Core material for high power high frequency coil/solenoid

[artis]

I read the limits of typical ferrite cores and it seems that most sources claim that they tend to work somewhere up to 1 Mhz. Surely they work great under that frequency, but is there any material that could provide a core material for a wide frequency range starting from few tens of Khz up to tens if not hundreds of Mhz?
The problem is I need a high strength B field at high frequency, and making a coil with air as the core material then demands extreme kA currents through the coil in order to achieve decent (up to 1T field strengths) I was wondering can this be somehow resolved by adding a core material to increase the permeability a bit so to decrease the required current through the coil itself?

Any ideas folks?

Thanks.

 

[DaveE]

Most all RF inductors are air or ferrite. There are about a million different types of ferrite to choose from; some are designed for RF applications. However, you will still have some core loss to deal with.

 

[berkeman]

I need a high strength B field at high frequency

What is the application?

(up to 1T field strengths)

Yikes, it's going to be hard to find a material that doesn't saturate at such high fields.

How big of a volume do you need to fill with this high-power RF EM field?

 

[berkeman]

And is this related to your previous thread?

https://www.physicsforums.com/threads/klystron-energy-conversion.968738/#post-6154787

 

[bobob]

No way. Not even with an air core.

 

[artis]

Yes it is related but only a little bit. I am not building a klystron.
I need the field to cover a circular area so it can be thought of as a 2d surface through which flux must go through , the surface can be put as close to the solenoid/coil as physically possible, but the problem as you know is that for a core material I can route the B field and provide the field with a higher permeability path so I need less current, sure you say core loss etc but that is a minimal problem compared to achieving decent field strength with an air coil that requires tremendous amounts of current, technically I could arrange and supply the kiloamp current but then cooling etc becomes a problem.

Well are there any known materials that are used in similar applications where both high frequency and high field strengths are required?

Also I can't seem to find any decent calculator or formula for calculating how much current would actually be necessary and what shape of coil could achieve the field strengths I need for the case if it was an air core coil.my area can vary but for example let's suppose 700cm2 area.

I have had ideas that maybe i could make this coil from multiple smaller coils in parallel , the ends at one side (the side of my surface) could be attached to a capacitor plate while the other ends tied together in a large wire, why capacitor ? because the surface that I need the field to run through is also flat so i could form a variable cap (plate separation) which would determine the frequency and the current running in the loop would pass through the coils creating the necessary field (hopefully) ?
what do you think?@bobob , No way what ?

 

[artis]

Just to bump this thread, is there any material (that I am not aware of) that could be used as a core material that has high magnetic permeability and is still effective at very high frequency?
I am checking google and wiki but it is hard to judge by those tables, the highest frequency limits are given for ordinary ferrites which are about 1 Mhz but then I read that there are core materials for RD transformers up to 100+ Mhz, also the typical ferrites have rather low permeability of about 400-600 μ

 

[berkeman]

Be sure to check the saturation flux for those materials you are reading about. I'm pretty sure they all saturate way below what you are wanting to generate...

 

[artis]

yes that is another problem , the saturation limit. but since the frequencies involved here would be high shouldn't that somehow lessen the saturation problem ?

Then again what is the average B field strength within a powerful smps ferrite core under operation ?

 

[berkeman]

what is the average B field strength within a powerful smps ferrite core under operation ?

Just look up some datasheets. Here is a list of part numbers from one SMPS transformer manufacturer with part numbers. Search the part numbers on their website to get the datasheets, which should have the saturation flux values listed...

http://premiermag.com/pdf/pmsm.pdf

 

[essenmein]

I read the limits of typical ferrite cores and it seems that most sources claim that they tend to work somewhere up to 1 Mhz. Surely they work great under that frequency, but is there any material that could provide a core material for a wide frequency range starting from few tens of Khz up to tens if not hundreds of Mhz?
The problem is I need a high strength B field at high frequency, and making a coil with air as the core material then demands extreme kA currents through the coil in order to achieve decent (up to 1T field strengths) I was wondering can this be somehow resolved by adding a core material to increase the permeability a bit so to decrease the required current through the coil itself?

