탄소나노튜브 전계방출엑스선원의 고속구동에 관한 연구: A Study of Fast and Stable Operation of Carbon Nanotube Field Emission X-ray Tubes
by
201-4호
제1과학관
A carbon nanotube (CNT) field emitter has been considered as one of the most promising electron sources for vacuum electronic devices because of its extremely large field enhancement factor and ease fabrication on a variety of substrates. Specifically, x-ray tubes with the CNT emitter can be digitally addressed with a rapid speed and low power consumption, which is expected to open a new era of digital x-ray sources over conventional thermionic emitter-based ones having been used since the discovery of x-ray by Röntgen. Since the thermionic electron emission from filaments in conventional x-ray sources is of typical analog behavior, the modulation of electron emission is difficult and thus, pulsed x-rays are in general achieved by direct pulse driving of extremely high acceleration voltage applied to the anode target in a diode mode. A grid configuration and control were rarely used in the thermionic pulsed x-rays due to its driving complexity and imperfection. Generally, the thermionic x-ray tube could be operated within a few tens of milliseconds for both diode and triode configurations.
In this study, CNT field emitters were fabricated by using a paste that was specially milled of synthesized CNTs for field emission, inorganic fillers for improving adhesion of CNTs to substrates, and organic binders for formulation. A point-contact method has been developed for a miniature or a high-brightness CNT emitter on a sub-mm-diameter-metal tip meanwhile conventional screen-printing was used for large-area, high-current electron sources.
The CNT-on-Tip cathode can provide high-brightness electron beams due to their point-like electron emission sites. The triode type emitter with a gate electrode is a prerequisite to an intensity-controllable x-ray. In order to design the improved triode structure, we firstly simulated the trajectory of the emitted electron beams in the triode CNT-on-Tip cathode using the commercial simulator and optimized the configuration. The lateral electric field between the cathode and the gate electrodes was minimized by optimizing the gate structure (opening and height), the diameter of metal tip, and the area of CNT emitter on the metal tip, which resulted in decreasing the emission current to the gate electrode. The developed triode CNT-on-Tip cathode showed an ideal gated field emission characteristics and could be useful to realize a intensity-controllable x-ray tube. To reduce charging at the insulating body of the x-ray tube along with a small focal spot, we proposed a focusing-functional gate structure.
We have studied and correlated failure in the x-ray tube with the field emission gate leakage current of CNT emitters. The x-ray tube, even with a small gate leakage current, exhibits an induced voltage on the gate electrode by the anode bias voltage, resulting in a very unstable operation and finally a failure. The induced gate voltage is apparently caused by charging at the insulating spacer of the x-ray tube through the gate leakage current. Thus, suppressing the gate leakage current could be a criterion for the successful fabrication of x-ray tubes.
Although field emission electron sources can provide the digital operation of vacuum devices, they have inherent demerits such as irregularity, instability, and short lifetime in field emission currents. These problems could be overcome to a great extent by an active-current control (ACC) using a voltage-controlled current source like a high voltage MOSFET. The control transistor is serially connected to the cathode node of the field emitters, controlling the field emission current directly. Unlike conventional pulse operation through the gate, a constant DC bias is applied to the gate, and cathode voltage is automatically modulated with a pulse by addressing pulse voltages through the control by MOSFET. We achieved the long-term stability of fully sealed x-ray tube with the ACC. There is no degradation of current during the measurement time of as long as 900 h, which is corresponding to 648,000 shots with a pulse width of 50 ms. Thanks to the ACC, very stable and reliable emission behavior was accomplished.
However, the current control driving in general gives rise to large retardation in the response time of electronic devices. The ACC would also deteriorate the response time of CNT field-emission x-ray tubes in spite of enhancing their stability and reliability. The slow response of CNT x-ray sources would not only generate a blurred x-ray image but also weaken their digital property severely. Therefore, an additional fast operation technique is needed when the ACC is used. The push-up voltage source, composed of a voltage supply and a resistor, was found to reduce markedly the response time of CNT x-ray tubes down to several tens nanoseconds. Therefore, the advanced active-current control (advanced-ACC) with a push-up voltage source is proposed for the fast operation of CNT field-emission x-ray tubes while preserving the advantages of improved stability and reliability. The very fast operation of CNT x-ray tube with the advanced-ACC could provide an improved x-ray imaging technology, including high-speed scanning and blur-less imaging, minimize the radiation exposure to patients reducing the risk of any radiation significantly, and optimize x-ray pulse resulting in the reduced anode heat content.
Thesis Advisor: Prof. Hyeong-Rag Lee