Coaxial Transmission Line Resonator Research Articles

Analysis in microwave-driven plasma for miniature space propulsion

Abstract Microwave-driven Coaxial Transmission Line Resonator (μ-CTLR) produces a small-volume high density plasma plume. In the previous study, we discovered that the plasma generated by the μ-CTLR remains stable even at low pressure around tens of mTorr, while consuming minimal power below 10 W [K.T. Kim et al., PSST 31 (2022) 105006]. In this study, we have investigated the capability of the μ-CTLR plasma operating at 900 MHz for micro-propulsion applications. At the argon gas flow rate of 100 SCCM (3 mg/s) and the power of 8 W, the plasma plume attains high gas temperature (> 3000 K), high electron density (> 1020 m-3), and electron temperature of about 2 eV. The estimated thrust is about 4 mN, demonstrating that the μ-CTLR has high thrust desirable for space micro propulsion system, together with the other merits of low power consumption and small size.

Plasma-Activated Media Produced by a Microwave-Excited Atmospheric Pressure Plasma Jet Is Effective against Cisplatin-Resistant Human Bladder Cancer Cells In Vitro.

Media exposed to atmospheric pressure plasma (APP) produce reactive oxygen and nitrogen species (RONS), with hydrogen peroxide (H2O2), nitrite (NO2-), and nitrate (NO3-) being among the most detected species due to their relatively long lifetime. In this study, a standardized microwave-excited (ME) APP jet (APPJ) source was employed to produce gaseous RONS to treat liquid samples. The source was a commercially available plasma jet, which generated argon plasma utilizing a coaxial transmission line resonator at the operating frequency of 2.45 GHz. An ultraviolet-visible spectrophotometer was used to measure the concentrations of H2O2 and NO3- in plasma-activated media (PAM). Three different types of media (deionized water, Hank's balanced salt solution, and cell culture solution Dulbecco's modified eagles medium [DMEM]) were utilized as liquid samples. Among these media, the plasma-treated DMEM was observed to have the highest levels of H2O2 and NO3-. Subsequently, the feasibility of using argon ME-APPJ-activated DMEM (PAM) as an adjuvant to enhance the therapeutic effects of cisplatin on human bladder cancer cells (T-24) was investigated. Various cancer cell lines, including T-24 cells, treated with PAM were observed in vitro for changes in cell viability using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. A viability reduction was detected in the various cancer cells after incubation in PAM. Furthermore, the study's results revealed that PAM was effective against cisplatin-resistant T-24 cells in vitro. In addition, a possible connection between HER expression and cell viability was sketched.

Magnetic confinement less microwave discharge gridded ion thruster

A watt-level microwave discharge is induced without magnetic confinement for a high-precision gridded ion thruster, making use of a coaxial transmission line resonator. The thruster is characterized by performance measurements and plasma diagnosis, and a description of the operating mechanism of this thruster is given in this paper. A Faraday probe and a retarding potential analyzer are employed for the diagnosis. The results show that the plume divergence increases due to the electric field formed between the plume and the ion-induced electron emission cathode. A discharge mode transition is found during the increase of the microwave power, in which the bulk heating mode is converted to the surface heating mode. The magnetic-less microwave discharge gridded ion thruster performs a continuously adjustable thrust range of 5–115 μN and a highest total efficiency of 17.2%. Compared to the common ECR ion thruster, this thruster is free from magneto-static interference on the instruments and the additional magnetic momentum on the spacecraft.

Development of an atmospheric fluorocarbon plasma jet generated by pulse modulated microwave discharge

Atmospheric fluorocarbon plasma plays an important role in the surface modification of insulating materials like polymers. The existing fluorocarbon plasma is usually generated by dielectric barrier discharge, which has a low concentration of reactive species and may cause insufficient surface fluorination. This work attempts to develop an atmospheric fluorocarbon plasma jet using a coaxial transmission line resonator by microwave discharge with locally enhanced electric field and high density. The gas temperature is reduced by pulse modulation technology. Three kinds of working gases, pure CF4, Ar/CF4 and He/CF4, are utilized to generate the atmospheric microwave fluorocarbon plasma jet. The discharge images, optical emission spectra, electron densities and gas temperatures are studied experimentally. The results show that the Ar/CF4 plasma jet has the best comprehensive performance, such as strong discharge intensity and controllable gas temperature. The electron density of the Ar/CF4 plasma jet has a magnitude of 1020 m−3, indicating a higher density than that of the frequently used dielectric barrier discharge. With the other conditions unchanged, the gas temperature at the end of the Ar/CF4 plasma jet can be reduced from 410.2 to 347.3 K by decreasing the duty cycle of the modulated pulse from 0.5 to 0.1. Thence, the microwave Ar/CF4 plasma jet is considered to be a promising fluorocarbon plasma source for surface fluorination of polymers.

