Flow characteristics of a cold helium arc‐jet plasma along open field lines

Abstract

We experimentally study plasma parameters including ion acoustic Mach number of expanding cold helium plasma jet with an electron temperature of less than 1 eV flowing along open field lines. It is experimentally found that the ion Mach number increases from 1 to 3, and that the plasma potential decreases by about 1 V. We discuss the experimental results based on a quasi one‐dimensional flow model in which the plasma is assumed to be quasi‐neutral and in a state of thermodynamic equilibrium. Our model describes the ion acceleration, the axial profiles of the potential drop, and the electron temperature/density. The model also shows that the helium ions are accelerated both by the electric field and by the increasing cross‐sectional area of the transonic flow. After the ion acceleration, the ion Mach number decreases and the electron temperature increases. These phenomena are discussed in terms of a shock wave. It is noted that the electron density decreases even in the shock wave. This is discussed in terms of rapid recombination because of the low electron temperature. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Numerical Study on Acceleration and Deceleration Mechanism of Weakly Ionized Plasma Flowing Supersonically Through Open Field Line

“…We would also like to calculate the electric potential self-consistently with the plasma flow. Finally, we would like to compare the numerical results with our previous experiments [10]- [12], and discuss the validity of the present numerical modeling. Fig.…”

“…We should describe such rarefied systems by the Boltzmann equations, which can be solved statistically by particle-based method, and typically, by the direct simulation Monte Carlo (DSMC) method [14]- [16]. Upon our previous experimental and numerical study [10]- [13], we must carry out numerical simulation of the expanding plasma jet as rarefied gas dynamics based not on fluid approximation but on particle modeling, which is precisely the objective of this paper. As will be shown later, whereas the ions and neutral particles can be treated by particle-based simulation, the electrons cannot be treated by particle model like particle-in-cell (PIC) scheme [17], since required spatial discretization becomes too fine, and the time step also too short.…”

“…We developed an arc-heated magnetically trapped expanding plasma jet generator where an arc plasma under atmospheric pressure discharge is continuously being ejected into a rarefied gas wind tunnel under weak magnetic field [9]. By this apparatus, we found supersonic velocity of ions of helium plasma up to Mach ∼3, and the electric potential drop at the end of the hollow magnets, where the magnetic field became diverging [10]. We also carried out a quasi 1-D numerical simulation based on a fluid dynamic treatment, and found that the ions in the plasma is accelerated both by aerodynamic effect and by electric field induced by the diverging magnetic field [11]- [13].…”

“…1 shows the geometry of the present simulation. In the figure, the plasma flow comes in from the left side through a uniform magnetic field in hollow electromagnets with about Mach number Ma ∼ 1.2 [10]- [12]. Of course, the flow field is cylindrically symmetrical.…”

“…In our experimental apparatus, we have six hollow magnets set coaxially with their gap 1 cm, each of which is of 30 turns with its inner diameter D = 8 cm, where the value of magnetic field inside the magnets is found to be 0.16 T with the magnet current 300 A [10]- [12]. Then, we can calculate the magnetic field B as shown in Fig.…”