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The ion-optical prototype of the low energy neutral atom sensor of the Interstellar Boundary Explorer Mission (IBEX)
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Image of FIG. 1.
FIG. 1.

Various calculated hydrogen ENA energy spectra together with available measurements (Ref. 15). The eight energy bins of IBEX-Lo superimposed on top fill the gap below where no measurements are available.

Image of FIG. 2.
FIG. 2.

IBEX-Lo prototype sensor concept. The sensor has a cylindrical symmetry. Neutral particles enter the sensor through a circular collimator, are converted to negative ions by reflecting from a conversion surface (CS), preaccelerated (PreAC), energyanalyzed and focused to the center by a poloidal energy analyzer (ESA), postaccelerated (PostAC), and finally registered in the detector section by an imaging microchannel plate (MCP) or a channel electron multiplier (CEM). For flight configuration the detector will be replaced by a time-of-flight section to get mass resolution.

Image of FIG. 3.
FIG. 3.

Three-dimensional cut through the IBEX-Lo prototype. Only one quarter of the circumference is equipped with conversion surfaces to reduce complexity of the prototype. The collimator is not shown in this drawing. The size of the prototype as shown is in diameter and in length.

Image of FIG. 4.
FIG. 4.

(a) Schematic representation of the collimator. Etched plates with a thickness of are stacked with spacers with a thickness of approximately equal the linewidth in the center. The distance increases geometrically toward both front and back sides. A precollimator on the front side improves further the suppression for large angles outside the field of view (FoV). (b) Complete IBEX-Lo prototype with collimator, which is about in diameter.

Image of FIG. 5.
FIG. 5.

Conversion surface support unit. The unit covers one quarter of the circumference of the prototype, divided into strips of about width. For the prototype measurements only the center tantalum strip was equipped with four ta-C DLC tiles (top) because the testing was done with a neutral beam collimated down to this size. The total area equipped with ta-C tiles for prototype measurements was . A heater attached to the back side of the tantalum strip (bottom) allowed to study the effects of heating the surfaces up to .

Image of FIG. 6.
FIG. 6.

Cut through the conversion surface assembly (top left), the electrostatic analyzer (bottom), and the imaging MCP in the center of the sensor (top right). The secondary electron suppression magnets embedded in the inner and outer preaccelerator electrodes are shown in white.

Image of FIG. 7.
FIG. 7.

(a) Secondary electron suppression magnets embedded in the preaccelerator electrodes (inner electrode is shown, view from the ESA side). For this configuration the electrode surface is coated with iridite and the magnets themselves are untreated AlNiCo. The outer electrode looks the same in this configuration. Individual magnet rods are in diameter. (b) Configuration with magnets hidden behind a blackened and serrated electrode surface (outer preaccelerator electrode is shown, view from the ESA side). The inner electrode surface (not visible) also has serrations in this configuration.

Image of FIG. 8.
FIG. 8.

Electron microscope images of copper sulfide-blackened microserration structure, Image by E. Krähenbühl, Institute of Applied Physics, University of Bern.

Image of FIG. 9.
FIG. 9.

Outer ESA electrode with fins. Particles from the conversion surface always hit the fins on a face with no direct line of sight from the impact point to the ESA exit. The complete assembly is in diameter.

Image of FIG. 10.
FIG. 10.

Setup for ion tests. The ion beam enters the prototype through a hole in the conversion surface plane. Different reflection angles of incident neutral particles are simulated by rotating the prototype relative to the incoming ion beam.

Image of FIG. 11.
FIG. 11.

Setup used for neutral particle tests. The prototype is mounted at a fixed position relative to the ion beam neutralizer (Ref. 28) which provides a neutral beam with similar angular characteristics as the separately evaluated collimator. The neutral beam energy is adjusted by changing the potential of the inner box of the neutralizer containing the neutralizer surface, while the energy of the incident positive ions is kept constant. The factor of 1.17 in the formula for the floating potential accommodates for the energy loss at the neutralizer surface.

Image of FIG. 12.
FIG. 12.

