Coupling Of Radio Frequency Power Research Articles

Analysis of spiral antenna for enhancing antenna-plasma coupling impedance for SST-1 tokamak

Abstract A detailed characterization of a high-power radio frequency (RF) broadband circularly polarized two-arm spiral antenna is designed to operate within the frequency range of 0.1–1.0 GHz. The impedance matching network technique is introduced to optimize its performance. The traditional spiral antenna is excited by a vertical or horizontal balun, whereas the proposed design is directly fed by a coaxial cable featuring a planar feeding section specially optimized to achieve broadband input impedance matching. The spiral antenna is designed as per the steady-state superconducting tokamak (SST-1) port space constraints. The simulated efficiency of the RF power coupling with the hydrogen plasma is ∼70 %. Through simulation, it was evident that the proposed antenna exhibited inherent resonance at 0.5 GHz with a reflection coefficient of −27.94 dB and an axial ratio is 3.39 dB respectively. The obtained outcomes unequivocally demonstrate the circular polarization of the designed antenna. Overall, the findings support the enhancement of plasma heating and current drive techniques in fusion research.

A compact radio-frequency ion source for high brightness and low energy spread negative oxygen ion beam production.

A high brightness and low energy spread (∆E) ion source is essential to the production of a high-quality primary ion beam applied in secondary ion mass spectrometry (SIMS). A compact 13.56MHz radio-frequency (RF) ion source with an external planar spiral antenna has been developed as a candidate ion source for the production of negative oxygen ion beams for SIMS application. This ion source is designed with a three-and-a-half-turn water-cooled planar antenna for RF power coupling, a multi-cusp magnetic field for effective plasma confinement, and a three-electrode extraction system. The experimental results show that more than 50 µA negative oxygen ion beams have been extracted, which consist of 56% O-, 25% O2-, and 19% O3-. The ion energy distribution of the negative oxygen ion beam exhibits a Gaussian distribution with a minimum ∆E of 6.3eV. The brightness of the O- beam is estimated to be 82.4Am-2 Sr-1V-1. The simulation, design, and experimental study results of this RF ion source will be presented in this paper.

Long-pulse high-performance H-mode plasmas achieved on EAST

A record duration of a 310 s H-mode plasma (H98y2 ∼ 1.3, ne/nGW ∼ 0.7, fBS > 50%) has been recently achieved on experimental advanced superconducting tokamak (EAST) with metal walls, exploiting the device's improved long-pulse capabilities. The experiment demonstrates good control of tungsten concentration, core/edge MHD stability, and particle and heat exhaust with an ITER-like tungsten divertor and zero injected torque, establishing a milestone on the path to steady-state long-pulse high-performance scenarios in support of ITER and CFETR. Important synergistic effects are leveraged toward this result, which relies purely on radio frequency (RF) powers for heating and current drive (H&CD). On-axis electron cyclotron heating enhances the H&CD efficiency from lower hybrid wave injection, increasing confinement quality and enabling fully non-inductive operation at high density (ne/nGW ∼ 70%) and high poloidal beta (βP ∼ 2.5). A small-amplitude grassy edge localized mode regime facilitates the RF power coupling to the H-mode edge and reduces divertor sputtering/erosion. The high energy confinement quality (H98y2 ∼ 1.3) is achieved with the experimental and simulated results pointing to the strong effect of Shafranov shift on turbulence. Transport analysis suggests that trapped electron modes dominate in the core region during the record discharge. The detailed physics processes (RF synergy, core-edge integration, confinement properties, etc.) of the steady-state operation will be illustrated in the content. In the future, EAST will aim at accessing more relevant dimensionless parameters to develop long-pulse high-performance plasma toward ITER and CFETR steady-state advanced operation.

Real-time measurement device of RF impedance and power for HUST RF negative ion source

A radio-frequency (RF) negative ion source is constructed at Huazhong University of Science and Technology (HUST). It is aimed to produce H− beam up to 30 keV and study the beam optics. For the tuning of matching network, and analyzing the RF power coupling efficiency of the ion source driver and the ratio of harmonics in the RF system, it is necessary to measure the RF impedance and power at the input port of the matching network. Therefore, a real-time measurement device of RF impedance and power is developed. Aimed at sampling the RF voltage and current, a kind of non-contacting sampling circuit is adopted. Parametric analyses are performed to evaluate the influence of coaxial cable on the sampled signal transmission, and then a preprocessing circuit is proposed. Based on FPGA, a high-speed signal processing unit is developed with the high-speed ADC acquisition, FFT analysis and discrete spectrum correction function. The experiments are carried out at 1, 3 and 5 MHz respectively to verify the performance of the measurement device. In general, the measurement device is of high precision and speed for RF impedance and power measurement.

