The Cassini Orbiter Ion and Neutral Mass Spectrometer (INMS) is designed to measure the composition and density variations of (low energy) ions and neutral species in the upper atmosphere of Titan, in the vicinity of the icy satellites and in the inner magnetosphere of Saturn where densities are sufficiently high for measurement. The sensor utilizes a dual radio frequency quadrupole mass analyzer with a mass range of 1-99 amu, two electron multipliers operated in pulse-counting mode to cover the dynamic range required and two separate ion sources. A closed ion source measures non-surface reactive neutral species which have thermally accommodated to the inlet walls such as N₂ and CH₄. An open ion source allows direct beaming ions or chemically active neutral species such as N and HCN to be measured without surface interaction. The instrument can alternate between these three different modes (closed neutral, open ion and open neutral). Characterization and calibration of each of these three modes is done using a low energy ion beam, a neutral molecular beam and a neutral thermal gas source. An onboard flight computer is used to control instrument operating parameters in accordance with pre-programmed sequences and to package the telemetry data. The sensor is sealed and maintained in a vacuum prior to launch to provide a clean environment for measurement of neutral species when it is opened to the ambient atmosphere after orbit insertion. The instrument is provided by NASA/Goddard Spaceflight Center, Code 915. Operation of the instrument and data analysis will be carried out by a Science Team.

Keywords: Cassini, Titan, INMS, Mass Spectrometer, Ion and Neutral Mass Spectrometer

The Cassini mission to Saturn consists of an Orbiter provided by Jet Propulsion Laboratory (JPL) for the National Aeronautical and Space Administration (NASA) and the Huygen's Titan probe, provided by the European Space Agency (ESA)^1,2,3. The measurement objective of the Ion and Neutral Mass Spectrometer (INMS) onboard the Saturn Orbiter is to:

• measure the in-situ composition and density variations, with altitude, of low energy positive ions and neutrals in Titan's upper atmosphere

• measure the in-situ composition of low energy positive ions and neutrals in the environments of the icy satellites, rings and the inner magnetosphere of Saturn, wherever densities are above the measurement threshold and ion energies are below ~100 eV.

The INMS measurements will contribute to the science objectives of the Cassini mission² by investigation of the: 1) upper atmosphere of Titan, its ionization and its role as a source of neutral and ionized material for the Saturn magnetosphere; 2) environment in the rings; 3) interaction of the icy satellites and ring systems with the magnetosphere, and possible gas injection into the magnetosphere; 4) effect of Titan's interaction with the solar wind and magnetosphere plasma; and 5) interactions of Titan's atmosphere and exosphere with the surrounding plasma.

The Cassini Saturn Orbiter spacecraft will utilize the massive Titan moon for gravity assisted orbit changes. During the four year nominal Cassini mission some 35-40 flyby's of Titan are currently scheduled with the majority targeted for 950 km altitude. There is a possibility of having several passes re-targeted as low as 850 km if the INMS requires lower altitudes for data interpretation. Titan is the only satellite known to possess a very dense atmosphere; it is comprised mainly of N₂ with percent levels of CH₄^12,10,4,5. Photodissociation of methane produces radicals that combine to form heavier hydrocarbons such as C₂H₂ and C₂H₆; reactions between the hydrocarbons and atomic nitrogen (from N₂) produce nitriles such as HCN and C₂N₂. Many trace level heavier hydrocarbons, nitriles (including HCN) as well as CO and CO₂ have been observed in the stratosphere⁵ and these may also be present in the upper atmosphere along with the previously detected N₂, CH₄ and traces of C₂H₂^10,5,7. The homopause altitude is estimated to be 750-925 km^7,10 and the exobase near 1500 km^7,8 so that INMS in-situ neutral measurements will be made in the diffusion equilibrium and exosphere regions of the upper atmosphere. Correlation with measurements in the lower atmosphere can be through Orbiter remote sensing measurements and direct measurements by the Gas Chromatograph Mass Spectrometer (GCMS) and other instruments on the Huygen's Probe during its entry phase. Models⁷ of the photochemistry suggest that the major neutral species in the upper atmosphere include H, H₂, N, NH, N₂, CH₄, HCN, C₂H₂, C₂H₄, C₂H₆ and heavier radicals such as C₄N₂. The exosphere temperature above 900 km is a result of the balance of the Ultraviolet (UV) heating, cooling by HCN¹¹ and heat conduction, and is a function of solar activity and the HCN mixing ratio. The average inferred scale-height temperatures during the Voyager 1 encounter¹⁰ ranged from 165 K (725 km to 1265 km) to 186 K (above 1265 km). In the upper atmospheres of both Earth and Venus there is evidence for superrotation of the thermosphere and upward propagating gravity waves, both of which are likely to be present in Titan's upper atmosphere. Variations with latitude, solar activity, magnetosphere energy input and local time can also be expected to be present. In-situ density and temperature measurements by the INMS can be used by the Cassini project to establish the lowest altitude for safe operation of the spacecraft where the effects of drag are within tolerable limits. Isotopic ratios are of interest as clues to atmospheric origin and evolution. However, these measurements will be made in a region that is diffusively separated. Correction for this effect requires an accurate knowledge of the homopause level which will require a chemical model and a comparison with Probe data.

