Abstract:
A system and method for separating ions in an ion mixture, such as a plasma in space. The ion mixture enters an electrostatic analyzer, whose ion path has at least two sections. A first section applies a DC voltage to the ions, and a next section applies an RF frequency voltage to the ions. Appropriate DC and RF voltages are applied, such that at least a portion of the lower mass ions are absorbed into the RF section of the analyzer. The heaver ions are transmitted out of the ion path and are readily available for further analysis.

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates to ion composition analysis, and more particularly to instruments designed for composition measurements of plasmas in space. 
   BACKGROUND OF THE INVENTION 
   The dynamic range of ion composition analysis instruments is limited by several factors. Two problems are saturation of particle counters and spillover of signals from highly dominant species into channels tuned to minor species. 
   Instruments designed for composition measurements of hot plasmas in space can suffer greatly from both of these problems because of the wide energy range required and the wide disparity in fluxes encountered in various regions of interest. To detect minor ions in regions of weak fluxes, geometry factors need to be as large as practicable. As a result, in dense plasma regions, problems with saturation by the dominant fluxes and spillover to minor-ion channels become especially acute. 
   Present-day ion composition instruments have been unable to provide meaningful measurement of minor ion species at the Earth&#39;s magnetopause (the boundary between the magnetosphere and the solar wind). This is due to high background levels caused by spillover of dominant proton fluxes. Specifically, this effect has precluded ion composition measurements at energies in the important keV range at the magnetopause. 
   Previous attempts to solve the dynamic-range problem have involved mechanical constrictions that reduce the throughput equally for all species. This approach alleviates the saturation and spillover problems, but reduces the minor ion throughput to levels that are often undetectable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
       FIG. 1  illustrates representative ion fluxes encountered in a magnetopause region by a spacecraft. 
       FIG. 2  illustrates a toroidal tophat electrostatic analyzer and a time-of-flight (TOF) mass analyzer in accordance with the invention. 
       FIG. 3  illustrates how the path of ions through the analyzer depends on the phase of the RF deflection voltage, using the system of  FIG. 2 . 
       FIGS. 4A and 4B  illustrate results of a laboratory test, with relative transmission of oxygen ions and protons as a function of applied RF frequency, using the system of  FIG. 2 . 
       FIG. 5  illustrates results of a laboratory test, with the transmission ratio of protons as a function of the peak-to-peak voltage of the RF deflection potential, using the system of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As stated in the Background, hot plasmas resident in the magnetospheres of the Earth and other planets present a challenging target for space-borne particle detectors, and particularly for ion composition instruments. These plasmas have source regions both in the solar wind and in the planetary ionospheres, so there is typically a mixture of ions such as hydrogen, helium, oxygen, nitrogen, and other minor species with density ratios that are in some cases very high. The varying mass/charge ratios and fluxes present a difficult challenge in attempting ion composition analysis. 
   This description is directed to systems and methods for solving the dynamic range problem in the few-eV to several-keV energy/charge range. This energy/charge range is important for space physics research, where the dominant ions are of low mass/charge (typically H+), and the minor ions are of higher mass/charge (typically O+). 
   The technique described herein involves using radio-frequency (RF) modulation of a deflection electric field in an electrostatic analyzer used with a time-of-flight (TOF) instrument. The analyzer reduces H+ counts into the TOF instrument by a controllable amount of up to factors of 1000, while reducing O+ counts by a known and calibratable several percent. 
     FIG. 1  illustrates representative ion fluxes that will be encountered in the Earth&#39;s magnetosphere region by a particle analyzer in space. Typical applications of such particle analyzers are the ion composition instruments used on a spacecraft, such as a spacecraft used for NASA&#39;s magnetospheric multiscale (MMS) mission. 
   The H+ fluxes are representative of a compressed dayside boundary region (magnetopause) with density of 80 cm −3  and 400 km/s bulk flow, similar to the high-speed flows observed in reconnection. The O+ fluxes are modeled after the beams observed in the Low Latitude Boundary Layer, which lies just outside the magnetopause. The magnetotail fluxes are modeled after plasma sheet encounters, including the O+ composition representative of disturbed magnetospheric conditions. 
   As shown in  FIG. 1 , the proton fluxes are often extremely high. In contrast, in the same part of the magnetosphere, important minor ions will have fluxes of only a few percent of the proton flux. Because of thermalization by the bow shocks, which slow the solar wind in front of the magnetospheres, and wave turbulence within the magnetospheres, the ion distribution function covers a wide energy range with significant flux at all entrance angles to a particle analyzer. 
   Two major problems limit the dynamic range of space-borne ion composition instruments (particle analyzers). The first is simply the requirement for very high counting rates, a requirement that results when an instrument that is sensitive enough to detect minor species in a tenuous plasma region must also measure major species in a more dense region. The second is the spillover from dense major species into channels tuned to minor species. 
