Microwave Launcher Research Articles

The ECRH-Power Upgrade at the Wendelstein 7-X Stellarator

The existing ECRH system at W7-X consists of 10 gyrotrons, with output power levels ranging from 0.6 MW up to 1.0 MW each at a frequency of 140 GHz, quasi-optical transmission lines and microwave launchers at the plasma vessel. Compared to other large fusion experiments, W7-X has a relatively low power-to-volume ratio. However high heating power is particularly necessary for achieving high plasma beta values, where the improved confinement of fast ions, one of the optimization criteria of W7-X, can be examined. It is therefore necessary to expand the ECRH systems in several consecutive steps. It is planned to increase the number of gyrotron positions from 10 to 12 and at the same time to evolve the gyrotron output power in several development steps from 1 MW to nominal 1.5 MW and, finally, up to 2 MW. At the same time, the transmission lines will also be upgraded for 2 MW operation. A special effort is also made to improve the reliability of the system by the fast control system.

The striplastron: A new circular stripline microwave launcher for plasma generation in capillary tubes.

Until now, microwave plasmas could be ignited in small capillary tubes (inner diameter less than 1mm) by generating surface waves propagating along their dielectric surfaces using mainly surfatron launchers or stripline-based launchers, such as "split-ring resonators." This work presents a novel launcher, which is called the "striplastron," because it is halfway between surfatrons and stripline launchers. It is a circular stripline directly excitated by a microcoaxial cable. Compared with the surfatron, it is smaller and the part of the plasma hidden by the launcher is limited to its thickness (∼1-2cm). Moreover, its frequency response presents an intense and large resonance. Eventually, its geometry could allow the simultaneous ignition of several microplasmas in capillaries arranged in parallel, which could be very interesting for applications, such as the use of multi-tube microplasmas as metamaterials.

Plasma power balance: methodology and investigations of microwave capillary discharges

Nowadays, where strong energy constraints are set by the environmental and economical context, the real power coupled in plasmas becomes a critical aspect particularly for applications where plasma technologies are facing competition. This issue is enhanced for atmospheric pressures micro-plasmas, where the local power density can be very high (e.g. 105 W cm−3) which implies fast heat exchanges. The precise knowledge of the power coupled to plasma is also a key for fundamental understanding of discharge properties. This is often used as an input parameter for plasma modeling and its inaccuracy can mitigate the predictive quality of plasma simulation tools. In the present paper the macroscopic power balance was established for continuous microwave (MW) plasmas generated in capillaries in argon gas flows. The macroscopic power budgets were performed based on measurements of microwave leak fluxes, heat fluxes (thermal radiation, convection, conduction), optical radiation and chemical powers. It was proven that the traditional power measurements at the MW generator would largely overestimate the real power coupled to plasmas, namely by at least a factor two. An important power fraction is found in the heat transfer through the MW launcher, the impedance mismatching being here better assessed. The proposed methodology, which is based on the physics of transport phenomena, can be employed for characterization of other (micro-) plasma sources.

Utilization of O-X-B mode conversion of 28 GHz microwaves to heat core electrons in the upgraded Proto-MPEX

The magnetic geometry of the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) was recently modified to enable more effective utilization of 28 GHz microwave auxiliary power, specifically: (1) to heat plasma electrons in the radial core of the device and (2) to deliver the heated plasma to the target plate of the device. To achieve this goal, the microwave launcher geometry and placement were significantly re-engineered, guided by previous experimental results and computational modeling. The core electron temperature in the launcher region is observed to increase from 3 eV to 11 eV with 30 kW of auxiliary power after the improvements, and an increase from 3 eV to 6 eV is concurrently measured in the target region (∼1 m from the launcher) at electron density above O-mode cutoff. Radially resolved measurements in the launcher region exhibit a strong dependence on the magnetic geometry. The results of a magnetic field scan reinforce the effectiveness of the intended O-X-B mode conversion scenario that is currently planned for microwave heating of the Material Plasma Exposure eXperiment (MPEX).

