A parametric study of the effect of discharge energy on PPT
performance
Citation for published version (APA):
Rezaeiha, A., Anbarloui, M., & Farshchi, M. (2011). A parametric study of the effect of discharge energy on PPT performance. In 28th International Symposium on Space Technology and Science (ISTS), Okinawa, Japan, June, 05-12, 2011 [ISTS2011-b-02]
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A Parametric Study of the Effect of Discharge Energy on PPT Performance
By Abdolrahim Rezaeiha, Mehdi Anbarloui, and Mohammad Farshchi
Sharif University of Technology, Tehran, Iran rezaeiha@alum.sharif.edu
Pulsed Plasma Thruster (PPT) is one of the promising space propulsion devices for the micro- and nano-satellites as the following advantages: simplicity, lightweight, robustness, and low power consumption. Moreover, PPT can generate very small impulse bits with high specific impulse. Therefore PPT has great advantages as a precision attitude control. Although μPPTs and their relevant issues have been investigated to a certain degree in recent years, but as a result of the fact that PPT performance is a function of a lot of different parameters like geometrical dimensions, discharge energy, capacitor characteristics, etc and also because of its sophisticated physics; it is felt that issues related to developing an optimized μPPT needs to be investigated in greater detail for a PPT with smaller dimensions and better performance. Therefore, following the development of a laboratory benchmark rectangular breech-fed PPT, a wide range parametric study has been conducted on investigating the effect of discharge energy on PPT performance. The PPT impulse bit varied from about 0.4 mN-s to more than 1.3 mN-s while discharge energy changed from less than 10 J to more than 50 J. Specific impulse also increased from 200 s to 110 s. This paper reviews the results of the mentioned parametric study in brief.
Key Words: Parametric Study, Discharge Energy, PPT Performance
Nomenclature 0
: Magnetic permeabilityh
w
0V
i
E
,L
t Ibit Isp h α mbit ce ηT: Distance between electrodes : Electrode width
: Capacitor voltage : Discharge current : Discharge energy
: Nozzle inductance gradient : Time
: Impulse bit : Specific Impulse : Thruster efficiency
: Angle between electrodes (Flare angle) : PPT mass bit per shot
: Average exhaust velocity : Thruster efficiency
1. Introduction
In spite of the simplicity in mechanical structure, pulsed plasma thrusters have such a sophisticated physics which is not thoroughly known yet. Probably because several different complicated processes (starting from the initiating spark and creation of the plasma puff between the electrodes, followed by the discharge of several kA current and its energy transfer to the surface of PTFE and PTFE ablation (phase change, depolymerization and ionization), the accelerating effect of induced magnetic field on the charged particles and the resulting thrust) all happen together and at a short period of several microseconds which makes the phenomena hard to be
modeled and simulated.
In addition, PPT performance is affected by various parameters like geometrical dimensions of electrodes, angle between electrodes, discharge energy, capacitor characteristics, discharge current amplitude and behavior, etc so that optimization of PPTs require extensive experimental studies to get to know the effect of each parameter. One of the most influential factors which can be essential in each PPT design is the required power. Required power is a function of discharge energy and working frequency. Thus, discharge energy is an important element in each design and the effect of must be completely known. Furthermore, discharge energy is the best parameter to be changed and adjusted, when a multi-mode PPT with different levels of thrust is to be designed.
Previously, the effect of discharge energy has been studied to some extent, for example during the development of
flight-model EO-1 PPT1) and the laboratory PPT2) as well; also
during the development of µPPT by Pottinger and
Scharlemann3).
In this research, a parametric study has been conducted on a
breech-fed PPT, previously designed and developed4), when
the discharge energy varied from less than 10 J to more than 50 J to study the behavior of impulse bit and specific impulse. Moreover, an investigation was carried out to see the PPT minimum working voltage.
The present paper reviews the results obtained from the experimental study and also compares it to the results of the
similar on the EO-1 PPT2), LES-8/9 PPT5), µPPT3), and
2. Essential factors in PPT design
PPT is made up of various parts which are listed below. Figure 1 shows a schematic of breech-fed PPT and its different parts are easy to distinguish. It also shows the mechanism of production of thrust.
Electrodes (Anode and cathode) Igniter plug
Capacitor
Power Processing Unit (PPU) Propellant (PTFE)
Spring
Supporting structure
Below comes in Table 1 a list of the most essential factors which need to be considered in PPT design as a result of the fact that they either affect the PPT performance or its life time, system size, mission capabilities, etc.
