Fig.3 shows the experimental setup for investigating the collection efficiency of the ion collector. It consisted of an ionizer, a DC voltage power supply, a Faraday cup, an electrometer, a high efficiency particulate-free air (HEPA)filter, and a flow system. A high concentration of ions was generated by corona discharge with the corona-needle ionizer developed by Intra and Tippayawong [14,15] as shown inFig. 4.