Space Potential Measurement of Laser Ablation Plasma forElectrostatic Acceleration
By Ayaka HAMADA,1) Kaede YANO,1) Takato OGASAWARA,1) Hideyuki HORISAWA,1) and Haruo SHINDO2)
1)

Department of Mechanical Engineering, The University of Tokai, Hiratsuka, Japan
2)
Plasma Research Laboratory CO.LTD., Sagamihara, Japan(Received June 21st, 2019)

In this study, development of a laser-electrostatic hybrid acceleration thruster was conducted, which is a combination ofa laser propulsion system and an electrostatic propulsion system. Since this system can additionally accelerates ions byaccelerating laser-ablation plasma by an external electrostatic field, highspeed ions can be generated. In this study, to obtainspatial and temporal plasma parameters for the optimum acceleration, Langmuir probe measurement of laser-ablation plasmas
was firstly conducted, where focused Nd:YAG laser pulses were irradiated on a copper target in a vacuum chamber. Temporalchanges of spatial distributions of the plasma parameters were obtained. As the results, the maximum electron temperaturesvaried from 6.4 eV to 1.5 eV at probe positions at 150 mm and 170 mm respectively

Key Words: Diagnostics, Langmuir Probe, Laser Propulsion, Plasma

1.

Introduction

Small-sized onboard laser plasma thrusters are undersignificant development with rapid evolutions of compact but～high-power laser systems.1) 4) One of the advantages of suchlaser thrusters is the use of solid-state materials for thepropellant. Since any solid material can be used for thepropellant, tanks, valves, or piping systems, which arenecessary for the thrusters with liquid or gaseous propellant, are
not required for the laser propulsion system. Therefore, thelaser thruster system can be very simple and compact. Also,
significant controllability of thrust is possible by changing theinput laser power. In order to further improve the thrust
performances and system simplicities of those conventionallaser propulsion systems, a preliminary study on a laser-electric
hybrid propulsion system was conducted.
2.

the electrode configuration for additional electric acceleration,and properly controlling a power source, or voltage and current,
adapting to each acceleration mechanism, the propulsion
system satisfying all the above acceleration schemes through i)
to iii) can be achieved with one thruster configuration. Namely,
this system enables a robust conversion between high specificimpulse operation and high thrust density operation in regard
with mission requirements. Each of two typical types of theacceleration schemes are currently being investigated as
described in following subsections.
In this study, to reveal the acceleration mechanism of thelaser-electrostatic hybrid thruster, the plasma plume near theablation target was characterized by the Langmuir probe.

Electrostatic acceleration by laser induced plasma

Our laser-electric hybrid acceleration system is depicted inFig. 1. The basic idea of these systems is that a laser-ablation
plasma, induced by the laser irradiation on a solid target, isadditionally accelerated by an accelerator grid. Because thelaser-ablated plasma has the initial velocity of about 10 km/s,
specific impulses can be significantly increased with further
acceleration of ions by an electrostatic regime. In this case,relating to electrode configuration, plasma density, electricalinput-power (voltage and current), etc., acceleration regimes
for the laser-ablation plasma can be classified into three types,i.e., i) electrostatic acceleration, ii) electromagneticacceleration, and iii) electrothermal acceleration. Especially forthe laser-ablation plasma, depending on laser conditions suchas pulse energy, fluence, etc., plasma density distribution canbe widely controlled. Moreover, this can also be increased
through additional electric discharges. Therefore, optimizing

Fig. 1. Schematics of laser ablation thrusters.
3.

