Effect of the magnetic field structure on the intensity of electron cyclotron emission from the plasma of the L-2M stellarator

Abstract

Results are presented from experimental studies of the influence of the stellarator magnetic field structure on the plasma behavior in electron-cyclotron resonance regimes with a high heating power per electron. The magnetic field structure was changed by varying the induction current I _p from −14 to +14 kA. The plasma electrons were heated at the second harmonic of the electron gyrofrequency by an X-mode microwave beam with a power of P ∼ 200 kW, the average plasma density being in the range n _e = (0.5–2) × 10¹³ cm⁻³. At I _p = 0, the rotational transform varies from $\rlap{--} \iota$(0) = 0.2 on the magnetic axis to 0.8 at the plasma boundary. At a positive current of I _p = 13.5 kA, the rotational transform was $\rlap{--} \iota$(0) = 0.8 on the axis and $\rlap{--} \iota$(a _p) = 0.9 at the plasma boundary. Experiments with a positive current have shown that the radiative temperature first increases with current. When the current increases to I _p = 11–14 kA, strong modulation appears in the electron cyclotron emission signals received from all the plasma radii, the emission spectrum changes, and the emission intensity decreases. At a negative current of I _p = −(6.5–13.5) kA, the rotational transform vanishes at r/a _p = 0.4–0.6. In this regime, the number of suprathermal electrons is reduced substantially and the emission intensity decreases at both low and high plasma densities.