2017-b-77p

Magnetic Field Measurement of Laser-Electromagnetic Hybrid Thrusters
Yuki Murayama1, Toshiaki Ohi2, Kentaro Kato3, Haruhito Kato4, Nao Akashi5, Hideyuki Horisawa6
1,2,3,4,5

Department of Mechanical Engineering Graduate school of Engineering, Tokai University, Hiratsuka, Japan
6

Department of Aeronautics and Astronautics, Tokai University, Hiratsuka, Japan

A small spacecrafts which mass and power limited need a new propulsion device. The device has to realize a highspecific impulse and simple structure. The paper proposed a new thruster, which is combination of laser propulsion andelectromagnetic acceleration like a pulsed plasma thruster. To realize the effects, the magnetic field distribution of plasma
flow is observed using a optics camera and magnetic field probe. Those probes are arranged along the plasma flow. We
confirmed that gradually moving magnetic field after 1200 ns at plasma generation.
Key Words: Pulsed plasma thruster, Laser ablation, Magnetic field measurement

Nomenclaturelwh1.

initial flow rate, generated by the laser irradiation on a solidpropellant. The plasma is accelerated by electromagnetic force
which is provided by correlation between discharge current
which flows if two electrodes are breakdown and self-induced
magnetic field. Figure 2. shows photograph of appearance of a
LA-PPT, and table 1. shows the size of a LA-PPT thruster
head. Two metal plates are made from molybdenum, Mo, andsupport body is made from aluminium oxide, Al2O3. Figure 3.
shows the moment of a LA-PPT operation. At the left side ofthis photo, the plasma shines brilliantly near by the surface ofpropellant. The discharge current between electrodes shine
two plate electrodes. The plasma is accelerated to the right. Inaddition, the circular device appears in this photo is a thruster
target made from macromolecule materials.

: electrode channel length: electrode channel width: electrode channel height(it means gaps of electrode)

Introduction

In recent years, the number of micro satellites, is superior incost performance, increase rapidly. Those small satellites havesevere limitation in mass and power, so they need highspecific impulse thruster. Therefore, electricity propulsion hasadvantage in specific impulse, furthermore, Pulsed plasma
thruster, PPT, which is kind of electricity propulsion has
advantage in the simplicity of construction and the ratio of its
weight per thrust power it can generate.
On the other hand, laser propulsion is a new system ofspace propulsion. In late years, laser is studied in each country,
a laser technology develops rapidly, many laser systems
became smaller and more high performance. Laser propulsion
system does not need some gas tanks, valves, and piping
because this system can use solid propellant. Therefore, uselaser for thruster system can simplify the structure of thepropulsion systems.
For compactification and high performance of propulsion
system, our group devised laser-electromagnetic hybridthruster. It has special advantage of simplifing the structureand has high specific impulse. We conduct basic study ofvarious type of new hybrid thruster.

Figure 1. schematic of a LA-PPT

Table 1. Size of a LA-PPT

2.

Method of Laser Assisted PPT, LA-PPT
Figure 1. shows the construction of 2D laser assisted PPT,LA-PPT. This thruster head is constructed by two electrodeplate and solid block propellant. One of the electrodeconnected with capacitor bank that is fully charged. The otherconnected with grand. The basic idea of LA-PPT system isuse of ablation plasma, has high plasma density and highPartlw
h1Width, mm
501015

