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Electron Temperature Probe An Introduction to Space Instrumentation, Edited by K. Oyama and C. Z. Cheng, 91–105. Electron temperature probe K.-I. Oyama and C. Z. Cheng Plasma and Space Science Center, National Cheng Kung University, No. 1 Tah-Hsueh Rd., Tainan 70101, Taiwan The electron temperature probe (ETP) was invented in Japan in 1970’s. The probe measures the electron temperature accurately and the measurement is not influenced by the electrode contamination. The instrument has low weight, low data transmission bit rate and low power consumption. The probe has been deployed in many sounding rockets, Earth orbiting scientific satellites, and Mars exploration spacecraft in Japan. The probe has also been deployed in sounding rockets in West Germany, India, Canada, USA, and Brazil. The probe has also been deployed in Brazilian satellites, Korean satellites, and recently as a Taiwan satellite payload. The manuscript describes the principle of the ETP instrument, the system configuration, the mechanical interface with respect to the sensor location, the control timing between data processing units; some useful information, the interference with other instruments, and future improvements and tasks. Some useful information for conducting performance check after the instrument fabrication and before the flight deployment is also presented in Appendix A. Keywords: Ionosphere, electron temperature, sounding rocket, satellite. 1. Introduction sounding rockets of India, Brazil, USA, and West Germany The electron temperature (Te)isone of the fundamen- with great success (Sampath et al., 1974; Smith et al., 1978; tal plasma parameters which are needed for ionospheric re- Schlegel and Oyama, 1987; Wilhelm et al., 1987; Piel et search. Electron temperature has been measured by the al., 1988, 1991, 1992; Rohde et al., 1993). The probe was ground based Incoherent scatter radar, satellites, and sound- installed in 5 Japanese satellites: TAIYO (Oyama and Hi- ing rockets since in 1947 when Reifman and Dow (1949) rao, 1975), HINOTORI (Oyama et al., 1982), KYOKKO measured the DC current—voltage curve by using Lang- (Oyama and Schlegel, 1984), OHZORA (Oyama et al., muir probes (Langmuir and Mott-Smith, 1924; Mott-Smith 1985; Oyama and Abe, 1987), and AKEBONO (Oyama, and Langmuir, 1926) on board the V-2 rocket. However, 1994). The probe was also deployed in the satellites of Ko- the electrode contamination is the most serious issue to rea, Brazil, and Taiwan (Muralikrishna et al., 2000, 2004; obtain accurate and reliable Te using Langmuir probes. Takahahsi et al., 2000; Lee et al., 2002). ETP has sim- Szuszczewicz and Holmes (1975) invented a pulsed probe ple design, is reliable and provides excellent data. Because to remove the electrode contamination effect for sounding the ETP system can be easily manufactured by university rocket experiments. The pulsed probe can give accurate students, it can be considered as one of the possible in- Te values even by using conventional unclean probes. The strument candidates for future student satellites, or even Langmuir probe, which was installed in the DE-2 satellite, small earthquake-studying satellites. The purpose of the was cleaned by a system that applies high negative voltage manuscript is to provide the technical information of the in- for ion bombardment as well as a heater inside the electrode strument, and encourage college students to build and study (Krehbiel et al., 1981). However, the electrode contami- the instrument. nation issue was not addressed in some of the Langmuir probes that were installed on recent satellites (Lebreton et 2. Electron Temperature Probe al., 2006). 2.1 ETP measurement principle The Electron Temperature Probe (ETP) was first invented When a voltage is applied to an electrode placed within by Hirao and Miyazaki (1965) and later improved by Hirao plasmas as shown in Fig. 1, the probe current, which flows and Oyama (1970) in Japan. ETP has very simple electric through the electrode, is expressed as (Mott-Smith and circuits, consumes low electric power, and has low weight. Langmuir, 1926) ETP provides accurate measurement of Te in spite of using conventional unclean electrodes (Hirao and Oyama, 1971, I p(V ) = Ies exp(−eV/kTe) − Ii (1) 1972a; Oyama, 1975). ETP can be operated without any ex- tra commands to the system from the ground stations dur- where V is the relative potential between the applied volt- ing operation, except a command to switch on/off the in- age and the space potential Vs . The space potential, also strument. ETP has been flown in more than 50 Japanese known as the plasma potential, is the electric potential sounding rockets including ones from the Showa Antarc- within the ambient plasma in the absence of any probes. tica station (Hirao and Oyama, 1980; Oyama and Hirao, Typically the space potential is approximately uniform out- 1982; Schlegel et al., 1983). ETP was also deployed in the side of the plasma sheath region. In the above equation, e is = . × −19 the electronic charge ( 9 1 10 C), and k is the Boltz- Copyright c TERRAPUB, 2013. −23 mann constant (= 1.38 × 10 J/K), Ie exp(−eV/kTe) is the electron current in the electron retardation region where 91 92 K.