Simulative research on the expansion of cathode plasma in high-current electron beam diode

Abstract

The expansion of cathode plasma has long been recognized as a limiting factor in the impedance lifetime of high-current electron beam diode. Realistic modeling of such plasma is of great necessity in order to discuss the dynamics of cathode plasma. Using the method of particle-in-cell, the expansion of cathode plasma is simulated in this paper by a scaled-down diode model. It is found that the formation of cathode plasma increases the current density in the diode. This consequently leads to the decrease of the potential at plasma front. Once the current density has been increased to a certain value, the potential at plasma front would then be equal to or lower than the plasma potential. Then the ions would move towards the anode, and the expansion of cathode plasma is thereby formed. Different factors affecting the plasma expansion velocity are discussed in this paper. It is shown that the decrease of proton genatation rate has the benefit of reducing the plasma expansion velocity.

Cold atmospheric pressure plasma jet prepares TiO2 coating on carbon fibre for field emission and explosive electron emission

“…where γ is the polytropic exponent, T i is the ion temperature, m i is the ion mass, k is the Boltzmann constant, and f (n 1 /n 0 ) is the reduced function of the concentration of cathode-flare peripheral layers. f (n 1 /n 0 ) can be described in two forms [51],…”

Current loss of magnetically insulated coaxial diode with cathode negative ion

“…In this model, the movements of neutral particles and their mutual interactions are neglected for simplicity. [29,30] Based on the electron impact-ionization model, [29][30][31] the cathode plasma including ions and electrons are generated when an electron beam propagates through the neutral gas layer and ionizes the gas. Similarly, the cathode negative ions (H − ) are produced when the backscattering ions (H + ) bombard and ionize the residual neutral gas according to the ion impact-ionization model as the dominant negative ion production mechanism.…”

“…[20] As a result, the negative ion current loss is determined by its ionization production rate. The ionization production rate is defined as [30] R ion = n t φ i σ ion ,…”

“…with k, P, and T representing the Boltzmann constant, the target neutral gas pressure, and temperature, respectively. The gas temperature T in the impact-ionization model is usually 300 K. [30] It is difficult to estimate the gas pressure and is limited by the PIC simulation calculation capability. Figure 5 shows the variations of the current loss coefficient K with gas pressure in the impact-ionization model.…”

“…Therefore, the average velocity is about 1.1 cm/ns. As the positive ion expansion velocity is just several cm/µs, [29,30] its expansion is not obvious during the period as shown in Figs. 7(c) and 8(c).…”