Plasma Preionization Research Articles

Microwave-assisted pre-ionization experiments on GLAST-III

GLAss Spherical Tokamak (GLAST-III) is a spherical tokamak with an insulating vacuum vessel that has a unique single-passage capability for incident microwaves. In this work, electron cyclotron resonance heating (ECRH)-assisted plasma pre-ionization in GLAST-III is explored for three radio-frequency (RF) polarizations (the O-, X-, and M-modes) at different toroidal-field (TF) strengths and filled gas pressures. The optimum hydrogen pressure is identified for efficient plasma pre-ionization. A comparison of the plasma pre-ionizations initiated by the O-, X-, and M-modes shows prominent differences in the breakdown time, location, and wave absorption. In the case of O-mode polarization, microwave absorption occurs for a relatively shorter duration, resulting in a bell-shaped electron-temperature temporal profile. Microwave absorption is dominant in the case of the X-mode, leading to a broader temporal profile. The M-mode discharge contains features of both the X- and O-modes. Efficient plasma pre-ionization is achieved in the X-mode polarization for the intermediate TF strengths (with a central toroidal magnetic field ). Traces of the electron-number density show a similar tendency, as revealed by These results suggest that the X-mode is the best candidate for efficient plasma pre-ionization at low filled gas pressures in small tokamaks.

Comparative study of spark pin-array and printed circuit board surface plasma pre-ionization systems for transversely excited atmospheric CO2 laser performance

The performance of a homemade transversely excited atmospheric CO2 laser, capable to be alternatively run using UV spark pin-array (SPA) and printed circuit board (PCB) surface plasma pre-ionization systems, is comparatively studied. The pin-array configuration is comprised of two rows of 5 mm spark gaps symmetrically located on either side of the discharge volume. The surface plasma pre-ionization system consists of a PCB of which, one side is patterned with isolated millimeter size circular pieces. The performance of both types of pre-ionization in different working conditions was experimentally investigated and the output energies were compared with each other as well as the simulation results based on the six-temperature vibrational-rotational model. It has been found that the SPA system provides better discharge stability and uniformity with higher output energies, while using the PCB surface plasma configuration allowed the laser to operate with higher gas lifetime and lowered the need for helium in its gas mixture.

The Effect of Removing the High Power Pulse Transformer on the Microwave Pulse Duration in the Preionization Phase in Taban Tokamak

In this study, a simple, economical and efficient preionization system (2.45 GHz, 1 kW) was constructed to assist the current startup in Taban tokamak ( $B_{T}= 0.7$ T, $R = 0.45$ m, a = 0.15 m). Power pulse transformer is a part of the preionization circuit, and its removal from the system can have impact on the performance of microwave duration and plasma preionization. In the present system, the magnetron was connected directly to the capacitor using a triggered spark gap. This new preionization system allows studying the electron cyclotron emission (ECR) preionization phenomenon at different toroidal magnetic fields. The toroidal magnetic field ripple was also used to determine the entrance moment of the ECR layer into the chamber. As a result, the exact moment of sending microwave can be determined using the moment of the ECR layer formation inside the chamber for maximum absorption of wave power.

Optimizing the Plasma startup through ECR plasma pre-ionization in Taban Tokamak via Triple Langmuir probe

In the present study, a pre-ionization using a 2.45 GHz microwave through Electron Cyclotron Resonance (ECR) waves was investigated to assist the current startup in Taban tokamak. Since the characteristics of the ECR plasma depend on the gas pressure, the influence of changing the pressure of the filling gas on the ECR plasma parameters has been studied both theoretically and experimentally. Based on the obtained results, the optimum condition has been determined and the tokamak plasma parameters measured in optimal conditions are presented. A proper Triple Langmuir Probe (TLP) has been constructed and used in the experimental tests of this study. Using this diagnostic, the plasma parameters such as electron temperature, floating potential and density were measured. Steps to design and manufacture of the employed TLP are presented in detail. Due to the importance of gas pressure in low frequency ECR startup, it is crucial to determine the optimal startup conditions through controlling the filling gas pressure.

0-D modeling of SST-1 plasma break-down & start-up using ECRH assisted pre-ionization

Electron cyclotron resonance (ECRH) assisted break-down and start-up is considered as useful tool towards the discharge initiation in superconducting tokamaks, where the vacuum vessels and the cryostats are usually electrically continuous with thick walls. ECH pre-ionizations are known to reduce the required central solenoid swing induced toroidal electric field, E significantly. The Steady state superconducting tokamak (SST-1, R=1.1m, a=0.2m) has achieved successful plasma break-down and subsequent current ramp-up with ECH pre-ionizations in both fundamental mode and second harmonic modes with E∼0.35V/m. This work has discussed an appropriate simulation model and validated its results with experiments for the ECRH assisted breakdown and start-up, for both 1st harmonic ordinary mode (O1) and 2nd harmonic extra-ordinary mode (X2), in SST-1 for hydrogen plasmas, where the loop voltage is limited to 0.35V/m. The simulation model is a zero-dimensional (0-D) model. In this model five temporal equations are solved for spatially-uniform plasma. The primary findings of this investigation has been the determination of the threshold ECRH power for successful pre-ionization of plasma in SST-1 and validations of the results with experimental findings in SST-1

Surface plasma preionization produced on a specially patterned PCB and its application in a pulsed CO2 laser

The performance of an atmospheric pressure pulsed carbon dioxide laser employing surface plasma preionization, produced on a specially patterned printed circuit board (PCB), is reported. The surface plasma is formed due to many tiny plasma channels produced in millimeter sized open circular gaps, made by lithography on one side of PCB. The preionizing plasma is mostly consisted of corona or glow stage and transition to spark one hardly occurs. This type of preionization allows a maximum of 220J/l energy deposition into the main plasma, while up scaling is yet possible by more optimization of PCB and the pattern. The laser output energy of 1.2J per pulse with overall efficiency of 7% has been obtained with gas mixture of He:CO2:N2=3:1:1. This type of surface plasma preionization is specifically appropriate for very large volumes and high pressures, where the conventional UV emitting preionizations like spark arrays or corona are not effective.

Plasma start-up results with electron cyclotron assisted breakdown on Frascati Tokamak Upgrade

Several experiments aimed at optimizing plasma pre-ionization using electron cyclotron (EC) waves have been carried out on many tokamaks in recent years as the basis of a multi-machine comparison study made to define the best operation scenarios for ITER, where the plasma breakdown will have to be achieved with a toroidal electric field of only 0.3 V m−1. The FTU (Frascati Tokamak Upgrade, R = 0.935 m, a = 0.3 m) contribution to this study is the main subject of this work. A reduction in electric field, as can be obtained with pre-ionization by ECH, can lower the transformer flux consumption in the start-up phase leading to a longer plasma current flat top. This point is of particular interest in the conceptual design of the steady-state scenario of the proposed FAST tokamak and has also been addressed. In the FTU experiment the scan in pre-filling pressure has evidenced the capability of EC power to increase, by a factor 4, the range of working pressure useful for plasma start-up. Varying the breakdown a minimum electric field of 0.41 V m−1 has been found with 0.8 MW of EC in perpendicular injection. A scan in magnetic field has evidenced that plasma start-up is likely insensitive to alignment between EC resonance and null position. A total transformer flux saving of 22% has been found acting on plasma resistivity (by increasing electron temperature) and on the plasma starting point (for an internal inductance reduction).