Development of a neutral velocity spectrometer for upper atmospheric research

中性质量和速度光谱仪（NMS）是将于2007年底发射的加拿大CASSIOPE小卫星上的e-POP（增强型极流探头）有效载荷仪器之一。NMS通过二维位置探测，测量上层大气中H_2、He、O、N_2等主要中性物质的密度和二维速度分布函数。大气中性物质的速度分布函数从未被测量过，但速度分布的信息对于理解高层大气非热成分的行为是重要的，这些成分被认为是由环境中性物质和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。我们修改了测量方法，以适应实际飞行实验，相对于实验室环境资源有限。中性粒子以航天器撞击速度进入NMS仪器，并被电子枪电离，然后被引入电子束后面的一个腔室。在腔室中，通过施加在腔室顶板上的脉冲负电压，离子在垂直于冲压速度的方向上向底部加速。一部分离子到达腔室底部的狭缝，进入离子加速区，在这里离子被恒定的均匀电场加速到检测器。在此过程中，入射粒子的加速度只发生在与撞锤垂直的方向上，并保持撞锤平原的入射速度。该探测器由微通道板（MCP）和位置敏感阳极组成。一个加速离子根据它的质量和速度到达一个位置。检测位置的分布表示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.