Part B
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been made in a parallel direction: the main purpose is to understand if the Alta's low- power Hall thruster HT-100 is compatible with a Direct-Drive configuration.
This is the first European experiment of a Direct-Drive and follows the experiment of J.A. Hamley et al. in 1997 [25] and the extensive experimental campaign carried out at NASA’s JPL in 2012 by J.S. Snyder et al. [58].
The main objective of this experiment, whose duration and level of detail was limited by the time of a thesis, was to realize a Direct-Drive system in which a safe ignition and a correct operation of the HT-100 supplied by a solar array are achieved and then to understand the throttleability of the system, its stability limits, operative points, etc.
The other goal, which is complementary to the main one, was to design an electrical filter and understand its effectiveness during both ignition and steady state operation. The current oscillations were measured on the panel and thruster side and then compared in order to understand the shielding capability of the filter. The current oscillations are particularly important because may impede the correct system operations or even damage the solar panels. The ignition is also of particular interest since it entails high current peaks that the solar panel cannot afford: in a Direct-Drive system, during ignition, the filter must provide the excess current in order to assure the proper thruster start up.
The filter designed and realized can be considered only a preliminary version; in order to optimize its performance, it was useful to gather data as a starting point for a future further development.
Another aspect of the experimental test was the selection, understanding and arrangement of the photovoltaic system since its I-V curve determines the thruster operating conditions. The solar panel performance depends on several factors such as the typology of the solar module, the irradiance level, the temperature of the solar cells, the plane inclination with respect to horizontal, etc. An important effort was made in this direction in order to take into account all these elements for a proper Direct-Drive system design.
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9 Test Setup
In this chapter the instrumentations and facilities employed during the whole test timespan are listed and described. They were provided by Alta S.p.A. and Alta S.p.A.
Divisione Energia.
9.1 Thruster
For the present study the Alta’s Hall thruster HT-100 [64] has been selected. The HT- 100 is the smallest and lowest power Hall Effect thruster ever developed in Europe and is designed to perform orbit control tasks on micro-spacecraft and attitude control tasks on mini-satellites with low power production capability. The development of this Morozov-type Hall Effect Thruster started in 2003 and it has been progressing in various phases.
Figure 9.1: HT-100 firing at Alta IV-4 test facility (left) and HT-100 mounted on Alta’s thrust balance (right)
The magnetic field is generated by permanent magnets. The EM (Engineering Model) thruster is equipped with a discharge chamber made of a mixture of BN/SiO2 (60/40) and a conventional anode and propellant distribution. The thermal load produced by the discharge plasma is dissipated by the outer case radiating surface and the thruster body is insulated from the thrust stand by means of a cylindrical hollow mounting interface. This
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element also provides accommodation to gas lines, power connections and electrical insulator protecting them from the contamination of the surrounding plasma.
The HT-100 hollow cathode has been recently re-designed and the Direct-Drive test was carried out in this early phase of cathode life during which its performance have not reached the desired one yet. This means that during tests the matching between the thruster and the cathode was not optimized.
For instance, although it was designed to work only during startup, up to now, for correct thruster operations the cathode is required to be always active, also after ignition.
The thruster performances and characteristics are summarized in table 9.1.
Power (W) 80 - 300
Thrust (mN) 5 - 12
Isp (s) 800 – 1200
Efficiency Up to 35%
Propellant Xenon (99.996%)
Thruster Feed Pressure (bar) 2.5
Thruster Unit Mass (g) 600
Thruster Envelope (mm) Ø 76 x 35 (Cathode excluded) Table 9.1: HT-100 performance and characteristics
The thruster operating voltage commonly ranges from 200–400 V whereas current goes from 0.4 to 1.4 A.
9.2 Solar Array
The solar panel configuration was chosen in order to match the HT-100 voltage and current carachteristic: the goal was to obtain an I-V curve that allowed to test the Direct- Drive system at the solar panel peak power point. The type of solar cell that best fits this requirement is the thin film technology, since it provides high voltages coupled with low current levels with respect to standard silicon cells. The module selected was the Schüco MPE 90 AL 01 whose main characteristics are shown in the table 9.2.
In order to achieve the desired performances an arrangement of 4 modules connected in series was selected, obtaining a panel whose nominal characteristics are summarized in table 9.3.
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Nominal Power (W) 90
Peak Power Voltage (V) 73.80 Peak Power Current (A) 1.25 Open Circuit Voltage (V) 95.30 Short Circuit Current (A) 1.56
Efficiency (%) 6.3
Table 9.2: Schuco solar module main characteristics at an irradiance of 1000W/m2, an air mass of AM 1.5 and a cell temperature of 25° C
In addition, an electrical box has been installed to ensure safe operation and manage the switching of the system. The box was interposed between the solar array and the filter unit and included a 1000 Vdc breaking and manoeuvering switch and a couple of 2 A fuses.
Nominal Power (W) 360
Open Circuit Voltage (V) 381.2 Short Circuit Current (A) 1.56 Peak Power Voltage (V) 295.2 Peak Power Current (A) 1.25
Table 9.3: Solar Panel nominal characteristics at an irradiance of 1000W/m2, an air mass (AM) of 1.5 and a cell temperature of 25° C.
9.3 Test Facilities and Instrumentation
9.3.1 Vacuum Chamber
The thruster was tested in the Alta’s IV4 facility (figure 9.2) [71] which represents the cornerstone of Alta’s Hall thruster test activities.
This chamber consists of a main vessel (Auxiliary Chamber, AC), 2 m in diameter and 2.5 in length connected through a 1 m gate valve to a service chamber (Small Chamber, SC), 1 m in diameter and 1 m in length. The chamber is made of stainless steel AISI 316 L characterized by a low magnetic relative permeability (μr<1.06).
