OT Question for Iggy

It is long gone. I have nothing similar. I have two HP supplies for my personal use, but 0-40 volt and lower amps. I also have one Lambda power supply 0-10v that we use for the kids' HO scale railroad. (finally something that does not fail all the time)

On a second thought I have a PP-1104C/G, you can google it. Up to

100a. However, I am not really in the mood to sell it, it is my last one of very many and it is usable sometimes.

The bottom line is, unfortunately I cannot help, I can only recommend owning a few power supplies that cover a range of uses.

Just curious why you need one, for electrolysis of something?

i

Oh well. I assumed it was long gone but just in case I figured buy from someone I know :-). There are several on ebay at a range of prices, I'll keep shopping and pick one up in the next few weeks. I told my old adviser I'd charge a 3 tesla supercon magnet for him when he gets an experiment ready to go, after I heard that he got a quote of $28k from the manufacturer. That's a bit extreme considering that they used to charge $5k

20 years ago and that included a day spent shimming the field with an nmr probe that they supplied - no shimming needed on this one. I actually charged this magnet once back when I was a student, after repairing a vacuum leak and pumping the cryostat back down, but we borrowed a supply back then. For this I need low ripple so I want the HP, the Lambda's I looked at were way noisier (5 mVpp vs 75 or 100 mV), and while for this I only need 38 amps or so, I figured I might as well get the 6260B at 100 A instead of a 6259B at 50 A since the prices are pretty similar (but 110 V is more convenient than 220, hmm, decisions, decisions :-)). Voltage will peak at maybe 4 V early on up to 15 or 20 A, then it gets tapered off as the current comes up and the permissible rate of change of field goes down. It'll be a fun outing if it works out. Thanks, anyway.

----- Regards, Carl Ijames

I do not see any on ebay in the range of prices. The one for $133.12 is bad.

Mine sold for $56.00 in March 2005. (and it was good) I did not mind the price.

Reviewing my emails... On the same day, I sold two brand new diesel engine heads for an Onan military genset. I bought both for $100 and sold for $910. The person who bought it, purchased it for EXPRESS PURPOSE of selling it back to the military, as part of a repair project, for a much greater price than what he paid me.

i

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Carl,

Could you give us the Cliff notes version of what you are doing? Is this a real magnet out of ferous material or a ring with current flowing around in it super cooled?

Just curious,

Wes

I guess I'm constitutionally incapable of a true cliff-notes length answer, but this is my short version:

It's a 6" horizontal room temperature bore 3 tesla persistent mode superconducting magnet with a homogenous cylindrical volume of about 2" diameter by 4" long with a uniformity of 10 ppm (parts per million). There are actually 10 superconducting shims that can improve this to 1 ppm but for this experiment that's not needed. It's the magnet for a Fourier transform mass spectrometer. That's the what for, the what is that it has about nine miles of superconducting wire on the order of 0.1 mm diameter wound into a solenoidal coil about 30" long, ID about 8", and a complete guess on the OD is12". This is in a dewar so it can be submerged in liquid helium at 4.2 kelvin, surrounded by a radiation shield thermally connected to the top of the He dewar boiloff tubes so it stays at about 20 K, which is then surrounded by a liquid nitrogen dewar at 77 K which is wrapped loosely with

20-50 layers of superinsulation (very thin aluminized mylar for thermal radiation shielding) which is then surrounded by the outer cryostat housing at room temperature. Down the bore there are three tubes within tubes - the smallest is at room temperature, the next is mounted at the ends to the nitrogen dewar, and the next is mounted at the ends to that 20 K radiation shield. The gaps between tubes and between the 20 K tube and the helium dewar are about 1/4" (on the radius) so the thermal gradient there is pretty steep and that's mostly what drives the helium boiloff rate. Takes about 30 liters of liquid He to keep the coil submerged, and the dewar holds another 30-40 L above that for a hold time of about 2 months. I forget the nitrogen capacity but a 160 L dewar provides a refill every 3-4 days and lasts 2 weeks. Go look at ebay item 310164162456 for a vertical bore version from the same company, Oxford Instruments (this magnet has higher field, smaller bore, and about the same homogeneity after shimming the superconducting shims). Outer cryostat diameter is about 36-40". New back in the early 1980's this was a $50,000 or so magnet.

