2019-b-093p

An Alternating Electric Field Accelerator
for Laser-Ablation Plasma Acceleration

By Ryosuke FUJII,1) Ayaka HAMADA,1) Kai ONOSE, 1) and Hideyuki HORISAWA2)
Tokai University, Hiratsuka, Kanagawa, 259-1292, JapanA fundamental study of a laser-electrostatic hybrid propulsion system with alternating electric field applied to anacceleration electrode was conducted, in which laser-induced plasmas were induced through laser beam irradiation onto a solid
target and accelerated by electrical means instead of direct acceleration using only laser beam. In the laser-induced plasma
generated from a solid target irradiated with a nanosecond laser pulses exceeding an ablation fluence, it was shown thatelectrons, primarily absorbing the laser energy, were emitted earlier and faster from the target and then ions follow. In thisstudy, to accelerate the temporally shifted particles, an alternating electric field accelerator, rather than a static field accelerator,
was developed, which can firstly accelerate electrons with a negative electric field pulse and then secondly accelerate ions withanother positive electric field pulse. Characterization of the generation of alternating short pulse voltages typically of 1µsduration between two electrodes was conducted. The acceleration characteristics of electrons and ions using alternating fields
were evaluated by a Faraday cup.

Key Words: Laser ablation plasma ,Electrostatic acceleration ,Alternative electrostatic field generation

Nomenclature
A
V1
V2

1.

: Plasma current: Voltage of a first acceleration electrode: Voltage of a second acceleration electrode

Introduction

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 Acceleration Thrusters
Schematics of laser-electric hybrid acceleration systems areillustrated in Fig. 1. The basic idea of this systems is that laserablation plasmas, induced through laser irradiation on a solid
target, are additionally accelweerated 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 byelectrical means, significantly high specific impulses can beexpected.

Fig 1.Schematics
thruster.

oflaser-electric

hybridA 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, orpropellant. 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 ambipolardiffusion. 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 can be expected.
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 2 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 figure 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 layer7. From the figure, a difference ofarrival times of electrons and ion can be assumed as a gap
between negative and positive peaks, which is about 10µs.

the current-voltage. The plasma current was estimated from thetime of signal arrival to the Faraday cup. Pulse generator (DG535) was used to transmit a signal to the IGBT and the Nd:YAGlaser. It synchronize the switching circuit with the Nd:YAGlaser.

Fig 3. Experimental setup.

Fig 4. A schematic illustration of a laserelectrostatic hybrid thruster

Fig 2.Temporal changes of electron and ion currents
measured in a laser ablation plasma current.
2.

Experimental Apparatus

Temporal evolutions of the plasma current were measured
by a Faraday Cup, as shown in Fig.3. The Faraday Cup consistsof three electrostatic grid and one collector grid. It was placed
at 100 mm away from the thruster (target plate position) along
the centerline. Temporal changes of the plasma currents wereacquired by an oscilloscope (Tektronix, spectral width 300
MHz, maximum sampling rate 2.5GS/s). Since the transient
probe data were highly reproducible, it was possible to obtain

2.1. Accelerator Electrodes
A schematic illustration of a laser-electrostatic hybrid
thruster is shown in Fig.4. 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 theelectrostatic field, effects of arbitrary delays and durations forthe application of applied voltages to the target and accelerationelectrode on ion emission characteristics were investigated.
2.2 Switching Circuit
To induce alternating pulse voltages to the electrodes, wehave developed a switching circuit, shown in Figure.5. 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 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 Fig.6. The voltages
and durations can be controlled to optimize acceleration
conditions.

3.

Result and Discussion

Temporal changes of plasma (electron and ion)current were observed by Faraday cup measurement.
The pulse voltage applied to electrodes were delayed from 0 µs
to 28 µs relative to a laser pulse. The temporal changes ofplasma current were plotted in Figs.8 to 15.
Figures 8 and 9 show current waveforms with constantacceleration voltages applied to electrodes. Figure 8 shows thecurrent with a steady acceleration voltage of +50V. A negative
current was mainly observed, and most of the positive sides arevery low. Figure 9 shows the current with a steady acceleration
voltage of -50V. Conversely, in the case of -50V, a positive
current waveform was observed, and most of the negative sides
were very low.
Figures 10 to 15 show current waveforms with pulsed
acceleration voltages applied to acceleration electrodes atdifferent delays from laser pulse irradiation.
It can be seen that the positive current increases with theincrease of the delays of less than 12 s. The maximum positive
current is observed at a delay of 12 µs, and the peak value ishigher than that obtained in the steady bias case shown in Fig.9.
After the delay of 12 s, the positive current decreases with

Fig 5. Configuration of switching circuit
This switching circuit can generate a short pulse
voltage, in which pulse width is less than 10µs. V2
provides positive voltages to electrodes. On the otherhands, V1 provides negative pulse voltages.

increasing delay.
Fig 7. Schematic of the pulse voltage being delayed from 0µs
to 28µs relative to a laser pulse.

Fig 6. Pulse voltage applied to accelerator gridThis figure shows an example of alternating pulse
voltages applied to electrodes. Pulse width is 5µs. V1
provides pulse voltage (-100V) and V2 provides constantbias (+50V).

Fig 8. Temporal changes of plasma current(constant bias +50V).

Fig 9. Temporal changes of plasma current(constant bias -50V).

Fig 10. Temporal changes of plasma current at0µs.

Fig 12. Temporal changes of plasma current at
8µs.

Fig 13. Temporal changes of plasma current at
12µs.

Fig 14. Temporal changes of plasma current at
16µs.
Fig 11. Temporal changes of plasma current at
4µs.

Fig 15. Temporal changes of plasma current at
24µs.

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. In order to find the optimum
timing of the alternating pulse voltages, the temporal changesof ion and electron currents wereevaluated by a Faraday cup. It was shown that the positive
current increased with the increase of the delays of the pulse
voltage of less than 12 s. The maximum positive current wasobserved at a delay of 12 µs, and the peak value was higher
than that obtained in the steady bias case. After the delay of 12
s, the positive current decreased with increasing delay.
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