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Figure 9.2: Alta’s IV-4 vacuum facility
9.3.2 Pumping System
The facility is equipped with two indipendent oil-free pumping system, one connected to the main vessel (AC) and the other connected to the SC for redundancy. The pumping system is constituted by a 25 mc/hr rotary pump, a 2000 l/s turbomolecular pump used for evacuation, a cryogenic high vacuum stage based on 1 x 3000 l/s cryopump and a 6 x 12000 l/s custom cold plates. The pumping speed reached is about 70000 l/s allowing a ultimate vacuum level less than 10-7 mbar. The pressure level inside the chamber is monitered by three Leybold-Inficon ITR90 Pirani/Bayard-Alpert sensors.
9.3.3 Laboratory power supply
The anode power supply used is the Delta Elektronika BV SM 600-10 which allows to regulate the output voltage up to 600 V and the output current up to 10 A, providing a maximum power capability of 6 kW.
The power supply used for cathode operations is the Hüttinger PFG-5000 capable to reach up to 1000 V.
9.3.4 Instrumentation
In order to measure and record current and voltage oscillations during the test was used the digital Tektronix DP4104 oscilloscope. The data were acquired with a clamp-on current probe with adjustable scale. The solar irradiance was measured with a pyranometer (LP PYRA 02, figure 9.3) with a sensitivity of 9.51 μV/(W/m2). It was
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connected to a multimeter whose voltage data (readable on the display) was the value to convert in W/m2. Then, a thermocouple was employed for solar cells temperature measurements.
Figure 9.3: The pyranometer (LP PYRA 02)
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The filter is an essential element of a Direct-Drive system since it has the function of damping the thruster current oscillations in order to ensure the correct functionality of the solar array. Therefore one of the first steps in the Direct-Drive implementation was the design of the filter. Arising from the NASA experiment at JPL [58], it was decided to use the PSpice software in order to electrically simulate the Direct-Drive system behavior under different filter configurations. The simulation allowed understanding whether or not a certain filter configuration was effective in damping the thruster oscillations even if it does not provide very accurate results. The difficulties in predicting precise results derive from the fact that, first of all, the thruster modeling is a very complex task and then, the measurement of all the circuit parasitic characteristics, whose contribution is not negligible, is difficult.
Once decided the filter configuration, it was initially tested with the laboratory power supply instead of the solar panels. This additional step was meant to understand if the filter was effective and compatible with thruster operations. In the next paragraphs, it is shown the procedure for the Pspice modeling of the Direct-Drive circuit in its components.
The performance of different filter configurations with different values of capacitance and inductance was studied in order to adopt the best solution. Then, a test of the filter- thruster coupling was carried out using the standard power supply.
10.1 Hall Thruster Modeling with Pspice
The Hall thruster behavior modeling with Pspice is certainly the most challenging part of the simulation. However the accurate modeling of the HT-100 during transients and steady-state operations and at all the possible I-V combinations is a demanding task which goes beyond the scope of the present analysis. In fact in this context the simulation is only useful to conduct a preliminary design of the filter architecture. In [58] the Hall thruster is modeled as a time-varying resistor described by a sine wave. The steady-state part of the resistor was chosen to fix the thruster operating point while the time-varying
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amplitude was decided in order to match the measured current oscillations. In this work a similar approach has been employed even if the parts constituting the circuit are completely different because the Pspice student version does not comprehend all the elements available in the full version. The idea is to treat the thruster as a current oscillations generator which produces current waves analogous to the ones experimentally measured. As a consequence, it behaves also like a time-varying resistor.
The HT-100 circuit schematization is illustrated in figure 10.1. The two blocks ESUM and GVALUE are contained in the ‘ABM’ (Analogue Behavior Modeling) library, in particular the first one is a voltage-controlled voltage source whose output is derived from the sum of the two input signals. The GVALUE is a voltage-controlled current source characterized by a proper transfer function, thus it generates a current wave proportional (according to its transfer function) to the input voltage signal.
Figure 10.1: Hall thruster schematization with Pspice
The voltage generators (VPULSE and VSIN) are the two sources through which the current oscillation profile are modeled, in fact by suitably choosing their parameters it is possible to reproduce the shape of current fluctuations. In particular, the part VPULSE (in the ‘Source’ library) can generate several voltage waves of different shapes (triangular, square, trapezoidal, etc.) and in this analysis its parameters are selected in order to produce a triangular wave. The part VSIN (in the ‘Source’ library) generates voltage sine waves. The sum of these two voltage signals (by means of ESUM) enters into the GVALUE block whose transfer function (Iout/ΔVin) is simply 1/Rt where Rt is the thruster steady-state resistance. Therefore, the GVALUE outputs into the circuit the desired
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current fluctuations. Summarizing, by selecting the amplitude and frequency of the voltage generators together with the term Rt, the circuit produces the desired current oscillations.
The choice of the parts VSIN and VPULSE arises from the fact that the measured current oscillations (see figure 10.2) can be well reproduced by the sum of a sine plus a triangular wave. The values of the parameters that define these two functions must be selected according to the oscillations’ characteristics and thruster operating conditions:
Imax (current peak), Imin (current minimum), Rt, Vop (operating thruster voltage).
As already said, the schematization presented here is different from the one carried out at NASA where the current wave shape has a sine pattern. Here it was decided not to adopt this kind of wave because it was too far from the real HT-100 behavior. The figure 10.3 shows the thruster current oscillation simulated with Pspice for the same operating conditions of the figure 10.2. It is possible to notice that the two current waves have a very similar shape.
Figure 10.2: HT-100 current oscillation at anode voltage of 300 V and anode flow rate of 1 mg/s (scale 100mv/A)
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Figure 10.3: HT-100 simulated current oscillation
In this simulation, only the thruster ‘breathing’ frequency is considered and all the other fluctuations at higher frequencies are neglected. However, current oscillations at higher frequency can be easily implemented in the model. Moreover, thruster behavior modeling at start-up or shut-down is not taken into account. In the figure 10.1 it is possible to notice some ground connections; it is due to the fact that, for a correct simulation with Pspice, all the circuit elements must have at least one path to ground with respect to which the voltage is computed. The 100g (g stands for giga) resistors are placed where a current passage toward ground must be avoided.
