Instrument Overview
  ===================
    The Galileo Probe Mass spectrometer (GPMS) is an instrument
    designed to measure the chemical and isotopic composition in the
    atmosphere of Jupiter, including the vertical variations of the
    constituents.  The measurements were performed by in-situ
    sampling of the ambient atmosphere in the pressure range from
    approximately 0.5 to 21 bars.  In addition, batch sampling was
    performed for noble gas composition measurement and isotopic
    ratio determination, and for sensitivity enhancement of
    non-reactive trace gases.
 
    The instrument consists of a gas sampling system that is
    connected to a quadrupole mass analyzer for molecular weight
    analysis.  In addition two sample enrichment cells and one noble
    gas analysis cell are part of the sampling system.  The mass
    range of the quadrupole analyzer is from 2 amu to 150 amu.  The
    maximum dynamic range is 1.E8.  The detector threshold ranges
    from 10 ppmv for H2O to 1 ppbv for Kr and Xe.  It is dependent on
    the instrument background and ambient gas composition because of
    spectral interference.  The threshold values are lowered through
    sample enrichment by a factor of 100 to 500 for stable
    hydrocarbons and by a factor of 10 for noble gases.  The gas
    sampling system and mass analyzer were sealed and evacuated until
    the measurement sequence was initiated when the Probe entered the
    upper atmosphere of Jupiter.  The instrument weights 13.2 kg and
    the average power consumption is 13 W.  Since the probe is not
    pressurized the instrument is enclosed by a pressurized housing
    made of titanium to save weight.  Pressurization (~1 atmosphere
    of N2) prevents corona, condensation, and collapse of the
    electronics package during the descent.
 
    The instrument followed a pre-programmed sampling sequence of
    8192 steps with a sampling rate of two steps per second.
 
    Instrument Id                      : GPMS
    Instrument Host Id                 : GP
    Pi PDS User Id                     : HBNIEMANN
    Instrument Name                    : GALILEO PROBE MASS SPECTROMETER
    Instrument Type                    : MASS SPECTROMETER
    Instrument Manufacturer Name       : GODDARD SPACE FLIGHT CENTER
    Build Date                         : 01-JAN-1985
 
    Mass                               : 13.2 kg (29.11 lbs)
    Outline shape                      : cylinder
    Length                             : 45.2 cm
    Diameter                           : 18.5 cm
    Instrument power                   : 13 W
    Pumps and heater power             : 12 W
    Housing                            : pressurized
    Housing pressure                   : 1 atm. N2
    Data rate                          : 32 bps
    Ambient pressure range             : 0.1 to 20 bar nominal
    Operating temperature range        : -20C to +50C
    Sample inlet viewing               : stagnation point
    Sample outlet viewing              : minimum pressure point
    Sample inlet/outlet deployment     : metal ceramic breakoff cap
    mechanism                            pyrotechnically activated
    Inlet system direct leaks          : 2 glass capillary arrays
    Inlet system noble gas analysis    : 1 scrubber
    Inlet system enrichment cells      : 2
    Inlet system valves                : solenoid operated
    Sensor pumps                       : non-evaporable getter
                                         sputter ion pump
    Ionization type                    : electron impact
    Electron ionization energy         : 75 eV, 25 eV, 15 eV
    Number of filaments                : 2
    Ion source pressure range          : 1.E-13 to 1.E-4 mb
    Mass analyzer                      : quadrupole
    Mass range                         : 1 to 150 amu
    Mass analyzer resolution           : unit mass, flat topped peaks
                                         1.E-8 nominal crosstalk for
                                         adjacent masses 1-60 amu more
                                         crosstalk for higher masses
                                         and ions with non-thermal
                                         energies
    Ion detector                       : secondary electron multiplier
                                         pulsecounter
    Dynamic range                      : 1.E8
    Sample scan format                 : 1, 0.5, 0.125 amu per 0.5 sec
                                         step
    Electronics                        : Read Only Memory (ROM)
                                         controlled descent sequence
                                         8192 steps (16 bits/step)
                                         ground command override
                                         capability during checkout
                                         and cruise
 
    For more information on the GPMS instrument see [NIEMANNETAL1992].
 
 
  Principal Investigator
  ======================
    The Principal Investigator for the GPMS instrument was Hasso B.
    Niemann, NASA Goddard Space Flight Center.
 
