Research on discharge effect of solar array with power supply subjected to hypervelocity impact

In the early exploration to space, the solar array was directly fixed on the main structure of space vehicles. With the development of space technologies and the rising demands for electron equipment, Paddle type solar arrays and most parts folding type solar array appear sequentially.¹ Nowadays, the solar array of orbiting space vehicle comprises mainly of the front coverglass used to absorb sunlight, the silica gel in the middle-layer as an insulation to connect the coverglass with the aluminum liner supporting the upper layers and damping. In 2005, L. X. Jiang and Y. Bai from Beijing Institute of Spacecraft Environment Engineering used an explosive accelerator to accelerate aluminum powder in micrometer level and impact on K8 optical glass and produce phenomena of abrasive wear due to collision craters. The transmittance of optical glass is measured using a CΦ-46 type of spectrometer. Results show that the transmittance in the abraded area was remarkably decrease.^2–6 In 2008, J. G. Huang, H. W. Li from Center for Space Science and Applied Research of Chinese Academy of Sciences investigated cumulative damage to the exposed spacecraft materials such as solar cells by the micro debris of Low Earth Orbit,^7–9 the results indicate that the transmittance of coverglass decreases after the micro-debris impact on the solar array, which was one of the main causes that weaken the solar array of space vehicle. In 1999, R. G. Nicholas utilized two-stage light gas gun to conduct the impact test that a 50μm sphere soda-limit glass projectile with impact angle from 0° to 75°obliquely impacted the solar array of space vehicle, results showed that impact velocity influenced minimally the diameter of shell-shaped crater on the condition that the impact angle was less than 45°, and if the impact angle exceeded 45°, the crater diameter decreased with the increasing of the incidence angles. In 2010, H. W. Li studied experimentally the solar array subjected to hypervelocity impact by micro-debris and analyzed the variation of short-circuit current, open-circuit voltage and maximum output power after impact.^10–13 F. Shinya employed a two-stage light gas gun in Laboratory of Spacecraft Environmental Interaction Engineering, Kyushu Institute of Technology to research the electrostatic discharge generated by the solar array impacted by LY11 aluminum projectile.^14–16 In the experiments, the operational conditions of orbiting space vehicle were simulated, the characteristic parameters of hypervelocity impact-induced plasma was diagnosed and electrostatic discharge was measured experimentally, the aforementioned data verified the detectability of the discharge phenomenon of plasma generated by solar array subjected to hypervelocity impact, moreover, the initial experimental data was obtained. Toyoda¹⁶ consider that the key factor selecting power supply was natural capacitance and response time in terms of the research on electrostatic discharge test system. Y. Akahoshi¹⁵ investigated experimentally the discharge generated by the solar array subjected to hypervelocity impact from LY11 aluminum projectile, and they extracted the relationship between ion current and impact velocity in the process of projectile perforating the liner. Extensive experimental data extracted by Y. Akahoshi indicated that continuous discharge occured when the solar array input power exceeded 110W.^17–20 The plasma discharge phenomenon induced during the solar array encountering the space debris were the main issues in the design of space vehicle. In this paper, discharge characteristics of the plasma during hypervelocity impact on solar array were analyzed by the experiments, which would contribute to raising the research level about hypervelocity impact, assessing the discharge of solar array and protective measurements should be taken, and thus provided the worthy references to the construction of shield for solar arrays against debris.

The experiments were conducted by using a two-stage light gas gun at Intense Dynamic Loading Research Center of Shenyang Ligong University, China. The gas gun can accelerate a projectile with the mass of 0.15g to a velocity of 7km/s. The chamber is arranged with a set of vacuum system that can eliminate the effect of gas to the plasma. Fig. 1 listed the layouts of sensors and target in the chamber.

In experiments, the impact angle was defined as the angle between the ballistic trajectory and target plane. Table I lists the basic experimental parameters.

The collision point was set as the coordinate origin O(0,0,0), the direction pointing to the upper ballistic trajectory was set as the positive direction of axis Y and the direction vertical upward to the target plane was set as axis Z. Axis X direction meets the right-handable rules. The Triple Langmuir Probe made of tungsten wire with the size of Φ0.28×20mm² was set perpendicularly to the 2A12 aluminum target to measure the characteristic parameters of the plasma and the central coordinates of the Triple Langmuir Probe are set as (-16,0,50) in all experiments. Fig. 2 lists the spatial rectangular coordinate system.

The Triple Langmuir Probe diagnostic system is used to measure the plasma characteristic parameters, which comprises of the Triple Langmuir probe, the DC power supply and an oscilloscope. Diagram of Triple Langmuir Probe diagnostic circuit is showed in Fig. 3.

