Instrument Overview:
    ====================
    The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis
    (ROSINA) consists of two mass-spectrometers for neutrals and
    primary ions with complementary capabilities and a pressure sensor
    . ROSINA will answer important questions posed by the mission
    main objectives. To accomplish the very demanding objectives
    through all the different phases of the comet   activity, ROSINA
    has unprecedented capabilities, including very wide mass range
    (1 amu to >300 amu); very high mass resolution (m/dm >3000, i.e.
    the ability to resolve CO from N2 and 13C from 12CH); very wide
    dynamic range and high sensitivity; as well as the ability to
    determine cometary gas velocities, and temperature.
 
    Scientific Objectives:
    ======================
    The spectrometer primary objective is to determine the elemental
    , isotopic and molecular composition of the comet   atmosphere and
    ionosphere, as well as the temperature and bulk velocity of the
    gas and the homogenous and inhomogeneous reactions of the gas and
    ions in the dusty cometary atmosphere and ionosphere.
    In determining the composition of the atmosphere and ionosphere,
    the following prime scientific objectives, also set by the Rosetta
    Science Definition Team, will be achieved:
    -   Determination of the global molecular, elemental, and isotopic
        composition and the physical, chemical and morphological
        character of the cometary nucleus.
    -   Determination of the processes by which the dusty cometary
        atmosphere and ionosphere are formed, and characterization of
        their dynamics as a function of time, heliocentric and
        cometocentric position.
    -   Investigation of the origin of comets, the relationship
        between cometary and interstellar material and the
        implications for theories on the origin of the Solar System.
    -   Investigation of possible asteroid outgassing and establishing
        the relationships between comets and asteroids.
 
 
 
    SENSORS:
    ==========
 
    DFMS
    ====
    DFMS (double focusing mass spectrometer) is a state of the art
    high resolution Mattauch - Herzog mass spectrometer
    (resolution m/Dm > 3000 at 1% peak height) with a high dynamic
    range of 10^10 and a good sensitivity of 10^-5 A/mbar. It is based
    on well proven design concepts which were optimized for mass
    resolution and dynamic range using modern methods for calculating
    ion optical properties. The DFMS has two basic operation modes: a
    gas mode for analyzing cometary gases and an ion mode for
    measuring cometary ions. Switching between the gas and ion modes
    requires a change of only a few potentials in the ion source and
    suppression of the electron emission that is used to ionize the
    gas. All other operations are identical in the two modes.
 
    Ion Optics
    ----------
    Ion source: The design of the ion source is based on the electron
    bombardment source used in modern laboratory rare gas mass
    spectrometers. This source combines high sensitivity
    (10^-3 A/mbar) with good linearity over a very wide gas pressure
    range (from several 10^-5 mbar to below 10^-14 mbar), small energy
    dispersion and low background.
    The source has two viewing directions with different field of
    views (FOV).
 