 
  Scientific Objectives
  =====================
    The goal of the GPMS was the in-situ measurement of the chemical
    composition of Jupiter's upper atmosphere down in altitude to
    ambient pressures of 20 bar or more.  The instrument was designed
    to detect gases but any solid or liquid that can be vaporized
    will also be detected.  With in-situ measurements and sample
    enrichment, the GPMS provides the measurements necessary to
    address very fundamental questions of the origin and evolution of
    the Jovian atmosphere.  It does this by measuring the abundance
    and isotope ratios of 'major' (i.e.  mole fractions > 1.E-07)
    constituents as a function of altitude with high accuracy.
    Repeated species sampling during the descent permits the
    determination of the altitude dependence of constituent abundance
    and allows correlation with cloud layers, and the regions of
    thermodynamic and photochemical activity.  The best precision is
    achieved from the most abundant gases.  To search for trace
    constituents at concentrations on the order of 1 ppbv, a sample
    enrichment system has been added to the basic mass spectrometer,
    and for noble gases a purification cell is included.
 
    A mass spectrometer was chosen because of its impartiality.
    Within its mass and sensitivity range it detects everything
    admitted to it, and it is therefore ideal for an exploratory
    mission like the Galileo Probe to Jupiter.
 
 
  Calibration
  ===========
    The GPMS was calibrated before flight as described in
    [NIEMANNETAL1992].  The procedure uses a high-pressure flow
    system to provide gas mixtures to the inlet system entrance to
    simulate flight conditions.  Temperature, pressure and gas
    composition can be varied.
 
 
  Operational Considerations
  ==========================
    In order for the electronics to provide the proper voltages for
    successful operation the temperature must be within the operating
    limits.
 
 
  Electronics
  ===========
    Commands and timing events are accepted and processed by the
    logic system.  The primary measurement is stored in an output
    register for interrogation by the spacecraft telemetry system.
 
    The instrument is under control of the programmer that is an
    array of Read-Only-Memory (ROM) devices.  The programmer has an
    8192-word, 16-bit look-up table and an output register to hold
    the 16-bit word for the current data sample.  Each one half
    second the ROM address is incremented and the instrument is
    configured for the next measurement.  During each of the 8192 one
    half second intervals, six instrument variables e.g.  mass
    number, ionization energy, inlet system configuration, etc.  can
    be configured to any allowable state.  The application of power
    causes the instrument to begin executing a programmed 256 step
    test sequence.  The instrument remains locked in this test mode
    until the SEQUENCE START command is received from the probe.
 
    Mass number selection in a quadrupole is a function of amplitude
    and frequency of the RF (Radio Frequency) signal applied to the
    rods.  Two frequencies are used to cover the mass range from 2 to
    150 amu.  In each frequency range, the mass number is
    proportional to RF amplitude.  The actual mass range is slightly
    larger than this to allow scanning past the center of the peaks
    in order to verify tuning and resolution.  A constant resolution
    over the mass range from 2 to 150 amu is maintained by proper
    choice of the DC and RF voltages.
 
    The electron-beam ion source requires an electrode supply of
    well-regulated voltages and a feedback controlled emission
    regulator.  Three different ionization energies are programmer
    selected and are accomplished by changing the appropriate ion
    source potentials.  The ion source is provided with two redundant
    filaments, powered by redundant emission regulators.  This
    implementation simplifies the design and increases reliability.
 
    A high voltage supply of nominal 3 KeV operates the
    detector-multiplier.  Command capability to optimize the
    secondary electron multiplier gain through the selection of one
    of four values is provided.  The ion arrival rate of the detector
    during each one half second of the descent constitutes the
    primary measurement.  Pulse counting ranges from a rate of about
    3E7 counts/sec down to rates as low as 0 or 1 count per half
    second integration.  At ion arrival rates exceeding the upper
    count limit the instrument will be desensitized automatically.  A
    logarithmic (base 2) compressor is used with a 9-bit mantissa and
    4-bit exponent.  This provides a full scale of 3.3E7 per range
    and a resolution of one part in 512.
 
    The electronics system was constructed of multi-layer printed
    circuit board technology.  Weight and size constraints for the
    Probe required that approximately 90% of the electronics
    components be packaged in the form of multi-layer hybrid
    circuits.  To meet structural requirements, the circuit boards
    were mounted on a cross-web structure enclosing the quadrupole
    analyzer and ion detector section.
 
 
  Location
  ========
    The GPMS is centered in the Probe and the two gas inlets placed
    near the apex of the Probe exterior shell.  The corresponding
    exit ports are placed at a minimum pressure point inside the
    Probe.
 
