Plasma Characterization of a Short-pulse
Laser-assisted Pulsed Plasma Thruster
By Kentaro KATO1) , Toshiyuki OI 1) and Hideyuki HORISAWA2)

1) Department of Mechanical Engineering Graduate school of Engineering, Tokai University, Japan
2) Department of Aeronautics and Astronautics, Professor Tokai University,JapanTo observe the effects of the acceleration channel length of the rectangular electrodes of a short-pulse laser assisted
pulse plasma thruster (LA-PPT), measurement of magnetic field distributions by magnetic probes were conducted.
Temporal variations of magnetic field distributions were measured by magnetic field probes located at various positions for
Vc = 8,000 V (C = 50 nF) with thrusters of channel lengths of 50 mm and 5 mm. The magnetic field of shorter channel
length was larger than longer one by about 30%. Moreover, by calculating delays when the magnetic field strength reachedthe local maximums for various positions, traveling speeds of the strong magnetic field regions for channel lengths of 50mm and 5 mm were 93 km/s and 125 km/s, respectively.
Key Words: Electric propulsion, laser propulsion, pulsed-plasma thruster, laser-assisted pulsed plasma thruster

Nomenclature: Acceleration channel lengthl: Acceleration channel widew: Acceleration channel highh: Capacitance
C𝑉𝑐: Charging voltage of capacitor

1.

Introduction

In resent year, increasing demand of the following small
satellite mass 100 kg or less is seen. Especially, the demandfor new small propulsion device system development for Drag
free control and formation flight is while growing. Generalpropulsion system of small satellites is not only mass limited
but also power limited. 1-4) The benefit of using electric
propulsion for the reduction of spacecraft mass will likely beeven more significant for mass limited micro- spacecraft
missions. 2-4) It is effective to use the electrical propulsion
system which a specific impulse can expect as promotion
system to solve these. Research and development has beenpromoted for practical use in each country. For example ion
engine, pulse plasma thruster etc. pulse plasma thruster isexcellent as a propulsion system for attitude control because it
can accurately control the impulse. Moreover, it is notice whatis thruster suitable for compact satellite. 4)
On the other hand, small sized onboard laser plasma thruster isalso under significant development with rapid evolution ofnovel compact laser systems. The laser thruster has advantagethat they can induce high specific impulses. In addition, thesystem can be very simple and small with significant
controllability of the thrust.13-16)
2. Laser Assisted Pulsed Plasma Thruster
A schematic of the rectangular laser-assisted pulse plasma
thruster (LA-PPT) is illustrated in Fig.2.17-18)

An electrode connected with capacitor that is fully charged.
The electrode is configuration as to interpose the solid
propellant from the top and bottom. A basic idea of the systemis that a bunch of laser-ablation plasma, induced through laser
irradiation on a solid target, is supplied to an acceleration
channel and additionally accelerated by electrical means
between an anode and cathode. When the current between theelectrodes increases, the plasma can be heated and further
ionized through joule heating. Thus, the electro thermal
acceleration effect will be significant. With larger current
exceeding more than one thousand amperes, theelectromagnetic acceleration effect becomes significant. With
the interaction of the current and a self-induced magnetic field,a streamwise acceleration of the plasma will be provided.
Discharge current is generated by breakdown between theelectrodes.
Since any solid materials can be used for the propellant inthese systems, no tanks, no valves, or piping systems arerequired for the propulsion system. Also, various materials inany phases can be used for the propellant. Therefore, thesystem employing this technique can be simple and compact.
As the laser-ablation plasma has a directed initial velocity oftens of km/s, which will be further accelerated by electricalmeans, significantly high specific impulses can be expected.

3.

Research Motivation
From results of various measurements to date, comparing ofdischarge of nanosecond pulse and microsecond pulse,
nanosecond pulse is large performance of propulsion at thesame discharge energy.
Fig. 3 shows the typical impulse bit characteristics of thelaser-assisted pulsed plasma thruster replotted from ourprevious report, in which variations of impulse bit with chargeenergy are plotted for various acceleration channel

configuration. From the Fig 3, it can be seen all the cases that
impulse bit increased with the increase of the charge energy.
Moreover, thrusters with 10mm×5mm channel length showed
higher impulse bit in all the cases.
Therefore, an objective of this study is to observe themechanism on such a difference in the channel length is topropulsion performance. To observe the self-magnetic field
what is occur in acceleration process with magnetic probe.
From the time variation of the time-resolved value ofmagnetic field, time delay of a strong magnetic field area wereanalyzed and velocity of the plasma were estimated.
4. Experimental set up of Plasma Magnetic Field
Measurement
The LA-PPT is placed in vacuum chamber (6.7 ×10-3 Pa) andignited through a laser ablation plasma induced with a focusedlaser pulse at a center surface of the propellant. As for thelaser oscillator, an Nd:YAG laser (B.M.Industries., 5000 series,
wavelength: 1,064 nm, Pulse width: 5 ns, Pulse energy: 400mJ) was utilized. A specifications ns-LA-PPT of themeasurement object is shown in Table.1.
Temporal variations of magnetic field distributions in an
acceleration channel, between anode and cathode, wereestimated with magnetic probes. A schematic of theexperimental setup for the magnetic field measurement isshown in Fig.3. In this study, based on results of Fig.3,
thrusters of channel lengths of 50 mm and 5 mm werecompared. The magnetic probes were attached on a side wall
(glass wall) of the acceleration channel to insulate the probes
from the electrodes. Each probe consists of a coil of the core
diameter of 2.0 mm (3 turns of enameled copper wire of 1.2
mm in diameter).

