Multi-layer structure formation of relativistic electron beams in plasmas

A two-dimensional(2D) electromagnetic particle-in-cell(PIC) simulation model is proposed to study the density evolution and collective stopping of electron beams in background plasmas. We show here the formation of the multi-layer structure of the relativistic electron beam in the plasma due to the different betatron frequency from the beam front to the beam tail. Meanwhile, the nonuniformity of the longitudinal wakefield is the essential reason for the multilayer structure formation in beam phase space. The influences of beam parameters (beam radius and transverse density profile) on the formation of the multi-layer structure and collective stopping in background plasmas are also considered.

A 340 GHz High-Power Multi-Beam Overmoded Flat-Roofed Sine Waveguide Traveling Wave Tube

A phase shift that is caused by the machining errors of independent circuits would greatly affect the efficiency of the power combination in traditional multi-beam structures. In this paper, to reduce the influence of the phase shift and improve the output power, a multi-beam shunted coupling sine waveguide slow wave structure (MBSC-SWG-SWS) has been proposed, and a multi-beam overmoded flat-roofed SWG traveling wave tube (TWT) based on the MBSC-SWG-SWS was designed and analyzed. A TE10-TE30 mode convertor was designed as the input/output coupler in this TWT. The results of the 3D particle-in-cell (PIC) simulation with CST software show that more than a 50 W output power can be produced at 342 GHz, and the 3 dB bandwidth is about 13 GHz. Furthermore, the comparison between the single-beam sine waveguide (SWG) TWT and the multi-beam overmoded SWG TWT indicates that the saturated output power of the multi-beam overmoded SWG TWT is three times more than that of the single beam SWG TWT.

Magnetic field influence on the Penning discharge characteristics

In this work characteristics of pulsed penning ion source for miniature linear accelerators was investigated by experimental measurements and PIC (Particle-In-Cell) simulations. The paper presents dependences of the discharge current and extracted current on intensities of the uniform magnetic field for different pressure. Also, typical examples of the current pulse waveforms obtained by PIC simulation and experiment for different magnetic field are presented. The simulated electron and ion distributions inside discharge gap give qualitative explanation of the experimentally observed fluctuations in current pulses. These current fluctuations arise as a result of the violation of the electric field axial symmetry due to the electron spoke movement of the towards the anode.

Evaluation of Ion Beam Behavior in 50 W Class RF Ion Thruster

Phenomenological behavior of ion beam acceleration through the grid system in 50 W class RF ion thruster has been investigated using PIC simulation and evaluated by experimental test using Faraday probe. Beam trajectory for various grid voltages reveals that the metal engine cover of the ion thruster which is needed to seal RF coil around the discharge chamber affects the beam divergence angle. Simulation result shows that the divergence angle increases by 10.52% mainly because of the larger radial electric field in the presence of the metal engine cover. The divergence angle increases as the accelerator grid voltage increases. The current density distribution measured by the Faraday probe shows a bigger divergence angle with the engine cover installed. For the test cases with mass flow rates from 3 sccm to 4 sccm at the RF power of about 50 W, the current density distribution exhibits the 2nd peak at the radial position about 4 cm from the centerline.

FOCUSING OF POSITRON BUNCH WHEN MOVING IN ELECTRON BUNCH WAKEFIELD IN THE DIELECTRIC WAVEGUIDE FILLED WITH PLASMA

The results of numerical PIC-simulation of focusing accelerated (test) positron and drive electron bunches in the dielectric waveguide filled with radially heterogeneous plasma with the vacuum channel are given in the paper. The wakefield was excited by electron bunch in quartz dielectric tube, inserted into cylindrical metal waveguide. The internal area of dielectric tube has been filled with plasma different transverse density profiles, heterogeneous on radius, with the vacuum channel along waveguide axis. Plasma density for all studied cases was so low that plasma frequency was less, than the frequency of the main dielectric mode. Results of numerical PIC simulation have shown that possibility of simultaneous acceleration and focusing of the test positron bunch are possible in the wakefield. The best acceleration happens in case of plasma absence, however at that there is no focusing of test positron bunch.