C. da C. Rapozo

Lower hybrid electron cyclotron heating and associated parallel cooling

The authors have observed experimentally that during electron cyclotron heating the electron longitudinal temperature drops as the perpendicular temperature increases. An RF source of 2.45 GHz and 1.0 kW was used. When an additional RF source (lower hybrid wave) of 30 MHz and 0.35 kW was introduced, it heated the plasma via the lower hybrid resonance at r=5.0 cm without, however, destroying the anisotropy in the temperature. The experiment was carried out in a linear mirror machine with a low-density ( approximately 1010 cm-3) weakly-ionized (<1.0%) plasma.

Lower hybrid geometrical resonance in linear LISA machine

Plasma heating by radio frequency (RF) in the electron cyclotron and lower hybrid frequency ranges has been studied using Langmuir probe, magnetic probe, Faraday cup and optical spectrography in the linear mirror machine LISA. It is found that global modes (ECW) rather than WKB ones have been excited. The plasma heating was via the electron cyclotron resonance and via a linear mode conversion of the extraordinary mode into an upper-hybrid mode and this mode decays into a Bernstein wave (at r=+or-3.0 cm). An RF source of 2.45 GHz and 1.0 kW was used where r is the plasma inner radius. When an additional RF source of 30 MHz and 0.35 kW was introduced it heated the plasma via the lower hybrid resonance at r=5.0 cm and resulted in a 30%, or more, increase in overall electron temperature and plasma density.

Lower hybrid resonant layer excitation and mode conversion in the linear LISA machine

The radio frequency absorption near the lower hybrid resonance of an inhomogeneous plasma has been studied using Langmuir and magnetic probes and also a Faraday cup. The plasma was confined in the mirror linear device LISA, excited by a 28 MHz RF source. It has been shown that the absorption rate and the resonance region in space depends on the resonant volume created by the external magnetic field. Due to the collisional damping, the quality factor of the machine is comparable to a low-Q cavity resonator.

The negative r.f. conductance of the sheath in a low density plasma

In this work we use nonlinear sheath capacitance properties in low density plasma to produce parametric amplification of an r.f. signal in the high frequency band (H.F.). The experiment has been carried out in the linear mirror device LISA of the Universidade Federal Fluminense (UFF), where a helium plasma was produced by a radio-frequency source of 30 MHz with a power which can be varied from 10 watts to 100 watts. The experimental results show a r.f. signal amplification at 5 MHz with a transducer gain of 5.7. It shows good agreement with the theoretical results of a classical analysis of parametric amplification with a nonlinear device.

Electron cyclotron and collisional modulation heating in LISA machine

It has been produced a weakly ionized radiofrequency plasma by electron cyclotron resonance in the Mirror Linear Device LISA of the Universidade Federal Fluminense. The plasma is analyzed both analytically and experimentally. It has been shown that the plasma heating rate is a function of the resonant volume, where two volumes have been considered. The collisional plasma heating process due to the magnetic field modulation has also been studied.

EFICIENCIA DA RAZÃO DE AQUECIMENTO DE UM PLASMA EM FUNÇÃO DO VOLUME RESSONANTE E DO EFEITO DO BOMBEIO MAGNÉTICO

Observou—se, experimentalmente, na maquina linear tipo espelho LISA, que a razao do aquecimento de plasma, durante o aquecimento ciclotronico dos eletrons, e funcao do grande e pequeno volume de ressonância. Estes volumes ressonantes sao obtidos desconectando—se um certo numero de bobinas, que produzem o campo magnetico na maquina LISA. Utilizou-se, tambem, um codigo numerico para resolver duas equacoes que descrevem o processo de aquecimento colisional do plasma (bombeio magnetico colisional).

AQUECIMENTO DO PLASMA NA RESSONÂNCIA HÍBRIDA NA MÁQUINA “LISA”

Foi criado um plasma de Helio com uma fonte de micro—onda na faixa de fRF—2,45 GHz e PRF = 800 W e estudou—se a temperatura e densidade eletronica, quando o plasma e excitado por radio frequencia de fRF = 30 MHz e PRF = 0,1 kW. Observou-se um ganho de aproximadamente 20% tanto na temperatura como na densidade e os modos predominantes TE12 e TM03.

ONDAS ÍON-ACÚSTICAS NO PLASMA MAGNETIZADO

A defasagem entre os sinais da onda no plasma, na maquina linear LISA Foi obtido, medindo-se a variacao temporal entre os sinais de duas sondas eletrostaticas flutuantes, colocadas no eixo, distanciadas entre si de 70 cm (?Z= 70 cm). Para um plasma de helio cuja pressao esteve em torno de 6 x lO-5 Torr, os sinais detectados pelas sondas foram injetados num osciloscopio TEKTRONIX-485, a fim de serem fotografados e analisados. Esta medida nos levou a conclusao da existencia de onda ion-acustica lenta, com ?< ?ci cos?, propagando-se com ângulo ? diferente de 90o com respeito ao campo magnetico (experimentalmente ?=45o) com velocidade de fase Vf = (kbTe/mi.)½cos? , isto e, v = v cos?, em concordância com os trbalhos de Stepanov(1959) e Hirose e outros (1961).