2019-b-063

Acceleration Charactarisitc of Laser Ablation Plasma
by Alternating Electrostatic Field
By Takato OGASAWARA,1) Haruhito KATO,1) Yuki NAKAMURA,1) and Hideyuki HORISAWA1)
1)

The Graduate School of Engneering, Tokai University, Japan(Received June 21st, 2019)
A fundamental study of a laser-electrostatic hybrid propulsion system with alternating electric field applied to an
acceleration electrode was conducted. Plasma was induced by laser beam irradiation onto a solid target and accelerated byelectrical means. In this study, a nanosecond laser pulses exceeding an ablation fluence, it was shown that electrons, primarilyabsorbing the laser energy, were emitted earlier and faster from the target and then ions follow(-than ions). To accelerate thetemporally shifted particles, an alternating electric field accelerator was developed, which can firstly accelerate electrons witha positive electric field pulse and then secondly accelerate ions with another negative electric field pulse. The acceleration
characteristics of electrons and ions using alternating fields were evaluated by a retarding potential analyzer (RPA).
Key Words: Space Engineering, Electric propulsion, Double layer, Laser ablation plasma

1.

Introduction
1.2 Laser ablation plasma
Generation of a laser ablation plasma induced through aninteraction of an intense focused pulsed laser and a solid target
is a short-duration event. Plasma temperature and density
dramatically change with time and in space. Figure. 1 shows atypical output signal measured by a Faraday cup (FC) fixed at
100 mm from a Cu target, showing temporal changes ofelectron current (negative signals, or below zero ampere) andion current (positive signals, or over zero ampere) measured ina laser ablation plasma. This Fig. 1 shows that negative signalsof electrons, primarily absorbing the laser energy inside andoutside of the target, are reaching to FC earlier than positive
signals of ions, since electrons are emitted earlier than ionsfrom the target surface.
Then, ions follow foregoing electrons. Namely, a wavefrontof a laser ablation plasma consists of two layers of electrons
and ions, or a double laye7). From the Fig. 1,a difference ofarrival times of electrons and ion can be assumed as a gap
between negative and positive peaks, which is about 10μs.

Small-sized onboard laser plasma thrusters are undersignificant development with rapid evolutions of compact but
high power laser systems1) 2) 3). 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 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 power4) 5) 6). In order to further improve the thrust
performances and system simplicities of conventional laser
propulsion systems, a preliminary study on a laser-electric
hybrid propulsion system was conducted.
1.1 Laser-electrostatic hybrid acceleration thrusters
The basic idea of this systems is that laser-ablation plasmas,
induced through laser irradiation on a solid target, areadditionally accelerated by electrical means. As the laserablation plasma has the directed initial velocity, which isfurther accelerated by electrical means, significantly highspecific impulses can be expected.
A preliminary investigation on a laser-electrostatic hybrid
acceleration thruster is being conducted, in which a laserablation plasma is accelerated by an electrostatic field. A
focused laser pulse is irradiated on to a solid target, orpropellant. Then, a laser-induced plasma, or laser-ablation,
occurs at an irradiating spot of the propellant surface. In thelaser-ablation process, first of all, electrons are emitted fromthe surface, and, then, ions are accelerated through ambipolar
diffusion. In this study, those ions are further accelerated withan additional acceleration electrode. Since the laser-induced
plasma is further accelerated by an electrostatic field, highspecific-impulse can be expected.

Fig. 1 Temporal changes of electron and ion currents measured
in a laser ablation plasma

1steady positive bias voltage. By combing these negative pulse
voltage and steady positive bias, alternating voltages to theelectrodes can be generated. In this circuit, IGBT and capacitor(1μF) were used to generate a pulsed voltage wave. This pulsed
voltage wave was transmitted to a secondary circuit throughtransformer (1:1) and applied to the electrodes. A typical output
signal of alternating pulse voltages applied to the electrodes
synchronized with a laser pulse is shown in Figure. 5. Thevoltages and durations can be controlled to optimize
acceleration conditions.

Fig. 2 Experimental setup

2.

Expermental Appatratus

Temporal evolutions of the plasma current were measured
by a Faraday Cup, as shown in Figure. 2. The Faraday Cup
consists of three electrostatic grid and one collector grid. It wasplaced at 100 mm away from the thruster (target plate position)
along the centerline. Temporal changes of the plasma currents
were acquired by an oscilloscope (Tektronix, spectral width
300 MHz, maximum sampling rate 2.5GS/s). Since thetransient probe data were highly reproducible, it was possible
to obtain the current-voltage. The plasma current was estimatedfrom the time of signal arrival to the Faraday cup. Pulse
generator (DG-535) was used to transmit a signal to the IGBT
and the Nd:YAG laser. It synchronize the switching circuit with
the Nd:YAG laser.

