ISSN 00109525, Cosmic Research, 2014, Vol. 52, No. 6, pp. 481–484. © Pleiades Publishing, Ltd., 2014. Original Russian Text © O.L. Vaisberg, P.P. Moiseev, G.V. Koynash, L.A. Avanov, V.N. Smirnov, V.V. Letunovsky, A.K. Tonshev, V.D. Myagkih, A.V. Leybov, S.D. Shuvalov, 2014, published in Kosmicheskie Issledovaniya, 2014, Vol. 52, No. 6, pp. 521–525.
Panoramic Energy–Mass Ion Spectrometer for the PhobosGrunt Mission O. L. Vaisberg, P. P. Moiseev, G. V. Koynash, L. A. Avanov, V. N. Smirnov, V. V. Letunovsky, A. K. Tonshev, V. D. Myagkih, A. V. Leybov, and S. D. Shuvalov Russian Space Research Institute, Moscow, Russia email:
[email protected] Received February 10, 2014
DOI: 10.1134/S0010952514060070 1
INTRODUCTION Corpuscular diagnostics plays an important role in the study of the solar wind and the magnetosphere of Earth and other planets. Its goal is the investigation of the velocity distribution functions of ions and elec trons and the mass analysis of the ion component of plasma. The measurement of the energy distribution function must be carried out in a wide range of angles, namely ±30° for the solar wind and 2π for the plane tary magnetospheres. The specfic requirements of space research resulted in the development of many compact instruments with high metrological performance. The most frequently used instrument in space missions of other countries is the tophat analyzer [1], which is a modification of a spherical electrostatic analyzer. It carries out an anal ysis of the particle energy per charge, E/Q, over a 360° flat field of view. Measurement in a third dimension is provided either by spacecraft rotation or by using elec trostatic angular scanner. The analysis of ion velocity is performed by the timeofflight (TOF) analyzer with additional ion acceleration. It consists of a thin foil, knocked out electrons from which provide the initial impulse, and a positionsensitive particle detector used for receiving a stop signal and for measurements of the azimuthal distribution of ions. Precursors of the device being described here are the ion energyangular analyzer SKA1 [6], which operated on the Interball Tail Probe satellite and the wideangle energymass analyzer FIPS [7] for the MESSENGER satellite of Mercury. Our proposed electrostatic mirror with a 2π (or more) field of view was described in the previous papers [1, 2]. This mirror converts particle distribution from a hemisphere to a conical cylindrically colli mated beam. It is possible to record this beam on the positionsensitive detector or to analyze it according to mass or velocity before recording, saving the infor mation about the polar distribution of particles. This 1 The article was translated by the authors.
system is an electrooptical analog of an allsky cam era or a “fisheye” lens in optics. An instrument based on such a mirror allows us to record the instantaneous distribution of the particle flux density in a 2dimen sional cross section in the velocity space, thus leaving only one scan to do, that of energy. The described method of velocity measurement of particles in combination with analysis by TOF between the foil and the detector has the disadvantage that it is necessary to keep the whole TOF section at the high negative potential of –15 to –20 kV. This is in order to reduce straggling and, in particular, to increase energy resolution and to increase the sig nal/noise ratio in the semiconductor detector as an ion detector. ELECTROOPTICAL PLAN OF ENERGY–MASS SPECTROMETER In this paper we describe the results of physical tests of the instrument DIAries [4]. Initially, the described plan was developed for the PICAM energymass ana lyzer [5] for the BEPICOLOMBO mission of Euro pean Space Agency. The instrument DIAries, the tests results of which are described in this paper, was developed for the PhobosGrunt mission by Russian space agency, and this plan was later accepted for the instrument PICAM, with some modifications. The electrooptical plan of the instrument is shown in Fig. 1. A computer model of it was developed, using the program SIMION [6]. Mirror 2 was initially made up of elliptical surfaces, so that the ions that enter entrance window 1 would be focused on the entrance slit 4 of the electrostatic analyzer (ESA) 5. Later mir ror 2 was optimized to concentrate the image spot at the entrance of the ESA; this was achieved by locating the turning points of the trajectories in the mirror 2 on the elliptic curve. The increase in the gap between electrodes of the mirror led to the distortion of the ion trajectories. The correction for this field was per formed by introducing a convex bulge on the reflecting
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Fig. 1. Electrooptical plan of DIAries instrument (axial section of cylindrically symmetric configuration): three ion beam paths are shown from the entrance window to detector, with entrance angles of 0°, 45° and 90°. Calcu lated energy width of beams is 8–10%.