The Flexible Plasma Trap (FPT) for the production of overdense plasmas

Electron Cyclotron Resonance Ion Sources are currently the most efficient ion sources among those used in facilities dedicated to nuclear physics. The need for a more flexible magnetic field and RF injection system suggested to design and develop a different type of plasma trap, named Flexible Plasma Trap (FPT). The magnetic field of FPT is generated by means of three coils while microwaves in the range 4–7 GHz can be injected by three different inputs, one placed along the axis and two placed radially. FPT can work in different plasma heating schemes so it will be an ideal tool for studies of plasma and multidisciplinary physics. Moreover, a microwave launcher has been designed and installed to the FPT for launching microwaves with a variable tilt angle with respect to the magnetic field. This paper describes the characteristics of the FPT along with the preliminary results of plasma diagnostics.

Investigation of a nitrogen post-discharge of an atmospheric-pressure microwave plasma torch by optical emission spectroscopy

The species, N2(C), N2(B), and N2+(B), in the post-discharge of a nitrogen microwave induced discharge (2.45 GHz) at atmospheric pressure are investigated by means of optical emission diagnosis of the spatial distribution of emission intensities of N2(C-B), N2(B-A), and N2+(B-X) transitions. Correspondingly, the post-discharge can be divided into two distinct regimes, the early and late afterglows. It is found that not only atomic N survives in the late afterglow regime of the post-discharge but also the N2+ ions are produced even far from the microwave launcher. This is attributed to the fact that the vibrationally excited N2(X, υ) and ground state N(4S) with a long lifetime can be conveyed at far distance and act as the precursor for generating N2+ ions locally.

A new launching scheme for ECR plasma based on two‐waveguides‐array

ABSTRACTThis article describes the design, the simulations, and the tests of a microwave launcher for alternative plasma‐heating schemes in the Flexible Plasma Trap (FPT). A waveguide‐phased array has been designed to tilt the wave injection angle: this will allow modal conversion to generate Electrostatic Bernstein Waves and then to obtain overdense plasmas. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2629–2634, 2016

Comparative study of microwave radiation-induced magnetoresistive oscillations induced by circularly- and linearly- polarized photo-excitation.

A comparative study of the radiation-induced magnetoresistance oscillations in the high mobility GaAs/AlGaAs heterostructure two dimensional electron system (2DES) under linearly- and circularly- polarized microwave excitation indicates a profound difference in the response observed upon rotating the microwave launcher for the two cases, although circularly polarized microwave radiation induced magnetoresistance oscillations observed at low magnetic fields are similar to the oscillations observed with linearly polarized radiation. For the linearly polarized radiation, the magnetoresistive response is a strong sinusoidal function of the launcher rotation (or linear polarization) angle, θ. For circularly polarized radiation, the oscillatory magnetoresistive response is hardly sensitive to θ.

Effects of Mass Flow Rate on the Thermal-Flow Characteristics of Microwave CO2 Plasma.

In this study, the thermal-flow characteristics of atmospheric pressure microwave CO2 plasma were numerically investigated by simulation. The electric and gas flow fields in the reaction chamber with a microwave axial injection torch operated at 2.45 GHz were simulated. The microwave launcher had the standard rectangular waveguide WR340 geometry. The simulation was performed by using the COMSOL Multiphysics plasma model with various mass flow rates of CO2. The electric fields, temperature profiles and the density of electrons were graphically depicted for different CO2 inlet mass flow rates.

Large-Volume Plasma Device with Internally Mounted Face-Type Planar Microwave Launchers for Low-Temperature Sterilization

Large-Volume Plasma Device with Internally Mounted Face-Type Planar Microwave Launchers for Low-Temperature Sterilization

Study of microwave components for an electron cyclotron resonance source: Simulations and performance

A high power (2 kW, CW) magnetron-based microwave system operating at 2.45 GHz has been designed, tested, characterized, and used to produce plasma. The system consists of a microwave source, an isolator, a directional coupler, a three-stub tuner, a high voltage break, a microwave vacuum window, and a microwave launcher. These microwave components were simulated using microwave studio software. The low power and full term characterization of the microwave system has been done using vector network analyzer. The system was tested for 2 kW continuous wave of microwave power using glass-water load. The microwave system has been developed to study the microwave interaction with plasma at different operation regimes (Gases: Nitrogen, argon and hydrogen; Gas pressure : 10−5–10−3 mbar; Microwave power : 300–1000 W; Magnetic field: 875–1000 G) and to extract the proton beam current with hydrogen produced plasma. A plasma density ∼5 × 1011 cm−3 and average electron temperature of ∼13 eV was obtained. This article describes various aspects of the microwave system including design, fabrication, characterization and performance studies of the microwave components.

