A Coherent Tunable Smith-Purcell Radiator for the Far Infrared

0070491 walsh本项目的目的是开发一种用术语“光栅耦合振荡器”（GCO）来定义的器件[1,2]。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动力，并在整个太赫兹- fir光谱范围内（0.3-30太赫兹）充当本振。近一个世纪以来，由于缺乏可调谐的相干源，FIR光谱制度一直相对未被充分利用。尽管存在这些困难，但从生物物理学和凝聚态物质到等离子体物理学和射电天文学的广泛科学问题的重要性推动了FIR光谱区域的研究。贺卡业务的独特特点将为这些调查中使用的技术宝库增添一个强有力的新工具。贺卡业务是对旧思想的新颖改编。其基本组成部分是一个非常“明亮”的电子束和一个衍射光栅。大约50年前，史密斯和珀塞尔首次描述了电子束在光栅上移动的实验。通过光栅的单个电子会产生表面电流足迹，从而产生辐射尾迹。光栅对电子通过的响应是相干的，就像介电材料对快速粒子通过的响应是相干的一样。通过光栅的漫射电子束将产生一个总信号，这是每个电子贡献的非相干叠加。在这个极限下，辐射功率仅与束流成线性关系，产生的总功率也不大。当光束电流密度超过临界值时，光栅上的分布反馈将导致单个电子贡献也相干地增加。可用的功率急剧增加。在迄今为止的实验中，这个“开始振荡”的阈值已经在高达1.5太赫兹的频率上被跨越。贺卡业务在几个方面优于其他现有的FIR来源。首先，自由电子束的使用避免了限制固态器件光谱范围的大块材料响应。其次，与传统的微波管工程相反，使用明亮、低电流的电子束和合适的低损耗开放谐振器（光栅）结构，克服了传统电子管调谐范围小于1太赫兹的所有四个障碍。早在50年前[4-6]，这些障碍就被确定为：(1)制造精度的限制，(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，一个完整的贺卡业务将小于一个公文包，没有低温或复杂的支持硬件。GCO输出功率的理论估计为10mw (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1太赫兹以下的GCO输出功率为100 nW，效率为10-7。本课题旨在通过提高电子束质量和光栅耦合效率，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倍的辐射和分辨能力。在目前的实验中，一种改进的扫描电子显微镜（SEM）产生驱动光束。这种扫描电镜系统将得到改进，以产生更亮的电子束。将开发信号采集光学和仪器，以便更精确地监测实验条件。本文将从实验和理论两方面研究光栅谐振器的设计，以降低损耗，降低启动振荡阈值，提高输出耦合效率。GCO操作理论将进一步发展，以了解输出功率对电流的依赖。该项目的最终目标是使用基于紧凑dc-dc变换器的高压电源和单尖端场发射阴极来运行SEM，从而为微型GCO的原理运行提供证明

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.***