An experiment to measure the charge on dust grains in a plasma

Summary form only given. To measure the charge on individual grains, an existing double-plasma machine (30 cm dia /spl times/ 60 cm) has been modified to allow the introduction of dust grains through a small diagnostic porthole on top of the chamber. Typical plasma parameters in argon are n/sub e/ = 10/sup 8/ - 10/sup 10/ cm/sup -3/ and T/sub e/ = 3 eV. The charging time of the dust is short compared to the transit time to the bottom of the chamber. The charge on exiting grains is measured by a Faraday cup and a sensitive electrometer located just below the chamber. The integrating electrometer uses field-effect transistors in a cascode configuration and without feedback resistors or input resistors. The leakage current is below 0.1 pA and the accumulated leakage charge is periodically removed by a reed relay. A typical dust grain with 6000 electrons should generate a voltage step of 300 microvolts. The noise voltage within the 10 kHz bandwidth is a few microvolts; thus the charge on a grain should be determined with an uncertainty of about one percent.