Research on the Induced Electrostatic Discharge of Solar Arrays under the Action of ESD EMP

Abstract

In this paper, the electrostatic discharge of solar arrays in spacecraft energy systems is taken as the research object. The influence and internal mechanism of external electromagnetic radiation on electrostatic discharge is studied. Meanwhile, the charging and discharging test platform of spacecraft solar arrays under a strong field is first established. Then, the influence of irradiation field strength, electron beam energy and beam density on electrostatic discharge of solar arrays is analyzed and summarized. The results show that during the irradiation process of solar arrays using a high-energy electron beam under vacuum conditions, the higher the electron beam energy and the beam current density, the higher the discharge frequency of the solar array. When the intensity of the external electromagnetic radiation field increases, the discharge frequency also increases. Under the action of external radiation field with the same peak field strength, the larger the gap, the smaller the discharge frequency. With the increase in the field strength, the potential difference of each part of the solar array becomes smaller, and the peak of the discharge current decreases. The research results can provide technical reference for electrostatic protection of spacecraft solar arrays.

1. Introduction

Spacecraft electrostatic discharge is a relatively complex procedure in electromagnetic environments, since it is generally concerned with a space environment, spacecraft structure, size and material properties, etc. [1,2,3,4,5]. The solar array is an energy system widely used in spacecraft. With the continuous development of aerospace technology, the output power and bus operating voltage of solar array are increasing, which makes the electrostatic protection of spacecraft energy system impossible to ignore [6,7,8,9,10]. Under the influence of space electron irradiation and space plasma environment, the surface of spacecraft solar array in orbit will first generate unequal potential charging [11,12]. When the charge potential difference does not reach the discharge threshold, there will be no discharge hazard. However, it is very likely to induce the solar array to produce low-voltage electrostatic discharge due to the electromagnetic radiation environment of space, resulting in the working failure of the spacecraft energy system. Therefore, mastering the mechanism and law of electrostatic discharge induced by strong electromagnetic fields, in studying the electrostatic protection of space equipment, is of great significance [13,14,15,16].

However, the research on electrostatic discharges induced by strong electromagnetic fields is still in its infancy, mainly focusing on the study of metal electrodes [17,18,19,20,21]. Hefei Cao et al. obtained the variation in threshold voltage of needle-plate corona discharge in vacuum tube induced by electromagnetic pulse through the developed simulation experiment system of electromagnetic radiation field [17]. Hao Liu et al. used spark discharge to simulate electrostatic discharge electromagnetic pulse (ESD EMP) radiation source, and analyzed the influence of electromagnetic pulse radiation on the threshold voltage and discharge current characteristics of electrostatic discharge in vacuum tube [20]. Xining Xie et al. studied the basic discharge law of needle-ball electrode structure induced by electrostatic discharge radiation field in natural environment, and obtained the relationship between the induced discharge number and the voltage, position and distance of discharge gun [21]. Mengu Cho and others discussed the electrostatic discharge phenomenon, charging and discharging problems of spacecraft solar cell arrays [22]. Otherwise, we have investigated the charging and discharging characteristics of spacecraft typical dielectric materials, and obtained a certain law of electrostatic discharge induced by external electromagnetic field [23]. However, it is unclear about the discharge law of actual space equipment such as solar cell arrays induced by strong field.

In order to analyze the problem of electrostatic discharge induced by space strong electromagnetic field in solar array of space equipment energy system and improve the electrostatic protection performance of space equipment energy system, a series of related studies need to be carried out: firstly, the comprehensive simulation of space and its electromagnetic environment; secondly, the charging simulation method of the solar cell array; thirdly, research on the monitoring technology of the induced discharge.

In this paper, the solar array has been used as the test object. The high-energy electron beam has been used to simulate the space charged environment to irradiate and charge the solar array in the vacuum environment, and the ESD EMP has been used to simulate the radiation environment of the space strong electromagnetic environment, so as to study the characteristics of the solar array under the strong field. The basic law of the induced discharge under ESD EMP has been obtained, and the induced discharge mechanism is studied and analyzed, which can provide technical reference for electrostatic protection of space solar arrays.