Impedance Characteristic Analysis and Preliminary Experimental Results of a High-Power RF Plasma Source

To study the characteristics of radio frequency (RF) power coupling, RF plasma diagnostics, actively cooled tube, and negative ion production, a plasma source was designed as a small-scale test facility. The matching network was carefully designed and successfully applied to the 2-MHz RF plasma source. In this article, the design method of the matching unit will be presented; with the assistance of this matching unit, the RF plasma source was successfully lit and the reflected power was reduced to a very low level. A <inline-formula> <tex-math notation="LaTeX">$V\!\!-\!\!I$ </tex-math></inline-formula> probe was installed in front of or in the rear of the matching network, respectively, to <i>in situ</i> measure the RF voltage, current, and phase angle. The equivalent resistance of the RF plasma source was analyzed in detail. The RF power coupling efficiency was derived in two different methods, and the relationship between the coupling efficiency and the equivalent resistance of the plasma source is verified. At the same time, the equivalent resistance of the plasma source was also collected to optimize the matching unit.

Study on influence on plasma under current-generated magnetic filter using Langmuir probe in HUST negative ion source

The current-generated magnetic filter is widely used in negative ion sources to bring down electron temperature and optimize source operation. The effect of the current-generated magnetic filter on plasma generation and transport is important. The axial distribution of cesium-free plasma parameters under different magnetic filter configurations was measured by a movable Langmuir probe at HUST negative ion source. Abnormal changes of plasma parameters are discovered at the driver exit. When the magnetic filter field penetrates into the driver, plasma density inside the driver increase, and the high-density high-temperature region of plasma expands outside of the driver. And the radiofrequency power coupling is also influenced by the magnetic filter. These experimental phenomena indicate that a new magnetic filter configuration is required to optimize source operation.

Modeling inductive radio frequency coupling in powerful negative hydrogen ion sources: validating a self-consistent fluid model

Radio frequency (RF) negative hydrogen ion sources utilized in fusion and for accelerators use inductively coupled plasmas, which are operated at a low driving frequency, high power densities and gas pressures in the order of 1 MHz, 10 W cm−3 and 1 Pa, respectively. In this work a numerical fluid model is developed for a self-consistent description of the RF power coupling in these discharges. After validating the RF power coupling mechanism, such a model is a valuable tool for the optimization of RF power coupling and hence can help to increase the efficiency and reliability of RF ion sources. The model validation is achieved using measurements from the ITER RF prototype ion source. Steady state numerical solutions are obtained for the first time, where all modeled trends fit well. Remaining systematic quantitative differences could be caused by 3D effects such as highly non-uniform magnetic fields that cannot be captured in the current model formulation, which is 2D cylindrically symmetric. The coupling between the RF fields and the electrons is realized in the electron momentum transport equation, where approximations consistent with the operating regime of RF ion sources are applied. Here large magnetic RF fields lead to a plasma compression by the nonlinear RF Lorentz force. Using a local approximation for the electron viscosity, it is found that increased diffusion of the RF current density mitigates the compression. Navier–Stokes equations for the neutral atoms and molecules are used to capture neutral depletion. In this way it is shown that at high powers neutral depletion has a large impact on the power coupling via the viscosity of the electrons. The application of the self-consistent model for optimization of the RF power coupling will be described in a forthcoming paper.

Self-consistent fluid model for simulating power coupling in hydrogen ICPs at 1 MHz including the nonlinear RF Lorentz force

Radio frequency (RF) power coupling in inductively coupled plasmas is investigated numerically using a self-consistent fluid model. Hydrogen discharges are simulated at pressures from 0.3–10 Pa and at RF powers of around 1 kW. At the low excitation frequency of 1 MHz a high magnetic RF field of around 30 G is generated by the RF coil, meaning that discharges at low pressures are in the nonlinear skin effect regime. Therefore, a description of the RF power coupling by simple collisional Joule heating is not appropriate. Moreover, models that account for collisionless heating by means of a stochastic collision frequency or as diffusion of the RF current density (as is state of the art for discharges operated in the anomalous skin effect regime at higher frequencies of e.g. 13.56 MHz) are incapable of describing the RF power coupling in the nonlinear skin effect regime properly. This is due to their total neglect or simplified treatment of the RF Lorentz force. Instead, this work demonstrates that the RF power coupling mechanism for discharges operating at low RF in the nonlinear skin effect regime can be described by an electron momentum balance retaining the nonlinear RF Lorentz force as well as electron inertia and advection. The crucial role of the RF Lorentz force in generating the RF plasma current density and thus in shaping the plasma parameter profiles is validated successfully with experimentally obtained electrical and spatially resolved plasma parameters for pressures as low as 0.5 Pa. Below this pressure the results obtained from the model and the ones from the experiment diverge. Most likely this is caused by a sudden change in the electron distribution function at the lowest pressures.