Any ideas folks?

Thanks.

Unfortunately most ferrites saturate at B max ~500-600mT, this is reasonable if you think about the proportion of amperian loops available in pure Fe vs Fe2O3. Ferroxcube has materials that work out to ~100MHz (EMI suppression materials).

Is the purpose of this coil energy storage or the 1T flux density?

I think you may find that air is the only plausible choice for the flux density you are looking for if that is the goal.

 

[essenmein]

Also if the field doesn't need to be continuous you may be able to make the kA currents fairly "easily" by building an LC tank, charge the cap, connect the coil and let it ring!

 

[artis]

well the field should need to be continuous but it's an AC field, so if you think pulse operation then no. although what would pulse mode change , the saturation limit is still the same isn't it ?

the goal is simple, have as high as practically possible flux through a 2D surface.
Air sure does the job but at the cost of enormous thick pipe coil, water cooled at best and I have to somehow supply the coil with kiloamps of high frequency current, quite a task.

 

[anorlunda]

the goal is simple, have as high as practically possible flux through a 2D surface.
Air sure does the job but at the cost of enormous thick pipe coil, water cooled at best and I have to somehow supply the coil with kiloamps of high frequency current, quite a task.

What in the world are you trying to make?

 

[essenmein]

well the field should need to be continuous but it's an AC field, so if you think pulse operation then no. although what would pulse mode change , the saturation limit is still the same isn't it ?

Yes off course its an AC field!

The pulse operation won't change the saturation limit of ferrites or the currents needed to achieve a certain flux density, what it does is allow you to create those currents relatively speaking quite easily compared to continuous operation. With Mhz of continuous operation you'd probably still want an LC tank so you could drive it with a square wave rather than having to have some monstrous RF amplifier. 1Mhz square wave drive might even be in the realm of diy if you know what you are doing.

Keep in mind if you do put in ferrites that will get you to the 500-600mT quite easily, however above that you'll be basically back to air core and quite nonlinear currents. So I would think the drive requirements for a air cored coils while higher currents are probably a bit more manageable.

the goal is simple, have as high as practically possible flux through a 2D surface.
Air sure does the job but at the cost of enormous thick pipe coil, water cooled at best and I have to somehow supply the coil with kiloamps of high frequency current, quite a task.

Unfortunately this is reality. Even in electric machines air gap flux density is not generally above 1T and that's using a lot of really good magnetic material (Bsat ~1.6-1.7T) to get there (this flux density is what basically limits the performance of machines).

So the aim is flux normal to a 2D surface? Ie the interior field of a solenoid?

 

[artis]

@anorlunda asked the question , also you @essenmein , well yes I have also noticed that ordinary transformers at low frequencies as well as motors/generators have the flux somewhere around 1T in their cores and airgaps, 1T would also suffice for me the problem is getting that 1T at those frequencies.
yes the flux should simply cut a given circular 2D area.

as for what I'm building , a high frequency self resonant or driven (depends on the setup) generator, in the self resonant case I could use the B field from the generator to power the field coil, but as long as and this is important the field coil represents a very low impedance because the generator also has very low impedance and high current but low voltage , so technically I could pump kiloamps of current continuously through the field coils (assuming they are thick enough and cooled enough for such operation) but I would need the coils to hava matching low impedance othervise I have no chance of getting that current through.

sure air sounds better because if at those RF frequencies all the available materials have drawbacks in terms of saturation etc,
but looking purely from formulas the permeability drop that air represents is drastic, my output voltage would be about 4/5 volts if I could get 1T flux in the airgap, getting that with a material that has permeability around 500-1000 would be way easier but with air I would literally have to pump 10kA through a coil to get the field and that is if my math is correct if not it might be more. Drastic.
basically a superconducting field could would be needed.

