I would like to build an electromagnet to accelerate electrons into a circle (a precursor to a mass spectrometer). I would like to calculate the field strength, but I don't know how because I would like to wind it out of aluminum strap salvaged from a welder, and I can only find formulas for solenoids (and I don't fully understand how the formulas are derived, and therefore can't find the formula myself). The core resembles an E core, but with a large air gap in the center leg where the electrons (and hopefully ions eventually) will pass through. The middle leg is also round, while the others are rectangular. I can supply plans of the core if need be.