Experimental study of a microwave produced toroidal plasma

We report production of hydrogen plasma using a 2.45 GHz microwave source in a toroidal device BETA [2]. The equilibrium density, floating potential and plasma temperature in the poloidal cross section have been measured. Contours of equal density and floating potential exhibit a slab nature in the vertical direction of the poloidal cross section. Plasma is formed with a peak density of 4 x 10(10) cm(-3). The electron temperature is about 4-7 eV. Our measurements reveal that the X-mode wave is mode converted to electrostatic modes near upper hybrid resonance (UHR), which propagates close to the electron cyclotron resonance (ECR) region where the entire energy of the waves is deposited. Thus we observe plasma profiles broadened towards the weaker magnetic field region up to the UHR region, with an inner boundary near ECR region. This is further supported by numerical calculation. The fluctuating component of the density and floating potential is also measured. Low-frequency (LF) electrostatic instabilities are observed to be radially localized. The presence of both flute and drift modes is reported. Our measurements reveal that the excitation of LF fluctuations depends strongly on the sign of the product del n.del B where del n and del B are the radial density and magnetic field gradients, respectively. LF coherent fluctuating modes are observed in the region where del n.del B > 0 and a turbulent spectrum is observed where the condition del n.del B < 0 is satisfied. Our measurements reveal that collisional drift modes could be excited when n(e) < 0 (where eta(e) = del In T-e/del In n(e)), whereas Rayleigh-Taylor modes could be excited when n(e) > 0 and the density gradient is antiparallel to the effective gravity 'g'.