Coil Former Progress

15 02 2010

Did some more work on the coil formers. Stuart and I used the computer controlled milling machine to precisely drill the holes. First we mount the teflon coil former in a chuck:

Then we center the spindle:

Drill the holes:

Next we take some aluminum angle bracket:

Cut them down to size:

Drill the holes:

And tap:

Assemble:





Coil Power Supply

13 02 2010

Joe Khachan just sent me details on the power supply for the coils. They are designed to produce a brief high current pulse.

Joe says:

The part that took the most time to build was the power supply. I’ve attached a diagram that looks something like our circuit without dump resistor to dump charge of the capacitors when we need to service them. We used a hockey puck type of SCR (a type of thyristor) that can take 1000 A continuous or 10 kA pulsed. This may have been an overkill because we found that a maximum of 300 A was needed. However, we may need higher current as we increase the size of the polywell. The capacitor bank was make of 5 X 1500 microFarads electrolytic capacitors that can take a maximum of 450V all connected in parallel. You need some kind of transformer that can step up the voltage from the mains and be able to charge the capacitors within a couple of seconds. That means it shoud be a reasonably hefty transformer. We control the voltage output of the transformer with a Variac on the input side. You need to protect the SCR from back EMF with a diode across it. The diode should be able to take a few amps. The wire diameter about one millimeter and there were 10 turns per former.

Joe’s coil power supply looks like this:

Here is the preliminary bill of material (click for Mouser produce page):

1500 microFarads electrolytic capacitors max 450V

Silicon-controlled rectifier (SCR)

Diode Diode to protect the SCR from back EMF

Step-up Transformer Step-up Transformer

Variac

Power Rectifier

Trigger Isolation Transformer

18 AWG magnet wire

100 watt 2KΩ resistor for bleeding capacitor bank

Rack mountable chassis.

Please comment if you notice wrong parts.





Coil Formers

12 02 2010

My shop-mate Stuart machined these coil formers from a teflon rod:

Here is a time lapse video of Stuart machining the  formers on the lathe:

Next we have to drill four holes in each former and connected them with angle brackets.





Coil Former Dimentions

9 02 2010

Here are the final dimensions of the coil former:





New Copper Coil Polywell on the Scene

25 12 2009

Researchers at the University of Sydney have made a small Polywell device which looks like this:

Powerpoint slides of their research here.

Notice there is no metal exterior on the magrid. As far as I understand… instead of using a magrid with a shell at positive potential (like the WB6 does), they are shooting in electrons with kinetic energy from an electron gun.

This seems like a feasible way to build a copper coil polywell. If the researchers are willing and able to share the details of the experiment, I would explore replicating the device and results. It looks manageable:





Superconducting Magnet

15 10 2009

Gearing up for a second superconducting magnet test. This time computer controlled. Here is the new bobbin with 133 turns:

IMG_4262

Read the rest of this entry »





The Core

11 04 2009

I’m preparing to have the core fabricated. I have a number of considerations to consider.

chassis1

Welding. We have to weld the lids to the chassis.

Maybe TIG welding will work. My concern is that heat affected zone will damage the SC coils inside. We have ~2.5 mm from the surface to the coils. Laser welding has a much smaller heat affected zone. TODO: get a quote for laser welding from EB Industries. Can anyone comment of the viability of TIG welding for this sitation?

Surfacing. The product that comes back from prometal has a rough surface which we need to machine so that the lid mate well.

Previously I tried wet sanding. This worked decently. However, I wasn’t able to get the deeper surface imperfections, it took a lot of sanding. The outer ring of the torus half saw more material removed than the inner ring, which means that the inner rings mate very tightly, but the outer ring has about a 0.25 mm gap.

There is a surfacing machine here in the shop. It’s large enough to accommodate the lids, but not large enough to accommodate the chassis. The surfacing machine uses a magnetic vise, so the work piece must be magnetic. The sample parts we ordered from prometal are magnetic, however the next parts will be made with a less magnetic stainless steel alloy (the chassis should not be magnetic).

We may need to take surfacing into consideration for the design of the part. ie, we may need to include some extra material on the prometal part, so that after we surface it, we have a perfect half torus.

UPDATE: Stuart told me about Lapping which seems to be an advanced for of wet-sanding.

CAD problems

I’m using BRL-CAD to generate my parts. Lately I’ve been getting this error when I try to export to STL: class_lu_vs_s: loop transits plane of shell/face?  I can’t proceed until I overcome this bug.

Even when the STL export works, it takes forever to render an STL with the resolution I need for production (I’m talking days here). This is really cramping my flow.

Permeability of the Core

We are building a superconducting core. There will be liquid nitrogen at atmospheric pressures inside the core (and connected to outside of chamber via a fluid feedthrough). The core can’t be so permeable as to leek nitrogen into the vacuum which would poison the reaction. Speaking of pressure differentials, the core must withstand the pressure from the inside. To calculate this pressure, I think we need to know the internal surface area of the core.





