1. Mass [amu/q]
0 5 10 15 20 25
Intensity[electrons/ns]
10
10
0
1
2
3
4
5
Background Corrected Thermal Mode Mass Spectrum in CASIMIR - Neon
Mass [amu/q]
0 5 10 15 20 25
Intensity[electrons/ns]
10
8
-5
0
5
10
15
20
Background Corrected Neutral Mode Mass Spectrum in CASIMIR - Neon
H2
H1
20
Ne
22
Ne
H1
H2
20
Ne
22
Ne
Mass [amu/q]
0 5 10 15 20 25
Intensity[electrons/ns]
10
10
0
1
2
3
4
5
Background Corrected Thermal Mode Mass Spectrum in CASIMIR - Neon
Intensity[electrons/ns]
10
8
5
10
15
20
Background Corrected Neutral Mode Mass Spectrum in CASIMIR - Neon
H2
H1
20
Ne
22
Ne
H1
H2
20
Ne
22
Ne
MicroChannel Plates (MCP) are devices that work as signal
amplifiers.Theyareconstitutedoforcoatedwithahighlyconductive
material (i.e. gold) that releases electrons once hit by a charged
particle. Once a potential difference is applied across its ends, if
a charged particles hits a microchannel a cascade of electrons is
formed.
In NIM, MCPs are used to detect the ions after they have travelled
the drift tube. The observed pulses take the shape of a sum of Gaussians, given by the mismatching impedances
of the anode-acquisition card-oscilloscope transmission line. In fact, a non-perfect impedance matching implies
reflections, which are then observed at a later time.
Our aim was to minimize the bandwidth (in terms of Full Width Half Maximum) of the pulses to below 500 ns. To do
so, we had to optimize the impedance matching and time resolution. Three anode designs were tested. Also, each
acquisition card’s termination impedance was optimized for each anode.
Sources:
Cotter, J. R., Time-of-Flight Mass Spectrometry for the Structural Analysis of Biological Molecules, Analytical Chemis-
try, Vol.6, No. 21, November 1, 1992
Wurz, S. and Meyer, S. NIM Ion Source Test, Jupiter Icy Moon Explorer, Issue 1, 2014-09-05
European Space Agency, JUpiter ICy moons Explorer: Exploring the Emergence of Habitable Worlds around Gas Giants:
Definition Study Report, September 2014
http://www.dmphotonics.com/MCP_MCPImageIntensifiers/microchannel_plates.htm
Anode Type A
Worst type of anode. The time resolution is relatively good, but there is a
high impedance variation depending on where the electrons hit the anode.
Anode Type B
Best anode to reduce the impedance variations due to the hitting point.
However, there is a high time delay of around 200 ns.
Anode Type C
Trade off anode. The impedance varies by a non-negligible degree
depending on the hitting point, but the shorter trace effectively decreases
the time delay. Two type C anodes were tested, to check the impact of
weariness on the peaks.
Neutral and Ion Mass spectrometer (NIM) prototype for ESA’s JUpiter ICy moons Explorer (JUICE)
mission
Niccolo` Bigagli1
, Stefan Meyer2
, Matthias Lüthi2
, Prof. Dr. Peter Wurz2
1
Department of Physics and Astronomy, Bates College, 04240, Lewiston, ME
2
Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
The JUICE mission NIM modes and measurements MicroChannel Plates (MCP) detectors
Reflectron
Drift Tube Ion Source
Detector
Filament
Neon Isotope
Ne20 Ne22
RelativeAccuracytoTerrestrialAbundance[1]
10-4
10-3
10-2
10-1
100
101
Relative Accuracy Compared to Terrestrial Abundance of Neon Isotopes in Neutral Mode
Neon Isotope
Ne20 Ne22
RelativeAccuracytoTerrestrialAbundance[1]
10-4
10-3
10-2
10-1
100
101
Relative Accuracy Compared to Terrestrial Abundance of Neon Isotopes in Thermal Mode
Neutral and Ion Mass spectrometer (NIM)
The NIM is a Time-Of-Flight mass spectrometer, an instrument used to analyze the chemical composition
of gaseous samples measuring the mass to charge ratio of the particles composing it. At the beginning of
a trial, particles are accelerated to different velocities relatively to their charge to mass ratio. The velocity
reached in such a fashion is dependent on the specific charge and mass of a particle. The sample is
then let travel a known distance, which different particles will travel in different times. Then, to identify the
particles an analytic time to mass conversion is made.
Research Questions
What are the conditions for planet formation and the emergence of life?
How does the solar system work?
Mission Themes
Emergence of habitable worlds around gas giants
Jupiter’s system as an archetype for gas giants
Lifetime
7.6 cruise years starting on 2022
3.5 years of operation starting on 2030
Highlights
First ever in situ analysis of Jupiter moons’ exospheres
Analysis of the magnetic interactions between moons and planet
Understanding of geochemical processes of the moons
Study of the Jovian atmosphere
Components:
Filament
Through an electron discharge ionizes the neutral particles in the
sample.
