J. Plasma Physics (2013), vol. 79, part 3, pp. 311–314. c  Cambridge University Press 2012 doi:10.1017/S0022377812001018 311 Optical thickness corrections to ECE measurement of electron temperature in IR-T1 tokamak MONA AHMADI 1 , PEJMAN KHORSHID 1 and OMID JALILI 2 1 Department of Physics, Mashhad Branch, Islamic Azad University, Mashhad, Iran (khorshid@mshdiau.ac.ir) 2 Department of Physics, Noor Branch, Islamic Azad University, Noor, Iran (Received 10 June 2012; revised 5 September 2012; accepted 8 October 2012; first published online 26 November 2012) Abstract. The Electron Cyclotron Emission (ECE) radiation has been investigated, passing through plasma column with concerning absorption effect on the IR-T1 tokamak. The intensity of second harmonic X-mode is used to investigate changes in electron temperature profile. The results show that radiation temperature (T rad ) detected outside the plasma column is less than the electron temperature (T e ) in the core. The radiation temperature can be directly interpreted as T e to about 3–10% reduction. This implies that optical thickness and electron density effects are more considerable at lower temperatures. 1. Introduction Electron temperature measurements via the Electron Cyclotron Emission (ECE) system is a known diagnostic technique in tokamak plasmas (Bornatici et al. 1983; Jun et al. 2006). The theory of cyclotron emission and absorption is considered as a non-relativistic effect in low-temperature tokamak plasmas (v t /c  1). The con- tribution of second harmonic (n = 2) in non-relativistic plasma is expressed as ω = nω ce , where ω ce = eB t /m e c is the electron–cyclotron frequency, e and m e are electron charge and mass respectively, c is the speed of light, and B t is the toroidal magnetic field (Bornatici et al. 1983; Al- bajor et al. 2005). The relation between thermal plasma cyclotron intensity and some other plasma parameters can be expressed as (Pandya et al. 2007) follows: I (ω)= F (τ(ω),ρ) I BB , (1.1) where I BB is the black-body radiation intensity, and F (τ(ω),ρ) is the correction factor, so it can be defined as F (τ (ω) ,ρ)= 1 - exp (-τ (ω)) 1 - ρ exp (-τ (ω)) = T rad T e , (1.2) here τ (ω) ≡  l 0 α (ω) ds is the optical depth, where l is the length of the radiation path. The optical depth of tokamak plasma is related to the plasma absorption α (ω) (Hartfuss and H ¨ Ase 2004). For τ (ω) > 1, plasma is optically thick (Silva et al. 2004), and the correction factor is close to one (F (τ(ω),ρ) ≈ 1). The correction factor depends on both optical depth τ (ω) and reflection coefficient of the vessel walls (ρ) (Pandya et al. 2007). If τ (ω)  1, plasma will be optically thin (Boyd and Sanderson 2003). T e is the electron temperature emitted from the core of plasma, T rad is the electron temperature detected outside the plasma by receiver. For optically thick plasmas, T rad approaches the electron temperature T e if the plasma is optically thin so the T rad is smaller than T e (Janicki 1993; Peters et al. 1995). The paper is arranged as follows: Section 2 introduces the correction factor considering plasma parameters such as absorp- tion and electron temperature. Section 3 describes the results of electron temperature measurements and the last section presents our conclusion. 2. Analytical methods Measurements of the electron temperature in the IR- T1 tokamak main parameters are as follows: major radius R 0 = 45.0 cm, minor radius a = 12.5 cm, toroidal magnetic field B t = (0.6–0.8) T, maximum values of electron density and electron temperature are obtained, respectively, as follows: n e0 = (0.5 - 1.5) × 10 19 m -3 and T e0 = (180 - 200) eV. Since the plasma emission is detected perpendicular to the magnetic field, the second harmonic X-mode wave of ECE can be used as diagnostic tools. So in IR-T1 the intensity of ECE is detected by k α -band (30–40 GHz) heterodyne ra- diometer, antenna positioned in the low field side (LFS) of plasma in mid-plane of the tokamak. The ECE signal goes to the mixer and then to the IF (amplifier/filter). The mixer is coupled to an oscillator, operating in the frequency band of 30–40 GHz. The IF output of mixer is amplified by a factor of 40 dB in the frequency band of 130 ± 15 MHz and is sent to the detector. The video amplifier output signal goes to the video output. Figure 1 shows the block diagram of the k α -band radiometer for ECE measurements (Khorshid 1994). Figure 2 shows the time evolution of plasma current and ECE signal. Figure 2 illustrates discharge current Ip, soft X-Ray signal, loop voltage, and ECE signal channel 1 for a typical IR-T1 discharge. The frequency of the