cassini orbiter ion andneutral mass -...

Cassini Orbiter Ion andNeutral Mass Spectrometer Instrument

Wayne Kasprzalç H. Niemann, D. Harpold, J. Richards, H. Manning, E. Patrick, P. MahaffyNASA/Goddard Spaceflight Center, Code 915, Greenbelt, MD 20771

ABSTRACT

The Cassini Orbiter Ion and Neutral Mass Spectrometer (INMS) is designed to measure the composition and densityvariations of(low energy) ions and neutral species in the upper atmosphere ofTitan, in the vicinity ofthe icy satellites andin the inner magnetosphere ofSaturn where densities are sufficiently high for measurement The sensor utilizes a dual radiofrequency quadmpole mass analyzer with a mass range of 1-99 ainu, two electron multipliers operated in pulse-counthgmode to cover the dynamic range required and two separate ion sources. A closed ion source measures non-surface reactiveneutral species which have thermally accommodated to the inlet walls such as N2 and C. An open ion source allows directbeaming ions or chemically active neutral species such as N and HCN to be measured without surface interaction. Theinstrument can alternate between these three different modes (closed neutral, open ion and open neutral). Characterization andcahl,ration of each ofthese three modes is done using a low energy ion beam, a neutral molecular beam and a neutral thermalgas source. An onboard flight computer is used to control instrument operating parameters in accordance with pre-programmed sequences and to package the telemetiy data. The sensor is sealed and maintained in a vacuum prior to launch toprovide a clean environment for measurement ofneutral species when it is opened to the ambient atmosphere after orbitinsertiolL The instrument is provided by NASA/Goddard Spaceflight Center, Code 915. Operation ofthe instrument anddata analysis will be canied out by a Science Team.

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

1. INTRODUCTION

The Cassini mission to Saturn consists ofan Orbiter provided by Jet Propulsion Laboratory (JPL) for the NationalAemnautical and Space Administration (NASA) and the Huygen's Titan probe, provided by the European Space Agency(ESA)'. The measurement objective ofthe 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 inTitan's upper atmosphere

. measure the in-situ composition oflow energy positive ions and neutrals in the environments ofthe icy satellites, ringsand the inner magnetosphere of Saturn, wherever densities are above the measurement threshold and ion energies arebelow —400 eV.

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

The Cassini Saturn Orbiter spacecraft will utilize the massive Titan moon for gravity assisted orbit changes. Duringthe four year nominal Cassini mission some 35-40 flyby's ofTitan are currently scheduled with the majority targeted for 950kin. There is a possibility ofhaving several passes re-targeted as low as 850 km ifthe INMS requires lower altitudes for datainterpretation. Titan is the only satellite known to possess a very dense atmosphere which is comprised mainly of N2 withpercent levels of CH12"°'5. Photodissociation of methane produces radicals that combine to form heavier hydrocarbons suchas C2H2 and C2H6; reactions between the hydrocarbons and atomic nilrogen (from N2) produce nitriles such as HCN andC2N2. Many trace level heavier hydrocarbons, nitriles (including HCN) as well as CO andCO2 have been observed in thestratosphere5 and these may also be present in the upper atmosphere along with the previously detected N2, CH4 and traces ofC2H210'. The homopause altitude is estimated to be 750-925 kin"° and the exobase near 1500 km so that 1NMS in-situneutral measurements will be made in the diffusion equilibrium and exosphere regions of the upper atmosphere. Correlation

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separate closed and open source inlet, rather than a singlecombined quasi-open ion source, is used to optimize the

CLOSED SOURCE interpretation ofthe neutral species; the open ion source isFILAMENT BLOCKS

necessaiy to measure ions. The Appendix lists theOPEN ION SOURCE QUADRUPOLE instrument parameters in Table 1 and modes of possible

DEFLECTOR/TRAP- SWITCHING . . .LENS operationm Table 2. The neutral gas density environment

. ION LENS duringa Titan flyby is nearly optimai for direct samplingFOCUSING withoutambient 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 thespacecraft's motion also increases the detection range forthese species. For gas species that are not surface reactivethis geometry provides an accurate means of measuring

DUALSECONDARY — ambientdensity. Surface reactive neutral gases, such as N,MULTIPLIER DETECTOR measured in the "open-source" inlet which utilizes theASSEMBLY

spacecraft's motion to form a neutral beam which issubsequently ionized, with no surface interaction. The