 

[essenmein]

sure air sounds better because if at those RF frequencies all the available materials have drawbacks in terms of saturation etc,
but looking purely from formulas the permeability drop that air represents is drastic, my output voltage would be about 4/5 volts if I could get 1T flux in the airgap, getting that with a material that has permeability around 500-1000 would be way easier but with air I would literally have to pump 10kA through a coil to get the field and that is if my math is correct if not it might be more. Drastic.
basically a superconducting field could would be needed.

Sounds to me like you should add more turns!

Mag field is fundamentally driven by A-t, if you need 10kA with 4-5V, you could achieve the same A-t with 40-50V and 1kA or 4-500V with 100A, by simply increasing turns count by 10 or 100.

Since you are at 1Mhz, skin depth is what a few 10's of um? build your coils out of copper tube (like refrigeration line), run non conductive coolant down the tube.

 

[artis]

1Mhz is the lower limit , the upper might be tens of Mhz and higher, at such frequencies I can't make many turns or I will run into higher impedance
Ideally I could run large currents through the coils but they have to have very low impedance for a low voltage high current source to be able to "drive" them

well higher voltage at those frequencies might not be so easy to get.
then I would have to put my output through a transformer and I'm afraid there I run into additional problems.

 

[essenmein]

1Mhz is the lower limit , the upper might be tens of Mhz and higher, at such frequencies I can't make many turns or I will run into higher impedance
Ideally I could run large currents through the coils but they have to have very low impedance for a low voltage high current source to be able to "drive" them

well higher voltage at those frequencies might not be so easy to get.
then I would have to put my output through a transformer and I'm afraid there I run into additional problems.

With such a range of frequencies you'll likely need a few different coils.

Do you have a source in mind? The lower 1Mhz might be doable with MOSFET bridge operating in quasi resonant mode (at 1Mhz minimizing switching loss will be a high priority), but much above that I think you'll be into RF amplifier land and I don't know much about those...

What are you doing with this field? More importantly are you removing energy from it? Where I'm going is I wonder if building a high Q resonant tank and then having an exciter coil so you could build up energy slowly in a large resonating system, and if you are not removing energy from it, at that point you don't have to make kilo amps, you just have to over come the system losses. You'd be limited in frequency range though, you'd have to make variable capacitors, which might not be that hard.

 

[Baluncore]

is there any material (that I am not aware of) that could be used as a core material that has high magnetic permeability and is still effective at very high frequency?

High frequency performance is not so much chemical composition as it is the dimension of the insulated particles that make up the core. The magnetic field moves rapidly through the binding insulation. Skin effect then decides velocity and depth of penetration into the magnetic particles. Deeper penetration into the larger magnetic particles results in losses because the magnetic energy cannot be recovered before the field has reversed.

Your magnet coil will need few turns to self resonate as a tank circuit at higher MHz. That will require a very high circulating current at resonance to get the field required.

You may notice that the dielectric constant of a ferrite core affects the coil self capacitance. You may then take advantage of the fact that the dielectric constant of a ferrite core material is a function of frequency. You will need to make a test capacitor with your selected ferrite as dielectric, then measure the capacitance at different frequencies.

 

[Baluncore]

To physically survive a 1T field you will need to consider the topology of a bitter magnet plate and make your RF tank circuit like that.
https://en.wikipedia.org/wiki/Bitter_electromagnet#Construction

 

[artis]

@essenmein I believe is almost getting my idea. Yes in fact no energy is being taken from the exciting B field much like in an ordinary alternator or generator where the exciting field is just there but the energy is taken from the rotational inertia and prime mover of the rotor. So the current only has to overcome the losses presented by the coil.