IBEX-Lo prototype (cylindrical structure in the center) installed in the MEFISTO calibration facility of the University of Bern, Switzerland. The ion beam enters the chamber from the left through the fan-shaped opening. The ion beam is neutralized in the box left of the center of the image (Ref. 28) prior to entering the IBEX-Lo prototype. Most electrostatic shielding around the prototype setup was removed for this picture.

Image of FIG. 13.
FIG. 13.

IBEX-Lo prototype collimator (cylindrical structure in the center) installed in the CASYMS calibration facility of the University of Bern, Switzerland. The ion beam enters the chamber from the left through the rectangular opening. The imaging MCP detector is mounted behind the bottom section of the collimator.

Image of FIG. 14.
FIG. 14.

(a) Relative transmission (normalized to the maximum count rate) of the prototype collimator in a two-dimensional angular scan. The inset shows the orientation of the hexagon pattern relative to the scan directions. The full lines, separated by 60°, in the scan and the inset mark cuts through the pattern perpendicular to the grid lines. Nominal performance is obtained except in the direction of the solid lines, where some leakage is visible (see text). (b) shows a profile of the transmission along the dashed line in (a). The angular width of the center peak (solid line) is just under 7° FWHM. The observed leakage is shown as a dashed line. The dotted line indicates the limit of the dynamic range of the detector used.

Image of FIG. 15.
FIG. 15.

Measured beam cross section at the exit of the annular ESA when the conversion surface is illuminated with a low divergence neutral oxygen beam with average particle energy (left). The 50% intensity level is shown in gray. The larger scatter in the simulated image (right) is due to an overestimation of the angular scattering at the conversion surface in the simulation.

Image of FIG. 16.
FIG. 16.

Seven out of eight energy bins of the prototype (open circles) measured with two different detectors. Bins 1–5 were measured using an imaging MCP set to float potential and bins 6 and 7 with a CEM set to float potential. Bins 5 and 7 show some distortion as the center energy approached the maximum energy possible for the detector float voltage used. Nevertheless the energy bins remain well defined. The expected theoretical response for a monoenergetic neutral beam is shown for bin 4 (solid circles, bold line). The deviation from the measurement (dashed line) on the high energy side of the peak is due to the energy distribution created in the beam neutralizer.

Image of FIG. 17.
FIG. 17.

Ionization efficiency for hydrogen (a) and oxygen (b) off various ta-C DLC conversion surfaces as used in the prototype measured in different experiments in Bern (IBEX, ILENA), Essen (JUSO), and Denver (NICE). The data set labeled NICE was taken from (Ref. 35).

Image of FIG. 18.
FIG. 18.

Geometric factor of the prototype for different postacceleration voltages. At low energies the geometric factor is governed by the ionization efficiency at the conversion surface (closed black squares). At higher energies the geometric factor depends on the postacceleration (small open squares, for postacceleration). A postacceleration of will increase the geometric factor (open diamonds) for higher energies. The solid line shows the expected geometric factor over the whole energy range (fit at lower energies, simulation at higher energies) for postacceleration. For even higher postacceleration the geometric factor would increase further (dotted line).

Image of FIG. 19.
FIG. 19.

Transmission of prototype for energy bin 1 with center energy is shown with data from a sensor with (open circles) and without (closed circles) secondary electron suppression magnets. The difference between the two sets corresponds to the secondary electrons generated at the conversion surface (solid line). The contribution of sputtered particles was estimated (dashed line) and subtracted from the data obtained from a sensor with suppression magnets resulting in the fraction of converted hydrogen (dash-dotted line).


Generic image for table
Table I.

IBEX-Lo energy bins.

Generic image for table
Table II.

Optimization constraints and drives for the automatic ion-optics optimization. “Prior optimization” refers to the manual optimization of the ion-optical system.

Generic image for table
Table III.

Suppression of scattered particles.

Generic image for table
Table IV.

Neutral-to-negative ionization yield for different species.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: The ion-optical prototype of the low energy neutral atom sensor of the Interstellar Boundary Explorer Mission (IBEX)