Instrument Information |
|
IDENTIFIER | urn:nasa:pds:context:instrument:gp.gpms::1.0 |
NAME |
GALILEO PROBE MASS SPECTROMETER |
TYPE |
SPECTROMETER |
DESCRIPTION |
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. |
MODEL IDENTIFIER | |
NAIF INSTRUMENT IDENTIFIER |
not applicable |
SERIAL NUMBER |
not applicable |
REFERENCES |
Niemann, H.B., D.N. Harpold, S.K. Atreya, G.R. Carignan, D.M. Hunten, and T.C.
Owen, Galileo Probe Mass Spectrometer experiment, Space Science Reviews volume
60, pp 111-142, 1992 Niemann, H.B., S.K. Atreya, G.R. Carignan, T.M. Donahue, J.A. Haberman, D.N. Harpold, R.E. Hartle, D.M. Hunten, W.T. Kasprzak, P.R. Mahaffy, T.C. Owen, N.W. Spencer, and S.H. Way, The Galileo Probe Mass Spectrometer: Composition of Jupiter's Atmosphere, Science 272, pp 846-849, 1996 |