MEMS Electric-Field Probes for Laboratory Plasmas

This paper presents microfabricated sensors for directly measuring fine-scale plasma parameters in typical laboratory plasmas. Microfabricated probes have the potential to significantly advance basic plasma physics by enabling the measurement of fundamental processes under controlled conditions. Historically, the spatial scales of the finest electromagnetic-field fluctuations in laboratory plasmas have been too small for conventionally fabricated tools to sense. The new probes are arrays of electric-field sensors for measuring Debye-scale structures in the Large Plasma Device (LAPD) at the University of California at Los Angeles. Typical Debye lengths in the LAPD are 25 mum, with electron gyroradii of about twice that value. The probes are constructed of polyimide, chrome, and gold; have electrode widths ranging from 8 to 20 mum; are 23 mum thick; and are spaced 40 mum apart. The probes are wire bonded to a printed circuit board with commercial amplifiers, and the ensemble is placed inside the plasma chamber. The frequency response of the measurement system extends to 1 GHz. The probes have been used to measure the electric fields of interesting structures in the LAPD.