Charged plasma and ion grid in interaction in ion thrusters I was just wondering ..... 
In this Image of an ion thruster, when the positively charged particles pass through the grids, wouldn't they just bombard the negatively charged grid(a fraction of them). This means that there must be a constant adjustment to maintain the potential difference between the grids. Is this the reason for the high energy consumption of these engines (along with ionization..)?
 A: A good high power ion thruster uses a lot of energy to accelerate ions to high energies.
The image does not include any power source, which is a serious problem if a person looking at the image wants to understand how ion thrusters work. 
A simple ion thruster woks like this:
A small amount of energy is used to ionize a bunch of atoms, then a much larger amount of energy is used to move some of the electrons away from the plasma. Now the plasma is a positively charged plasma, from which positive ions tend to fly off. If there's some negative object nearby, it accelerates the approaching positive charges and decelerates the positive charges that are moving away, so the negative object does not really do anything to the ions that move past it, but it may prevent ions flying off into the opposite direction. 
A: You say "constant adjustment" as if you expect a motor with a PID controller to be spinning cat's fur on the grid to keep its charge ;P
The grid keeps it charge with 1.) a battery and 2.) by connecting both electron guns' filament to the positive grid. An electron gun is a special filament that when heated (by passing current through it) expels electrons.
As the gas ions are passing by the negative grid, the electrons are jumping off and neutralizing the gas as it exits.  This is just part of the circuit under normal operating conditions - the Thevenin equivalent voltage of the circuit would be such that it's adequately charged at all times.
Energy consumed by the system is 1.) work done to accelerate the ions and 2.) the work done to heat the electron gun filament.
Here's the circuit:

