Surfatron Plasma Source Model

The surfatron launcher is an electromagnetic structure designed to produce surface wave discharges (SWDs). Unlike other launchers like the surfaguide, which is coupled via a rectangular waveguide for high power (kW range), the surfatron is coupled to the microwave power supply via a coaxial cable. This design choice limits the operational power to about 400 W at 2.45 GHz but yields high flexibility, making it extremely suitable for active diagnostics such as Thomson scattering (TS) measurements.

Figure 1: The surfatron plasma setup. The launcher utilizes a coaxial structure to couple electromagnetic energy, establishing a rotational symmetric electric field distribution at the excitation gap.

Computational Domain

The electromagnetic and transport equations are solved in a two-dimensional axisymmetric cylindrical coordinate system. The computational domain layout defines boundary conditions such as Perfect Electrical Conductor (PEC) and Propagation Boundary Condition (PBC) to model the excitation gap and waveguide propagation.

Figure 5: Schematic of the computational domain for the surfatron source. PEC = Perfect Electrical Conductor, PBC = Propagation Boundary Condition.

2D Plasma Distributions (Inside and Outside the Launcher)

The two-dimensional contours below illustrate the distributions of electron density (a), electromagnetic power density (b), and gas temperature (c). The left column (Figure 7) displays the full discharge axis (z = 0 to 600 mm), highlighting the maximum values inside the launcher region (z = 72 - 111 mm). The right column (Figure 8) is a zoom focusing on the plasma column outside the launcher (z = 120 to 600 mm).

Figure 7a: 2D electron density (n_e) from z = 0 to 600 mm.

Figure 8a: 2D electron density (n_e) zoom (z = 120 - 600 mm).

Figure 7b: 2D power density from z = 0 to 600 mm.

Figure 8b: 2D power density zoom (z = 120 - 600 mm).

Figure 7c: 2D gas temperature (T_g) from z = 0 to 600 mm.

Figure 8c: 2D gas temperature (T_g) zoom (z = 120 - 600 mm).

Model Validation Against Experiments

To investigate the effect of increasing pressure on the structure and shape of the plasma, we developed a 2D self-consistent (EM and quasineutral plasma, not consistent with EEDF solver) physical model. The model couples electromagnetic wave propagation, transport of charged and neutral species, and detailed argon chemistry. Crucially, the simulation results were compared with experimental Thomson scattering data across different pressures. The simulations allow us to observe how the plasma behaves when surrounded by the Faraday enclosure that represents the surfatron itself ("inside" the surfatron).

Figure 17a: Axial electron temperature (T_e) comparison at 660 and 2000 Pa between model and Thomson scattering measurements.

Figure 17b: Axial electron density (n_e) comparison at 660 and 2000 Pa between model and Thomson scattering measurements.

Interconnection: Molecular Ions and Gas Temperature

As the operational pressure increases towards the intermediate range (2000–8800 Pa), volume recombination processes become increasingly dominant, comparable in magnitude to ambipolar diffusion. The formation of molecular ions (such as Ar₂⁺) plays a vital role in facilitating these volume recombination events.

The density of these molecular species and the resulting recombination rates are strongly coupled with the spatial distribution of the gas temperature. As the gas temperature rises, it alters both the neutral density profiles and the rate constants of the molecular ion chemistry, preventing the plasma from expanding to the discharge tube walls and causing constriction.

Figure 15a: Axial distribution of the electron density (n_e) at the center of the discharge. Note the non-linear decay at 8800 Pa due to Molecular Assisted Recombination (MAR).

Figure 15b: Axial distribution of the gas temperature (T_g) at the center of the discharge.

Figure 16a: Axial distribution of the molecular ion density (n_Ar2⁺) at the center of the discharge.

Figure 16b: Axial distribution of the electron temperature (T_e) at the center of the discharge.

Bibliography