Table 1. PPT essential design parameters
PPT Essential Design Parameters
Geometrical parameters Electrodes’ length/width/thickness Electrodes spacing
Flare angle (α)
Electrode shape (rectangular/tongue) Aspect ratio (h/w)
Capacitor characteristics Capacitance
Internal resistance/inductance Discharge behavior Mass
Life time Maximum voltage
Propellant Propellant face in thrust chamber Propellant type (PTFE or other ones) Plume contamination
Propellant temperature Igniter plug Type
Distance between spark and PTFE face Spark energy
Life time PPT working frequency
Electrodes’ material (conductivity and erosion resistance) PPT working power and discharge energy
PPT EMC/EMI issues
3. Experimental facility 3.1. Vacuum chamber
PPT experiments were performed in a middle-size high vacuum facility capable of achieving a chamber pressure of
10-6 mbar while the thruster was working. The bell-type
vacuum chamber has dimensions of 0.4 m in diameter and 0.4 m in length. It is evacuated by an oil diffusion pump in conjunction with a rotary centrifugal pump, while the pressure is monitored with different gauges. The chamber is equipped with a number of feedthrough flanges and a Plexiglas window for visual inspection of the PPT.
3.2. HV probes
Two high voltage probes which are capable of transmitting
high voltage up to 15 kV to oscilloscope with a reduction ratio of 100:1 were used to record the PPT capacitor discharge voltage and the PPT igniter plug arc voltage.
3.3. Rogowski coil
Rogowski coil was needed to record the PPT discharge
current pulse and to calculate the impulse bit of the thruster. Therefore several Rogowski coils with peak current measurement of 60,12,6,1.2 kA are used in the tests.
3.4. Power supply and digital oscilloscope
A 750 W power supply was used to power the dc-dc boost converters used to convert the 24 V input power from power supply to the desired voltage to charge the main capacitor and the PPT discharge initiating circuit. A four-channel digital oscilloscope was used to record three signals coming from the thruster.
4. Thruster and experimental set-up
The designed and developed PPT4) is a breech-fed PPT with
rectangular electrodes which are made of copper. The electrodes’ width is 31 mm while they are located with 31 mm distance between them and the flare angle (α) is considered to be zero (the electrodes are parallel). The PPT thrust nozzle length is 50 mm and the aspect ratio (h/w) equals 1. This
makes the propellant surface to be 9.61 cm2. PTFE
(Polytetrafluoroethylene) is used as the propellant when it is the routine propellant used in PPTs, although other
fluorocarbons have been tested in laboratories7).
An oil-filled 35 µf capacitor with maximum voltage of 2.5 kV is used as the PPT main capacitor. In our tests the capacitor varies from 750 V to 1750 V, makes the discharge energy rise from 9.54 J to 54 J. An annular semiconductor igniter plug with 2 mm diameter of center electrode is also used to produce plasma puff to initiate the capacitor discharge current between the electrodes in vacuum. The plug has been put inside the PPT cathode while its cathode was electrically isolated from the thruster cathode. The igniter plug cathode was connected to the thruster cathode via a 270 μH inductor. The inductor is used to decrease the coupling current flowing from the thruster cathode to the plug cathode, as a result of discharge chamber arc attachment to the plug face which has a strong bearing on the accumulated plug deposit. The value of inductance was chosen according to the results of the studies
made by Aston and Pless8). Furthermore, it was also observed
that the use of this inductor as a coupling element may also conclude to an increase in thrust though it is still under experiment to be proved.
The power processing unit (PPU) is capable of charging the main capacitor in a range of voltage starting from 50 V up to 2000 V. The discharge initiating circuit inside the PPU also transmits a pulse with 1500 V peak and several microsecond widths to igniter plug to produce the required arc. The detailed description of the PPT design and development is available at ref. 4. Table 2 summarizes the characteristics of the designed PPT.