MOUNIIING ambient gas density is sampled directly with no stagnationenhancement Eleciron impact ionization is used to createions from neutrals. The entrance ion collimator also servesas a trap for elecirons and ions which could cause spurious

ionization ofneulral species. Ambient thermal and suprathennal ions are sampled by collimating them using the ioncollimator system ofthe "open-source" with the open source filament off. Open source ambient ions or ions created byelectron impact ionization are focused into an electrostatic deflector or quadrupole switching lens9. The switching lensmultiplexes ions from either the open or closed source into a single Radio Frequency (RF) quadrupole mass analyzer, whichseparates the ions according to their mass-to-charge ratio. The ions are detected by two secondary electron multipliersoperating in pulse-counthg mode to cover the dynamic range required. The potential for measurement ofthe heavierhydrocarbon species and possil,le pre-biotic cyclic hydrocarbons such as CH has resulted in an increase in the INMS massrange from 1-66 to 1-8 and 12-99 amu (atomic mass unite).

The SensOr and electronics are packaged in the form ofa box (Figure 2). The sensor entrance apertures are containedin a single plate which is covered by a metal-ceramic breakoffhat that is pyrotechnically activated. During cruise the breakoffhat is protected by thermal shielding. The INMS instrument is mounted on the Fields and Particle Platform (FPP). Thenormal to both the open and closed source INMS orifices point in the spacecraft -X direction. The field ofresponse of thetwo gas inlets are different (see Table 1). The geometric field ofview ofthe open source is limited to about 8° cone halfangle which limits the angular response ofneutral and ions measured with this inlet The closed source has a much widerfield of response, approximately 2t steradians. Venting ofthe ion sources occurs at right angles to the -Xaxis. This is tolower the ion source and analyzer pressures (increasing the ion mean free path) during a Titan pass when the spacecraft ram

approximately along the -Xdirection.The elecironics aremounted on boards which are parallel to the FPP platform.The sensor is made mainly of titanium and the electronicsbox of aluminum to be strong, yet lightweight. The boxforms an electrostatic shield for the instrument as well asproviding micrometeroid and high energy particleprotection. The secondary electron multipliers areadditionally shielded with tantalum. The spacecraftmultilayer insulation (ML1) is used for thermal control andmicrometeroid protection. It is attached to the brackets asshown in Figure 2. The thermal radiator panel is used todissipate 1NMS internally generated heat and is not coveredby the MLL The package design is such that the entranceapertures of the sensor protrude beyond the edge of the FPPplatform and MLI insulation. This is to prevent gas

Figure 2. 1NMS sensor with electronics.

SPIE Vol. 2803 / 131

CLOSED iON SOURCEANTECHAMBER

NOPEN

Figure 1. Cross section of the INMS sensor.

ORCUff


132 / SPIE VoL 2803

Figure 3b. The INMS engineingmodel sens and electronicsassemblywith two sidepanels andtop panel removed. The electronicsbox height is about 19.1 cm, thelength about.32.4 cm and thewidth about 22.9 cm

Figure 3a. Close-up of INMS ion source showing closedsource (top), open source (bottom) and switching lenscube. The diameter of the large flange is approximately10.4 cm. Electrical leads from the ion sources and lenssystem attach to a mounting plate and are routed to aheader block shown at the bottom left hand side.


contamination from the spacecraft and provide the maximum field of view.The mass specirometer is consiructed oftitanium and is free oforganic materials. It can be baked to 3500 C for

vacuum cleanup and will be maintained below 108hPaby a getter pump during the launch and cruise phases. A miniatureion gauge and a thermistor are used to monitor the internal sensor pressure. In a sealed configuration there is a residual gasatmosphere (primarily helium, argon, methane, carbon monoxide, carbon dioxide, hydrogen and water) remaining that can beused for sensor testing. Maintaining the sensor and ion sources under a vacuum keeps these regions clean until ambientneulral atmosphere measurements can be made. Opening ofthe sensor to the external environment by ejection ofthe breakoffhat is planned to occur after Saturn Orbit Insertion (SOl) prior to the ring plane crossing. The pyrotechnically activatedmetal-ceramic breakoffdevice has been used successfully for many instruments onEarth orbiting satellites as weli as for thePioneer Venus Orbiter and Galileo Probe mass spectrometers. For the inner satellites close to Saturn, calculations'8 indicatethat measurements oflow densities in these regions will be difficult due to the magnetosphere radiation background in thesecondary 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 1(1" particles/cm2/sec18.

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

2.a Operation

Electron impact ionization is used to ionize neutral species region because it offers high sensitivity. It has virtues ofbeing species non-specific and production ofunique fractionation patterns which aid in the identification ofmolecules. Thepresence ofdissociation products, however, can lead to problems in interference where ionization ofan abundant moleculemasks the presence ofa trace constituent. Much ofthis disadvantage can be surmounted by the use oftwo ionizationenergies so thatthe resulting different fractionation patterns can be exploited to minimize interference.

