Magnetic Lens

Quote; "One could completely do away with ferrite cores and use only air core- but coils become bulky and need to carry very high current".

When there is no core, flux density will be low, hence for compensating apply current of high frequency and this is what practically being done.

Coreless high frequency induction furnace has reduced weight and increased flexibility.

You are right in saying that at present air-cored coils are being used - especially in long tube welding.

But using any kind of a core gives better flux control. You can operate at lower frequency. Higher frequency also leads to higher EMI. I was actually wondering whether arc welding / tig welding / laser cutting could be done with such sharply focused induction frequency torches. Total air core coils will result in lower efficiency lies, higher leakages.

Do you have an analogy to equate EMI with flow of gases ?

Most probably your query relates to Plasma arc welding, the functions you described would be fulfilled.

Check Wikipedia or google out Plasma arc welding.

It sounds like you're looking for a flux concentrator. For induction heating, these use various magnetic materials such as metal laminations (low frequencies), and at high frequencies, combinations of iron dust and a dielectric binder. The latter materials can reach significantly higher flux densities than ferrite alone can achieve - typically 1T or so. Here's a commercial example:

http://www.fluxtrol.com/index.php?action=product.ferrotron_559h

Unfortunately, even the best ferromagnetic materials saturate at about 1.9 T, so other techniques must be used to achieve 5-10T fields.

For example, specially shaped flux concentrators are often used to intensify pulsed magnetic fields in Electromagnetic Forming by means of concentrated eddy currents induced within relatively massive slotted shapes of copper or silver. For example:

http://www.google.com/patents?id=5bJNAAAAEBAJ

Perhaps this technique could be applicable to induction heating...

Thank you for those leading references. Fluxtrol seems to be an interesting material - capable of operating at high frequency with Bmax = 1 T.

The magnetic lens patent of 1966 also made interesting reading. Yes I am conceptualising a magnetic lens to be developed - hoping to find an application in daily life like - alternative to laser cutting or tig welding etc. In air - one could go to much higher Bmax by using magnetic lenses. As we know efficiency of lasers is very low, but concentrated beam power is very high.

Thank you for the very interesting hint to extract more energy from available material. Steel has limitations of operating frequency.