Abstract:
A spectrometer is provided for the energy analysis of charged particles. The spectrometer consists of a hemispherical capacitor energy analyzer, a collimator and entrance aperture that define the solid angle of acceptance and geometric factor of the spectrometer, and a charged particle detector. The entrance aperture and collimator are arranged to maximize the geometric factor of the analyzer while retaining high energy-resolution.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to spectroscopy and in particular relates to spectrometers and methods of spectroscopy for the energy analysis of charged particles. 
     BACKGROUND OF THE INVENTION 
     Charged particle spectroscopy is a powerful tool in space science. The energy analysis of the ambient charged particles in outer space provides an understanding of geophysical and extraterrestrial phenomena. Charged particle spectroscopy in space generally involves energy analyzing the charged particles that flow from various directions toward the spacecraft. The spectra collected help us understand atmospheric phenomena such as solar photoionization of the earth&#39;s upper atmosphere and extraterrestrial phenomena such as changes in the solar wind over the solar cycle. The knowledge gained from such instruments also helps us model conditions in outer space. 
     Since the flow of charged particles in outer space is generally low, it is of great importance to fly instruments with a large geometric factor in order to collect data as quickly as possible. The geometric factor is proportional to the product of the charged particle energy analyzer&#39;s entrance aperture area and its solid angle of acceptance. The sensitivity of the instrument (the rate at which particles are counted for a given ambient particle flux) is proportional to the instrument&#39;s geometric factor. 
     In general, there is an inverse relationship between geometric factor and energy resolution for electrostatic energy analyzers. In practice, slit width is often narrowed to increase energy resolution. By narrowing slit width, geometric factor and sensitivity are reduced due to the decreased area of the entrance aperture. High energy resolution instruments tend to have a low geometric factor and high geometric factor instruments tend to have low energy resolution. 
     The trend in space science has been to sacrifice energy resolution in favor of geometric factor to compensate for the low particle fluxes in outer space. High geometric factor instruments can energy analyze the ambient charge particles very rapidly—but at relatively low energy resolution. Spectrometers of inherently large geometric factor and low energy resolution now dominate the field, such as those classified as quadraspherical in design. Some details of the quadraspherical (quarter of a sphere), or “top hat”, design instruments are described by C. W. Carlson et al. in  Measurement Techniques in Space Plasmas: Particles , pp. 125-140, 1998. 
     Although the trend now is to fly compact, large geometric factor, quadraspherical charged particle analyzers, hemispherical electrostatic analyzers have flown in the past to provide very high energy-resolution spectra. Hemispherical electrostatic analyzers are preferred for high energy-resolution work because of their high charged-particle-optical efficiency and their lack of charged-particle-optical aberrations. One such instruments is described by Doering et al. in  Radio Science , Vol. 8, No. 4, 1973, pp. 387-392, flew on three satellites in the 1970&#39;s. The energy resolution of the instrument was 2.5% (change in energy divided by energy, full peak width at half maximum peak height). Charge particle analyzers now used for space flight rarely have energy resolution of better than 5%, and more commonly have energy resolution in the double digits. 
     There is now interest in collecting high energy-resolution spectra of charged particles in outer space. For instance, the determination of spacecraft floating potential is possible through an analysis of high energy-resolution electron energy spectra, as described in L. Goembel and J. Doering, Journal of Spacecraft and Rockets, Vol. 35, No. 1, pp. 66-72, 1998. It is important to measure spacecraft charge because even minor spacecraft charging biases scientific instruments (such as plasma spectrometers) and makes it difficult to interpret valuable data. In extreme cases rapid discharge from a spacecraft can cause costly system failures. Monitoring the charge and reducing it through a controlled discharge can prevent such damage. Other uses for high energy resolution electron spectra exist, such as in the determination of the ratio of ambient atomic oxygen to nitrogen in the upper atmosphere, as described by L. Goembel and J. P. Doering in Journal of Geophysical Research, Vol. 102, No. A4, pp. 7411-7419, 1997. 
