A Coherent Tunable Smith-Purcell Radiator for the Far Infrared

该项目的目的是开发一种由术语“光栅耦合振荡器”(GCO)[1，2]定义的器件。GCO是一种可调谐的相干辐射源，我们预计它可以为光谱研究提供能量，并在整个太赫兹-FIR光谱范围(0.3-30太赫兹)内充当本地振荡器。近一个世纪以来，FIR光谱系统一直未得到充分利用，这主要是由于缺乏可调谐的相干信号源。尽管存在这一困难，但从生物物理学和凝聚态物质到等离子体物理和射电天文学的广泛科学问题的重要性推动了对FIR光谱区域的研究。GCO的独特特性将为这些调查中使用的各种技术增加一个强大的新工具。GCO是对旧想法的新颖改编。其基本组件是一个非常“明亮”的电子束和一个衍射光栅。史密斯和珀塞尔[3]在近50年前首次描述了电子束在栅上移动的实验。单个电子通过栅会产生表面电流足迹，进而产生辐射尾迹。光栅对电子通过的响应是相干的，这与介质材料对快速粒子通过的相干响应是相同的。一束弥散的电子束通过一块栅会产生一个总信号，它是每个电子贡献的非相干叠加。在此限制下，辐射功率仅与束流成线性关系，并且产生的总功率不大。当束流密度超过某一临界值时，光栅上的分布式反馈将导致单个电子贡献也相干地相加。可用的电力急剧增加。到目前为止的实验中，这种“启动-振荡”阈值已经在1.5THz的频率下超过。GCO在几个方面优于其他可用的FIR源。首先，自由电子束的使用避免了限制固态器件光谱范围的整体材料响应。第二，与传统的微波管工程相反，使用明亮、低电流的电子束和适当的低损耗开放谐振器(光栅)结构克服了将传统电子管的调谐范围限制在1太赫兹以下的所有四个障碍。早在五十年前[4-6]，这些障碍就被确定为：(1)制造精度的需要设定的极限，(2)热稳定性，(3)“电路”损耗，以及(4)启动电流密度随工作频率的快速增加。最后，一个完整的GCO将比一个公文包更小，没有低温或复杂的支撑硬件。理论估计GCO的输出功率为10‘S MW(CW)，效率超过0.01。工作频率范围仅受电子束质量的限制。目前的GCO输出功率和效率分别为100NW和10-7。提出的研究过程旨在通过改善电子束质量和光栅耦合效率来降低启动电流，从而提高功率和工作频率。这一目标需要广泛的理论和实验研究，以实现预期的1000辐射和分辨率增加。在目前的实验中，改进的扫描电子显微镜(SEM)产生驱动光束。这种扫描电子显微镜系统将得到改进，以产生更明亮的电子束。将开发信号采集光学装置和仪器，以便更精确地监测实验条件。本文将从实验和理论两个方面对光栅谐振腔的设计进行研究，以降低损耗，降低启动振荡阈值，提高输出耦合效率。我们将进一步发展GCO的运行理论，以了解输出功率与电流的关系。该项目的最终目标是使用紧凑型DC-DC转换器高压电源和单端场发射阴极来操作扫描电子显微镜，从而为微型GCO的原理操作提供证据。

0070491WalshThe purpose of the project is to develop a device which is defined by the term "grating coupledoscillator" (GCO) [1,2]. The GCO is a tunable source of coherent radiation that, we anticipate, can provide power for spectroscopic investigations and function as a local oscillator over the entire THz-FIR range of the spectrum (0.3-30 THz). For nearly a century the FIR spectral regime has been relatively under-exploited, largely due to the lack of tunable, coherent sources. Despite this difficulty, the importance of a broad range of scientific questions extending from biophysics and condensed matter to plasma physics and radio astronomy have motivated research in the FIR spectral region. The unique characteristics of the GCO will add a powerful new tool to the arsenal of techniques used in these investigations.The GCO is a novel adaptation of an old idea. The essential components are a very "bright"electron beam and a diffraction grating. Smith and Purcell [3] first described experiments withelectron beams moving over a grating nearly fifty years ago.A single electron passing over a grating induces a surface current footprint which in turn produces a radiative wake. The response of the grating to the passage of the electron is coherent in the same sense that a dielectric material will respond coherently to the passage of a fast particle. A diffuse beam of electrons passing over a grating will generate a total signal that is an incoherent superposition of the contributions from each electron. In this limit, the radiated power scales only linearly with beam current and the total power produced is modest. When the beam current density exceeds a critical value, distributed feedback on the grating will cause individual electron contributions to also add coherently. The power available increases dramatically. In experiments to date, this "start-oscillation" threshold has been crossed at frequencies up to 1.5 THz.The GCO is superior to the other available FIR sources in several respects. First, the use of a freeelectron beam avoids the bulk material response which limits the spectral range of solid state devices. Second, contrary to conventional microwave tube engineering, the use of bright, low current electron beams with a suitably low loss open resonator (grating) structure surmounts all four impediments that limit the tuning range of conventional electron tubes to less than 1 THz. As early as fifty years ago [4-6], these impediments had been identified as: (1) limits set by the need for precision in fabrication, (2) thermal stability, (3) "circuit" losses, and (4) the rapid increase of the start current density with the operating frequency. Lastly, a complete GCO will be smaller than a briefcase, without cryogenics or intricate supporting hardware.Theoretical estimates of the GCO output power are 10's of mW (CW) with efficiency exceeding 0.01. The operating frequency range is limited only by the quality of the electron beam. Current GCO output power and efficiency below 1 THz are 100 nW and 10-7 respectively. The proposed course of research aims to lower the start current, thereby increasing the power and operating frequency, through improved electron beam quality and grating coupling efficiency. This goal requires extensive theoretical and experimental studies to achieve the anticipated 1000 increase in radiated and resolving powers.In current experiments, a modified scanning electron microscope (SEM) generates the drivingbeam. This SEM system will be improved to produce a much brighter electron beam. The signal collection optics and instrumentation will be developed in order to facilitate more precise monitoring of the experimental conditions. The grating resonator design will be investigated experimentally and theoretically to reduce losses, in order to lower the start oscillation threshold, and enhance the output coupling efficiency. The theory of GCO operation will be developed further to understand the dependence of output power on current. The final goal of the project is to operate the SEM with compact dc-dc converter based high voltage supplies and a single tip field emission cathode, thus providing proof of principle operation of a miniature GCO.***