Although the limited availability of electronic components and the difficulties in simulating the thruster physics behavior, this electrical scheme has been very useful in understanding how the filter had to be designed.
10.2 Solar Array Modeling with Pspice
In order to design the filter unit and study its effectiveness in shielding the oscillation produced by the Hall thruster it is necessary to model the behavior of the solar panel that feeds the Direct-Drive system. As seen before, the solar array is made of four thin film modules developed by Schüco, each one providing 90 W of nominal power. The overall characteristics of the solar panel are reassumed in the table 10.1. To replicate the behavior of the solar array in the Pspice environment a piecewise linear model was used [2]. This model utilizes three parallel diodes which are used to simulate the I-V curve of the solar array dividing it into several segments.
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Nominal Power (W) 360
Open Circuit Voltage (V) 381.2 Short Circuit Current (A) 1.56 Peak Power Voltage (V) 295.2 Peak Power Current (A) 1.25
Table 10.1: Solar panel performance at an irradiance of 1000 W/m2, an air mass (AM) of 1.5 and a temperature of 25°C
Each diode is modeled as simple piecewise linear (PWL) voltage controlled resistor with two states ON and OFF as shown in figure 10.4. Of course the PWL model can be improved by approximating the curve with more segments by connecting more diodes in parallel with suitable values of series resistances. In addition to these resistances other two are added: a shunt resistance (Rsh) and a series resistance (Rs), both shown in figure 10.4, which are present in the classical schematization of a solar cell or more in general in a circuit modeling of a photovoltaic system.
Figure 10.4: Solar array circuit schematization with Pspice
The model is primarily developed to simulate single PV cell. However, it is easy to scale up the model to account for the overall photovoltaic (PV) panel. Scaling up to simulate the bulk PV panel can be carried out using two methods:
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1. Bulk PV Model: Where the experimental data of the solar panel are used to compute directly the values of diodes ON-voltages (Von1, Von2, and Von3) and series resistors (Ron1, Ron2, and Ron3).
2. Single PV cell Model: the basic PV solar cell, is modeled, then the total voltage produced from the panel is computed from: Vpanel = nVcell, while the current through all cells is identical.
For the purpose of this study the first method is more suitable since we want to simulate the entire solar panel coupled with the Hall thruster. The procedure adopted here is a slightly modified version of the one presented in the reference [2] since it includes in the calculations also the shunt resistance.
At this point it is possible to calculate the values of the diodes ON-voltages and series resistors utilizing the data in the table 10.1. The calculation procedure is almost straightforward: with the three diode structure the I-V curve obtained is divided into four segments. To obtain these segments it is necessary to decide the locations of their vertex points; one point will be positioned in correspondence of the peak power voltage VPP and the others will be symmetrically arranged with respect to this value at 0.95VPP and 1.05VPP
(this is an arbitrary choice). These values correspond to the ON-voltages of the diodes (Von1, Von2, and Von3). To evaluate the series and the shunt resistors it is necessary to consider the operation in each segment:
Segment 1: VD < Von1 (Von1 = 0.95*VPP = 280.4 V)
When the generated voltage (VD) is less than the ON-voltage of the first diode (Von1), all diodes are off and no current flows through them. In this way the generated current flows through the load and a small portion can flow through the shunt resistance Rsh. Considering that in this segment the I-V curve goes from the short circuit current point (Vsc=0; Isc=1.56 A) to the point corresponding to the On-voltage of the first diode (Von1=280.4 V; Ion1=1.35 A), it is possible to determine the shunt resistance with the following equation:
1 1 on 1335
sh
sc on
R V
I I
Segment 2: Von1 ≤ VD < Von2
The diode D1 is ON. In this segment the current goes from the Ion1 current (1.35 A) to the current (1.25 A) in the real I-V curve of the panel corresponding to the voltage Von2 that
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is the peak power point of the panel. The current through the shunt resistance is computed from:
2 0.221
on sh
sh
I V A
R
The current through the diode D1 (ID1) is:
1 1 0.089
D sc sh on
I I I I A
Than the first series resistance Ron1 is:
2 1
1
1
164.4
on on
on
D
V V
R I
Segment 3: Von2 ≤ VD < Von3
Diodes D1 and D2 are ON. The current through the shunt resistance is now:
3 0.232
on sh
sh
I V A
R
The current through D1 is calculated as follows:
3 1
1
1
0.180
on on
D
on
V V
I A
R
Since the output current (Ion2) of the solar panel corresponding to the voltage of Von2 = 310 V is 1.10 A, the current through the diode D2 is:
2 1 2 0.048
D SC sh D on
I I I I I A
Ron2 is then computed from the equation:
3 2
2
2 on on 296
on
D
V V
R I
Segment 4: Von3 ≤ VD < VOC
All diodes are ON. The panel load current is zero at the open circuit point. The current through Rsh is obtained from the equation:
0.286
OC sh
sh
I V A
R
The current through D1 is calculated from equation:
1 1
1
0.613
OC on
D
on
V V
I A
R
The current through D2 is computed from:
2 2
2
0.290
OC on
D
on
V V
I A
R
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The current through the diode D3 is finally evaluated:
3 1 2 0.371
D SC sh D D o
I I I I I I A
Where Io is the output current of the solar panel, which in this case is zero because we are considering the open circuit point.
Ron3 is then computed from the equation:
3 3
3
191.9
OC on
on
D
V V
R I
Then, the series resistance (Rs) was fixed to 1 mΩ; this is not a physical realistic value, however this choice was made thinking Rs only as a curve fitting parameter in order to well reproduce the I-V characteristic.
Substituting the values obtained from the above calculations in the electrical circuit (figure 10.4), the solar array I-V curve can be derived by running the Pspice simulation (figure 10.5). It must be noticed that this panel performance, used for the following simulations, reproduce the solar array behavior in standard atmospheric conditions.