The ionosphere of Titan has not been directly measured. Voyager 1 constraints estimated the electron density^16,15 as (3-5)x10³ cm^-3. An induced magnetosphere¹³ was observed consistent with the magnetic field being draped over an obstacle ("ionosphere" ) and in the wake heavy ions with a mass-to-charge ratio of 28 (N₂⁺ or H₂CN⁺ from the ionosphere) were observed¹⁴. Models of the ionosphere⁸ show a complex hydrocarbon ion composition that varies with altitude. In the model of Keller et al.⁸ the ionosphere peak near 1050 km is dominated by H₂CN⁺ and C₂H₅⁺ whereas near the "ionopause" (if it exists) CH₅⁺, CH₃⁺, CH₄⁺ are dominant with plasma convection becoming more important at altitudes above 2000 km. Many heavier hydrocarbon ions C_mH_n⁺ (m=4,5..) are also produced. The ionosphere is the obstacle for the interaction of Titan with the solar wind and Saturn's magnetosphere⁶. The interaction between the ionosphere and magnetosphere of Saturn or solar wind will result in ions being swept into the plasma wake. Saturn's magnetosphere can also serve as an energy input to the ionosphere and neutral atmosphere. Variations of the ionosphere between day and night, with Titan in the solar wind or in Saturn's magnetosphere, and with the orbital position of Titan relative to Saturn are expected.

As the spacecraft passes through closest approach to Titan it carries out an altitude scan. The location of the scan relative to Titan's surface depends on the particular configuration chosen for the orbit tour. During a typical pass both ions and neutral species concentrations will be measured. The neutral density scale height can be used to derive an atmospheric temperature assuming that the atmosphere is in diffusion equilibrium and that the horizontal density variations are small. Ion density scale heights can yield information on the plasma temperature and flow. The ion and neutral densities can provide information on the ion-neutral chemistry in the upper atmosphere as well as the heat budget.

The particles in Saturn's rings are thought to be made up of mainly (water) ice and mixed with other (rocky?) material. Other ices such as ammonia and methane may also be present. Many of the satellite's of Saturn have densities that suggest a composition of (water) ice and some darker material. Infrared remote sensing confirms the presence of water ice (or frost) on the surfaces of many of the major satellites. The neighborhood of the rings and icy satellites is expected to contain a tenuous exosphere of water vapor and ions derived from the water or other ices as a result of ion sputtering, meteorite impact and sublimation. For example, H, OH and O are likely to be present along with various ions such as H⁺, H₂⁺, O⁺, OH⁺, H₂O⁺, H₃O⁺ and O₂⁺. The OH radical has been detected in Saturn's magnetosphere¹⁹ (~ 160 cm^-3) and in the atmosphere of the rings²⁰ (~700 cm^-3). Measurements can be made for the targeted icy satellites and in the space between Titan and Rhea (0.87 R_S) covered by the Orbiter. A H torus¹² (10-20 cm^-3) has been found in the vicinity of Titan's orbit, assumed to be due to photolysis of CH₄. A H₂ torus may also be present along with a N torus²¹. These species may be measured by INMS if the density is above the detection threshold (using either a one integration period sample or by combining multiple integration period samples). The tenuous "clouds" of neutrals provide a source of ions which can mass load the local magnetic field. Other possible magnetosphere measurements include the thermal co-rotational plasma composition near the equator (if the INMS pointing is appropriate), the ion wakes of Titan and other objects (icy satellites and rings), and low energy ion "polar-wind" outflows (e.g. H₂⁺ and H₃⁺) from Saturn.

