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 righthand 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 São 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 B O = 1:14T, the first cutoff of ECE occurs for a radial position r <FONT FACE=Symbol>@</FONT> 5 cm. From these measurements the radial electron density was determined. For a symmetric parabolic profile n e = n eo [1 - (r/a)²]alpha , values of alpha between 0.86 and 0.97 were experimentally obtained. A good agreement of these values with those obtained from the microwave interferometer measurements (<FONT FACE=Symbol>a »</FONT>0:85) was found. Therefore, the ECE right-hand cutoff constitutes an independent method to obtain information about the electron density profile.

A millimeter/microwave detection system, in operation in the TCABR Tokamak is described. The system is used for electron cyclotron measurements. The main part of the system is a heterodyne sweeping radiometer based on a BWO oscillator that operates in the frequency range of 52 to 85GHz. The system operates in two modes : fixed frequency (maximum resolution of 10 mus) and sweeping mode (50 mus per frequency step). The radiometer is calibrated in frequency and in radiation intensity. The frequency calibration is made by means of a precision harmonic oscillator. The absolute calibration was done using a blackbody (microwave absorber) immersed in liquid nitrogen (77 K) and also put in an oven with adjustable temperature up to 1470 K. Two others components are also used for periodic intensity calibration check and sensibility measurements : a Criogenic Matched Load and a Noise Source. A Gaussian antenna is used for better space resolution measurements. Between the antenna and the radiometer, oversized waveguides are used to reduced the signal attenuation. The antenna axis is in the equatorial plane of the machine and perpendicular to the plasma column axis. The accessibility and absorption conditions are discussed. Results showing time and radial profi les of the detected ECE radiation for the TCABR are presented. For a magnetic fi eld of B TO = 1.14T it was verifi ed that the maximum permissible density to access the second harmonic in the X mode is n e0 <FONT FACE=Symbol>@</FONT> 2.3 × 10(19) m-3.