2. Materials and Methods

2.1. Test Setup and Test Method

The vacuum tank, made of quartz glass (wave transmitting material, φ400 mm, 730 mm high), is mainly used as the experimental setup of electrostatic discharge of solar arrays induced by ESD EMP shown in Figure 1. Vacuum degree exceeds 10⁻³ Pa. The range of electron beam energy is 10–15 keV, and beam density is 9–14 nA/cm². The detailed test setting is revealed in Figure 2. The test sample is a 3J-GaAs solar cell dedicated to aerospace with each measuring 3 cm × 4 cm, and its structure diagram is shown in Figure 3a. The solar cells with 3 × 2 structure are installed in a two-stage series connection (see Figure 3b). The parallel gap between solar cells is adjusted to the specified value (0.5 mm, 1.0 mm and 1.5 mm).

Before the test, the fabricated sample was placed vertically in a fixed position of the vacuum tank, 35 cm away from the vertical coupling plate. During the test, the external DC voltage regulator added 12 V DC voltage to the solar array to simulate its working voltage. A certain electron beam was used to irradiate the solar array. The CT-1 current probe (Bandwidth 25 kHz–1 GHz, volt-ampere output characteristic 5 mV/1 mA) was used to collect the current signal on the discharge ground circuit. The oscillography was used for measuring the induced discharge waveform with the discharge frequency (discharge times per unit minute). Firstly, the sample was irradiated by ESD EMP radiated using the discharge gun and coupling plate. Then, the above methods were used to record the induced discharge under different ESD EMP field intensity irradiation conditions. The contact discharge mode was used between the discharge gun and the coupling plate to ensure the stability of the electrostatic field during the radiation, which was conducive to the repeatability of the test measurement. We repeated the test 5 times and taken the average value. The influence of electron beam energy, beam current density and radiation field intensity on the electrostatic discharge of solar array was studied. The test object is shown in Figure 4.

2.2. Calibration Result of ESD EMP Radiation Field

Figure 5 is the connection diagram of the ESD EMP radiation field strength test. Firstly, the relative position of the vacuum tank and the vertical coupling plate was fixed according to the actual test situation. The broadband ESD radiation field test system (3.5 Hz~1 GHz) was placed at the position of the electrode, and then the ESD Simulator voltage was set at −5~−30 kV with the interval of 5 kV. The broadband ESD radiation field test system and oscilloscope were used to test the radiation intensity of ESD EMP produced by the discharge gun. The communication between the field strength meter and the oscilloscope was carried out through optical fiber to prevent electromagnetic pulse from interfering with the oscilloscope and affecting the test accuracy. The peak field strength of ESD EMP under different discharge gun voltages is listed in

Table 1. Peak values of ESD EMP produced by different discharge gun voltages.

Discharge Gun Voltage (kV)	−1	−5	−10	−15	−20	−25	−30
Radiation peak field intensity (kV/m)	−0.5	−3.6	−6.6	−8.1	−10.2	−11.4	−12.3

, and the corresponding change trend is shown in Figure 6.

From the above chart, it can be clearly seen that the peak value of ESD EMP gradually increases with the discharge gun voltage increasing. Therefore, it can be deduced that the discharge gun voltage is directly related to radiation field strength of ESD EMP, which is an almost proportional relationship.

3. Results

3.1. Discharge Current Law of Solar Array under Electron Beam Irradiation

3.1.1. Typical Current Waveform Characteristics and Discharge Mechanism Analysis without ESD EMP Field

According to the connection test equipment shown in Figure 2, the solar cell array was charged by adjusting the electron energy of the electron gun to 6 keV and the beam current density to 6 nA/cm² at the vacuum of 10⁻³ Pa. The discharge waveforms are measured without external field irradiation. There are four main typical waveforms shown in Figure 7.

