S3-4
Plasma-materials interactions in spacecraft propulsion
宇宙機用プラズマ推進機におけるプラズマ-壁相互作用
Naoji Yamamoto
山本直嗣
Kyushu University
6-1, Kasugakoen, Kasuga,Fukuoka, 816-8580, Japan
九州大学 〒816－8580 福岡県春日市春日公園6-1
To understand interactions between plasma and the wall is important for space application, especiallyadvanced propulsion system, electric propulsions. For example, the lifetime of the ion engines and Hall
thrusters is limited by the erosion of the wall by the ion sputtering. The thrust performance in Hall thruster
depends on the wall materials, that is, the plasma parameters were changed by the wall characteristics.

1. Introduction
This paper shows examples of thePlasma-materials interactions in spacecraft
application. We focus on the topics in thermal
protection system and electric propulsion,
especially Hall thrusters and ion thrusters.
2. Thermal Protection System
When the space vehicle enter the earth from thespace, the shock layer appears and rare atmospherein the shock layer is dissociated and ionized and be
weakly ionized plasma. From the shock layer, hugeheat flux enter the space vehicle as convective,
catalytic and radiative heat flux. The thermalprotection system (TPS) is crucial technique atre-entry to the earth against severe aerodynamic
heating [1-3]. For example, the re-entry speed ofHAYABUSA mission is about 12 km/s, theestimated heat flux was 15 MW/m2 for 30 seconds.
For the heat protection, several materials,
ceramic tiles, C/C (Carbon/Carbon composite)materials CFRP ablative materials, etc., were used.
C/C materials can keep sufficient stiffness at hightemperature, therefore, they apply structural parts.
Ceramic tiles have features of low density and low
thermal conductivity, they were adopted as athermal insulator.
Ablative materials are used as a functionalmaterial. The principle is as follows [4]. If thetemperature of the ablator increase due to theaerodynamic heating, pyrolysis reaction occurs in
the resin of the ablator, a pyrolysis gas is generated
and the resin is carbonized. The carbonized resin
forms char layer and the gas goes through the layer.
The pyrolysis gas brow to the hot shock layer gas
and reduced the heat flux from the shock layer as
convective heat flux. In addition the pyrolysis gas
absorb heat from the char having passing throughthe char and cool down the vehicle. At Hayabusa

mission, they used Tilt-layered and Lattice Layered
carbon phenolic was used for the protection of thecapsule.
3. Hall Thruster
Hall thrusters [5] are a class of electric
propulsion device in which a propellant gas isionized and accelerated to produce thrust. Hallthrusters show considerable promise for satellite
station keeping and orbit transfer applications[6,7]
since they offer an attractive combination of highthrust efficiency, exceeding 50%, with a specificimpulse range of 1,000-3,000 s and a higher ion
beam density than ion thrusters, due to the existenceof electrons in the ion acceleration zone. This isbecause a moderate magnetic field is applied in theacceleration zone, causing the magnetization of theelectrons and not the ions [8]. Several types of Hallthrusters are available, but they can be categorized
into two general groups; the magnetic layer type
and the anode layer type[9,10]. One example ofthe former type is the Russian “Stationary PlasmaThruster” (SPT). It has a ceramic wall and itsacceleration channel length is longer than its
channel width.
In the Hall thruster, the interactions between
plasma and the acceleration channel wall wereobserved. One of the interesting interaction isconcerned with the secondary electron emission
effect of dielectric wall.
The thrust performance of the thruster depends
on the wall material [11], and this dependency is
due to the difference in the secondary electronemission coefficient. The increase in secondary
electron emitted from the wall reduce the potential
drops on the wall and the temperature in the thruster.
The SEE coefficient has positive interaction withelectron temperature[12], therefore, the wall keepthe adequate electron temperature in the Hall
thruster and the moderate potential drop on the walland this contributes the adequate ion acceleration,that is high efficiency and long lifetime of the Hallthruster. Indeed, the thrust performance using low
SEE coefficient ceramic wall, e. g., BN/ ALN isworse than that using high SEE coefficient BN.
This is one of the example of self-functional
appearance by the plasma-materials interactions ina spacecraft propulsion.
It is interesting that the secondary electronemission leads to anomalous electron diffusion
across the magnetic field, so-called near wall
conductivity, and degrade the thrust performance,
so the adequate SEE material, (BN/SiO2 or BNceramic) is used as a wall material in Hallthrusters.

Fig.1. Cross-sectional view of a Hall thruster

4. Ion Engine
An ion engine works by pushing ions away fromthe spacecraft [13]. Ions in the discharge chamber
plasma accelerates to 30,000-50,000 m/s(normalion engine) by the strong electric field Ion thrusters
show high energy transfer efficiency, exceeding70%, with good specific propellant consumption.
Therefore, ion thrusters have already been used
extensively in space missions, such as Deep Space
1[14], HAYABUSA[15], and other missions[16].
A Neutralizer is one of the key component of ion
engine, it emits electrons for neutralization.
Lifetime limiter of the ion engine is lifetime ofneutralizer, and the sputtered wall material
deposition on the insulator would degrade theneutralizer performance. The reason why
molybdenum wall was sputtered by the ions, though
the potential drop on the wall is almost 20-30 V,had been unknown. However, several study [17,18]

reveal that the double ionized ions would play animportant role. And in Hayabusa 2, there are
improved neutralizers and they will show the goodperformance.
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