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Summary forn only given. Study of Langmuir turbulence (LT) in plasma and LT-induced nonlinear phenomena remains an important topic of the fundamental plasma science. A distinctive LT's feature is possibility of electromagnetic radiation emission on plasma frequency (vp) and dual plasma frequency (2vp) due to processes of plasmon conversion on plasma density fluctuations and plasmon-plasmon merging respectively. Well-identified in solar radio-flares, these processes remained weakly studied in laboratory plasmas. In this report we present the first results on experimental investigation of 100-550GHz-emission from hot (Tc~1 keV) and dense (n~1014-1015 cm-3) plasma at the GOL-3 multi- mirror open magnetic trap operating in conditions of turbulent heating by a powerful high-current relativistic electron beam (Ee~0.8 MeV, Ie ~30 kA, je~2 kA/cm2, tau~12 mus, Q~130kJ). An original multi-channel quasi- optical radiometric diagnostics developed for spectral measurements at frequencies 100-550GHz corresponding to aforementioned vp- and 2vp-emission processes is described. The diagnostics includes fast Shottky-diode-based receivers with internal preamplifiers for registering 1muW-10 mW radiation at integration time < 5 ns, and a set of quasi-optical band-pass metal mesh filters for selecting spectral bands at typical FWHM 20-60GHz and out-of-band rejection 20-40dB. The performed REB-plasma interaction experiments revealed a significant level of subterahertz emission with duration 7-10mus, which in some cases exceeded a saturation level of Shottky-detectors at "plasma-receiver" distance 2-3 m. It was observed a strong correlation of emission spectra with dynamics of plasma density, as well as spectra broadening and high-frequency shifting during REB-injection. The data of radiometric diagnostics are compared with combined data of laser scattering and oth- r GOL-3 diagnostics for detailed analysis of LT-physics and verification of theoretical models.