The closed ion source uses a spherical antechamber with an entrance oiifice for the ambient gas flux. A longcylindrical tube connects this antechamber to the entrance ofthe ionization region. Two redundant electron gun assembliesprovide a collimated electron beam used for ionization. The ions formed are focused into the quadrupole switching lens by aseries ofcylindrical eleefrostatic lenses. The incoming gas makes many collisions with the antecbamber surfaces andthermally accommodates to the wall temperatures. A ram enhancement is achieved by limiting the gas conductance from theantechamber into the ion source while maintaining a high particle flux into the entrance aperture.

The entrance aperture ofthe open ion source leads to an cylindrical antechamber which consists of4 plates in 4equally spaced segments. The ion collimator/trap can be used to trap ions and electrons during neutral measurements or beused to focus ambient ions into an exit aperture. The neutral beam is ionized by one oftwo redundant opposing electronguns and focused into the quadrupole switching lens. With the exception ofthe entrance plate, which is at spacecraft ground,the open source electrostatic lenses and the quadrupole switching lenses are programmable. This allows the lens system tomeasure both neutrals and ions in an optimized manner following the spacecraft equivalent energy. In neutral mode they canbe used to discriminate gas particles that have thermally accommodated to the ion source walls from the direct beamingcomponent at spacecraft energies. In ion mode they can be used to "steer" the incoming ions and perform a coarse energyscan.

The electron guns have filaments (0.0076 cm, 97% tungsten-3% Rhenium) in a coiled configuration, which areheated to provide an eleciron beam that is collimated and focused by electrostatic lenses. Two electron impact ionizationenergies are provided: 70 and 25 eV. The electron emission is 40 pa. The quadrupole switching lens9 uses four circular rodsectors mounted in a cube assembly to provide a nearly hyperbolic electrostatic field for a 900deflection. This device is usedto multiplex ions from either the closed source or the open source into the common entrance lens system of the quadrupolemass analyzer. The fractional energy transmission width, i.E/E, is 0.3 for a 0.3 cm entrance and exit aperture. The switchinglens potentials can be scanned (see Figure 4) to provide an estimate of the ion and neutral energy distribution up to about150 volts since the potential on each rod can range from 0 to -300 volts. Doubly charged species require a different potentialon the rod segments than do singly charged species. The quadrupole mass filter uses four precision ground hyperbolic rods

SP!E Vol. 2803 / 133


mounted in a mechathcal assembly. The rod spacing parameter ,ro, is 0.58 ciii and the rod length Is 10 cm. The quadrupolerods are excited by Radio Frequency (RF) and Direct Current (DC)potentials which together create a dynamic elecirostaticfield within the quadrupole region that controls the transmitted mass (mass/charge ratio), the resohition, and the transmissionefficiency. A mass scan is effected by v&ying the RF potential amplitude, V., to satisfy the relationship M =0.55 VJfwhere is in volts, f is the RF frequency in megahertz, and M is in atomic mass units (amu). The nominal mass range forthe INMS is 1-8 amu and 12-99 amu using two separate frequencies. The frequency, f is counted and is used to compensatefor drift The resolution is controlled over each mass range by programming the VdV. ratio to maintain the resolving poweras defined by a cmsstalk criteiion appropriate for that ss range. The resulting flaMopped peaks allow a mass scan mode inwhich each mass is monitored by a single step to achieve the lowest detection limit in a specified period. Another operatingmode reduces V to zero and creates a high pass filter giving the sum ofall masses greater than, for example, 90 smu. Theions exiting the qpadrupole are detected by two secondary electron multipliers, diffeiing in signal detection level by about afactor of2000. Charge poises at the anode ofthe multiplier are amplified and counted. The detection threshold is determinedby background noise in the multiplier (approximately 1 count per 100 seconds in the laboratory). The upper count rate ofeach detector system is about 10 mHz limited by the product ofthe multiplier pulse width and gain bandwidth ofthe pulseamplifier counter system. Ion counts above this value can be measured directly as an analog current Multiple sample periodscan be combined to lower the detection threshold to the backgmund noise level (signal/noise ratio =1). Assuming amaximum 1 mHz counting rate, the dynamic range ofthe two detector system is

