HELIX: A Versatile Plasma Composition Spectrometer Robert Sheldon, Dennis Gallagher, Oleg Vaisberg, Mark Adrian, Wheaton College/NSSTC/MSFC. Abstract
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Robert Sheldon, Dennis Gallagher, Oleg Vaisberg, Mark Adrian, Wheaton College/NSSTC/MSFC
Composition instruments have made themselves indispensable in understanding the complex plasma environment of both magnetospheres and the solar wind. The combination of energy, mass and charge state effectively paint plasma constituents and provide valuable information on sources, acceleration, transport and decay of ions. However, future mission opportunities are likely to put severe constraints on present instrument designs, requiring a radical rethinking of mass spectrometry techniques.
We present a plasma composition spectrometer prototype based on flight-proven time-of-flight technology with a novel helical design that greatly extends the flight path length, and hence the resolution of the compact spectrometer. In addition, the design enables very high geometric factors and duty cycles, making this instrument highly sensitive for both rare species and fast sampling. The prototype has demonstrated M/dM=50 at 2keV for ions and molecules, with the promise of M/dM > 3000, all in a package that is estimated to be 1kg, 1 liter, and consume 1 Watt. Scaling this package down to a “standard” resolution instrument with M/dM=100, reduces these sizes to 100g 125cc spectrometer ideal for multiple space probes or a Pluto flyby.
In this poster, we will try to trace the history of Space Mass Spectrometry, indicate where we are, and what the future holds. It is our contention that we are at the threshold of high resolution space spectrometers based on TOF technology, that will make the quantitative measurements needed for 21st century models, both solar plasma origins and of biological origins.
-Suppose one uses a pinhole camera to take gamma-ray pictures of galaxies. Clearly, two pinholes give twice the sensitivity, but make for overlapping images. With some effort, we can deconvolve the image achieving the same resolution at twice the sensitivity. Extrapolating then to a coded mask made of 50% pinholes allows the maximal sensitivity which can still be deconvolved. This is standard practice in high energy astrophysics.
-A paper in ROSI  showed how this can be applied to TOF systems. Instead of the spatial domain, the time domain can have a coded mask. Thus 50% open gates can be used to give the same resolution with maximal sensitivity. The cost is in S/N ratio, and subtleties of the “kind” of noise in the system. Signal independent noise can be reduced, but signal dependent noise (such as scattered ions or neutrals) can not be addressed this way.
-We have not done the calculation, but it seems feasible that space and time can be deconvolved separately, leading to imaging TOF MS. This permits magnetic sector MS to be simultaneous with TOF MS at 50% spatial and 50% temporal duty cycle. The advantage is to permit one technique to provide, say, 4 low bits of resolution, and the other 4 high bits of resolution in a very compact MS.
In-line HELIX TOF Mass Spectrometer Concept
Limitations of Space MS (other than resolution)
- Highly integrating measurement, adding all species and directions together. It does fine if moments of the distribution function are sufficient information, rather than f itself.
- Unfortunately much of the solar wind has the same E/q range, so that little composition information is possible. Also, most analyzers are narrowband E/q filters that must be scanned, with energy resolution inversely proportional to duty cycle. 5% dE/E is about as good as it gets.
- Magnets separate on rigidity, or M/q, so they do a better job than E/q filters on composition. However, the weight of magnets (or power) limits the highest energies to ~20keV/q, which isn’t high enough to see the heavies in the solar wind. Again, resolution is inversely proportional to sensitivity, giving a practical resolution limit of ~10.
- Linear TOF measure velocity, which can be combined with an energy measurement to determine E, m, and q separately. However, C-foils need > 2keV/nuc to work effectively, which typically requires 10’s of kV post-acceleration before the C-foil. This reduces the TOF, so that relative timing uncertainty grows. Thus there is a tradeoff between dE/E energy straggle from the foil, and dT/T timing uncertainty that give a “sweet spot” for post-acceleration around M/dM~15.
-The highest mass resolution yet flown (Rosina not yet operational), but pushing the limits of C-foil technology. Uses a hyperbolic field to eliminate dependence of TOF on E, thus immune to energy straggle in the foils. However, 80+% of ions exit the foil in a neutral state, and become noise rather than signal. This limits the efficiency and the dynamic range. Post-acceleration can partly mitigate this effect, but is limited by the deflection voltages available in space to V<30kV. At these voltages, TOF is also severely reduced, with the same problems as above. Nor is this voltage high enough to permit a larger efficiency with C-foils. Nothing better than C-foils is likely to exist.
Advantages of Time-of-Flight Technology
-used extensively since at least Fizeau, ~1800
-used in laboratory/space TOF, e.g. ROSETTA TOF
-ENA is a different story but may have a MEMS solution
- Ambiguities are generated for multiple packets.
-This can be partially solved with Hadamard transforms.
-Alternatively, combining magnetic sector & chopped TOF can remove the ambiguity with better noise tolerance.
-Position sensing also can be used to remove ambiguities, giving this design a number of alternative paths toward higher sensitivities.
