The Jet Propulsion Laboratory (JPL) Space Science and Technology Series expands the range of the ongoing JPL Deep Space Communications and Navigation Series to include disciplines other than communications and navigation in which JPL has made important contributions. We would like to thank the Chief Scientist and Chief Technologist Office for their encouragement and support.
Ion Thruster Geometry
The plasma generator has a high positive voltage compared to the spacecraft or space plasma and is therefore enclosed in a "plasma shield". The performance of the thruster depends on the plasma generator efficiency and the ion accelerator design.
Hall Thruster Geometry
Photograph of the NEXlS ion thruster [27] showing the 57-cm-diameter multiaperture gratings and plasma screen surrounding the thruster body. A portion of the electrons emitted from the hollow cathode also leave the thruster with the ion beam to neutralize the outgoing charge.
![Fig. 1-2. Photograph of the NEXlS ion thruster [27] showing the 57-cm-diameter multiaperture grids and plasma screen enclosing the thruster body](https://thumb-ap.123doks.com/thumbv2/123dok/10290635.0/19.648.143.508.115.430/photograph-nexls-thruster-showing-diameter-multiaperture-enclosing-thruster.webp)
Kawaguchi, "Asteroid Rendezvous of Hayabusa Explorer Using Microwave Discharge Ion Engines," 29. International Electric Propulsion Conference, IEPC Princeton, New Jersey, 3. oktober 1-4. november 2005. Hart, "L-3 Communications ETI Electric Propulsion Overview," 29. International Electric Propulsion Conference, IEPC Princeton, New Jersey, 3. oktober - 4. november 2005.
Thruster Principles
- The Rocket Equation
- Force Transfer in Ion and Hall Thrusters
- Thrust
- Specific Impulse
- Thruster Efficiency
- Power Dissipation
- Neutral Densities and Ingestion in Electric Thrusters
The total mass of the spacecraft at any time is the given mass, md , plus the mass of the thruster:. The thrust is proportional to the beam current to the square root of the acceleration voltage.
Problems
Chapter 3
Basic Plasma Physics
Introduction
Maxwell’s Equations
However, by convention, we will occasionally return to other metric units (such as Ncm2, mg/s, etc.) commonly used in the literature describing these devices. E = - V where the negative sign comes from the convention that the electric field always points in the direction of ion motion.
Single Particle Motions
3.3 - 13) with 1/2 mv, 2 = eVI for the energy of the singly charged particles in the direction perpendicular to the magnetic field. This lengthens the trajectory by half a cycle and shrinks the trajectory by the opposite half cycle, causing the net motion of the particle in the E × B direction.
Particle Energies and Velocities
-8) The flux of particles in one dimension (say in the .? direction) for a Maxwellian distribution of particle velocities is given by n < vz >. In this case, the average of the particle velocities in the positive vz direction is taken because the flux is only considered in one direction.
- Momentum Conservation
- Particle Conservation
- Energy Conservation
By using momentum conservation, it is possible to evaluate how the electron fluid behaves in the plasma. -6) where vz is the electron velocity in the z direction and p represents the term electron pressure.
Diffusion in Partially Ionized Gases
- Collisions
- Diffusion and Mobility Without a Magnetic Field
- Diffusion Across Magnetic Fields
- Classical Diffusion of Particles Across B Fields. The fluid equation of motion for isothermal electrons moving in the perpendicular
- Ambipolar Diffusion Across B Fields. Ambipolar diffusion acros,s magnetic fields is much more complicated than the diffusion cases just
-9) where v is the neutral particle velocity and the reaction rate coefficient in the denominator is the average over all the relevant collision cross sections. In ion and Hall thrusters, the ion current in the plasma is typically much smaller than the electron current due to the large mass ratio, so the ion current term in Ohm's law, Eq. -47) is the product of the zero-order Bessel function of the first kind times an exponential term in the axial direction:.
Also assume that the maximum electric field occurring in the plasma due to Debye shielding is proportional to the electron temperature divided by the plasma radius:. Solving for vy and eliminating the E x B terms and the diagmagnetic displacement in the x direction, the transverse velocity of the electrons is given by.
Sheaths at the Boundaries of Plasmas
- Debye Sheaths
- Pre-Sheaths
- Child-Langmuir Sheaths
- Double Sheaths
- Summary of Sheath Effects
In the previous section, the sheath characteristics were analyzed for the case where the potential difference between the plasma and an electrode or boundary is small compared to the electron temperature (e@ << kT, ), resulting in Debye shielding sheaths. Conservation of energy means that the ions arrived at the edge of the sheath with an energy given by . This potential drop between the center of the plasma and the edge of the mantle, @, is called the pre-mantle potential.
Therefore, the plasma density at the edge of the sheath is about 60% of the plasma density at the center of the plasma. If one boundary of the bilayer is the mantle edge at a thermionic cathode, Eq. 3.7.
Ion Thruster Plasma Generators
Introduction
Proper design of the magnetic field is critical for providing adequate confinement for high efficiency while maintaining sufficient electron loss to the anode to produce stable discharges over the thruster operating range. Electrical schematic of a DC discharge ion thruster with the cathode heater, keeper and discharge power sources. RF and microwave ion thrusters use ion accelerator and electron neutralizer implementations almost identical to those of the DC discharge ion thruster.
