PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 1

The Entrance System for An Advanced Mass and Ionic ChargeComposition Experiment (AMICCE) for Heliospheric Missions

M. I. Desai∗, F. Allegrini ∗, R. Livi ∗, S. Livi∗, D. J. McComas∗ and B. Randol∗

∗Southwest Research Institute, 6220 Culebra Road, San Antonio, 78238, United States.

Abstract. Electrostatic analyzers (ESA) are usedextensively to characterize plasmas in a variety ofspace environments. They vary in shape, geometryand size and are adapted to the specific particlepopulation to be measured and the configurationof the spacecraft. Their main function is to selectthe energy-per-charge (E/q) of the particles withina passband. An E/q range larger than that of thepassband can be sampled by varying the voltagedifference between the ESA electrodes. The voltagesweep takes time and reduces the duty cycle for aparticular E/q passband. Our design approach foran Advanced Mass and Ionic Charge CompositionExperiment (AMICCE) comprises a novel electro-static analyzer (ESA) that essentially serves as aspectrograph and selects ions simultaneously over abroad range of E/q. Only three voltage settings arerequired to cover the entire range from ∼10 to 270keV/q, thus dramatically increasing the product ofthe geometric factor times the duty cycle when com-pared with existing instrument designs. In this paper,we describe the AMICCE concept with particularemphasis on the prototype of the entrance system(ESA and collimator), which we designed, developed,and tested. We also present comparisons of thelaboratory results with electrostatic simulations.

Keywords: Energetic Particles, Instrumentation,Acceleration,

I. I NTRODUCTION

Until a decade or so ago, the large gradual SEPevents and the Energetic Storm Particle (ESP) eventswere believed to occur when fast shock waves driven byCMEs accelerated material out of the ambient corona orthe thermal solar wind (see e.g., [1]). However, mea-surements obtained by sophisticated new instrumentson board Wind, SoHO, & ACE spacecraft have shownthat CME-driven shocks can routinely accelerate tracerion species like3He near 1 AU ([2]; [3]) and nearthe Sun ([4]; [5]; [6]). In addition, the seed populationfor particle events associated with corotating interactionregions or CIRs also comprises ions from a varietyof different sources that include the solar wind andinterstellar pickup He+ ions ([7]; [8]).

The above observations have provided compellingevidence that IP shocks accelerate particles out of asuprathermal (ST) pool of material whose energy regionlies above that of the solar wind. Unfortunately, ourcurrent understanding of the origin and evolution of

the suprathermal ion population is limited primarily bythe fact that very few heliospheric instruments havebeen designed specifically to obtain measurements inthis energy regime. This is because the suprathermalenergy region (∼2-200 keV/nucleon) lies between thatsampled by solar wind instruments, which require longintegration times (≥ 1 day) to acquire adequate statisticsat these energies, and that by the energetic particleinstruments, which typically do not extend down intothe ST regime due to the low-energy thresholds (∼50-100 keV) of solid-state detectors. Consequently, manykey properties of the suprathermal ion population remainunknown, such as for example, the relative contributionsof different sources, their angular and velocity distri-butions, and their temporal and spatial evolution. Fullycharacterizing these properties is the first critical stepneeded for improving our knowledge of the physics ofparticle acceleration in our solar system and throughoutthe Universe.

II. SCIENCE OBJECTIVES

Our new instrument concept, AMICCE, will enableus to achieve the following goals:

• Identify the main sources of suprathermal ions andcharacterize their properties.

• Measure the charge and/or mass-dependent frac-tionation during SEP, IP shock, and CIR events.

• Constrain and test acceleration models for so-lar flares, CME-driven and CIR-associated shocks,with charge, mass, and spectral observations span-ning nearly two orders of magnitude in energy.

• Improve understanding of the origin, injection, ac-celeration, and propagation of suprathermal and en-ergetic particles near the Sun and in the heliosphere.

III. I NSTRUMENT REQUIREMENTS

In order to achieve the above goals AMICCE mea-sures the charge state, elemental, and isotopic compo-sition of a variety of heavy ions from H-Fe over thesuprathermal energy range from∼10 keV/nucleon upto ∼300 keV/nucleon. AMICCE is designed to havesufficient mass and charge state resolution to distinguishboth3He and He+ ions from the more abundant4He ionsat times when both ratios fall below the∼10% level.

