Measurement and analysis of current density distribution in thermionic emission, 열음극의 전류밀도분포 측정 및 분석

by Hyung-Sup Kong (MS Candidate)

Wednesday Nov 15, 2017, 2:30 PM → 3:00 PM Asia/Seoul

119호KongK (제1과학관)

Vacuum electronic devices are operated by electron beam. Current density distribution of the beam as well as total quantity of current density is important for proper operation of the device. If current density distribution affects to the efficiency of the device, the regulation of current density distribution can increase device’s efficiency. To find the way to control the current density distribution, we measured current density distribution in circular solid beam. Our experimental setup has an electric focusing structure which is used in general.

The experimental setup was installed in a vacuum chamber and the electrons were extracted from the cathode. The cathode was heated by an AC heater power supply and high voltage pulse was applied in the meantime. Current density of the electron beam was measured by a faraday cup. To measure the distribution, an additional screen which has a small aperture was installed at the beam entrance. A step-motor system which is controlled by a computer was used for mapping the current density distribution. An 

electrical focusing structure is adapted on our cathode holder. It makes the electrons focused or defocused while they travel from the cathode to the faraday cup and screen. Because of azimuthal symmetry of our experimental setup, we investigated 1D-current density distribution of the beam profile in the polar coordinate system. From the results, we observed an edge peak of the distribution at the outer beam radius.

The shape of the distribution (beam size or edge shape) has a dependency on the emission regime (space charge limited or temperature limited). We tried to analyze the dependency by computer simulation.

In addition, we changed the gap voltage and temperature to check the tendency of the distribution change. The change of the gap voltage (while temperature is constant) or the change of temperature (while the gap voltage is constant) leads to change of the relative edge peak height with certain tendency. In SCL regime, the changing of the gap voltage (temperature is constant) or temperature (the gap voltage is constant) does not affect significantly on the relative edge peak height. On the other hand in TL regime, the increasing of the gap voltage (temperature is constant) leads to make the relative edge peak higher and the increasing of temperature (the gap voltage is constant) leads to make the relative edge peak lower.

Thesis Advisor: Prof. Hyeongcheol Ri