Progress of physics understanding for long pulse high-performance plasmas on EAST towards the steady-state operation of ITER and CFETR

Recently, the first ever 100 s long, steady-state H-mode discharge with good control of impurities, core and edge MHD stabilities, and heat exhaust was demonstrated in the Experimental Advanced Superconducting Tokamak (EAST) using the ITER-like (International Tokamak Experimental Reactor) tungsten upper divertor. Using both radio frequency (RF) power and neutral beam injection (NBI) heating, EAST has demonstrated fully non-inductive scenarios with an extension of fusion performance at high density and low rotation: β P ∼ 2.5, β N ∼ 2.0, H98,y2 ∼ 1.2, bootstrap current fraction fBS ∼50% at q95 ∼ 6.8. With pure RF power heating, plasmas have been maintained for up to 21 s (over 40 times the current relaxation time) with zero loop voltage and small edge localized modes (ELMs) at high density (ne/nGW ∼ 0.6–0.8), β P ∼ 2.0, β N ∼ 1.6, and ƒBS ∼47%. Experimental investigations show how plasma current profiles, turbulent transport and radiation properties self-consistently evolve toward fusion relevant steady state conditions. Modeling and physics experiments have confirmed the synergistic effects between electron cyclotron heating (ECH) and low hybrid wave (LHW), where ECH enhances the heating and current drive from LHW injection, enabling fully non-inductive operation at higher density. Small/no ELMs facilitate the RF power coupling in the H-mode phase and reduce divertor erosion. A low tungsten concentration was observed at high β P with a hollow profile in the core. Reduction of the peak divertor heat flux with f rad of up to 40% was compatible with the high β P scenario by using active radiation feedback control. With features such as dominant electron heating, zero/low NBI torque and an ITER-like tungsten divertor, fully non-inductive high-performance experiments on EAST offer unique contributions towards the succesful operation of ITER and CFETR (the Chinese Fusion Engineering Testing Reactor).

Design of 6 MeV X-band electron linac for dual-head gantry radiotherapy system

A compact 6 MeV electron linac is being developed at Sungkyunkwan University, in collaboration with the Korea atomic energy research institute (KAERI). The linac will be used as an X-ray source for a dual-head gantry radiotherapy system. X-band technology has been employed to satisfy the size requirement of the dual-head gantry radiotherapy machine. Among the several options available, we selected a pi/2-mode, standing-wave, side-coupled cavity. This choice of radiofrequency (RF) cavity design is intended to enhance the shunt impedance of each cavity in the linac. An optimum structure of the RF cavity with a high-performance design was determined by applying a genetic algorithm during the optimization procedure. This paper describes the detailed design process for a single normal RF cavity and the entire structure, including the RF power coupler and coupling cavity, as well as the beam dynamics results.

Compact dual‐polarised microstrip patch antenna with high interport isolation for 2.5 GHz in‐band full‐duplex wireless applications

In-band full duplex (IBFD) wireless operation can be accomplished by mitigating the self-interference (SI) at the receiver that is caused by coupling of radio-frequency power from its own transmitter. Maximum amount of SI cancellation (SIC) should be achieved at antenna stage to enable full-duplex wireless communication operation at same frequency. This study presents the design and implementation of a compact two-port dual-polarised microstrip patch antenna which uses three-port microstrip patch antenna with 3 dB ring hybrid coupler as SIC circuit to achieve high interport isolation. The implemented antenna provides 57dB interport isolation between transmit and receive ports at centre frequency of 2.5GHz with linear orthogonal polarisation and interport isolation is better than 40dB for 50MHz bandwidth. The fabricated compact microstrip patch antenna has 3.1 and 3.7 dBi gains for Tx and Rx modes, respectively. The proposed antenna has comparatively wider 10dB input bandwidth (50MHz) along with high interport isolation.

Triple-probe Diagnostic Measurements in Plasma of GLAST Spherical Tokamak

Triple-probe has been developed and operated successfully to characterize ECRH-assisted argon as well as hydrogen microwave plasmas in GLAST Spherical Tokamak. This technique enables to determine transient plasma parameters such as floating potential, electron temperature and electron number density in rapidly time-varying plasmas. An effective electron heating mechanism is applied to produce microwave plasma by injecting radiofrequency (RF) radiation at a frequency of 2.45 GHz in the presence of resonant toroidal magnetic field. Plasma parameters and corresponding fluctuations are measured as a function of time in different gas fill pressures for various applied magnetic fields. The results demonstrate the dependence of plasma parameters such as Vf, Te, ne and their fluctuations on gas fill pressure during the pre-ionization phase of the GLAST operation. Plasma behavior is observed to be closely depending on the coupling of RF power during microwave discharge. Additionally, the hydrogen plasma shows pronounced fluctuations in comparison with argon plasma with some decrease in electron temperature and densities.