 
  Operational Modes
  =================
    A detailed description of the GPMS pre-programmed data sampling
    and processing has been described in [NIEMANNETAL1992].  Each
    step is 0.5 seconds in length.  The full 8192 step sequence is
    shown below.  Step 0 corresponds to GPMS internal time of 0
    seconds.
 
      Event     Step #    Beginning   Length     Comments
    Sequence    Range     Time(sec)    (sec)
    ========   ========   =========   ======  ========================
       A            N/A         N/A      N/A  power on initiates 256
                                              step test cycle
       B              0         0.0      N/A  receive Start of Sequence
                                              command from Probe
       B           0-81         0.0     40.5  background & tuning,
                                              instrument sealed
       B1           0-7         0.0      3.5  tuning check (high
                                              resolution scan mass
                                              16 amu)
       C        90-1809        45.0    859.5  Direct Leak 1 open
       C1      329-1330       164.5    500.5  fill Enrichment Cell 1
       D      1814-2159       907.0    172.5  Direct Leak 1 closed,
                                              measure gas background
                                              while pumping down
       D1     2128-2159      1064.0     15.5  tuning check (high
                                              resolution scan mass
                                              28,44,16,4 amu)
       E      2164-2450      1082.0    143.0  analyze contents of rare
                                              gas cell, reactive gases
                                              removed by getter
       F      2465-3095      1232.5    315.0  analyze contents of
                                              Enrichment Cell 1; cell
                                              heated to release
                                              ad/absorbed contents
       G      3100-3480      1550.0    190.0  close off Enrichment Cell
                                              1, measure gas background
                                              while pumping down
       H      3492-8192      1746.0   2350.0  Direct Leak 2 open
       H1     3565-3758      1782.5     96.5  fill Enrichment Cell 2
       H2     4446-5329      2223.0    441.5  analyze contents of
                                              Enrichment Cell 2; cell
                                              heated to release
                                              ad/absorbed contents;
                                              contents ADDED to Direct
                                              Leak 2 flow
       H3     5640-6015      2820.0    187.5  tuning check (high
                                              resolution scan mass
                                              2-46 amu)
       H4     6168-6535      3084.0    183.5  tuning check (high
                                              resolution scan mass
                                              47-90 amu)
       H5     6688-6847      3344.0     79.5  tuning check (high
                                              resolution scan mass
                                              121-140 amu)
       ***         6851      3425.5      N/A  last step of Probe descent
                                              data
       H6          8192      4096.0      N/A  last step in programmed
                                              sequence
       I              0      4096.5      N/A  go back to 256 step cycle
 
    Most data were taken at 75 eV electron impact energy but data at
    lower electron energy were also taken.
 
    25 eV data      15 eV data
    Step range      Step range
    ==========      ==========
    2346-2364       2294-2302
    3794-3959       2615-2773
    4161-4210       3960-3967
    4977-5085       4104-4119
                    4211-4119
                    4608-4767
 
    The detailed mass sequence is variable with selected mass groups,
    unit mass scans from 2-50 and higher resolution (1/8 amu) scans.
    The detailed final descent sequence is contained in the count
    data file.
 
 
  Subsystems
  ==========
    Several subsystems are part of the GPMS instrument:
 
    A) Gas sample inlet and gas processing system
    B) Pressure reduction system
    C) Ion source
    D) Quadrupole mass analyzer
    E) Detector
    F) Pumping System
 
    A complete description can be found in [NIEMANNETAL1992].
 
    A) Gas sample inlet and sample processing system
 
    The inlet system consists of two fully self-contained units that
    operate in time sequence as the probe descends through the
    atmosphere.  Both units contain an ambient atmosphere flow system
    whose gas inlet side is placed near the apex of the probe, and
    whose exit ports are placed at the minimum pressure point inside
    of the probe.  Ambient pressure at the exit port is assumed as a
    worst case condition.  The pressure difference (approximately 6
    millibar) between the stagnation point and the low pressure point
    causes a flow past the pressure reducing leaks.  Inlet and outlet
    ports are sealed by metal-ceramic tube and kept under vacuum
    prior to entry.  They are opened in sequence after entry by
    redundant pyrotechnic actuators.  The materials used for the
    inlet system plumbing are primarily nickel and inconel.  A
    silinizing process passivated the surfaces in contact with the
    gas.  The solenoid-operated microvalves are manufactured by Aker
    Industries of Oakland, California.  The inlet system also has
    heaters to warm the gas and evaporate any condensates that might
    clog the inlets.
 
