I'm working on a VAN DE GRAAFF generator and I need to connect the bottom comb to the earth ground. But in my country we do not have a ground line . Can i use the neutral line instead and connect the bottom comb to the neutral ?
The bottom comb is biased against the bottom housing, and for shock safety it is usual to ground the bottom housing (the base) and consider the top (ball) housing to be the 'live' electrode. That's only a convention, however; electrostatics was practiced for years before grounding stakes and house wiring existed. It is not generally safe to connect your neutral wire directly to the comb, nor to the base. This is because wiring of buildings is not immune to confusions (like, misidentified neutral wires).
Connecting the base through a resistor (1 Mohm) to any available ground is good practice; that's how technicians' antistatic workstations are set up.
If the motor in the base works on AC power, maybe an indirect neutral connection is appropriate There are safety-approved components that can connect the base to the neutral wire, which are usually reliable mega-ohm resistors or resistor/capacitor combinations. These were used commonly at the antenna terminals in inexpensive hot-chassis cathode-ray tube televisions.
Actually you do not need to connect the bottom of the Van De Graaff generator to ground. But you will need to complete the circuit from the top ball to the bottom in order to get fun sparks.
For example if you use a soup ladle to get a nice spark from the top then the handle of the ladle should be connected to the bottom.
A very convenient sort of conductor to use for connecting the bottom to experiments is a ball-chain. You would never use a ball-chain as a wire for low voltage but it works very nicely for high voltages.
That would not be a good idea because you would most likely end up with a broken machine. Since outlets are AC, the "negative" terminal will be positive for the amount of time it is negative. This switching depends on your country. In the US, it's 60 times a second, in Europe it's 50. So not only would it not be a good ground source, you could end up breaking the components in the machine /get some (unintended) nasty shocks. It would be better to simply take a metal rod and shove it in the ground and connect the ground line to that.