**4. Spacecraft issues**

The first issue is that the divergence angle of these devices is about 60°, which relatively large and cause problem related to erosion of the channel walls and outer surfaces of the thruster. The erosion of the walls decreases the lifetime of the device. This channel usually has a length of the order of centimeters. In addition, densities in the channel are typically in the range between 10<sup>17</sup> and 10<sup>18</sup> m<sup>3</sup> for the plasma, and 1018 and 10<sup>20</sup> m<sup>3</sup> for the neutral gas [4]. The plasma in a Hall thruster does not stay uniform and an inhomogeneous plasma immersed in the external electric and magnetic fields is not in the thermodynamically equilibrium state, this deviation in general is a source of plasma instabilities. The amplitudes of the waves and instabilities are attributed by the density scale lengths of plasma and magnetic field and other parameters. These waves/oscillation and instabilities may affect the efficiency of the device, hence forth research on studies on oscillation/instabilities always attracted the investigators.

## **5. Types of Hall plasma thruster**

Two types of Hall thrusters have been developed: a thruster with closed electron drift and extended acceleration zone or stationary plasma thruster and a thruster with a very short acceleration channel or thruster with anode layer.

In **Table 1**, typical values of some of the pertinent properties are listed at the thruster exit for the SPT-100.

#### **5.1 Dielectric wall thruster or stationary plasma thruster**

Such thrusters have a wall made up of dielectric of boron nitride or silicon carbide and extended channel compared to its width. The role of the wall is that the collisions of the electrons and ions with the wall generate low energy secondary electrons. These secondary electrons keep tending the electron temperature low in the discharge plasma. By reducing the discharge electron energy, a smooth and continuous variation in plasma potential between the anode and the cathode is


**Table 1.**

*Typical plasma parameters for Hall Thrusters.*

obtained. Since the dielectric walls are not conductive, charge builds up along the length of the acceleration channel that leads to a variable potential profile along its length.