For whatever reason Oxford favored small diameter wire and lots of turns, which has the effect of making the inductance of the coil very large. I think this magnet is about 30 henries and it reaches full field at 36 amps or so. This inductance limits the rate of change of current because you want to keep the voltage as low as possible. The superconducting wire in this case is single filament niobium titanium embedded in copper. Start with a tube of niobium, put a titanium rod down the middle, and draw it down to some mm diameter (I could have the metals reversed). Put that in a copper tube and draw that down to under 1 mm diameter and maybe 1-5 miles per spool. Put that in a furnace for days and form a cylindrical shell of the superconducting niobium-titanium alloy, which is used because it works and is flexible :-). Okay, it can be used up to maybe 5 tesla in this form at current densities of 10^5 amp/cm^2. To go higher do all this but make multiple filaments which gets you to 7-11 tesla depending on bore size. To go higher make a smaller coil wound with similar wire but niobium-tin and fire that coil after winding because that alloy can't be bent without breaking and put that coil inside a larger coil made from the first wire (to minimize the amount of expensive wire) to get to 15-18 tesla and $2-4 million per magnet at the upper end for a room temperature bore of 6". Okay, back to the single filament wire - wind that on an aluminum bobbin, bringing the ends out each time a spool runs out. After winding, splice the ends by crimping and maybe soldering with pure lead and support these on stalks so they stick up like a porcupine. This gets them to a low enough magnetic field that they stay superconducting - the critical current density and magnetic field of the joints is markedly inferior to the wire. Somewhere along the way vacuum impregnate the coil with epoxy so the wire cannot move under the hoop stress induced by the magnetic field. Take the very ends of the solenoid and connect a special piece of superconducting wire which has a heater wrapped around it and support this somewhere towards the top of helium dewar so when you heat up the heater you don't boil off all the liquid. Wire some power resistors in parallel with the switch, put the coil in the dewar, weld everything together, pull a vacuum on the dewar, cool everything down, and connect a power supply across the coil. Other manufacturers chose bigger wire and fewer turns, like ebay item 120465624954 made by Magnex that runs at 296 amps and can be charged in 5 minutes but now all the wires from power supply to coil have to be that much bigger and will carry that much more heat into the dewar, boiling off that much more helium during charging. In the end I think they all work and none is obviously better :-).

Okay, back to charging our magnet. First connect everything and set the power supply to zero volts and zero amps. Then set the voltage to maybe 2-3 volts and slowly raise the current limit so that over maybe 10-30 seconds you get to full current of 36 amps. At this point the current flows from power supply to the junction at one end of the coil, through the superconducting switch which is still cold at this point and takes the current because the coil inductance makes that current rise very, very slow at this point, through the switch, and back to the power supply. Lock the current limit knob if possible and turn the voltage back to zero. This sets the supply so you don't try to run the current up too high and tests all the connections. Now warm up the switch by applying about 5 V and 20 mA to the switch heater and waiting 10-30 seconds for it to go non-superconducting. Now slowly turn the output voltage up to 3.5-4 V (I'm working from old memories so could be off a volt or an amp here or there). Now the current will flow from supply through the coil and back to supply, with a tiny bit going through the switch since it's resistance is now tens of ohms, and the current will rise at a rate set by the inductance and the applied voltage. The copper matrix that the superconducting alloy is embedded in will also take a little current since it is also in parallel, and this current will make a little heat which must be minimized so that the wire doesn't warm up and go non-superconducting. Stare at the meters and the clock for a couple of hours until the current gets up to 20-25 A and the field up to about 2/3 of full value, then turn the voltage down to about 2.5 or 3 V to slow down the rate of rise and stare some more. Somewhere near 32 A out of 36 A total slow down again, and let the current get to the final value. Now gird your loins and grab the current limit knob and tweak it slightly to bump the current up about 0.005%, leave it for a few minutes, and then turn it back down to 36 A. This slight overcurrent empirically was found to make the final field much more stable. The fear is from knowing that you are already right at the edge of the critical current density at that field and now you are creeping a little closer. (If you weren't that close to the edge they would rate the magnet higher and put you right back on the edge; wire is expensive :-).) If you go too far the wire goes normal in some spot which then gets hot which then makes the rest of the coil normal, all in about a millisecond. Enough heat is released from the energy stored in the magnetic field to boil off all the liquid helium and blow it into the room in about