These thrusters also use applied or self-generated magnetic fields to improve the ionization efficiency of the system. The principles of these different classes of plasma generators are described in the following paragraphs after a discussion of the plasma generator efficiency that can be expected in an idealized case.
Idealized Ion Thruster Plasma Generator
To first order, the power injected into the plasma goes to ionization and excitation of the neutral gas, heating of the electrons, and current carried to the walls and grids of the ions and electrons. 4.2 -4) where U+ is the ionization potential of the propellant gas, U* is the excitation potential of the gas, z is the average electron confinement time, &i is the ion energy carried to the walls, and E is the electron energy carried to the walls by the electrons leaving the plasma. The discharge loss is defined as the power into the plasma divided by the beam current out of the thruster, which is a value for the efficiency of the plasma generation mechanism.
-6 and 4-7, where the energy loss rates for ionization and excitation have not changed with the best electron. closure, but the energy convected from the plasma in the form of ion and electron power at the boundaries is reduced. In reality, the discharge loss is significantly higher than that found in this idealized example due to the imperfect confinement of ions and electrons in the propellant and due to other loss mechanisms that will be described below.
DC Discharge Ion Thruster
- Magnetic Multipole Boundaries
- Electron Confinement
- Ion Confinement at the Anode Wall
- Ion and Excited Neutral Production
- Power and Energy Balance in the Discharge Chamber
- Discharge Loss
- Discharge Stability
- Recycling Behavior
- Limitations of a 0-D Model
Some of the primary electrons are lost directly to the anode at the magnetic tip (ZL). Ions produced in the discharge chamber can flow back to the hollow cathode, to the anode wall, or to the plane of the accelerator. 4.3 -9) where ni is the ion density at the center of the discharge and A is the total ion loss area.
Ions in the discharge chamber are produced both by the primary electrons and by the tail of the Maxwellian distribution of the plasma electrons. The electron temperature in the discharge chamber can be found using particle balance of the ions. Example of the discharge loss versus mass utilization efficiency for three discharge voltages in the NEXlS thruster [38].
Having a representative exhaust model allows environmental changes in the propellant to be understood as well.
Kaufman Ion Thrusters
-61) as the plasma electron current minus the ion current and plus the primary current, the discharge current is now correct. Equating the current in to the current out and solving for the discharge loss gives. Assuming that the average magnetic field strength in the thruster is about 50 G, the discharge loss of Eq.
In addition, the lower discharge voltage causes the plasma potential to become significantly negative relative to the anode potential ( = T, ), which will cause the discharge to become unstable. If the field is too strong or the anode area in contact with the plasma is too small, the plasma potential becomes negative relative to the anode to pull the electrons from the discharge.
Once the plasma source is turned on, the necessary electrons collide to provide the rf heating. If the antenna in rf thrusters is directly exposed to the plasma, ions in the discharge can be accelerated by the rf voltage at the surface and corrode the antenna. In the energy balance equation, it is assumed that the power absorbed by the plasma is simply given by Pabs.
The discharge loss performance of rf ion thrusters commonly reported in the literature [50] is much lower than that found in our example. Ion confinement factor (fraction of Bohm current at the wall) as a function of the magnetic field induced in the discharge chamber volume. Discharge loss of the rf ion thruster versus mass utilization efficiency for three values of induced magnetic field in the discharge chamber.
Microwave Ion Thrusters
Microwave energy is coupled to the plasma by electron cyclotron resonance heating, where the microwave frequency corresponds to the cyclic frequency of the electrons in the magnetic field. The use of microwave radiation allows direct heating of plasma electrons, but for the wave to add energy to the electrons, collisions must occur. Due to the difficulty of generating these high magnetic fields throughout the volume of the discharge chamber, the resonant region is often localized to a small area within the thruster volume and the plasma is allowed.
Coupling of the plasma from the resonance region or surface magnetic layer to the thruster volume is problematic due to the reduced transverse transport in the field. The propellant volume is also minimized, with the plasma production region close to the grids.
- Neutral Atom Model
- Primary Electron Motion and Ionization Model
- Discharge Chamber Model Results
- Chapter 5
The ion diffusion model uses the magnetic field information and plasma properties to determine the motion of the plasma. The spatial distribution of the neutral density is dependent on the gas injection regions (sources), gas reflux from the walls, loss. An example of the axisymmetric boundary ("wall") and internal mesh for the NSTAR ion thruster of Wirz and Katz [58] is shown in Fig.
Gas enters from the hollow cathode in the rear center and the propellant injection manifold in the front of the exhaust chamber. The neutral gas is lowest on-axis near the grids due to the NSTAR supply adjustment; however, as discussed below, the high primary electron density found in this propellant region produces significant ionization and "excites" the neutral gas.
Ion Thruster Accelerator Grids
![Fig. 4-11. Comparison of measured (dashed) and calculated (solid) magnetic field contours in the six-ring NEXIS thruster [20]](https://thumb-ap.123doks.com/thumbv2/123dok/10290635.0/118.648.121.526.558.834/comparison-measured-dashed-calculated-magnetic-contours-nexis-thruster.webp)
![Fig. 4-15. Ionization and excitation cross sections for xenon [28,29].](https://thumb-ap.123doks.com/thumbv2/123dok/10290635.0/128.648.126.527.594.862/fig-ionization-excitation-cross-sections-xenon.webp)
