**3. Hall thruster operation**

Hall effect thrusters (HETs) were originally developed in United States and Russia about 60 years ago, and the first working devices were reported in U.S. in the early 1960s. Now a days, most of the countries using the Hall thruster technology in their space mission. Unlike chemicals and electric rockets, the propulsive thrust in a Hall thruster is achieved by an ionized inert gas (Xenon) which has high atomic number and low ionization potential. For this Xenon is mostly used. In a Hall thruster, the propellant is ionized and then accelerated by electrostatic forces.

**Figure 1** shows the internal parts of a plasma Hall thruster. Generally, the discharge channel is cylindrical shape made up with metallic material. The magnetic field of the order of 150 Gauss is applied to produce closed drift of electrons inside the channel. The applied magnetic field which is strong enough so that the electrons get magnetized, i.e. they are able to gyrate within the discharge channel, but the ions remain unaffected due to their Larmor radius much larger than the dimension

of the thruster. Thus the electrons remain effectively trapped in azimuthally *E* ! � *B* ! drifts around the annular channel and slowly diffuse towards the anode. This azimuthal drift current of the electrons is referred to as the Hall current. The propellant enters from the left side of the channel via anode and gets ionized through hollow cathode of the device. The electric field of strength �1000 V/m gets

**Figure 1.** *Schematic diagram of a typical Hall plasma thruster.*

generated inside the discharge channel along the axial direction of the device [3]. In addition, these kind of devices have implication in partially ionized plasmas (tokamaks), in ionosphere (base of the solar photosphere), in protoplanetary discs, circum nuclear discs in active galactic nuclei and neutron stars. Hall thruster has high thrust resolution, it is being used for the adjustment of the location of the satellite onboard.

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

Thruster with anode layer also developed in Russia has a narrow acceleration zone associated with the narrow electric field region near the anode. This geometry considerably shortens the electric field region in the channel, where the ion acceleration occurs. However, this configuration does not change the basic ion generation or acceleration method. The channel wall made up of conductor, which is usually also a part of the magnetic circuit, is biased negatively (usually cathode potential) to repel electrons in the ionization region and to reduce electron-power losses. This reduces the loss caused by the ion and electron collisions with the walls. Since the walls are conductive, a constant potential (same as that of the cathode) is observed along the entire wall. Very high electron temperatures, i.e. more than

**6. Review of status of current research and development in the subject**

The range of the oscillations lies from few kHz to MHz in the acceleration channel of the thrusters and has been given in **Table 2**. Rayleigh-Taylor (RT) instability takes place when a lighter fluid supports a heavy fluid. The plasma in the Hall thruster possesses Rayleigh-Taylor instability, resistive instability, transit time instability, electromagnetic instability and sheath instabilities [5–11]. These systems are rampant with plasma instabilities and fluctuations, many of which are responsible for performance, driving electron transport across magnetic field lines and contributing to propellant ionization. Over the last decade several studies have been carried out with HET to characterize the low frequency azimuthal and axial oscillations and optimizing magnetic field profile for a wide range of operating conditions for better efficiency and performance. Singh and Malik [10, 11], investigated that temperature of the ion and drift velocity profiles of the electron modifies the conditions for Rayleigh type instability under the effects of thermal motions of ions. The plasma resistivity induces resistive instabilities (electrostatic and electromagnetic) [6–9] associated with azimuthal and axial directions. High-frequency (1–10 MHz) instabilities have been studied in the Hall-effect thruster [6–9], where it was found that these instabilities have the highest level near the thruster exit plane. These oscillations in the Hall thruster determine the efficiency of the system and may affect the divergence of the ion beam and electron transport across the

**Range (kHz) Type Driving mechanism**

5–25 Rotating spokes Ionization process

*Range and classification of oscillations in a Hall Thrusters.*

10–20 Loop or circuit oscillations Magnetic field, discharge voltage and electron

20–60 Azimuthal modes or drift instability Gradient of density and magnetic field 70–500 Transient time oscillations Plasma density gradient and low ionization

0.5–5 MHz Azimuthal waves Drift velocity of plasma species

wall collision frequency

length.

**Table 2.**

**27**

**5.2 Thruster with anode layer**

*Hall Thruster: An Electric Propulsion through Plasmas DOI: http://dx.doi.org/10.5772/intechopen.91622*

50 eV, are typically observed in such thrusters [1].
