High-frequency noise on antennas in plasmas

Abstract

Fluctuations in plasmas at frequencies near the electron plasma frequency (ωp ) have been measured with in situ wire antennas. Such observations are important to the understanding of basic plasma properties (discrete versus collective effects) and the use of antennas in plasmas. The experiments are performed in a large (1 m diameter ×2 m) pulsed dc discharge and afterglow plasma (ne<1012 cm−3, kTe<5 eV) with a weak axial magnetic field (B0=5 G). The fluctuations are detected from wire antennas (length L≫radius a≳Debye length λD) with a low-noise microwave receiver. The observations reveal three different physical processes that determine the noise spectra: (i) single particle shot noise, (ii) collective oscillations by bounded sheath–plasma resonances, and (iii) noise enhancements by longitudinal plasma waves. The first phenomenon (shot noise) gives rise to broadband noise (ωωp) on both electric and magnetic antennas. In the evanescent regime (ω<ωp) , the shot noise is induced by random electron motions through distances of, at most, a collisionless skin layer (c/ωp) around the antenna. Recalling that electron transit-time effects cause absorption of waves in a collisionless skin layer (known as ‘‘anomalous’’ skin absorption) the present observation of a collisionless skin emission effect can also be understood by the equivalence of blackbody absorption and emission coefficients. The second phenomenon (sheath–plasma resonance) is observed as a narrow resonant enhancement in the shot noise below cutoff (ω<ωp) on electric antennas surrounded by sheaths or dielectrics. The series sheath–plasma resonance, usually identified from reflection/absorption measurements with incident waves, is established here, for the first time, as a feature of thermal emission spectra from antennas. The third phenomenon (plasma dielectric ε(ω,k)→0) produces a broad noise enhancement at ω≳ωp on electric antennas. It can only be observed in the open-loop voltage of dipoles measured with a high-impedance transformer between antenna and 50Ω transmission line. The enhanced noise is electrostatic, randomly polarized, but not significantly enhanced by the presence of energetic electron tails. Existing theories can describe the new observations qualitatively but not quantitatively.