Assuming the nominal neutral ion source SenSitiVity and a maximum count rate of 1 mHz yields an ion sourcedensity ofabout 3xlO cm for detector 1 and about 6x10" cm for the lower signal gain leveldetector 2. The lower limitonthe ion source density is about lOcm for 1 countper integrationperiod. Inthe mass range below about 50amu this isusually not realized in the closed source because ofbackground gases, identical to those being measured and emitted from thesmTounding surfaces or, for both sources, interference at some mass numbers ofother ambient gases present in highconcentrations. In the closed somte the calculated ambient density is lower than the ion source density due the velocity ramenhancement The maximum ion source density, ofabout 1012 -3 (1O hPa), is limited by mean free path conditions in theion source and analyzer regions. The Sensitivity is established by instrument tharacterization and varies with the species dueto different ionization efficiencies for neutrals in the ion sources, the transmission ofthe quadrupole switching lens and massfilter, and the conversion efficiency at the secondary electron multiplier.

The ion flux sensitivity is about iO particles/cm2/sec. For 1 count per integration period the minimum flux isabout 3x104 Using a spacecraft speed of6 km/sec (Titan flyby at 950kmaltitude) the corresponding density is0.05 cies/cm3, assuming that the spacecraft potential does not interfere with the ion measurement The sensitivity isestablished by sensor characterization and will vary with the ion species due to spacecraft equivalent energies for the differentspecies masses, the transmission ofthe quadrupole switching lens aixi filter, and conversion efficiency at the secondaryelectron multiplier detector. The spacecraft equivalent energy at zero degrees angle ofaliack and 6km/sec speed is 0.188eWamu. The effect ofspacecraft potential is modify the incoming flux and modify the ion trajectory directkms relative to the

In order for the INMS to make valid ion and0.9 neutral density measurements, the spacecraft velocity vector0.8 phis any atmosphere or ionosphere drift velocity must be

: 0.7 within the field ofresponse ofthe appropriate source.0.6 During Titan flybys the spacecraft can operate either in

:. °.s RadarMode with the radar tracking local nadir or in INMS0.4 mode with the spacecraft velocity vector tracking the INMS0.3 axis. In Radar Mode the INMS angle ofattack ranges from

: 0.2 0 to 8° within 2.4 minutes ofclosest approach at aI.' spacecraft speed of 6 km/sec. The altitude change is 60 km

over a spacecraft track length of 870 km.

Ion EnergyLb Electrical

Figure 4. Relative transmission of the quadrupole switchinglens9 as a function of the incoming ion energy and the The electronics system of the INMS is based ondifference in potential between the rod segments. designs used for the Huygen's Probe GCMS instrument A

134 I SPIE Vol. 2803

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block diagram ofthe electronicsis shown in Figure 5. The lowvoltage (LV) power supplyconverts the spacecraft power towell regulated DC voltagessupplied to the instrumentelectronics. A pulse widthmodulated converter allowsefficient generation of multiplesecondary voltages whileproviding secondary-to-piimaryisolation. A large number ofvoltages are required for biasingthe various focus electrodes aswell as to supply DC voltagesfor the secondary electronmultipliers. Analog modules areused for regulating the emissionofthe electron guns, forproviding fixed andprogrammable voltages to setlens potentials, for supplying RFand DC for the quadrupole mass

analyzer, for supplying high voltages for the detectors, and for the pulse counting circuits. The digital electronics includes asingle microprocessor, the spacecraft bus interface cfrcuit and interfaces between the CPU and the analog modules. Majorportions ofthe electronics are packaged in hybrid circuits to save weight and space.

The radio frequency generator drives the quadrupole attwo resonantfrequencies in order to reduce the need for alarge amplitude potential for the mass range (1-99 amu) required. Frequency selection is performed by a solid-state switchedbandpass 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 ofthe electron multiplier are converted by the pulse amplifier into voltage pulses whichare counted ifthey are above a preset threshold. An analog measurement ofthe multiplier current is also provided which isused to determine the inilight multiplier gain.

. The instrument computer (MA31750) controls the measurement sequence, counts the detector pulses, provides theanalog current to digital conversion ofthe detector current and monitors instrument housekeeping parameters. The computeris programmed in ADA as the target language with some use ofassembly language to handle time critical functions,input/output and interrupts. The instrument PROM/RAM will also contain the default measurement and test sequenceswithout requiring memoiy uploads.

Data from the INMS to the spacecraft consists of: 1) housekeeping packets which contain normal analog-to-digitalconverter channel data or memory dump data if science packets are not being collected; and 2) science packets which containeither 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 spacecraftvelocity vector) is given in Figure 6. Particles entering through the orifice collide many times with the surfaces of the ionsource electrodes and enclosure, and thermally accommodate to the surface temperature before leaving again through theorifice. For angles of attack less than 900this results in an enhancement of the number density in the antechamber over thatin the ambient atmosphere which varies approximately with the cosine of the angle of attack. The number density in theantechamber is determined by a balance of the incoming number flux at the spacecraft speed and the outgoing gas flux at the

SPIE Vol. 2803 / 135

BsksbIeVsciiuA.aibly

Figure5. INMS Electrical Schematic.


