Interpulse plasma of a high-power narrow-bandwidth pulsed microwave discharge

The plasma produced by high-power (60–100 kW), narrow-pulse-width (0.5–1 μs) microwaves with a repetition frequency of 0.3–1 kHz has been studied experimentally. Particular attention is paid to the plasma between pulses (interpulse plasma). The plasma was created by launching microwave pulses from a magnetron operating at 3 GHz into a magnetic confinement device in a vacuum chamber. The gas used was a rare gas (Ar), a molecular gas (N2), and a molecular gaseous mixture (O2+N2), with a volume mixing ratio of 1:4. The temporally decaying ion current measured with a Langmuir probe showed different features for the three kinds of gases, which are attributed not only to the effects of recombination and diffusion but also those of metastables and negative ions. The plasma density at the end of each microwave pulse was of the order 1010 cm−3 and the electron temperature was 6–8 eV, much higher than that of the usual afterglow. Numerical simulation of the decay characteristics showed a fair agreement with the experimental results.