Detection of a low-frequency ion instability in a magnetic nozzle

Abstract A low-frequency ion instability, with frequency <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">I</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> between the ion gyrofrequency and the lower hybrid frequency <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> <mml:mo>,</mml:mo> <mml:mi mathvariant="normal">i</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">I</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≪</mml:mo> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi>LH</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , is detected in an argon plasma expanding in a magnetic nozzle for magnetic fields between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>240</mml:mn> <mml:mo>&lt;</mml:mo> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mi>z</mml:mi> <mml:mrow> <mml:mo>,</mml:mo> <mml:mi>max</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>700</mml:mn> </mml:mrow> </mml:math> G. The frequency of the instability exhibits a linear dependence with magnetic field strength, and the wave amplitude has a radial maximum that would match the location of a conical density structure, i.e. high-density cones. For all of the magnetic field cases analysed, the high-frequency spectra showed upper and lower sidebands centred around the driving frequency and at a separation equal to the instability frequency, 27.12 MHz <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mstyle scriptlevel="0"/> <mml:mo>±</mml:mo> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">I</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> kHz. Measurements of the perpendicular wavenumber would satisfy, for certain magnetic field strengths, the dispersion relation of both an electrostatic ion cyclotron wave (ICW) and of an ion acoustic wave. It is hypothesised that the observed low-frequency wave could be an acoustic-like instability propagating perpendicular to the magnetic field, which develops as an ICW at some magnetic field strengths. From the data collected, it is suggested that the high-frequency sidebands may be caused by modulation of the low-frequency wave.