Even in the billion dollar systems, they have ground problems.
Short Circuit Delays Restart of the Large Hadron Collider - Scientific American
Short Circuit Delays Restart of the Large Hadron Collider - Scientific American
These novices need to catch up with the world of quantum hifi physics 🙄
A ground lift switch and a cryo treated mains cable would have them up and running in no time 😀
A ground lift switch and a cryo treated mains cable would have them up and running in no time 😀
Ya gotta love it.
When they say diode box, what they really mean is DIODE box.
The diode is a 100 mm diameter piece of silicon, designed to bypass the magnet in the event the magnet quenches. It operates normally at the 1.9 Kelvin temperature, and acts like an SCR.
The forward voltage of the diode essentially goes away, and instead, the device can sustain somewhere between 3 and 30 volts in the forward direction without conducting forward current.
When a magnet quenches, it's IR drop goes way up, and the diode clamps the voltage first at whatever the diffusion parameters were set for, and then the diode instantly heats up, and reverts back to a forward bias junction device. It conducts the current of the rest of the sector around the heating magnet, and they very quickly turn off the power supply. And when I say instantly, I mean instantly..13 kiloamps and 30 volts ain't no biggie in itself😱, but at 1.9 K there really is no heat capacity in the silicon nor in the copper anvils they are compressed between.
Unfortunately, that switching operation only occurs at liquid helium temperatures, so they have to enclose it deep in the cryostat, with everything welded for high pressure resistance.
QA for these big objects is a PITA, as it can take days (or weeks) to bring the system back up to room temperature..as I recall, it took weeks to bring the tunnel up to room temperature after the accident..
jn
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bummer if they have to warm it up to let a meatbag in to inspect.
I have to admit the whole concept of cooling the whole ring to 1.9K is mind boggling.
I have to admit the whole concept of cooling the whole ring to 1.9K is mind boggling.
They are CERN. They have ways.
When they did the splice work, the first thing they did was come up with a way to X-ray every solder splice. sheesh.. Now, they are going to use that to view the diode box. The only thing they have to worry about is oxygen deficiency hazard in the tunnel while the magnets are cold. The tunnel looks pretty much like the one in terminator 3, but less of a curve...and no chick robots.
In ODH training here, they have one very important rule. If you hear the whoosh of a big liquid helium or liquid nitrogen leak, RUN the other way.
Run. Do not walk. This IS a race. The loser....well, loses..
Yes. Most people do not know what it takes, nor how weird or dangerous it can be. They do have the 8 biggest refrigerators on the planet though.
jn
Yah. Their idea of a fuse turned out to be a 50 foot long, 45 ton iron based object which played leapfrog in the tunnel during the "accident".
But at least they only use lead free solder..
jn
Really? Lead free? Hmmm...
Cryo treatment all around, that's - uh - cool. Wonder what the thing sounds like?
Thanks for the info on the diode box.
Cryo treatment all around, that's - uh - cool. Wonder what the thing sounds like?
Thanks for the info on the diode box.
Historically, these superconducting accelerators use lead free because lead itself goes superconducting at about 10K IIRC. They were worried that there would be lead grains in the solid alloy that would carry the current preferentially over the rest of the alloy matrix. When you solder to copper, it forms two copper tin intermetallics at the copper boundary layer, and that pulls the tin out of the alloy, leaving lead grains. (at least, that's the thinking)
For a while, I was the worlds leading consumer of lead free solder for electrical splices (I had to place the orders), and by accident, the world's expert in actually using the stuff, especially with the R type flux we were limited to.. CERN has since surpassed the quantities I used. by about an order of magnitude..
And no, the machines don't sound different as a result...😀
jn
I assume its rather hard to get a microphone working at those temperatures to even hear them? and do they evacuate the tunnel as well as the ring? I would expect a 'purposeful hum' from all the MW being consumed down there.
Whilst mentioning ground problems I found this CERN fact
Ohms law always wins!
Whilst mentioning ground problems I found this CERN fact
Ohms law always wins!
They generally evacuate the tunnel of people for safety. But the tunnel temp is typical of the dirt in the surrounding earth.
All the supplies are above ground. The radiation spill with beam tends to upset silicon devices. RAM tends to change logic states due to radiation, and even linear stuff can latch up.
As memory transistors kept getting smaller, it eventually became possible for the natural radiation of ceramics to flip memory cells states. They solved that somehow. But the shielding in the tunnel would be too much.
