How to calculate the wind speeds produced by an ionic thruster

[Jon Doe]

So I am working on a paper where I discuss the above-mentioned issue. The situation I end up with is that I need to find the speed produced by an ionic thruster functioning at the atmospheric level (thus drag on the sped-up particles can not be ignored). The thruster is operating with 712 volts and the gap between the electrodes is 20mm (the electrodes form a uniform electric field).

My current theory is given in images below:

 

 

 

The issue is that my teacher says it's wrong and thus can not be in my final draft but I don't know what the issue is... My calculations with the given numbers are also an image below:

 

Seriously any help will be appreciated!