Fig. 3. Variation of impulse bit with charging energy for thrusters withvarious electrode geometries (electrode length ×electrode height)

Positions of the magnetic probes for the measurement are
shown in Fig.5. The first magnetic probe was placed at nearsurface region of propellant. As shown in this figure, tomeasure temporal and spatial variations of the magnetic fieldstrengths in the acceleration channels, magnetic probes wereattached on an x-stage and displaced to x-direction with
arbitraly intervals.

Fig.4. Schematic of the experimental setup for the magnetic fieldmeasurement

Fig.1. Pulsed plasma thruster

Fig.5. Positions of magnetic probes on side wall of acceleration channel

Fig. 2. Schematic of illustration of a rectangular laser-assisted pulsed
plasma thruster

Fig.6. Magnetic probe

Table 1. LA-PPT
l[mm]50mm&5mm

w[mm]

10mm

h[mm]

15mm

Material (electrode)

Copper

Propellant

Al2O3

Table 2. Parameter of magnetic probe
Turn3

Probe diameter𝟐×𝟏𝟎−𝟑[m]Enamel wire

Materialof copper

5.

the magnetic field, or the current sheet, moving in thedistances between the probes. Moreover, it can also be seenthat temporal variations of the magnetic fields in regions atnear propellant surface are almost in phase with a discharge
current waveform with a delay of about 100 ns. This implies
that a portion of the initial discharge current is probably
running at near surface of the propellant.
As can be seen in these figures, the magnetic fields in awhole region of the acceleration channel are induced at 100 ns
after induction of discharge current between electrodes. Then,similar to the discharge current waveform, the magnetic field
waveforms periodically change with the maximum peakvalues at propellant surface of ±0.24 T in Fig.7 (for channellength of 50 mm) and ±0.33 T in Fig.8 ( for channel length of5 mm). From these results, it is shown that larger magnetic
fields were induced with shorter channel cases.
Values of peak magnetic fields monitored at various positions
from these figures are replotted in Fig.9.
As can be seen, values of the magnetic field strengths ofshorter channel case are much larger than that of longer
channel case. Moreover, a decrease of the magnetic fieldstrength with increasing distances from the propellant surface
is more significant in longer channel case than that of shorter
channel case. On the other hand, in a shorter channel case, thedecrease of the magnetic field strengths with distances isinsignificant. This is probably due to larger local current
densities in shorter channel case.
By estimating times and delays when the magnetic fieldstrength reach the local maximums at 0 mm and 2 mm,traveling speeds of the strong magnetic field regions can becalculated. From the results, average speeds for channel lengthof 50 mm and 5 mm are 93 km/s and 125 km/s, respectively.
This is probably due to larger local current densities andmagnetic field strength, or namely larger Lorentz force, inshorter channel case.

Results and Discussion

Temporal variations of magnetic fields measured by themagnetic probes located at various positions for Vc = 8,000 V,
C = 50 nF are shown in Figs.7 (for channel length of 50 mm, x= 0 ~ 8 mm, ⊿x = 2 mm) and 8 (for channel length of 5 mm,
x = 0 ~ 5 mm, ⊿x = 1 mm), respectively.
As can be seen, the magnetic fields in the acceleration
channel were changing temporally and spatially. In general, it
is presumed in PPT operation that the discharge current runs
between electrodes forming a current sheet, and the motion of
the current sheet is induced by an interaction of the discharge
current in the current sheet and the self-induced magnetic field,
or namely, Lorentz force.21) Associated with the motion of thecurrent sheet, the motion of the magnetic fields in theacceleration channel will be induced. Since the magnetic
probes are fixed on a side wall of the acceleration channel inthis study, temporal variation of the magnetic field monitoredwith each probe includes positive and negative waveforms ofthe magnetic field.
From Fig.7, it is shown that there are delays for the first
positive (or negative) peaks at probes located at further
downstream positions. These delays can be the durations forFig. 7. Probe 3 signal at 0~50mm from propellant surface (Vc =8000V,C= 50nF)

By estimating times and delays when the magnetic fieldstrength reached the local maximums, traveling speeds of thestrong magnetic field regions for channel lengths of 50 mm
and 5 mm were 93 km/s and 125 km/s, respectively.
This is probably due to larger local current densities andmagnetic field strength, or namely larger Lorentz force, inshorter channel case.
References
1)
2)

3)
4)
Fig. 8. Probe 3 signal at 0~5mm from propellant surface (Vc =8000V,
C= 50nF)
5)

6)
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9)
10)

11)
12)
Fig. 9. Peak value of each measurement point (Vc=8000V, C = 50nF,
channel length: 5 and 50mm)

6.

Conclusion

To observe the effects of the acceleration channel length of
the rectangular electrodes of a short-pulse laser assisted pulse
plasma thruster (LA-PPT), measurement of magnetic field
distributions by magnetic probes were conducted. Temporalvariations of magnetic field distributions were measured bymagnetic field probes located at various positions for Vc =8,000 V (C = 50 nF). As for thrusters, channel lengths of 50mm and 5 mm were compared.
From the result, it was shown that magnetic field of ashorter acceleration channel thruster (5 mm) was larger thanthat of a long one (50 mm) by about 30%.
Moreover, for the channel length of 50mm, the strength ofmagnetic field was decreasing with distance from thepropellant surface. On the other hand, for the channel lengthof 5 mm, the strength of magnetic field was constant with allmeasurement positons.

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