The mass spectrometer sensor consists of two separate inlets for measurement of ambient ions and neutral particles (Figure 1), the electrostatic quadrupole deflector (switching lens), the quadrupole mass analyzer and the two detectors. A separate closed and open source inlet, rather than a single combined quasi-open ion source, is used to optimize the interpretation of the neutral species; the open ion source is necessary to measure ions. The Appendix lists the instrument parameters in Table 1 and modes of possible operation in Table 2. The neutral gas density environment during a Titan flyby is nearly optimal for direct sampling without ambient pressure reduction. Surface non-reactive neutral gases such as N₂ are measured in the "closed-source" inlet where a velocity ram enhancement as a result of the spacecraft's motion also increases the detection range for these species. For gas species that are not surface reactive this geometry provides an accurate means of measuring ambient density. Surface reactive neutral gases, such as N, are measured in the "open-source" inlet which utilizes the spacecraft's motion to form a neutral beam which is subsequently ionized, with no surface interaction. The ambient gas density is sampled directly with no stagnation enhancement. Electron impact ionization is used to create ions from neutrals. The entrance ion collimator also serves as a trap for electrons and ions which could cause spurious ionization of neutral species. Ambient thermal and suprathermal ions are sampled by collimating them using the ion collimator system of the "open-source" with the open source filament off. Open source ambient ions or ions created by electron impact ionization are focused into an electrostatic deflector or quadrupole switching lens⁹. The switching lens multiplexes ions from either the open or closed source into a single Radio Frequency (RF) quadrupole mass analyzer, which separates the ions according to their mass-to-charge ratio. The ions are detected by two secondary electron multipliers operating in pulse-counting mode to cover the dynamic range required. The potential for measurement of the heavier hydrocarbon species and possible pre-biotic cyclic hydrocarbons such as C₆H₆ has resulted in an increase in the INMS mass range from 1-66 to 1-8 and 12-99 amu (atomic mass units).

The sensor and electronics are packaged in the form of a box (Figure 2). The sensor entrance apertures are contained in a single plate which is covered by a metal-ceramic breakoff hat that is pyrotechnically activated. During cruise the breakoff hat is protected by thermal shielding. The INMS instrument is mounted on the Fields and Particle Platform (FPP). The normal to both the open and closed source INMS orifices point in the spacecraft -X direction. The field of response of the two gas inlets are different (see Table 1). The geometric field of view of the open source is limited to about 8º cone half angle which limits the angular response of neutral and ions measured with this inlet. The closed source has a much wider field of response, approximately 2p steradians. Venting of the ion sources occurs at right angles to the -X axis. This is to lower the ion source and analyzer pressures (increasing the ion mean free path) during a Titan pass when the spacecraft ram approximately along the -X direction. The electronics are mounted on boards which are parallel to the FPP platform. The sensor is made mainly of titanium and the electronics box of aluminum to be strong, yet lightweight. The box forms an electrostatic shield for the instrument as well as providing micrometeroid and high energy particle protection. The secondary electron multipliers are additionally shielded with tantalum. The spacecraft multilayer insulation (MLI) is used for thermal control and micrometeroid protection. It is attached to the brackets as shown in Figure 2. The thermal radiator panel is used to dissipate INMS internally generated heat and is not covered by the MLI. The package design is such that the entrance apertures of the sensor protrude beyond the edge of the FPP platform and MLI insulation. This is to prevent gas contamination from the spacecraft and provide the maximum field of view.

The mass spectrometer is constructed of titanium and is free of organic materials. It can be baked to 350º C for vacuum cleanup and will be maintained below 10^-8 hPa by a getter pump during the launch and cruise phases. A miniature ion gauge and a thermistor are used to monitor the internal sensor pressure. In a sealed configuration there is a residual gas atmosphere (primarily helium, argon, methane, carbon monoxide, carbon dioxide, hydrogen and water) remaining that can be used for sensor testing. Maintaining the sensor and ion sources under a vacuum keeps these regions clean until ambient neutral atmosphere measurements can be made. Opening of the sensor to the external environment by ejection of the breakoff hat is planned to occur after Saturn Orbit Insertion (SOI) prior to the ring plane crossing. The pyrotechnically activated metal-ceramic breakoff device has been used successfully for many instruments on Earth orbiting satellites as well as for the Pioneer Venus Orbiter and Galileo Probe mass spectrometers. For the inner satellites close to Saturn, calculations¹⁸ indicate that measurements of low densities in these regions will be difficult due to the magnetosphere radiation background in the secondary electron multipliers (SEM). A 0.23 cm (0.09 in.) thick tantalum shield has been placed around the SEM region, providing an adequate shield by lowering the estimated background radiation level to less than 10^-11 particles/cm²/sec¹⁸.

Figure 3a is a photograph of the INMS engineering unit ion source and Figure 3b the sensor tube with the support electronics. The INMS instrument is a modification of the Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument designed for the Comet Rendezvous Asteroid Flyby Mission (CRAF). It has a heritage of similar instruments designed by GSFC for upper atmosphere measurement missions such as Atmosphere Explorer, Dynamics Explorer, Pioneer Venus and Galileo Probe Mass Spectrometer.