Density of atoms in Ar*(3p54s) states and gas temperatures in an argon surfatron plasma measured by tunable laser spectroscopy

This study presents the absolute argon 1 s (in Paschens’s notation) densities and the gas temperature, Tg, obtained in a surfatron plasma in the pressure range 0.65<p<100 mbar. The absorption signals of 772.38, 772.42, 810.37, and 811.53 nm lines, absorbed by atoms in 1s3, 1s4, and 1s5 states, were recorded with two tunable diode lasers. Tg is deduced from the absorption line shapes when scanning the laser wavelengths. The line profile, which is a Doppler broadening dominated Gaussian at gas pressures of p<10 mbar, changes to a Voigt shape at p>10 mbar, for which the pressure broadening can no more be neglected. Tg is in the range of 480-750 K, increasing with pressure and decreasing with the distance from the microwave launcher. Taking into account the line of sight effects of the absorption measurements, a good agreement is found with our previous measurements by Rayleigh scattering of Tg at the tube center. In the studied pressure range, the Ar(4 s) atom densities are in the order of 1016−1018 m−3, increasing towards the end of the plasma column, decreasing with the pressure. In the low pressure side, a broad minimum is found around 10<p<20 mbar and hence the Ar(4 s) atom densities increase slightly with rising pressure. For the studied pressure range and all axial positions, the density ratio: 1s5/1s4/1s3 is very close to a Boltzmann equilibrium by electron impact mixing at the local Te, which was previously measured by Thomson scattering. The Ar(4 s) densities are successfully compared to a detailed Collisional Radiative Model.

Sterilization Method for Medical Container Using Microwave‐Excited Volume‐Wave Plasma

AbstractWe demonstrate a novel sterilization technique that sterilizes medical instruments stored in medical containers by generating a microwave‐excited volume‐wave plasma inside medical containers using a planar microwave launcher. We confirmed that a plasma was generated inside the medical container by the microwaves transmitted through the heat‐resistant plastic lid of the container. A Langmuir probe was used to study the characteristics of the microwave‐excited volume‐wave plasma generated inside the container. The inactivation characteristics of Geobacillus stearothermophilus spores set inside the medical container were also investigated. 2.3 × 106 spores were inactivated after irradiation for 40 min or longer by a plasma generated in a simulated air mixture. This inactivation time could be reduced to 30 min by adding water vapor to the air‐simulated plasma.magnified image

Thomson scattering on argon surfatron plasmas at intermediate pressures: Axial profiles of the electron temperature and electron density

The axial profiles of the electron density n e and electron temperature T e of argon surfatron plasmas in the pressure range of 6–20 mbar and microwave power between 32 and 82 W have been determined using Thomson Scattering of laser irradiation at 532 nm. For the electron density and temperature we found values in the ranges 5 × 10 18 < n e < 8 × 10 19 m − 3 and 1.1 < T e < 2.0 eV. Due to several improvements of the setup we could reduce the errors of n e and T e down to 8% and 3%, respectively. It is found that n e decreases in the direction of the wave propagation with a slope that is nearly constant. The slope depends on the pressure but not on the power. Just as predicted by theories we see that increasing the power leads to longer plasma columns. However, the plasmas are shorter than what is predicted by theories based on the assumption that for the plasma-wave interaction electron–atom collisions are of minor importance (the so-called collisionless regime). The plasma vanishes long before the critical value of the electron density is reached. In contrast to what is predicted by the positive column model it is found that T e does not stay constant along the column, but monotonically increases with the distance from the microwave launcher. Increases of more than 50% over 30 cm were found.

Application of an antenna excited high pressure microwave discharge to compact discharge lamps

A novel type of high pressure microwave discharge has been investigated to feed the microwave power at the centre of the compact high pressure discharge lamps using the antenna effect. This method of microwave discharge is named as the antenna excited microwave discharge (AEMD). The 2.45 GHz microwave of around 50 W from the solid state microwave generator can sustain a stable plasma column in the small gap between a couple of antennas fitted on the compact lamp filled with discharge gases at a pressure higher than atmosphere. The AEMD has been applied to a compact metal halide lamp and an extremely high pressure mercury discharge lamp. As a result, the metal halide lamp showed high luminous efficacy of around 130 lm W−1. The excellent lamp properties obtained here can be explained by the low heating loss at the antennas and the lamp wall. The profiles of the microwave electric field in the lamp and the microwave launcher have been numerically calculated to consider the microwave power supply into the lamp.