It can be seen from Figure 7 that the first pulse direction of current waveform (a) is different from (b–d), which is caused by the different charging potential polarity of different dielectric materials on the solar array. The specific description is as follows.

Negative charge discharge of PI film on aluminum (Al) honeycomb substrate: in the charging process of the high-energy electron beam to the PI film dielectric material on the surface of the solar cell, the PI film surface accumulates electrons continuously, namely negative charges due to the secondary electron emission coefficient of the material below 1. When the Al honeycomb substrate is grounded, with the accumulation of negative charges on the PI film, the negative potential on the surface of the film increases gradually, and the electric field strength from the material to the Al honeycomb substrate increases continuously. When the electric field strength exceeds the breakdown electric field strength of PI film, the electrostatic discharge will be generated, and the discharge current as shown in Figure 7a will appear, that is, the discharge current of negative charge on honeycomb substrate after PI film charging.

Positive charge discharge of solar cell on the Al honeycomb substrate: under the irradiation of electron beam with a certain range of energy, the secondary electron emission coefficient of glass material will be greater than 1 [24]. The main reason why the discharge current waveforms in Figure 7b,c appear in this experiment is that during the irradiation of electron beam on the glass cover of the solar cell, the amount of electrons lost on the surface of the glass cover is larger than that obtained, which leads to a positive charge. With the accumulation of positive charges on the surface, the surface potential increases continuously, and the electric field strength between the cell sheet and the honeycomb base increases continuously. When the electric field strength exceeds the breakdown electric field strength of the PI film, the positive charge electrostatic discharge will be generated. The main pulse shown in Figure 7b,c is the positive discharge current waveform.

Surface flashover discharge between the glass cover and PI film of solar cell: the positive potential on the surface of the glass cover and the negative potential on the surface of PI film of the solar cell increase simultaneously under the irradiation of the electron beam. Furthermore, the electric field strength between the glass cover and PI film increases continuously. When the electric field strength exceeds the breakdown electric field strength of desorption gas on the surface of PI film, the surface flashover discharge occurs. In the dark environment, the surface flashover discharge of solar array was also observed. However, as there is no current released through the ground, it cannot be measured by measuring the ground current.

Regarding the electrostatic discharge between the glass cover of solar cell and the Al honeycomb substrate and PI Film: the positive potential on the surface of the glass cover of the solar cell and the negative potential on the surface of the PI film increase simultaneously under the irradiation of the electron beam, both of which increase the electric field strength of the honeycomb substrate simultaneously. It is likely to induce the negative charge discharge between the nearby PI film and the Al honeycomb substrate. The first pulse is positive and the main pulse is negative as shown in Figure 7d. However, it is not found out that the first pulse is negative and the main pulse is positive in the experiment, which is supposed to be related to the structure of the solar array. That is to say, as the PI film lies between the solar cell and the Al honeycomb substrate, the electrostatic discharge of the charge on the Al honeycomb substrate easily induces the electrostatic discharge of the charge on the PI film. On the contrary, the charge on the PI film does not easily induce the positive charge on the cell to produce electrostatic discharge.

3.1.2. Analysis of Discharge Current Law under External Field

The electron energy of the electron gun was fixed to 12 keV and beam density to 9 nA/cm², and the peak value of ESD EMP field was adjusted to the specified value (0, 1.2, 2.4, 3.6, 6.6, 8.1, 10.2, 11.4 and 12.3 kV/m). The electrostatic discharge waveforms of the solar array under different ESD EMP radiation intensities are shown in Figure 8. The variation trend of electrostatic discharge current amplitude under different radiation field strengths is shown in Figure 9.