electrode. Generally, this modification of mirror 2 allowed reduction of the size of the image at the ESA’s entrance by about a half. The toroidal electrostatic analyzer 5 is primarily used for selecting particles according to energy. An analyzer rotation angle of 241° was chosen in order to focus the beam’s exit from the entrance window 4 on the aperture of the diaphragm 6, by the angle and energy. Obviously, the analyzer does not change the angular distribution of the beam formed at the entrance of the primary mirror. After leaving the ESA, ions in the passband fall on the secondary mirror 7, which reflects them onto the positionsensitive parti cle detector 8. Thus, the image on the detector repre sents the angular distribution of particles on the hemi sphere, and, with allowance of angular resolution of the electrooptics, there is an unambiguous corre spondence between the direction of motion of parti cles in the hemisphere and their position on the parti cle detector. The instrument uses TOF method for separating ions by mass. As known, ions in the electric field are analyzed with respect to their particle energy per charge E/q, and the speed of selected ions is inversely propor tional to the square root of the mass to charge ratio (m/Q)–1/2. Using the timeofflight method, one can analyze these ions by mass (more precisely, by the mass tocharge ratio, m/q). In the most commonly used method of mass analysis in space research, a thin foil is used to generate a start signal from the secondary elec tron and ion detector for generating a stop signal (see, e.g., [9]). However, the use of foil to generate a start signal requires a sufficiently high ionacceleration potential and a normal incidence of the ions on the foil. For TOF analysis on the instrument, it was decided to use an electrostatic gate to inject a short ion beam into the interstice of the TOF and a device for syn
chronizing the flight of particular ions from gate to the detector. A comb gate [10] was used as an electrostatic gate. It turned out that a toroidal electrostatic analyzer can be used as a synchronizing element, wherein faster ions move along a longer trajectory. By choosing the size of the analyzer, it is possible to equalize the flight times of ions of one type but at different velocities (in the energy passband) at intervals of 3–4 plus 6–8, on one side, and at ESA, on the other side. Thus, ions with different ratios E/Q will reach the detector at times inversely proportional to their velocities Vi = (2E/Q)–1/2, and at the same time the mass line’s width will be substantially reduced due to the synchro nization of each type of ion. One can optimize the electronoptical scheme in such a way that divergence of ions electricfieldfree gaps 3–4 and 6–8 would be compensated during flight by ESA ions. Thus, ions of different masses are collected in narrow packets, which allows one to improve mass resolution (m/q) substantially. This method is somewhat analogous to one described in paper [11]. An electrostatic gate of the comb gate type described in [8], consists of thin electrodes, which are biased alternatively by bipolar voltages (“+” on even, “–” on odd, or vice versa) for locking the gate. When the gate is closed, ions entering the instrument through input window 1 do not fall into the dia phragm 4. The gate for the beam is opened by apply ing zero potential to the gate electrodes. The shape of the gate was chosen with reference to the time of flight of the whole range of polar angles in order to ensure their arrival at the detector at the same time. The instrument can operate in two modes: the energy spectrum mode (with an electrostatic gate opened) and the mass spectrum mode (using the elec trostatic gate). In both cases, setting the instrument to a specific energy, a 2dimensional distribution of the intensity of the ion fluxes of energy in the hemisphere can be obtained from a positionsensitive detector. This helps avoid the effect of changes of the flux during mea surement and reduces the period of measurement. The threedimensional velocity distribution function of par ticles is obtained by an energy scan. The complex trajec tory of the ions in the instrument prevents ultraviolet radiation from entering the detector. The construction of the instrument is described in paper [4]. INSTRUMENT TESTS IN A VACUUM CHAMBER Testing of the instrument installed on the space craft PhobosGrunt, was carried out in the laboratory 546 of department 54 of the Space Research Institute, Russian Academy of Sciences (IKI) (Fig. 2). The lab oratory bench consists of: a vacuum chamber (length 60 cm and diameter 60 cm), an ion source with energy of 100 eV and a manipulator device for rotation of the instrument relative to the ion beam in polar and azi muthal angles. An automatic vacuum formation sys COSMIC RESEARCH
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PANORAMIC ENERGY–MASS ION SPECTROMETER Polar angle 20°
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Fig. 3. Mass spectra obtained by illuminating the instrument with an ion beam at the polar angle of 40° for azimuth angles of 0°, 90°, 180° and 270°. Ion peaks of N+, N2+, Ar+ and trace of another unidentified ion are marked. N2+ peaks are approximated by a Gaussian distribution and a ratio of the peak position to a full halfwidth of the Gaussian approximation in units of time scale is shown for each graph.
tem has been installed in a chamber, which allows the attainment of a pressure of ~1 × 10–5 Torr (with the instrument placed inside the chamber and the ion source working). All tests were performed at close to this value of pressure in the chamber. The limited time between the manufacture of the flight model of the COSMIC RESEARCH
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instrument and its transfer to the spacecraft for instal lation did not allow thorough testing of the instrument and correction of the identified deficiencies. However, the performed tests did allow us to show that the cho sen plan of the instrument works and that its perfor mance can be improved for future missions.