Ion Source
Through a non-uniform electric field (electrical lenses) accelerates
particles proportionally to their charge to mass ratio.
Drift Tube
Allows the charges to travel at constant speed for a given length. The
particles will separate according to the velocity reached in the IS.
Reflectron
Through a gradually increasing electric field deflects the particles and
cancels the effects of the initial kinetic energy distribution.
Detectors
Detects the arrival time of the particles through MicroChannel Plates
detectors
Neutral Mode - Neon Tests
Tests were run to assess the behavior and precision
of the prototype. To do so, mixtures of known gases
were injected in a hypobaric chamber simulating
exospheric pressure where the prototype was placed.
The sample was then analyzed and the measured
isotopic abundances of the gases were compared to
the known terrestrial abundances.
Somewhat high noise levels were experienced. Unexplained noise cluster at mass= 5 amu, as no particle
with such mass exists. Relative accuracies below 10% and 1% for 22
Ne and 20
Ne respectively.
Extremely low noise levels were experienced. Relative accuracies below 0.1% and 0.01% for 22
Ne and
20
Ne respectively. Extremely successful mode. Such high precision is extremely difficult to achieve
Testing was impossible. Some instrumentation necessary to ionize the sample was being used by the
Rosetta team for most of the period, and when we acquired it we noticed that the setup did not allow
enough ions to reach the Ion Source. Also due to parasitic capacitance (around 20 pF) between specific
electrodes in the Ion Source, unsustainable noise levels were observed. We attempted to isolate the
noise with a High Pass Filter, but it was inconclusive. New tests will be made in October.
The voltages of the Ion Source electrodes were set to optimize mass transmission, mass resolution and
particle density detection limit.
TOF Mass Spectrometer schematics
Peak Amplitude [V]
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
Time[s]
× 10-10
3
4
5
6
7
8
9
Anodes Comparison with Optimized Termination Impedances
Peak Amplitude [V]
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
Time[s]
× 10-10
3
3.5
4
4.5
5
5.5
6
6.5
7
Peaks' FWHM for the C Anode with Varying Termination Impedances
MCP Voltage[V]
880 900 920 940 960 980 1000 1020 1040 1060 1080
Time[s]
× 10-10
3
3.5
4
4.5
5
5.5
6
6.5
7
Peaks' FWHM for the C Anode with Varying Termination Impedances
The type B anode exhibits the best
impedance matching, with minimal FWHM.
However,asourmeasurementsdonotshow
the effects of time delay, the type C anode
will ultimately have better performances.
The comparison of the average
waveform (2000 trials) of the
pulses with varying termination
impedances shows the effects
of mismatched impedances. If
the termination impedance is
higher than the oscilloscope’s, we
experience a secondary peak, if it
is lower, we observe an overshoot.
ThecomparisonofFWHMforvaryingMCP
voltages for the type C anode confirms
the choice of optimized impedance we
had made, and shows the effect on the
bandwidth of mismatching impedance in
the transmission line.
Another comparison of the peaks’
bandwidth as a function of their
amplitude shows the misleading effect
of an underestimated termination
impedance. The overshoot in the
waveform seems to decrease the
FWHM, but modifies its Gaussian
profile, hence effectively worsening
resolution.
Plot Axes
Time [ps]
-500 0 500 1000 1500 2000 2500 3000
Signal[mV]
-35
-30
-25
-20
-15
-10
-5
0
5
Termination Impedances Comparison
CombineInPlotShow Files
Impedance Calculation
Baseline [mV] Pk Ampl [mV] Reflection [mV] Termination [Ohm] Z0 [Ohm]
0 0 0 51 ?
0 0 0 51 ?
0 0 0 51 ?
Control
6 / 8 (Sorted by voltage: 1012.5)
Allows the in-situ analysis of the exospheres. Ionizes
neutral gases’ particles to +1 cations and performs
measurements
Analyzes ions already present in the sample. Limited
to positive ions, but a negative sub-mode is under
development
Acknowledgments:
My research was supported by Bates College through the Student Summer Research Fellowship, and by the Hoffman
Foundation through the Hoffman Research Support Grant.
My most sincere thanks go to all the Physikalisches Institut of the University of Bern for allowing me to take part
in their research and teaching me a whole set of valuable skills and notions. In particular, I thank Stefan Meyer,
Matthias Lüthi and Peter Wurz for their guidance and the rest of the team for the insights they gave me into their
work.
Thermal Mode - Neon Tests
Stores neutral samples in a Pitot sphere placed above
the Ion Source to allow analysis during non-flyby periods
Ion Mode