A method for determining plasma density by use of a solenoid is proposed. The calculation of the distributions of the electric and magnetic field in a homogeneous plasma placed in the solenois is made and the current flowing in the solenois or the impedance looking into the solenoid is expressed in terms of the plasma density. The method is to apply a radio frequency voltage across the solenoid and to measure the impedance by either a Lissajous' figure on a X-Y scope or an impedance bridge.
Upon measuring the magnetic field distribution and axial-electric field in a coaxial plasma gun, we could obtain the charged particles density distribution in the gun.