It will depend on the degree of ionization you require, as well as the pressure of the air passing through the ionization region. Your gas will not likely be at STP if the air is moving through a reducing tunnel.
Ionization and breakdown (the common name for the process of forming a plasma by ionization) occurs more easily at lower pressures (so a quickly moving flow will be easier to break down) and can be initiated by DC voltage, RF, microwave, laser power, or sufficient heat. In terms of efficiency, microwave frequency power will cause breakdown at lower power levels than RF or DC, but the equipment is in general more expensive. It can also be a complex engineering problem to deliver sufficient microwave power in the right configuration.
DC discharges are less efficiently generated but are relatively simple: apply 3 kV across a 1 mm gap and you should be near the threshold of breakdown. You can get very high temperature gas this way, but your anode and cathode will get ablated.
The more strongly your gas is ionized, the more power will get generated by passing through the magnetic field.
In general you're going to spend a lot of power ionizing the air. For large flow rates, you will require thousands of watts. You will likely recapture a very small portion of it from running it through a magnetic field. I think it is important to realize you will need to pump more electrical power into the system than you will capture from it.
The wikipedia page for Paschen's law, which shows the curve of applied electric field to pressure required to break down a gas (for DC), is a good starting point: https://en.wikipedia.org/wiki/Paschen%27s_law
If you're interested in reading more, I recommend "Gas Discharge Physics" (back in print!) by Yuri Raizer. Sanborn Brown also has a good book, likely in most large university libraries, "High Frequency Gas Discharge Breakdown," which is pretty readable.