The mechanism of ion extraction from flowing plasma using an external electrostatic field is investigated experimentally and computationally. The plasma is generated by electron impact ionization of a copper atomic beam. The plasma, along with the atomic beam vapor, flows upward and ions from the plasma are extracted using an external electrostatic field set up by a pair of parallel-plate electrodes. The extraction mechanism is studied using in-house developed hybrid 2D Particle-In-Cell (2D-PIC) code. The experimental results are found to be in good agreement with the computationally obtained values.