Development of Neutral Mass Spectrometer using Time-of-flight Technique for Space Flight

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Quadropole type and magnetic deflection type neutral mass spectrometer are commonly used for space flight. However both type have basic problems for measuring neutral mass distribution in space. Those are 1. evaluation of the effect with the recombination of target gas and atomic oxygen, which is major component of the several hundred km height atomosphere and absorb on the surface of the instrument, 2. loss of velocity idstribution function information of target gas. Recently we proposed a new tuype neutral mass spectrometer using time-of-flight technique to eliminate the effect of the recombination inside the instrument and to achieve the measurement of distribution function of target gas. With the support of this grant, we have developed the key components of this detector (pulse ion source, acceleration part for time-of flight (TOF), and detector which measures time-of-flight and 2 dimensional position, simultanously) and then integrate them into a system for rocket flight. The dev … More eloped elements are :1. Filament power supplySupply constant power with low voltage to filament which emits electron beam for ionization of target neutral gas. Output power is controled by the input control voltage. Leak current between input and output is minimized to be able to measure the emission current correctly.2. Acceleration part for time-of-flightA mesh is placed between ionization part and acceleration part. Mesh potential is set to OV during ionization period using pulse electon beam. This mesh is effective to suppress extra beam divergence due to the potential gradient which exists when the mesh is not exist. Mesh potential is changed to -10V when ionization period is finished to accelerates the ion torward detector. High speed switching (0->-10V within 20nS) of the mesh potential enabled to minimize the dispersion of TOF caused by the difference of the ionizing point.3. Detector3 stage MCP is used for detector. A mesh is placed between MCP and position sensitive anode. TOF pulse is measured by the signal from this mesh. Using this mesh with position sensitive anode enables simultanous measurement of TOF and position of the ionized particle.Further improvement :The method which we are developed has a limitation of the sampling frequency of the distribution function of target gas. This limitation comes from the fact that the simultanous measurement of TOF and position detection requires that number of ions per 1 pulse beam should be less than 1.To overcome this limitation, we have starting the feasibility study of the system which uses CCD camera as position detection and switch MCP HV for mass selection. Less