How Should High-Value, Fragile Equipment Be Transported Safely?

[Moonbear]

It would be very inefficient use of resources to have a grad student build a magnet power supply. There are companies (like Oxford Instruments) that do nothing but this, and they will take custom jobs. Humanino (I suspect) has been designing the overall experiment (of which the magnet is a small, yet essential part).

Ah, then I probably just misunderstood the issue. I had the impression this was going to set him back a full two years, but perhaps it's not quite that bad...or maybe he was considering switching to theoretical physics anyway and this is a convenient "excuse" to help him make up his mind.

 

[humanino]

Don't worry, I'm aware of this. It's common to keep and eye on the voltage to confirm beginning and end of ramping. Also, if your magnet quenches from low fields, the only indicator may be the voltage across the magnet leads.

Yes indeed Gokul, you are right. You could be of precious help I guess. Indeed the power supply itslef is controlled through voltages measurement. Any beggining of quench triggers a system to remove the liquid helium in the vessel, so the entire coils become resistive. You know all of this I'm certain.

How high a field do you go up to ?

5 T in the center, but it is especially designed to be a confined field.

Just out of curiosity, who did you buy the power supply from ?

Danphysics.

 

[humanino]

Ah, then I probably just misunderstood the issue. I had the impression this was going to set him back a full two years, but perhaps it's not quite that bad...or maybe he was considering switching to theoretical physics anyway and this is a convenient "excuse" to help him make up his mind.

You are not totally wrong...

Yet, if we do not find any solution within a week or so, the administration is going to start threatening us with postponing, which for me implies the need for a new project. It took me a while to decide between theory and experiment. Now, I would go for theory in order to avoid all the issues not depending on me. In theory you are more lonely, but at least you do not depend on the iron market in China

 

[Gokul43201]

5 T in the center, but it is especially designed to be a confined field.

Our magnet has been used last at a little over 16T. At these fields, it's quite tricky to build a magnet that can maintain the desired homogeneity over more than a few cc of volume around the magnet center.

Oh, and a quench from 16T is quite a spectacular event. Our persistent current switch allows low-field quenches that are not disastrous.

One more question, if you don't mind : What kind of cryostat do you use - and how low do you go ?

 

[humanino]

Our magnet has been used last at a little over 16T.

Impressive ! Homogenous 16T, actually I guess it is tricky ! For us, homogeneity was not among the specifications. We needed an intense field in the forward direction, that decrease rapidely in the transverse direction. This field map is logarithmic :

http://www.jlab.org/~fxgirod/field_map.jpg

The blue points correspond to constrained spots, where another magnet should not receive too high mechanical constraints.

What kind of cryostat do you use - and how low do you go ?

We use a standard liquid helium vessel. We work at 4.2 K.

 

[humanino]

And just out of curiosity, who did you buy the power supply from ?

Here is the http://www.danfysik.dk/pdf/custom_designed/SYSTEM-8500-Brochure.pdf from Danphysics of our power supply. It is standard but has to be customized.

 

[Gokul43201]

We needed an intense field in the forward direction, that decrease rapidely in the transverse direction.

Usually this is done by running a shorter second winding backwards, whose center is displaced from the main winding along z.

I should have remembered that nuclear/high energy folks use high current magnets. I think ZEUS has (or had) a 1000A magnet...

The blue points correspond to constrained spots, where another magnet should not receive too high mechanical constraints.

What are the distance units in the field map ?

We use a standard liquid helium vessel. We work at 4.2 K.

Hope you don't mind if I shoot a few more questions at you. Does your cryostat have a liquid helium level sensor ? If it does, is it a superconducting sensor or a capacitive sensor, or something else ?

 

[humanino]

I should have remembered that nuclear/high energy folks use high current magnets. I think ZEUS has (or had) a 1000A magnet...

Actually, our main magnet can run up to 3300A
It is not shunt, so this current must be constantly delivered. The shunt would require a too complicated security system in case of quench.

What are the distance units in the field map ?

those are meters. I don't know how to convert to inch or feet

Does your cryostat have a liquid helium level sensor ? If it does, is it a superconducting sensor or a capacitive sensor, or something else ?

Actually, we monitor the temperature, the pression and the liquid level in the helium vessel. The level is controlled by a supeconducting sensor, it is just a wire whose superconducting part lies inside the liquid, so its resistance simply gives the level.

 

[Gokul43201]

The level is controlled by a supeconducting sensor, it is just a wire whose superconducting part lies inside the liquid, so its resistance simply gives the level.

I suspected this may be the case. I just completed building a capacitive level sensor for one of our cryostats. The problem with a superconducting sensor is that (i) it makes RF noise (at the step edge) every time you turn it on or off (our signals are very small ~pV), (ii) if you leave it on continuously, you radiate too much heat into the cryostat (this cryostat operates at ~0.3K, so we have to be careful about not putting in heat). The advantage of a capacitive sensor is that it is not dissipative ; the tricky part is that the dielectric constant of liquid helium is only about 4% greater than that of helium gas. So, the signal is pretty small.

 

[Evo]

Since humanino's been so busy, I will give an update. (he's been in a hole all week pulling cable with a Russian guy)

The power supply was repaired and the magnet works!

 

[humanino]

thank you my precious Evo

indeed, it is fixed now, and this required much pain. But we like pain

Unfortunately, alcohol is forbiden here at the lab
when is the next PF party ? there is much to celebrate !

 

[Moonbear]

You're just in time for the PF tour bus to Hawaii. We're going to Honolulu to examine the conditions surrounding the tree where Tsu photographed the virgin mary in smoke. Ivan's even bringing along the grill for some fabulous steaks.

Glad you got it all fixed, even if it was a pain to do. *phew*