Ji-Hwan Yeon

Electrostatic Focusing Lens Module With Large Focusing Capability in Carbon Nanotube Emitter-Based X-Ray Sources

We fabricated and analyzed an electrostatic focusing lens (FL) module comprising a micro-FL with many holes near the gate and a macro-FL of single aperture toward the anode for carbon nanotube (CNT) emitter-based X-ray sources. The micro-FL suppresses the divergence of the electron beams whereas the macro-FL governs the shape of the beam bundle. The FL module can be controlled over a wide range of gate and anode voltages while maintaining the focal spot. We achieved a small focal spot of 300 μm from CNT emitter dot arrays of 10 mm × 1.6 mm with line pairs of 3 lp/mm maintained for different anode voltages.

A high-performance carbon nanotube-based field emission x-ray tube for commercial dental imaging

We developed a fully vacuum-sealed carbon nanotube (CNT)-based field emission x-ray tube with commercial-level performance for dental imaging application. The CNT x-ray tube was designed in a triode configuration with the focus-functional gate and had a compact size, much smaller than the conventional hot-cathode tubes. The small size of CNT x-ray tubes along with no cathode heating enables us to make a very compact portable dental imaging system. We have obtained a quite good x-ray image of a human tooth phantom by operating the CNT x-ray tube in an anode voltage of 65 kV, an emission current of 3 m A, and a pulse duration of 0.5 s, which is comparable to that of the conventional commercial hot-cathode tube.

Fast and Stable Operation of Carbon Nanotube Field-Emission X-Ray Tubes Achieved Using an Advanced Active-Current Control

The advanced active-current control (ACC) with a push-up voltage source is proposed for the fast operation of carbon nanotube (CNT) field-emission x-ray tubes while preserving the advantages of improved stability and reliability. 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. 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.

Digital properties of field emission X-ray tubes with carbon nanotube field emitters

We describe reliable CNT x-ray tubes and their digital properties such as easy modulation of x-ray dose, high-speed operation, and low-voltage and low-power consumption control with an active-current control unit (ACCU) developed. The simple and advanced ACCU was found to shorten the response time of CNT x-ray tube down to sub-microsecond along with stable and reliable operation. The fast digital CNT x-ray tubes with the ACCU could provide improved x-ray imaging like blur-less and short-time scanning.

A fully vacuum-sealed miniature x-ray tube with carbon nanotube field emitters for compact portable dental x-ray system

There have been many efforts to develop x-ray sources using field electron emitters instead of conventional thermionic cathodes for digital controlling of x-rays in medical imaging. Specially, portable x-ray systems need a miniature x-ray tube with less-power consumption, easy insulation of high voltage and light shielding of x-rays. Carbon nanotube (CNT) has attracted much attention as the most promising field emitter due to its geometric high aspect ratio, high physical and chemical inertness. To date, however, CNT field emitters have not been satisfactorily incorporated into a fully vacuum-sealed x-ray tube due to their instability and/or unreliability. We successfully developed a fully vacuum-sealed, miniature x-ray tube with CNT emitters for portable dental x-ray systems. The x-ray tube was designed in a triode configuration with a self-electron focusing gate and reliable CNT emitters, and was fully vacuum-sealed within a miniaturized volume of 15 mm in diagonal and 65 mm in length, very tiny and small as compared with conventional thermionic one. The nominal focal spot size of the x-ray tube is 0.4 mm with an operational tube voltage of 65 kV and a current of 3 mA, which offers quite good x-ray images of a human tooth phantom. No heating the miniature x-ray tube for electron emission leads to easy insulation of high tube voltage and light shielding of x-rays, giving a compact and light portable x-ray system. Furthermore, digital operation of the x-ray tube through an active-current control could provide a commercial lifetime along with pretty good stability.

Optimization of carbon nanotube paste emitter by controlling the size distribution of SiC nanofiller aggregates

We tried to improve field emission properties of carbon nanotube (CNT) paste emitters by controlling the size distribution of aggregated SiC fillers. The SiC fillers used as an adhesion promoter strongly affects the dispersion of CNT paste, determining uniformity and reproducibility of the field emitter. We found, to date, that the optimum distribution is about 1 μm of D50, the value of the particle diameter at 50% in cumulative distribution. The CNT emitters with the optimized paste showed uniform and reproducible emission performances along with high adhesion to the cathode. The improved paste formulation and emitter fabrication could lead to successful field emission devices and applications.

An active current-control unit composed of cascade transistors for extremely stabilizing emission currents from carbon nanotube field emitters

We propose an advanced active current-control (ACC) unit composed of two cascade transistors for extremely stabilizing field emission currents. The upper transistor connected to the cathode modulates high voltages for field emission while the other one regulates emission current stably. Using the ACC unit developed, very stable emission currents were achieved from the fully vacuum-sealed x-ray tube with carbon nanotube field emitters for long pulse sequence. The advanced ACC unit could provide commercial stability and reliability of field emission x-ray sources even in high voltage and/or high duty environments.

A field emission nano-focus x-ray source with effective electron beam focusing module

We developed a field emission nano-focus x-ray source using carbon nanotube (CNT) emitters and electron beam (E-beam) focusing modules, which could be used in the applications requiring the high-resolution x-ray images. For the nano-size focusing of E-beam from CNT emitters, we designed special focusing modules consisting of electrostatic and magnetic lenses. The geometries and operating conditions of focusing modules were optimized to give the desired demagnification characteristics by performing the electron optics simulations of the designed x-ray source under external electric and magnetic fields. We constructed and operated an open-type nano-focus x-ray system using the focusing modules under a high-voltage condition to attain the high-resolution x-ray images. The results showed a possibility of field emission nano-focus x-ray sources with a relatively high current and controllable magnification through the effective focusing modules.

Fast active-current control with a push-up voltage source for carbon nanotube field-emission X-ray tubes

The advanced active-current control with a push-up voltage source is proposed for the fast operation of a carbon nanotube (CNT) field-emission X-ray tube while maintaining the advantages of improved stability and reliability. The push-up voltage source, composed of a resistor and a voltage supply, was found to reduce the response time of CNT X-ray tubes down to sub-microsecond.

Brightness of electron beam from a small carbon nanotube emitter fabricated using the probe contact method

A novel method for fabricating a small-sized carbon nanotube (CNT) emitter was developed. The emitter was fabricated by contacting the tungsten probe covered with CNT paste to the surface of Kovar rod, and then the brightness of electron beam generated from the CNT paste emitter was measured. We obtained the virtual source size of 1.13 × 10^-10 m² and the brightness of 1.2 × 10⁷ A m^-2sr^-1 at an applied voltage of 3,100 V.