    The sample enrichment systems are an integral part of the ambient
    atmosphere flow systems.  Atmospheric gas, after passing by the
    direct flow capillary leaks, is also conducted to sample
    enrichment cells.  The enrichment cells contain
    zirconium-graphite getters for binding the reactive gases and a
    porous carbon adsorbing material, Carbosieve (80-100 mesh,)
    chosen to adsorb complex hydrocarbons.  The gas purification cell
    is a small volume of gas, isolated by microvalves, and exposed to
    a getter.  It removes the reactive gases, and allows a pure noble
    gas analysis.  The hydrogen pressure in the volume is reduced by
    approximately by five orders of magnitude and the remaining
    partial pressure is determined by the equilibrium vapor pressure
    of the hydrogen dissolved in the getter.  The gases absorbed by
    Carbosieve Enrichment Cells are released by a programmed heating
    cycle during the probe descent.  During these cycles the cells
    are isolated from the flow system by the solenoid operated micro
    valves and connected through separate capillary leaks to the
    ionization region.  Two independent leak systems are employed for
    sample enrichment.  The sample enrichment leaks for Direct Leak 1
    can be isolated from the ion source by redundant ball closures to
    prevent the ion source pressure from exceeding its optimum value,
    and to permit repeated observation of system background pressure
    after the initial sampling and enrichment sequences are
    completed.  The second independent inlet system is opened to the
    atmosphere after the first system has been isolated from the ion
    source.  Prior to analysis the enrichment cells are heated to
    approximately 200C for 5 minutes to desorb the gases.
 
    B) Pressure reduction system
 
    A small fraction of the gas flowing through the gas inlet system
    is conducted through the pressure reducing leaks into the
    ionization region.  The leaks, which are arrays of micron size
    glass capillaries (typically seven capillaries per leak) with
    inside diameters ranging from 1.5um to 6um, have a conductance
    chosen so that the pressure in the ion source region does not
    exceed 1.E-04 mb.  The Galileo Electro Optics Corp.  of
    Sturbridge, Massachusetts fabricated the capillaries in a
    proprietary process.  The gas flow path for Direct Leak 2 was
    designed to minimize clogging of the capillary array by
    condensable gases (e.g.  water droplets) through the use of a
    droplet trap.  A similar feature was also designed for Direct
    Leak 1.
 
    C) Ion source
 
    The importance of minimizing gas-surface interactions in the high
    vacuum side of the sensor after the pressure reduction stage
    requires that the ion source be very compact and an integral part
    of the sample inlet system.  Electron impact ionization is used
    in a miniature, dual filament ion source.  The second filament
    provides redundancy and is turned on automatically should the
    first filament break or burn out.  A collimated electron beam is
    directed through the ionization region past the end of capillary
    Direct Leak 2.  Sample distortion caused by gas-surface
    interactions is minimized by directing the high-pressure flow
    against the capillary leak and by locating the leaks in the ion
    source so that the gas leaves the capillaries on the ion source
    side directly through the ionizing electron beam.  The gas
    emitted from the capillaries can now be ionized and analyzed
    without experiencing previous surface collisions with the ion
    source walls.  Direct Leak 1, the leak for the Enrichment Cell 1
    and noble gas purification system are connected via short tubes
    to the ionization region.  Chemical reactions on the surfaces of
    the hot filament are minimized by isolation through narrow slits
    and by separate pumping of the filament region.  The electron
    beam energy is varied to permit species identification and
    discrimination by observing spectra of fragmentation patterns at
    several different electron energies.  Ions are focused into the
    mass analyzer by a 3-element electrostatic ion lens system.
 
    The pumping speeds in the flight system are limited by weight and
    power restrictions that do not permit instant removal of the
    gases from the ion source after they initially pass the
    ionization region for Direct Leak 2.  A component of this
    randomized gas contributes to the measurement.  The ratio of the
    direct beaming to the randomized component strongly depends on
    the system geometry.  A ratio of 5:1 was achieved with the ion
    source design of the flight unit.  The most critical parameters
    are the distance between the electron beam and the capillary
    exit, and the cross section of the electron beam.  The
    high-pressure operation of the mass spectrometer ion source is
    limited by mean free path considerations leading to losses due to
    ion molecule collisions.  A higher density in the ionization
    section can be tolerated as a result of beaming because the
    ionization volume in which this high density exists is extremely
    small, having only a small effect on the ion path.
 