2.a Operation

Electron impact ionization is used to ionize neutral species because it offers high sensitivity. It has virtues of being species non-specific and production of unique fractionation patterns which aid in the identification of molecules. The presence of dissociation products, however, can lead to problems in interference where ionization of an abundant molecule masks the presence of a trace constituent. Much of this disadvantage can be surmounted by the use of two ionization energies so that the resulting different fractionation patterns can be exploited to minimize interference.

The closed ion source uses a spherical antechamber with an entrance orifice for the ambient gas flux. A long cylindrical tube connects this antechamber to the entrance of the ionization region. Two redundant electron gun assemblies provide a collimated electron beam used for ionization. The ions formed are focused into the quadrupole switching lens by a series of cylindrical electrostatic lenses. The incoming gas makes many collisions with the antechamber surfaces and thermally accommodates to the wall temperatures. A ram enhancement is achieved by limiting the gas conductance from the antechamber into the ion source while maintaining a high particle flux into the entrance aperture.

The entrance aperture of the open ion source leads to an cylindrical antechamber which consists of 4 plates in 4 equally spaced segments. The ion collimator/trap can be used to trap ions and electrons during neutral measurements or be used to focus ambient ions into an exit aperture. The neutral beam is ionized by one of two redundant opposing electron guns and focused into the quadrupole switching lens. With the exception of the entrance plate, which is at spacecraft ground, the open source electrostatic lenses and the quadrupole switching lenses are programmable. This allows the lens system to measure both neutrals and ions in an optimized manner following the spacecraft equivalent energy. In neutral mode they can be used to discriminate gas particles that have thermally accommodated to the ion source walls from the direct beaming component at spacecraft energies. In ion mode they can be used to "steer" the incoming ions and perform a coarse energy scan.

The electron guns have filaments (0.0076 cm, 97% tungsten-3% Rhenium) in a coiled configuration, which are heated to provide an electron beam that is collimated and focused by electrostatic lenses. Two electron impact ionization energies are provided: 70 and 25 eV. The electron emission is 40 ma. The quadrupole switching lens⁹ uses four circular rod sectors mounted in a cube assembly to provide a nearly hyperbolic electrostatic field for a 90º deflection. This device is used to multiplex ions from either the closed source or the open source into the common entrance lens system of the quadrupole mass analyzer. The fractional energy transmission width, DE/E, is 0.3 for a 0.3 cm entrance and exit aperture. The switching lens potentials can be scanned (see Figure 4) to provide an estimate of the ion and neutral energy distribution up to about 150 volts since the potential on each rod can range from 0 to -300 volts. Doubly charged species require a different potential on the rod segments than do singly charged species. The quadrupole mass filter uses four precision ground hyperbolic rods mounted in a mechanical assembly. The rod spacing parameter , r₀, is 0.58 cm and the rod length is 10 cm. The quadrupole rods are excited by Radio Frequency (RF) and Direct Current (DC) potentials which together create a dynamic electrostatic field within the quadrupole region that controls the transmitted mass (mass/charge ratio), the resolution, and the transmission efficiency. A mass scan is effected by varying the RF potential amplitude, V_ac, to satisfy the relationship M = 0.55 V_ac/f² where V_ac is in volts, f is the RF frequency in megahertz, and M is in atomic mass units (amu). The nominal mass range for the INMS is 1-8 amu and 12-99 amu using two separate frequencies. The frequency, f, is counted and is used to compensate for drift. The resolution is controlled over each mass range by programming the V_dc/V_ac ratio to maintain the resolving power as defined by a crosstalk criterion appropriate for that mass range. The resulting flat-topped peaks allow a mass scan mode in which each mass is monitored by a single step to achieve the lowest detection limit in a specified period. Another operating mode reduces V_dc to zero and creates a high pass filter giving the sum of all masses greater than, for example, 90 amu. The ions exiting the quadrupole are detected by two secondary electron multipliers, differing in signal detection level by about a factor of 2000. Charge pulses at the anode of the multiplier are amplified and counted. The detection threshold is determined by background noise in the multiplier (approximately 1 count per 100 seconds in the laboratory). The upper count rate of each detector system is about 10 mHz, limited by the product of the multiplier pulse width and gain bandwidth of the pulse amplifier counter system. Ion counts above this value can be measured directly as an analog current. Multiple sample periods can be combined to lower the detection threshold to the background noise level (signal/noise ratio = 1). Assuming a maximum 1 mHz counting rate, the dynamic range of the two detector system is ~10⁸.