As for semiconductors, I don't need them I think, because the very generator itself is a LC tank circuit, so in theory I can use the large B field of the generator to power simply another LC circuit which would be my excitation field coil, providing a variable capacitor would not be a problem in order to control frequency in a given range, as already stated the largest concern for me is to have the coil with a very low impedance (preferably in mili to micro Ohm range) because my output of the generator is low voltage high current so in order to utilize a wide range of frequencies I would need the coil to have very low impedance in that range of frequencies otherwise current and resulting B field would be affected/limited.
So high current supply in my case is not a problem unless the impedance gets high.Thanks @Baluncore, I have read about Bitter magnets before , now they have high current capabilyt sure I see why that is the case but due to the large flat parallel surfaces isn't the capacitance higher than in an ordinary coil? what I'm saying is isn't the impedance of such a flat coil higher at the same frequency as with an ordinary coil? Also is the B field concentrated only in the circular opening in the middle of the Bitter coil or does the field exists also in the space of the copper itself? Because if it is in the middle mostly then I would need a very large diameter coil which would seem impractical

 

[Baluncore]

... but due to the large flat parallel surfaces isn't the capacitance higher than in an ordinary coil?

Yes. The capacitance depends on the overlap and separation between plates. But you can make a transmission line resonator rather than a coil and capacitor.

The magnetic field is concentrated in the centre hole. You can have a strong field or a large area.

For the fields you are demanding a normal wire coil will be forced outward and stretched by the field. How will you cool the coil?

 

[essenmein]

I don't know for sure but it looks like bitter magnets are mainly DC?

As mentioned the inter winding capacitance would make this difficult to work at higher frequency.

Looks like basically a helical coil made of individual plates? Sort of like how some planar transformers are build?

 

[artis]

Well the bitter coil is basically a very thick flattened wire wound in the shape of a coil with holes to allow for cooling to run through. but as already mentioned the capacitance would be very great compared to other coil types.
Also the reported field strengths with bitter magnets seem huge (record about 45T) but I think @essenmein is correct in the assumption that it is a static DC field with static DC current through the coil? maybe low frequency AC, also the problem is that it seems the field is concentrated in a rather small circular area essentially the middle of the coil, which doesn't suffice for my application sadly.

@Baluncore can you say more about the transmission line resonator ? I am googling it as we speak but any additional input would be nice.

 

[Baluncore]

You must trade area for field strength = lines per square metre.
The strongest field requires the smallest possible area.
You must have amp * turns.
You must wind a conductor around the field area.
Resistance, R is proportional to number of turns. Losses are I squared * R.
Weight of conductor? One thick turn = R, or two turns, half as thick = 4 * R.
You want a higher current, so you need a lower impedance.
Transmission line; Z=√(L/C). High current, implies higher C, lower L.
A coaxial line has higher capacitance than a ladder line, but still works at RF.

Inductance, L is proportional to number of turns squared.
At RF you must reduce turns, so you can have higher C, for more amps.
Magnetic self-forces on an inductor are square law, so rapidly become destructive.
Stray self capacitance is lower when inductor sheet currents are edge-on.

So don't dismiss a one or two turn bitter magnet plate as a tank circuit.

 

[artis]

but since my area is circular and the strongest field would actually be needed closer to the circumference while at the very center where my rotor axis is I could have no field at all , maybe I can compose my coil like an arrangement of multiple smaller coils all switched parallel.

the thing that maybe helps here to minimize the inductance is this that for each coil I can connect one end of each coil/coils to a flat conducting surface which would be one plate of a adjustable capacitor , the other plate would be located on my rotating part, this way I both get the B field running through both plates which I need (copper or aluminum permeability is the same as air so they won't distort the field lines I assume)
and at the same time I can build maybe a very thick and powerful electromagnet?



maybe arrange the coil like the axial shaped stator coils found in motors like the one in the link, only make each coil from a single thick but hollow copper strip or bar instead of multiple turns like for DC.

another weird shape i found is this
https://www.google.com/search?biw=1....0j0i24j0i30.qvg_1doNUZ8#imgrc=Io8qGh3hU6ZH0M:

would such a coil formed from a single thick turn bent in this shape form a homogeneous B field ?surely the impedance should perfectly match the impedance of my generator output which is very low , around 1 ohm and less in order for me to be able to push maximum current through.

 

[Baluncore]

... , the other plate would be located on my rotating part, this way I both get the B field running through both plates which I need (copper or aluminum permeability is the same as air so they won't distort the field lines I assume)
and at the same time I can build maybe a very thick and powerful electromagnet?