Fig. 3 A schematic illustration of a laser-electrostatic hybrid thruster

2.1. Accelerator electrodes
A schematic illustration of a laser-electrostatic hybrid
thruster is shown in Figure. 3. The hybrid thruster consists of a
Cu target (propellant), an acceleration electrode, and a laser
oscillator. The propellant target and accelerator electrode areconnected to a switching circuit. This circuit provides
alternating voltages (positive and negative voltages) between
the target and the acceleration electrode. The Cu target wasmounted on an X-Y stage to refresh irradiating spots of thetarget surface. For the laser oscillator, an Nd:YAG laser(wavelength: 1064nm, pulse energy: 340mJ/pulse, peak power:
0.17GW, pulse width: 5ns, repetition rate: 10Hz ) was used.
Laser ablation plasmas or ions were accelerated by theacceleration electrode made of a 0.3 mm-thick Cu plate with ahole of 10 mm in diameter. To evaluate the effects of theelectrostatic field, effects of arbitrary delays and durations forthe application of applied voltages to the target and accelerationelectrode on ion emission characteristics were investigated.

Fig. 4 Diagram of switching circuit

Fig. 5 Output voltage of switching circuit

2.2. Switching circuit
To induce alternating pulse voltages to the electrodes, wehave developed a switching circuit, shown in Figure. 4. In thisFig, V1 and V2 are bipolar power sources. V1 provides anegative pulse voltage. On the other hands, V2 provides a22.3.
RPA
To obtain ion energy (or velocity) distribution, a retarding
potential analyzer (RPA) was employed. Values of ion currents
for constant retarding potentials (or ion energies) are recorded,and then relations of potential (grid voltage, or ion energies)
versus ion currents are plotted, as shown in Figure. 6. Taking
first derivatives of the plot, or namely dI/dV, energy, or velocity,
distribution of ions can be obtained.

acceleration voltage of 0~-150V by the switching circuit. The
vertical axis is the number of ions, and the horizontal axis is theion velocity. From the Fig, as the acceleration voltage isincreased, the most probable velocities gradually increased.
The most probable velocities at each acceleration voltage andthe average velocity can be obtained, as Figure. 8, from velocity
distributions of Fig. 7 From Fig. 8, the ion velocity increasedby the potential difference between acceleration electrode andpropellant target. The most velocity of the ions increased from
32.5km/s (acceleration voltage is 0V) to 38.9km/s (acceleration
voltage is -150V). The average velocity of the ions increased
from 22.3km/s (acceleration voltage is 0V) to 36.1km/s(acceleration voltage is -150V).
4.

Conclusion

A preliminary investigation of an alternating electric field
accelerator for a laser-electrostatic hybrid thruster wasconducted, in which a laser ablation plasma was accelerated byan alternating electrostatic field. A newly developed pulse
generating circuit successfully generated alternating pulse
voltages between electrodes. From the ion energy distribution
measurement with RPA, it was confirmed that ions wereaccelerated by alternating electrostatic field.

Fig. 6 Ion current vs grid voltageReferences
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2)

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1)

Fig. 7 ion velocity distribution of each pulse voltage for acceleration

2)

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4)

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6)
Fig. 8 Most probable and average velocity in case of pulse voltageacceleration

3.

Results and Discussion

7)

Figure. 7 shows ion velocity distributions when applying

3

T. M. Cover and J. A. Thomas, Elements of Information
Theory, Wiley-Interscience, 2006.
S. Arimoto, "Computation of Random Coding Exponent
Functions," IEEE Trans. Information Theory, Vol. 22, No. 6,
pp. 665-671, 1976.
T. S. Han and S. Verdú, "Approximation Theory of Output
Statistics," Proc. of 1993 International Symposium onInformation Theory, p.153, 1993.
Phipps, C. Luke, J. ”Diode laser-driven microthrusters: A
new departure for micropropulsion,” AIAA Journal, Vol. 40,2000, pp. 310-318.
Pakhomov, V. A., Gregory, D. A., “Ablative laserpropulsion: An old concept revisited,” AIAA Journal; Vol.
38,2000, pp. 725-727.
Phipps, C. Luke, R. J., Lippert, T. Hauer, M. Wokaun,
A.“Micropropulsion using a laser ablation jet,” A. Journal ofPropulsion and Power, Vol. 20, 2004; pp. 1000-1011
Horisawa, H. Igari, A. Kawakami, M. Kimura, I.
“Dischargecharacteristics of laser-electric hybrid thrusters,”
AIAA Paper,2004, 3937.
Horisawa, H. Kawakami, M. Kimura, I. “Laser-assistedpulsedplasma thruster for space propulsion applications,”
Applied Physics A, Vol. 81, 2005; pp. 303-310.
Horisawa, H. Sasaki, K. Igari, A. and Kimura, I. “Laserelectrichybrid acceleration system for space propulsionapplications”, The Review of Laser Engineering Vol. 34,
2006, pp. 435- 441
S. Elezer and H.Hora. “Double Layer in Laser-ProducedPlazma”.Physic Reports 172, No. 6 1988, 339-407.