Figure 10.5: Nominal solar panel I-V curve simulated with Pspice
10.3 Direct-Drive System Simulation for Filter Design
The following step was the selection of the filter configuration; in fact there are several examples in electric propulsion literature of passive filter arrangements where the only common element is a capacitor, placed in parallel with respect to the thruster and the power supply. The capacitor works as a reservoir of charge; this means that it provides the current peaks to the thruster, attenuating in this way the power supply task. In order to optimize the filter performance, resistors and/or inductors are sometimes added to the
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filter scheme as it is possible to found in lots of examples of DC-DC converters which supply Hall thrusters.
In the previously mentioned work at JPL [58] the baseline filter architecture for Direct- Drive operations is the same utilized during operations with the standard power supply: a paralleled 80 F electrolytic capacitor. According to following experimental tests, this simple scheme is not satisfactory for the HT-100 case. The most effective architecture has turned out to be a LC layout, the one characterized by a paralleled capacitor and a series inductor (figure 10.6).
Figure 10.6: Filter architecture. Yellow circles represent the location where measurements were taken
A bleed resistor (1 MΩ) was placed in parallel to the capacitor for safety reasons; it does not affect the filter behavior but it is needed for capacitor discharging once the system is turned off.
Another crucial point in the simulation was the assessment of the parasitic properties of the circuit. Idealizing the wirings by setting up as zero their parasitic resistance, inductance and capacitance provides misleading results, in fact their contribution is not negligible at all. For what concerns the parasitic resistance, the evaluation was not a demanding task and it has been added in the circuit with a good precision. On the inductor side the parasitic resistance is greater since the coil’s wire determines a significative contribution. For what concerns the parasitic inductance, the measurements were not possible, therefore their values have been chosen according to coherently hypothesized values; then the parasitic capacitance has been neglected. The complete Direct-Drive circuit is shown in figure 10.7 where it is possible to recognize (from left to right) the schematization of the solar array, the filter and the thruster.
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Figure 10.7: Direct-Drive electrical circuit (Pspice)
Obviously the simulations have been useful not only for understanding which filter configuration was the most suitable but also for evaluating the size of the elements composing the filter, in particular the capacitance (C) and the inductance (L). The operating point at which the investigation was carried out is the solar array peak power point where the solar array produces a voltage of 295.2 V and a current of 1.25 A (Rt = 236 Ω). It is reasonably assumed that the peak power point is the most critical situation for the system operations. In this condition, the thruster current oscillates (mainly at 40 kHz) between 3 A and 0.2 A.
The simulation was run for different values of L and C in order to assess the most suitable combination which ensures proper filter operations. The Pspices results are shown in the table 10.2 in terms of current peak-to-peak value (sometimes called ripple factor) which is an easily measurable quantity. The data are also reported in the double- logarithmic plot of figure 10.8.
The analysis of the results evidences that higher values of both C and L generally give better peaks damping. While capacitance of 0.01 and 0.1 F are to discard, 1 and 10 F provides essentially the same performance. Also increasing the inductance has the effect to slightly reduce the peak-to-peak value.
The ultimate choice of the filter elements had to take into account also other specifications: for the capacitor the voltage and the ripple factor, for the inductor the current flowing in its wire. According to these considerations and in order to adopt a conservative arrangement, a 10 F electrolytic capacitor and a toroidal choke of 1 mH were selected. The last value, after a dedicated measurement, came out to be quite higher that is 1.4 mH. Despite the good performance shown also by the 1 F capacitor, this choice was not possible since any capacitor with that value of capacitance and suitable value of voltage and ripple factor were available.
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In principle, the filter could be constituted by only a 10 F capacitor; however this configuration was rejected after experimental tests. In fact a preliminary test with the laboratory power supply showed that this arrangement was not able to dampen enough the current peaks.
Capacitance C
( F) Inductance L (mH)
Current Peak-to-Peak value on Solar Array Side (A)
0.01 0 4.80
0.1 0 0.11
1 0 0.02
10.0 0 0.01
0.01 0.1 3.30
0.10 0.1 0.10
1 0.1 0.01
10 0.1 <0.01
0.01 1 4.80
0.1 1 0.08
1 1 0.01
10 1 <0.01
Table 10.2: Pspice simulation results for different capacitance and inductance values
Figure 10.8: Double-log plot of the simulation results
The picture of the filter unit is reported in the figure 10.9; it can be noticed also the presence of the bleed resistor in parallel with the capacitor.
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Even if the voltage oscillations were considered and analyzed in the previous simulations, no results are here provided because they are, by far, of minor concern in a Direct-Drive system. First of all their intensity, with respect to the mean value, is low (typically less than 10%) and then, while the solar panel has a stringent limitation in current it is not true for voltage. In fact, in the thruster voltage range the solar panel can easily afford small voltage variations.
Figure 10.9: Filter unit
10.3.1 Limits of the Simulations
As already anticipated, the simulations have been useful in understanding how the filter should be arranged and in selecting the filter elements size. However there are two schematizations that limit the physical veracity of the analysis. The first is the thruster modelization which takes into account only the breathing mode of oscillations, neglecting the higher frequency modes. The second is the approximate knowledge of the parasitic characteristics of the circuit.
The influence of the parasitic characteristics of the circuit is clearly observable if the frequency response of the filter transfer function is considered. In figure 10.10 the Bode diagram of the ideal filter transfer function is plotted, where the parasitic resistance and especially the parasitic inductance of the circuit are neglected. The plot trend represents the one of an ideal second order low-pass filter with a cut-off frequency lower than 1 kHz.
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Figure 10.10: Bode plot of the filter transfer function (neglecting circuit parasitic characteristics)
Figure 10.11: Bode plot of the filter transfer function (including circuit parasitic characteristics)
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However if the parasitic resistances and inductances are added to the simulation the result is quite different as it is possible to verify from figure 10.11 where the Bode plot of the real circuit is shown.
The two plots are evidently different, especially in the high frequency region, even if both cases show a ‘safe’ frequency response. Nevertheless it is obvious that the parasitic characteristics of the circuit affect the system performance and the difficulties in measuring the wires’ inductance is a limiting factor in providing very accurate results.
10.4 Filter Test with Laboratory Power Supply
After the preliminary design, the successive step was to experimentally verify the filter effectiveness and its compatibility with thruster HT-100 ignition and operations.