Assuming the nominal neutral ion source sensitivity and a maximum count rate of 1 mHz yields an ion source density of about 3x10⁸ cm^-3 for detector 1 and about 6x10¹¹ cm^-3 for the lower signal gain level detector 2. The lower limit on the ion source density is about 10⁴ cm^-3 for 1 count per integration period. In the mass range below about 50 amu this is usually not realized in the closed source because of background gases, identical to those being measured and emitted from the surrounding surfaces or, for both sources, interference at some mass numbers of other ambient gases present in high concentrations. In the closed source the calculated ambient density is lower than the ion source density due the velocity ram enhancement. The maximum ion source density, of about 10¹² cm^-3 (10^-4 hPa), is limited by mean free path conditions in the ion source and analyzer regions. The sensitivity is established by instrument characterization and varies with the species due to different ionization efficiencies for neutrals in the ion sources, the transmission of the quadrupole switching lens and mass filter, and the conversion efficiency at the secondary electron multiplier.

The ion flux sensitivity is about 10^-3 particles/cm²/sec. For one count per integration period the minimum flux is about 3x10⁴ part/cm². Using a spacecraft speed of 6 km/sec (Titan flyby at 950 km altitude) the corresponding density is 0.05 particles/cm³, assuming that the spacecraft potential does not interfere with the ion measurement. The sensitivity is established by sensor characterization and will vary with the ion species due to spacecraft equivalent energies for the different species masses, the transmission of the quadrupole switching lens and filter, and conversion efficiency at the secondary electron multiplier detector. The spacecraft equivalent energy at zero degrees angle of attack and 6 km/sec speed is 0.188 eV/amu. The effect of spacecraft potential is modify the incoming flux and modify the ion trajectory directions relative to the spacecraft.

In order for the INMS to make valid ion and neutral density measurements, the spacecraft velocity vector plus any atmosphere or ionosphere drift velocity must be within the field of response of the appropriate source. During Titan flybys the spacecraft can operate either in Radar Mode with the radar tracking local nadir or in INMS mode with the spacecraft velocity vector tracking the INMS axis. In Radar Mode the INMS angle of attack ranges from 0 to 8º within 2.4 minutes of closest approach at a spacecraft speed of 6 km/sec. The altitude change is 60 km over a spacecraft track length of 870 km.

2.b Electrical

The electronics system of the INMS is based on designs used for the Huygen's Probe GCMS instrument. A block diagram of the electronics is shown in Figure 5. The low voltage (LV) power supply converts the spacecraft power to well regulated DC voltages supplied to the instrument electronics. A pulse width modulated converter allows efficient generation of multiple secondary voltages while providing secondary-to-primary isolation. A large number of voltages are required for biasing the various focus electrodes as well as to supply DC voltages for the secondary electron multipliers. Analog modules are used for regulating the emission of the electron guns, for providing fixed and programmable voltages to set lens potentials, for supplying RF and DC for the quadrupole mass analyzer, for supplying high voltages for the detectors, and for the pulse counting circuits. The digital electronics includes a single microprocessor, the spacecraft bus interface circuit, and interfaces between the CPU and the analog modules. Major portions of the electronics are packaged in hybrid circuits to save weight and space.

The radio frequency generator drives the quadrupole at two resonant frequencies in order to reduce the need for a large amplitude potential for the mass range (1-99 amu) required. Frequency selection is performed by a solid-state switched bandpass filter. The DC voltage is created by high-voltage operational amplifiers and is superimposed on the RF amplitude. Both the RF and DC amplitudes are programmed by digital to analog converters.

Charge pulses at the anode of the electron multiplier are converted by the pulse amplifier into voltage pulses which are counted if they are above a preset threshold. An analog measurement of the multiplier current is also provided which is used to determine the inflight multiplier gain.

The instrument computer (MA31750) controls the measurement sequence, counts the detector pulses, provides the analog current to digital conversion of the detector current and monitors instrument housekeeping parameters. The computer is programmed in Ada as the target language with some use of assembly language to handle time critical functions, input/output and interrupts. The instrument PROM/RAM will also contain the default measurement and test sequences without requiring memory uploads.

Data from the INMS to the spacecraft consists of: 1) housekeeping packets which contain normal analog-to-digital converter channel data or memory dump data if science packets are not being collected; and 2) science packets which contain either normal science data, memory dump data or special test data.

2.c Instrument Response

The closed source response as a function of the angle of attack (angle between the orifice normal and the spacecraft velocity vector) is given in Figure 6. Particles entering through the orifice collide many times with the surfaces of the ion source electrodes and enclosure, and thermally accommodate to the surface temperature before leaving again through the orifice. For angles of attack less than 90º this results in an enhancement of the number density in the antechamber over that in the ambient atmosphere which varies approximately with the cosine of the angle of attack. The number density in the antechamber is determined by a balance of the incoming number flux at the spacecraft speed and the outgoing gas flux at the antechamber wall temperature. The relationship between the ion source density, incoming flux and ambient density is predictable from kinetic theory, and will be verified by laboratory characterization of the instrument. Non-reactive species (e.g. N₂ and CH₄) will be measured in this mode. For the closed source, ambient density measurements can be made from about 0 to 90º angle of attack, depending on the gas background. Certain reactive species may also be measured in this mode as surface recombined reaction products (e.g. H as H₂).