     FIG. 2  illustrates an ion composition analyzer  110 , which provides improved dynamic range in accordance with the invention. Analyzer  110  is a “tophat” type electrostatic analyzer  110 , and provides ions to the entrance of a time-of-flight (TOF) mass analyzer  120 . Together, analyzer  110  and TOF analyzer  120  measure three-dimensional composition-resolved distribution functions of hot plasmas in space. 
   More specifically,  FIG. 2  is a planar section view of a toroidal analyzer  110  with deflection plates  116  and  118  that create an ion path within the analyzer  110 . The ion path is segmented into a DC entrance section  112  and an RF exit section  114 . An example of a suitable deflection plate gap is 4.1 mm, constant throughout the analyzer  110 . 
   In the example of  FIG. 2 , analyzer  110  has a curved-plate and toroidal configuration for the ion path. In other words, deflection plates  116  and  118  form a curved toroidal path. Other configurations may be used, such as the more common spherical tophat geometry, or such as various non-tophat geometries (cylindrical, hemispherical, etc.), or parallel plate geometries. 
   The technique involves the incorporation of a radio-frequency (RF) deflection voltage in the exit segment  114  of the analyzer  110 . A DC deflection voltage is applied to the entrance segment  112  as a pre-filter to the RF section. The entrance section  112  applies the DC deflection electric field to the ions within, and presents a nearly monoenergetic beam (ΔE/E ˜0.2) to the exit section  114 . The same DC deflection voltage is applied to the exit section  114 , but an additional RF voltage is superimposed on it. Without the DC pre-filtering, the RF deflection section  114  would simply sample adjacent parts of the wide energy spectrum typically encountered in magnetospheric environments. 
     FIG. 2  further illustrates ray tracing of the ion paths through the analyzer  110 . Appropriate software can be used to simulate ion paths through analyzer  110 . An example of such software is the SimIon™ software, available from Scientific Instrument Services, Inc. 
   For purposes of example, the paths of H+ and O+ ions are shown. Analyzer  110  can reduce the H+ flux to extremely low levels while keeping the O+ flux nearly unaffected. 
   In the electrostatic analyzer  110 , the RF deflection voltage causes only slight deflections of slower-moving heavy ions (such as oxygen), which execute several oscillations about the center line between the deflection plates as they transit the RF deflection section. These ions will tend to remain within the deflection plates during an RF period. Lighter, faster-moving ions (such as hydrogen) will strike the deflection plates in a time short compared to the RF period of the deflection voltage. 
   Thus, the analyzer  110  acts as a high-pass mass/charge filter (or equivalently, a low-pass velocity filter). By varying the frequency and magnitude of the RF deflection voltage, the filtering can cover a fairly wide range of energies and can be tuned to transmit known fractions of ions at all masses. In this way it solves both of the dynamic range problems (count saturation and major species spillover) mentioned above. 
   In the example of  FIG. 2 , a uniform mixture of H+ and O+ ions enters the “tophat” of the analyzer  110  from the left and is deflected into the entrance region  112  by the DC field. H+ and O+ ions at an energy/charge of 1 keV fill the field of view of the analyzer  110 , which has a DC potential difference of 189 V across the deflection plates in the DC section. 
   The entrance section  112  of the ion path, with its DC field, is an “energy filter”. All ions within a selected narrow energy band regardless of mass, are transmitted through this section  112  and enter the exit section  114 . The DC potential is selected to choose the energy to be transmitted. In volts, the potential is typically about 15% or so of the energy in electron volts. 
   The ions then travel into the exit region  114  where RF is applied. In the RF section, a 5 MHz 150 V signal is added to the DC deflection potential. The applied voltage may vary. In the RF section, the O+ ions are transmitted by the analyzer  110  and enter the TOF analyzer  120 . For the deflection voltage and RF frequency used, the H+ ions are seen to be totally absorbed by the plates  116  and  118 , while the O+ ions exhibit a high transmission fraction. 
   In the exit section  114  of the ion path, the RF frequency is chosen so that a heavy ion will undergo many cycles of low-amplitude spatial oscillation as it traverses the deflection plates. Lighter ions will undergo a much smaller number of higher-amplitude oscillations along their paths. The result is a high transmission of heavy ions (e. g., O+) and a successively lower transmission as the ion mass decreases and the ions begin to strike the deflection plates. 
   By varying the RF frequency and voltage, ion filtering can be optimized for certain combinations of ions at various energies. For a fairly narrow energy range, such as that for H+ at the magnetopause, a single frequency RF deflection voltage is sufficient to allow accurate O+ measurements while reducing the H+ count rate to a known and manageable level. 
   The specific electrostatic analyzer  110  used is a variation on a conventional tophat analyzer. Instead of spherically symmetric deflection plates, the analyzer  110  has a toroidal geometry, which is somewhat more efficient by volume and has focusing characteristics that are better suited to coupling with a TOF mass-analyzer  120 . 
   To illustrate the technique in mathematical terms, consider the effect of a peak RF voltage V 0  at frequency f applied across a deflection gap Δy in a parallel-plate analyzer, in which the DC applied deflection voltage is zero. The deflection from the central plane of the analyzer as a function of time is given by: 
   