    The one parallel to the source axis has a wide FOV of +/-20DEG,
    the one orthogonal to it a narrow FOV of +/-2DEG. Most of the
    measurements will use the wide FOV, allowing cometary gas with
    wide angular spread in the flow direction to enter the ionization
    region. The narrow FOV will be used for determining the exact flow
    direction of the cometary gas jets. The axis of the wide FOV is
    parallel to the axis of the cameras, and normally directed towards
    the nucleus.The FOVs are determined by a set of electrodes
    upstream of the ionization region. Suitable potentials
    applied to these electrodes prevent the entry of low energy
    ambient ions into the DFMS operating in the gas mode. Cometary
    ions with higher energies
    (>60 eV) cannot pass through the analyzer and it is not necessary
    to prevent their entry into the ion source. In the ion mode the
    potentials on these electrodes are changed to attract the cometary
    ions even in case of positive charging of the S/C and to focus
    them into the gas ionization region of the source. A negatively
    biased coarse meshed grid surrounding the ion source area up to a
    distance of 15 cm is used to augment the ion sensitivity.
    The instrument outgassing could lead to serious interference while
    measuring the cometary gases. To keep the interference as low as
    possible the entire ion source region is built to UHV standards
    and degassed before launch and also during flight. Since the
    narrow analyzer entrance slit has a very low vacuum conductance
    (the only connection between the source and analyzer regions)
    outgassing from internal sensor parts is efficiently suppressed.
    The cometary gases entering the source are ionized with an
    electron beam parallel to the slit direction. A weak magnetic
    field (0.02T) is used to guide the electrons. Two filaments are
    provided to give redundancy. The electron energy can be varied
    between about 10 and 90 eV. At higher electron energies (>60 eV)
    the ionization cross section is maximal and hence the instrument
    sensitivity at its optimum. At low electron energies
    the cross section is reduced but there is much less fragmentation
    of the more complex molecules. This can be used to facilitate the
    identification of unknown species. The ion source can be operated
    with electron currents of 2 uA, 20 uA or 200 uA to provide three
    sensitivity levels which differ by a factor of 10. By means of a
    small gas tube calibrated amounts of a noble gas mixture can be
    leaked into the ionization region and will be used for in-flight
    tests and calibration of the DFMS.
    A mass scan is achieved by varying the ion energy. To minimize
    mass and sensitivity discrimination the ion source is operated at
    a fixed acceleration potential of 1 kV. After the first focus
    point (line width typical 150 um) a transfer lens is used to
    accelerate and/or decelerate and focus the ions onto the entrance
    slits of the analyzer section. Two entrance slits are used, a
    narrow slit (14 um) and a wide slit (200 um).
    The ion beam can be guided through the narrow slit in the high
    resolution mode or through the wide slit in the low resolution
    mode by electrostatic deflection. The axis of the transfer lens
    is tilted by 6 degrees relative to the ion source axis to protect
    the narrow entrance slit of the analyzer from cometary dust
    particles.
    The final ion energy is established in the transfer section of the
    ion source. To pass through the analyzer with its fixed magnetic
    field the ion energy must be changed from 6 keV at mass 12 amu to
    430 eV at 140 amu.
    Thus, the 1 keV ions from the source are either accelerated or
    decelerated in this section and at the same time focused on the
    entrance slit of the analyzer.
    The mass analyser: For the Rosetta DFMS the following key
    requirements were considered to optimize the analyzer geometry:
    -   Mass resolution m/dm > 3000 for a mass range 12 to 150 amu/q
        at the 1 % peak level
    -   Good energy focusing properties to allow dE/E up to 1%,
        (important if lower ion energies are used).
    -   High mass dispersion to allow the use of a position-sensitive
        focal
        plane detector.
    -   A large free viewing angle (preferably 2 pi) for the ion
        source
        acceptance.
    -   A small overall analyzer size and a radius of curvature in the
        magnet below 10 cm.
    The resulting optimal field geometry is a combination of a 90
    degree toroidal electrostatic analyzer (ESA) with a 60? sector
    magnet for momentum analyses. High mass dispersion can be achieved
    by using an electrostatic zoom lens system.
    At the high mass resolutions the detector and focal plane coincide
    only at one specific mass number either in the center of the
    multichannel plate (MCP) or at the channel electron multiplier
    (CEM) detector. High resolution can thus only be obtained for the
    mass multiplets at one mass number and the mass lines from
    neighbouring mass numbers will show less mass resolution.
    To obtain a full high resolution mass spectrum from 12 to 150
    amu/q it is thus necessary to record a mass spectrum at each
    integer mass number.
    The analyzer can also be operated in a low resolution mode which
    allows the simultaneous recording of several mass lines on the
    position-sensitive detector with a resolution of m/?m of several
    hundred. Neighboring integer mass numbers are well separated at
    this mass resolution. In this mode the zoom system is used to
    rotate the focal plane into the plane of the position-sensitive
    detector.
 
    Detectors:
    ----------
    The instrument has three independent ion detectors.
    The main imaging detector is located in the center of the detector
     package,
    as indicated by the position of the two Chevron MCP's. The Chevron
    MCP's with a rectangular form were adapted to the geometry of the
    focal plane.
    Its pore size is 6 um, the inclination of its tubes 13 degrees
    and the maximum total gain at saturation is about 10^6. In order
    to keep the maximum resolution the MCP front face should have been
    located exactly at the focal plane. However, the energy of the
    ions collected on the front face of the MCP should be larger than
    ~1 keV in order to guarantee a large enough MCP detection
    efficiency. For this reason, the front face of the MCP can be
    biased up to a negative post-acceleration voltage of -3 kV.
    In order to prevent large perturbations of the ion trajectories,
    which would totally deteriorate the focusing properties of the
    spectrometer, the MCP must be approximately perpendicular to the
    average ion trajectories.
    Extensive numerical modeling has shown that with such geometry the
    global resolution of the instrument is adequate and reaches the
    specified value. The CEM is located at the upper left part of the
    detector package. A 20 um wide slit is positioned about 1 cm ahead
    of it and coincides with the location of the end of the focal
    plane. At the same time it prevents the high voltage on th CEM
    entrance to leak out and affect ion trajectories in the drift
    space before the focal plane. The CEM may be operated in a
    counting and an analog mode.
    The Faraday cup (FC) has a 0.35 mm wide slit in front of the cup
    and coincident with the right end of the focal plane. It provides
    the capability of absolute instrument and detector calibration and
    the medium resolution measurements of the water peak in a current
    range 10^-14 to 10^-8 A.
 