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

Figure 7. Response of the neutral opeu source as afunction of the angle of attack.

( I •1

Figure 8. Effect of applying a retarding potential toneutral ions.

antechamber wall temperature. The relationship between the ionsource density, incoming flux and ambient density is predictablefrom kinetic theory, and will be verified by laboratorycharacterization ofthe instrument Non-reactive species (e.g. N2 andC) wifl be measured in this mode. For the closed source, ambientdensity measurements can be made from about 0 to 90° angle ofattack depending on the gas background. Certain reactive speciesmay 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 thespacecraft with respect to the ambient atmosphere is used todisciiminate between fast and slow moving ionized neutral particleswhen the ion source is approximately pointed in the direction ofmotion. The quasi-open source geometry allows ambient gas toenter the ionization region directly and permits measurements ofchemically active species. The angular response is limited by thegeometric field ofview to a cone ofabout 8° halfangle. Thetheoretical response ofthe open source as a function ofangle ofattack are given in Figure 7. The ambient particle density ismeasured directly by this methocL Successful measurements of theopen source neutral ambient density requires discrimination of thethermally accommodated gas from the direct beaming componentby: 1) reducing the velocity ram stagnation density enhancement bymarimizing the gas conductance from the ioninig region into thevent region which is at right angles to the open source axis; 2)applying a slight retarding potential (Figure 8) to the ions after theyleave the ionization reglon and 3) using the quadrupole switchinglens fransmission (Figure 4) which is a function ofenergy to filterthe ion energy. Numerical studies'7 using only (3) indiCate that lessthan 2% ofthe 28 amu ions from thermally accommodated gas aretransmitted to the mass analyzer when the switching lens potential isset to transmit a 28 amu ionized neutral beam at spacecraft energies(6 km/sec spacecraft speed is equivalent to 5.22 eV compared to0.015 eV for a thermally accommodated gas at 300 K temperature).This ratio will be determined during sensor characterization. Theopen source neutral mode measurements will be confined to closestapproach where the density is sufficiently large and the angle of

.5 attack is approximately within the geometric view cone.

The open source ion mode response has been studied bynumerical simulation using the SamoffBEAM 3D'7 software tomodel the lens system from the entrance aperture to the entrance ofthe quadrupole mass analyzer. An example ofthe angular response isgiven in Figure 9 (left hand scale) for spacecraft equivalent ionenergies and no thermal energy spread. ffthe transmission isoptimized only at O angle ofattack then halfWidth half maximum(HWHM) point is about 3° for 28 amu.. Optiiithing the transmissionby adjusting the potential on paired segments of the collimator ateach angle of attack results in a HWHM of about 15° for 28 amu.The difference in potential between pairs of plates needed to optimizethe transmission is approximately linear with increasing angle ofattack (Figure 9 right hand scale). For ion mode the advantage ofbeing able to program the collimator plate voltages to increase the

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0.5

136 1 SP1E Vol. 2803


angular response beyond the geomeiric view cone can beimportant for increasing the altitude range ofcoverage forthermal ions dining a Titan pass. For example, optimizationofthe angle coverage for N2 results m either the inbound oroutbound leg ofthe pass being about 4.7 minutes in lengthwith an altitude change ofabout 230 km and a spacecrafttrack length of 1700 km for conservative 15° angle of attack.For a 950 km closest approach altitude to Titan, this issufficient to permit measurements over the ionosphericpeak8 near 1 100 km Suprathennal ions can also bemeasured up to about 100 eV. The energy resolution of thequadrupole switching lens is coarse and ion direction canonly be inferred by simultaneous scans ofthe collimatorplates. The suprathermal ion mass to charge ratio can beuniquely determined. This will aid the interpretation of lowenergy ion data obtained by insiruments doing energy-per-charge scans using the spacecraft velocity which havedifficulty in resolving closely spaced mass groups.