The greatest challenge is to make the chopper fast enough for the energetic plasma ions of the solar wind, ~ 1-3 keV/nuc.
- 1us gate widths are standard, but 10-100ns gatewidths are needed for >10keV ions
- Recent space instruments are beginning to achieve these higher frequencies without a huge penalty in power. One has to be careful, however, of radiating too much EMI.
TOF Resolution and UV Rejection
-This may be the best of both worlds.
Therefore, with a long, helical path, TOF may be increased by 10-100x, resulting in 10-100 improvement in dT/T, and resolution, while simultaneously excluding photons.
The Challenge of Space Mass Spectrometers vs Lab
A Short History of Space MS vs Resolution (M/dM)
Science attainable at various Resolutions
- protosolar nebula composition, solar mixing ratios
Shocks transport, magnetic holes, cosmochemistry, biomolecules
Why aren’t dynamic range & sensitivity important?
Laboratory measurements often are looking for rare species and need lots of sensitivity, ppm to ppb. Space plasmas come pre-ionized and pre-energized, and thus are a fixed quantity. Since space plasma densities are orders of magnitude below laboratory “ionizers”, the critical factor for space MS is geometric factor: how big an opening is available for collecting space plasmas. Since UV photons come through the same opening, generally the limitation for space is determined by the collimator/ light trap.
In the spectrum above, a mixture of gases was introduced through the leak valve into a electron impact ionization source, and accelerated to 2 keV. The beam was primarily carbon dioxide, but with some atmospheric and “P40”, a mixture of argon and methane, in the system from previous runs. Notice that without a C-foil, the molecular gases do not dissociate. Note also, that the width of the peaks appears to depend on mass, whereas strict TOF gating indicates that the width ought to be independent of mass. We interpret this effect to be due to finite width of the comb-gate, interacting with the slower moving molecular species.
Also note the near absence of background. This is remarkable, since our gating technique is spewing ions all over the chamber, and we are intercepting only 5% or so of entering ions. We attribute this to very little scattering within the TOF system itself at pressures of about 2-3x10-6 Torr. With proper collimation, the noise contribution from the deflected beam can be eliminated giving an additional 10-100x improvement in our already excellent S/N ratio. We expect the dynamic range of this instrument to be more than sufficient to measure the 16/17/18Oxygen isotope ratios.
TOF has the promise to revolutionize space mass spectrometry. We have achieved M/dM=50 without C-foils, and without magnets, for molecules as heavy as carbon dioxide. The mass resolution is exactly as designed, with a 10us TOF and a 100ns gatewidth. (Note that T/dT=100 is equivalent to M/dM=50). We had expected to increase this mass resolution to several hundred by lowering the energy, and thereby raising the TOF. Unfortunately, the finite thickness of our comb-gate prevented this, since slower moving ions interacted with the gate even when relatively far away. Heuristically, we were trying to chop the ion beam with a dull knife, and despite feeding the beam in more slowly, could not achieve thinner slices. This necessitates a second generation comb gate that is currently under development, but which should have a 1/10 thickness compared to our current hand-wound gate. This suggests that 10ns might be close to the limit of achievable gate widths. Accordingly our second generation design will have a 100us flight time, for a combined resolution of ½ (dT/T) = 5000.
In this concept, ions are collimated through a cylindrical collimator that has baffles to exclude UV sunlight. This collimator can also work as an “ion buncher” which operates like “chirping” in radar, to take a collection of ions entering in time period dt, and compress dt by spreading dE, such that dt*dE=constant. Since our instrument is an isochronous instrument, dE has little effect on the TOF. Thus a buncher can improve the efficiency of a synchronous gate to nearly the same efficiency as an asynchronous C-foil.
Annular Channel Plate
The annular multichannel plates permit the beam to pass through the gap, but on the return, they detect the ions. The cylindrical electric field is maintained continuously through this region by oversized outer cylinder kept at a higher potential, such that the field is smooth. Position sensing anodes can be implemented if the dual-Hadamard system is employed, though all such improvements do make the design more complicated. Likewise, should total energy, E (and hence, q) be desired, the annular MCP can be replaced by a annular SSD with a cylindrical MCP (not shown) used to record the stop time. Such complications are only necessary if E/q information proves ambiguous, which is rarely the case when M/dM>1000.
Annular Comb Gate
The modulation is effected with a comb gate, which has the advantage of very small capacitance and inductance, and therefore very fast switching times. Laboratory prototypes have been switched at 80ns, and probably 10ns is easily obtained with a more advanced pulser. The gate operates with alternating wires of high potential that deflect the ions into the walls of the TOF region (OFF state). When the voltages are grounded (ON state), the ions proceed through. On return of the reflected ions, the OFF state deflect the ions onto the two annular detectors. Since deflection depends on voltage, one can also have an OFF’ state that deflects ions away from the detectors. Should the detectors be swamped by H+, one can put the comb gate into an OFF’ state to avoid saturation.
The TOF region
A patented reflection region confines the ions and provides a hyperbolic reflecting field that makes for an energy independent TOF. This is the heart of the device, and the part we have tested in our first prototype shown above.