     To date, there have been no compact, large geometric factor instruments capable of collecting high energy-resolution charged particle spectra in outer space. The high energy-resolution hemispherical analyzer-based instrument described by Doering et al. in Radio Science, Vol. 8, No. 4, pp. 387-392, 1973 would be considered bulky by today&#39;s standards. It would also be considered slow to collect spectra by today&#39;s standards since its geometric factor was small compared to the quadraspherical spectrometers that are currently in use. Designers of charged particle spectrometers appear to have reached an impasse in efforts to design a compact, high geometric factor, high-energy resolution instrument. Although the fully focusing charged particle optics of the hemispherical condenser design make it the preferred configuration for high energy-resolution spectroscopy, the large hemisphere that would be needed to collect data quickly with a spectrometer of the traditional design rules out the deployment of such an instrument. The accepted rule in the design of space flight charged particle spectrometers has been “if sensor optics are focusing then little can be done to improve performance short of increasing sensor dimensions”, as quoted from D. T. Young, “Space Plasma Particle Instrumentation and the New Paradigm: Faster, Cheaper, Better”, p.8, Measurement Techniques in Space Plasmas: Particles, R. T. Pfaff, J. E. Borovsky, David T. Young, Editors, (Geophysical Monograph; 102), American Geophysical Union (Washington, D.C. 1998). 
     Much development of hemispherical charged particle energy analyzers has been done in fields outside of space science. The double-focusing property of the hemispherical analyzer has long been utilized in the field of surface imaging electron spectroscopy (XPS or ESCA). Hemispherical analyzers with extended arcuate slits such as shown in FIG. 6 of U.S. Pat. No. 3,733,483 to Green et al. (1973), FIG. 4a of U.S. Pat. No. 5,285,066 to Sekine et al. (1994), and FIG. 1 of U.S. Pat. No. 6,104,029 to Coxon et al. (2000) have been used to maximize the sensitivity of such instruments. In such imaging spectroscopy, focusing multi-element fore-optics are used to transmit an electron-spectroscopic image of the surface to the entrance plane of the hemispherical analyzer. The resulting image on the detector has one direction representing energy, and the perpendicular direction representing position on the original surface, as described by U. Gelius et al. in J. of Electron Spectroscopy and Related Phenomena Vol. 52, 1990, p. 761. 
     Traditional hemispherical charged particle analyzers for space flight have contained a circular entrance aperture, such as that of Doering et al. in Radio Science, Vol. 8, No. 4, pp. 387-392, 1973. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes an arcuate entrance slit on a charged particle analyzer to retain energy resolution while increasing aperture area, and, thus, geometric factor. Unlike imaging spectrometers that have contained arcuate slits, the present invention does not utilize imaging fore-optics but has an arcuate collimator that defines the solid angle of acceptance of the instrument. The present invention maximizes the solid angle of acceptance of the instrument and maximizes the aperture area of the instrument so that the ambient charged particles can be collected with greatest efficiency. The double focusing property of the hemispherical analyzer is used to maximize the solid angle of acceptance and charged-particle-optical filling of the space between the hemispherical electrodes while retaining the superb energy resolution of the hemispherical design. 
     The present invention breaks through the perceived impasse in efforts to design a compact high energy-resolution, high geometric-factor charged particle analyzer. The present invention retains the energy resolution of instruments that have flown in the past, but vastly increases geometric factor, by using an arcuate slit for both the collimator and entrance aperture. It is possible to increase the geometric factor by nearly two orders of magnitude over the instrument in Doering et al. with no increase in instrument size. Such a dramatic increase in the geometric factor of the instrument with no increase in bulk makes the instrument of the present invention competitive with similarly sized space science instruments of quadraspheric or other lower resolution design. This invention makes it possible to collect the quality data needed to determine, for example spacecraft floating potential, with a compact instrument and with high temporal resolution. 
     The present invention provides a charged particle spectrometer with a large geometric factor and high energy resolution that is capable of obtaining charged particle spectra of the environment under investigation in a relatively short period of time. 
     The above object is achieved by a charged particle spectrometer containing a coaxial hemispherical charged particle energy analyzer having an input slit extending in the direction perpendicular to a radial direction of the hemispherical electrodes included in the energy analyzer, an input collimator for defining the field of view of the spectrometer which is also extending in the direction perpendicular to a radial direction of the hemispherical electrodes included in the energy analyzer and a detector placed at the output end of the hemispherical analyzer that is capable of detecting the charged particles that pass through the hemispherical analyzer. 
     Other objects and features of the invention will become obvious upon an understanding of the illustrative embodiment about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Examples of embodiments of the present invention will now be described with reference to the drawings, in which: 
     FIG. 1 is an isometric view of the assembled invention. 
     FIG. 2 is a sectional isometric view of the assembled invention. 
     FIG. 3 is an exploded view of the invention with parts labeled. 
     FIG. 4 is a front view of the invention with detector absent. 
     FIG. 5 is an enlarged detail of the area marked in FIG.  4 . 
     FIG. 6 is a schematic drawing of the solid angle of acceptance of the spectrometer as defined by the input collimator and slit. 
    