The electron temperature T_e (eV) of plasma generated by impact can be calculated from the following equations

Where I₁, I₂ and I₃ are probe current, e is basic electronic charge, k is Boltzmann constant. Applied voltage V_d2=18V, V_d3 = 3V, resistance R₁=R₂=10kΩ. Plasma density N_e (m^-3) is expressed as²¹ 

Where M is ion mass and its unit of gram; S is the area of bare part of the single tungsten probe of Triple Langmuir Probe and its unit of mm²; I_i is current and its unit of μA; $ΔV=V3−V2$⁠, V2 and V3 can be determined via 4 channels of the oscilloscope.

In order to reveal the discharge phenomenon generated by solar array subjected to debris and mimic effectively the orbiting solar array operational state, additional power supply was employed to supply power inversely for the solar cells in laboratory, the solar array in the state was thus called power-supply solar array. The external circuit supply power to the solar array, data acquisition system of current and voltage in the external circuit and branch discharge current acquisition system in the branches were established.

The external circuit of the solar array comprises of the sensitive constant voltage source, resistance, voltage probe, current probe and the oscilloscopes. The constant voltage source acts on the solar array, supplying power to the solar array and its output voltage was constant. The discharge voltage, discharge current of the external circuit and the branch discharge current were collected by one voltage probe and three current probes. The voltage probe V_P was used to collect the discharge voltage in the external circuit of the solar array, the current probe C_p1 was used to gather the discharge current in the external main circuit of the solar array, the current probe C_p2 was used to gather the discharge current in the branches of the solar array, the current probe C_p3 was used to collect the discharge current in the branch between the solar array and liner, the data collected was stored and recorded via the oscilloscopes. Discharge voltage and current in the external circuit as well as the discharge currents in the branches was gathered and recorded by the four channels of the DPO4104B oscilloscope. Fig. 4 shows the power supply and discharge test system of the solar array.

In the circuit, to ensure the test circuit run smoothly, the resistance R was selected 100$Ω$ with high power; the voltage and current probes are produced by Tek.LTD, America, the TMDPO200 type voltage probe has a bandwidth of 200MHz, and the P6021A type current probes have a bandwidth of 60MHz. Fig. 5 illustrates physical diagram of the voltage and current probes. According to the experimental and data acquisition requirement, the gear of 2mA/mV was applied for the current probes and the discharge currents were recorded by Oscilloscope. The ratio of the measured value to the real value is 1:2, and the 75Vpk gear was used for the voltage probe to collect the discharge voltage in experiments.

Fig. 6 shows the solar arrays after impact in experimental shot 1.

It can be seen from Fig. 6 that the damage to solar array are observed in experiment shot 1. Rapid fracture mode of coverglass occurs due to the brittleness and the failure are determined by using maximum tensile stress theory. The process is explained that shock wave is induced by the hypervelocity impact, which reflects and transmits at the coverglass and silica gel interface. The reflected stress wave, in the form of inversely tensile stress, propagates in the coverglass while the rarefaction wave transmitting into the silica gel reflected at the silica gel and aluminum liner interface, and changes into inversely compressive wave. The remaining transmitted wave propagates into the 2A12 aluminum liner and strong shock wave forms in the plate. When the shock wave reaches the liner and its surface at certain vacuum, inversely tensile wave forms, and part of the wave returns into the liner, acting on the silica gel and coverglass propagating periodically in the coverglass and silica gel and aluminum liner system, then a complicated shock wave structure forms. Superposition enhancement and damping effect may occur in this shock wave structure, leading to numerous arc-shaped micro-cracks, which is due to the stress enhanced by the superposition effect exceeds the maximum tensile strength. The 4 pieces of contacting coverglasses fail since the projectile impacts at the center of the four-coverglass. The 2 adjacent coverglasses below are damaged more severely than the upper two slice because the projectile impacts the solar array downward.

The experimental photos at the different moments were acquired by superhigh speed camera of HSFC-Pro, which was made of PCO company, Germany. Fig. 7 shows the photos of solar array at the different moments in experiment shot 3.

One can see from Fig. 7(a) that the projectile had contacted the aluminum liner and a bulge was observed in the back of the liner at t=10μs. In Fig. 7(b), the projectile had perforated the solar array, meanwhile the debris cloud had not dispersed. From Fig. 7(c) to Fig. 7(f), the debris cloud dispersed gradually, the rarefaction wave generated during the impact reflected and acted on the coverglass again, forcing the coverglass splashed inversely and the compressed coverglass moved together with the target material through the perforation tunnel, and it is extracted from Figs. 7(d), 7(e), and 7(f) that with the sustainable impacting on solar array, forward debris cloud persisted expanding and broke the coverglass further, inducing continuous expanding axially and radically, such as the black substance in the photos.