Figure 9. The angular response'7 to ions, masses 1, 28, 75atenergies 0.187, 5.224, 12.993 eV respectively, optimizing 2.d Instfllfllent Modes

the lransmission at O and optimizing using the 4-segmentcollimator plates.

the measurements in terms ofa pulse counter output is doneduring sensor characterization of each of the three instrumentoperating modes. The three modes are mirrored in the singlevacuum station (Figure 10) which incorporates: 1) thermalgas source for characterization of the closed source modesensitivity and mass spectral fragmentation patterns; 2)neutral beam for characterization of the open source neutralbeam flux response; and 3)an ion beam source forcharacterization of the ion mode flux response up to about150 eV. Two ion beam systems are available: an alkalimetal ion source and Colutron plasma source. Theinstrument is attached externally by a flexible bellows withtwo degrees of rotational freedom for angular responsecharacterization up to about 250. The instrument can betranslated to place each of the open and closed sources at theproper center of rotation. The all metal system is pumpedwith oil free turbomolecular and backing pumps. In additionto the characterization of the flight unit, the backupengineering unit will be cross-referenced to the flight unitfor future investigation of new species data or otherinstrument uncertainties as the flight data is collected andanalyze

=0

EU)=LU

=U-=0LU>LULU

o 5 10 15 20 25 30 35Angle of Attack (Q)

The function of the microprocessor is to providethe neutral/ion measurement sequence along with the massnumbers (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 inwhich the mass is sequentially is stepped from 1 to 99 (or some subset) in unit or 1/8 amu steps. A high pass mode isimplemented to allow measurement of the total signal above a given mass number. Table 2 (Appendix) contains an exampleof modes of operation. The electron energy can also be changed between scans along with filament on and ofL

2.e Sensor Characterization

An overall check of the instrument operation starting with a neutral/ion density or flux and ending with results of

HIGH VACUUM TEST STATION

ION BEAM

MOLECUR IFLOW I

CAPILLARIES

NEUTRhL BE

25 DEGREE SUPERSONIC BEAMANGULAR 5 iONSROTATION _______________

METASTABLE EXCiTATIONSOURCE

MOVABLE RPANEUTRAL GAS

QUADAUPOLE ION" /Au'Au METAL ION I TRANSFER STANDARD

1'

DEFLECTOR / SOURCEMOLECULAR DRAG

FIXED RPA PROBE MOVABLE METASTABLEDETECTOR

GAUGE

Figure 10. The INMS test system. The static (thermal) gas,neutral beam and ion characterization are carried out on onevacuum station.

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A staticpressure dividing technique is employed to characterizethe closed source forthe Chemically inactivespeciessuchas N2, H2, CH, C2H2, C2H., CO,He, Ne, Ar, Kr, Xe, CO and HCN eitheras single gases or pre-determined gas mixtures. During calibration, the gas flows through a capillazy leak operating in the molecular flow regimeand is pumped out ofthe test chamber through a small diameter orifice. Chamber pressures above 1O hPa are referenced toa molecular drag gauge, and lower pressures are determined by exirapolalion using abigh pressure capacitance manometer.AbsOlUte sensitivities will be established for each electron energy along with fragmentation patterns which aliow interferencecorrections to be applied to ifight data.

A neulralmolecular beam is used to establish the characteristics ofthe open source operating in neutral mode. Thesystem will provide beam speeds from 1 km/sec to about 5kmisec (nominal spacecraft speed at periapsis is 6 km/see) by asupersonic expansion ofa high pressure heated gas (H2) seeded with calibration gas species (He, Ne, N2, Ar, Kr). The neutralbeam will establish the angu]ar characteristics ofthe open source and its sensitivity relative to the closed source. Open sourcemeasurements during fight can be compared to closed source measurements ofthe same species for a cross check ofthe opensource response. An empirical relationship between the two responses will be determined as a function ofspecies mass,velocity and angle ofattack The beam speed is measured with a metastable time offlight detector.

An ion beam system is used to characterize the open source ion mode flux response. The ion beam is generated froma plasma discharge, accelerated through a potential difference, passed through a Wein filter to select the ion mass of interestand then decelerated down to energies ofinterest (1-100 eV). The ions are deflected thmugh a quadrupole switching leus. Analternative method is to use a direct in-Line alkali metal ion source. The ion beam flux into the instrument is determined witha separate retarding potential analyzer (ItPA) detector. The ion beam will also be used to characterize the INMS quadrupoleswitching lens and ion trap response as a function ofthe potentials used. Possible species used include H, H, He, Li,N24, Ne, K, Ark, + Xe'.

