No abstract

A technique was developed to measure the desorption time for ion emission from a heated surface using phase-sensitive lock-in detection of modulated alkali metal atom beams. This technique is described, together with the derivations of equations which relate the desorption time to experimental parameters. The relationships obtained show the dependence of the amplitude and phase of the ion emission ac signal upon the desorption time (i.e., the mean time that adsorbed particles spend upon a heated surface before being emitted as ions.) The measurable effects upon the signal as the desorption time increases are the attenuation of the amplitude and the phase shift of the modulated wave. A description of ac ion emission characteristics is included along with the results of ac modulation measurements of cesium and rubidium ion desorption times from tungsten at various temperatures. The ac modulation technique can be used to determine other mean times involving particle-surface interactions and in general the effective time constants for the passage of ac signals.

An ion source that utilizes a continuously oxygenated porous tungsten surface to generate indium ions is described. Indium vapor supplied to the back of the tungsten ionizer from a reservoir percolates through to the front surface where the indium is evaporated as ions. An accel-decel electrode extraction system is used to increase the ion current capability of the source. The ionizer and reservoir are designed to operate at temperatures in excess of 1000°C and at voltages up to 20 keV. The peak ion current attained was 0.57 mA at an ion energy of 10 keV.

No abstract

The concept of a self-rectifying engine is based on the fact that a probe immersed in a plasma forms a quasi-rectifying contact with the plasma. A cesium bombardment engine was modified by replacing the cylindrical anode with multiple anode segments. These segments were driven from a center-tapped transformer with the center tap connected to the engine cathode. Alternate segments functioned as the anode when they were positive with respect to the plasma. This maintained the discharge and a continuous ion beam was extracted. The efficiency of this engine was found to be the same as that of the engines previously used on the 3700 and 8200 h life tests. The necessary circuitry for starting and power control was determined, and a method for automatic discharge power control was developed which maintained engine operation at maximum efficiency while correcting for input line voltage variations as well as shifts in engine performance. Excellent stability was achieved by using an error-integrating feedrate control loop. Turn-on time from standby to full thrust was less than 60 sec.