15 Threshold Ionisation in Nuclear Fusion
In nuclear fusion, where helium is a by-product of the fusion reaction, the accurate determination of the helium/deuterium ratios is an essential requirement for process characterisation. Quantifi cation is not possible within the limitations of mass resolution of a conventional quadrupole mass spectrometer due to the unresolvable mass overlap of both D2
and He at m/z 4. The mass separation is 0.025 amu. In conventional quadrupole mass spectrometry, unit mass resolution is used.
When the two gases are present simultaneously, a typical electron energy spectrum for m/z 4 is as shown for an example 1:1 mixture of D2
:He. The onset of an ion signal at around
15.4 eV is due to production of deuterium ions and the sharp increase in the signal at around 24.5 eV is due to the onset of ionisation of the helium in the mixture. The ionisation threshold energies for D2
+ [NIST Standard Reference Database 107, Kim Y-K, Irikura KK, Rudd ME, Ali MA,]. from D2 and He+
to be quantifi ed to levels of <10 ppm. The TIMS technique is not confi ned only to the separation of He/D2
been proven to be a routine measurement using TIMS.
The TIMS spectrum shows a deconvolution of these two species. Applying rudimentary calculations to TIMS spectra such as that shown below, allows the presence of D2
and has seen
further use to 3 amu using the same methodology. For any conventional type of mass spectrometry this proves even more challenging due to the mass separation (0.0058 amu) of the helium 3 isotope (3
He) and any hydrogenated deuterium (HD). However, this has in Helium . from He are consistent with published data
However, with increasing pole diameter, power requirements to maintain the RF and DC components increase exponentially, meaning that conventional power supply electronics cannot be used. For many typical applications and where unit mass resolution is required, such as leak detection and precursor analysis, a pole diameter of 6 mm the most effi cient solution. Where power requirements are modest and performance is high.
For more demanding applications, such as nuclear fusion fuel species characterisation, overlapping species require enhanced mass resolution to resolve these overlaps. A mass overlap is defi ned by an unresolvable peak at adjacent masses, where the unit mass remains the same. The below table shows typical nominal and exact masses for common nuclear fusion species.
An example of a typical mass interference in nuclear fusion is 4
a mass separation of 0.026 amu. This is unresolvable in conventional mass spectrometry. The developments of larger quadrupole diameter mass fi lters, up to 20 mm, more stable and powerful power supplies and operation in the second stability region ‘Zone-H’ have all contributed to improvements in ultra-high resolution quadrupole MS systems. An example spectrum of a 20 mm road diameter quadrupole, operating in zone-H can be seen below. The system was able to resolve the adjacent peaks, with the ability to quantify both 4 + mixtures to 1 ppm for all components by mass.
and D2
He+ and D2 , which have He+ Showing threshold ionisation energies of He and D2 at 4 amu. Ultra-High Resolution Mass Spectrometry
As discussed earlier, the resolution offered by conventional mass spectrometry techniques is not adequate to resolve the mass overlaps found in typical techniques.
In conventional mass spectrometry, the quadrupole mass fi lter allows the ions produced at the ionisation source to be selected by mass and transported to the detector. A quadrupole mass fi lter consists of two pairs of parallel, equidistant poles which are biased at equal and opposite potentials. The twin potentials contain fi xed DC and alternating RF components. The resulting electric fi eld allows ions of a single m/z ratio onto the detector.
The equations below show the relationship between the applied voltages on pairs of poles on both the x and y planes:
Y plane -(Vdc X Plane +(Vdc
+ Vrf- + Vrf-
cos ωt) cos ωt)
Data showing the separation of 4He+ and D2 + by mass.
Further examples include the quantifi cation of 3
He+ even lower, 0.0058 amu.
Recent improvements in RF technology have allowed the introduction of ultra-high resolution quadrupole mass spectrometers, these have the capability to resolve the low mass species found in nuclear fusion reactions by mass.
Showing a series of mass spectrometer fi lter assemblies (from L-R 20 mm, 9 mm, 6 mm triple fi lter, 6 mm single fi lter, 6 mm shortened for high pressure).
Improvements in performance can be gained by increasing the quadrupole pole diameter, allowing enhancements in stability, sensitivity and mass resolution. Commercial quadrupole mass spectrometers are available with diameters ranging from 6 mm to 20 mm.
Showing the improvements in resolution using the fi rst and second stability regions of RF/DC quadruple mass spectrometers.
Commercial quadrupole mass spectrometers are normally operated in what is termed ‘Stability Zone 1’ where the pole voltages are relatively low, this is shown in the red shaded region in the fi gure (left).
+
in HD, where the mass separation is
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