Hypervelocity impact induces phase transition, leading the material gasified accompanied with plasma. Electron temperature and density are representative characteristic parameters. Fig. 8 shows the typical time history curves of electron temperature and density. Ionized vapor appeared at the shock interface of the aluminum projectile and target, the correspondingly generated plasma cloud and debris expanded and splashed, which was reflected as a dramatic increase of the electron temperature and density. With the expansion of plasma, reconstruction and inverse bremsstrahlung of the plasma, plasma electron temperature and density at the probe damped rapidly. It is noticeable in Fig. 8 that both of the electron temperature and density oscillate with multi-peaks, which is probably attributed to the unloads inversely after impacting, compelling the plasma to attach the crater secondarily or multiply expand and was caught by the Triple Langmuir Probe, and the other explanation is that the high-velocity impact induced plasma splashing inversely was obstructed by obstacles such as the Triple Langmuir Probe during its expansion.

In experiment, it is seen clearly that the waveforms of the discharge voltage and current in the main road as well as each branch circuit of the solar array are from the data collected by the oscilloscope. Fig. 9 shows the waveforms of the discharge voltage and discharge current from oscilloscope.

It can be seen from Fig. 9 that the voltage drops to zero at the moment that the projectile impact the target, this is because the shock generated during the projectile impacting the solar array leads to shock polarization at the interface of coverglass and aluminum liner, and polarizing the insulation layer between the solar cell and liner, besides, initial discharge in the cell-liner and the cell-cell is induced because the cell-cell was conducted by the plasma. At the moment, the external circuit turns into closed circuit, the external circuit current got right 2.2ms later, the current disappear and the circuit become open again. Fig. 10 illustrates the variation of discharge current against time extracted by Origin 9.0 software, where C_P1 is the discharge current in the main road, C_P2 is the discharge current in the cell-cell of solar array, C_P3 is the discharge current in the cell-liner of solar array.

One can see from Fig. 10 that the initial discharge in the cell-liner of solar array was induced by high-density plasma. The peak of a primary discharge current collected by C_P1, C_P2 and C_P3 were 1.0A, 0.6A and 0.7A, respectively. It can be seen from Fig. 10 that a secondary discharge was induced by the plasma in the main road and between the cell-liner and the discharge lasted for a long time, suggesting the severity of a secondary discharge. In a secondary discharge, the discharge currents peak by C_P1 and C_P3 were 0.9A and 0.8A, respectively. In Fig. 10(d), the occurrence time and deadline of discharge current by C_P1, C_P2 and C_P3 showed high synchronism regardless of a secondary discharge or a primary discharge, meanwhile the trend of the discharge current collected by C_P1 and C_P3 were identical in a secondary discharge. The occurrence of a secondary discharge was attributed to that the impact-induced plasma moved and gathered along the solar array direction before regrouping because of the voltage of 192V applied in the main circuit. When the plasma on the solar array were accumulated to a certain density, a secondary discharge of the adjacent cells with a high gradient potential was induced, and the discharge current amplitude was more evident than that of a primary discharge and the duration was longer. However, the high density plasma would probably induce arc discharge, which will result in the solar array carbonized and eroded. The typical primary discharge current history is illustrated in Fig. 11.

It can be seen from Fig. 11 that a primary discharge was induced by high-density plasma and conducting the cell-cell of solar array at the start of the projectile impacting the solar array, the peaks of discharge current acquired by C_P1, C_P2 and C_P3 were 0.42A, 0.53A and 0.18A, respectively. And a secondary discharge was not observed in shot 3, which was due to that bits of substance in low temperature splash at the rim of contact site of the projectile and solar array during the process of impact while most of the substance in high temperature was compressed in the molten pool by the residual projectile and not exposed enough, consequently, ionized particles cannot expand enough, and low-density plasma cannot induce a secondary discharge.

Experiments were conducted to investigate the hypervelocity impact on solar array by the projectile made of 2A12 aluminum with a various of impact velocities and angles. An independent-built Triple Langmuir Probe diagnostic system, data acquisition systems of the discharge current and voltage and an external circuit power supply for the solar array were applied to diagnose the plasma characteristic parameters and measure the discharge current and voltage, The following conclusions can be drawn:

1. The time histories of electron temperature and density were given, and lasting time of plasma and discharge duration were both about 2ms, however, discharge currents were less than 0.8A at the given experimental conditions.

2. Meanwhile, the characteristics of a primary and a secondary discharge current were revealed, especially, 4.2km/s may be the critical impact velocity induced a secondary discharge at the given experimental parameters and layout of solar array.

The authors would like to acknowledge National Natural Science Foundation of china (Grant Nos. 10972145, 11272218, 11472178), Open Foundation of Hypervelocity Impact Research Center of CARDC (Grant No. 20180201) and Open Project of State Key Laboratory of Explosion Science and technology in Beijing Institute of Technology (Grant No. KFJJ18-04M) to provide fund for conducting experiments.