The test was carried out the 11th June 2013 in the Alta IV4 vacuum facility with the laboratory power supply SM600-10 as power source. This power supply can provides up to 6 kW, thus it is clearly oversized for the HT-100 needs.
The electrical circuit schematic is presented in figure 10.12 where the yellow circles identify the locations where the current measurements were taken. As seen, the filter is constituted by a 10 μF electrolytic capacitor and a 1.4 mH inductor.
Figure 10.12: Filter test with laboratory power supply - electrical circuit
The thruster was correctly ignited at a anode voltage of 350 V, a xenon flow rate of 1 mg/s and a discharge current of 0.85 A, then the anode voltage was reduced to 300 V and the discharge current to about 0.6 A. The current measurements were acquired with an oscilloscope set up with a scale of 100 mV/A.
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The figure 10.13 shows the thruster current oscillations whose peak value reaches 1.4 A at a frequency of about 40 kHz. In the figure 10.14, the current oscillations on the power supply side can be observed: the current peak has been reduced by the filter to 0.9 A; a reduction of about 35% with respect to the peaks on the thruster side. It is interesting to notice how the filter cuts off the 40 kHz fluctuations, however it seems that the oscillations at higher frequency are slightly enhanced. This phenomenon can also be seen in figure 10.15 and 10.16.
Figure 10.13: Thruster current oscillations (scale 100 mV/A)
The figure 10.15 shows the thruster current oscillations (yellow trace) and its Fast Fourier Transform (FFT, red trace) where the RMS current value is provided as a function of frequency. The figure 10.16 shows the same physical quantities but measured on the power supply side.
It can be noticed that the filter reduces the 40 kHz RMS value by a factor of six, eliminates the 80 kHz oscillations, however it amplifies the 100 kHz. Despite this occurrence the thruster did not show any anomalous behavior and worked properly during the whole test.
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Figure 10.14: Power supply current oscillations (scale 100 mV/A)
The measurements at thruster start up were also recorded. The figure 10.17 shows that the thruster current inrush reaches about 68 A, whereas the power supply current peak is only 1.2 A (figure 10.18). The filter’s capacitor supplies the charge needed by the thruster for ignition, reducing a lot the burden for the power supply. This task is of particular importance for a Direct-Drive system because the solar array cannot afford this very high current value.
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Figure 10.15: Current oscillations (yellow) and its FFT (red) on the thruster side
Figure 10.16: Current oscillations (yellow) and its FFT (red) and at the power supply
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Figure 10.17: Ignition transient. Thruster current peak (scale: 10 mV/A)
Figure 10.18: Ignition transient. Current peak on power supply side (scale: 100 mV/A)
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The figures 10.19 shows the ignition transient in a different time scale (2μs per division), in particular the thruster current peak. The figure 10.20 presents again the thruster current peak once the filter is removed so that the power supply is directly connected to the thruster. The comparison between the two figures (10.19 and 10.20) highlights that the filter does not affect the thruster behavior during start up, in fact the two figures are practically identical.
The first result of the test is that the filter is compatible with thruster operation since no anomalous behavior was detected. It must be enlightened that in this test the power source was the common power supply and not the solar array, therefore, not only these are two different devices with different behavior but the filter has been designed only for Direct-Drive system operations.
In the present work, the filter behavior with power supply was not investigated further because the primary goal was not the optimization of the filter but the demonstration that the filter worked in a satisfactory way.
Figure 10.19: Ignition transient. Thruster current peak (scale: 10 mV/A)
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Figure 10.20: Ignition transient. Thruster current peak (scale: 10 mV/A). Filter removed
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The understanding of solar array performance is of predominant interest in the planning of a Direct-Drive system test. The effect of several parameters such as the solar panel orientation and inclination, the period of the year, temperature, pressure, etc can influence the output of a solar panel; the thruster operating conditions are consequently affected by all these factors. In this direction, a way to relate the nominal performance and the actual one must be developed. Then, for the accurate knowledge of the system characteristics in terms of voltage, current and power the ultimate act is the measurement of these values.
11.1 Solar Irradiance Prediction Model
The solar irradiance plays a driving role in a photovoltaic system; in fact the solar array performance directly depends on the amount of solar flux incident on the cells’ surface.
Therefore an important step in the selection and in the arrangement (in particular the slope angle) of the solar modules has been the one involving the study of a solar irradiance prediction model.
Figure 11.1: Solar array orientation angles
Using the Bird model [6] and taking into account the variation of solar position [17], it has been developed a Matlab code whose output is the solar irradiance incident on a generic surface as a function of time (in a day). For convenience’s sake the results are
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valid for a specific location, in particular Ospedaletto (Pisa, Italy) where the solar panels were installed; however it can be easily modified taking into account also generic locations. The input parameters are simply the date, the atmospheric pressure, the slope (or tilt angle) of the panel surface and the azimuth angle (see figure 11.1).
The Bird model is valid for a clear sky day and is characterized by several assumptions and atmospheric data. In the present model the values of aerosol optical depth and precipitable water have been averaged from the Aeronet website [67] which, in their huge database, shows also atmospheric measurements carried out in Rome. Then, other values or model constants were chosen according to Bird’s suggestions (see the Matlab code below).
The model precision in a clear-sky day is about ±10% and it generally slightly underestimates the irradiance. It is thought that it is essentially due to two factors: first of all the presence of thin layers of particles or clouds, when they do not cover directly the sun beam light, act as a reflective surfaces which thus increase the irradiance and this effect is not included in the model. Then, the uncertainties in some atmospheric values and the fact that the Bird’s suggestions about model constants may be obsolete since they were proposed more than thirty years ago could imply a little imprecision of the model.
Despite this remark, the code has been really useful in the selection of the panels tilt angle. In fact, even if the magnitude of solar irradiance is not very accurate, the influence of the surface orientation can be anyway observed. The period dedicated to the tests was the first week of September, so the tilt angle has been optimized in order to get the maximum irradiance during the central hours of these days. With an azimuth of zero degrees (panels oriented toward south), the optimal tilt angle came out to be 35°, therefore the ancillary structure was built in order to obtain the desired inclination.