-I. OYAMA AND C. Z. CHENG: ELECTRON TEMPERATURE PROBE Fig. 1. Basic concept of the Langmuir probe measurement system for sounding rockets and satellites. U indicates the power source, j indicates the current measurement device, Cs indicates the satellite or rocket speed, and Collector indicates the probe electrode. Fig. 3. The current-voltage characteristic curve shifts to the negative voltage side when the high frequency sinusoidal voltage signals with amplitude of 400 mV and 800 mV are added to the sweeping voltage of aDCLangmuir probe. The floating potential shift is a function of Te. When a high frequency sinusoidal voltage is added to the sweeping voltage of a DC Langmuir probe, the current- voltage characteristic curve shifts to the negative potential side as shown in Fig. 3. The phenomena was first found by Takayama et al. (1960), and later studied by many people (e.g., Buckley, 1966; Dote and Ichimiya, 1967). 6 The shift of the current-voltage characteristic curve de- Fig. 2. One example of the current-voltage curve, where ne = 2 × 10 3 els/cm and Te = 1500 K are assumed. The horizontal axis is the applied pends on the amplitude of the high frequency sinusoidal probe voltage relative to the space potential. wave,acos ωt, which is superposed on the sweeping volt- age of a DC Langmuir probe. The probe current is ex- pressed as the space potential is positive with respect to the electrode, 1/2 Ies = ene S(kTe/2πme) , ne is the electron density, S is IP (V + a sin ωt) the surface area of the electrode, and I is the ion current. i =−Ii + Ie exp(−e(V + a sin ωt)/kTe) When the probe voltage is lower than the space potential, =−I + I exp(−eV/kT ) exp(−ea sin ωt/kT ). (4) electrons are retarded, and ions are accelerated. When the i e e e probe voltage is much larger than the plasma potential, there Note that because the ions are too heavy to respond to the is no longer a sheath potential to reflect electrons and the high frequency oscillating voltage, the ion current is not electron current saturates, and Ies is called the electron sat- affected by the oscillating voltage and remains the same uration current. For sounding rocket experiments Ii is ex- value. Employing the Jacobi-Anger expansion eizsin θ = = (v + v ) v ∼ ∞ pressed by Ii ene S r t , where r ( 1–2 km/s typi- ( ) inθ ( ) = n=−∞ Jn z e , and making use of the relations J0 iz cally) is the rocket velocity, vt (∼300–500 m/s typically) is n I0(z), and J−n(z) = (−1) Jn(z), where Jn(z) is the n- the thermal velocity of ions (mainly NO+, and O+,inthe 2 th order Bessel function of the first kind, and I0(z) is the ionospheric E and F regions), and the surface area S is in- modified Bessel function of the zeroth order, we have fluenced by a sheath modified by the motion of the probe. When the applied probe voltage equals to the space poten- ea sin ωt = exp − tial (i.e., V 0), Eq. (1) reduces to kT e ∞ I (V ) = I − I . (2) ea ea inωt n −inωt p es i = I0 + Jn i e + (−1) e .(5) kTe n=1 kTe When the probe current I p(V ) is zero, the applied probe voltage is called the floating potential. The floating voltage Then, the total probe current has two parts, the DC compo- relative to the space potential is calculated from Eq. (1) as nent and the oscillating AC component. If we filter out the oscillating AC part, the probe current becomes V f =−(kTe/e)[ln(Ii /Ies)] (3) IP =−Ii + Ies exp(−eV/kTe)Io(ea/kTe). (6) where ln is the natural logarithm. The probe is floating be- cause the applied probe voltage relative to the space poten- Then, the floating potential Vfa at which the probe current tial is sufficiently negative to repel some lower speed elec- is zero for the DC component can be obtained as trons and attract ions to maintain zero net probe current as =−( / ) { / ( / )}. shown in Fig. 2. V fa kTe e ln Ii Ies Io ea kTe (7) K.-I. OYAMA AND C. Z. CHENG: ELECTRON TEMPERATURE PROBE 93 Fig. 5. The basic principle of the ETP system. Fig. 4. The ratio of two floating potential shifts as a function of Te. The ratio is calculated for four different amplitudes of the high frequency sinusoidal voltage signal (a = 0.1 V, 0.2 V, 0.4 V, and 0.8 V).
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