The emission characteristics of a point cathode thermionic electron gun have been studied with numerical method. The method is based on the integral form of Poisson equation. The field including the space charge is obtained by the iterative procedure, consisting of the field calculation, the electron ray tracing in 3D, and the determination of the space charge from the rays. The space charge distribution was determined from the rays traced with random emitting conditions considering energy and angular distributions and treated as many coaxial ring charges. The use of random emitting conditions leads to a self-consistent result after several iterations. The emission currents were calculated for different bias voltages and different cathode temperatures. The method provides the emission currents close to the measured values.

Space charge in a point cathode thermionic emission gun has been studied with numerical method. The method is an iterative one, consisting of the field determination using the integral form of Poisson's equation, the direct ray tracing in 3D, and the estimation of space charge from the traced rays. The rays are traced for limited emitting conditions. The estimated space charge is treated as a number of the coaxial charged rings with different radii and positions. Using the method, the potential distribution around the cathode tip was examined at different cathode temperatures. The numerical results at the cathode temperatures of 2800 K and 2950 K are given, and the influence of space charge is discussed.

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Point cathode electron gun with high brightness and long cathode life has been developed. In this gun, a straightened tungsten wire is used as the point cathode, and the tip is locally heated to higher temperatures by electron beam bombardment. The high brightness operation and some findings on the local heating are presented.Gun construction is shown in Fig.l. Small heater assembly (annular electron gun: 5 keV, 1 mA) is set inside the Wehnelt electrode. The heater provides a disk-shaped bombarding electron beam focusing onto the cathode tip. The cathode is the tungsten wire of 0.1 mm in diameter. The tip temperature is raised to the melting point (3,650 K) at the beam power of 5 W, without any serious problem of secondary electrons for the gun operation. Figure 2 shows the cathode after a long time operation at high temperatures, or high brightnesses. Evaporation occurs at the tip, and the tip part retains a conical shape. The cathode can be used for a long period of time. The tip apex keeps the radius of curvature of 0.4 μm at 3,000 K and 0.3 μm at 3,200 K. The gun provides the stable beam up to the brightness of 6.4×106 A/cm2sr (3,150 K) at the accelerating voltage of 50 kV. At 3.4×l06 A/cm2sr (3,040 K), the tip recedes at a slow rate (26 μm/h), so that the effect can be offset by adjusting the Wehnelt bias voltage. The tip temperature is decreased as the tip moves out from the original position, but it can be kept at constant by increasing the bombarding beam power. This way of operation is possible for 10 h. A stepwise movement of the cathode is enough for the subsequent operation. Higher brightness operations with the rapid receding rates of the tip may be improved by a continuous movement of the wire cathode during the operations. Figure 3 shows the relation between the beam brightness, the tip receding rate by evaporation (αis the half-angle of the tip cone), and the cathode life per unit length, as a function of the cathode temperature. The working life of the point cathode is greatly improved by the local heating.