Table 2. Our PPT characteristics PPT feeding type Breech-fed
Electrode width 31 mm
Electrode spacing 31 mm
Flare angle 0 deg
Electrode shape Rectangular
Aspect ratio (h/w) 1
Thrust nozzle length 50 mm Electrode material Copper Propellant face area 9.61 cm2
Propellant type PTFE
Capacitor 35 µf, oil-filled, 2.5 kV Igniter plug Annular semi-conductor Igniter plug voltage 1500 V Igniter plug arc energy < 0.2 J
Working power 10-50 W
Discharge energy 10-50 J Capacitor voltage 750-1750 V
A picture of the laboratory benchmark PPT is shown in figure 2 when various parts of the PPT can be easily seen and figure 3 shows the PPT while working in the vacuum chamber. When the capacitor is charged by PPU, the igniter plug fires and makes a small arc discharge. This discharge makes a plasma puff between the electrodes causing the main capacitor to discharge. The main discharge with high current, transfers energy to PTFE surface, hence makes it to ablate and ionize. This makes a plasma sheet and a conductive path for current for the continuation of capacitor discharge. The passing of current creates a magnetic field. The magnetic field perpendicular to the plasma current leads to a Lorentz force accelerating the plasma. Thus, an impulse is obtained by the thruster. When the entire energy stored in the capacitor is discharged, the thruster pulse ends. The duration of the discharge depends on the electrical properties of the oscillation circuit and can be observed from the voltage pulse measured from the PPT discharge.
A schematic of the test plan used to measure the PPT current and voltage which are used to parametrically study the effect of discharge energy on PPT performance is shown in Figure 4. PPU output to capacitor is adjustable between 500-1750 V through the tests.
The test matrix comes below in table 3:
Table 3. PPT test matrix
Test Capacitor voltage Discharge energy
1st test 750 9.84 2nd test 1000 17.5 3rd test 1250 27.3 4th test 1500 39.3 5th test 1750 54 5. Experimental results
The PPT discharge current curves are analyzed to provide
an estimate of impulse bit, Ibit. The Ibit is related to the
discharge current via Eq. 1 and is determined by integrating the discharge current curve using a numerical method.
(1.)
Fig. 2. A picture of the PPT
Fig. 3. PPT firing in vacuum chamber
Fig. 4. PPT test plan
t bit i dt L I 0 2 ' 2Where the inductance gradient L’ is approximated by Eq. 2 and expressed in terms of permeability of free space also known as the magnetic permeability constant (Eq. 3), the
electrode separation (h) and electrode width (w).3)
(2.)
(3.)
Specific impulse (Isp) is calculated according to Eq. 4 which is
taken from Ref. 9. This equation is valid only for breech-fed
PPTs and gives an estimate of the system Isp. Ibit in Eq. 4 is in
μlb-s unit. Mass bit (mbit), Thruster efficiency (ηT), and
average exhaust velocity (ce) can be calculated from Eq. 5-7.
(4.)
(5.)
(6.) (7.)
Fig. 5. Discharge current at 750 V, 9.84 J
Fig. 6. Discharge current at 1750 V, 54 J
Figure 5 shows the discharge current measured for the first test at 750 V (9.84 J) and figure 6 shows the discharge current for the fifth test at 1750 V (54 J).
Figure 7 and 8, each shows three curve on one graph, respectively for the 750 V test and 1750 V test. In each figure, the yellow curve belongs to main discharge current, the cyan curve shows the main capacitor voltage history, and igniter plug voltage pulse is depicted by magenta curve.
Fig. 7. First test curves at 750 V, 9.84 J (main discharge current (yellow), main capacitor voltage history (cyan), igniter plug voltage pulse
(magenta))
Fig. 8. Fifth test curves at 1750 V, 54 J (main discharge current (yellow), main capacitor voltage history (cyan), igniter plug voltage pulse
(magenta))
The design of the PPT is in a way to provide a basis for comparison with earlier models in Ref. 1,2,3, and 5. The PPT was tested at discharge energies of 54 J, 39.3 J, 27.3 J, 17.5 J, 9.8 J, 4.3 J, 1.09 J, and 0.7 J. The tests with discharge energies of 4.3 J, 1.09 J, 0.7 J and even 0.175 J were done only to prove the operation of PPT at these low voltages. The PPT could successfully work at a capacitor charge voltage as low
bit sp I E I 6 . 1 * 560 w h L'0 7 0 4 *10
sp bit bit I g I m . E m I bit bit T . . 2 2
bit bit e m I c as even 100 V but stopped working when the voltage came below this and could not perform at 50 V.
Impulse bit measurements for each discharge energies between 9.8 J to 54 J were taken at 10 different tests. The results are shown in Table 4. The results show that there is almost a linear relationship between impulse bit and discharge energy in this range as seen in Figure 9. The same trend is
observed in ref. 1, 2, 3, and 5. Ibit for 4.3 J, 1.09 J, and 0.7 J
are also seen in Figure 9 but they are not integrated in the curve fitting process. Table 4 shows the average impulse bit, specific impulse, discharge energy, mass bit, thruster efficiency, and mean exhaust velocity for each of five tests. Each data is the average of 10 data measured in 10 tests.