    Electronics:
    =-----------
    The ROSINA DFMS electronics described here provides power and
    controls the cover mechanism, the ion source and GCU, all elements
    of the ion optics, and the detectors. The instrument control is
    provided through an interface with the ROSINA DPU.
    Commanding and acquiring of housekeeping and science data is done
    by the ROSINA DPU. The DFMS electronics does not need to store
    data or commands. The ion source is protected by its cover. Once
    the vacuum seal is broken after launch, it can be opened and
    closed and placed in intermediate positions. This capability is
    required to protect the instrument from contamination (for example
    from very high pressures near the comet) and it provides a shutter
    , which can be partially closed, blocking the cometary ion and
    neutral influx. This second feature will allow inflight
    calibration and a determination of the residual gas in the
    spectrometer.
    The cover motor and the ion source are on spacecraft ground
    potential.
    The ion source contains two filaments (for redundancy), which are
    powered by the ion source controller. The ion source controller
    regulates the current to the filaments and also receives
    housekeeping information on the filament current and temperature
    in the vicinity of the filament.
    Starting at the entrance to the ion source, there is one ion
    source voltage commandable from 0 to +/-300 V with 12 bits
    accuracy. This voltage repels the ions coming from the comet.
    After the ion suppression grid, two power supplies provide
    voltages to prevent ions created in the ion source from escaping
    back through the entrance aperture.  Another two power supplies
    provide the ionization box with potentials to accelerate the
    electrons from the filament. The ions formed in the ion source are
    extracted from the ionization region, accelerated, and sent
    through the transfer optics section using high voltages from
    additional five power supplies. Two of these power supplies in the
    transfer optics section require 0 to -2000 V with 16 bit accuracy.
    The accuracy of all power supplies in the ion source and ion
    optics is determined by the mass resolution requirements of the
    DFMS. After the transfer optics section, ions pass through a wide
    range of ion optical elements which ultimately focus a mass
    dispersed ion beam onto several possible detectors including a
    high resolution, position sensitive detector.
    Since the ion source potentials are referenced to spacecraft
    ground, the ion optics in the analyzer must float at a high
    voltage acceleration potential (Vacc). This floating acceleration
    potential is provided by a 14 bit 0 to -6500 V power supply.
    Because all the ion optical elements float at this high potential,
    they are also electrically isolated from the power supplies and
    instrument controllers that reside on ground.  Communication to
    and from these isolated power supplies is provided by a serial
    interface across several fiber optics channels.
    The power to these units is supplied across a high voltage
    transformer. The electrical controlled double slit system is
    powered by two 1000 V 12 bit power supplies located after the
    transfer optics.  Following a corrective lens element accomplished
    by a pair of plates biased at low voltage (0 to 50 V), the ions
    enter the electrostatic analyzer.
    This analyzer is biased from two 10 to 550 V 18 bit power supplies
    .This high accuracy is needed to select specific ion energies, to
    focus specific masses on the channel electron multiplier in the
    detector and to achieve a good peak shape in the CEM high
    resolution scan.
    The ion beam is corrected first by a Matsuda plate pair controlled
    by 0 to +/-110 V. After the passage through the magnet three
    0 +/- 50 V power supplies are used to bias three individual zoom
    lenses (one hexapole and two quadrupoles). The magnet is a static
    element in the ion path but the temperature is monitored by the
    DFMS electronics.
    In case the optical elements in this section are not active, the
    DFMS remains in the low mass resolution mode and the mass
    dispersed ion beam impinges on the detector according to the
    optical steering.
    When the four optical elements (powered by two 0 to +/-500 V 14
    bit and two 0 to +/-2000 V 14 bit supplies) are active, the DFMS
    is in the high mass resolution mode, and the ion beam that
    impinges on the chosen detector.
    Through a high voltage transformer interface, the DFMS electronics
    also provides high voltages to the CEM detector and its repeller
    grid, the repeller grid for the Faraday cup detector and to the
    front and back side of the MCP's.
    Detector electronics: The very large dynamic range is achieved
    with an analog detector system. In this system, charges are
    accumulated and/or currents are measured on a collector at the
    exit of the MCP.
    In this analog mode, the gain of the MCP can be varied over more
    than 6 orders of magnitude using an appropriate adjustment of the
    supply voltage, as used, for example, on the NMS instrument of the
    ESA Giotto mission.
    This gain control is added to the normal dynamic range of the
    collector electronics itself.
    In order to meet the resolution requirement of  ~25um a new ASIC
    chip, the LEDA512 (Linear Electron Detector Array) was developed.
    This chip integrates two identical but independent detector
    systems, each consisting of a collector under the form of a row of
    512 anodes (or pixels) collecting the electrons emitted from the
    back face of the MCP and of the associated charge integration
    electronics. With a pixel width of 22 um and a 3 um separation
    between neighboring pixels this collector has a width of 12.8 mm
    in the focal plane over a height of 8 mm and each mass peak covers
    approximately 6 pixels.
    The back face of the MCP is separated by a distance of 0.2 mm from
    the plane of the LEDA-collector. In the standard mode of operation
    electrons exiting from the MCP are accelerated by a potential of
    ~250 Volts, limiting the spreading of the space charge and
    maintaining the required resolution. Each individual pixel
    operates as a floating electrode of a capacitor with its second
    electrode at ground; the capacitance of a pixel is approximately
    4 pF. In the read-out sequence, which can be made as fast as 10 ms
    for the 512 pixels, each pixel is connected sequentially through
    an analog multiplexer to a charge amplifier. This amplifier
    provides at its output a pulse with an amplitude proportional to
    the amount of electron charge collected on each pixel. A 12 bit
    ADC then converts the pulse height into a 12 bit digital word
    stored in a spectrum accumulation register. This register is
    ultimately read out by the instrument DPU through an opto-coupler
    link.
    As a consequence of the accelerating voltage applied to the front
    face of the MCP and of the variable HV polarization between the
    front and the back faces of the MCP which controls its gain, the
    LEDA is at a  Floating Detector Package  potential (FDP) which can
    reach several kV with respect to the DFMS reference level.
    In order to avoid leakage currents problems on the Faraday cup and
    difficulties associated with two different high voltages in the
    electronics installed in the detector package, the Faraday cup
    electronics is polarized at the same floating voltage as the LEDA.
 