4. Experimental Setup
4.1. Experimental specimen of thruster
Figure 4. shows schematic of experimental setup of a LAPPT. Figure 5. shows Appearance of experimental setup
around the vacuum chamber. A LA-PPT and capacitor bankfor LA-PPT are set in a vacuum chamber. We operated a LAPPT when the chamber’s back pressure is 2.7 × 10-3Pa.
We use a Nd-YAG laser, CONTINUUM inc, Surelite II,wave length : 1064nm, pulse width 5ns, and energy of one
pulse is 400mJ, to ignite our thruster. The laser unit is set
outside the chamber. The laser needs lens for attenuation andlight condensing. The laser has excessive energy. we have touse an optical beam attenuator unit when the laser irradiates
the high power beam through windows of the chamber. On theother hand, a condenser lens is placed in the chamber, because
the lens have to be set near the surface on the propellant tofocus the laser beam.
The thruster electrodes of LA-PPT require high voltage andsevere current. Considering Paschen law, high voltage isnecessary for the electrical breakdown between two electrodes,
anode and cathode. Considering generating high Lorentz force
in the thruster head, severe current is necessary for theelectromagnetic acceleration. Hence two electrodes need highpower supply. A regulated DC power supply cannot supply
the large power, because it has a problem of power
consumption and excessive fever. In this case, the large
capacitance capacitor is superior to A regulated DC power
supply. The electrodes connected to a capacitor bank. Thecapacitor bank is made of high voltage capacitors, TDK, Ultrahigh voltage ceramic capacitors, UHV-3A. Each capacitor has
4.6nF capacitance, and total capacitance of the capacitor bankhas 24.50nF. The capacitor bank can release electric current
during the operation of a LA-PPT. The operation is extremely
a short time. As for the operating time of a LA-PPT, the time
which all event is over is shorter than 3us. All events move bynano seconds unit. The capacitor bank is suitable for such acondition.
If we operate some devices for short time, we have toprepare a timing of pulse. We needed a delay generator andinfrared photo diode. The delay generator send a pulse signal
to Nd-YAG laser and ICCD camera. The infrared photo diodecan catch the laser beam, and send immediately a pulse signal
to an oscilloscope. The oscilloscope recodes the discharge
current of a LA-PPT by the current monitor when theoscilloscope receives the signal.

Figure 2. Appearance of a LA-PPT

3.

Motivation
One of the most important parameter of electricity
propulsion system is specific impulse. Figure 3. shows thevarious case of typical characteristics when the laser assisted
PPT operated high specific impulse. They were operated indifferent electrode conditions, various electrode geometries,that means multiplied electrode length and electrode height.
The plot in this figure is from our previous report. From thefigure 3., specific impulse increased in proportion to energythat the capacitor bank charged. Furthermore, it shows that themore thruster heads are long, the specific impulse is high. OurLA-PPT recoded about 7200sec with 10mm electrode lengthand 50mm electrode height, at that time, the energy that thecapacitor bank charged is about 8.6J. It is a notable recode,
the specific impulse of our LA-PPT is higher than electricity
propulsion systems were already put to practical use, like ion
and hall thruster. A LA-PPT has assessed operations that use
short pulse and high voltage operation and has recoded
outstanding performance. In addition, LA-PPT has advantagesof compact construction and weight.

Figure 3. the various case of typical characteristics when
the laser assisted PPT operated high specific impulse2Figure 7. A typically observation data of the plasma in a
LA-PPT by the ICCD camera
Figure 4. Appearance of a Nd-YAG laser

4.3. Magnetic field probe
We could not completely understand the status of plasma
for only the light intensity of plasma flow. Moreover, we hadto estimate the relationship between the light intensity andother plasma parameters. Therefore, we examined whether thecorrelation between the light intensity and magnetic fields in a
LA-PPT channel.
We performed directory a magnetic field measurement by
Rogowski coil to determine the self-inducted magnetic field ina LA-PPT channel between two electrodes. Figure 8. shows amagnetic field probe we made and used. The diameter of thisprobe is only 1mm, and this probe is made of copper wire of
0.6mm in diameter. The surface of copper wire we used iscoated with enamel. This coating protected the copper wirefrom discharge current between electrodes. In other words, thecoating behaves the electrical insulator.
If any current flows the probe, it means being magnetic
fields at the point. We have to do the time integration of thecurrent which flows in the probe to determine magnetic fields
at the point. A simple circuit can integrate a wave input andcan output it. Therefore, we prepared an integrating circuit.
Figure 9. shows schematic of experimental setup of a LA-PPT
when we measure magnetic fields. An integrating circuit ismade of electric resistances and capacitors. We prepared eightcircuits, because one integrating circuit is necessary in one
probe. The probes and the circuits are same quantity. Theoscilloscope display and recode the output signal, the signal
has been integrated.

