PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = " ORIGINAL 2012-09-17 SBN:T.Barnes Fixed INSTRUMENT_TYPE case" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "RO" INSTRUMENT_ID = ROSINA OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ROSETTA ORBITER SPECTROMETER FOR ION AND NEUTRAL ANALYSIS" INSTRUMENT_TYPE = "MASS SPECTROMETER" INSTRUMENT_DESC = " 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: ========== " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = BALSIGERETAL2007 END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = ALTWEGGETAL2004 END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END