Visualization Study on the Bubble Suppression Characteristics of $\hbox {LN}_{2}$ for Developing a High Voltage Superconducting Magnet System

Electrical breakdown voltages of liquid nitrogen (LN₂) are measured by dielectric experiments, according to the gap between electrodes, and the diameter of electrodes. Also, the distribution of electric field intensity at sparkover is calculated and analyzed by the finite element method. The dielectric characteristics of LN₂ depend on the amount of bubbles, as well as the duration time of the bubbles. Therefore, it is important to observe and analyze the bubble suppression characteristics of LN₂ according to the conditions of the applied voltage type, electrode material, and system pressure. The bubble suppression characteristics of LN₂ according to these conditions are experimented by measuring the duration time. Also, the amount of bubbles generated after the electrical breakdown is monitored by a high-speed camera system. AC and lightning impulse voltages are applied to the sphere to plane electrode system made of stainless steel, copper, and iron to compare and analyze the bubble suppression characteristics of LN₂. It is found that the electrical breakdown and bubble suppression characteristics of LN₂ are determined by the electrode material. Also, the bubble suppression characteristics of LN₂ with the system pressure (from 0.1 MPa to 0.4 MPa) are observed and analyzed.