A method for generating an ion beam in an ion implantation process is provided. The method includes supplying a working gas into a first portion of an arc chamber which is separated from a second portion of the arc chamber by an intermediate plate. The method further includes guiding the working gas into the second portion of the arc chamber via a plurality of gas outlets formed at two opposite edges of the intermediate plate. The method also includes generating an ion beam from the working gas in the second portion of the arc chamber.