Experimental Study of Spectral Index, Mode Coupling, and Energy Cascading in a Turbulent, Hot-Ion Plasma

Abstract

The spectrum of electrostatic potential fluctuations was measured with a capacitive probe in a steady‐state, turbulent plasma confined in a magnetic mirror geometry. The plasma conditions ranged over ${\text{5×10}}^7{\mathrm{\le n}}_e{\text{≤5×10}}^8/{\mathrm{cm}}^3{\text{,8≤T}}_e{\text{≤38eV,350≤T}}_i\text{≤930eV}$ , and $B_{\max }\text{\hspace{0.17em}=\hspace{0.17em}1.0\hspace{0.17em}T}$ . When oscillation peaks were absent, the amplitude spectrum of electrostatic potential fluctuations had a power‐law dependence on frequency of the form ${\mathrm{\theta =\theta }}_0{\nu }^{\mathrm{-n}}$ over the range 0.2‐1.0 MHz. The value of the spectral index $n$ depended on the probe position in the magnetic field, and, at a given position, depended on the plasma characteristics. The spectral index was generally below the value $\text{n\hspace{0.17em}=\hspace{0.17em}2.5}$ predicted by some turbulence theories formulated in $k$ space. Enhancement of the turbulent spectrum on the high‐frequency side of oscillation peaks suggests that the direction of net energy flow in the turbulent spectrum is from low frequencies to high frequencies in this plasma. The effects on the amplitude spectrum of a sinusoidal external modulation of the anode voltage were assessed over the range 0‐100 kHz. The spectra exhibited several mode coupling phenomena, including enhanced harmonics of the externally imposed frequency, and sideband modulation of plasma generated oscillation peaks. External excitation of the plasma at frequencies below 50 kHz resulted in only a small degree of ion heating.