Theory and analysis of operating modes in microplasmas assisted by field emitting cathodes

Abstract

Motivated by the recent interest in the development of novel diamond-based cathodes, we study microplasmas assisted by field emitting cathodes with large field enhancement factors using a simplified model and comparisons with particle-in-cell with Monte Carlo collision (PIC-MCC) simulations and experiments. The model used to determine current-voltage characteristics assumes a linearly varying electric field in the sheath and predicts transition from an abnormal glow to arc mode at moderate current densities in a 1 mm argon gap. The influence of an external circuit is also considered to show the dependence of current as a function of the applied voltage, including potential drop across external resistors. PIC-MCC simulations confirm the validity of the model and also show the significant non-equilibrium nature of these low-temperature microplasmas with electron temperatures $1 eV and ion temperatures $0:07 eV in the quasi-neutral region. The model is also used to explain experimental data reported for a 1 mm argon gap at a pressure of 2 Torr using three different diamond-based cathodes with superior field emitting properties. The comparison shows that operating conditions in the experiments may not result in significant field emission and the differences observed in current-voltage characteristics can be attributed to small differences in the secondary electron emission coefficient of the three cathodes. However, the model and simulations clearly indicate that field emission using novel cathodes with high field enhancement factors can be used to enhance microplasmas by significantly decreasing the power requirements to achieve a given plasma number density even in gaps at which field emission is traditionally not considered to be a dominant mechanism. V C 2015 AIP Publishing LLC.

Incorporating Resistance Into the Transition From Field Emission to Space Charge Limited Emission With Collisions

“…Similarly, one may add an external resistor to control device current in microplasma devices [21]- [23]. These experiments [21] motivated one-dimensional planar particle-in-cell (PIC) simulations incorporating an external resistor [22] with good agreement with experimental results [21]; however, the conditions were not in the field emission dominated regime.…”

“…When R < Z tp , electron emission transitions from FN to CL directly, while electron emission transitions from FN to OL when R > Z tp . Thus, the resistance plays a critical role in the significance of space charge on emission, which has important effects on field emitters in both vacuum [20] and microplasma generation [22]. This becomes particularly important as gap size is reduced below ∼1 µm [17], which is close to the triple point for nitrogen [18], which should be close to that of air.…”

Microplasma device architectures with various diamond nanostructures

“…Simulation done by Venkatraman et al [44][45][46][47] suggests that ion number density near the cathode causes the ion-enhancement in microelectromechanical devices. On the other hand, in the breakdown of a gas, usually termed Townsend's breakdown, there are two primary mechanisms that contribute to a significant rise in charge carriersgaseous charge production through electron impact ionization (the α process) and cathode charge production through secondary emission (the γ process) [41,47,48]. Secondary emission is the electron emission from cathode materials due to bombarding particles/photons and is typically dominated by ions through Auger processes, though incident photons and metastable ions can also be a factor.…”

Stable Field Electron Emission and Plasma Illumination from Boron and Nitrogen Co‐Doped Edge‐Rich Diamond‐Enhanced Carbon Nanowalls

“…Additionally, it has been simulated that materials showing high FEE characteristics can markedly improve the performance of microplasma device characteristics. [64,65] Figure 9c shows the plasma illumination (PI) photographs of the microplasma devices, signifying that the D-ECNW nanostructures as a cathode ignited the plasma at a low voltage of 330 V corresponding to a threshold field of 0.33 V µm −1 . Also, the intensity of the PI photographs increases systematically with the applied voltage.…”