Figure 5. Appearance of experimental setup

Figure 6. Appearance of Nd-YAG laser

4.2. Plasma behavior observation by ICCD camera
A ICCD camera was used time-resolved optical imaging oflight intensity of plasma flows. The time scale of plasma
behavior is nano seconds order. We need a short expose timeand continuous imaging camera to observing plasma behavior.
From the observation of plasma behavior, we clarified that afront of plasma flows and plasma speed. We used an ICCD
camera, ANDOR TECHNOLOGY, minimum gate width : 2ns,
to verify the plasma LA-PPT generated. Figure 7. shows theplasma when momentary the laser fired.
Figure 8. photo of magnetic field probe3magnetic fields were moving downstream. Moreover, it was
shown that temporal variations of the magnetic fields inregions at near propellant surface are almost in phase with adischarge current waveform.
Figure 12 shows output signals of magnetic field measured
on center axis at 0 to 50 mm from propellant surface (chargevoltage of Vc = 1,000 V). From these signals taken at several
positions in the discharge channel, temporal variations ofmagnetic field distributions in the discharge channel werereproduced, as shown in Fig.13. In this figure, corresponding
temporal variation of plasma images taken by an ICCD
camera are also shown. From the figures, it is shown that thepart of light intensity where is stronger moves dawn stream.
The timing which the plasma generated and the timing which
the magnetic fields appearing are not same.
In this present study, we have demonstrated that the timing
the strong magnetic fields appears was 1,600ns late behind thetiming the plasma generated. Moreover, the strong magnetic
fields in a LA-PPT channel moves downstream in the same
way as the light intensity of plasma flow. The severe current
between anode and cathode and the self induced magnetic
fields by the severe current provides the plasma flow with thevelocity for the effect of an electromagnetic acceleration.

Figure 9. photo of magnetic field probe setup

4.3. Magnetic field measurement
Our magnetic field probe can determine magnetic fieldstrength at the point, so we prepared this probe eight todetermine a magnetic field distribution. This experimental
study was performed to determine the association between
light intensity of plasma flow and magnetic field of plasma. In
this paper, elucidating correlation between plasma behavior
and magnetic fields, we focus on plasma flow direction. Thoseprobes were arranged flow direction in this experiment.
However, the position which the plasma generated from thepropellant and the position of thruster head exit, it means theterminal of the plate electrode, were arranged lengthwise,
those probe lines that were this two positions wereperpendicularly at plasma flow. The plasma carried out acomplicated behavior at the position where the plasma
generated. The terminal of thruster head is the end of theelectrodes. The electrodes give the plasma the energy, so theplasma is the fastest at the point. Figure 10. shows probe
arrangement this experiment for this paper.

Figure 11. Operating a LA-PPT

Figure 10. probe arrangements when we conduct
experiment in the past

5.

ResultFigure 11. shows a LA-PPT in operation. We observed
relatively stronger magnetic fields, and the magnetic fields
were induced at near propellant surface region, cathode lowerside, after 1,200 to 1,600 ns. After 1,600 ns, regions of strong
Figure 12. Output signals of magnetic fieldmeasured on center axis at 0 to 50 mm frompropellant surface (charge voltage: 1,000V).
4plasma. We found out experimentally that a magnetic fieldappears on the point where the plasma was after movement of
the plasma.
In addition, after moving the stronger magnetic fields,
negative direction magnetic field has been observed near theterminal of electrodes. In this experiment, we determine only
one dimension magnetic field. If we want to the strength ofthe magnetic fields in a LA-PPT channel, we have to measure
three dimensional magnetic fields per one point, and we haveto take geometric mean.
The magnetic fields behavior is very complex. However,
we need to clarify the behaviors to electromagnetic
acceleration mechanisms and to design more excellent andmore robust electromagnetic thruster, like MPD or PPT.
References
1)
2)

3)
4)

5)
7)

Figure 13. The light intensity of plasma, left andthe magnetic fields in a LA-PPT channel, right.
Therefore, the time division is 400us.

8)
9)

6.

Conclusion
We estimated the experimental results why the appearing
magnetic fields is late behind the light intensity of plasma
flow. Compared the light intensity of plasma and the magnetic
fields, the strong magnetic fields appeared the space between
the propellant side surface and the brilliant light intensity ofthe plasma. Moreover, the plasma flows from the propellant
surface through the exit of electrodes, and considering
Maxwell’s equations, the magnetic fields are made by thecurrent. Hence, we found out the deeply association between
the discharge current and the light intensity of the brilliant

11)

12)

5

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