    Operational Modes
    -----------------
    The instrument has a large number of operational parameters which
    can be individually adjusted to fit any specific measurement
    requirements.
    However, a certain number of predetermined modes and measurement
    sequences are now implemented and it is expected that most
    measurements will use these modes.
    From time to time it will be necessary to retune voltages of the
    instrument to optimize the performance and to compensate for
    mechanical, thermal, etc. drifts which could occur in space.
    We expect that the basic retuning can be done autonomously, but
    some manual adjustments might still be necessary requiring
    extensive ground command sessions.
    For any given instrument setting we will use a basic integration
    time of approximately 20 s (MCP only).The accumulated spectra will
    be transferred to the DPU for further data processing.
    The adjustment of the instrument to a new setting, for instance a
    new value for the central mass, requires about 10 s.
    This includes the time necessary to optimize the detector gain.
    A full high-resolution mass spectrum from 12 to 150 amu/q can thus
    be recorded in 79x30 s = 2370 s = 40 min. A complete low
    resolution spectrum from 12 to 150 amu/q can be acquired in
    12x30 s = 600 s. Several 20 s spectra with the same settings will
    then be recorded either in sequence or cyclically and transferred
    one by one to the DPU. After statistical analysis, spectra
    recorded with identical settings will be added, compressed and
    transmitted as full mass spectra. This procedure optimizes the
    scientific data return from the instrument.
 
 
    Location:
    ---------
    DFMS is located on the +z platform close to the +y edge on
    the Rosetta spacecraft.
 
 
 
    Measured Parameters:
    --------------------
    The DFMS measures the ion current on the detector as a function of
    mass.
    Ions are either primary ions from the comet or ionized neutral gas
    .In both cases the geometric factor is a function of the mass, the
    detector efficency is a function of the ion(s) contributing to the
    peak and of the detector gain. In the neutral gas mode it is
    furthermore a function of th electron current and the ionization
    cross section. The mass itself is a function of the commanded
    voltages (acceleration, ESA voltages,zoom).
 
    RTOF
    ====
 
    The Reflectron Time-Of-Flight (RTOF) mass spectrometer was
    designed for an extended mass range and high sensitivity to
    complete the instrument requirements of the ROSINA package.
    TOF instruments have the inherent advantage that entire mass
    spectra are recorded at once, without the need of scanning the
    masses by varying some particular instrument parameter like the
    magnetic field. A storage ion source stores the continuously
    produced ions until their extraction into the TOF section.
    With high transmission into the TOF section and a sensitive
    detector, it is possible to record a very large fraction
    (>60% in the case of RTOF) of all ions produced in the ion source.
    These factors contribute to the overwhelming sensitivity of TOF
    instruments. Another reason to use TOF instruments in space
    science is their simple mechanical design (their performance
    depends on fast electronics rather than on mechanical tolerances)
    and easy operation.
    A time-of-flight spectrometer operates by simultaneous extraction
    of all ions from the ionization region into a drift space in form
    of short ion packets.
    The temporal spread of such an ion packet is compressed from about
    800 ns at the exit of the ionization region to about 3 ns at the
    first time focus plane (for mass = 28 amu/e) at the beginning of
    the drift section. These very short ion packets then pass the
    first leg of the drift section, the gridfree reflectron, and the
    second leg of the drift section until they arrive at the detector.
    Because different m/q packets drift with different velocities, the
    length of the drift section determines the temporal separation of
    ion packets of different m/q when arriving at the detector.
    If properly matched to the fieldfree drift section the ion mirror,
    i.e. the reflectron, establishes the isochronity of the
    ion-optical system, which means that the flight time of ions is
    independent of their initial energy. The mass resolution is
    determined by the total drift time and the temporal spread of the
    ion packets at the location of the detector, which is placed at
    the last time focus. Unlike other types of mass spectrometers, TOF
    spectrometers have no limit to the mass range. In practice, the
    mass range is limited by the size of the signal accumulation
    memory.
 
