Powering up a 9.6 T magnet in a 400 MHz NMR - Page 1
Forum section: electronics / projects

author: TheAmmoniacal
author_rank: Unknown
timestamp: February 23, 2016, 03:41:27 pm
content: I'm just curious how one would magnetize the 9.6 T magnet used inside NMR instruments for spectroscopy, anyone have some information on this? Even better, how they do it in practice?Attached is a picture of the one I use in the lab, but I wasn't there when they powered it up some years ago.The magnet stores about 0.4 MJ of energy, and uses a very long superconductor at LHe temperatures. Figures I've found say the wire carries ~100 A, but I'm not sure.How would you power it up? AC or DC? Capacitor bank? Large mains rectifier? Would it take a long time or quickly? How is the coil then disconnected?Would be nice if anyone had some info!

author: Nauris
author_rank: Regular Contributor
timestamp: February 23, 2016, 04:17:26 pm
content: Most likely there is two charging wires connected to the superconductive coil and then short piece of the superconducting coil between the charging wires is heated so it stops being superconducting (so called superconductive switch). Then coil can be charged/discharged with any suitable dc power supply.

author: 6thimage
author_rank: Regular Contributor
timestamp: February 23, 2016, 04:26:55 pm
content: As it is a superconducting magnet, you use a low voltage, high current DC supply and you slowly ramp up the current. Only a low voltage is used as the only resistance to the current is the wires connecting to the magnet - superconductors have no resistance (and why is really quite interesting). Obviously, prior to injecting a current the magnet needs to be in its superconducting state - i.e. cooled to its operating temperature, otherwise you will damaging it.Commercial NMRs will already have the control circuitry (and power supplies) so they are more a case of installing them then pressing a button in the GUI.How the coil is disconnected depends on the design, but more than likely they will switch in an additional piece of superconductor, so the coil forms a complete loop, which, as there is no resistance, will mean the current continues flowing, sustaining the magnetic field.Unfortunately, I mainly know the theory behind them, so I'm not sure how long they take to cool down or how long it takes to increase the magnetic field to the coil's maximum.

author: trojanrabbit
author_rank: Newbie
timestamp: February 23, 2016, 04:30:55 pm
content: Yikes!I used to work with RF amps for 1.5T machines that ran at 63.6MHz, 10kW pulsed, back then a 4CX5000 specially rated for MR service was used, when I left the solid state version was nearly ready.So this runs at 407MHz, does the power scale up too?

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 23, 2016, 04:42:38 pm
content: Reminded me of this

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 23, 2016, 05:30:24 pm
content: I used S.C. magnets about 20..25 years ago, up to 7T.The system from Oxford Instruments consisted of a big, blue metal dewar, the S.C. coil, a big, really massive DC power supply/sink, as already described, i.e. low voltage of maybe +/-10V max., current on the order of 100A, i.e. 63A for 7T, and also providing a safe and controlled ramp-up / ramp-down of current, plus steering of the S.C. switch.Cooling down to 4.2K took several hours. First pre-cooling with liquid Nitrogen to 77K, then removing the lq. N2 and filling in the liq. He4 from another transport dewar vessel.The current always has to be ramped up and down very carefully, at first, to limit the induced voltage, 2nd and most important, that the magnetic flux lines inside the superconducting wire (type II S.C.) could settle properly and slowly. In my documents, I found a max. rate of 2.8T/min. for my former apparatus. I think, I always chose about 10..15 minutes for complete ramp-up/down, instead.(I just found the complete steering procedures, which I had to write on my own, at that time)Too high a change rate would create excessive dissipation and heat, causing complete breakdown of the super-conductivity, so that the whole stored magnetic energy would be dissipated all of a sudden into the Helium. The sudden generation of He gas could break the whole dewar..In case of an accident, some safety bleeding resistors in series with several diode pairs, directly at the magnetic coil, would absorb the energy, to avoid destruction of the expensive S.C. coil.So this special power supply is exactly tuned to the coils specification, to avoid such catastrophic failures. As far as I remember, the coil winding was optimized to make "fast" magnetic field sweeps possible, in contrast to have long operating time (There's always a certain decay of the S.C. current, due to flux jumps, etc.)This power supply also included steering of the S.C. switch.After complete ramp-up, the heater of the superconducting switch was switched off, letting the current inside the coil run indefinitely, as long as enough liq. He4 was present.The - now - external current of the power supply could be switched off rapidly.Ramping down would happen analogously.In your picture, there's displayed only the NMR steering system.I assume, that nowadays, with much more sophisticated cryogenic systems, the power supply / steering unit for the S.C. magnet would only be used during initialization, and then removed by the service technician.Frank