Large Volume and High Density Surface Wave Plasmas Sustained by Two Microwave Launchers

Surface wave plasma (SWP) is an electromagnetic excitation along the planar interface between a dielectric and plasma medium when plasma density is so large that its permittivity becomes negative. An experiment SWP system consisting of two microwave launchers (upper and side microwave launcher) has been developed for producing large volume surface wave plasmas in our laboratory. The experimental investigation shows that comparable uniformity plasma with not only large volume but also high density properties has been obtained by the two launchers.

Production of large-area surface-wave plasmas with an internally mounted planar cylindrical launcher

We studied the characteristics of two different types of planar cylindrical microwave launchers installed inside the vacuum chamber for the production of a large-area processing plasma. The microwave launcher consisted of a coaxial waveguide and a cylindrical planar cavity, inside which a large-area thin quartz plate was inserted. We tested two configurations of microwave launchers; one was a closed cylindrical cavity structure and the other was a leaky open cavity structure. With a 1.5 kW microwave source, we demonstrated the production of large-area, surface-wave plasma (SWP) using Ar gas. Using the microwave launcher with a leaky open cavity structure, a spatial uniformity of SWP within ±3.5% over 160 mm in radius was achieved at a pressure of 20 ∼ 30 Pa. Numerical analyses using the electromagnetic wave simulator, MAFIA, were also carried out to study the electric field distribution radiated from the microwave launcher and to explain the experimental results.

Characteristics of surface-wave and volume-wave plasmas produced with internally mounted large-area planar microwave launcher

We studied discharge characteristics of microwave plasmas excited with a large-area planar microwave launcher installed internally in a 600-mm-diam cylindrical vacuum chamber. With the microwave power less than roughly 400W, we demonstrated the large volumetric volume-wave plasma (VWP) spread in the entire chamber at a pressure of 14–27Pa in He. Above 400W, the plasma discharge made a sudden transition to higher-density, uniform surface-wave plasma (SWP) having a spatial uniformity of ±3.5% over 300mm in diameter. Electron energy probability functions in the downstream region were studied using Langmuir probe measurements with Druyvesteyn method in both the SWP and VWP discharges.

Production and control of high-pressure surface-wave plasmas for water-repellant fluorocarbon film deposition

The production of large volume fluorocarbon plasmas by surface-wave at high pressure (200–3600 Pa) has been achieved. To this end, a microwave launcher made of slot antennae and of an air layer between the antennae and the quartz plate has been used. The influence of the shape of the antennae and of the thickness of the air layer has been studied in terms of homogeneity of the plasma on the quartz plate, as the applied power was varied, and simple models allowed for the understanding of the observed trends.For the configuration enabling the best balance between pressure and homogeneity, fluorocarbon polymer films have been deposited on silicon substrates for different reactive gas mixtures (CF4+C4F8) in argon. The water-repellency property of the deposited polymer films has been determined by contact angle measurements, and linked to observations of the gas phase by OES measurements in the visible spectral range. The absence of film when pure CF4 was used as a reactive gas is explained by the presence of oxygen in the discharge, which is probably released in the gas phase through etching of the quartz plate.

Characterization of Quasi-Optical Launcher for ECRH in SST-1

The design and characterization of a high-power microwave launcher have been discussed. The launchers have been designed for electron cyclotron resonance heating of the plasma in the Steady-State Superconducting Tokamak (SST-1). High-power microwave beam launchers mainly consist of a focusing mirror and a plane mirror combination to focus the beam at a specified location. Two microwave beam launchers are fabricated to launch the waves from the low magnetic field side (radial port) as well as from the high magnetic field side (top port). The frequency of operation is 82.6 GHz, and the power is 200 kW/continuous wave. A gyrotron capable of delivering 200 kW power at 82.6 GHz is under fabrication at M/s. Gycom. The mirrors of the launchers are based on quasi-optical Gaussian beam theory. The mirrors provide cooling for long-pulse (1000-s) operation. Low-power microwave characterization is done to check the performance of the launchers. A low-power microwave beam incident on the focusing mirror of the launcher and focusing action is analyzed at the output of the launcher.