    Ion Optics
    ----------
    The RTOF sensor consists of five ion-optical components: the ion
    sources, the drift tube, the reflectron, the hard mirror and the
    detectors. The sensor includes two almost independent mass
    spectrometers in one common structure.
    The spectrometers share the reflectron and the hard mirror;
    however, the ion sources, the detectors, and the data acquisition
    systems are separate.
    For the analysis of cometary neutral particles there is the
    electron impact storage ion source with associated ion-optical
    elements and data acquisition (the storage-channel), and for the
    analysis of cometary ions there is the orthogonal extraction ion
    source with associated ion-optical elements and data acquisition
    (the ortho-channel). Both channels are optimised for their
    distinct purpose but have the feasibility to perform the other
    measurement as well. This configuration guarantees high
    reliability through almost complete redundancy.
 
    Detector:
    ---------
    Detecting single ions as well as ion bunches with up to 105 ions
    arriving within nanoseconds time requires a detector with high
    detection efficiency.
    Furthermore, the detector has to have the ability to linearly
    amplify the incoming particles over a wide dynamic range. In order
    to minimize the time spread of the ion bunches registered on the
    detector, sufficiently fast detectors with an internal time
    response for single-ion events of less than one nano second have
    to be used. A narrow time width not only improves the mass
    resolution but also increases the peak amplitude and therefore
    improves the signal-to-noise ratio. The geometry of the
    ion-optical system of RTOF limits the diameter of the ion beam to
    12 mm on the detector.
    For mass saving reasons the active area of the detector is
    therefore only 18 mm. Micro-channel plates (MCPs) of imaging
    quality have been selected for registering the ions.
 
    Gas Calibration Unit:
    ---------------------
    The Gas Calibration Unit (GCU) is used to inject a defined
    quantity of a known gas mixture (He, CO2, and Kr) into either the
    storage source or the orthogonal source. By feeding a source with
    a known gas mixture (with well known masses), the sensor
    parameters can be optimized, the detection efficiency can be
    calibrated, and the performance can be evaluated in flight.
    For the two ion sources two independent GCUs are implemented,
    which are controllable by remote commands. Both GCUs are
    accommodated in a common housing and mounted on one electronic
    board. Each GCU consist out of gas tank of 5 cm3 filled with about
    5 bars of the calibration gas mixture, a high pressure gauge, a
    valve, a low pressure gauge (mini pirani) and a capillary tube
    with a standard CAJON vacuum connection at the GCU exit.
    From there the gas is routed with regular gas tubing into the
    ionization region of the ion sources. All sub components were
    fabricated very clean to avoid any gas contamination. Leakage
    rates for all components and mounted capillary tube with closed
    valve are 10^-10 mbar l/s.
    The controllable leak rate using the low pressure gauge can be set
    between 10^-3 mbar l/s and 10^-1 mbar l/s. With this leak rate,
    the apparent pressure in the ionization region of ion sources is
    in the range from 10^-9 to 10^-6 mbar.
 
