Characterization and Control of an Ion-Acoustic Plasma Instability Downstream of a Diverging Magnetic Nozzle

Abstract

The study and control of resonant instabilities in magnetized plasmas is of fundamental interest over a wide range of applications from industrially relevant plasmas to plasma sources for spacecraft propulsion. In this work electrostatic probes were employed to measure a 4-20 kHz instability in the ion saturation current downstream of an electric double layer (DL) in an expanding helicon plasma source. The amplitude and frequency of the instability were found to vary in inverse proportion to the operating argon gas pressure (0.2-0.6 mTorr) and in direct proportion to the applied rf power (100-600 W) and applied solenoid current (3-8 A). A spatially resolved characterization of the maximum instability amplitude determined two radial maxima, corresponding to the locations of most positive radial ion density gradient. Control and inhibition of the instability were achieved through the application of a kHz voltage amplitude modulation to the 13.56 MHz radio-frequency (rf) power supplied to the helicon antenna. Through the application of voltage amplitude modulations in the frequency range 2-12 kHz the instability was reduced by up to 65%, exhibiting a greater reduction at higher applied modulation frequencies. This effect is described through a variation in the radial ion density gradient via asymmetrically attenuated ion acoustic density perturbations induced by the applied voltage modulation. The application of voltage amplitude modulations has been demonstrated as a potential control mechanism for density gradient driven instabilities in magnetized plasmas.

Physics of E × B discharges relevant to plasma propulsion and similar technologies

“…better detachment in a way) is achieved with intermediate magnetic strengths yielding highly-magnetized electrons and weakly-magnetized ions (i.e. easier to detach) [365], [434]. The actual relevance of electron demagnetization on the downstream plume divergence requires to be confirmed experimentally.…”

Vector resolved energy fluxes and collisional energy losses in magnetic nozzle radiofrequency plasma thrusters

“…The plasma powers contributing to the thrust ℎ are 0.07, 0.97, and 1.59 W for the solenoid currents of 0.1, 0.4, and 2.0 kA, respectively, which are calculated from Equation (6). Then, power ratios to the plasma power contributing the thrust ℎ are calculated and shown in Figure 13.…”

“…Magnetic nozzle radiofrequency (rf) plasma thrusters have been developed worldwide for future high-power electric propulsion systems [1][2][3][4][5][6][7]. The main components of the magnetic nozzle rf plasma thruster are an rf antenna, a dielectric tube, and the solenoid.…”

Nonlinear interactions of ion acoustic waves explored using fast imaging decompositions

“…IAW may also be useful tools to probe sheath criteria in multiple ion plasmas [11][12][13] . Technological plasmas may also trigger IAW, that, in return, affect their operation, as reported for Hollow Cathodes 14,15 , Hall thrusters 16 and diverging magnetic nozzle thrusters 17 .…”