IV. D ESIGN CONCEPT

AMICCE combines an innovative design for an ESAwith time-of-flight (TOF) and residual kinetic energy(E) measurements. A schematic view of the concept is

2 DESAI et al. ESA FOR AMICCE

Fig. 1: Principles of operation of AMICCE.

shown in Figure 1. The ESA is shaped like a rhinoceros’sear with different radii of curvature along the outeredges. The unique shape of the ESA ensures that ionswith different E/q ratios exit the ESA and the collimatorat different azimuthal locations. The selected ions entera cylindrical TOF/E section by passing through an ultra-thin (∼0.5 µg/cm2) carbon foil. Secondary electronsemitted from the exit surface of the foils trigger a STARTpulse for the TOF measurement when they strike a ring-shaped position-sensing microchannel (MCP) detectorinside the TOF/E section.

Azimuthal mapping of the secondary electrons is usedto determine the E/q ratio of the ion. This novel designallows the ESA to select ions over a broad E/q rangewith a single voltage setting. The ions selected by theESA/collimator assembly strike a solid-state detector(SSD) at the end of the TOF/E section that measurestheir residual kinetic energyE. Secondary electronsproduced when the ion hits the SSD are accelerated ontoanother MCP where they trigger the STOP signal for theTOF measurement. The TOF measurement yields thevelocity of the ionv, which is related to the energy-per-mass ratio E/m via the relationshipE = 1/2 mv2. TheE/q selection, combined with the velocity,v and residualkinetic energy,E measurements provide a completecharacterization of each detected ion.

V. L ABORATORY PROTOTYPE

We designed, developed and tested a laboratory proto-type of the AMICCE entrance system, consisting of theESA and the collimator (see Figure 2). We mounted the

Fig. 2: Prototype of the AMICCE entrance system priorto assembly.

Fig. 3: ESA and collimator assembly.

prototype on our 4-axis positioning system and testedit in our vacuum chamber (see Figure 3 with singlycharged 10 keV ion beams.

VI. RESULTS

We used a 2-D imaging detector to map the locationof the ions after they exit the collimator. A comparisonbetween these measurements and results from a Simionmodel of the AMICE entrance system for an azimuthof 210, as shown in Figure 4, demonstrates excellentcorrespondence.

For each ion species at a given energy, we mea-sured the voltage-angle (or energy-angle) response ofthe entrance system by scanning the direction of theincoming ions and the voltage applied to the electrodes.Figure 5 represents the measured response at a particularazimuthal look direction (∼210). We repeated this typeof measurement at other azimuthal look directions anddetermined the overall energy-angle response of theAMICCE entrance system.

These results are summarized in Figure 6 and com-pared with simulation results from our Simion model.The AMICCE analyzer constant varies from∼10-30

PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 3

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Fig. 4: Comparison between laboratory measurementsand simulation results for azimuth of 210.

eV/V for a single voltage setting and there is a uniquerelationship between the azimuthal location on the MCPand the energy-per-charge (E/q) of the ion. We note thatin order to cover the desired energy range from∼10-270keV/q, only three voltage settings are necessary, namely,1 kV (∼10-30 keV/q), 3 kV (∼30-90 keV/q) and 9 kV(∼90-270 keV/q). The gap between the ESA electrodesis 2.5 mm, which results in a∼3.6 kV/mm electricfield for the highest voltage setting if the electrodeswere flat, infinite plates. Since the surfaces are curvedthe maximum electric field is somewhat higher. For themeasurements presented here, the ESA electrodes werebiased with opposite voltages, but the ESA also worksif only one electrode is biased at a voltage and the otheris left at ground.

The voltage (or E/q) resolution of the ESA-collimatorassembly varies from∼13%-18% (not shown), whichis sufficient to resolve He+ from He2+ when the cor-responding abundance ratio is≤10%. The novel aspect

Fig. 5: Voltage-angle response for azimuth of 210.

Fig. 6: Voltage-angle response for the AMICCE entrancesystem.

of AMICCE is that it selects ions in a very broad E/qrange for a single voltage setting of its electrodes. Theanalyzer constant of the entrance system multiplied bythe voltage difference between the electrodes yields theE/q of the ion. Figure 6 clearly shows that each azimuthcorresponds to a single analyzer constant or a uniqueE/q for a given voltage.

