| 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:
==========
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