Acceleration of Laser-Ablation Plasma by Alternating Electric Fields
By Haruhito Kato,1) Yuki Nakamura,1) Kaede Yano,1) Hideyuki Horisawa, 1)1)Tokai University, Kanagawa, JapanA fundamental study of a laser-electrostatic hybrid propulsion system with alternating electric field applied to an
acceleration electrode was conducted, in which laser-induced plasmas were induced through laser beam irradiation onto asolid target and accelerated by electrical means instead of direct acceleration using only laser beam. In the laser-inducedplasma generated from a solid target irradiated with a nanosecond laser pulses exceeding an ablation fluence, it was shownthat electrons, primarily absorbing the laser energy, were emitted earlier and faster from the target and then ions follow(than ions). In this study, to accelerate the temporally shifted particles, an alternating electric field accelerator, rather than astatic field accelerator, was developed, which can firstly accelerate electrons with a negative electric field pulse and thensecondly accelerate ions with another positive electric field pulse. Characterization of the generation of alternating variable
pulse voltages between two electrodes was conducted. The acceleration characteristics of electrons and ions using
alternating fields were evaluated by a retarding potential analyzer (RPA). Ion current, the most probable velocity and theaverage velocity was increased by alternating electrostatic field. The most probable velocities and average velocity
increased to 39 km/s and 36 km/s at -150 V.
Key Words: Space Engineering, Electric propulsion, Double layer, Laser ablation plasma

1.

Introduction

Small-sized onboard laser plasma thrusters are undersignificant development with rapid evolutions of compact but
high power laser systems[1][2][3]. One of the advantages ofsuch laser 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, arenot 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[4][5][6]. In order to further improve thethrust performances and system simplicities of conventionallaser propulsion systems, a preliminary study on a laserelectric hybrid propulsion system was conducted.
Fig. 1.

2.

Schematics of laser-electric hybrid thruster

Laser-electrostatic acceleration thruster
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 having the directed initial velocity is further
accelerated by an electrostatic field, high specific-impulse canbe expected.

Schematics of laser-electric hybrid acceleration systems areillustrated in Figure.1. The basic idea of this systems is thatlaser-ablation plasmas, induced through laser irradiation on asolid target, are additionally accelerated by electrical means.
Since any solid material can be used for the propellant in thesecases, no tanks, no valves, or piping systems are required forthe propulsion system. Also, various materials can be used forthe propellant. Therefore, the system employing this technique
can be very simple and compact. As the laser-ablation plasma
has the directed initial velocity, which is further accelerated
by electrical means, significantly high specific impulses canbe expected.

1

3.

negative and positive charges, alternating electric fields areapplied. To accelerate foregoing electrons passing through anaccelerator electrode, a negative electric field is applied. Then,
to accelerate subsequent ions, a positive electric field isapplied at targeted delay and in a controlled duration.

acceleration with alternating electrostatic field

4.

Experimental Apparatus

Temporal evolutions of the plasma current were measured
by a Faraday cup (FC), or retarding potential analyzer (RPA),
as shown in Fig.3. The RPA consists of three electrostatic
grids and one collector grid. Potentials of the grids were 0 V,-15 V, and 0 to 1000 V, respectively, from its intake. TheRPA was placed at 100 mm away from the thruster (targetplate position) along the centerline. Temporal changes of theplasma currents were acquired by an oscilloscope (Tektronix,spectral width 300 MHz, maximum sampling rate 2.5GS/s).
The plasma current was estimated from the time of signal
arrival to the Faraday cup. A pulse generator (DG-535) wasused to transmit a signal to a switching circuit and a laser andto synchronize them with arbitrary durations.
A schematic illustration of a laser-electrostatic hybrid
thruster is shown in Fig.4 to Fig.8. The hybrid thruster
consists of a Cu target (propellant), an acceleration electrode,and a laser oscillator. The propellant target and accelerator
electrode are connected to a switching circuit. This circuitprovides alternating voltages (positive and negative voltages)
between the target and the acceleration electrode. The Cu
target was mounted on an X-Y stage to refresh irradiatingspots of the target 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, orelectrons and ions, were accelerated by the accelerationelectrode made of a 0.3 mm-thick Cu plate with a hole of 10mm in diameter.

