Time-of-flight Method With Double Probe For Spark Plug Plasma Plume

As a micro-electric propulsion device,Pulsed Plasma Thruster(PPT)has been applied to many space missions.However,as a kind of pulse arc discharge with nature of fluctuation,for the tasks such as attitude adjustment and orbit control of micro spacecrafts,number of ignitions of PPT is limited and therefore the impact of the pulse to pulse fluctuation of PPT on the total impulse is serious.As the trigger of PPT,the discharge of spark plug is an important factor on the fluctuation of PPT discharge.Improving the stability of spark plug plasma plume is an important technical way to improve the PPT main discharge.The spark plug plume velocity is an important discharge parameter because it is the result of the spark discharge evolution process.Thus,measuring the plume velocity is a method to evaluate the spark plug discharge characteristics and the pulse uniformity of the plume can be reflected.Similarly,the PPT plume velocity can also reflect the repeat-ability of PPT discharge pulses.At present,diagnostic on PPT plumes are extensive,including plume morphology,plume velocity and density.However,there is little study on spark plug plume because spark plugs,as the trigger of PPT,is generally believed to be important on the PPT discharge ignition but on the development of PPT discharge.Compared with PPT plume,spark plug plume possess smaller volume and shorter duration.However,in fact,tiny diversity of PPT discharge ignition may cause a huge offset of PPT plume feature.Therefore,Studying the pulse uniformity of spark plug discharge is expected.This paper develops a TOF method for spark plug plume velocity measurement.The spark plug discharge pulse duration is order of microseconds and therefore its plume velocity can be obtained based on the time-of-flight measurement.In this paper,two sets of dual probes are used as time-of-flight sensors to form a velocity measurement system.The operation modes of the system are investigated and compared to find the respective advantages and disadvantages.In conclusion,the so called breakdown and conduction modes are verified both suitable for spark plug plume measurement,which can also be applied to diagnose PPT plumes.The velocity measuring system is composed of sensor probes and driving modulus.The probes includes two sets of typical Langmuir double probes,which are positioned with certain distance on the plume flight route.The driving modulus has two independent RC discharge circuits for the two probes.The capacitors are charged during the off stage of the plug plumes and discharge during the on stage through the probes.The set off moments of the two probe discharges are recorded to calculate the plume flight time and get the velocity.The probes could be biased by positive and negative voltage.A commercial spark plug is used to compare the probe performance with positive and negative bias.The results show that(1)for positive bias,the potentials of probes are higher than the space potential of the spark plug plume,which attract the electrons from the plume to the probes and form addition currents in the driving circuits,resulting in serious signal distortion.Therefore the capacitor discharge current signals are difficult to determine the plume flight time.On the contrary,when the probes are negatively biased,because the probe potentials are lower than the plume space potential,the electrons of the plume are repelled and the driving circuits are almost isolated from the plume.So negatively biased probes are suitable for the flight time sensing.(2)changing negative bias voltage,the probes experience two different discharge modes,namely conduction and breakdown modes.Both modes are applicable for measuring the plume velocity and give similar results.However,the conduction mode has strict requirements for the bias voltage and the breakdown mode is easier to implement.Negatively biased TOF probe was used to obtain the spark plug plume velocity and investigate the dependence of the plume velocity on the spark plug discharge conditions.Firstly,the effects of the spark plug discharge conditions such as the charging voltage,storage capacitor and discharge duration on the park plug discharge energy are experimentally revealed.The results show that changing the three parameters can effectively change the plug discharge energy.Secondly the spark plug plume velocity obtain with breakdown mode is correlated to the spark plug discharge energy.The plume velocity is of certain random even if the spark plug discharge conditions remain constant.For example,when the plug discharge energy is 31 mJ and the TOF probe flight gap is 2 cm,the measured flight velocity distributes between 4.10 km/s-13.55 km/s,the average value is 8.37 km/s,which shows the spark plug discharge is also of random as expected.The spark plug discharges in energy range of 2 mJ-31 mJ,generate plume average velocity ranging in 8.37 km/s-11.35 km/s and the plume velocity scatter in range of 4.04 km/s-24.21 km/s for any discharge energy.In order to reveal the origin of the velocity discreteness,the conduction mode is used for some selected cases of plug discharges.It is confirmed that the discretion of the flight times measured with breakdown mode comes from two origins: the discreteness of plume velocity and the randomness of breakdown occurrence in the probe gap.The random occurrence of breakdown can be suppressed by increasing the probe bias voltage and shortening the distance of the probe to the spark plug.