Transition between continuous and discrete spectra of high-wavenumber turbulence in neutral gas pressure scan experiments on a linear magnetized plasma

Abstract

Spectral transition between continuous and discrete spectra has been observed in high-wavenumber turbulence excitation experiments. The transition appeared in neutral gas scanning, which shows that continuous spectra dominate at low gas pressures and discrete spectra dominate at high gas pressures. Both spectra have well smaller spatial scale than the ion effective Larmor radius. The discrete spectra have 6-11 peaks and maximum peak values at 4th or 5th peak. Correlation length analysis reveals that the continuous component is turbulent and the discrete component is coherent. The bicoherence analysis shows that the turbulent spectra have finite nonlinear coupling, whereas the discrete spectra have no corresponding coupling.

Mass dependency of high-wavenumber turbulence in a linear partially magnetized plasma

“…For reference, indicators of wavenumber normalized to the ion Larmor radius evaluated from ion thermal velocity have been included at the top of Fig.3. We can see that the discrete spectra are in the range of k h q i % 1-6, which is consistent with the spatial scales of ion cyclotron range fluctuations, including ion Bernstein waves.The previous study suggested that the peak interval frequency, denoted as f interval , of the discrete spectrum is comparable to ion cyclotron frequency f ci 29. If this is the case, it aligns with characteristics associated with instabilities related to ion cyclotron motion, which includes ion Bernstein waves and ion cyclotron drift waves [31][32][33].…”

“…The previous study suggested that the peak interval frequency, denoted as f interval , of the discrete spectrum is comparable to ion cyclotron frequency f ci . 29 If this is the case, it aligns with characteristics associated with instabilities related to ion cyclotron motion, which includes ion Bernstein waves and ion cyclotron drift waves. [31][32][33] To confirm this possibility, we depicted the peak interval frequency as a function of ion cyclotron frequency in Fig.…”

“…Previous studies have shown that high-wavenumber broad spectra transition to discrete spectra as the pressure of neutral gas increases. 29 These fluctuations propagate at approximately the speed of sound, and there is an indication that the peak-to-peak frequency of the discrete spectrum aligns with the ion cyclotron frequency. If these propagation characteristics and frequency features are indeed accurate, it is expected that they would vary with ion mass.…”

“…In a previous study, neutral gas pressure scans were performed to identify spectral transitions between continuous and discrete spectra. 29 The dependence on pressure, magnetic field, or mass may be more direct and important for the investigation of the dispersion relation. Accurate scanning of pressure is difficult, and controlling magnetic field over a wide range while maintaining specific experimental conditions is challenging.…”

“…The interval between each probe tip is 3:9 mm, and the measurable wavenumber extends up to k h ¼ 16 cm À1 when assuming a specific propagation direction. 29 Typical plasma profiles obtained in the mass scan experiment are shown in Figs. 1(a)-1(d).…”