Pu-Shih Lu

Field-emission based vacuum device for the generation of terahertz waves

Terahertz (THz), waves i.e., electromagnetic radiation in the frequency extending from 0.1 to 10 THz (wavelengths of 3 mm down to 0.03 mm), have been used to characterize the electronic, vibrational, and compositional properties of solid, liquid, and gas phase materials during the past decade. More and more applications in imaging science and technology call for the well development of THz wave sources. Amplification and generation of a high frequency electromagnetic wave are a common interest of field emission based devices. In the present work, we propose a vacuum electronic device based on field emission mechanism for the generation of THz waves. To verify our thinking and designs, the cold tests and the hot tests have been studied via the simulation tools, SUPERFISH and MAGIC. In the hot tests, two types of electron emission mechanisms are considered. One is the field emission and the other is the explosive emission. The preliminary design of the device is carried out and tested by the numerical simul...

Modeling a thermionic energy converter using finite-difference time-domain particle-in-cell simulations

A thermionic energy converter (TEC) is a static device that converts heat directly into electricity by boiling electrons from a hot emitter surface across a small inter electrode gap to a cooler collector surface. The main challenge in TECs is overcoming the space charge limit, which limits the amount of current that can transmit across a gap of a given voltage and width. In this preliminary work, it is found that the OOPD1 simulation results is in good agreement with analytical results. We have verified the feasibility of studying and developing a TEC using a bounded finite-difference time-domain particle-in-cell plasma simulation code, OOPD1 developed by PTSG, UC Berkeley.

Interaction Mechanism of a THz Wave Generator using a Field Emission Cathode

In the previous meeting held at MIT (IVNC2004), we proposed a vacuum electronic device based on field emission mechanism for the generation of THz waves. A preliminary result was obtained. However, the detail interaction mechanisms were under study to improve the efficiency. In the present work, the MAGIC code is used to investigate the interactions between the electrons and the THz waves. To understand the interaction mechanism, the cathode has been trimmed to emit electrons. The simulation results show that the efficiency of the case corresponding to the trimmed cathode is higher than that of the original planar cathode. The phase space diagram of the case corresponding to the original cathode has been observed. The downstream current is oscillating with the THz radiation. The bunching electrons between the gaps contribute most of the generation of THz waves. The phase space diagram of the case corresponding to the trimmed cathode shows almost no downstream current but the corresponding efficiency is higher than that of the untrimmed case. This indicates that the interaction mechanism is strongly related to the gaps. The AC electric fields of the THz waves not only velocity-modulate the electron beam but also cause the density modulation of the field emission current. This pre-bunching effect provides the feedback loop as required by an oscillator

Space Charge Effects on Thermionic Emission: the Knee of the Transition from TL to FSCL Operation

We study the space charge effects on thermionic emission via a self-consistent approach. The Richardson-Dushman equation, corrected for the Schottky effect and the Poissons equation including relativistic effects are solved iteratively. The knee of the transition curve from TL to FSCL operation is obtained self-consistently

An improved self-consistent fitting model for characterizing field emitters

We propose a new fitting model for better characterizing field emitters such as a field emission array (FEA) that may be operated at high current density. In this improved model, we employ both effective work function and effective enhancement factor approximations. The experimental data can be fitted perfectly and therefore the discrepancy between the experimental results of FEA at high current density and FN theory can be explained. It indicates the space charge effects play the key role.

Influence of ion effects on a space charge limited field emission flow: From non-relativistic to ultra-relativistic regimes

Influence of ion effects on a space charge limited field emission flow has been studied systematically, by employing both analytical and numerical approaches. The analytic calculations are carried out self-consistently to yield the steady states of the bipolar flow. It is found that the field emission currents in the presence of saturated ion currents can be enhanced to be nearly 1.8, 1.5, and 1.4 times of the cases with no upstream ion current in non-relativistic, intermediate, and ultra-relativistic regimes, respectively. The solutions have also been verified using 1D PIC simulations, as implemented in the OOPD1 code developed by PTSG of UC Berkeley.

An injection locked millimeter wave oscillator based on field emission cathodes

The frequency range of the millimeter wave is from 30 up to 300 GHz. Millimeter waves have been extensively studied for many applications including high resolution radars, communications, materials processing, plasma heating, high energy particle accelerators and power transmission. In this paper, an injection-locked millimeter wave oscillator based on field-emission cathodes is proposed and demonstrated the well phase/frequency controllability. The simulation results are consistent with the prediction of Adlers equation. The preliminary results show that the design of the injection-locked oscillator seems to promise useful and compact millimeter wave sources.

An Injection-Locked Ka-Band Oscillator Based on Field-Emission Cathodes

An injection-locked Ka-band oscillator based on field-emission cathodes is investigated. The phase and frequency controllability has been demonstrated by the finite-difference time-domain particle-in-cell simulations. The results show the consistency with the prediction of Adlers equation.

Study on the Generation of THz Waves in a Vacuum Electronic Device

We proposed a vacuum electronic device based on field emission mechanism for the generation of THz waves in a previous work. The preliminary simulation results showed that an electronic efficiency up to 4% can be achieved with no external magnetic fields applied. However, the interaction mechanism is not clear. In the present work, the interaction mechanisms are studied. The MAGIC code is used to investigate the interactions between the electrons and the THz waves. To understand the interaction mechanism, the cathode has been trimmed to emit electrons. The simulation results show that the efficiency of the case corresponding to the trimmed cathode is higher than that of the original planar cathode. The interaction regions are located among the gaps between the cathode and the anode. The AC electric fields of the THz waves not only velocity-modulate the electron beam but also cause the density modulation of the field emission current. This pre-bunching effect provides the feedback loop as required by an oscillator

Study on the generation of THz waves in a vacuum electronic device

In the present work, the MAGIC code is used to investigate the interactions between the electrons and the THz waves. In the study, two types of electron emission mechanisms are considered for comparisons. One is the field emission and the other is the explosive emission. The simulation results show that an electronic efficiency up to 4% can be achieved with no external magnetic fields applied. The field emission based vacuum device seems to be a good candidate for the interaction mechanism involved in the system, the energy distribution of the electrons throughout the interaction structure has been observed. The electrons emitted from the cathode surfaces are accelerated by the applied voltage, and are bunching and synchronizing with THz waves through the well-tailored metallic structures. In the MAGIC simulation, the output power and efficiency corresponding to the cases for the field emission and the explosive emission are calculated, respectively.