Study on the ionization and acceleration of a microwave discharge cusped field thruster

The microwave discharge cusped field thruster is a novel concept electric micro propulsion device, also a candidate thruster for the gravitational detection mission. A coaxial transmission line resonator is utilized to feed the microwave into the thruster to generate Xe plasma steadily with a mass flow rate as low as 0.1 sccm. Due to the separation of ionization and acceleration, the thruster performs high operation mode stability over a wide range of voltage in low mass flow conditions. Experimental and simulation methods are carried out to study the ionization and ion acceleration of the thruster. The results show that in operating conditions with a mass flow rate of 0.1 sccm, an anode voltage of 0 V to 1000 V, and a microwave power of 2 W, the right-hand circularly polarized wave (R wave) and the ordinary wave (O wave) play the most important role in the ionization process. The ion acceleration region locates around the exit magnetic separatrix, and the acceleration region tends to converge toward the separatrix as the anode voltage increases, resulting in an increased focus of the thruster plume and concentration of the ion energy distribution. Due to the separation of the ionization and acceleration regions, the thruster performs a divergence efficiency of 0.5–0.8, and an acceleration efficiency of 0.9.

Cathode position effects on microwave discharge cusped field thruster

The microwave discharge cusped field thruster is a novel concept of electric micropropulsion device, which operates μN level thrust in low mass flow rate conditions, making use of a coaxial transmission line resonator. With its advantages of low thrust noise and high thrust resolution over a wide range of thrust, the thruster has emerged as a candidate thruster for the space-borne gravitational wave detection mission. The cathode effects commonly exist in many kinds of electric propulsion, and they are typically significant in micropropulsions. In order to find out the cathode position effects on a microwave discharge cusped field thruster, a thermionic cathode is mounted on a cross-slider for coupling. Under different cathode positions, the plume is analyzed by a Faraday probe and a retarding potential analyzer to analyze the performance and discharge characteristics. The results show that the magnetic mirror effect leads to significant degradation of anode current and an increase in low-energy ion ratio as the cathode moves away from the thruster exit. The electron conduction route also significantly impacts anode current efficiency, related to the cathode-exit distance and the thruster magnetic topology.

Ionization process and distinctive characteristic of atmospheric pressure cold plasma jet driven resonantly by microwave pulses

In the present study, a coaxial transmission line resonator is constructed, which is always capable of generating cold microwave plasma jet plumes in ambient air in spite of using argon, nitrogen, or even air, respectively. Although the different kinds of working gas induce the different discharge performance, their ionization processes all indicate that the ionization enhancement has taken place twice in each pulsed periods, and the electron densities measured by the method of microwave Rayleigh scattering are higher than the amplitude order of 1018 m−3. The tail region of plasma jets all contain a large number of active particles, like NO, O, emitted photons, etc, but without O3. The formation mechanism and the distinctive characteristics are attributed to the resonance excitation of the locally enhanced electric fields, the ionization wave propulsion, and the temporal and spatial distribution of different particles in the pulsed microwave plasma jets. The parameters of plasma jet could be modulated by adjusting microwave power, modulation pulse parameters (modulation frequency and duty ratio), gas type and its flow rate, according to the requirements of application scenarios.

Dual microwave-excited atmospheric-pressure plasma jets with a single power source

In this paper, we propose dual microwave-excited atmospheric-pressure plasma jets (DME-APPJs) that can be generated with a single power source. A T-junction power divider that evenly distributes the input power to two ports is used, and two coaxial transmission line resonators are used as plasma generators for the DME-APPJs. The power divider distributes the incoming power in port 1 and supplies 46% of this power to port 2 and port 3, respectively. Surface treatments are conducted by exposing the multiple plasma jets to polypropylene (PP) specimens and then confirming the degree of hydrophilic change. Using the proposed DME-APPJs, it is possible to perform surface treatments on areas that are up to 50% larger compared to using two single ME-APPJs with the same power consumption. The DME-APPJs thus show more advanced performance for surface treatments by comparison with the single ME-APPJ.