Figure 7 summarizes the average XY locations (dots)and their Full-Width at Half-Maximum (FWHM) in theplane of the imaging MCP for all 19 azimuth lookdirections. A least squares fit is used to determine acircle through the data points. The two data points insidethe fitted circle were not included in the fit because theywere determined from a smaller and different impactlocation distribution than the remaining data points.Note that this difference was also predicted from thesimulations. The radius and the coordinates of the center(with respect to the detector coordinate system) of thecircle are annotated, as well as the azimuth locations ofthe peak of the distributions which are calculated fromthe center of the circle. All the points lie on a circlewith very little deviation. The azimuth locations in thisfigure are derived from horizontal axis values of the datapoints in Figure 6.

4 DESAI et al. ESA FOR AMICCE

Fig. 7: Location of the center of spots for 19 azimuthslocations. The bars represent the FWHMs of the impactlocation distributions such as shown in Figure 4.

From the voltage-angle measurements (Figure 5) wealso inferred the geometric factor for a particular lookdirection (or energy-per-charge) and a total geometricfactor for all directions and the full E/q range. Thegeometric factor is given by the product of the field-of-view, Ω, with the active area,A, and with the energyresolution,∆E/E (see [9] for more details). Figure 8shows the geometric factor of the AMICCE entrancesystem as a function of azimuth angle on the MCP.

Since the ESA is symmetrical with respect to the 0-180 azimuth, we expect a symmetrical response anda symmetric geometric factor. In Figure 8, we noticethat there is an asymmetry (up to factor of∼2.4)between the 20-90 and 270-340 azimuths. Afterinvestigation we determined that the asymmetry was dueto a misalignment of the ESA electrodes. Our laboratoryprototype design did not have alignment features to en-sure parallelism of the electrode surfaces to sufficientlyreduce this asymmetry.

We find a total geometric factor of∼1.17×10−2 cm2

sr eV/eV, integrated over all azimuthal angles. If all threevoltage settings are swept in 1 minute, then the productof the geometric factor and the duty cycle is 0.233 cm2

sr eV/eV s.

VII. C ONCLUSIONS

We have designed, developed, and successfully testeda laboratory prototype of the entrance system comprising

a novel electrostatic analyzer and a collimator for an Ad-vanced Mass and Ionic Charge Composition Experiment(AMICCE). Our innovative design approach for enablesAMICCE (see [9]) to essentially serve as a spectrographand selects ions simultaneously over a broad range ofenergy-per-charge (E/q). Only three voltage settings arerequired to cover the entire range from∼10-270 keV/q,thus dramatically increasing the product of the geometricfactor times the duty cycle by about a factor of∼20when compared with existing suprathermal instrumentdesigns ([10]; [11]).

Fig. 8: Geometric factor versus azimuth on the MCP.

VIII. A CKNOWLEDGEMENTS

This work was partially supported by the InternalResearch and Development Program at the SouthwestResearch Institute (SwRI) and National Aeronautics andSpace Administration (NASA) Grant NNX08AG45G.Figures 1-8 are reprinted with permission from [9];copyright 2009, American Institute of Physics.

REFERENCES

[1] D. V. Reames,Space Sci. Rev.,1999,90, 413.[2] M. I. Desai, et al. Astrophys. J. Lett.,2001,553, L89.[3] M. I. Desai, et al. Astrophys. J.,2003,558, 1149.[4] C. M. S. Cohen,et al. Geophys. Res. Lett.,1999,26, 2697.[5] G. M. Mason,et al. Astrophys J. Lett.,1999,525, L133.[6] M. I. Desai, et al. Astrophys. J.,2006,649, 470.[7] K. Chootoo,et al. Journal of Geophys. Res.,2000105, 23107.[8] H. Kucharek,et al. Journal of Geophys. Res.,2003, 108, A10,

doi: 10.1029/2003JA009938.[9] F. Allegrini, et al. Rev of Scientific Inst.,in press.

[10] G. Gloeckler,et al. Space Sci. Rev.,1995,71, 79.[11] S. M. Krimigis, et al. Space Sci. Rev.,2004,114, 233.