You cannot move a conductive plate through a magnetic field without causing eddy currents in the plate, which partially cancels, and so distorts the field.

 

[berkeman]

surely the impedance should perfectly match the impedance of my generator output which is very low , around 1 ohm and less in order for me to be able to push maximum current through.

Sorry, I haven't been following this thread for a bit. What is the energy input into your generator? What is the energy source, and what is the purpose of using this generator?

 

[essenmein]

You cannot move a conductive plate through a magnetic field without causing eddy currents in the plate, which partially cancels, and so distorts the field.

Even more so since it is an alternating B field at >1MHz

Any conductive material in this alternating B field will be consuming energy from this field, basically they will act like a shorted turn in a transformer.

I don't know what kind of simulation tools the OP has access to but I'd be throwing some geometries into HFSS or Maxwell and see what it does, its pretty hard to mentally imagine what these fields would do with complicated geometry...

 

[Baluncore]

Any conductive material in this alternating B field will be consuming energy from this field, basically they will act like a shorted turn in a transformer.

Indeed, an AC magnetic field will be blocked and reflected by eddy currents in a conductive plate if the plate is thicker than the skin depth at the frequency of the AC field. At 1 MHz aluminium and copper have a skin depth of less than 0.1 mm. I would expect induced eddy currents to quickly melt a thin metal shim.

Aluminium foil is stamped into shape for food packaging by the use of a magnetic pulse that induces currents in the foil and so propels the foil against a mold.

 

[artis]

Ok fair points about the metal plate blocking my B field, I was just hoping I could utilize two discs , one would be the disc in which the rf current is generated while the other could simple be a capacitive disc that serves as both coil end plate and also as one more turn for my generator.

yes i guess i need some simulation software these geometries that I have thought up are rather complex with respect to B fields as there would actually be two fields, one would be the excitation field the other would be a toroidal field created by the generator itself, this second field would be the one from which I would extract the RF output power.
well it seems I can use only one disc or plate for a given B field in order for the disc to be able to cut the field lines and the flux density be high enough. I frankly still have little clue as to how I could make up this electromagnet for the excitation field.
I basically need two things, I need the coil impedance to be very low in order to achieve high currents and I need the field to be perpendicular to my rotating surface, well and strong enough (high flux density) that's about it.@berkeman simple, input power is an electric motor driving a shaft, further down the line lorentz force acts on electrons due to the presence of a B field and I collect my output.

 

[berkeman]

and I collect my output.

What's the output? Another shaft? Or RF EM?

 

[artis]

RF EM, why would the output be another shaft? (whatever that means I'm not sure)

 

[essenmein]

Are you making the RF with the generator? If so inquiring minds would love to know how many poles and what rpm you are spinning at to get 1-30MHz electrical frequency out!

 

[artis]

This is the fun part guys, I can understandably sense some irony @essenmein in your last post, the generator doesn't have any poles or pole pairs, it's not a synchronous or induction or any other type of commercial type of generator you would see in a power station.

I know this sounds lame but i thought some of you already knew what type of machine I'm building (trying to) based on all my threads here.
i'm using a conducting disc (Faraday disc) , due to Lorentz force the force felt by the electrons in the conducting disc is directly proportional to the rpm and B field strength so changing the B field strength at any time instant will also change the current/voltage amplitude in the disc, in other words the output directly follows the B field input which means that I'm getting out an amplified input signal, no poles no induction just Lorentz force.

Well induction would come in at other parts of the machine obviously since I'm using AC but the main generating part uses Lorentz force and Special relativity as it's theoretical working principle.
PS. back to the topic, so at Mhz frequencies my rotating surface should basically be very thin as using a thick one would be wasting metal as the B field would be stopped at the "skin depth" at the particular frequency ?

