Search

Magnetic islands are known to degrade confinement of energy and particles in tokamak plasmas. They are also present in the precursor of density limit disruptions being observed until the onset of the energy quench of the disruption. The subject of the effect of plasma flow on tearing mode stability has been studied both theoretically [1] and experimentally [2]. However the experimental work has focused on plasmas with Neutral Beam Injection (NBI), which adds high momentum to both the plasma and the island, deviating them from their natural velocities. It is important to know the plasma/island rotation behavior without NBI, not just to know their natural values but because future large tokamaks, like ITER, are expected to have low plasma rotation due to both large plasma inertia and low applied torque. In this work we will show experimental values of the ion velocity and the magnetic island velocity measured in TCABR ohmic plasmas, without NBI, and during the precursor of a density limit disruption. In this way it was possible to compare the natural plasma velocity with the island velocity. The ion velocity was inferred from Doppler spectroscopy of the impurity lines of CVI. Details of this diagnostic system and an error analysis of the measured signal can be found in [3]. With the present set up, the toroidal velocity can be measured with time resolution equivalent to 600 Hz and precision somewhat better than 5 km/s.

In this work we analyze the transport processes in solar loops considering a collisional plasma and high longitudinal plasma flow. The general theory of the neoclassical transport in toroidal configurations with a noncircular cross-section is applied to explain the transport processes in some kinds of solar loops, modeling the solar loop as a toroidal plasma column. The plasma is assumed to be in the collisional regime and to have particle longitudinal flow along the solar loop axis. The poloidal velocity and the radial fluxes of the particles and the ion heat flux are derived in this article. It is shown that the particle poloidal velocity can be measured giving rise to the possibility of having additional connections between the plasma macroscopic parameters, which are very important for the solar loops diagnostic. It is also shown that the particle and heat fluxes are on the ''classical'' level, within the factor of an order of magnitude. We hope that such an approach (together with other theories) can help to explain the transport processes in solar loops, whose shapes are similar to toroidal configurations.

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 @ 2.3 × 10(19) m-3.

In this work, we discuss the preliminary analysis of some disruptive plasma discharges in the TCABR tokamak, operating in the high density limit. The Fourier analysis of the MHD activity was greatly facilitated because the magnetic coils inside the TCABR were installed as to take into consideration the toroidal geometry of the system, in a straight forward manner. What we have observed is that the m = 3, 4 and 7 MHD components dominate during almost the whole discharge duration, whereas the m = 2 MHD mode increased substantially just before the occurrence of a major disruption. Also, we could estimate the angular velocity of the magnetic islands, which was observed to increase up to three times just before the major disruption.

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.