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.

The experimental results on the Alfvén mode structure identification by the microwave reflectometry in TCABR tokamak are presented. The knowledge of the spatial spectrum of the excited waves is crucial for optimization of Alfvén wave plasma heating and noninductive current drive scenarios in tokamak plasmas. No less important is the possibility to use the Alfvén wave excitation for diagnostic purposes. A microwave reflectometer with fixed frequency was used to register plasma density oscillations driven by the excited Alfvén waves, under the condition of the spectrum scanned by a controlled plasma density rise. It is shown that when the position of the local Alfvén resonance, rA, which is defined by relation omega = k||(rA)C A(rA), is close to the plasma zone where the microwave signal is reflected, the high-frequency modulation of the output signal of the reflectometer at the RF generator frequency increases. The possibility to use the observed effect for finding the plasma current profile in tokamaks is discussed.

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.

The rotating low frequency (RLF) field penetration and dissipation and the effect of ponderomotive forces driven by Ergodic Magnetic Limiter (EML) on the poloidal/ toroidal flow in tokamak plasmas are discussed. EML coils are represented as a sheet current expanded in Fourier series with poloidal/toroidal wave numbers M/N depending on coil shape and feeding. The Alfvén wave mode conversion effect in the RLF range is found responsible for wave dissipation at the rational magnetic surfaces qr = -M/N = 3 typical for EML coil design. Analytical and numerical calculations show maximums of LF field dissipation at the local Alfvén wave resonance omega = <FONT FACE=Symbol>½</FONT>k||cA<FONT FACE=Symbol>½</FONT> near the rational magnetic surface qr = 3 in Tokamak Chauffage Alfvén Brésilien. The poloidal rotation velocity U, taken into account in the dielectric tensor, can strongly modify the LF field and dissipated power profiles. Even stationary EML fields can dissipate at the local Alfvén wave resonance (UM/rA = k||cA). Preliminary estimations show that the stationary EML fields can decelerate the plasma rotation.

Using two dimensional numerical codes, we show that the direct electron heating found with electron cyclotron emission (ECE) radiometry is related to local Alfvén wave resonances in Tokamak Chauffage Alfvén Brésilean (TCABR) plasmas. The m/N =1/2, 0/2 and 0/3 modes are dominant in the observed wave heating, due to Alfvén wave continuum absorption. In particular, the resonant absorption of the m = 0 sideband harmonic plays an important role in the central core and peripheral heating. The increase of the electron temperature during the radio frequency pulse was accompanied by a local density rise that produced a cut-off of the ECE emission. The observed heating profile is also consistent with the reflectometer measurements of the density fluctuations induced by radio frequency fields in the local Alfvén wave resonance of the m/N =-1/-2, 1/-3 modes in TCABR.

The preliminary design study of the Alfvén wave system for the ETE spherical tokamak is presented. The main objective of the proposed RF system is generation of travelling Alfvén waves, which can be used for the study of the non-inductive current drive and the plasma heating in a wide range of operational regimes, which are specific for tokamaks with small aspect ratio R/a <IMG SRC="/img/revistas/bjp/v34n4b/til.gif">1.2 - 1.5. The antenna system consists of four modules, which are separated by 90º in toroidal direction. Each module has two sets of RF current carrying poloidal straps that are positioned symmetrically in relation to the equatorial plane of the vacuum chamber at the poloidal coordinates ±45º. The poloidal extension of each strap is approximately 90º. The excited mode numbers can be controlled by the phasing of the feeding RF current. In the basic regime of operation, the straps in the same toroidal cross-sections are fed by the RF currents with (0, ±pi/2) phasing so that they can excite travelling modes M = ±1, N = ±1. In order to decrease the periphery RF power deposition and to improve the selectivity of the Alfvén mode excitation, the inclination of the antenna straps in toroidal direction can be adjusted in the range of angles ±30º. The dependence of the antenna efficiency on the antenna orientation is studied numerically in the frame of 1-D MHD model.

Plasma resistivity is estimated in runaway discharges on TCABR tokamak from the decay of positive voltage spikes using method developed in I. El Chamaa Neto et al., Phys. Plasmas 7, 2894 (2000). In this regime, the plasma detachment from the limiter is observed experimentally indicating that plasma recombination is the main process of plasma particle losses. In order to investigate if this process is actually taking place, measurements of electron temperature in the plasma are necessary. The electron temperature deduced from the plasma resistivity is in the range 0.2 - 1.5 eV.

There is a renewed interest in using Alfvén waves (AW) in tokamak plasmas. Previously, AW were actively explored mostly for current drive and plasma heating in tokamaks. Presently, the possibility of the anomalous and neoclassical transport suppression by AW in tokamak plasmas is being vividly discussed. AW can also induce poloidal and toroidal plasma rotation. Toroidal plasma rotation can reach the subsonic level. These flows can substantially affect neoclassical transport both in collisional and weakly collisional plasmas. In this paper, the effect of plasma subsonic toroidal flows induced by Alfvén waves on transport processes in the edge of elongated tokamak is investigated. The dependence of poloidal plasma rotation and ion heat conductivity on the elongation parameter and the ratio of induced toroidal velocity to the sonic speed are analytically obtained.

Beryllium foils of different thickness have been employed for measuring the electron temperature in tokamaks using the absorbing method based on ratio of the bremsstrahlung intensities, emited by the plasma. However, in the case of small tokamaks with low electron temperatures, such as TBR-1, the foils must be very thin, becoming very fragile, hard to handle and with insufficient absorbing power of line radiation from impurities. Also they are very expensive. In this work, we have investigated whether other absorber materials, obtained by depositing Aluminium on polypropylene films, for example, could give similar results. These new kind of filters have good mechanical properties and are light tight, as opposed to the thin beryllium foils which have shown, very often, the presence of pinholes. Also, the attenuation of the line emission from impurities, with the use of such filters, is more effective. Silver foils have also been manufactured and used, but with unsatisfactory results due to the TBR-1 low plasma temperature. However, silver foils should be considered a good candidate for measuring Te through the absorbing method in large tokamaks with higher plasma temperatures.