30 seconds, and to warm the coil so somewhere near 100 K or warmer. Stopping the current that fast through that large an inductor leads to horrific voltage spikes, potentially thousands of volts, which in the old days lead to arcs and destroyed coils. Remember those power resistors? That's why they are there, to snub the voltage in case of a quench. Okay, now the current is back down that smidge and the field is stable where you want it, so turn off the heater on the switch and let it go superconducting again. Now the current flows out of the coil, through the switch, and back into the coil, around and around with no resistance. The power supply current flows to the switch, through the switch in the opposite direction (so the net switch current is zero but this is the best way to visualize things), and back to the power supply. Slowly turn the voltage down to zero, and then the current down to zero, and disconnect the supply from the magnet. Voila! a persistent mode superconducting magnet lives. With an Oxford magnet the total time is 2-3 hours. Single filament joints like this can be very, very good so that the power dissipated in their residual resistance makes the magnetic field decay maybe 50-100 parts per billion per hour. Joints with multifilament niobium-titanium aren't as good and there are usually more of them because you only use that wire when you need higher field which means more turns, so those magnets can decay a part per million per hour or so. Remember the superconducting shims? Those are windings of various shapes to produce orthogonal field gradients to correct defects in the field from the main coil. Some of them are shaped so that they couple to the main coil like the secondary of a transformer. Each shim has its own switch and heater, and some of those must be turned on and left on during the entire charging process with a resistor across the switch to dissipate the energy to keep from inducing too large a current in the shim. If needed, after the main coil is persistent you then connect a separate supply to each shim and set the currents to the appropriate values, then make those switches superconducting as before.

Oh, no ferrous material anywhere inside the coil because it would saturate near 2 tesla and distort and limit the field. This is the same technology used in MRI magnets. The cost of a magnet is mostly the cost of the wire, and the amount of wire is some function of the room temperature bore size, the homogenous volume, and the field strength. MRI magnets tend to be much bigger bore so people can fit into them, and to keep the cost under control they tend to be lower field. I think early ones were 1.5-2 tesla and cost

1/2 to 1 million. Nowadays they are 3-4 tesla and cost less - the miracle of volume production. Anyway, that's enough for now. Be glad to answer any other questions.

Iggy, that was a great price. I'm hoping for under $200 and I have some time to be patient.

----- Regards, Carl Ijames

"Wes" wrote in message news:A5Ynm.725189$ snipped-for-privacy@en-nntp-03.dc.easynews.com...

(Snip very interesting and highly thought - provoking explanation of applied superconducting electro magnetics)

Thank you Carl!

Your post is one of those rare gems that make it all worthwhile.

--Winston

Note that sometimes "bad" means that the vendor does not know how it is supposed to work.

I got a pair of "bad" Power Designs 2005 power supplies (20 V,

5 A) which were set by multiple concentric switches in steps of 1, 0.100, 0.010, and 0.001 V, plus a toggle switch which adds 10 V to the total. There is also a pot for tweaking the output from 0.000 to 0.001 V.

Anyway -- what was "wrong" with the supplies, and what I suspected was the problem was that the "sense" terminals on the back were not connected to the V+ and V- terminals. Normally, they are connected, and the front panel terminals are used -- but for use in rack-mounted test setups (and these two were in a dual rack mount frame) it is common to run the power and the sense wires from the back of the supply, to leave the front panel uncluttered by cables. Running the sense wires to where the load is assures that the regulation produces the set voltage *where the sense and power leads join*, instead of at the panel, allowing the loss of several milivolts or more depending on how long the cables are. The sense leads should be run back in a shielded pair cable to keep noise pickup from affecting the output voltage.