That train current/lep chamber was a really cool factoid. Thanks, I never knew that.
jn
I was intrigued more so went to check CERN
In pictures: X-rays probe LHC for cause of short circuit | CERN
As you can see, warm enough to work in short sleeves even with the system at somewhat chilly!
And they fixed it with brute force and POWER LHC restart back on track | CERN . Just in time to announce they had found evidence of the force 🙂
But now I need to adjust my brain. If the LHC consumes around 120MW when running, most of which is going into superconducting magnets that have zero resistance, where is the power being dissipated? Stupid question I know but never thought about it before
In pictures: X-rays probe LHC for cause of short circuit | CERN
As you can see, warm enough to work in short sleeves even with the system at somewhat chilly!
And they fixed it with brute force and POWER LHC restart back on track | CERN . Just in time to announce they had found evidence of the force 🙂
But now I need to adjust my brain. If the LHC consumes around 120MW when running, most of which is going into superconducting magnets that have zero resistance, where is the power being dissipated? Stupid question I know but never thought about it before
To make liquid at 4.5 K, you need about 1000 watts of electrical power from the grid for every watt of power dissipated into the liquid. A brutal consequence of Carnot.
To make 1.9 K liquid requires about 2000 watts from the grid per watt in the liquid.
Our cryostats on this side of the pond absorb about 1 watt of room warmth for every 50 feet, LHC is 27 kilometers long. There is an intermediate thermal shell around the magnets, it is at 50 K, so most of the heat goes into a 50K refrigerator which is more carnot friendly, the magnets have to be insulated from the 50 K shield. Still, there will always be insulation losses.
Conduction thermal loss is also there, as the magnets have to be held in position. Convection is stopped by the vacuum cryostat.
Penetrations to the cryo require warm to cold connections which conduct heat also.
Beam synchrotron losses warm everything it hits.
Power supply to tunnel connections may be via copper, I'm not sure there. We use supers, but CERN may not, or at least not all the way.
Oh, don't forget about a bazillion computers, vacuum pumps, displays, lights, for several thousand people.
The biggest hit? The keurigs...lots of them..major power usage..there is a night shift.
jn
The power supply for the magnets I suppose? Like an amp trying to feed a short circuit.
We use two supplies. Both are 7000 amp, but one is 30 volts, one is 400.
The 400 volt one is used to ramp from zero current to max. That is because it is a VERY large inductor. At 83 amps per second, much of the 400 volt supply is needed.
Once the current is at 6300, the second supply can take over, maintaining the current.
I suspect CERN does the same basic thing, albeit far larger numbers.
jn
jn -- which accelerator were/do you work at? (Privacy respected if you'd rather not share)
Fascinating stuff. Really cold matter gets interesting!
I had access to a liquid helium-cooled deep-layer transient spectroscopy system for some of my previous work (defect energy levels in semiconductors), but that's minor in comparison to this.
Fascinating stuff. Really cold matter gets interesting!
I had access to a liquid helium-cooled deep-layer transient spectroscopy system for some of my previous work (defect energy levels in semiconductors), but that's minor in comparison to this.
I live in NY, nuff said.
Bill, thanks for the links. I must say, that is a really really scary fix.
There are some PV conditions where helium gas becomes electrically conductive. As such, we make sure that there is at least a 200 mil air gap between metals that will have a potential difference.
They clearly have some kind of gap between conductive surfaces which was bridged. By blowing it with a 400 amp pulse, they do not know what the surface looks like after. If they are left with two spiky points, they've made a spark gap. If that gap is too small, an arc could occur during a quench of the dipole string.
Best case would then be, a continuous short to ground could flash up, requiring opening up the magnet..
Or, the diode could be damaged by fast slew current/inductance killing the diode in reverse direction, again forcing them to open the magnet. (even a 100 mm diode can be killed by a milliamp current in reverse direction if it avalanches at a small spot on the bevelled edge.)
Or, the transient punches a hole in the insulation of the magnet, requiring replacement of the magnet. Quarter to half a million dollars give or take, and perhaps a day longer than simply opening the box.
A distinct combination of engineering evaluation, tradeoffs, and monetary consequence no matter which way they go. To shut down for 6 weeks costs far more than a damaged magnet. People costs are probably over a meg a day.
jn
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