1. ESS 265Low Energy Particle Instruments 1 Particle Measurements Types of particle measurements –Langmuir Probes –Retarding potential analyzers –Magnetic spectrographs –Energetic neutral atom imagers –Electrostatic analyzers http://www.igpp.ucla.edu/public/vassilis/ESS265/20080421

2. ESS 265Low Energy Particle Instruments 2 Langmuir Probes, Overview In the ionosphere, where D is ~1-10cm (Ti, Te ~eV, Ne~ 10 2 -10 6 ) Scan in voltage, get Ni, Te, Ne Boom away from spacecraft body, normal to flow Need to ensure sensor is small relative to ion gyroradius Missions: AE, DE-2, AE-C, PVO

3. ESS 265Low Energy Particle Instruments 3 Langmuir Probes, Issues Non exponential fit reveals issues to deal with: –Cleanliness: Spacecraft conductive to avoid charging –Surface materials clean (Oxygen bombardment causes permanent non conductive layer, asymmetry of sensor) Avoid by heating up to outgass early in the mission Avoid by allowing thermal (high energy) electrons to bombard surface –High altitude missions do not have this problem –Work function patchiness low Order of 100mV is too high (corresponds to 1160K) for E-region temperatures of 300K (0.03eV) Use vitreous carbon (Weismann & Kintner 1963 on rockets) or highly oriented metals (PVO: rhenium, molybdenum) –Minimize VxB in high fields Venus, Mars OK (low fields) but at Earth, cross field antenna motion causes E along boom –Minimize by tipping boom along B Further reading: –Brace, L. H., Langmuir probe measurements in the ionosphere, in Measurement Techniques in Space Plasmas: Particles, Geophys. Monogr. Ser. 102, AGU, 1998 –Krehbiel, J. P., et al., The DE Langmuir probe instrument, Space Sci. Instrumentation, 5, 493, 1981. –Mott-Smith, J. M. and I. Langmuir, The theory of collectors in gaseous discharges, Phys. Rev., 28, 727, 1926.

4. ESS 265Low Energy Particle Instruments 4 Retarding Potential Analyzers In the ionosphere, mount along ram velocity, measure species densities –Ram speed (7.5km/s) is high or supersonic relative to ion thermal speed or motion –Spacecraft charging is negative and small relative to motional energy –I-V curve has steps at qV ret = ½m(V sr +V r ) 2 – q  s ; where:  s = sensor potential relative to plasma, V sr = ram speed –Homework #1 Show that the thermal width of the steps is m V sr V th, where V th is the ion species thermal speed. Show that for sensor potential of –0.8V, the step functions are at 1.1V for H + and 6V for O +. Ions can be further differentiated with mass spectrograph behind RPA –See: Chappell et al., The retarding ion mass spectrometer on DE-1, Space Sci. Instr. 4, 477, 1981 Heelis and Hanson, 1998

5. ESS 265Low Energy Particle Instruments 5 RPA/Ion Drift Meters In the ionosphere, mounted along ram velocity, measure species velocity –G2 retards lower energy H +, but allows higher energy O + through –Collimated beam comes through and falls asymmetrically on collectors –G6 suppresses electrons, G3-5 are grounded to remove distortions –Homework #2: Determine transverse velocity V t as function of ram speed, W, D. Issues: V t error can be significant when ram direction angle is large Further reading: –Heelis and Hanson, Measurements of Thermal Ion Drift Velocity and Temperature Using Planar Sensors, in Measurement Techniques in Space Plasmas: Particles, Geophys. Monogr. Ser. 102, AGU, 1998 Heelis and Hanson, 1998

6. ESS 265Low Energy Particle Instruments 6 RPAs in tenuous plasmas In the solar wind, rely on supersonic motion –G1, G3 are shields (ground); G4 is suppressor (-200V) –G3 is modulator between V1-V2, resulting in dif. Current –North-South RPA measurement results in transverse speed –Spinning (2.7sec) results in azimuth sectors (11.25 o, 45 o ) –Scanning in velocity/energy results in temperature –Full scan (11V-1.3 kV) initially tracking mode after lock on Vram allows data compression Further reading: –Lazarus and Paulanera, A comparison of solar wind parameters from experiments on the IMP8 and WIND spacecraft, in Measurement Techniques in Space Plasmas: Particles, Geophys. Monogr. Ser. 102, AGU, 1998 Lazarus and Paulanera, 1998

7. ESS 265Low Energy Particle Instruments 7 Magnetic Spectrographs For low energy particles (left): –post-acceleration V pa behind an RPA provides V, T and m/q –Homework #3 Show that in LIMS: m/q=(Br c ) 2 /(2V pa ), where B is magnetic field, r c magnet curvature For higher energy particles (right): –Broom magnet clears electrons –High field bends high energy ions –Ions that were not bent assumed neutrals (ENAs) Further reading: –Reasoner et al., Light ion mass spectrometer for space-plasma investigations: Rev. Sci. Instr. 53(4), p. 441, 1982. LIMSMagnetic Spectrograph on CRRES

8. ESS 265Low Energy Particle Instruments 8 Electrostatic Analyzers Electrostatic deflection analyzes velocity distribution –Analyzer constant, K=R 1 / , where  =R 2 -R 1 ; Outer shell is at 0 Volts, inner shell at potential V. –Electrostatic deflection at entrance aperture can measure incoming ions from different directions if spacecraft non-spinning –Homework #4 Show that the energy E of the particles of charge q, incident on the MCP is E=-K q V /2 Further reading: –Carlson et al., The electron and ion plasma experiment for FAST: Space Sci. Rev. 98, 33, 2001. –McFadden et al., The THEMIS ESA plasma instrument and in-flight calibration, Space Sci. Rev., in press

9. ESS 265Low Energy Particle Instruments 9 Time of Flight Electrostatic deflection => energy per charge: E/Q. Time of flight, , => energy per mass E/M –Post-acceleration U ACC provides sufficient energy for optimal McP operation and timing electrons at foil –Electrons generated at carbon foil result in energy loss  –Homework #5. Show M/Q=2(E/Q + qU ACC )/(d/t) 2 *  Further reading: –Moebius et al., 3D plasma distribution analyzer with time-of-flight mass discrimination for Cluster, FAST and Equator-S, in Space Sci. Rev., in Measurement Techniques in Space Plasmas: Particles, Geophys. Monogr. Ser. 102, AGU, 1998

10. ESS 265Low Energy Particle Instruments 10 Neutral Particle Flux measurements Variant of RPA to perform neutral flux measurements Further reading: –King and Gallimore, Rev Sci Instr. 68(2), 1997

11. ESS 265Low Energy Particle Instruments 11 Energetic Neutral Atom imaging First ENA image from ISEE-1 Medium Energy Particle Analyzer Roelof et al., 1987 ENA at Saturn from CASSINI/MIMI ENA at Earth from IMAGE/HENA ENA at Jupiter from CASSINI/INCA

12. ESS 265Low Energy Particle Instruments 12 Energetic Neutral Atom imaging Ion Neutral Camera (INCA) on Cassini to Saturn –FOV = 90°×120° OV –Serrated deflector plates at 0 & 66kV Deflect 500keV/q particles –Three layer foil at entrance slit suppresses UV –Secondary electrons at entrance steered to McP Determine ENA entrance coordinate normal to plane Determine START signal to TOF –Two-dimensional imaging MCP provide: Particle position at other end of flight path STOP for TOF –Time of flight and energy (McP) result in: Energy and velocity -> mass of ions Can distinguish between hydrogen and Oxygen Further reading: –Mitchell et al., J. Geophys. Res., 109, 2004