Speaker
Description
For long pulse gyrotron operation, the beam current of the gyrotron must be maintained at a target with high accuracy. In conventional method, the beam current has been adjusted with a pre-programmed control of the heater voltage to the cathode. The reason for changing the heater voltage during the pulse is to warm up the cathode as it cools down due to electron emission. This method requires many trials of adjusting a waveform of the pre-programmed voltage signal. Besides, the temperature of the cathode before operation depends on the previous pulse width and other factors. Hence, it is difficult to maintain the beam current with high accuracy even if the pre-programmed waveform of the cathode heater voltage is optimized. To solve these problems, beam current feedback system was developed. It automatically controls the heater voltage with a feedback loop of the beam current. The difference from the target to measured current was amplified by a factor of 10 with an error amplifier. The amplified signal was integrated with an integration circuit with a time constant of 10 seconds. In this system, a proportional control, which is aiming for faster control compared to the integration circuit, wasn’t installed because the reaction time of the beam current is more than one second by large heat capacity of the cathode. A feature of the system is that the integrated signal is added to the pre-programmed signal with a summing circuit. It reduced the control amount by the feedback, thereby increasing the stability of feedback control. It also allows operation with only the pre-programmed control, i.e., without feedback control. For example, pre-programmed control can be used instead of feedback control for beginning (50 seconds, typically) of the pulse where feedback control is inappropriate due to a significant decrease in current due to the emission cooling. The beam current feedback system was tested with ITER gyrotron and its test stand in QST. The feedback control was turned on at about 50 seconds, and automatically adjust the beam current to a target of 47 A by about 50 seconds. It was accurately maintained for 100~1000 seconds with an error of ±0.5%. An output power of gyrotron was stably maintained at about 1 MW because of the small error on beam current.