Fig. 2. Temporal changes of electron and ion currents measured in alaser ablation plasma

Fig. 3. Experimental setup

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 anddensity dramatically change with time and in space. Figure 2
shows a typical output signal measured by a Faraday cup (FC)fixed at 100 mm from a Cu target, showing temporal changesof electron current (negative signals, or below zero ampere)
and ion current (positive signals, or over zero ampere)measured in a laser ablation plasma. This figure shows thatnegative signals of electrons, primarily absorbing laser energy
at inside and outside of the target, are reaching to FC earlier
than positive signals of ions, since electrons are emitted
earlier than ions from the target surface. Then, ions follow
foregoing electrons. Namely, a wavefront of a laser ablation
plasma consists of two layers of electrons and ions, or a
double layer[7]. From the figure,a difference of arrival timesof electrons and ion can be assumed as a gap between
negative and positive peaks, which is about 10 μs.
In this study, to accelerate both electrons and ions withFig. 4. A schematic illustration of a laser-electrostatic hybrid thruster

2Fig. 8. fourth step of acceleration mechanism, Laser irradiation
Fig. 5. First step of acceleration mechanism, Laser irradiation

To induce alternating pulse voltages to the electrodes, wehave developed a switching circuit, shown in Fig.9. In thisfigure, V1 and V2 are bipolar power sources. V1 provides anegative pulse voltage. On the other hands, V2 provides asteady positive bias voltage. By combing these negative pulse
voltage and steady positive bias, alternating voltages to theelectrodes can be generated. In this circuit, IGBT andcapacitor (1μF) were used to generate a pulsed voltage wave.
This pulsed voltage wave was transmitted to a secondary
circuit through transformer (1:1) and applied to the electrodes.
A typical output signal of alternating pulse voltages applied tothe electrodes synchronized with a laser pulse is shown in
Fig.10. The voltages and durations can be controlled tooptimize acceleration conditions.

Fig.6. Second step of acceleration mechanism, double layer formation

Fig. 9. Diagram of switching circuit

Fig. 7. third step of acceleration mechanism, electron acceleration
Fig. 10. Output voltage of switching circuit

3

5.

Results and Discussions

Plots of peak ion currents versus grid voltages of RPA forvarious acceleration voltages are shown in Fig.11. Taking
derivatives of these plots of ion currents with respect to gridvoltages, or dI/dV, ion energy distributions, or namely ion
velocity distributions, can be obtained.
Fig.12. shows the results of ion velocity distributions forvarious acceleration voltages of 0 to - 150 V. From the figure,it can be seen that peaks of the distributions of ion velocities,
or the most probable velocities, shift toward higher velocities
with increase of acceleration voltages.
Plots of the most probable velocities of ions and averagevelocities of ions, derived from integrations of ion velocities
weighted with ion numbers, are shown in Fig.13. for various
acceleration voltages. It can be seen from the figure that ion
velocities increase with increasing acceleration voltages.
The most probable velocities change from 33 km/s at 0 V to
39 km/s at -150 V. In addition, the average velocities increase
from 22 km/s at 0 V to 36 km/s at -150 V.

Fig. 13. Most probable and average velocities for various acceleration
voltages.

6.

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
by an alternating electrostatic field. A newly developed pulse
generating circuit successfully generated alternating pulse
voltages between electrodes. To measure temporal evolutionsof the plasma currents and ion velocity distributions, aFaraday cup (FC), or retarding potential analyzer (RPA), wasemployed. It was shown that ion velocities increase withincreasing acceleration voltages. The most probable velocities
increased from 33 km/s at 0 V to 39 km/s at -150 V. In
addition, the average velocities increased from 22 km/s at 0 V
to 36 km/s at -150 V.
References

Fig. 11. Ion currents vs grid voltages of RPA for various acceleration
1)

voltages.

2)

3)

4)

5)

6)
Fig. 12. Ion velocity distributions for various acceleration voltages.
7)

4

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. Journalof Propulsion 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.
orisawa, H. Sasaki, K. Igari, A. and Kimura, I. “Laserelectrichybridacceleration system for space
propulsion applications”, The Review of Laser Engineering
Vol. 34, 2006, pp. 435- 441.
S. Eleser and H. Hora. “Double layer in laser-inducedplasmas”. Physics Reports, Vol. 172, 1989, pp. 339-407