how could i make a linear electromagnetic field to a great distance
What exactly do you mean with "linear"?
Make a large, strong source. The question is way to broad for a more specific answer.
A Electro magnetic field that is in a straight line that could go to a great distance
Are you talking about something like a laser?
no a electromagnetic field
"A electromagnetic field" is extremely vague. You're going to need to be more specific if you want help.
something like this.that could work in open air
No, nothing like that can be built to create an electric or magnetic field that doesn't fall off in strength over such a large distance in the open air. You'd need a large tunnel filled with electromagnets in order to generate a 1 km long magnetic field, and I'm not sure it's even possible to do something similar with an electric field.
What about Hall effect thruster
A linear electromagnetic field might be interpreted as one that did not fall off with distance. When radiated through space from a point source, the field will fall off as the inverse square of the distance. By confining the EM field to a waveguide, a 'linear' field can be achieved. A waveguide is simply a long skinny cavity bounded by metal.
A Hall effect thruster is still rather short range, you're probably better off trying a high energy cutting laser as a guide for pulsed em waves (I say a high energy cutting laser because it should have a slight atmospheric blooming effect) but I don't know if it would work at a distance or even work at all.
EM pulse will not follow ionized channel. Ionized channel will absorb the EM wave and simply convert it to heat. However, if you were to make a ring of lasers that could ionize a cylindrical shell maybe 10 cm in diameter extending to outer space, then the interior might be ionization free for a few milliseconds. We would have a conductive waveguide that could efficiently couple an EM pulse of comparable wavelength to 10 cm, or about 3 GHz. Once the EM wavefront left the far end of the tube, it would then obey the law of squares in falling off in intensity.
I didn't think about the conversion of the em wave to heat but the laser may work.
That is assuming the lasers don't absorb and convert the em wave before it reaches its destination.
To get a field that is not completely different from your sketch, you would need objects significantly larger than 100m on both sides (~500m is better). It is possible, but you would have to construct something like multiple large towers.
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