It can be seen from Figure 8 and Figure 9 that in the process of using electron beam to irradiate the solar array, with the increase in the peak field strength of the external radiation field, the peak value of the discharge current will gradually decrease. When there is no external field, the peak value of discharge current of solar array reaches 1.4 A. When the peak field strength of the external electromagnetic field reaches 12.3 kV/m, the peak value of the discharge current is 0.3 A, and the decrease in amplitude is 7/10 of that without the external field. According to the previous analysis, when the external electromagnetic field is coupled to the solar array bus bar, Al honeycomb and other metal parts will form a local highly electric field, which is superimposed on the self-built electric field of the deposited charges on the dielectric surface. On the one hand, the field-induced carrier’s mobility increases gradually, and the conductivity of the material increases. According to the charge discharge balance principle of dielectric material, the surface charge of dielectric material easily dissipates in the process of electron beam charge discharge, and the surface potential to reach the balance is reduced. At this time, if electrostatic discharge occurs, the discharge current becomes smaller. On the other hand, the superposed strong field increases the desorption gas of PI film, which forms a thin surface gas layer, and ionizes into plasma under the action of strong field, thus reducing the surface potential threshold of electrostatic discharge of the solar array [25]. At the same time, the superposition of the strong electric field may directly enhance the field emission effect at the triple junction, which makes the surface of interconnector release electrons. Under the acceleration of the electric field, more electrons are released again by electron bombardment on the surface of the glass (secondary electrons). So, the secondary electron avalanche is formed repeatedly. With the participation of the desorption gas, the flashover channel is finally formed. The stronger the radiation field is, the stronger the three effects are. The lower the breakdown potential, the weaker the intensity of electrostatic discharge, and the smaller the peak value of the electrostatic current.

3.2. Effect of Different Conditions on Discharge Frequency

3.2.1. Effect of Electron Beam Energy on Discharge Frequency

The electron beam with a beam density of 9 nA/cm² has been used to irradiate the solar array with a parallel gap of 1 mm. The discharge frequency of the solar array varies with different electron energy in different ESD EMP radiation field strengths as shown in Figure 10.

It can be seen from Figure 10 that in the process of using a high-energy electron beam to irradiate the solar cell, increasing the energy of electron beam will increase the discharge frequency of the solar cell. When the higher the energy of electron beam is, the more positive and negative charges are accumulated on the surface area of the battery array, the greater the potential difference between different materials, and the easier it is to produce electrostatic discharge.

Analysis: according to the structure of the solar array shown in Figure 3a, the PI film and the Al honeycomb structure can be equivalent to the dielectric metal electrode structure. When the high-energy electron beam is used to irradiate the solar array, electrons will gradually accumulate on the surface of the dielectric material, making its surface potential gradually increase. At this time, an electrostatic field is formed between the dielectric surface and the back grounded AI honeycomb plate; when the electrostatic field exceeds the breakdown field of PI film, electrostatic discharge will occur. In the dynamic process of charge and discharge, with the increase of electron energy, the surface potential of PI increases and the discharge frequency increases. Similarly, the external radiation will also increase the discharge frequency of solar array, and with the increase of electron energy, the induced discharge frequency will also increase.

3.2.2. Effect of Electron Beam Density on Discharge Frequency

When the solar array with a parallel gap of 1 mm is irradiated by an electron beam with an electron energy of 12 keV, the variation trend of the discharge frequency of the solar array with the electron beam density, under different ESD EMP radiation field strengths, is shown in Figure 11.

It can be seen from Figure 11 that in the process of using a high-energy electron beam to irradiate the solar cell, increasing the electron beam density will increase the discharge frequency of the solar cell. The higher the electron beam density is, the faster the rate at which positive and negative charges accumulate on the surface area of the battery array. More charges can be accumulated in unit time, so that the potential difference between different materials increases and it is easier to produce electrostatic discharge.