     
       
         
           
             
               
                 
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   Because of the difference in velocity between light and heavy ions with the same energy/charge (E), it is more relevant to examine the dependence of the deflection (y) from the central plane of the analyzer on the distance down the segment of the analyzer that has the RF voltage applied. Because v x =√2E/m is constant, we can substitute t=x/v x  in Equation (2). 
     FIG. 3  illustrates plots of y as a function of distance (x) for H+ and O+ with equal energies/charge of 1 keV for the following selected analyzer parameters: V 0 =150 V, Δy=4 mm, and f=ω/2π=5 MHz. Phases of the RF voltage when the ions enter the analyzer  110  are noted on the H +  curves. 
   As shown in  FIG. 3 , the path of the ions through the analyzer depends on the phase of the RF deflection voltage at t=0. This effect is illustrated for phase angles between 0° and 90°, the results of which are representative of the full range of angles. It is evident from  FIG. 3 , that for analyzer segments of a few cm in length, the H+ ions will be deflected into the plates while the O+ ions will not. Eventually, of course, the O+ ions will also strike the plates if they are too long. 
     FIGS. 4A and 4B  illustrate results of a laboratory test using the system of  FIG. 2 , with relative transmission of 1 keV singly-charged oxygen ions ( FIG. 4A ) and protons ( FIG. 4B ) as a function of applied RF frequency. The optimum response is seen to be at about 5 MHz. At this frequency, the proton counts are reduced by nearly three orders of magnitude while the O+ counts are reduced by only about 25% as compared to the response with a DC deflection voltage. 
     FIG. 5  illustrates results of another laboratory test using the system of  FIG. 2 , with the transmission ratio of 1 keV protons as a function of the peak-to-peak voltage of the 5 MHz deflection potential. A thousand-fold reduction in proton throughput is possible. The throughput can be regulated to intermediate values. 
   In sum, the above described system and method solves the problem of spillover of major ion signals in mass analyzers, which results in contamination of minor ion signals. It provides a controllable reduction of major ion throughput with little or no reduction in minor ion throughput. 
   The RF technique described herein can be tailored for effective use in many space and laboratory environments. The method will separate high mass ions from low mass ions regardless of flux differences, and is particularly useful when the light ions have significantly higher fluxes than the heavy ions of interest, a situation that would otherwise cause measurement problems. In space applications, the heavy ions of interest have lower fluxes than the lower mass ions, which favors application of the method herein. 
   The use of analyzer  110  with a TOF analyzer  120  is but one application of the invention. Analyzer  110  could be used without TOF analyzer, acting as a lower resolution mass spectrometer. Also, TOF analyzer  120  could be replaced by other types of mass analyzers, such as a magnetic sector mass analyzer.