脉冲羽流速度的双探针飞行时间测量模式对比

Measurement Modes of Time-of-Flight Method Using Double Probe for Pulsed Plume Velocity

关键词:  双探针  脉冲等离子体推力器  羽流速度  飞行时间法  击穿模式

Key words:  Double probe  Pulsed plasma thruster  Plume velocity  Time of flight （TOF）  Breakdown mode

摘要:

用于飞行时间（TOF）测量的双探针可以根据偏压不同而工作在导通和击穿模式。本文以脉冲等离子体推力器（PPT）的火花塞羽流作为测量对象，研究双探针TOF方案不同模式的优缺点和功能适用性。结果表明，导通模式的双探针信号微弱且易受干扰，导致羽流到达时刻（PAT）测量不可靠。击穿模式可以正、负电压驱动，只要重复测量次数足够多，两种击穿模式测得的羽流平均速度一致。然而两种击穿模式的测量离散性差别很大，正击穿模式的结果离散性很小，因为正电压探针的击穿电压低，而且击穿时刻相对于火花塞点火时刻的延迟比较稳定。与之相反，负电压探针击穿电压高，击穿不稳定，延迟起伏大，虽然提高驱动电压，可以使击穿延迟的起伏一定程度减小。因此，利用正击穿模式，通过多次测量可估算击穿时刻延迟的离散分布，进而分析火花塞羽流飞行时间的离散。正击穿模式的双探针TOF方案可以作为脉冲羽流速度随机性的一种评价手段。

Abstract:

The double-probes for time of flight （TOF） measurement can be operated with conduction or breakdown modes depending on bias. A spark plug， which usually triggers pulsed plasma thruster （PPT）， is applied as a plasma plume generator to analyze the operation modes of a TOF double-probe and explore its function. The results show that the double-probe signals in conduction mode are too weak to achieve reliable plume arrival time （PAT） measurement. On the other hand， if the probes are driven to breakdown mode by either positive or negative bias， the plume velocity measurement can be achieved and the resultant average velocity agrees to each other if enough measurements are repeated. However， the measured PAT scatters in very different range because positive biased probes can breakdown at lower voltage and with more stable relative delay to plug fire than negative biased probes. On the contrary， negative breakdown of probe needs much higher voltage and goes more unsteadily， although bigger biases can make it stable in something. Via positive breakdown mode， the scattering of TOF can be analyzed by eliminating the delay scattering of probe breakdown. In conclusion， the double probe TOF with positive breakdown mode can be a method to approach the velocity randomness of plasma plume pulses.