Compact cantilever force probe for plasma pressure measurements.
A simple, compact cantilever force probe (CFP) has been developed for plasma pressure measurements. It is based on the pull-in phenomenon well known in microelectromechanical-system electrostatic actuators. The probe consists of a thin (25 mum) titanium foil cantilever (38 mm of length and 14 mm of width) and a fixed electrode separated by a 0.75 mm gap. The probe is shielded by brass box and enclosed into boron nitride housing with a 9 mm diameter window for exposing part of cantilever surface to the plasma. When the voltage is applied between the cantilever and the electrode, an attractive electrostatic force is counterbalanced by cantilever restoring spring force. At some threshold (pull-in) voltage the system becomes unstable and the cantilever abruptly pulls toward the fixed electrode until breakdown occurs between them. The threshold voltage is sensitive to an additional externally applied force, while a simple detection of breakdown occurrence can be used to measure that threshold voltage value. The sensitivity to externally applied forces obtained during calibration is 0.28 V/microN (17.8 VPa for pressure). However, the resolution of the measurements is +/-0.014 mN (+/-0.22 Pa) due to the statistical scattering in measured pull-in voltages. The diagnostic temporal resolution is approximately 10 ms, being determined by the dynamics of pull-in process. The probe has been tested in the tokamak ISTTOK edge plasma, and a plasma force of approximately 0.07 mN (plasma pressure approximately 1.1 Pa) has been obtained near the leading edge of the limiter. This value is in a reasonable agreement with the estimations using local plasma parameters measured by electrical probes. The use of the described CFP is limited by a heat flux of Q approximately 10(6) W/m(2) due to uncontrollable rise of the cantilever temperature (DeltaT approximately 20 degrees C) during CFP response time.
Nedzelskiy
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Centro de Fusão Nuclear, Instituto Superior Técnico, Associação EURATOM/IST, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. igorz@cfn.ist.utl.pt
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