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In tokamak machines with low toroidal magnetic fields and high plasma densities, the accessibility conditions impose restrictions to the detection of the Electron Cyclotron Emission ( ECE ). In these machines, the right- hand cutoff condition can be used as an independent method to determine the local electron density from the ECE data in thermal discharges. In this paper is shown the results obatined from the detection of ECE radiation in the TCABR tokamak, in operation at the Institute of Physics of University of Sao Paulo. The effect of the ECE radiation cutoff was observed for different radial positions of the plasma column. To reach the ECE cutoff condition, the electron density was increased monotonically by the use of an external gas puffing system. For sufficient high densities, the emission at some frequencies is cutoff and the first and the last frequency to be cutoff depends on the shape of the density profile. These measurements do not require the plasma to be optically thick. It was observed that, for a toroidal field BO = 1:14T , the first cutoff of ECE occurs for a radial position r » = 5cm . From these measurements the radial electron density was determined. For a symmetric parabolic profile ne = neo (1 ¡ (r=a) 2 ) fi , values of fi between 0.86 and 0.97 were experimentally obtained. A good agreement of these values with those obtained from the microwave interferometer measurements (fi … 0:85 ) was found. Therefore, the ECE right-hand cutoff constitutes an independent method to obtain information about the electron density profile. ECE radiometry is a well established diagnostic for the elec- tron temperature measurements in Tokamak machines ( 1 ). In some cases, when the ECE radiation in X mode attains the cutoff region, temperature measurements cannot be per- formed, but, in despite of this, the radiometer can be used to obtain information about the density profile. In this pa- per, we experimentally obtain the electron density from the ECE right-hand cutoff in the TCABR tokamak. This tech- nique was first used by Lohr ( 2 ) to determine the electron density in the DIII-D tokamak. Above the minimum value of the electron density, when the ECE signal begins to cu- toff, only the frequency used in the ECE radiometer and the magnetic field ( neglecting the poloidal magnetic field ) are needed to determine the local electron density. This proce- dure constitutes an independent method to obatin the local electron density and the results can be compared with those obtained with the microwave interferometer. From the value of ECE frequency that is first/last in the cutoff, the electron density profile can be determined, providing that this can be described by only one or two parameters ( 3 ). In Section 2, a simple description of the experimental arrangement used for the measurements in the TCABR is shown. Some theoretical points of ECE cutoff method are discussed in Section 3. In Section 4, the experimental re- sults are presented and discussed. 2 Experimental aspects related to the

Electron density measurements from right-hand cutoff of ECE in the TCABR tokamak