The Matlab code used is the following:
function [Gpvamax] = SolarIrradianceBird (beta,gamma,month,n,P)
% Gpva: Solar irradiance incident on a surface as a function of time (given the date);
% P: pressure (mbar)
% n: day of the month;
% month=month's number;
% beta: tilt angle, surface inclination angle with respect to horizontal plane (radians);
% gamma: azimuth, angle between horizontal projection of surface normal and south (radians, west positive);
%NOTES:
% ALTA (Ospedaletto, PI, Italy) coordinates: 43.68N 10.43E;
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%t: italian solar time --> hours,minute (minutes in decimals)
t=8:0.01:20;
if month==1 N=n;
elseif month==2 N=31+n;
elseif month==3 N=59+n;
elseif month==4 N=90+n;
elseif month==5 N=120+n;
elseif month==6 N=151+n;
elseif month==7 N=181+n;
elseif month==8 N=212+n;
elseif month==9 N=243+n;
elseif month==10 N=273+n;
elseif month==11 N=304+n;
elseif month==12 N=334+n;
end
B=(N-1)*(2*pi)/365;
E=229.18*(0.0000075+0.001868*cos(B)-0.032077*sin(B)-0.014615*cos(2*B)- 0.04089*sin(2*B));
CorrST=4*(345-349.565)+E; %CorrST: Solar time-STDTime (in minutes) Gs=1367*(1+0.033*cos((2*pi*N)/365)); %Gs: Extraterrestial irradiance delta=23.45*sin(2*pi*(284+N)/365)*(pi/180); %delta: declination w=(15*(t+(CorrST/60))-180)*(pi/180); %w:hour angle
costeta=sin(delta)*sin(0.762)*cos(beta)-
sin(delta)*cos(0.762)*sin(beta)*cos(gamma)+cos(delta)*cos(0.762)*cos(beta )*cos(w)+cos(delta)*sin(0.762)*sin(beta)*cos(gamma)*cos(w)+cos(delta)*sin (beta)*sin(gamma)*sin(w); %teta: incidence angle
costetaz=sin(delta)*sin(0.762)+cos(delta)*cos(0.762)*cos(w); %tetaz:
sun zenith angle;
for i=1:length(costeta) if costeta(i)<0 costeta(i)=0;
end end
for i=1:length(costetaz) if costetaz(i)<0 costetaz(i)=0;
end end
M=(costetaz+0.15*(93.885-acos(costetaz)*(180/pi)).^(-1.25)).^(-1); %M:
AIR MASS
M_=M.*P/1013; %M_: pressure corrected AIR MASS UO=0.31; %UO: Ozone layer thickness (cm)
XO=UO*M; %XO: Total amount of ozone in a slanted path (cm) if month==6
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UW=2.17; %UW: Amount of precipitable water in a vertical column from surface (cm)
elseif month==7 UW=2.20;
elseif month==8 UW=2.36;
elseif month==9 UW=2.26;
else UW=1.64;
end
XW=UW*M; %XW: Total amount of precipitable water in a slanted path (cm) TR=exp(-0.0903*((M_).^0.84).*(1+M_-(M_).^1.01)); %TR: Rayleigh
Transmittance
TO=1-0.1611*XO.*(1+139.48*XO).^(-0.3035)-
0.002715*XO.*(1+0.044*XO+0.0003*XO.^2).^(-1); %TO: ozone transmittance TUM=exp(-0.0127*(M_).^0.26); %TUM: Transmittance of absorptance of uniformly mixed gases (CO2 and O2)
TW=1-2.4959*XW.*((1+79.034*XW).^0.6828+6.385*XW).^(-1); %TW:
Transmittance of water vapor absorptance if month==6
Taua038=0.22; %Taua038: aerosol optical depth from surface in a vertical path at 0.38 um wavelenght
Taua050=0.25; %Taua050: aerosol optical depth from surface in a vertical path at 0.50 um wavelenght
elseif month==7 Taua038=0.25;
Taua050=0.16;
elseif month==8 Taua038=0.25;
Taua050=0.15;
elseif month==9 Taua038=0.28;
Taua050=0.18;
else Taua038=0.23;
Taua050=0.15;
End
Taua=0.2758*Taua038+0.35*Taua050; %Taua: broadand aerosol optical depth from surface in a vertical path
TA=exp((-Taua^0.873)*(1+Taua-Taua^0.7088)*M.^0.9108); %TA:
transmittance of aerosol absorptance and scattering
K1=0.0933; %Constant assosiated with aerosol absorptance
TAA=1-K1*(1-M+M.^1.06).*(1-TA); %TAA: tranmittance of aerosol absorptance
TAS=TA/TAA; %TAS: transmittance of aerosol scattering
Ba=0.84; %Ba:forward scattering ratio (value suggested by Bird) rs=0.0685+(1-Ba)*(1-TAS); %rs: atmospheric albedo
Gd=Gs*costetaz*0.9662.*TR.*TO.*TUM.*TW.*TA; %Gd: direct irradiance on horizontal surface
Gas=Gs*costetaz*0.79.*TO.*TUM.*TW.*TAA.*(0.5*(1-TR)+Ba*(1-TAS))/(1- M+M.^1.02); %Gas: diffuse irradiance on a horizontal surface rg=0.3; %rg: ground albedo
Gt=(Gd+Gas)/(1-rg*rs); %Gt: global irradiance on a horizontal surface Gpva=Gt.*(costeta./costetaz); %Gpva: global irrandiance of a oriented surface
plot(t,Gpva);
Gpvamax=max(Gpva); %Gpvamax: max of Gpva End
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11.2 Solar Array I-V Curve Plot
The solar array performances are mainly affected by two parameters: the irradiance and the cells temperature. By knowing these two values, the solar array I-V curve is easily derivable; manufacturer provides the temperature coefficients which take into account the temperature effect on solar array voltage and current. Then, the losses (in the wires, connections and the ones due to cells mismatching factors) can be estimated during the photovoltaic system operations and their effect, in order to obtain a realistic I-V curve, must be included in the equations.