Experimental apparatus

Figure. 2 shows a schematic of the experimental setup forlaser induced plasma diagnostics. A laser beam oscillated by an
Nd: YAG laser oscillator (wavelength 1064 nm, pulse energy174 mJ / pulse, pulse width 5 ns) was introduced into a vacuum
chamber after being expanded by a beam expander. At this time,scattered light generated in the laser introducing window wasacquired by a photodiode, and this was used as a trigger (anorigin of time) of an oscilloscope for recording a time historyof a probe output current. The vacuum chamber was maintained
at a degree of vacuum of 10 -3 Pa order by a turbo molecularpump. The laser light introduced into the chamber was reflectedby a mirror, focused by a final stage lens, and irradiated on thesurface of a solid propellant (oxygen-free copper). At this time,
the laser induced plasma generated was diagnosed with a flat
single Langmuir probe (size: 5 mm × 5 mm) installed on auniaxial stage movable on an axis perpendicular to the surfaceof the propellant from the focusing point. In addition, as shownin the figure, a vector of the probe surface was arranged to thedirection (parallel) which parallel the plasma flow. An arbitrary
bias voltage was applied to the Langmuir probe, and the timechange of the ion / electron current was acquired for each laser
pulse. In addition, the probe was mounted on a movable
uniaxial stage so that the spatial distribution of plasma could
also be diagnosed. In addition, the focusing point on the surface
of a propellant was able to move on a movable stage in order torefresh an irradiated surface after each laser pulse irradiation.
Further, for comparison, the diagnosis was also performed in
the case where the probe surface was facing vertically to theplasma flow (the direction rotated 90 ° from the direction of theprobe in the figure).

pulse irradiation, and the reproducibility of the phenomenon isassumed to be high.
Firstly, while the probe bias voltage (arbitrary) at an arbitrary
position was fixed, the temporal change of the probe current isacquired with an oscilloscope. The multiple waveforms can beobtained by repeating the measurement with changing the bias
voltages, as shown in Fig.3. From the temporal changes ofprobe currents at various probe voltages, the voltage-current
characteristics at arbitrary times can be extracted by slicing thecurrent waveforms at an arbitrary time, and extracting a
relationship between the bias voltage and the probe current (Fig.
4). From the voltage-current characteristics, the electrontemperature and density at that time can be estimated. By
performing this process for all the time targeted, the temporalchanges of the plasma parameters were estimated6). In thisstudy, the time interval for extracting the voltage-current
characteristics was defined as 100 ns, and various probe
positions and quantities at each time were estimated. In addition,
since the data obtained in this process is very huge, thecalculation processing was made more efficient by adopting
parallel processing using MATLAB. The electron temperature
can be derived by finding the slope of the current-voltage
characteristic from the following equation. 6), 8)
𝑑𝑙𝑛𝐼𝑒(𝑉)𝑑𝑉=−𝑒(1)𝑘𝑇𝑒Here, e is an elementary charge, k is a Boltzmann constant,and Te is an electron temperature. In the processing using
MATLAB, the current-voltage characteristics for the numberof time plots (for example, 500) per 100 ns were extracted, andthe temporal variation of the electron temperature wascalculated. The electron density (Eq. (2)) and Debye length (Eq.
(3)) were additionally calculated using the estimated electron
temperature and the following formula, and then the temporalvariation of the plasma parameters was estimated.

Fig. 2. Experimental Setup of Single Probe Measurement.

4𝐼𝑒0𝑒𝑆𝑣(2)

𝜀0𝑘𝑇𝑒
𝜆𝑒 = √ 2𝑒 𝑁𝑒(3)

𝑁𝑒 =𝑒

4.

Results and Discussion

4.1 Probe surface facing parallel to the plasma flowFigure 3 shows the temporal variation of the probe current(probe position: 150 mm from the surface of the propellant) at
each bias voltage when the probe surface is facing parallel tothe plasma flow. In this figure, the temporal changes of theprobe currents at various bias voltages are displayed in different
colors. Generally, in single-probe plasma diagnosis, a relatively
steady (equilibrium state) plasma having a characteristic time
sufficiently longer than the plasma frequency is targeted. In thiscase, a change in probe current when the probe bias voltage iscontinuously changed is measured. On the other hand, the laser
induced plasma handled in this research requires relatively hightime resolution diagnosis. In this experiment, diagnosis wasmade according to the following procedure using the fact that
there is no big difference in a plasma condition for each laser

