1. Introduction

Because of the advantages of low-cost, low-mass, and high specific impulse, electric propulsion thrusters (EPTs) for spacecraft orbit correction and interplanetary spacecraft acceleration have recently become the front subject and focal point in space propulsion fields [1, 2]. As a

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

member of EPTs, pulsed plasma thruster (PPT) has a broad prospect on small satellites for its small, compact, and low mass [3–5]. PPT has been studied decades on its performance and lifetime, and has been successfully applied to a number of satellites [6–8]. However, the problem of low efficiency and ignition failure still restricts the development of PPT.

energy, and causes the density, temperature, pressure and components in the ablation plasma plume dramatically vary, which has an significant effect on the thrust performance. During the years, the expansion dynamics of pulsed laser generated plasma plume has been widely investigated through experimental and numerical methods [19–23]. Especially, several models have been proposed to describe the expansion of ablation plasma and the formation of plasma in the plume [21]. Aden et al. utilized hydrodynamic equations to describe the plasma expansion, which is called hydrodynamic model [24]. Afterwards, the hydrodynamic model had been widely applied to study various laser-solid interactions [25–28]. In order to calculate the laser energy loss caused by plasma absorption and shielding, the formation of plasma in the plume was also considered in several hydrodynamic models [22, 29–32]. Therefore, we establish a hydrodynamic model to investigate the ablation plasma expansion and ionization in the ceramic tube. The numerical model can give insight in the plasma dynamics and plasma behavior of LA-PPT, which is sometimes difficult

Plasma Generation and Application in a Laser Ablation Pulsed Plasma Thruster

http://dx.doi.org/10.5772/intechopen.77511

191

In this paper, a novel laser ablation plasma thruster with a ceramic tube is developed and investigated. This thruster is expected to overcome the shortages of conventional laser-electric hybrid acceleration systems in the above paragraph. And the ionization rate, specific impulse and thrust efficiency of the thruster are expected to be increased. Specially, a numerical model for nanosecond laser ablation of aluminum propellant is presented, which contains the target ablation and plasma expansion in the ceramic tube. Heat conduction model is established to calculate the target ablation, taking into account temperature-dependent material properties, phase transition, dielectric transition and phase explosion. Meanwhile, Hydrodynamic model is established to calculate the plasma expansion, taking into account ionization, plasma absorption and shielding. Then both calculations of the target ablation and plasma expansion are coupled in each time step. Afterwards, the plasma properties (such as velocity, temperature and electron number density) in the ceramic tube are numerically investigated utilizing the model. Moreover,

the thrust performance of the LA-PPT is also investigated by experimental methods.

As shown in Figure 2, when the laser beam irradiates the target (e.g., aluminum), the temperature in the target rises, and melting occurs when the surface temperature reaches the melting temperature Tm. Then a part of molten materials begins to vaporize when the surface temperature reaches the boiling temperature Tb. With sufficient laser fluence, the target temperature may approach the critical temperature Tc, where dielectric transition occurs, and the dielectric layer is formed near the surface. Furthermore, the ablation plasma expands in the opposite direction of the incident laser beam, and absorbs part of laser energy before the incident laser beam reaches the target surface. The absorption of the laser energy in the plasma accelerates the plasma expansion, at the same time, the shielding of the laser energy by the plasma

to obtain from experiments.

2. Physical model

2.1. Heat conduction model

significantly affect the heat conduction of the target.

Early in the year 2000, Horisawa et al. proposed a laser-assisted plasma thruster (LS-PPT), in which a laser-induced plasma was induced through laser beam irradiation onto a solid target and accelerated by electrical means [9–16]. Compared with the conventional PPT, the LS-PPT combines the laser ablation with electromagnetic acceleration means, which can significantly enhance the thrust performance. However, the phenomenon of "late ablation" is still inevitable in the LS-PPT, which significantly reduces the thrust efficiency of the thruster. In order to overcome the shortage of "late ablation," a novel laser ablation plasma thruster (LA-PPT) is proposed [17]. The LA-PPT separates the laser ablation from electromagnetic acceleration through a ceramic tube. As shown in Figure 1, the LA-PPT consists of a pair of rectangular electrodes, a ceramic tube and an insulator. The propellant is placed inside the ceramic tube. Because of the unique structure of this thruster, almost all types of solid matter can be applied as the propellant, such as metals, polymers and so on. Because laser-ablation plasma can has a directed initial velocity of tens of kilometers per second, which will be further accelerated by electrical means, a significant specific impulse can be expected [9]. Hence the LA-PPT is a promising candidate for small satellites propulsion, and the physical mechanisms of the thruster should be further investigated. The working process of the LA-PPT can be divided into two stages: laser-induced ablation and plasma-induced discharge. The ablation plasma expansion and ionization in the ceramic tube is the combination of the two stages, and it is crucial to understand the working process of the LA-PPT. However, the ablation plasma expansion and ionization is difficult to be experimentally investigated, especially when it occurs in a ceramic tube. Therefore, we utilize numerical method to investigate the ablation plasma expansion and ionization in the ceramic tube.

By using the numerical method to simulate the ablation plasma expansion and ionization in the ceramic tube, the relevant physics of propellant ablation needs to be implemented. The heating process within a propellant material during the irradiation of the laser pulse can be calculated by taking into account temperature-dependent material properties, melting, phase transition, dielectric transition, phase explosion, and the reflection of the laser beam at the surface of the propellant [18]. In addition, the ablation plasma absorbs part of laser

Figure 1. (a) Front and (b) right view of the laser ablation plasma thruster.

energy, and causes the density, temperature, pressure and components in the ablation plasma plume dramatically vary, which has an significant effect on the thrust performance. During the years, the expansion dynamics of pulsed laser generated plasma plume has been widely investigated through experimental and numerical methods [19–23]. Especially, several models have been proposed to describe the expansion of ablation plasma and the formation of plasma in the plume [21]. Aden et al. utilized hydrodynamic equations to describe the plasma expansion, which is called hydrodynamic model [24]. Afterwards, the hydrodynamic model had been widely applied to study various laser-solid interactions [25–28]. In order to calculate the laser energy loss caused by plasma absorption and shielding, the formation of plasma in the plume was also considered in several hydrodynamic models [22, 29–32]. Therefore, we establish a hydrodynamic model to investigate the ablation plasma expansion and ionization in the ceramic tube. The numerical model can give insight in the plasma dynamics and plasma behavior of LA-PPT, which is sometimes difficult to obtain from experiments.

In this paper, a novel laser ablation plasma thruster with a ceramic tube is developed and investigated. This thruster is expected to overcome the shortages of conventional laser-electric hybrid acceleration systems in the above paragraph. And the ionization rate, specific impulse and thrust efficiency of the thruster are expected to be increased. Specially, a numerical model for nanosecond laser ablation of aluminum propellant is presented, which contains the target ablation and plasma expansion in the ceramic tube. Heat conduction model is established to calculate the target ablation, taking into account temperature-dependent material properties, phase transition, dielectric transition and phase explosion. Meanwhile, Hydrodynamic model is established to calculate the plasma expansion, taking into account ionization, plasma absorption and shielding. Then both calculations of the target ablation and plasma expansion are coupled in each time step. Afterwards, the plasma properties (such as velocity, temperature and electron number density) in the ceramic tube are numerically investigated utilizing the model. Moreover, the thrust performance of the LA-PPT is also investigated by experimental methods.