3. ACKNOWLEDGMENTS

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. OtherGSFC personnel involved are: digital electronics design and programming (P.. Frost, F. Tan, M. Paulkovich, I. Stuart);sensor design and assembly (S. Dixit, H. Powers, R. Arvey, It Abell); sensor testing (C. Carison, E. Raaen); and sensorcharacterization (H. Mnniig, W. Kasprzalç E. Patrick). Analog electronics are designed and built by the University ofMichigan (G. Carignan, B. Block, K. Arnett, J. Maurer). 3-Dimensional ion trajectoiy simulations were performed by V.Swaminathan ofPrinceton Electronics Systems, and R. Aug and W. Murray ofDavid SamoffResearch Center as part of aNASA contract NAS5-32823 (P. Mabaffy, NASA Technical Representative).

The INMS is supplied to Jet Propulsion Laboratory (JPL) as a facility instrument on the Cassini Orbiter. TheInstrument Engineer is D. Boyd (JPL) and the Investigation Scientist is V. G. Anicich (JPL). The INMS instrument will beoperated after launch by a Facility Team. The facility Team Leader is J. H. Waite, Jr (Southwest Research Institute). TheTeam Members are T. E. Cravens (University ofKansas), W.-H. Ip (Max-Planck-lnstitute fur Aeronomie), W. T. Kasprzak(NASA/GOddard Spaceflight Center), I. G. Lubmaim (Uthversity of California, Berkeley), R. L. Mc Nutt Jr. (John HopkinsUniversity) and It Yelle (Boston University). The team is responsible for the instrument commanding, data processing andscience interpretation. The team has been involved in other aspects of INMS support: spacecraft attitude thrustercontamination; increase of the sensor maximum mass value from 66 to 99 amu addition of tantalum shielding to lower thesecondary electron multiplier magnetosphere radiation backgronad; determination of the form of the operational commandingsoftware; and orbital tour studies emphasi7ing Titan passes with optimum 1NMS pointing and low altitude sampling.

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4. APPENDIX

Table 1. Cassini Inn nd Neutral Mass Snectrometer (INMS) Parameter SummaryNeutral Gas Sampling Systems 1) Open source (molecular beaming) with energy discrunmati 2) Closed sourceIon Sampling System Thermal and suprathermal positive ions

Sample System Switching Electrostalic quadrupole deflector

Viewing Angle (Angle ofResponse)

1) Open Source cone halfangle; 2) Closed Source '-2ic steradians;3) Exhaust vent -2ir steradians

Neutral Mode Ion Sources Electron impact ionization electron energy (nominal): 25 eV and 70 eVMass Analyzer Quadrupole mass filter, 0.5 cm field radius, 10 cm rod length

Radio Frequencies: 1.77 mHz and 3.65 mHzMass Range 1 to 8; 12 to 99 amu nominal; high pass filter modeScan Modes 1) Survey: scan mass range in 1/8 or 1 amu steps; 2) Adaptive Mode: select mass valuesResolution/Crosstalk 1O for adjacent massesDetector System . Two secondary eleciron multiplier detectors operating in pulsecounting mode

(detector noise < 1 count/minute in laboratory)Dynamic range oftwo detector system for 1 integration period sample ' 108

Sensor Sensitivity 1) Ion flux sensitivity i0 (counts/sec)/(ions/cm2/sec), maximum energy 100 eV2) Neulral mode sensitivity 2.5x103 (counts/sec(pathcle/cm3) (dosed and open source)

Density/flux for 1 count perintegration period,no background

Minimum neutral ion source density (both sources) = 1.2xl04particles/cm3Maximum neutral ion source density P4012 particles/cm3Maximum closed source ram enhancement factor for N2 @6km/sec =50Minimum ion flux = 3.2x104 ion/cm2/sec

Data Rate Sample integration period =31.1 ms; total sample period 34.0 msSpatial resolution 200 meters along spacecraft track per sample periodInsirument conirol Microprocessor: MA3 1750 RAM: 128 Kbytcs PROM: 128 KbytesTelemeliy Science data rate 1498 bps; Housekeeping data rate 12 bps

Reduced science packet production mode implementedDeployment Mechanism Metal ceramic breakoffcap, pyrotechnically activated

Power (Current Best Estimate) Neutral Mode: average 233 W Ion Mode: average 20.9 WSleep: average 13.1 watts Off: 4 watts replacement heater

Size Maximum envelope (cruise): Height 20.3 cm (8.0"), Length 42.2 cm (16.6"), Width36.5 cm (14.4")

Weight (Current Best Estimate) 9.25 kg + 1.4kg for tantalum radiation shield 0.23 cm (0.090") thick

Table 2. Possible Modes of INMS Operation*

TYPE LENGTH (sec) MASS SELECTIONSOURCE__J__ION/NEUTRAL

0 2.3 68 Programmed masses Closed Neutral

1 2.3 68 Programmed masses Open Ion2 ' 2.3 68 Programmed masses Open Neutral

3 2.3 Unit aniu sweep 68 steps Closed Neutral4 2.3 Unit amu sweep 68 steps Open Ion5 2.3 Unit amu sweep 68 steps Open Neutral6 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. Instrumentprogramming flexibility allows a fraction ofType 1 to be followed by a fraction ofType 2 etc. Switching between the opensource neutral and ion modes involves turning the filament on and off, and is likely to be done only on a much broader timescale, on the order ofseveral tens of seconds.

