Enhanced diamagnetic perturbations and electric currents observed downstream of the high power helicon

Abstract

The high power helicon (HPH) is capable of producing a high density plasma (1017−1018 m−3) and directed ion energies greater than 20 eV that continue to increase tens of centimeters downstream of the thruster. In order to understand the coupling mechanism between the helicon antenna and the plasma outside the immediate source region, measurements were made in the plasma plume downstream from the thruster of the propagating wave magnetic field and the perturbation of the axial bulk field using a type ‘R’ helicon antenna. This magnetic field perturbation (ΔB) peaks at more than 15 G in strength downstream of the plasma source, and is 3–5 times larger than those previously reported from HPH. Taking the curl of this measured magnetic perturbation and assuming azimuthal symmetry suggests that this magnetic field is generated by a (predominantly) azimuthal current ring with a current density on the order of tens of kA m−2. At this current density the diamagnetic field is intense enough to cancel out the B0 axial magnetic field near the source region. The presence of the diamagnetic current is important as it demonstrates modification of the vacuum fields well beyond the source region and signifies the presence of a high density, collimated plasma stream. This diamagnetic current also modifies the propagation of the helicon wave, which facilitates a better understanding of coupling between the helicon wave and the resultant plasma acceleration.

Helicon-type radiofrequency plasma thrusters and magnetic plasma nozzles

“…This technique has been performed previously for identifying the plasma acceleration processes in the MPD thrusters, e.g., in Tobari et al (2007). Two dimensional measurement of the plasma-induced axial magnetic field has been performed in some experiments (Roberson et al 2011;Corr and Boswell 2007;Shinohara et al 2016) and shows the negative value of the change of the axial magnetic field ( B z < 0 ), being consistent with the electron-diamagnetic model. The azimuthal electric current p is obtained from ∇ × = 0 p , where the azimuthal component can be written as The relation of B r ∕ z ≪ B z ∕ r was confirmed in the experiment .…”

“…In plasmas, the motion of the charged particles can sometimes generate the internal current and the resultant magnetic field even in current-free plasmas. Measurement of the internal-current-induced magnetic field can give important information to discuss electromagnetic plasma acceleration processes induced by a Lorentz force and plasma-induced modification of the magnetic field structure, e.g., as performed in Tobari et al (2007) and Roberson et al (2011). Hence the measurement of the plasma-induced magnetic field is also an important technique to discuss the thrustgeneration physics and the plasma dynamics in the magnetic nozzle.…”

Influence of Electron and Ion Thermodynamics on the Magnetic Nozzle Plasma Expansion

“…These assumptions are avoided in [10], where we formulate a 2-D two-fluid model that illustrates the main acceleration mechanisms in a hot-electron and cold-ion plasma, and shows that the thrust generated by the MN is the magnetic reaction force caused by the diamagnetic plasma currents on the MN generator. The diamagnetic nature of the plasma currents was observed in [20], and the magnetic character of thrust was later confirmed in [21].…”

Plasma detachment mechanisms in a magnetic nozzle

“…Additionally, some of the simulations exhibit a saddle-point within this zone where the field actually cancels out, followed by a separatrix surface that extends downstream and divides the magnetic field in two disconnected regions, internal (upstream) and external (downstream). The existence of a point where B = 0 has been observed in recent experiments by Roberson et al 16 Since our current model cannot predict the electron currents that might exist beyond the separatrix line (where ψ < 0), we have taken j θe = 0 in this region. The actual magnetic field may therefore differ in this zone and its neighborhood for the simulations with larger β 0 .…”

“…The diamagnetic nature of the plasma has been recently confirmed experimentally. 16 Indeed, the applied and the induced magnetic fields need to repel each other to produce thrust, requiring a diamagnetic character of the latter.…”