Proposal for Relative Calibration of ECE Diagnostic System in HT-7 Tokamak. BY S. Sajjad Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui , China. HT-7

Outlines  EC Emission in Tokamaks.  Accessibility and Absorption Conditions.  Radial Profiles of ECE Radiation in HT-7.  Process for Relative Calibration.

ECE Emission in Tokamaks For a tokamak of major radius and toroidal field,the harmonic n of the ECE frequency emitted at radius r, in the equatorial plane, is given by where and are respectively the electron charge and mass. The relativistic factor γ, for the usual temperatures found in tokamaks can be considered as unity. The transport of the electron cyclotron radiation in a thermal plasma is given by where I(ω), the intensity of radiation, and is the radiative power per unit area per unit solid angle per unit of angular frequency, τ(ω) is the optical depth given by

where α(ω) is the absorption coefficient andis the Rayleigh-Jeans approximation for the blackbody radiation given by. For an optically thick plasma τ(ω) >1 substantial absorption occurs and which gives the results Therefore, for a given frequency, the detected radiative power is proportional to the electron temperature.

Accessibility and absorption conditions The conditions for accessibility and absorption for a radial profile of the following frequencies can be given as: The first and the second harmonics of the electron cyclotron frequency The right and left-hand cutoff frequencies The upper-hybrid resonance frequency and the plasma frequency

Optical Depth To determine absorption conditions of the plasma, the most important parameter is the optical depth τ(ω).For the extraordinary mode ( n ≥ 2 ) and perpendicular propagation ( θ = π/2 ) parallel to the equatorial plane of the torus τ(ω) is given as, and Where

Radial Pofiles of : Fundamental ECE frequency ( ), second ( ) and third ( ) harmonics of ECE radiation, Plasma frequency. = 2 T, = and = 700 eV. Radial Profiles of ECE Radiation in HT-7 Tokamak

Process for ECE System Calibration  It is proposed that the relative calibration of ECE signals can be done by small change in the toroidal field in consective shots at same plasma conditions.  Where the magnetic field varied a little in each shot to make a superposition for measured frequency points or channels on ECE resonant positions.  Electron temperature profiles and plasma conditions are supposed to be changed very little in these shots.  In this way, relative calibration factors for each measurable ECE frequency points of the receivers can be easily obtained.

Schematic of HT-7 ECE Diagnostic System Pre Amp

ECE Channels VS Position

Plasma parameters for Experiment B T = 2.0 T, 1.99T, 1.98T, 1.97T, 1.96T, 1.95T. (ΔB T =0.01T)= 100 Gauss I p = kA n e = *10 19 m -3 R = 122 cm a = 27 cm

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