    
     REFERENCE NUMERALS IN DRAWINGS 
     The following reference numerals appear in the drawings: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 20 
                 Central Bolt 
               
               
                   
                 22 
                 Offset Bolt 
               
               
                   
                 24 
                 Radial Bolt 
               
               
                   
                 26 
                 Magnetic Shield Base 
               
               
                   
                 28 
                 Collimator Plate 
               
               
                   
                 30 
                 Entrance Collimator 
               
               
                   
                 32 
                 Exit Collimator 
               
               
                   
                 34 
                 Base Plate 
               
               
                   
                 36 
                 Aperture Plate 
               
               
                   
                 38 
                 Entrance Aperture 
               
               
                   
                 40 
                 Exit Aperture 
               
               
                   
                 42 
                 Alignment Ring 
               
               
                   
                 44 
                 Offset Alignment Peg 
               
               
                   
                 46 
                 Central Alignment Peg 
               
               
                   
                 48 
                 Inner Hemispherical Electrode 
               
               
                   
                 50 
                 Outer Hemispherical Electrode 
               
               
                   
                 52 
                 Spacer 
               
               
                   
                 54 
                 Magnetic Shield 
               
               
                   
                 56 
                 Bolt Ring 
               
               
                   
                 58 
                 Charged Particle Detector 
               
               
                   
                 60 
                 Spectrometer Solid Angle of Acceptance 
               
               
                   
                 62 
                 Trajectory of Particle through Spectrometer 
               
               
                   
