Diagnostic Techniques For The Randomness Of Pulsed Plasma Plume

Pulsed plasma thruster(PPT)generates impulse pulse by its pulse plume.Due to the inherent randomness of the discharge process of plume,the impulse amplitude of PPT fluctuates to a certain extent.The fluctuation of impulse will reduce the accuracy of the thruster and affect the control precision of space mission.As the ignition starting device of PPT,the discharge plume of spark plug provides the initial plasma source for the main discharge of PPT.Due to the randomness of pulse plume discharge of spark plug,its operating parameters will affect the main discharge,plume repeatability and thruster life directly.At present,the research on spark plug mainly focuses on the electrode configuration,electrode material,life test and other aspects,diagnostic for the randomness of spark plug pulse discharge plume is still expected,because diagnostic methods for plume randomness is also very limited.Plume velocity,electron temperature and electron density are the basic parameters of plume plasma,the temporal and spatial distribution of these state parameters can characterize the temporal and spatial evolution behavior of pulsed plume.In this paper,the Langmuir electrostatic probe is used to design a diagnostic scheme which can track and measure the electron temperature,electron density and the velocity of spark plug plume.The above three plume parameters were evaluated to characterize the randomness of spark plug plume plasma.Time-of-flight(TOF)technology is a common method for measuring the pulse plume velocity.A TOF plume velocity measurement system is designed by using two sets of electrostatic double probes separated by a certain distance.Based on the experiment of spark plug plume,the different working modes of the measuring system were studied deeply,and the system was applied to the randomness evaluation of spark plug plume velocity,which proved that the measuring system could be used as a technical mean for the measurement of pulse plume velocity and the randomness evaluation.The results show that the signal of the conduction mode is weak and susceptible to interference,which makes the time of flight measurement of plume unreliable.The breakdown mode of double probe can be realized by using positive bias voltage and negative bias voltage.When the positive voltage is applied,the probe breakdown voltage is low and the breakdown time is stable.When the negative voltage is applied,the breakdown voltage is high,the breakdown is not stable,and the breakdown time fluctuates greatly.By increasing the negative driving voltage,the breakdown behavior can be stabilized and the breakdown time tends to be stable.Regardless of the positive or negative drive voltage,the plume multi-pulse averaged velocity of the measured by the two breakdown modes is consistent,although the dispersion of the measured results using positive voltage driving is obviously smaller.For verification,the following tests carried out:(1)adopting the positive breakdown mode,the effects of flight distance and ignition energy on plume velocity were studied by changing flight distance(1 cm,2 cm,3 cm)and spark plug energy(20 m J,35m J,50 m J).The results show that the measured plume velocity was close for the flight distances of 2 cm and 3 cm.The plume velocities measured with flight distance of 1 cm varied greatly,which may due to the short flight time increases the measurement error.(2)With the positive breakdown mode keeping flight distance as 2 cm and the spark plug energy as 35 m J,by statistics over 30 plug discharge pulses the breakdown time distributions of P₁ and P₂ probe were analyzed.The analysis showed that the width of the flight time distribution was 0.26?s,and the distribution width of the probe breakdown time is 0.064?s.It can be seen that the randomness of probe breakdown is much smaller than the randomness of flight time.The randomness of the TOF probe has little effect on the measurement of the flight velocity fluctuation under positive breakdown mode,the randomness of plume flight velocity is the main reason for the randomness of flight time.Thus,the TOF double probe driven by positive voltage under breakdown mode can be used as a method to measure the plume velocity and its randomness.The electrostatic triple probe,which is composed of two pairs of double probes in the same position,is an effective mean to track the time evolution behavior of the basic parameters of plasma.Especially for short pulse discharge plasma,the triple probe has high enough time response performance and can track the plasma state parameters in real time quickly.In this paper,a current type triple probe circuit is designed to track and measure the electron temperature and electron density in the spark plug plume.The electron density and temperature evolution were calculated by tracking the current of probe P₂ and P₃ and the voltages of them to the reference probe P₁.The results show that the evolution of electron density is almost proportional to the spark plug discharge current,while the temporal variation of electron temperature is significantly smaller than that of electron density.The measured electron temperature is in the range of 2-5 e V and the electron density is in the order of 10¹¹-10¹²/cm³.Fixing the discharge energy of the spark plug and changing the positions of the triple probes in the plume(the distance from the end face of the spark plug is 2 cm,3 cm,and 4 cm,respectively),The results show that the electron density at different positions along the plume path is negatively correlated with the distance.In conclusion,electron density shows a similar temporal evolution behavior to spark plug discharge current,there is a delay between the peak of spark plug current and the electron density,while electron temperature and plume velocity are not highly correlated with the spark plug current waveform.Although the current waveform of spark plug pulse discharge can reflect the fluctuation of electron density,it cannot reflect the fluctuation of electron temperature and plume velocity.The randomness of 3 plume parameters can be tracked by using the TOF probe based on two sets of double probes and the electrostatic triple probe.