magnet, etc), but is also utilized to "quech" when the magnet (MRI) is being taken out of service.

[infernoacid]

@bernadettelkay

I hope that this button is easy to reach for us button-philes?

[daufas]

@bernadettelkay I work for Siemens on MR's The quench button is not the preferred method for decommissioning these things , normally we would prefer to ramp the magnet down to avoid all this helium loss as that stuff is very expensive. Must admit it makes a great show though.

[carl95125]

What did you do to cause the magnet to quench...turn up the current or do something to the vacuum insulation? Could you have drained the magnet of some of the LHe first...before the quench?

Carl

[bernadettelkay]

the "quench" button activates a release valve that allows the liquid helium to escape.

[mickd01]

The quench button brings current to a heater element in the cryostat which makes the liquid helium turn to gas, magnet pressure goes up. High pressure causes burst disk in the vent to rupture allowing the helium gas to evacuate. Quench button normally is used only in an emergency because it wastes liquid helium and magnet can potentially be damaged. Usually the magnetic field is removed by connecting the power supply to the magnet, matching current of the magnet, then dialing down the current.

[daufas]

@mickd01 Mick, that is exactly right, the other thing not mentioned is that depending upon the power of the quench the coil can be damaged (magnet coil). It is much more preferred to ramp it down with the power supply. I am wondering what type of magnet this was. WE don't normally use that type of quench cap anymore , we use one that is better equipped to keep water out of the quench pipe.

[dsmous]

Being an electrical engineer, and frequent MRI patient, I have a technical question...

Is it truly non-magnetic after a quench?

I understand there isn't an electrically induced field after a quench, however, would there be any residual field??

I mean, if you drag a nail across a bar magnet, there is a slight magnetic field in the nail...

...or is an MRI unit constructed entirely of nonferrous metals and other materials...

[SpinUpDownUnder]

The cryostat and surrounding material has negligible magnetic susceptibility so it will not retain magnetic properties after the electromagnet is turned off.

Good question though, no material is perfect I guess

[Aquarello7]

A superconducting magnet stays magnetic even when the power is off. Trying to move an operational magnet that can pick up a bus is just asking for trouble. Removing the liquid helium (shown here) renders the coils non-superconductive, and, with the power shut off, therefore non-magnetic.

[bernadettelkay]

even with the power on, after releasing the helium, it is non-magnetic

[bernadettelkay]

We were demolishing the building and then will rebuild and install a new magnet.

[MooglyGuy]

So why, specifically, did you guys have to quench the MRI machine?