III' Applied Plasma Physics: theory, simulation, experiments

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Spacecraft Charging Hazard (II)

• The ISS has large surfaces (MMOD shields) covered
  by a thin (1.3 mm) anodized aluminum as a
  dielectric insulator
• Voltages as low as 70 V have been found to
  produce arcing on the dielectric coating
• Long-term exposure of the dielectric surface to
  the space environment can produce local damages
  (due to micro-meteorites or debris) of the
  dielectric and enable arcing at even lower
  voltages

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Spacecraft Plasma Hazard (III)

• EVA space suits have a safety threshold of 40 V
  (Marshall Space Flight Center test showed arcing
  through the suit at 68 V with new fabric)
• Beyond the 40 V value it is possible that a
  circuit close through the astronauts thorax
  cavity with a current in excess of 1 mA
• This current limit is generally accepted as
  safety threshold to prevent heart fibrillation.

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Spacecraft Plasma Hazard (IV)

• Potentially Lethal Hazard
• EVA Suit Specified to 40 V
• anodized coating arcing occurred at 68V in MSFC
  test
• Possible Sneak-Circuit
• 1 mA safety threshold

Display and Control Module (DCM)
Safety Tether
Plasma Arc through Ionosphere
Shunt path .5 to 2.5A
Crew member
ISS/MMOD 6000 ufd 35 to 120V 3.6 to 43.4 J
EMU Hardware
Body Restraint Tether (BRT)
Mini Work Station (MWS)
Tether MWS DCM etc
Imax CM lt 1mA NASA STD 3000
Primary path 100 to 500A
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Spacecraft Plasma Hazard (V)

• ISS Program Position Prior to Flight UF2
  charging is a potentially catastrophic hazard
• Requires two-fault tolerant control
• 2 PCUs (plasma contactors)
• Solar Arrays to wake or shunting at dawn
• One-fault tolerant EVAs has been occasionally
  allowed

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Spacecraft Plasma Hazard (VI)

• A series of measurements were performed with the
  ISS Floating Potential Probe between December
  2000 and April 2001 to identify the actual
  conditions of ISS charging and its relationship
  with the ionospheric plasma parameters
• The instrument ceased to respond after April
  2001, for unidentified causes, possibly related
  to its battery power supply system.
• The FPP data showed a much lower magnitude of the
  ISS floating potential than what was predicted
  based on first estimates
• The worst-case charging that was observed
  produced a negative potential of 26 V, much
  smaller than the 140 V negative predicted from
  pre-Flight 4A worst-case models

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ISS Floating Potential Probe
Spacecraft Plasma Hazard (VII)
FPP
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Spacecraft Plasma Hazard (VIII)

• Comparison between FPP and Ionosonde/IRI data

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Spacecraft Plasma Hazard (IX)

• The FPP data show that the ISS is charging at a
  much lower level than expected from its ability
  to collect electrons with the 140 bias of the on
  the solar arrays.
• Possible explanations for this discrepancy are
  the following
• - better modeling of the solar array cell
  collecting surface is required
• - a significant additional ion collection area
  (bare metal, grounded in the ram direction) is
  present on the ISS but has been unaccounted for
  (the ion collection offsets the effect of the
  solar array electron collection)

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Spacecraft Plasma Hazard (X)
FPP ne, Te
Ionospheric Variability Analysis
Boeing/SAIC Model
Discarded
FPP Potential
Data Fitting
Worst Case Potential
Ion Collection Area
Plasma Hazard Analysis Algorithm
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12.2.2 Plasma Contactors

• Plasma contactors are devices that allow to
  control the maximum floating potential of a
  spacecraft by providing a discharge path to the
  ionosphere for the excess electrons
• Essentially, the plasma contactor is a plasma
  source that establishes an electrically
  conducting path (the plasma) between the
  spacecraft ground and the ionosphere.
• The floating potential of the spacecraft is then
  clamped down to safe values (in the order of
  -10 V for the current ISS implementation)
• ISS plasma contactors are Xenon sources
  (hollow-cathode design, maximum current of 4 A,
  much larger than the present requirements)

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Plasma Contactor (II)

• In steady-state conditions a plasma sheath is
  formed between the contactor plasma and the
  spacecraft conducting surface
• For large values of the spacecraft floating
  potential the current in the sheath can be
  computed through the Child law and is independent
  on the spacecraft floating potential
• Corrections to the Child law can be introduced
  for collisional sheaths in this case there is a
  dependence of the current on the potential.
• For example a (ion) plasma current of about 12 A
  can be sustained in a Hydrogen plasma with
  density of 1018 and temperature of 1 eV with a
  plasma radius of 5 cm.

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Plasma Contactor (III)

• If transients occur (for example a sudden
  variation of the spacecraft potential at orbital
  sunrise) the sheath thickness adjust itself to
  new the value of the potential causing variations
  of the current that are also dependent on the
  potential. 
• If the plasma contactor is effectively lowering
  the floating potential to small values (compared
  to the ionospheric plasma temperature) the sheath
  becomes much smaller (few Debye lengths) and a
  calculation of the equilibrium conditions
  according to the Bohm sheath criterion should be
  performed.

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Plasma Contactor (IV)

• If a high-density plasma is produced near a
  conducting surface of a spacecraft in the Earth
  orbit an additional current path to the
  ionosphere will be established (in addition to
  the path represented by the interface between the
  ionospheric plasma and the spacecraft exposed
  conducting surfaces).
• On the ISS, the charging due to the solar panels
  produces an electron excess on the station
  structure and brings it to a potential energy
  that is significantly larger than the thermal
  energy of the ionospheric plasma.
• This is often expressed in less rigorous terms by
  saying that the floating potential is much
  higher than the plasma temperature.

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Plasma Contactor (V)
Plasma Source

• is current through the sheath supported by the
  ISS floating potential that discharges plasma
  electrons to the ionosphere

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Plasma Contactor (VI)

• In these conditions on the interface between the
  contactor plasma and the ISS conducting surface a
  plasma sheath is formed that can be described by
  the Child law
• The Child law essentially provides the current
  for a space-charge limited planar diode.
• Since the (negative) potential on the conducting
  surface is significantly higher than the plasma
  temperature, the sheath region is essentially
  depleted of electrons and filled only with ions.

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Plasma Contactor (VII)

• This situation can also be understood by
  considering a Boltzmann distribution for the
  electrons.
• For a large negative potential the electron
  density tends to zero, then the current flow is
  space-charge limited (as opposed to be partially
  neutralized).

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Plasma Contactor (VIII)

• If V is the potential across the sheath (assumed
  equal to the floating potential) the Child law
  for a sheath of thickness s and ions of mass M,
  gives the current density through the sheath as

where the temperature is expressed in eV and
the thickness of the sheath s is given by
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Plasma Contactor (IX)

• By substituting the expression of s the (ion)
  current density through the sheath can be written
  in a more familiar way as an ion saturation
  current (temperature still in eV)

• For a Hydrogen plasma with n1018, Te1 eV the
  current density is 1.5?103A/m2 and for a plasma
  radius of 5 cm the total current across the
  plasma contactor is 12 A.