Experiments on strong Langmuir turbulence (LT) driven by electron beam are reported. The technique of cold high-current relativistic electron beam (REB) permits to set up experimental conditions that are practically important but difficult for theoretical treatment of LT. These conditions include strong kinetic effects of plasma non-Maxwellian electrons, ion-acoustic oscillations which are weakly damped due to plasma non-isothermality and dispersion of Langmuir waves that are considerably modified by external magnetic field. A relatively dense plasma permits the use of the Thomson scattering method for observation of spectra of plasma fluctuations, electron distribution function, and local dynamics of plasma density. LT is studied in two operating modes that are characterized by moderate and increased current of REB. The experimental results with moderate REB current do not support the widely accepted picture when most of the Langmuir oscillations are trapped in density cavities. The energy flow through turbulence to plasma electrons is explained without major contribution of fully developed collapse, whereas, with increased REB current dynamic density cavities of spatial scale much less than the size of turbulent region are directly observed.
Core density turbulence in the HSX Stellarator