Spectrum of Langmuir fluctuations trapped in a cavity of a collisional plasma

The Langmuir fluctuations trapped and enhanced inside a cavity of the electron concentration in a plasma are considered. Stationary fluctuation spectrum is calculated with regard to finite collisional damping of the Langmuir waves and nonequilibrium fluctuation source. It is shown that the fluctuation spectrum obtained exhibits a frequency modulation due to the reflection and superposition of the elementary Langmuir waves in the irregularity. The modulation depth and period as a function of the irregularity size and the electron–ion collision frequency is calculated. These results may help to find from experimental observations such important plasma parameters as the irregularly size and lifetime, as well as the electron collision frequency.