Analysis: according to the above analysis, the PI film and Al honeycomb structure in the solar array structure can be equivalent to the dielectric metal electrode structure. When the high-energy electron beam is used to irradiate the solar array, the surface of the dielectric material will gradually accumulate charges and the surface potential will gradually increase. At the same time, the electrostatic field is formed between the dielectric surface and the Al honeycomb plate grounded on the back. When the electrostatic field exceeds the breakdown field of PI film, electrostatic discharge will occur. When the charging current of solar cell array by electron beam is equal to the electrostatic discharge current generated by the cell array, the solar array will reach a balance process of electrostatic charge and discharge, in which the increase of electron beam density will inevitably lead to the acceleration of the electrostatic charge rate on the surface of the solar array, and the accumulated charge on the surface of the solar array will increase in unit time. To achieve a new equilibrium state of electrostatic charge and discharge, the discharge rate of charge must be accelerated. One of the ways is to accelerate the electrostatic discharge rate of the solar array. Therefore, with the increase in electron beam density, the discharge frequency of the solar array increases.

3.2.3. Effect of Radiation Field Intensity on Discharge Frequency

The electron beam has been used to irradiate the solar array with a parallel gap of 1 mm. Under different electron beam density, the variation trend of discharge frequency of the solar array with ESD EMP radiation field strength is shown in Figure 12.

It can be seen from Figure 12 that in the process of using a high-energy electron beam to irradiate the solar cell, the ESD EMP radiation field will increase the discharge frequency of the solar cell; that is, induced electrostatic discharge will be generated, and the larger the ESD EMP radiation field is, the more easily the induced discharge will be generated.

Analysis: when using the external field to irradiate the solar array, on the one hand, due to the strong field radiation effect, the surface material of the solar array polarizes the internal molecular structure of the medium, increases the conductivity of the medium, and reduces its dielectric properties and the breakdown field strength of the surface medium. On the other hand, due to the coupling effect of electromagnetic radiation, the electromagnetic radiation generates a coupling voltage on the bus bar, which establishes an electrostatic field between the bus bar and the grounded Al honeycomb substrate. It will be superimposed on the original electrostatic field due to the irradiation of the electron beam, resulting in induced electrostatic discharge, thus increasing the discharge frequency of the solar array.

3.2.4. Effect of Parallel Gap of Solar Array on Discharge Frequency

An electron beam with energy of 12 keV and beam density of 9 nA/cm² is used to irradiate the solar array. Under specific parallel gaps of the solar array, the change trend of discharge frequency with different ESD EMP radiation intensity is shown in Figure 13.

It can be seen from Figure 13 that under the action of no external field, the discharge frequency of 0.5 mm parallel gap of battery array is 11 times/min, while the discharge frequency of 1.5 mm parallel gap of battery array is 20 times/min, which is nearly doubled; When the peak value of ESD EMP is 12.3 kV/m, the discharge frequency of 0.5 mm parallel gap is 22 times/min, while the discharge frequency of 1.5 mm parallel gap is 30 times/min, which is nearly 1/2 times faster. In this case, the potential difference between the left and right adjacent solar cells is 6 V. When the electrode gap is reduced from 1.5 mm to 0.5 mm, the average electric field intensity between the two electrodes decreases from 4.5 × 10³ to 1.2 × 10³ V/m. The average field strength increases by an order of magnitude. When the gap of parallel cells is reduced from 1.5 mm to 0.5 mm, under the simulated working voltage of the solar array, the variation of the extremely uneven field strength between the left and right adjacent cells is greater. While the parallel gap of the solar array becomes smaller, other conditions remain unchanged, and the increased electric field is superimposed on the electrostatic field between the PI film or the solar array and the grounded Al honeycomb plate, which makes the solar array easier to discharge. As the gap of the solar array decreases, the frequency of electrostatic discharge of the solar array increases.

4. Conclusions

In this study, we have successfully explored the charging and discharging characteristics of solar arrays in spacecraft energy systems via establishing an experimental research platform. The influence of electron beam irradiation with or without external ESD EMP field on electrostatic discharge law of solar arrays have been studied. Especially, we achieve the electrostatic discharge law under different influence factors including the electron beam energy, beam density, irradiation field strength and parallel gap of solar arrays. The obtained discharge law can provide theoretical support for the structure design of solar arrays, as well as provide technical reference for spacecraft electrostatic safety and protection.