@Baluncore, maybe a stupid question but why do you think eddy currents would form because remember that my field is homogeneous and the conductor is fully immersed in it , so all rotating parts see the same field going in the same direction which should push electrons in a single perpendicular direction , current loops shouldn't form here , so that leaves me to the question would such a rotating conducting material in a homogeneous B field at 90 degree angle to it experience the so called skin depth or would the field be able to pass through , because the field at any given instant of time can be thought of as a DC B field if frozen in time while the disc spins perpendicularly to the field and due to Lorentz force sideways electron current arises.
well maybe I'm wrong just thinking..
Because as far as I know in order for there to be eddy currents opposing the applied B field the B field either needs to cut the same conductor in opposite directions on the same plane (as when dropping a magnet through a copper pipe) or the B field needs to only cut part of a conductor leaving other parts field free where return currents can form loops, but when the field only cuts the conductor once and everywhere is the same there shouldn't form loops of current instead there should be a single current direction driven by the lorentz force.

 

[Baluncore]

... but when the field only cuts the conductor once and everywhere is the same there shouldn't form loops of current instead there should be a single current direction driven by the lorentz force.

Is your applied B field AC or DC?

 

[artis]

AC , just that it is homogeneous , in other words covers the whole rotating area, so in theory all electrons being on the same radial distance from the center feel the same force and direction no matter where they are located on the disc, the flux of course is also everywhere in the same direction at any given time instant and when it reverses it reverses all together.as far as I'm aware the Lorentz force principle on a moving conductor cut by flux is the same both for static and time changing B fields.

 

[Baluncore]

AC , just that it is homogeneous ,

As I see it, the reversing B field induces a peripheral current in the disc.
The disc becomes a single shorted turn.

 

[artis]

I am not entirely sure what you mean by "reversing B field and peripheral current"
the disc is essentially a single turn , and given it's extremely low resistance we can consider it "shorted"
all that matters is that all around the disc there is at some point between the center of it and the circumference a field which is homogeneous so that electrons get accelerated from center to circumference or vice versa and so a potential develops between rim and center.So the question becomes in this scenario are there eddy currents ? To the best of my understanding there shouldn't be eddy currents not even with an AC B field , if this is true then what changes in the so called skin depth of the B field passing through the conductor in this case? Because as I understand this , this is different from an EM wave in a waveguide or wave reflecting on a sheet of foil where circulating currents form due to the field looping within the conductor, here we only have flux , B field lines cutting a conductor even though the field changes at high frequency but at any given time instant the flux through the disc is homogeneous and only in one direction, so electrons in the rotating disc get deflected altogether either one way or the other. Is this reasoning holding water?

 

[essenmein]

I believe what you are proposing is commonly called a hysteresis brake.

 

[Baluncore]

The artis hypothetical model has excessive complexity in that not only is the B field alternating, but the disc is spinning. We can reduce that complexity since the B field alternates much faster than the disc rotates. What happens if the disc is stopped? It then comes down simply to the thickness of the disc compared to the skin effect depth at the B field frequency.

If the disc is thick, the B field will not be able to penetrate the conductive material because the induced ring current around the disc will cancel the incident field and so reflect the B field energy.

If the disc is thin, then induced currents during B field reversals, multiplied by the frequency of the reversals, will melt the disc.

 

[artis]

I feel that you have gotten this wrong, let me explain,
1) there aren't any disc rim/circumference currents around the edge of the disc because the whole edge is covered in the same flux going in the same direction as the middle of the disc, the return flux comes back outside the conducting material either loops through surrounding air or a higher permeability core material (depending on application/frequency etc)Secondly, I just set up a small experiment , I attached a brass disc with glue to an old brushless DC fan motor. I spun the disc , when i place a toroidal speaker magnet close to it there is no braking of the disc whatsoever, then i place a small neodymium magnet that covers only one small plane of the disc leaving the rest of the disc uncovered and now (due to eddy current forming loops creating an opposite field) the disc brakes down and spins much slower.
this is why the field homogeneity is key here, if the field is the same at each radial point in the disc all around it then the disc performs as if there would be no field at all, then the only braking effect can be made if the circumference is electrically connected to the center axis forming a current loop where Lorentz force can drive the electron current and this then becomes the Faraday disc. If the Faraday disc is placed within a proper homogeneous field there in theory should be no eddy currents forming in it.

