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Parametric generation of second sound in superfluid helium: Linear stability and nonlinear dynamics

Rinberg, D; Steinberg, V
We report experimental studies of parametric excitation of second sound (SS) by first sound (FS) in superfluid helium in a resonance cavity. The results on several topics in this system are presented: (i) The linear properties of the instability, namely, the threshold, its temperature and geometrical dependencies, and the spectra of SS just above the onset were measured. They were found to be in good quantitative agreement with the theory. (ii) It was shown that the mechanism of SS amplitude saturation is due to the nonlinear attenuation of SS via three wave interactions between the SS waves. Strong low-frequency amplitude fluctuations of SS above the threshold were observed. The spectra of these fluctuations had a universal shape with exponentially decaying tails. Furthermore, the spectral width grew continuously with the FS amplitude. The role of three and four wave interactions are discussed with respect to the nonlinear SS behavior. The first evidence of Gaussian statistics of the wave amplitudes for the parametrically generated wave ensemble was obtained. (iii) The experiments on simultaneous pumping of the FS and independent SS waves revealed several effects. Below, the instability threshold, the SS phase conjugation as a result of three wave interactions between the FS and SS waves was observed. Above the threshold two interesting effects were found: a giant amplification of the SS wave intensity and strong resonance oscillations of the SS wave amplitude as a function of the FS amplitude. Qualitative explanations of these effects are suggested
ISSN: 1098-0121
CID: 800732

Acoustic phase conjugation in superfluid helium

Rinberg, D; Steinberg, V
We present the first experimental observation of phase conjugate (PC) second sound (SS) waves superfluid helium. The main feature of a PC wave is that its phase is complex conjugate to the Incident wave phase. It is generated and amplified as a result of nonlinear interaction between an incident SS wave and a first sound (FS) pumping wave. At FS amplitudes larger than the threshold value, a parametric instability, i.e., spontaneous decay of a FS wave into two SS waves, takes place. Three main, theoretically predicted, features of phase conjugate waves were verified experimentally
ISSN: 0031-9007
CID: 800772

Universal broadening of frequency spectra in parametrically driven systems

Rinberg, D; Cherepanov, V; Steinberg, V
We report the experimental observation of the broadening of the frequency spectra of parametrically generated second sound waves by first sound in superfluid He-4. The first experimental evidence of Gaussian statistics of amplitudes of parametrically generated waves is presented. Using the kinetic theory of parametric excitation we suggest that the broadening and exponential tails of spectra result from four-wave resonance interaction in a close analogy with similar-evidence in parametrically generated spin waves. Universality of the exponential tails in the frequency spectra is suggested
ISSN: 0031-9007
CID: 800782