In this work, the curve has been plotted from the knowledge of the values of the short circuit current (Isc), the voltage (Vpp) and current (Ipp) at the peak power point and the open circuit voltage (Voc). The equations relating these quantities to the irradiance (G), cells temperature (Tc) and losses are the following [58]:
0 0
0
0 0
0
0 0
0
0 0
0
1 ( ) (11.1)
1 ( ) (11.2)
1 ( ) (11.3)
1 ( ) (11.4)
sc sc I Isc C
pp pp I Ipp C
pp pp V Vpp C
oc oc V Voc C
I I G f T T
G
I I G f T T
G
V V G f T T
G
V V G f T T
G
Where the subscript 0 refers to standard conditions (see paragraph 9.2) and the loss coefficients (f) must be evaluated during solar array operations. The temperature coefficients (β) are provided by manufacturer in 1/°C and listed in the table 11.1; a peculiarity of the amorphous silicon thin film cells is that the temperature impact on their performance is less significant with respect to the common silicon cells.
βIsc βIpp βVpp βVoc
+0.0008 +0.001 -0.003 -0.003
Table 11.1: Solar module (Schuco MPE 90 AL 01) temperature coefficients (1/°C)
Once the four values (Isc, Vpp, Ipp, Voc) are derived from the equations, a simple curve fitting can be performed in order to obtain the solar array I-V curve. Furthermore, also the power-voltage curve can be plotted considering that the power is simply the voltage times the current.
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11.3 Real Solar Array Performance
The losses in the solar array power system were estimated by measuring its real performance. After the analysis of several data, the mismatch coefficients (fI and fV) were set to 1. Although the assumption for the voltage coefficient is very common, the one involving the current coefficient may seem unrealistic. However this decision arises from the fact that these solar modules, in the first weeks of their operational life have higher performance with respect to the nominal ones [69]. Since the Direct-Drive system was tested in this phase of solar modules life, the above assumption is coherent with this consideration and it was made to well reproduce the actual I-V characteristics of the arrays.
Analyzing the data of the measurements, while the open circuit voltage of the solar arrays was practically the nominal one (380 V) the short circuit current in standard condition was found to be 1.69 A which is about the 8% higher than the nominal value (1.56 A). This circumstance confirms that these modules perform better in their early life.
The figure 11.2 shows the real I-V curve of the photovoltaic panel in standard conditions whereas the figure 11.3 is the power-voltage characteristics. It can be seen that the four modules can provide nearly 400 W at their peak power point.
Figure 11.2: Real solar panel I-V curve in standard condition (1000 W/m2, 25°C)
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Figure 11.3: Real solar panel power-voltage curve in standard condition (1000 W/m2, 25°C)
145
12 Direct-Drive Demonstration
The experimental tests target was to understand and analyze the Direct-Drive system operations. First of all, one may investigate whether any differences in thruster operating characteristics emerge when supplied by a Direct-Drive system. Thus, Direct-Drive thruster working features (I-V curve, etc.) should be evaluated and compared to those measured when the laboratory supply is utilized as the power source. In addition, a method for the regulation of thruster operating condition must be identified. It is evident that the anode supply removal limits the system flexibility: in fact the anode voltage can be simply varied when the laboratory supply is used, but this is not anymore feasible when the thruster is directly connected to the solar array. In the Direct-Drive configuration, a way to actively regulate the system operating conditions is by varying the propellant flow rate: the variation of the mass flow rate changes the operational point of the system on the I-V curve of the solar panel thus modifying the discharge current and voltage at which the system operates. Therefore the current experiment has to explain how the thruster performance regulation can be performed and how the thruster behaves in different regions of solar array I-V curve.
The start-up and the shut-down represent two critical phases of the test. The thruster requires certain conditions for the ignition in terms of voltage and mass flow rate;
furthermore violent current overshoots occur at the inception and extinction of the discharge. The experiment must investigate how a safe start-up and shut-down can be performed and how the system reacts in these phases. Arising from the study of J.S.
Snyder et al. [58], two ignition procedures had been experimented: the soft start and the hard start. In the first procedure the open circuit array voltage is applied between the anode and the cathode and then a mass flow rate is established. In order to avoid unwanted current peaks the mass flow rate should be sufficiently low (a value of 0.5 mg/s is considered to be enough for this case).
On the other hand, the hard start is carried out by first establishing the mass flow rate at the desired value and then, using a switch, the array power is instantaneously delivered to the thruster. This distinction between the two procedures is characterized by the
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magnitude of the current overshoot; in fact during the hard start the current peak is more demanding.
In addition to the high current peak during the transient, the Hall thruster exhibits current and voltage oscillation also during steady-state operations. As already shown in the paragraph 10.1 the HT-100 current peaks can reach values in the order of 3-4 A with a main oscillation frequency of 30-40 kHz; instead during start-up the observed peaks reach up to 80 A. The designed filter enables the compatibility between thruster requirements and solar array performance. The Alta’s HT-100 laboratory power supply does not need such a filter because it is oversized and can easily supply the thruster in this range of current and voltage. A part of the experimental data analysis had been thus dedicated to the understanding of filter behavior.
All the mentioned issues are summarized below:
Are there differences in the thruster operating characteristics when supplied by a Direct-Drive system?
How is the thruster operation at different points in the I-V solar array characteristic?
o Constant voltage region o Peak-power point
How the operating point of the thruster is regulated?
How do you start and stop a thruster in a Direct-Drive system?
How do you attenuate current inrush at start-up and shut-down?
What are the impacts of thruster oscillations on the solar array?
12.1 Experimental Test
The test was performed the 6th of September 2013 in the Alta’s laboratory (Pisa, Italy).
The four solar modules were arranged on movable structures inclined of 35 degrees with respect to the horizontal and oriented toward south in order to better exploit the solar irradiance (figure 12.1). They were positioned near the IV-4 vacuum chamber in order to reduce the path losses. The pyranometer was placed close to the solar panels and inclined of 35 degrees in order to accurately and faithfully measure the solar irradiance.
