Accurate Control Method of Low Pressure in Plume Characteristics Test of NanoSat Electrothermal Plasma Microthruster
Abstract: This article focuses on various nanosat electrothermal Plasma microthrusters. Taking the pocket rocket as an example, which works in the low pressure range of 0.1 to 10torr, the characteristics of different working gases and different low pressures on the plume are analyzed. The resulting effects illustrate the importance of precise control of low pressure. Regarding the technical problem of precise control of low air pressure of the propeller, this paper introduces the specific implementation method in detail, and conducts an assessment test. The test results show that the fluctuation degree of low air pressure control can reach within ±1%. In the end, this paper optimizes the test method, and proposes a more practical technical scheme for full-scale low pressure precise control.

1.Soru
In recent years, with the rapid development of NanoSat, there is an urgent need for small-volume, light-weight, low-cost and high-efficiency micro-thrusters. Therefore, it is necessary to carry out the test and evaluation of physical properties such as the characteristics of thruster’s plasma plume. The characteristics of plasma plume are significantly affected by the working gas and environment pressure. Taking the performance test of foreign pocket rocket plume as an example, the necessity and importance of precise control of low pressure are analyzed.
As a typical representative of NanoSat applications, Pocket Rocket is an electrothermal radio-frequency plasma thruster that can achieve thrust in the order of μN to mN. Because of its compact size and the use of capacitive radio frequency discharge, pocket rockets can obtain high-density plasma jets under low power conditions. It’s light-weight, low-cost, low-thrust, and large in specific impulse, also can work in the form of arrays, which are especially suitable for microarrays and provide power for the long term.
As shown in Figure 1, the horizontal vacuum chamber provides a low pressure environment for testing the characteristics of pocket rocket plasma plume. The vacuum chamber is a multi-functional low-pressure environment simulation test chamber, which can integrate a variety of test equipment for the performance test and evaluation of various plasma thrusters. As shown in Figure 2, in order to form a low-pressure environment, the vacuum chamber is equipped with molecular pump, mechanical pump, ionization vacuum gauge and capacitance pressure gauge. Vacuum chamber can reach a reference vacuum of 0.93mPa. The gas working fluids in the test are usually nitrogen and argon.

Under the condition of RF power and frequency of 20W and 13.56MHz respectively, the plume characteristics of pocket rocket were tested under different low pressures. Figure 3 is an experimental photo of plasma plume injected to different working gases at different pressures. Figure a is about 1.5torr low pressure argon, Figure b is about 4.0torr high pressure argon, Figure c is about 1.0torr low pressure nitrogen, and Figure d is about 7.0torr high pressure nitrogen.
It can be seen from the figure that both nitrogen and argon plumes spread at a certain cone angle at high pressure. Beams are collimated at the low pressure, but the effect of these features on thrust generation is not clear and requires further study.

To sum up, different working gases and different low pressures will have a significant impact on plume characteristics. Micro-thruster of pocket rocket works in the low pressure range of 0.1 to 10torr. Testing and evaluating plume characteristics within this range requires precise control of low pressure. This article will introduce the specific implementation method in detail for low pressure control, test and evaluate the implementation method. Finally, the implementation method will be optimized, and the accurate control technical scheme of low pressure full range is proposed.

2. The precise control method and test assessment of low pressure
The so-called low pressure generally refers to the absolute pressure lower than 1 standard atmospheric pressure and the range is 0.1~760torr. Capacitance pressure gauge is commonly used to accurately measure low atmosphere pressure. It usually adopted two different ranges of 10torr and 1000torr of capacitance pressure gauge to cover the measurement of entire low pressure range. The vacuum chamber of simulation test device usually needs to be controlled by low pressure through intake and exhaust. According to the airflow direction, air intake end is generally defined as the upstream, and exhaust end of vacuum pump is defined as the downstream. According to the control accuracy, two control modes of upstream and downstream are generally used to achieve accurate control of low air pressure with different ranges (10torr and 1000torr).
As shown in Figure 4, the upstream mode is to maintain the upstream pressure and outlet flow, and control chamber pressure by adjusting inlet flow. As shown in Figure 5, the downstream mode is to maintain the upstream pressure and inlet flow, and control chamber pressure by adjusting exhaust flow.