So having this in mind , would the AC magnetic flux really stop at skin depth in this case or would it penetrate the copper as it normally would since the permeability of copper itself as metal is the same as that of air?
Because the way I see it the only reason why a changing flux can't penetrate metal with respect to static flux is because of the formation of eddy currents that oppose the incoming field, correct?

 

[Baluncore]

Because the way I see it the only reason why a changing flux can't penetrate metal with respect to static flux is because of the formation of eddy currents that oppose the incoming field, correct?

Yes, eddy currents oppose and delay changes in the magnetic field through conductors.

Mount your disc on a vertical axis that is parallel to the lines of the B field. Forget disc rotation. Now generate a 1 MHz current and use it to drive a single turn loop in the same plane as, but now just outside the disc. That creates your alternating B field. You clearly have an air-core transformer with the disc as the secondary, but that disc is a shorted turn.

Imagine lines of B, passing through a copper coin. As the B field increases each line will induce a current circulating around that line. The currents between adjacent B lines flow in opposite directions so they cancel. The only currents that do not cancel make up the peripheral ring current, your shorted turn.

 

[artis]

Ok , I visualized and now see your point, fair point.
Right I see that the disc (aka the shorted turn) against the B field is stationary in electrical terms even though it might rotate so it will get induced current both whether it stands still or rotates.

Ok one remedy might be to cut one or two thin and long slots in the radial direction so that they end at some minor radius close to center, this should make any large summed circumferential currents impossible and only radial Lorentz force currents possible, is this a sound reasoning?

but one question though, even without the slots with the coin analogy that you made, if the coin is fully within the flux so that even the circumference is covered , how can the circular current form because the field lines are everywhere the same and as you said yourself the small loop current that each line makes cancels due to the lines all having the same direction.?
If the middle would be empty like in a loop of wire the flux loops through the loop and each field line sums up to form current in the loop but here the disc is as you said shorted so the individual field lines can't make net effort as the individual small currents destroy one another as you said yourself ,

 

[Baluncore]

Ok one remedy might be to cut one or two thin and long slots in the radial direction so that they end at some minor radius close to center, this should make any large summed circumferential currents impossible and only radial Lorentz force currents possible, is this a sound reasoning?

If you break up the area of the disc with a tree of radial cuts then the field will propagate through the gaps at close to the speed of light. The field will reach the conductor faster from those cuts than it would without them. The disc can be much thicker if the thin cuts are separated by two skin depths. That is analogous to the way the laminations of a transformer function, they have a thin insulator between them that provides a path for the magnetic field from the windings to quickly reach the full surface area of the laminations. So that the entire volume of the core can be reached, the transformer laminations, or the particles in a ferrite are smaller than twice the skin depth at the operating frequency.

but one question though, even without the slots with the coin analogy that you made, if the coin is fully within the flux so that even the circumference is covered , how can the circular current form because the field lines are everywhere the same and as you said yourself the small loop current that each line makes cancels due to the lines all having the same direction.?

The circular eddy currents in the conductor only cancel if they are next to another line that penetrates the conductor. The line inside the edge has an eddy current, but the line just outside the disc does not, so there is an imbalance that makes the peripheral current.

If the middle would be empty like in a loop of wire the flux loops through the loop and each field line sums up to form current in the loop but here the disc is as you said shorted so the individual field lines can't make net effort as the individual small currents destroy one another as you said yourself,

The best conductors make the best magnetic mirrors = magnetic insulators. It does not matter if the disc is replaced by a ring, the peripheral current will still be the same in the ring. If the flux that passes through the ring changes, it will induce a loop current in the ring. The middle of your disc is irrelevant.