A Single-chip Resonance-frequency Tracking Microwave-excited Plasma Driver IC for Portable Biomedical Applications

This paper presents a first-time single-chip microwave-excited plasma driver IC with resonance-frequency tracking and output-power-regulation scheme for biomedical applications. The power module includes a driver IC and a power amplifier, and is designed to provide stable microwave power that is required for generation of a plasma jet. A 2.4- GHz coaxial transmission line resonator is used to generate the plasma jet using input microwave power. The proposed system continuously monitors the resonance frequency, and controls it by adaptively changing the microwave frequency. To prevent the plasma jet from becoming high enough to damage human skin, the system monitors the power entering the resonator and keeps it constant. The proposed microwave plasma driver IC is implemented in a 0.18-μm complementary metal-oxide-semiconductor process. The system generates stable 1 W with a frequency tracking.

Cell viability and measurement of reactive species in gas- and liquid-phase exposed by a microwave-excited atmospheric pressure argon plasma jet

A microwave-excited atmospheric pressure plasma jet (ME-APPJ) was generated utilizing coaxial transmission line resonators with the resonance frequency of 788 MHz. We investigated the effects of the operating parameters such as input power and gas flow rate on the electron excitation temperature and electron density using optical emission spectroscopy. The amounts of various reactive species in the plasma-activated media (PAM) exposed by the ME-APPJ were estimated using colorimetric methods. The PAM was applied to A549 cultured cells and subsequently incubated for a given duration. The concentrations of NO2− and H2O2 in the PAM incubating A549 cells were monitored until 24 h. The reduced cell viability rate was observed on A549 cells incubated in the PAM. The concentrations of NO2− and H2O2 and the viability of A549 cells depended on the operating conditions of ME-APPJ. The cell viability rate exhibited a decrease with increasing power and decreasing gas flow rate. Correlation between the concentrations of gaseous reactive species in the plasma and in the PAM and the cell viability is discussed.

Increase of Power Efficiency in Coaxial Transmission Line Resonator by Using a Spacer to Compensate for Plasma Impedance

This article presents a method to improve the power efficiency of microwave plasma generators which have a resonator structure after plasma ignition. A coaxial transmission line resonator, one of the microwave plasma generators, was used to design to have an input impedance of 50 Ω in the absence of plasma. However, after plasma ignition, the plasma impedance affects the input impedance of the resonator, and the device efficiency decreases. The proposed resonator includes a spacer at the shorted end; the spacer has the necessary impedance to compensate for the plasma impedance. The spacer increased the device efficiency by up to 27.2% and increased the length of the plasma jet by 2.5 mm compared to the conventional one. Optical emission spectrum analysis confirmed that the intensities of Ar spectrum and reactive oxygen/nitrogen species spectra were also increased by 50% and 30% by using the spacer, respectively.

Five gaseous reactive oxygen and nitrogen species (RONS) density generated by microwave plasma jet

Microwave plasma jets based on a coaxial transmission line resonator show outstanding advantages such as portability, lower power consumption, and low-temperature operation. In this manuscript, the most important of the five gaseous reactive oxygen and nitrogen species (RONS) densities including OH, O, O3, NO, and NO2 densities are measured synchronously, while the plasma jet is treating a biological tissue under different discharge parameters (gas composition, gas flow, skin humidity, and output power). It has been found that the gas composition and the output power are two key factors to regulate the dose of RONS concentrations. For the OH concentration, water vapor in the gas flow plays an important role. By changing the microwave power and the gas composition, the OH concentration changes from 0.4 × 1014 cm−3 to 5.54 × 1014 cm−3. O is mostly affected by the O2 percentage in Ar, and 1% percentage is the optimal value. O3 is proportional to the O2 percentage in Ar, and 2% O2 can get a maximum O3 density of 6.9 × 1016 cm−3. NO is in the range of 2 × 1013 cm−3 to 5.4 × 1014 cm−3, and NO2 is in the range of 5.5 × 1014 cm−3 to 5 × 1015 cm−3. The increase in the humidity of the skin has a positive effect on the concentration of OH, O3, and NO while a negative effect on the O and NO2 concentration. For the normal skin with a humidity of 40%, the concentrations of the long lifetime species like O3 and NO2 are almost 101–2 times those of the other reactive species. This research provides parameter setting guidance for different RONS requirements.

Development and plasma characterization of an 850 MHz surface-wave plasma source

This work presents a simple design of surface wave plasma (SWP) source based on cylindrical cavity excited by ∼ 850 MHz ultra high frequency (UHF) wave. The cavity equipped with four rectangular slots demonstrates the usefulness of a large aperture coupling for plasma generation using top-wall excitation. The UHF power is coupled to the plasma through an upper dielectric wall placed at the open end of a coaxial transmission line that is short-circuited at the other end to construct a coaxial transmission line resonator. Using high-frequency microwave simulation the structure of the cylindrical cavity and the slots are designed and fabricated. Numerous plasma diagnostic methods are used to study the plasma characteristics and the mechanism of surface wave (SW) plasma generation. The plasma generation capability of the source is investigated at various operating pressures and UHF powers. It is seen that reasonably radial uniform plasmas with a very high plasma density ∼ 1011 cm-3 can be produced by a wide variation of pressures from 10 mTorr to 180 mTorr. Further, the usefulness of this UHF plasma source for large area plasma application is realized.