For the open source mode, the relative motion of the spacecraft with respect to the ambient atmosphere is used to discriminate between fast and slow moving ionized neutral particles when the ion source is approximately pointed in the direction of motion. The quasi-open source geometry allows ambient gas to enter the ionization region directly and permits measurements of chemically active species. The angular response is limited by the geometric field of view to a cone of about 8º half angle. The theoretical response of the open source as a function of angle of attack are given in Figure 7. The ambient particle density is measured directly by this method. Successful measurements of the open source neutral ambient density requires discrimination of the thermally accommodated gas from the direct beaming component by: 1) reducing the velocity ram stagnation density enhancement by maximizing the gas conductance from the ionizing region into the vent region which is at right angles to the open source axis; 2) applying a slight retarding potential (Figure 8) to the ions after they leave the ionization region; and 3) using the quadrupole switching lens transmission⁹ (Figure 4) which is a function of energy to filter the ion energy. Numerical studies¹⁷ using only (3) indicate that less than 2% of the 28 amu ions from thermally accommodated gas are transmitted to the mass analyzer when the switching lens potential is set to transmit a 28 amu ionized neutral beam at spacecraft energies (6 km/sec spacecraft speed is equivalent to 5.22 eV compared to 0.015 eV for a thermally accommodated gas at 300 K temperature). This ratio will be determined during sensor characterization. The open source neutral mode measurements will be confined to closest approach where the density is sufficiently large and the angle of attack is approximately within the geometric view cone.

The open source ion mode response has been studied by numerical simulation using the Sarnoff BEAM 3D¹⁷ software to model the lens system from the entrance aperture to the entrance of the quadrupole mass analyzer. An example of the angular response is given in Figure 9 (left hand scale) for spacecraft equivalent ion energies and no thermal energy spread. If the transmission is optimized only at 0º angle of attack then half width half maximum (HWHM) point is about 3º for 28 amu. Optimizing the transmission by adjusting the potential on paired segments of the collimator at each angle of attack results in a HWHM of about 15º for 28 amu. The difference in potential between pairs of plates needed to optimize the transmission is approximately linear with increasing angle of attack (Figure 9 right hand scale). For ion mode the advantage of being able to program the collimator plate voltages to increase the angular response beyond the geometric view cone can be important for increasing the altitude range of coverage for thermal ions during a Titan pass. For example, optimization of the angle coverage for N₂⁺ results in either the inbound or outbound leg of the pass being about 4.7 minutes in length with an altitude change of about 230 km and a spacecraft track length of 1700 km for conservative 15º angle of attack. For a 950 km closest approach altitude to Titan, this is sufficient to permit measurements over the ionospheric peak⁸ near 1100 km Suprathermal ions can also be measured up to about 100 eV. The energy resolution of the quadrupole switching lens is coarse and ion direction can only be inferred by simultaneous scans of the collimator plates. The suprathermal ion mass to charge ratio can be uniquely determined. This will aid the interpretation of low energy ion data obtained by instruments doing energy-per-charge scans using the spacecraft velocity which have difficulty in resolving closely spaced mass groups.

2.d Instrument Modes

The function of the microprocessor is to provide the neutral/ion measurement sequence along with the mass numbers (scan) to be sampled. The mass scan can contain arbitrary sets of values from 1 to 99 amu for either neutral or ion mode in 1/8 amu steps. A survey mode is implemented in which the mass is sequentially is stepped from 1 to 99 (or some subset) in unit or 1/8 amu steps. A high pass mode is implemented to allow measurement of the total signal above a given mass number. Table 2 (Appendix) contains an example of modes of operation. The electron energy can also be changed between scans along with filament on and off.

2.e Sensor Characterization

An overall check of the instrument operation starting with a neutral/ion density or flux and ending with results of the measurements in terms of a pulse counter output is done during sensor characterization of each of the three instrument operating modes. The three modes are mirrored in the single vacuum station (Figure 10) which incorporates: 1) thermal gas source for characterization of the closed source mode sensitivity and mass spectral fragmentation patterns; 2) neutral beam for characterization of the open source neutral beam flux response; and 3) an ion beam source for characterization of the ion mode flux response up to about 150 eV. Two ion beam systems are available: an alkali metal ion source and Colutron plasma source. The instrument is attached externally by a flexible bellows with two degrees of rotational freedom for angular response characterization up to about 25º. The instrument can be translated to place each of the open and closed sources at the proper center of rotation. The all metal system is pumped with oil free turbomolecular and backing pumps. In addition to the characterization of the flight unit, the backup engineering unit will be cross-referenced to the flight unit for future investigation of new species data or other instrument uncertainties as the flight data is collected and analyzed.