For the above two control modes, two capacitive pressure gauges of 1torr and 1000torr and a 24-bit high-precision pressure controller were used to conduct the assessment test. The test device is shown in Figure 6 and Figure 7.


During the upstream mode test process, the vacuum pump was first turned on and then pumped at full speed, and PID parameters of controller were self-tuned at about 68Pa. After the self-tuning is completed, 8 set points of 12, 27, 40, 53, 67, 80, 93 and 107Pa are controlled respectively. Air pressure change during the whole control process is shown in Figure 8.
During the downstream mode test process, the vacuum pump was firstly turned on and then pumped at full speed, and air intake valve was adjusted to the position of micro intake, and then PID parameters of controller were self-tuned at about 300torr. After the auto-tuning is completed, 5 set points of 70, 200, 300, 450 and 600 Torr are controlled respectively. Air pressure change during the whole control process is shown in Figure 9.

By expressing the above control effects at different low pressure constant points as fluctuation rate, the fluctuation rate distribution in the entire range shown in Figure 10 and Figure 11 is obtained. It can be seen from fluctuation rate distribution diagram that fluctuation rate can be accurately controlled within the range of ±1% in the whole range of low pressure, and the large fluctuation at 12Pa is due to PID parameters obtained by self-tuning at 68Pa. It is unreasonable, and a separate PID parameter self-tuning is required.

3. Full-scale low pressure precise control implementation
It can be seen from the above air pressure precise control methods that different control modes can be selected according to actual needs. For example, the upstream mode can be selected for low air pressure control below 10torr, and the downstream mode can be selected for high air pressure control in the range of 10 to 1000torr.
In most low-pressure environment simulation test equipment, especially for the performance test of thrusters, it is necessary to reach precise control of air pressure and automation in the entire low-pressure range, so it is not the best choice to use or switch the upstream and downstream control modes alone.
In order to reach automatic and precise control in the full range of low pressure, we integrated the upstream and downstream modes, and proposed a technical scheme for two-way control mode. The overall scheme layout is shown in Figure 12.

In the full-scale control process of low-pressure, two capacitive vacuum gauges with different measurement ranges are required to cover the full-scale range. A continuous capacitive vacuum gauge and a continuous ionization vacuum gauge can also be used to cover a wider low-pressure range. For further information about our electronic needle valve, please visit https://www.genndih.com/proportional-flow-control-valve.htm

In the technical scheme of two-way control mode, higher requirements are put forward for controller and electronic needle valve, which are mainly reflected in the following aspects:
(1) It is required to have the ability to connect two vacuum sensors at the same time, and to switch between two vacuum sensors according to low pressure measurement value, so as to accurately measure and control the low pressure in time.
(2) The controller needs to have high measurement accuracy, such as 24-bit A/D sampling accuracy, to meet measurement accuracy requirements of different vacuum gauges and give full play to measurement capability of vacuum gauge.
(3) In two-way control mode, vacuum pressure controller is also required to have forward and reverse control functions, that is, reverse control for the upstream electronic needle valve and reverse control for the downstream electronic ball valve.
(4) In two-way control mode, electronic needle valve and electronic ball valve are responsible for the adjustment of upstream and downstream gas flow need to work alternately, so these electronic needle valves need to have the fastest possible response speed. The smaller the vacuum chamber, the smaller the air pressure inertia, the faster the response speed is required, the general requirement is that the time from fully closed to fully open of valve is within 2 seconds or even lower.
To sum up, by adopting low pressure control scheme of the above two-way mode, especially after adopting the new high-performance vacuum pressure controller and high-speed electronic needle valve, the precise control of low pressure full range can be reached.