author: tonyh88
author_rank: Regular Contributor
timestamp: February 23, 2016, 05:56:59 pm
content: Hi,I work in a NMR facility right now and we have pretty much the same magnets as in your picture. The installation is always handled by the manufacturer usually but from what I remember from the last time we did this it's pretty much how Nauris explained it.Quote from: Nauris on February 23, 2016, 04:17:26 pmA charging rod is inserted inside the magnet. The power supply wich is connected to this charging rod can send a small current in a superconductive switch to open the loop on the coil and start the charging procedure. The switch is then closed once the magnetic field as reached the desired strenght and stability. There is also a bunch of control signal on this charging rod for different purposes.There is alot of rules to respect as charging rate goes since you need to energize the magnets with a lot juice as Dr.Frank explained. The higher you go in field strenght the more current you need. Usually the magnet will come with a charging rate chart and you need to follow exactly this rate if you don't want your magnet to quench and lose its superconductivity.The power supply also has a lot of safety options( big ass diodes and bleeding resistor if ever there is a big inductive kick or sudden discharge of the magnet), measurement of the voltage drop on the coil at all time during the charging ( your aiming for 0V here. If there is voltage drop it means you are losing superconductivity and need to slow down on the charging) and so on.Those power supply are surprinsingly not that big nowadays and are also used to energize the cryoshim coils inside the magnet but that is another story

author: knotlogic
author_rank: Regular Contributor
timestamp: March 01, 2016, 03:44:31 pm
content: Quote from: Dr. Frank on February 23, 2016, 05:30:24 pmInteresting to hear that! From what I know Oxford Instruments stopped making them blue quite a while ago and the "new" look is polished steel.We had a clinical MRI system powered up at work some time ago. I wasn't there when they did the actual ramp up, but after that I saw the kit they used to power up the magnet - a box made by OI that just about fit on a cart. Interestingly, I was told the magnets ship with liquid helium already inside. Not full of course.I was also told ages ago that when ramping up the magnet, theres a chance that the coils might shift due to the applied field. If that happens you get a bit of friction, which heats the wire, which can cause it to go resistive, which generates heat, which.... you get the idea. For that reason they do test ramping them up at the factory.

author: AndreasF
author_rank: Frequent Contributor
timestamp: March 01, 2016, 07:56:28 pm
content: Quote from: knotlogic on March 01, 2016, 03:44:31 pmLiquid nitrogen is used during transport.

author: knotlogic
author_rank: Regular Contributor
timestamp: March 02, 2016, 02:45:31 pm
content: Quote from: AndreasF on March 01, 2016, 07:56:28 pmI thought that would be the case too, but the service engineer said no, liquid helium. Sort of makes sense, because it's a newer design with no nitrogen jacket AFIK. It's also a "zero boil off" system, though in reality what that means is that there's a helium recycler that recondenses(?) the helium and returns it to the system.

author: tonyh88
author_rank: Regular Contributor
timestamp: March 02, 2016, 04:15:14 pm
content: Yes Oxford is no longer in the superconducting magnet industy as far as I know. Nowadays It is mostly Bruker and Agilent. Agilent recently stopped the NMR system sale so it is pretty much only Bruker for now in Canada anyway.NMR magnets are shipped empty of any liquid. No liquid helium or liquid nitrogen inside. The way they are constructed from inside to outside (simplified explanation here):1) The coil which will be energized2) Liquid helium that submerge the coil in the tank3) an insulator air pocket4) liquid nitrogen tank5) another air pocket6) shielding materialWith a new magnet the liquid helium compartment is first cooled with liquid nitrogen. When it is cool enough it is purged from the liquid Nitrogen and then filled with liquid Helium. The the liquid Nitrogen compartment is then filled and you can start to energize the magnet. The whole process takes a while. It would waste way too much helium to directly cool the liquid Helium compartment with liquid helium and this stuff is expensive (10$ per Liter around here) so hence the liquid Nitrogen cooling first which is cheap.