    RTOF electronics
    ----------------
    The entire electronics of the RTOF instrument consists of the
    following 9 functional blocks:
    . Main Controller (MC):
    The MC handles the commands coming from the DPU and the data and
    housekeeping going to the DPU. It contains the following blocks:
    -   Motor mechanism for the cover
    -   Backplane heater unit
    -   Gas calibration unit
    -   Gas Extraction Pulser, Ion Extraction Pulser, and Hard mirror
        pulser
    -   ETS and ETSL latch up disable
    -   Filament emission
    -   Differential serial interface to the DPU
    -   Gateway switches for ETS, ETSL, and Digital Board
    -   Housekeeping unit
    -   Power switching unit
    Equivalent Time Sampler (ETS)
    -   Data acquisition system for fast and non-repetitive signal
    pulses for the storage channel.
    Equivalent Time Sampler Light (ETSL)
    -   Data acquisition system for fast and repetitive pulses for the
    orthogonal channel.
    High Voltage Board #1 (HV#1)
    ?   High-voltage supplies for ion sources, hard mirror,
    acceleration, lens, reflectron and drift tube.
    High Voltage Board #2 (HV#2)
    -   Supply for extraction pulsers, detectors and hard mirror
    pulser voltages.
    Low Voltage Power Supply (LVPS)
    -   Supply for analog ?5V, dig. +5V, +8V, analog ?15V, +24V, +40V,
    +70V.
    Digital Board for power supplies
    -   Backplane, entrance lens and entrance supplies, controller for
    the supplies, HK and MC, temperature sensors.
    Filament Emission Controller (FEC)
    -   The FEC regulates the emission current of the storage and the
    orthogonal source filaments, for the main and the redundant
    filament sets. Ion, Gas and Hard Mirror Pulser
    -   The Ion and Gas pulsers perform the extraction with a negative
    pulse with a fast falling edge (tf < 5 ns) and a medium fast
    rising edge (tr ~100 ns). The amplitude is programmable.
    -   The Hard Mirror Pulser deflects charged particles before they
    hit the detector with a positive pulse from a positive Hard Mirror
    potential.
    Pulse width, delay from trigger and pulse amplitude are
    programmable.
    Equivalent Time Sampling (ETS) and the Equivalent Time Sampling
    Light (ETSL) system
    The ETS and the ETSL are the two data acquisition systems in the
    RTOF sensor. ETS is dedicated to the storage channel and ETSL to
    the orthogonal channel. Both data acquisition systems serve as
    time to digital converters (TDC); that is, whenever a signal
    exceeds a preset 3 bit programmable trigger level (10-100 mV), the
    time of this event is stored in a memory.
    However, because the ion density is much higher in the storage
    source due to the longer duty cycle leading to multiple ions
    arriving at the same time on the detector, ETS is additionally
    capable of converting the signal height into a digital value thus
    serving as fast ADC (analogue to digital converter) paired with
    its TDC function.
    Parameters like trigger level, TOF, etc. of the ETS and ETSL
    systems are serially commanded by the DPU. The data acquisition
    of events starts with a command. The systems generate then the
    periodic trigger for the source extraction pulsers. A 13bit
    (max. 217us) start delay time can be programmed prior to the
    start. Only after the elapsed time the data acquisition system
    starts accepting signals from the detector. This avoids
    overflow of the FIFO memory if there is a high event rate from the
    pulser crosstalk or for low masses. The maximum allowed time of
    flight ( ?? 217 ?s ) depends on the extraction frequency which can
    be selected between 1 and 10 kHz and is further limited by the
    size of the data FIFO (512 x 18bit) memory
    The ETS is a multiple ADC high-speed data acquisition system that
    is designed to record TOF spectra of fast and non-periodic pulses
    registered by the MCP detectors. Histograms of the spectra are
    recorded on the ETS.
    16 high speed, 8 bit low power ADC units are fired with a 1.65 ns
    tapped delay, after an input signal exceeds the trigger level of a
    high speed comparator. Its delay is as small as 0.5 ns to
    minimize jitter, Thus 16 data points cover 26.5ns . The delay
    between the sampled waveform and the 1st ADC start is less than
    0.5 ns The analogue signal bandwidth is ~ 1 GHz
    to record waveforms with minimal signal distortion. The input is
    terminated to 50 ? and is AC coupled. The input is protected
    against voltages greater than +-1.2 V. An 8 bit conversion takes
    2.5 clock cycles at 50 MHz. The ADC units are designed for
    asynchronous operation to save power. Each unit contains a sample
    and hold circuit with an aperture time of 0.35 ns and an ADC as
    well as the control logic and clock generation.
    The 8 bit ADC data with an increment of 1.65 ns are accumulated
    to a 30 bit wide word for each TOF channel. The number of events
    per TOF channel is accumulated to 18bits. The dead time between
    two trigger events, generated from an incoming waveform, is ~ 133
    ns in the standard mode.
    In mass spectrometry mass peaks occur only at discrete flight
    times corresponding to integer mass numbers. Therefore the dead
    time of the ADC has to be shorter than the difference in flight
    time for adjacent mass numbers. For the case that a time gap free
    sampling is required (e.g. to detect multiple charged ions), the
    system can be set into the 'Delayed Time Sampling Mode', where the
    acquisition start delay value is not fixed, but increased
    automatically by 26.5ns after each extraction.
    For cases where a higher mass resolution is required (e.g. triplet
    at mass/ charge 28 amu/e) the ETS can be commanded to a high
    resolution mode where the start of the ADC firing is delayed by n
    times 0.55 ns (n=0-2) relative to the trigger event. Thus one
    spectrum is recorded during three (3x0.55ns=1.65ns) extractions
    thus increasing the measurement time by the factor of three for
    the same statistics.
    There is the option to run the ETS in a half synchronized way with
    a trigger pulse from the ETSL. Instead of starting the system
    periodically by the internally generated extraction clock, the
    circuitry waits for the external trigger from the ETSL to get
    started. A jitter of approximately 26.5 ns relative to the
    external trigger will occur to get the ETS internal state machine
    synchronized. This synchronized mode will be used when both
    channels are active because otherwise cross talk between the two
    channels due to the fast pulsers is inevitable.
    For testing the electronics, a stimulator pulse is available that
    generates an analogue signal from 1 ns ... 250 ns width, and from
    10 mV ... 500 mV height. Width and amplitude are 8 bit
    programmable. This internal calibrator generates ADC and TDC data
    during ground tests and in space to verify the time scale, the
    trigger levels and the ADC conversion. The occurrence of the pulse
    in the TOF test spectrum is 13bit programmable (~185 ns-217 us).
 
