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.

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