The cells temperature was measured with a thermocouple disposed on the backside of the module cover-glass. It is assumed that the cells temperature is very close to the one of the module.
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Figure 12.1: Alta’s Direct-Drive solar array test-bed
The HT-100 thruster have been mounted in the IV-4 facility on its supporting structure two days earlier and, once the optimal vacuum condition was reached, the thruster has been preliminarily ignited with laboratory supplies in order to verify the correct functionality.
As already anticipated in the paragraph 11.3, the photovoltaic system performance were measured in order to obtain (from relations 11.1-11.4) its real performance; in particular the short circuit current in standard conditions came out to be 1.69 A while the open circuit voltage was 380 V.
Figure 12.2: Direct-Drive system electrical scheme. Green circles represent locations of current measurements
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The voltage loss in the wires and connections from the arrays to the thruster was measured and resulted to be 4 V (this value was taken as constant). Then, during system operations, the thermocouple recorded a panel temperature ranging from 50 °C to 51 °C.
The Direct-Drive system comprehending the solar array, the electrical box, the filter unit and the thruster is schematically depicted in figure 12.2.
The current measurements were performed on the thruster side and on the solar panel side that is before and after the filter unit (see figure 12.2).
12.2 Test Sequence and Results
The first attempt to turn on the HT-100 in a Direct-Drive mode was performed with the soft start procedure: the open circuit voltage was applied between anode and cathode and then the propellant flow rate was gradually increased until the thruster ignited at a value of 0.5 mg/s. In this operating condition the anode current was 0.45 A and the anode voltage was 333 V. The ignition occurred at the first attempt with no issues observed; the operation was stable for several minutes. The cathode, always operative during the whole test, was set to a flow rate of 1 mg/s corresponding to a current of 1.5 A; these values were maintained constant for the whole first phase of the test.
Then, the operating conditions were changed by stepwise increment of the anode flow rate. The figure 12.3 shows the sequence of the operating conditions and the trend of the irradiance during the whole test timespan (Italian summer time, GMT+2).
After the proved stability of operations the xenon flow rate was increased to 0.6 mg/s corresponding to a discharge voltage of 330 V and a anode current of 0.50 A. Then, after few minutes the flow rate was again consequently increased at 0.7 mg/s, 1 mg/s, 1.25 mg/s and 1.5 mg/s whose corresponding values of current and voltage are shown in table 12.1. In particular, at 1.5 mg/s the solar array peak power point was reached with a current of 1.37 A, a voltage of 269 V and thus a discharge power of 368 W. Since the HT- 100 was not designed to operate at such high values of power and anode current, the propellant was not further increased and this operative condition was maintained for just few minutes. At this point, the flow rate was decreased, first at 1 mg/s, and then gradually up to the extinction of the discharge which occurred at 0.7 mg/s. Thrust measurements were performed at the ignition (8 mN) and in the peak power point condition (17 mN). The latter value is quite high for the HT-100 and indicates that the thruster was operating in a very high power condition.
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Figure 12.3: Test sequence of the HT-100 Direct-Drive operation
150 Time G
(W/m2) Tp (°C) Ma
(mg/s)
Va
(V)
Ia
(A)
I RMS - TS (A)
I RMS - PS (A)
Ic
(A) Mc
(mg/s)
Thrust (mN)
13:08 967 51 0.50 333 0.45 0.213 0,108 1.5 1 8 13:15 970 51 0.60 330 0.50 0.348 0.101 1.5 1 / 13:28 975 51 0.70 328 0.62 0.594 0,085 1.5 1 / 13:37 979 51 1.00 315 0.90 0.588 0.084 1.5 1 / 13:40 979 50 1.25 300 1.12 0.526 0.117 1.5 1 / 13:45 976 50 1.50 269 1.37 0.537 0.109 1.5 1 17 13:51 973 50 1.00 311 0.92 0.590 0.087 1.5 1 13.8
Table 12.1: Direct-Drive test data
The system performance control via stepwise xenon flow rate regulation came out to be a simple and reliable method for thruster throttling. The HT-100 responded correctly and quickly to the imposed variations of the operative conditions, even when the mass flow rate change was remarkable (e.g. from 0.7 mg/s to 1 mg/s and from 1.5 mg/s to 1 mg/s).
In the figures 12.4 and 12.5 the tested operating conditions are depicted on the solar array I-V and P-V curves (see paragraph 11.3). The test was carried out during solar noon; in the whole test timespan, the irradiance and cells temperature variation was small and thus the solar panel output performance remained almost constant. In fact, the irradiance varied between 967 and 979 W/m2 whereas the cells temperature ranged from 50 °C to 51 °C.
Figure 12.4.: Current-Voltage curve during test runtime and thruster operative conditions
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Figure 12.5: Power-Voltage curve during test runtime and thruster operative conditions
12.3 Ignition
In the second phase of the tests, the system performance investigation at ignition was studied both for the soft start and the hard start procedure and measurements of current waves were gathered before and after the filter. First, the soft start was investigated at a propellant flow rate of 1.25 mg/s. The figures 12.6 and 12.7 show respectively the current wave on the thruster side and on the solar array side.
Figure 12.6.: Ignition transient during Soft Start on thruster side
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Figure 12.7: Ignition transient during Soft Start on panel side
It is possible to notice that the current peak required by the thruster is about 45 A whereas the solar panels “see” only 4.5 A, the charge difference is in fact provided by filter.
Then, the hard start was tested: while the thruster chamber was filled by 1.25 mg/s of gas propellant the switch of the electrical box was closed and the thruster started. The figure 12.8 and 12.9 show the current wave for the hard start case for both the thruster and panel side. In this situation the thruster current peak reached almost 80 A while the arrays current was 18 A. These values are significantly higher than the ones measured for the soft start, meaning that hard start is the most demanding ignition procedure.
Figure 12.8: Ignition transient during Hard Start on thruster side