    RTOF operation modes
    --------------------
    The RTOF flight instrument provides several operation modes to
    assure optimal scientific data return under diverse mission
    conditions. The fundamental modes are the storage channel and
    orthogonal channel modes, with their dedicated ion sources and
    their own optimized data acquisition system. The RTOF sensor on
    the Rosetta spacecraft has the following operational modes:
    Storage channel mode: The storage channel mode is assigned to the
    electron impact storage ion source and analyzes initially neutral
    particles.
    During the storage period up to 10^5 ions will be accumulated in
    the ion source and extracted by a high-voltage pulse into the TOF
    analyzer section.
    The Equivalent Time Sampling (ETS) data acquisition system, which
    is described below, records the detector signal proportional to
    the number of incoming ions.
    Orthogonal channel mode: The orthogonal channel mode is performed
    with the orthogonal extraction ion source optimised to analyze
    cometary ions. The Equivalent Time Sampling Light (ETSL) data
    acquisition system, which is described below, counts the
    registered ions extracted from the orthogonal extraction ion
    source. Moreover, the orthogonal extraction ion source also has
    the ability to ionize incoming neutral particles with a filament
    assembly using electron impact ionization. Both ion sources could
    therefore be used to detect as well as neutrals or ions.
 
    Single- and triple-reflection mode: The single-reflection mode
    refers to the ion trajectories starting at the ion source, being
    one time reflected in the reflectron and the trajectories ending
    at the detector .
    This mode produces a large instantaneous mass range with medium
    mass resolution.
    In the triple-reflection mode, the ions leave the ion source,
    reverse their direction of motion for the first time in the
    reflectron, and experience a second reflection in the hard mirror.
    After a third reversal of their direction of motion in the
    reflectron, they hit the detector. The reflectron is used twice in
    this mode and the hard mirror is passed once.
    This mode produces high mass resolution with a smaller
    instantaneous mass range.
    Switching between the single- and triple-reflection mode is
    performed by changing the voltage of the reflectron lens.
    The single-reflection mode requires a typical reflectron lens
    voltage of about -2500 V below the drift potential, whereas the
    triple-reflection mode operates with a reflectron lens voltage of
    -4000 V below the drift potential. There is no mechanical tilt
    elements operated in flight nor are there electrical deflection
    plates, which could redirect the ion beam between the single and
    triple reflection mode. The storage and the ortho channel can be
    used simultaneously but they always must operate in the same mode
    (single- or triple-reflection) because of the commonly used
    reflectron structure and the differing voltage sets for the
    single- and triple-reflection mode.
    Blank mode: The blank mode allows suppressing selected mass lines
    to prevent overloading of the detector in case of very intense
    mass lines (e.g., water ions) . This mode is available only
    together with the triple-reflection mode since the blank pulse
    operation is performed with the hard mirror and requires
    synchronization of the extraction pulse with the hard mirror blank
    pulse.
    Calibration mode: The calibration mode allows the calibration of
    the detection efficiencies and the sensor optimization during
    flight by using the RTOF Gas Calibration Unit (GCU). Upon DPU
    command, the GCU system releases a defined quantity of a
    calibration gas into the selected ion source. In addition, it has
    been foreseen to self-optimize the RTOF sensor by a software
    module of the DPU. To achieve optimal performance of the RTOF
    sensor the electrical parameters (e.g. voltages on the ion optical
    elements etc.) have to be fine-tuned carefully. In flight, the
    RTOF sensor is initially operate with a preset adjustment of the
    electrical parameters derived from the sensor calibrations. To
    achieve optimal performance in space (e.g. at a given sensor
    temperature) an automatic optimization algorithm will be used for
    fine-tuning the sensor parameters involving either the calibration
    system for the initial optimization or using cometary gas for
    routine optimization. The optimization process has to be performed
    autonomously on board the spacecraft by the ROSINA DPU due to a
    limited command and data transfer rate during the mission
 
    Location:
    ---------
    RTOF is located inside the S/C with the exception of the sensor
    head which is exposed to space near the +z/-y corner on the +x
    platform. The view direction is towards +z.
 
 
 
    Measured Parameters:
    --------------------
    The RTOF measures the ion current on the detector as a function of
    mass.
    Ions are either primary ions from the comet or ionized neutral gas
    . In both cases the geometric factor is a function of the mass,
    the detector efficency is a function of the ion(s) contributing to
    the peak and of the detector gain. In the neutral gas mode it is
    furthermore a function of th electron current and the ionization
    cross section. The mass itself is a function of the commanded
    voltages (drift voltage mainly).
 
 
    COPS
    ====
    COPS consists of two sensors based on the extractor-type
    ionization gauge principle . The nude gauge measures the total
    pressure (more exactly, the total neutral particle density) of the
    cometary gas. The ram gauge analyzes the ram pressure which is
    equivalent to the cometary gas flux.
    The generated ion currents are measured by corresponding high
    sensitivity electrometers. Depending on the measurement mode of
    the electrometer, COPS has a time constant of about 1 to 10
    seconds.
 