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5. REFERENCES

1. C. Kobihase, "Meeting with a majestic giant The Cassini Mission to Saturn," The Planetcay Report,VoL XII, No. 4, pp. 4-11, The Planetary Society, July/August 1993.

2. J-P. Lebreton, 'The Cassini Mission to Saturn and Titan, an overview," European Space Agency, ESA SP-.241, pp. 225-232, December 1985.

3. Current information on the Cassini mission with its study of Saturn, rings, magnetosphere, icy satellites and Titan isavailable on the World Wide Web at address hftp//www.jpLnasa.gov/cassini/.

4. The Atmospheres ofSaturn and Than, European Space Agency, ESA SP-241, December 1985.5. D. M. Hunten, M. G. Tomasko, F. M. Raser R. E. Samuelson, D. F. Strobel and D. J. Stevenson, "Titan," Saturn, ed.

T. Gehrels and M. S. Matthews, University ofArizona Press, pp. 671-759, 1984.6. F. M. Neubauer, D. A. Gumett, J. D. Scudder and R. E. Hartle, "Titan's inagnetospheric interaction,"

Saturn, ed. T. Gebrels and M. S. Matthews, University ofArizona Press, pp. 760-787, 1984.7. D. Toublanc, J. P. Parisot, I. BI-iIICt, D. Gautier, F. Raulin, and C. P. Mc Kay, "Photochemical modeling of Titan's

atmosphere," ICARUS, vol. 1 13, pp. 2-26, 1995.8. C. N. Keller, T. E. Cravens, and L. Gan, "A model ofthe ionosphere ofTitan," I Geophys. Res., voL 97, No. A8, pp.

12117-12135, August 1, 1992.9. P. B.. Mabaffy and K. Lai, "An electrostatic quadrupole deflector for mass spcc1imeter applications," J. Vac. Sd., vol. A

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atmosphere: Composition and temperature from the EUV solar occultation results," J. Geophys. Res., voL 87, No. A3,pp. 1351-1359, 1982.

11. IL V. Yelle, "Non-LTE models ofTitan's upper atmosphere," The Astrophysicai Journal, voL 383, pp. 380-400, Dec.1991.

12. A. L. Broadfoot et aL, "Extreme ultraviolet observations from Voyager 1 encounter with Saturn," Science, vol. 212, pp.206-211, April 10, 1981.

13. N. F. Ness, M H. Acuna, K. W. Bebannon and F. M. Neubauer, "The induced magnetosphere ofTitan," J. Geophys.Res., vol. 87, pp. 1369-1381, 1982.

14. It E. Hartle, Ct aL, "Titan's ion exosphere observed from Voyager 1," J. Geopliys. Res., voL 87, pp. 1383-1394, 1982.15. R. L. Mc Nutt, Jr. and J. D. Richardson, "Constraints on Titan's Ionosphere," Geophys. Res. Lett., voL 15, No. 7, pp.

709-712, July 1988.16. G. F. Linda], et a!., 'Ilie atmosphere ofTitan: An analysis ofthe Voyager 1 radio occultation measurements," Icarus,

voL 53, pp. 348-363, 1983.17. V. Swaminathan, Princeton Eleclromcs Systems, and R. Aug and W. Murray, David Sarnoff Research Center, "Design

of an Improved Miniature Ion Nentral Mass Spectrometer for NASA Applications," NASA Contract NAS5-32823, 1996.18. R. Mc Nutt, Applied Physics Laboratory, John Hopkins University, Private communication, 1996.19. D. E. Shemansky, P. Matheson, D. T. Hall, H.-Y. Hu, and T. M. Tripp, "Detection of the hydroxyl radical in the

Saturn magetosphere," Nature, vol. 363, pp. 329-33 1, 1993.20. D. T. Hall, P. D. Feldman, J. B. Holberg, and M. A. Mc Grath, "Fluorescent hydroxyl emissions from Saturn's ring

atmosphere," Science, vol. 272, pp. 516-518, 1996.21. W.-H. Ip, "Titan's Hydrogen Torus," European Space Agency, ESA SP-241,pp. 129-141, December 1985.

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