Development of a neutral velocity spectrometer for upper atmospheric research

中性质量和速度谱仪(NMS)是将于2007年底发射的加拿大小卫星CASSIOPE上的E-POP(增强型极地流出流探测器)有效载荷仪器之一。NMS利用二维位置探测技术测量了高层大气中H_2、He、O和N_2等主要中性物质的密度和二维速度分布函数。中性大气物质的速度分布函数从未被测量过：然而，速度分布的信息对于理解高层大气中非热成分的行为是重要的，这些成分被认为是由环境中的中性物质与高能等离子体之间的各种碰撞过程产生的。完成了NMS仪器的基本设计，并通过实验验证了其可行性。针对实验室环境资源有限的实际飞行实验，对测量方法进行了改进。中性粒子以航天器的冲压速度进入NMS仪器，并被电子枪电离，然后被引入电子束后面的腔室。在腔体中，通过施加于腔体顶板的脉冲负电压，离子沿垂直于冲压速度的方向向底部加速。一部分离子到达腔体底部的狭缝，进入离子加速区，在那里离子被恒定的均匀电场加速到探测器。通过这一过程，入射粒子的加速度只发生在垂直于冲头的方向上，并保持了冲头平面内的入射速度。该探测器由微通道板(MCP)和位置灵敏阳极组成。加速的离子根据其质量和速度到达某一位置。探测位置的分布代表了中性粒子的二维速度分布函数。

Neutral Mass and velocity Spectrometer (NMS) is one of the e-POP (enhanced Polar Outflow Probe) payload instruments on a Canadian small satellite CASSIOPE which will be launched in the end of 2007. NMS measures density and 2-dimensional velocity distribution function of each major neutral species such as H_2, He, O, and N_2 in the upper atmosphere using 2-D position detection. The velocity distribution function of neutral atmospheric species has never been measured: However the information of the velocity distribution is important to understand the behavior of nonthermal components of the upper atmosphere, which are considered to be generated by various collisional processes among ambient neutral species and high-energy plasmas. Basic design of NMS instrument has been done and its feasibility has been confirmed experimentally. We modified the measurement method to adapt to the actual flight experiment, which has limited resources relative to the laboratory environment. Neutral particles enter the NMS instrument with spacecraft ram velocity and ionized by an electron gun, then introduced to a chamber behind the electron beam. In the chamber the ions are accelerated towards the bottom in the perpendicular direction to the ram velocity by a pulsed negative voltage applied to the top plate of the chamber. A part of ions reach a slit in the bottom of the chamber and enter the ion acceleration region where ions are accelerated by a constant uniform electric field to a detector. Through this process, acceleration of incident particle occurs only in a direction perpendicular to the ram and incident velocity in ram plain is maintained. The detector consists of a Micro Channel Plate (MCP) and a position sensitive anode. An accelerated ion arrives at a position according to its mass and velocity. The distribution of detected positions represents 2-D velocity distribution function of neutral particles.