Anyway -- once I got them home and replaced the missing sense jumpers, both worked just as designed. And I got them for what I considered to be a particularly good hamfest price for the period. (It helped that I used to use these at work, and knew what they would do so I wanted them. :-)

Not any good on eBay -- unless the vendor includes photos of the back panel and you can see that there are no jumpers between the power terminals and the sense terminals there. On something capable of 100 A, the sense terminals will probably be normal sized Jones barrier strips, and the power terminals on the back will be on an extra heavy duty barrier strip -- or even on stud terminals on the back.

Of course -- if you can see the sense wires are connected in the photo -- don't bother bidding. :-)

Good Luck, DoN.

Carl, it was a bit of a hard to read (due to its scientific nature), but extremely fascinating writeup. I did not realize how much energy you can store in a superconducting coil and how fragile (unstable) the superconducting state is at the edge. I appreciate you taking time to explain it in simple terms.

I wish I could buy liquid nitrogen somewhere.

i

Sad to say, it happened to me once where I was in the role of the seller.

yep

Congrats.

This is the fun part of ebaying, for sure. I did also buy several such "bad, will not turn on" things and had good luck with them, UNLESS they were gasoline powered.

Which is the case with that bad HP 6260 power supply for $133.

yep...

i

brillaint post.

I was always wondering how they inject current into a superconducting magnet.

Ignoramus7829 fired this volley in news:W7qdnRDLCOeHtzzXnZ2dnUVZ snipped-for-privacy@giganews.com:

You can, Ig. If you can buy or build a small Dewar flask (easy), you can buy it at many medical gasses supply houses, or in a large town, and a welding gas supply.

I get mine from a local college that does electron spin resonance research. They have an LN2 generator on-site.

LLoyd

Lloyd, this is awesome. One thing that it will probably let me do, is machining of rubber pieces. Thanks a lot. I will explore this a little more in depth.

i

Where are you? I have a few supplies in that size. Mostly 3 phase input however.

Ping me off list at cyberzl1 AT yahoo DOT com

JW

Thanks for the kind words, guys. Iggy, you can get liquid nitrogen at most welding gas suppliers. If they fill nitrogen gas cylinders there they will have a liquid bulk station (big tank that gets refilled by semi-trucks), and if not they probably keep a 160 L transport dewar on hand for small sales. Just get a stainless steel thermos bottle, like the Stanley brand, that holds a quart or two (bigger is much better). It will probably keep liquid nitrogen for a couple of hours, at least, maybe up to six hours. A good 2 L glass dewar will hold LN2 for 8-12 hours. For freezing rubber to machine it, I bet dry ice would do nearly as well and it will keep much longer in an ice chest or thermos bottle.

Whoever suggested checking the sense wires, thank you, they show one view of the back and you can see that they are present, sigh :-). Like I said, I have time so I'll keep looking.

----- Regards, Carl Ijames

OK, cool, I will definitely get some.

Now, what about dry ice, can that be found easily?

I once bought it in Florida right in a supermarket, but cannot find it here in IL.

i

Krogers

Holy smokes, that was the best thing I've read on this newsgroup since, well, ever. This really seems like a job that could be well handled by a decent programmable power supply and controller. You could turn it into a "set it and forget it" deal. I have to believe that, with the demise of so many electronics companies in the past few years, that such a thing would be available pretty cheap.especially if you've got time to shop.

I've found it easily on the Gulf Coast (Texas). Very useful for returning with frozen shrimp or fish. Actually, I've never looked for it here in Michigan. I've never wanted to carry any foodstuffs the other direction.

Pete Keillor

I was in Meijer a few weeks ago and I'm pretty sure the people ahead of me bought some at the checkout. Don't know where they have it stashed though, other than I would look around the regular ice cooler or just ask the cashier about it.