    The nude gauge
    --------------
    Free electrons emitted from the 17 mm hot filament at the
    potential of +30 are accelerated toward the cylindrical anode grid
    (22 mm diameter and 34 mm in height) placed at 180 V. Ionized gas
    atoms and molecules are collected by the cathode hidden below the
    base plate. To increase the ion current yield a hemispherical
    reflector is mounted around the cathode. This reflector is set to
    a potential of 110 V. The measured ion current is directly
    proportional to the particle density in the ionization volume of
    the nude gauge. The gauge is decoupled from the surrounding plasma
    by an outer grid maintained at -12 V compared to the spacecraft
    potential.
 
    For redundancy two 3ReW filaments are available, addressable by a
    switch.
    Each filament can emit up to 1 mA regulated on the current trapped
    by the anode grid. Taken into account the sensitivity of the
    electrometer and the X-ray limitation, the nude gauge can measure
    pressure values between 4*10^-11 to 10^-5 mbar.
    Laboratory calibration of the flight model yields a sensitivity of
    20 mbar-1 for nitrogen at 100 ?A electron emission.
 
    The ram gauge
    -------------
    A spherical cavity, 60 mm diameter with a 6 mm aperture facing the
    comet, stands on a hollow boom. A screen prevents the gas from
    directly impinging into the boom where the density is measured.
    The conductance of the top aperture is 3.4 s-1 for water at 200 K,
    giving an equilibrium time of less than 200 ms for the system.
    The real response time of the instrument is longer as it is driven
    by the electrometer used to measure the ion current.
    This configuration allows the gas to be isotropized and
    thermalized to the wall temperature before entering the ionization
    volume.
    The created ions are collected by a three element lens-like
    configuration consisting of the anode grid, the base plate and the
    reflector. The reflector is a hemisphere of 8mm radius with an
    apex aperture, through which is mounted the collector
    (0.15 mm diameter, 3 mm long).The anode grid
    (16 mm diameter and 19 mm in height) is at 180 V, the base plate
    with an aperture of 3.4 mm diameter at its centre is at 0 V.
    The nitrogen sensitivity of the flight model is 5 mbar-1 for
    pressure values between 10^-10 and 10^-4 mbar.
    As the wall temperature of the equilibrium sphere and the boom
    should be the same as the surrounding cometary coma a cold
    electron source has to be used for the electron impact ionization
    process. Therefore, the usual hot filament design was replaced by
    a microtip field-emitter device.
 
    The microtips
    -------------
    The microtips have a resistive layer that increases emission
    stability and serves as ballast in case of arc generation. For
    this type of micro-emitters a lifetime of 20 000 h is given by the
    manufacturer. Tests have been carried out to evaluate their
    resistance to the cometary environment, and the influence of
    certain gases (O2, H2).
    The emitter is made of more than 1.8 million tips arrayed in
    32 x 32 pixels, representing an emitting area of 14 by 14 mm2.
    The 1024 pixels were grouped by bonding on a ceramic with gilded
    tracks in eight interlaced groups of vertical lines. This special
    arrangement gives eight independent emitters that can be addressed
    separately, either sequentially or jointly. Each group can deliver
    1 mA electron emission current at 70 V extraction voltage. Such an
    electron emitter is of particular interest for space applications
    because of its low power consumption.
 
    Mechanical / Structure /Electronics
    -----------------------------------
    The nude and the ram gauge are each mounted at the end of a boom
    to avoid direct gas reflections from the payload platform or the
    nearest instruments.
    For mechanical stiffness, and accommodation for the launch, the
    booms are limited to lengths of 25 cm. In order to preserve
    cleanliness, the two gauges were constantly purged with nitrogen
    until the launch.
    The three electronic boards are housed in a 165X140X75 mm box that
    also supports the booms. The instrument mass is 1.5 kg.
    The digital board controls the link with ROSINA   DPU. The other
    two boards contain the high voltage supplies for both gauges and
    the corresponding electrometers.
    Each electrometer has three ranges (10 MOhm, 1 GOhm and 100 GOhm
    with 1 mikroF integration capacitor), switched by DPU commands.
    The measured value is converted by a 12 bit ADC and stored as a
    housekeeping value.
    Depending on the selected electrometer range, ion currents between
    0.1 pA to 1 ?A can be measured.
    Two sensors are used to measure the temperatures of the
    electronics the ram gauge. The total nominal power consumption is
    7 W at 28 V primary, with 2.4 W for the nude gauge and 0.7 W for
    the ram gauge.
    The ram gauge boom points toward the comet, while the nude gauge
    boom is parallel to the solar panels. Half of COPS will never
    be exposed to the Sun,so half of each boom is sandblasted and the
    other half is gold-plated.
    On the spacecraft the electronics box is protected by a
    multi-layer insulation.
 
    Location:
    ---------
    COPS is located on the +z platform close to the +y /-x corner off
    the Rosetta spacecraft.
 
    Measured Parameters:
    --------------------
    COPS measures the ion current . This current is proportional to
    the pressure and the emission current. It is dependent on the gas
    species. The nude gauge measures the total pressure, the ram gauge
    the ram pressure.
 
 
    References:
    ==========