Bullet-shaped ionization front of plasma jet plumes driven by microwave pulses at atmospheric gas pressure

Ionization waves (propagating bullet-shaped plasma) are always present in atmospheric-pressure plasma jets generated by a pulsed DC power supply or low-frequency voltages. Nevertheless, whether these ionization waves exist for pulsed microwave plasma jets remains unclear. In this paper, a coaxial transmission line resonator driven by microwave pulses is capable of generating atmospheric pressure plasma jet plumes. Depending on the discharges, these plasma jet plumes exhibit distinctive characteristics, such as bullet-shaped ionization fronts for argon plasma and ball-shaped for helium plasma. Fast images show argon plasma plumes generating several small branches but only one dominant ionization front travels more distance along the jet axis. Both ionization-wave images and electromagnetic simulation results indicate that the bullet-shaped ionization front forms a plasma jet plume immediately. The dominant ionization wave is resonantly excited by the local enhanced electric field, which originates from the local net charge of the streamer plus surface plasmon polariton located at the open end of the resonator.

Characterization of a microwave-excited atmospheric-pressure argon plasma jet using two-parallel-wires transmission line resonator

This paper presents a method to produce a microwave-excited atmospheric-pressure plasma jet (ME-APPJ) with argon. The plasma was generated by a microwave-driven micro-plasma source that uses a two-parallel-wire transmission line resonator (TPWR) operating at around 900 MHz. The TPWR has a simple structure and is easier to fabricate than coaxial transmission line resonator (CTLR) devices. In particular, the TPWR can sustain more stable ME-APPJ than the CTLR can because the gap between the electrodes is narrower than that in the CTLR. In experiments performed with an Ar flow rate from 0.5 to 8.0 L·min−1 and an input power from 1 to 6 W, the rotational temperature was determined by comparing the measured and simulated spectra of rotational lines of the OH band and the electron excitation temperature determined by the Boltzmann plot method. The rotational temperature obtained from OH(A-X) spectra was 700 K to 800 K, whereas the apparent gas temperature of the plasma jet remains lower than ∼325 K, which is compatible with biomedical applications. The electron number density was determined using the method based on the Stark broadening of the hydrogen Hβ line, and the measured electron density ranged from 6.5 × 1014 to 7.6 × 1014 cm−3. TPWR ME-APPJ can be operated at low flows of the working gas and at low power and is very stable and effective for interactions of the plasma with cells.

Microwave Plasma Generation With Resonance Frequency Tracking and Power Regulation

This paper presents a palm-sized power-providing module for microwave plasma generation with resonance-frequency tracking and output-power-regulation schemes. A 2.4-GHz coaxial transmission line resonator is used as a microwave plasma generator. The power module covers the frequency range from 2.3 to 2.5 GHz with the maximum power capability of 24 W. The power module adaptively sets the output frequency so that the forward power to the resonator is minimally reflected in response to the resonance frequency shift due to plasma formation. At a forward input power of 2 W, the resonance frequency shift by plasma formation is 41 MHz. With the proposed adaptive frequency tracking scheme, the power-transfer efficiency improves from 56 to 88 %. In addition, production of $\cdot $ OH radical, which is one of the key species in biomedical applications, increases by 33 % at 2 W. The power regulation function is verified by measuring the plasma gas temperature, which is an important indicator of safety for biomedical applications. With a feedback loop control, the power module stably generates 1 W and maintains a constant plasma gas temperature of 40 °C for 300 s. The measurements reveal that the proposed schemes can improve the power efficiency and controllability of microwave plasma generation, and thereby increase the feasibility of producing a portable low-power microwave bio-plasma device.

Design of catheter radio frequency coils using coaxial transmission line resonators for interventional neurovascular MR imaging.