Table 4. The results of the tests
Vo E (J) Ibit (μN-s) Isp (s) Mbit (μg) η Ce (km/s) 750 9.84 476 200 242 5% 1.96 1000 17.5 663 366 184 7% 3.60 1250 27.3 943 525 183 9% 5.15 1500 39.3 1118 800 142 11% 7.87 1750 54 1323 1100 122 13% 10.84
Fig. 9. Linear relationship observed between Ibit and discharge energy
Moreover, a linear relationship seems to exist between the specific impulse and discharge energy which can be seen in Figure 10. It also conforms to the results of ref. 6.
Fig. 10. Linear relationship observed between Isp and discharge energy
Table 5 compares our thruster performance with the µPPT
developed by Pottinger and Scharlemann3). It can be seen that
when the thruster electrodes’ width and electrodes’ spacing has been scaled up by 3.1 factor and discharge energy has been scaled up by 3.2 factor and at the same aspect ratio (h/w), the PPT impulse bit has increased by 6.88 factor while the specific impulse has remained quite constant. This comparison can be useful in deriving scaling laws for µPPTs, although it is considered only as a starting point and needs so much further research. Table 6 also compares our PPT performance with EO-1 PPT and LES-8/9 PPT. It is observed that our PPT compared to EO-1 PPT could produce 3 times more impulse bit at the expense of having half specific impulse. This is achieved when both have the same propellant face area and operated approximately at the same discharge energy. We
guess that the increase in Ibit is a result of the coupling element
utilized but it is still under further investigation.
Table 5. A comparison between our PPT and a µPPT
Our PPT μPPT PPT Parameters 35, 2.5 kV 31.1 , 2 kV Capacitor (μF) 1250 725 Charge voltage (V) 26.5 8 Stored energy (J) 9.61 1 Propellant area (cm2) 31 10 Electrode width 31 10 Electrode spacing 0 0 Flare angle 1 (31/31) 1 (10/10) h/w ratio 50 - Electrode length (mm) Rectangular, breech-fed Rectangular, breech-fed PPT type 524 506 Specific Impulse (s) 943 137 Impulse bit (μN-s)
Table 6. Comparison of our PPT with EO-1 PPT and LES-8/9 PPT Our PPT LES-8/9 PPT EO-1 PPT Parameter
35 , 2.5 17 , 1.5 kV 41.1 , 2 kV Capacitor (μF) 1250 1500 1090 voltage (V)Charge 27.3 20 24.4 E (J) 9.61 6.25 9.6774 area (cm2)Propellant 1 (31/31) 1.17 (27.1/23) 1.5 (38.1/25.4) (mm/mm)h/w ratio 50 50 38.1 L (mm) Rectangular, breech-fed Rectangular, breech-fed Rectangular, breech-fed PPT type 1.75 1.900 (26% of total mass) 1.465 (30% of total mass) Capacitor mass (kg) Oil-filled Oil-filled Oil-filled Capacitor type
Coaxial semiconductor
Coaxial semiconductor
Coaxial
semiconductor Igniter type
500 625 - voltage (V) Igniter
0.125 0.4 - energy (J) Igniter
IGBT SCR IGBT Switching
524 1000 1150 Isp (s)
A picture of the engineering model PPT developed in smaller dimensions is also shown below in Figure 11.
6. Conclusion
The test results show that there is quite a linear relationship between discharge energy and both impulse bit and specific impulse and this shows a good level of correspondence with other research mentioned in ref. Also our tests show that the PPT can operate at a voltage as low as 100 V.
Furthermore, our PPT results compared to a scaled-down µPPT can lead to a beginning of deriving scaling laws for them. It was observed that when ” h” and “w” was scaled up by a factor of 3.1 with constant aspect ratio (h/w) and “E” was
increased by a factor of 3.2, Ibit enhanced by a factor of 6.88
while Isp remains constant.
In addition, the comparison of our results with EO-1 PPT and
LES-8/9 PPT showed an increase in Ibit at the same power and
propellant face area which is guessed to be a result of utilizing
a self-inductor as a coupling element to connect the PPT
cathode to igniter plug cathode. Although the Isp became half.
However to prove this theory, further research is going to be carried out.
Acknowledgments
The authors would like to thank Mr. Majid Nazarian for his assistance with the vacuum facility.
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Fig. 11. The ENG-model PPT developed
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