A static pressure dividing technique is employed to characterize the closed source for the chemically inactive species such as N₂, H₂, CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₈, CO_2, He, Ne, Ar, Kr, Xe, CO and HCN either as single gases or pre-determined gas mixtures. During calibration, the gas flows through a capillary leak operating in the molecular flow regime and is pumped out of the test chamber through a small diameter orifice. Chamber pressures above 10-⁶ hPa are referenced to a molecular drag gauge, and lower pressures are determined by extrapolation using a high pressure capacitance manometer. Absolute sensitivities will be established for each electron energy along with fragmentation patterns which allow interference corrections to be applied to flight data.

A neutral molecular beam is used to establish the characteristics of the open source operating in neutral mode. The system will provide beam speeds from 1 km/sec to about 5 km/sec (nominal spacecraft speed at periapsis is 6 km/sec) by a supersonic expansion of a high pressure heated gas (H₂) seeded with calibration gas species (He, Ne, N₂, Ar, Kr). The neutral beam will establish the angular characteristics of the open source and its sensitivity relative to the closed source. Open source measurements during fight can be compared to closed source measurements of the same species for a cross check of the open source response. An empirical relationship between the two responses will be determined as a function of species mass, velocity and angle of attack. The beam speed is measured with a metastable time of flight detector.

An ion beam system is used to characterize the open source ion mode flux response. The ion beam is generated from a plasma discharge, accelerated through a potential difference, passed through a Wein filter to select the ion mass of interest and then decelerated down to energies of interest (1-100 eV). The ions are deflected through a quadrupole switching lens⁹. An alternative method is to use a direct in-line alkali metal ion source. The ion beam flux into the instrument is determined with a separate retarding potential analyzer (RPA) detector. The ion beam will also be used to characterize the INMS quadrupole switching lens and ion trap response as a function of the potentials used. Possible species used include H⁺, H₂⁺, He⁺, Li⁺, N₂⁺, Ne⁺, K⁺, Ar⁺, Kr⁺ and Xe⁺.

The INMS instrument sensor is designed, built, tested and characterized at NASA/Goddard Spaceflight Center (GSFC). The Instrument Development Task Manager is H. Niemann and the Instrument Manager is J. Richards. Other GSFC personnel involved are: digital electronics design and programming (R. Frost, F. Tan, M. Paulkovich, J. Stuart); sensor design and assembly (S. Dixit, H. Powers, R. Arvey, R. Abell); sensor testing (C. Carlson, E. Raaen); and sensor characterization (H. Manning, W. Kasprzak, E. Patrick). Analog electronics are designed and built by the University of Michigan (G. Carignan, B. Block, K. Arnett, J. Maurer). 3-Dimensional ion trajectory simulations were performed by V. Swaminathan of Princeton Electronics Systems, and R. Alig and W. Murray of David Sarnoff Research Center as part of a NASA contract NAS5-32823 (P. Mahaffy, NASA Technical Representative).

The INMS is supplied to Jet Propulsion Laboratory (JPL) as a facility instrument on the Cassini Orbiter. The Instrument Engineer is D. Boyd (JPL) and the Investigation Scientist is V. G. Anicich (JPL). The INMS instrument will be operated after launch by a Facility Team. The facility Team Leader is J. H. Waite, Jr (Southwest Research Institute). The Team Members are T. E. Cravens (University of Kansas), W.-H. Ip (Max-Planck-Institute fur Aeronomie), W. T. Kasprzak (NASA/Goddard Spaceflight Center), J. G. Luhmann (University of California, Berkeley), R. L. Mc Nutt, Jr. (John Hopkins University) and R. Yelle (Boston University). The team is responsible for the instrument commanding, data processing and science interpretation. The team has been involved in other aspects of INMS support: spacecraft attitude thruster contamination; increase of the sensor maximum mass value from 66 to 99 amu; addition of tantalum shielding to lower the secondary electron multiplier magnetosphere radiation background; determination of the form of the operational commanding software; and orbital tour studies emphasizing Titan passes with optimum INMS pointing and low altitude sampling.