    D) Quadrupole Mass Analyzer
 
    The quadrupole analyzer filters the ion beam produced by the ion
    source, transmitting ions of a chosen charge to mass ratio only.
    The transmitted ions are focused onto a secondary electron
    multiplier ion detector.  The radius of the quadrupole field is 5
    mm and the field length is 150 mm.  Mass selection is
    accomplished by application of radio frequency and static
    potentials of varying magnitude to diagonal rod pairs.  The
    selected mass value is determined by the relation m =
    0.55xV/(fxf) where m is the mass in amu, V is the amplitude of
    the applied radio-frequency voltage and f is the frequency in
    MHz.  To allow voltage scanning over a sufficient amplitude
    range, two separate radio frequencies were used, 2.83 MHz for 2
    to 19 amu and 1.13 MHz for 20 to 150 amu.  The only dimensionally
    critical element in the system is the precision rod assembly.
    The rigid and compact design has been proven to be extremely
    stable in previous flight experiments, and in vibration and
    thermal testing.
 
    E) Detector
 
    A continuous dynode secondary electron multiplier detected ions
    exiting the analyzer.  The multiplier was a rugged version of the
    standard Model 4770 manufactured by Galileo Electro Optics of
    Sturbridge, MA.  Charged pulses at the anode of the multiplier
    were amplified and counted.  The background noise of the
    multiplier was approximately one count per minute.  The upper
    count rate of approximately 3E7 counts/sec was limited by the
    multiplier anode pulse width.
 
    F) Pumping system
 
    The pumping system establishes a flow of sample gas though the
    ion source at a particular pressure when a sampling device is
    opened, and, after analysis and closure, removes the sample from
    the ion source region.  Non-evaporable getter pumps and a sputter
    ion pump are used.  The getter pumps are activated prior to
    instrument delivery and required no further Probe power.  The
    sputter ion pump requires only electrical power for operation and
    has no moving parts.  Their use in the GPMS requires care,
    because hydrogen and helium are the major gases in the atmosphere
    of Jupiter.  Getter materials absorb hydrogen at a very high rate
    but helium is absorbed very little, if any.  Sputter ion pumps
    also pump hydrogen with high efficiency but hydrocarbons are
    synthesized in the pump by reactions of hydrogen ions with carbon
    trapped in the pump surfaces.  The effective pumping speed for
    helium is usually small because of the low ionization cross
    section of helium and the requirement that helium be buried
    physically in the pump elements since it does not become
    chemically bound or to go into solution like hydrogen.  This
    requires sputtering of comparatively large amounts of cathode
    material which tends to release larger quantities of gases
    previously entrapped in the pump surfaces.  To eliminate the
    synthesis of hydrocarbons in the sputter pump, a cascaded pump
    system is used.  A high capacity baffled getter pump is operated
    in cascade with a sputter ion pump.  The gas flow from the mass
    spectrometer into the getter pump is conductance limited to
    maintain a constant pumping speed during the measurement phase.
    The getter pump absorbs hydrogen and other reactive gases before
    they can reach the sputter pump.  Gases emitted by the pump must
    pass back through the getter pump first before they can enter the
    mass spectrometer.  Thus, their contribution to the gas in the
    ion source gas is significantly reduced.  The preceding getter
    chamber buffers small sputter pump instabilities.
 
    The getter material used is sintered zirconium-graphite available
    from SAES Getter of Milan, Italy (type ST171).  They are
    activated by heating to approximately 900C for 45 minutes while
    being connected to a laboratory pumping system.  The cathode
    materials of the sputter ion pump are tantalum and titanium.  The
    electrode geometry has been optimized to enhance the pumping
    speed for helium.  Pumping speed is limited to about 2 liters/sec
    at the flange.  The magnetic field of the sputter ion pump is 0.2
    Tesla over an area of 35 square centimeters.  The yoke is
    designed to minimize the stray field and magnetic shielding is
    provided to the ion source housing to cancel the stray field of
    the pump because of its location directly above the ion source.
 
 
  Measured Parameters
  ===================
    The ion arrival rate from the mass analyzer into the detector
    during each one half second of the descent constitutes the
    primary measurement.  At ion arrival rates exceeding the upper
    count limit of the discriminator/pulse-counter system the signal
    is desensitized automatically.  The descent sequence also
    contains steps where a desensitized mode has been pre-programmed.
    The counting register value can be expressed either as
    counts/sample-integration-period or counts/sec.