It is technically challenging to design compact yet sensitive miniature catheter radio frequency (RF) coils for endovascular interventional MR imaging. In this work, a new design method for catheter RF coils is proposed based on the coaxial transmission line resonator (TLR) technique. Due to its distributed circuit, the TLR catheter coil does not need any lumped capacitors to support its resonance, which simplifies the practical design and construction and provides a straightforward technique for designing miniature catheter-mounted imaging coils that are appropriate for interventional neurovascular procedures. The outer conductor of the TLR serves as an RF shield, which prevents electromagnetic energy loss, and improves coil Q factors. It also minimizes interaction with surrounding tissues and signal losses along the catheter coil. To investigate the technique, a prototype catheter coil was built using the proposed coaxial TLR technique and evaluated with standard RF testing and measurement methods and MR imaging experiments. Numerical simulation was carried out to assess the RF electromagnetic field behavior of the proposed TLR catheter coil and the conventional lumped-element catheter coil. The proposed TLR catheter coil was successfully tuned to 64 MHz for proton imaging at 1.5 T. B1 fields were numerically calculated, showing improved magnetic field intensity of the TLR catheter coil over the conventional lumped-element catheter coil. MR images were acquired from a dedicated vascular phantom using the TLR catheter coil and also the system body coil. The TLR catheter coil is able to provide a significant signal-to-noise ratio (SNR) increase (a factor of 200 to 300) over its imaging volume relative to the body coil. Catheter imaging RF coil design using the proposed coaxial TLR technique is feasible and advantageous in endovascular interventional MR imaging applications.

Donut shape plasma jet plumes generated by microwave pulses even without air mole fractions

It is well known that the plasma jets driven by lower frequency voltages or pulsed DC power supply normally present with donut shaped cross sections, especially at where the diffused air mole fractions are less than 0.01. Thence, it is interesting to further study whether the donut shape is still in truth for the pulsed microwave plasma jet or not. In this letter, the cross sectional structures of atmospheric pressure plasma jet plumes driven by pulsed microwaves have been experimented on a cylindrical coaxial transmission line resonator. The plasma jet plumes demonstrate particular characteristics, like argon plasma with a donut shape but helium plasma with an uniform lighten cross section, despite whether the air mole fraction exists or not. For argon discharge, the fast images show that the donut shaped cross section only occurs at the end of each microwave pulses. In combination with helium discharge, the cross sectional patterns are immediately determined by the dominant ionization front of the plasma jet plumes, which are resonantly generated by the local enhanced electric field of ionization waves.

Passive Wireless Sensor for Measuring AC Electric Field in the Vicinity of High-Voltage Apparatus

A passive wireless sensor for the measurement of ac electric fields emanating from high-voltage (HV) apparatus using radio-frequency (RF) resonant cavities is presented. The sensor is composed of a coaxial transmission line resonator, where the resonant frequency is perturbed by capacitively coupled varactors. HV apparatus induces a bias voltage on the varactors through capacitively coupled electric fields. A printed circuit board on the top of the cavity provides coupling between the cavity and varactors and also between the varactors and the external electric field. The sensor is designed with a resonant frequency in the range of 2.4–2.5 GHz in the industrial, scientific, and medical band. Using capacitive coupling to an HV transmission line with 60 Hz at 1.7 kV, a resonant frequency shift of 7.2 (kHz)/12.5 (V/m) was obtained. The resonance frequency is determined remotely by sending pulses of RF signal to the sensor and recording the ring back of the resonator. RF pulses, with a frequency of 2455 MHz, are used to remotely interrogate the sensor at a repetition rate of 2.5 MHz and a pulsewidth of 100 ns. Downconverting the received signal, a ring back signal is recorded whose analysis determines the resonant frequency.

Pulsed microwave-driven argon plasma jet with distinctive plume patterns resonantly excited by surface plasmon polaritons**Project supported by the National Natural Science Foundation of China (Grant Nos. 11105002 and 61170172), the Natural Science Foundation of Anhui Province, China (Grant Nos. 1408085QA16 and 1408085ME101), the China Postdoctoral Science Foundation (Grant No. 2014M551788), and the Open-end Fund of State Key Laboratory of Advanced

Atmospheric lower-power pulsed microwave argon cold plasma jets are obtained by using coaxial transmission line resonators in ambient air. The plasma jet plumes are generated at the end of a metal wire placed in the middle of the dielectric tubes. The electromagnetic model analyses and simulation results suggest that the discharges are excited resonantly by the enhanced electric field of surface plasmon polaritons. Moreover, for conquering the defect of atmospheric argon filamentation discharges excited by 2.45-GHz of continued microwave, the distinctive patterns of the plasma jet plumes can be maintained by applying different gas flow rates of argon gas, frequencies of pulsed modulator, duty cycles of pulsed microwave, peak values of input microwave power, and even by using different materials of dielectric tubes. In addition, the emission spectrum, the plume temperature, and other plasma parameters are measured, which shows that the proposed pulsed microwave plasma jets can be adjusted for plasma biomedical applications.