A DOUBLE PENDULUM PLASMA THRUST BALANCE AND THRUST MEASUREMENT AT A TANDEM MIRROR EXHAUST

For the purpose of measuring the plasma momentum flux in a plasma syst em, a highly sensitive and precision balance has been developed. It ca n measure a force, an impulse, or thrust as low as 0.1 mN free of mech anical noise, electrical and magnetic pickups. The double pendulum sys tem consists sf two parallel conducting plates. One or both of the pla tes can be suspended by needles. The needle suspended plate (or plates ) can swing freely with negligible friction because of the sharp point s of the needles. When one of the plates is impacted by an impulse it will swing relatively to the fixed plate or other movable plate. The c apacitance between the plates changes as a result of such a motion. Th e change of capacitance as a function of time is recorded as an oscill ating voltage signal. The amplitude of such a voltage signal is propor tional to the impacting force or impulse. The proportional factor can be calibrated. The forces can thus be read out from the recorded value of the voltage, The equation of motion for the pendulum system has be en solved analytically. The circuit equation for the electronic measur ement system has been formulated and solved numerically. Using this ba lance the thrust at the exhaust of a Tandem Mirror plasma thuster has been measured. The analytical solution of the overall characteristics agrees greatly with the measurement. (C) 1995 American Institute of Ph ysics.