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Figure 2 shows three sets of combined images on coordinatesensitive detector with illumination of the instrument with ion beam at three polar angles 20°, 40° and 60° at four azimuths 0°, 90°, 180° and 270°. Contours were drawn at half of the maximum density distribution of ion registration. Subsequent analysis and comparison with numerical simulations of the instrument showed that some image distortions occurred because measurements were not at the maxi mum transmission of the instrument. Obtained results show that the width of the transmission function is ~45° at the azimuth and ~15° at the polar angle. This provides 48 independent elements in the image of the hemisphere, which is an acceptable number for the display of hemispherical crosssections of a 3dimen sional velocity distribution of ions. Given that the nominal number of energy degrees is not less than 32, the threedimensional space velocity distribution will have 1536 elements, which is more than enough for space experiment. Figure 3 shows the measurements of the mass (timeofflight) spectrum of the residual gas in vac uum chamber with the addition of argon in an ion source. The recorded noise on the detector has been subtracted out. The line of N2+ is recognized clearly, and N+ and Ar+ are also seen. Peaks that are due to N2+ are approximated using gauss curves. The ratios of the maxima of those distributions to their full width at half maximum in terms of the time of flight are given on each of 4 graphs. This ratio is equal to the mass res olution M/ΔM and is ~35 for 0° azimuth. For other azimuths, these values vary near 15. It can be seen that at these three azimuths the maxima have peaks above the approximating curves at 1.5–2 times the heights of the curves. Thus, the effective mass resolution of these angles is ~25. Unfortunately, the tight schedule for the develop ment and delivery of instruments for the spacecraft did not allow us to investigate the flight instrument in suffi cient detail or to modify it according to test results. This explains the flaws that can be seen in the above data, in particular, the image distortion and low mass resolution (this last could be partly due to the performance of the electronic components). Also, it was not possible to measure the energy resolution of the instrument due to the large width of the energy distribution of the ion source, namely ~20%. A numerical simulation of the instrument’s ion optics shows that the energy resolution is 8–10%, depending on the polar angle.
Nevertheless, the results of physical tests of the flight instrument for the PhobosGrunt mission have shown its operability, in particular, at an acceptable value of mass resolution and satisfactory image char acteristics, especially considering the ability to elimi nate image distortion during data processing. Further work on the instrument for subsequent experiments should result in an improved performance. REFERENCES 1. Vaisberg, O., Goldstein, B., Chornay, D., et al., Ultra fast plasma analyzer—an allsky camera for charged particles, The First Solar Orbiter Workshop, Tenerife, Spain. ESA SP493, 2001, pp. 451–454. 2. Vaisberg, O.L., Advanced method for exploration of plasma velocity distribution functions: allsky camera for very fast plasma measurements, Adv. Space Res., 2003, vol. 32, pp. 385–388. 3. Vaisberg, O.L., Avanov, L.A., Leibov, A.V., et al., A pan oramic plasma spectrometer: an allsky camera for charged particles, Cosmic Research, 2005, vol. 43, no. 5, pp. 373–376. 4. Vaisberg, O.L., Koinash, G.V., Moiseev, P.P., et al., A panoramic energymassspectrometer of ions DIAries for the PhobosGrunt project, Astron. Vestn., 2010, vol. 44, no. 5, pp. 485–497. 5. Carlson, C.W., Curtis, D.W., Pashmann, G., and Michael, W., An instrument for rapidly measured dis tribution functions with high resolution, Adv. Space Res., 1985, vol. 2, pp. 67–70. 6. Vaisberg, O.L., Leibov, A.W., Avanov, L.A., et al., Com plex plasma analyzer SCA1, in Interball Mission and Payload. RKAIKICNES, 1995, pp. 170–177. 7. Zurbuchen, T.H., Gloeckler, G., Cain, J.C., et al., Conference on missions to the Sun II, Korendyke, C.M., Ed., Proc. of the Society of PhotoOptical Instrumenta tion Engineers, Bellingham, Wash., 1998, vol. 3442, pp. 217–224. 8. Manura, D.J. and Dahl, D.A., SIMION. Scientific Instrument Services, Inc. Idaho National Laboratory, Revision, 2008. 9. Gloeckler, G., Ipavich, F.M., et al., The charge energymass spectrometer for 0.3–300 keV/e ions on AMPTE CCE, IEEE Trans. Geosci. Remote Sensing. GE23, 1985, pp. 234–240. 10. Stoermer, C.W., Gilb, S., Frederich, J., et al., A high resolution dual mass gate for ion separation in laser desorption/ionization time of flight mass spectrometer, Rev. Sci. Instrum., 1998, pp. 1661–1664. 11. Mamyrin, V.A. Pozhunkov, A.A., et al., Massreflec tron: a new nonmagnetic highresolution massspec trometer, Zh. Eksp. Teor. Fiz., 1973, vol. 37, pp. 45–48.
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