Here, Ie0 is a saturated electron current, S is a surface area ofa probe, ve is an electron thermal velocity, and ε0 is adielectric constant of vacuum.
Figure 5 shows a temporal evolution of the electron
temperature at each probe position. It was confirmed that thearrival time of the highest temperature of the electron
temperature at each probe position tends to be delayed to reachthe highest temperature as the probe position moves
downstream from the propellant. Also, the value of themaximum temperature decreases with the increase of thepropellant-probe distance. On the other hand, the electrontemperature at each probe position after reaching the maximum
temperature tends to rise slightly as it gets farther from thepropellant.
Figure. 6 shows the temporal change of electron density ateach probe position. It was confirmed that the electron density
reaches lowest values at the time when the electron temperaturepeaks at each probe position. In addition, the minimum valueof the electron density at each probe position increases as thedistance between the propellant and the probe increases, andafter reaching the minimum density, the temporal evolution ofdensity (gradient of each waveform) is relaxed as the distancefrom the propellant.
Figure. 7 shows the time variation of Debye length at eachprobe position. The change in Debye length with time shows
the same tendency as the electron temperature, and themaximum temperature decreases with the increase in thedistance between the propellant and the probe, and at the sametime there is a delay in the time to reach the maximum value.

Fig. 5. Temporal Evolution of the Electron Temperature.

Fig. 6. Temporal Evolution of the Electron Density.

Fig. 3. Temporal Evolution of Probe Currents for Various
Probe Biasing Voltage. (Probe in the Parallel to the Plasma)Fig. 7. Temporal Evolution of Debye Length.
4.2 Probe surface perpendicular to the plasma flow

Fig. 4. I-V Characteristics. (Probe in the Parallel to thePlasma)

Figure. 8 shows the time variation of the probe current at
each bias voltage obtained when the flat plate probe is placed
vertically to the plasma flow (probe position 150 mm). The
absolute value of the probe current tended to decrease at eachtime as compared to the case where the probe surface was set
parallel to the plasma flow. The V-I curve was extracted fromthe temporal change of the probe current (Fig. 9). From thisfigure, the temporal change of the electron temperature
calculated as described above is shown in Fig. 10. The peaktemperature tends to be lowered by about 4 eV at a position of150 mm as compared with the case where the probe is placedparallel to the plasma flow. The decrease in electrontemperature in the case of vertical facing is considered to bedue to the fact that the projected area to the plasma flow is small
and the influx of high energy electrons is small, as can be seenfrom the decrease of the electron flow.

Fig. 8. Temporal Evolution of Probe Currents for Various
Probe Bias Voltage. (Probe in the Perpendicular to thePlasma)

5.

Conclusion

In order to realize the effective ion acceleration in the laser /electrostatic acceleration combined thruster, a single Langmuirprobe is used in order to understand the spatial structure andtemporal behavior of the laser-induced plasma as a propellant
in detail. The diagnosis of the laser-induced plasma was carriedout. As a result of measurement, when the probe position wasmoved away from the propellant (moved from 150 mm to 170mm), the electron temperature decreased from 6.4 eV (at 0.91μs) to 1.5 eV (at 1.26 μs). In addition, it was confirmed that theelectron density tends to be the lowest at 1.5 × 10 −19 m −3 (at0.92 μs) when the electron temperature reaches a peak. In
addition, it was found that the electron temperature at 150 mm
was about 1/3 the size of that at equilibrium when the flat plateprobe was installed vertically to the plasma flow. Moreover, thetime change of Debye length showed the same tendency as theelectron temperature. The peak value of the Debye length was
4.9 μm (9.3 μs). In the future, we plan to conduct more detailedmeasurements by increasing the number of measurement points.
References

Fig. 9. I-V Characteristics. (Probe in the Perpendicular to thePlasma)Fig. 10. Temporal Evolution of the Electron Temperature.
(Probe in the Perpendicular to the Plasma)
From these results, for effective electrostatic acceleration of
the high energy ion component of the laser ablation plasma, it
is desirable to apply an accelerating voltage at an early time, ex.
less than 10 s, at a position relatively close to the propellant.

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