YBCO Superconducting Cable

8 04 2009

Very exciting. The YBCO arrived today. This is what $1,105.00 worth (13 Meters) of  insulated YBCO superconducting cable looks like:ybco3

The ribbon itself is very very thin and flexible. Much more flexible than I was expecting.  Here is a close up of the ribbon:

ybco2

That’s just some scotch tape at the end.

The lead time for this YBCO is about 3 months. So you really have to order in advance.

Today I plan to order the first prototype of the chassis.

Also going to get the dewar flask filled with some liquid nitrogen and attempt to build a superconducting coil!

While we’re on the topic of superconducting cable. Would it be possible to buy a used MRI machine and extract the superconducting cable? Even if this is possible, it would be using low temp superconducting cable, which of course requires both liquid helium and liquid nitrogen.





Manifest

20 02 2009

The parts from ProMetal just arrived. These parts feel heavy, dense, and strong. You really have to hold these in your hands to believe it. They hold water without leaking. Cost ~$30 each. img_3245img_3249

There is some texture, but overall these parts are highly conformal to the design. img_3264

lids

This is very exciting. The first prototype core is within reach.

Next I will grind the touching surfaces of the lids and try to laser weld them together.

 

On the down side this material is magnetic:

magnetic_steel

I wonder if ProMetal can adjust the composition of the alloy. Stainless steel can be either magnetic or non magnetic depending on the alloy:

There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures.

Update: Looking at the ProMetal Materials Spec Sheet, it looks like they offer 316SS, which is non magnetic, although less strong than the 420SS. Bingo.





Decawell Moon Shot

3 02 2009

I’m starting to think it makes sense to take a moon shot, and build a superconducting core from go. This would allow for continuos operation, smaller chassis, higher field strengths per turn, and is generally Badass.     

I have 13 meters of superconducting YBCO tape on the way (was more expensive with copper matrix and insulation). That’s enough to build a one turn dodecahedral polywell, with one meter of superconducting cable for each of 12 coils.  I’m not exactly sure how to apply the manufacturers specs for the cable. This is my guess:

Superconducting properties (@ 77K) 

Critical current IC*: 200 – 250 A/cm 

Engineering crit. current*: 200 – 250 A/mm2 Density:   9 g/cm3 

Icvalues range from 80 –110 Amps at 77 K in 4 mm width 

Engineering Current Density (Je) = 21 –29 kA/cm2 

It looks like I take the Critical current IC* 200 – 250 80 –110 A/cm and multiply it by the length of cable 1300cm, which gives us 26,000 104,000 Amps. I’m working out the B field calculations using Ampère’s force law


Now regarding the chassis.

chassis

A few weeks ago I contacted POM Group, requesting a quote for fabricating the chassis and lids in stainless steel using direct metal deposition, based on my EGIS files of the parts. POM wrote back: “unfortunately it would not be feasible to manufacture the chassis with our process. It would be feasible to DMD the lids”. I’m eagerly awaiting more information on what makes it infeasible. My guess would be too great an angle of overhang. Although I’m not writing off DMD just yet, I want to explore other options such as casting.

I came across an interesting technology called  ZCast Metal. This uses the Z Corporation rapid prototyping technology to directly produce a ceramic negative for metal casting. Currently this process only works for aluminum, brass, zinc and magnesium which have lower melting points than steel. It cost far less than traditional rapid prototype casting, where the negative is made on a CNC machine, followed by a wax injection mold, followed by a ceramic coating, followed by the actual casting. Another limitation is the size of the Zcorp printer, the maximum build envelope being 254 x 381 x 203 mm.

This raises the question: Does the chassis _have_ to be made of stainless steel? Bussard indicated that stainless steel was the ideal material, but other metal may also be suitable (keep in mind we are being a prototype). The chassis must do a few things. It must be strong enough to resist the mechanical forces exerted by the electromagnetic coils. It must be electrically conductive so it can be set to a high positive electric potential. An in the case of superconducting coils, it’s must be sealed well enough to prevent the liquid nitrogen boil off from poisoning the vacuum. I’m not sure if the magnetic properties of the metal are important. Depending on the alloy, stainless steel can be either magnetic or non magnetic. 

Here is useful tensile strength comparison chart. This shows Aluminum with a tensile strength of 145 Mpa. Stainless Steel’s tensile strength can range from 200 to 700 Mpa depending on alloy and annealing. So it may be possible to use aluminum for the chassis, though with a reduced operating envelope.

In terms of outgassing, aluminum varies widely depending on treatment, and seems comparable to stainless steel. 

I’m considering designing the chassis so that the coils are serviceable, ie have the lids bolted on not welded together. This way you can upgrade the coils with more turns later on.  I really like this idea. We could get it working with a single turn of superconducting cable, then when more money comes in, upgrade the unit to 10 or 20 turns. On the down side this would introduce issues with bolts breaking the smooth profile of the chassis, ie not being conformal to the coils. Also, it would be more challenging to seal in the liquid nitrogen with a non welded design. 

Today I’m getting quotes to have the chassis and lids cast in stainless steel using rapid prototype casting as well.








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