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1,  2 , and  3 , in which the same reference numerals are used to designate like parts, the preferred embodiment of the charged particle analyzer or spectrometer in accordance with the present invention is illustrated. The analyzer includes a pre-energy analysis entrance collimator  30 , an entrance aperture  38 , an inner hemispherical electrode  48  and a coaxial or concentric outer hemispherical electrode  50 . The base of hemispheres  48  and  50  define the plane of focus for the analyzer, approximately the location of an aperture plate  36 . The center of aperture plate  36  therefore defines the spherical center of the charged particle analyzer. A base plate  34  separates a collimator plate  28  from aperture plate  36 . Aperture plate  36  contains entrance aperture  38  and an exit aperture  40 . Collimator plate  28  contains entrance collimator  30  and an exit collimator  32 . A magnetic shield base  26 , collimator plate  28 , base plate  34 , aperture plate  36 , and inner hemisphere  48  are held together with a central bolt  20  and a central alignment peg  46 . The collimators  30  and  32 , apertures  38  and  40 , openings in magnetic shield base  26 , and base plate  34  are held in alignment by an offset alignment peg  44  and an offset bolt  22 . Outer hemisphere  50  is aligned with inner hemisphere  48  by use of an alignment ring  42  that centers outer hemisphere  50  in a magnetic shield  54 . Multiple radial bolts  24  attach a bolt ring  56 , magnetic shield  54 , a spacer  52 , outer hemisphere  50 , and alignment ring  42  to magnetic shield base  26 , collimator plate  28 , base plate  34 , aperture plate  36 , and inner hemisphere  48 . A charged particle detector  58  is located at the output end of the spectrometer as shown in FIGS. 1,  2  and  6 . 
     All of the parts of this embodiment of the inventions are constructed from conductive metal with the exception of  42 ,  44 ,  46 ,  52 , and  58 . Parts  42 ,  44 ,  46 , and  52  are constructed of a non-conducting plastic to electrically isolate the conductive parts they separate. Charged particle detector  58  is constructed from a combination of conducting and non-conducting materials. Magnetically shielding parts  26  and  54  are constructed from 80% permeability mu-metal sheet. Collimator and aperture plates  28  and  36  are constructed from molybdenum sheet in this embodiment of the invention. 
     FIGS. 4 and 5 illustrate the arrangement of entrance aperture  38  and entrance collimator  30 . FIG. 4 is a front view of the preferred embodiment of the invention with detector  58  absent and FIG. 5 is an enlarged view of the circular area marked in FIG. 4. A hemispherical energy analyzer contains two concentric hemispherical electrodes  48  and  50  defining a hemispherical space between. Entrance aperture  38  is in the shape of an arcuate slit whose center of curvature coincides with the spherical center of coaxial or concentric hemispherical electrodes  48  and  50 , with the slit lying on a circle whose radius is substantially midway between inner and outer hemispherical electrodes  48  and  50 . Entrance aperture  38  in this embodiment of the invention extends in an arc by 60 degrees. Collimator plate  28  lies in a plane parallel to, but some distance from, aperture plate  36 . In the preferred embodiment of the invention illustrated in FIGS. 1-5, collimator plate  28  is separated from aperture plate  36  by a distance that is equal to approximately 15% of the radius of inner hemisphere  48 . Collimator plate  28  contains entrance collimator  30 , an arcuate slit whose center of curvature coincides with the spherical center of coaxial hemispherical electrodes  48  and  50 , with the slit lying on a circle whose radius is substantially midway between inner and outer hemispherical electrodes  48  and  50 . Entrance collimator  30  in this embodiment of the invention is an arc that extends somewhat more than 60 degrees and is somewhat wider in the radial direction than entrance aperture  38 , as illustrated in FIG.  5 . It is the combination of entrance collimator  30  and entrance aperture  38  that defines the solid angle of acceptance of the spectrometer. 
     FIG. 6 is provided to illustrate the function of the preferred embodiment of the invention and is not drawn to scale. Some shapes have been simplified and some distances have been exaggerated for clarity. A voltage is applied between hemispherical electrodes  48  and  50 . Aperture plate  36  and collimator plate  28  are electrically isolated from hemispherical electrodes  48  and  50 . Entrance collimator  30  restricts the angle of acceptance into entrance aperture  38 . The dotted outline to the left of entrance collimator  30  approximates the solid angle of acceptance  60  of this embodiment of the spectrometer. A trajectory of a charged particle through the spectrometer  62  appears as a dashed line. The electrostatic potentials of surfaces  28 ,  36 ,  48 , and  50  are set to pass a particle with trajectory  62 . A charged particle enters the spectrometer through entrance collimator  30  and entrance aperture  38  and follows a semicircular path with its center of radius at the hemispherical center of the instrument. The particle is then free to pass through exit aperture  40  and exit collimator  32  and continue to charged particle detector  58 . If a particle has more or less energy than the band-pass of the spectrometer it will not strike detector  58 . If the particle does not enter the spectrometer solid angle of acceptance  60  it will not pass through the analyzer and strike detector  58 . Thus charged particle detector  58  will only detect charged particles of the band of energies selected by setting the electrostatic voltages of collimator plate  28 , aperture plate  36 , inner hemisphere  48 , and outer hemisphere  50  to electrostatic potentials known to those skilled in the art of hemispherical electrostatic charged particle energy analysis. The invention will also only detect particles that enter the spectrometer through its solid angle of acceptance  60  as defined by entrance collimator  30  and entrance aperture  38 . Exit collimator  32  serves to reduce the spectrometer noise due to scattered secondary charged particles produced within the space between inner hemisphere  48  and outer hemisphere  50  in this embodiment of the invention. Exit aperture  40  serves to narrow the energies of charged particles that are allowed to reach the detector in this embodiment of the invention. 
     Thus, the reader will see that the invention provides for a hemispherical charged particle energy spectrometer with a larger aperture area than that with a circular entrance aperture and provides for a large solid angle of acceptance in order to have a large geometric factor. The invention will reduce the time needed to gather a charged particle energy spectrum at a given ambient flux. The invention will be especially important in the field of space science instrumentation where high-speed data collection with compact, light weight instruments is needed. 
     The above description is not intended to limit the scope of the present invention, but rather is an exemplification of an embodiment thereof. Many other variations are possible that are within the scope of the present invention and produce the unexpected results and advantages thereof, For examples in another embodiment exit collimator  32  can be eliminated and the analyzer retains its functionality. Likewise, exit aperture  40  can be replaced with a position sensitive charged particle detector to retain energy resolution with the added advantage of multiple channels of energy detection at a single setting of electrostatic potentials at surfaces  28 ,  36 ,  48 , and  50 . The section of arc of collimators  30  and  32  and apertures  38  and  40  could be less, or more, than the 60° in the preferred embodiment of the invention and the advantages of this invention would be retained. In another embodiment, inner hemisphere  48  and outer hemisphere  50  can be very nearly hemispherical. In yet another embodiment, arcuate entrance collimator  30  could have a shape that very nearly, rather than exactly, follows an arc. 
     Having thus described my invention with the detail and particularity required by the patent laws, what is claimed to be protected by Letters Patent is set forth in the following claims: