L4 Charging

ELEC3225 - SPACECRAFT SYSTEMS ENGINEERING - SPACE ENVIRONMENT

• Course: ELEC3225 Spacecraft Systems Engineering

• Topic: Space Environment and Spacecraft Charging

• Professor: S.B. Gabriel

• Institution: University of Southampton

• Date: October 2021

Topics Covered

• Basics of Space Environment

  • Understanding the differences between surface charging and internal charging.

  • Overview of design practices related to charging.

  • Examples of charging effects.

Natural Environments Contributing to Charging

• Surface:

  • Thermal Plasma: Ionized particles influencing surface charging.

  • High Energy Electrons: Ranges between 1-100 keV impacting the spacecraft.

  • UV/EUV Radiation: Ultra-Violet and Extreme Ultra-Violet photons affecting charging properties.

  • Magnetic Field: Interaction with the spacecraft's materials can alter charge distributions.

  • Neutral Particles: Contributes to the atmospheric interactions that can result in charging.

• Internal:

  • High Energy Electrons: Energies greater than 100 keV can cause internal charging issues.

Role of Environments in Charging (1)

• Surface Charging Mechanisms:

  • UV/EUV Photons: Lead to the emission of photoelectrons, affecting charge on surfaces.

    • High energy photoelectrons can accumulate charge.

    • Thermal plasma electrons contribute to surface charge but can help reduce accumulation through secondary electron ejection.

  • Magnetic Fields: Can inhibit the escape of secondary electrons, leading to altered properties.

  • Contaminants: Change surface charging properties by affecting secondary emission coefficients.

Role of Environments in Charging (2)

• Internal Charging Mechanisms:

  • Trapped charge can occur due to high energy electrons,

  • Effects of ungrounded cables or isolated conductors create electric fields leading to breakdown.

  • Cable Insulation: Lack of proper shielding can exacerbate this phenomenon.

Theory of Spacecraft Charging

• Charging Equation:

  • JE - JI - JPH = 0

    • Where:

      • JE: Electron current density

      • JI: Ion current density

      • JPH: Photoelectron current density

• Interpretation of the equation reveals balance conditions for current within the spacecraft system under sunlight exposure.

Calculation of Surface Potential

• Potential Equation: CAdV/dt = J(V) + σV = 0 where ΣI = 0

  • Notable Challenges: Capacitance (CA) and conductivity (σ) are difficult to determine accurately.

  • Current density (J) also presents complications due to its non-linear and non-local nature.

• Surface Potential Calculation Components:

  • J = - Je + Jb + Jse + Jsi + Jp ± Jc + Ji

    • JE: incident electron current density

    • JB: back-scattered electron current density

    • JSE: secondary electron current due to JE

    • JSI: secondary electron current due to Ji

    • JP: photo-electron current

    • JC: conduction current

    • Ji: incident ion current density

  • In eclipse, V ~ - Te for high temperatures (Te > ~ 1000eV).

Internal Charging in the Outer Zone

• Key Points:

  • High fluence of 0.5 – 5.0 MeV electrons (> 1011 MeV cm-2) leads to significant internal charging.

  • These energies can penetrate thin shielding materials (<1.5 mm Al equivalent).

  • Thin insulating dielectrics with low conductivity can lead to electrostatic discharge paths.

  • Connection between internal charging and sensitive circuits.

Spacecraft Charging Effects

• Impacts on Spacecraft Operations:

  • Distortion of plasma measurements.

  • Enhanced risk of contamination.

  • Potential for arcing, resulting in circuit failures.

  • Loss of power and logic upsets.

Observations of Spacecraft Charging Anomalies

• **Recent Studies: **

  • Comprehensive analyses of ESD-related anomalies reviewed various notable spacecraft incidents (e.g., TELSTAR-401, INTELSAT-511).

  • Highlighting that many failures remain unexplained and require better monitoring and reporting.

Design Guidelines for Spacecraft Charging Effects

1. Grounding:

   • Ensure all conducting elements are connected to a common ground via charge bleed-off resistors.

2. Exterior Surface Materials:

   • Employ at least partially conductive materials for exterior surfaces to control differential charging.

3. Shielding:

   • Design spacecraft to have continuous shielded structures around electronics (Faraday cage principles).

4. Filtering:

   • Implement electrical filters to protect circuits from discharge-related upsets.

5. Procedures:

   • Establish rigorous procedures to maintain electrical continuity in the vehicle grounding system.

Susceptibility of EVA Equipment

• Challenging Environments:

  • ESD risks identified in polar-aurora orbiting, influencing space suit charging conditions.

  • Testing environment outlined with electron beams assessing arc discharge risks.