 

[artis]

Even though this does not directly relate to my case let me ask, are you saying that if I have a wire loop through which a certain amount of flux passes through that only the flux closest to the physical wire induces current in it while the flux that is at the center (furthest away ) contributes nothing to the induced current?Ok so judging by your statements I can conclude that it's only the circumference part of the disc that has the potential to create the circumferential current in the disc, so my slots shouldn't run far down the radial direction just say for a 30cm diameter disc they would have to be some 1/2cm long measuring from the rim? Because deeper down all lines are in the same direction and any current loops should be canceled before they can form ?Two skin depths at Mhz frequencies sounds like an awfully small number , I don't think it's practically possible to cut the circumference or any part of the disc wth slots that close to one another or am I getting something wrong here?

 

[Baluncore]

Even though this does not directly relate to my case let me ask, are you saying that if I have a wire loop through which a certain amount of flux passes through that only the flux closest to the physical wire induces current in it while the flux that is at the center (furthest away ) contributes nothing to the induced current?

No. All flux that passes through a loop induces current in the loop. Eddy currents flowing in a sheet conductor follow the periphery of the sheet.

Ok so judging by your statements I can conclude that it's only the circumference part of the disc that has the potential to create the circumferential current in the disc, so my slots shouldn't run far down the radial direction just say for a 30cm diameter disc they would have to be some 1/2cm long measuring from the rim? Because deeper down all lines are in the same direction and any current loops should be canceled before they can form ?

Cutting short slots just moves the peripheral current inwards slightly. One slot at least must run to the disc centre and form the trunk of a radial tree of slots that block the induced eddy currents.

Two skin depths at Mhz frequencies sounds like an awfully small number , I don't think it's practically possible to cut the circumference or any part of the disc wth slots that close to one another or am I getting something wrong here?

A numerically controlled 'Wire EDM' could fill a 6” (150mm) THICK copper disc full of 0.020” (0.5mm) wide slits. A UV laser could do the same to a much thinner disc.

I don't think you should be worrying about construction until you have actually modeled the device and shown that it would work. It is cheaper and easier to optimise the design in a simulator than in a workshop.

 

[artis]

Well I would want if possible only one radial slit that goes halfway or more to the center of the disc because having more of those would disturb the Lorentz current created in the disc because the current is not straight radial instead it bends like a snake from the center somewhat like a cork screw until it reaches the rim so having too many long radial slits would disturb it and cause losses.

Well sure we have fancy CNC laser cutting etc but the thing that worries me here is that the disc has to be spinning and at rather high rpm, I'm afraid it will simply not hold together and break by the centrifugal forces when it will be cut into the slots.
Why such a thick disc as 150mm or is that just an example? and what you mean by saying the disc full of 0.5mm slits, so the 0.5mm is the slit width but what is the distance between each adjacent slit and how far do you think they should run say percentage wise since disc diameter can vary. As in percentage with respect to disc radius.

Sure if it were just one vertical slit that goes through the radius (although that is also bad) but here , well maybe the best way to do it is to create multiple thinner discs each having two or three vertical slits that run say 2/3 radius length and then join the discs together in such a way that each next disc covers the previous disc in such a way as to join the slitted part so that no two discs have their slits lining up and creating a weak point. each disc could be laminated and then joined together by copper or other metal rivets under pressure, since all discs have the same radius the rivets could be conductive because the potential would be the same at that place on each disc. this is the best I could come up with thinking about it now.

 

[Baluncore]

Why such a thick disc as 150mm or is that just an example? and what you mean by saying the disc full of 0.5mm slits, so the 0.5mm is the slit width but what is the distance between each adjacent slit and how far do you think they should run say percentage wise since disc diameter can vary. As in percentage with respect to disc radius.

150mm is an example beyond your requirements.
Transformers have up to 10% insulation between laminations. You could have 10% cuts so no conductor was more than 5 cut widths away from air. Alternate cuts go deeper than others so as to cover the area with skin depth metal.

Many before you have discovered that their idea is impractical. There is a reason why you cannot buy one of these on the web. The problem here is not machining, but is to simulate a model design that will work, or to understand why it cannot.