Plasma diagnostic approach for high rate nanocrystalline Si synthesis in RF/UHF hybrid plasmas using a PECVD process

Hydrogenated nanocrystalline silicon (nc-Si : H) films intended for efficient nc-Si : H solar cells are usually made at the transition to the nanocrystalline regime using the plasma-enhanced chemical vapor deposition (PECVD) process. This change occurs within a sensitive process window and is affected by various deposition parameters. This paper reports a study of nc-Si : H films' fabrication by utilizing systematic plasma diagnostics. This work presents a novel approach for plasma processing using radio frequency (RF), ultra high frequency (UHF) and RF/UHF hybrid plasmas. Using careful analysis, efforts are made to investigate the radicals and plasma formation by changing the operating source power and silane (SiH4) concentration. The aim of this work is also to investigate the PECVD process and conditions favorable for the synthesis of nc-Si : H film. For the present study, we systematically use the optical emission spectroscopy (OES), normal, and RF-compensated Langmuir probe (LP) and vacuum ultraviolet absorption spectroscopy diagnostics. Measurements reveal that the OES diagnostic is consistent with the LP measurements. Investigation reveals that UHF power in addition to RF enables higher dissociation of H or SiH radicals and the production of higher plasma density. The combined effect of both RF and UHF sources is used as the hybrid plasma source. Measurements also reveal that inbetween SiH4 flow rates ∼20–30 sccm, there is significant change in the plasma characteristics that denotes the nc-Si : H−a-Si : H transition region. An atomic hydrogen density (nH) in the range ≈(8 − 10) × 1011 cm−3 and plasma density n0 ≈ (2 − 3) × 1011 cm−3 with a silane to hydrogen ratio of 1–2% with high crystallinity has been obtained. Along with the discussion on the effect of frequency on plasma chemistry, this explains the RF power coupling and the role of electrons and ions in plasmas with increasing frequency.

Spectral broadening of lower hybrid waves produced by parametric instability in current drive experiments of tokamak plasmas

In order to explain the results of the non-inductive current produced in the lower hybrid current drive (LHCD) experiments, a broadening of the radiofrequency (RF) power spectrum coupled to tokamak plasma needs to occur. The presented modelling, supported by diagnostic measurements, shows that the parametric instability (PI) driven by ion sound quasimodes, which occur in the scrape-off plasma layer located near the antenna mouth, produces a significant broadening of the launched LH spectrum. Considering the parameters of LHCD experiments of JET (Joint European Torus), and other machines as well, the PI growth rate is high enough for producing the compensation of the convective losses and, consequently, the broadening of a small fraction (of the order of 10%) of the launched power spectrum. Such a phenomenon is identified to be intrinsic to the RF power coupling in the LHCD experiments. As the principal implication of considering such spectral broadening in modelling the LH deposition profile, experiments of LHCD-sustained internal transport barriers in JET were successfully interpreted, which evidenced the effects of a well-defined LH deposition profile. The present work is important for addressing the long-lasting debate on the problem of the so-called spectral gap in LHCD. The design of LHCD scenarios relevant to the modern fusion research programme, an important requirement of which is the control of the plasma current profile in the outer half of plasma, can be properly achieved by considering PI-induced spectral broadening.

Superconductivity in high energy particle accelerators

The basics of superconductivity are outlined with special emphasis on the features which are relevant for the application in magnets and radio frequency cavities for high energy particle accelerators. The special properties of superconducting accelerator magnets are described in detail: design principles, magnetic field calculations, magnetic forces, quench performance and persistent magnetization currents. The design principles and basic properties of superconducting cavities are explained as well as the observed performance limitations and the countermeasures. The ongoing research efforts towards maximum accelerating fields are addressed and the coupling of radio frequency power to the particle beam is treated.

Analysis of the RF Energy Coupling into a Tissue Medium through a Spherical Ferrite Implant

The coupling of radio frequency power into a semi-infinite lossy tissue medium with an embedded lossy ferrite sphere is analysed. A magnetic loop near the surface of the lossy medium is taken as the external excitation source. The field induced inside the spherical ferrite implant is determined by solving an integral equation. The kernel of this integral equation is the Green's function of the semi-infinite medium and is expressed in terms of Sommerfeld type integrals. Absorbed power densities inside the ferrite spherical implant and the surrounding lossy medium are computed and are compared for several cases. The efficiency of using ferrite implants in hyperthermia applications is examined and conclusions are presented.