making a mag field

Maybe the dosimetry (weber's) they were referring to wasn't at that level.

Jamie

Didn't stop the EU trying to ban them:

After they gave up on banning high static fields they tried to set the limit for changing magnetic fields so slow you could be near an MRI, as long as you could get there without walking in the process: ?Estimates have shown that movement close to a scanner at normal walking speed results in induced currents that breach the ELVs by a factor of almost 10.?

I guess the staff and patients were suposed to be supported on an array of trained snails during transport.

Chris

The MAGNET for NMR is essentially DC, and a low level RF field inside. Of course, the MAGNET seems to be "unshielded" as any additional iron keeper or equivalent would be too big and too expensive and impractical to properly enclose the field.

On Fri, 20 Dec 2013 02:43:09 -0800 (PST), Bill Sloman Gave us:

The similarity is when the 'static field' gets its initial generation, and also when it collapses. If done by a switch, the slew rate is so fast that it is EXACTLY what an EMP pulse is, just not as powerful.

If the field is natural, it will always be there. If artificially generated, the rate at which power is applied will determine if there is a single pulse generated or not, before the standing field is 'up'.

I can't immagine it being better than filling the gap with iron.

the iron is going reroute field-lines until it's saturated and then it'll let some though

it might leak some between the lid and the box having nesting boxes with different orientations may help there.

these electromagnets are superconductors, it's cheaper and more convenient to leave them "turned on". as I understand it special equipment is needed to power them up.

You mean apart from being a fairy story?

You don't get much current induced into a good coax.

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Static fields of sufficient strength cause problems in transformers.

For example, some years ago, I had a case where little wideband ferrite transformers used in the signal processing circuitry of beam position pick-ups in the CERN Proton Synchrotron were saturated by the field of the bending magnets about halfway through acceleration. At that point, the beam trajectory appeared to suddenly become a prefect circle. Not very credible. ;-)

The assembly holding the transformers was mostly aluminium with a light (too light!) iron shielding box inside. I had to add a considerable amount of soft iron to get sufficient shielding. That was a pity, because the iron is heavy and gets radioactive. But it did solve the saturation problem.

Jeroen Belleman

From Bill's old stomping grounds, a European health spa for frogs:

Cheers, James Arthur

We want to run an autimatic BIST on 48 isolated differential channels, so the easy way is to use 48 DPDT relays, so we can switch each channel onto a test bus. Like this:

600 gauss isn't very powerful. The weird thing is that the customer is going to use a bunch of our stock "COTS" VME modules in his system, and they are full of these same relays. But the spec for the new custom module requires it to work in a 600 gauss field. Maybe physics is different down in Australia.

The field would have to switch very fast to induce much voltage into PCB traces. That's hard to do. EMP comes from large pulse generators, like nuclear weapons and very specialized capacitive-discharge or explosion-pumped gadgets.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

Magnetic shielding is tricky. A typical steel Hoffmann-type enclosure isn't very effective. The field would just as soon penatrate the walls and go straight through as to take the long way around through the side walls. 2:1 reduction is typical for a bread-box sized steel electrical equipment box.

My customer didn't specify.

A little maybe.

Sure, but not on a VME module.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

Nah, you just need to place a "-" in front of the math!

Jamie

Sounds like there wasn't enough ferrite used in the scan magnetic coils to start with?

The electron accelerators we deal with have $12k worth of ferrite as part of the scan coil assemblies. Reluctance of the field is very low and external fields in that area do not really bother it that much..

Each coil is ~ 10 turns of copper tape wire wrapped with fiber glass tape and then lacquer. THe coils are about 10 inches in diameter and they sit into a molded section of the ferrite which is ~ 50 ibs worth. These accelerators are not large units in comparison to others I've seen, they generate up to 2.5 Mev give or take a little.

However, beyond the scan coils (magnets) external diamagnetic and ferromagnetics can cause issues.

In your case as to the circle being generated, sounds like a diamagnetic effect, where the returning scan is going in the opposite direction and thus bends the beam the other way, which forms a circle after a complete scan is done..

Jamie

Now THAT's a cheap drunk!

Have to be careful in the construction. Helps a great deal to have the box completely enclosed. Actually can get worse if the box is not complete.

shallow gaps, no real advantage. larger gaps, well starts depending on a lot.

magnetic fields tend to 'retransmit' right through a multi-layer sructure. Especially if the field has a strong gradient [translate that to mean a close dipole.]

Magnetic shielding is very different from electrostatic shielding. The key is to provide a low-reluctance path to guide the flux around the volume you need to protect. The orientation of the shielding w.r.t. to the direction of the local flux is crucial. However, there is no need at all to have a closed box.

Jeroen Belleman

Based upon experience, yes you do. Especially around some of those R/W heads to protect them from 'unknown' orientation fields.

Actually, corrected a client's primary consultant's mistake in applying the concept that magnetic field is NOT electric field and there is no need to 'close' a shielding box. Very simple solution won me a new client.

What I said is "Have to be careful in the construction. Helps a great deal to have the box completely enclosed. Actually can get worse if the box is not complete." I stand behind that comment/suggestion.

Not really. Typically, a gaussmeter (in the G-mG range) will come with a little mu-metal cap for the sensor for zeroing. Obviously, it's a cylinder open on one end, which is absolutely fine.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

You snipped my comment about 'careful construction' but is ok

A matter of degree, depends on just 'how' much shielding is required.

A 'Faraday cage' open on one side shield will provide some degree of shielding, too. just a matter of degree.

Yes.

Best regards, Spehro Pefhany

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