Table 1. Cassini Ion and Neutral Mass Spectrometer (INMS) Parameter Summary
Neutral Gas Sampling Systems	1) Open source (molecular beaming) with energy discrimination; 2) Closed source
Ion Sampling System	Thermal and suprathermal positive ions
Sample System Switching	Electrostatic quadrupole deflector
Viewing Angle (Angle of Response)	1) Open Source ~8º cone half angle; 2) Closed Source ~2p steradians; 3) Exhaust vent ~2p steradians
Neutral Mode Ion Sources	Electron impact ionization; electron energy (nominal): 25 eV and 70 eV
Mass Analyzer	Quadrupole mass filter, 0.5 cm field radius, 10 cm rod length Radio Frequencies: 1.8 mHz and 3.7 mHz
Mass Range	1 to 8; 12 to 99 amu nominal; high pass filter mode
Scan Modes	1) Survey: scan mass range in 1/8 or 1 amu steps; 2) Adaptive Mode: select mass values
Resolution/Crosstalk	10-6 for adjacent masses
Detector System	Two secondary electron multiplier detectors operating in pulsecounting mode (detector noise < 1 count/minute in laboratory) Dynamic range of two detector system for 1 integration period sample ~ 108
Sensor Sensitivity	1) Ion flux sensitivity 10-3 (counts/sec)/(ions/cm2/sec), maximum energy ~100 eV 2) Neutral mode sensitivity 2.5x10-3 (counts/sec)/(particle/cm3) (closed and open source)
Density/flux for 1 count per integration period, no background	Minimum neutral ion source density (both sources) = 1.2x104 particles/cm3 Maximum neutral ion source density ~1012 particles/cm3 Maximum closed source ram enhancement factor for N2 @ 6 km/sec = 50 Minimum ion flux = 3.2x104 ion/cm2/sec
Data Rate	Sample integration period = 31.1 ms; total sample period = 34.0 ms
Spatial resolution	~ 200 meters along spacecraft track per sample period
Instrument control	Microprocessor: MA31750 RAM: 128 Kbytes PROM: 128 Kbytes
Telemetry	Science data rate 1498 bps; Housekeeping data rate 12 bps Reduced science packet production mode implemented
Deployment Mechanism	Metal ceramic breakoff cap, pyrotechnically activated
Power (Current Best Estimate)	Neutral Mode: average 23.3 W Ion Mode: average 20.9 W Sleep: average 13.1 W Off: 4 W replacement heater
Size	Maximum envelope (cruise): Height 20.3 cm (8.0"), Length 42.2 cm (16.6"), Width 36.5 cm (14.4")
Weight (Current Best Estimate)	9.25 kg + 1.4 kg for tantalum radiation shield 0.23 cm (0.090") thick

Table 2. Possible Modes of INMS Operation*
TYPE	LENGTH (sec)	MASS SELECTION	SOURCE	ION/NEUTRAL
0	2.3	68 Programmed masses	Closed	Neutral
1	2.3	68 Programmed masses	Open	Ion
2	2.3	68 Programmed masses	Open	Neutral
3	2.3	Unit amu sweep 68 steps	Closed	Neutral
4	2.3	Unit amu sweep 68 steps	Open	Ion
5	2.3	Unit amu sweep 68 steps	Open	Neutral
6	26.1	1/8 amu scan 1-8 & 12-99 amu	Closed	Neutral

* Modes can be combined such as Type 0 followed by Type 1 followed by Type 6 in the simplest example. Instrument programming flexibility allows a fraction of Type 1 to be followed by a fraction of Type 2 etc. Switching between the open source neutral and ion modes involves turning the filament on and off, and is likely to be done only on a much broader time scale, on the order of several tens of seconds.

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Figure 1. Cross section of the INMS sensor.

Figure 2. INMS sensor with electronics.

Figure 3a. Close-up of INMS ion source showing closed source (top), open source (bottom) and switching lens cube. The diameter of the large flange is approximately 10.4 cm. Electrical leads from the ion sources and lens system attach to a mounting plate and are routed to a header block shown at the left hand side.

Figure 3b. The INMS engineering model sensor and electronics assembly with two side panels and top panel removed. The electronics box height is about 19.1 cm, the length about 32.4 cm and the width about 22.9 cm.

Figure 4. Relative transmission of the quadrupole switching lens⁹ as a function of the increasing ion energy and the difference in potential between the rod segments.

Figure 5. INMS Electrical Schematic.

Figure 6. Closed source response as a function of angle of attack. MW=molecular weight.

Figure 7. Response of the neutral open source as a function of the angle of attack.

Figure 8. Effect of applying a retarding potential to neutral ions.

Figure 9. The angular response¹⁷ to ions, masses 1, 28, 75 at energies 0.187, 5.224, 12.993 eV respectively, optimizing the transmission at 0_ and optimizing using the 4-segment collimator plates.