Injected Microwave Power Research Articles

Intense vanadium ion beam production for super-heavy element research experiments

Long-term production of an intense 51V13+ ion beam was imperative for synthesizing a new element (Z=119) at RIKEN. We systematically measured the beam intensity as a function of microwave power and material consumption rate for effectively producing an intense beam. Consequently, we generated a 1 emA of 51V13+ ion beam at an injected microwave power and extraction voltage of 3.5 kW and 12.6 kV, respectively.

Producing intense uranium ion beam for RIKEN RI beam factory

Intense highly charged uranium (U) ion beams were produced using RIKEN 28 GHz superconducting electron cyclotron resonance (ECR) ion source in a high-temperature oven (HTO). We used a double-oven system to ensure ample vapor supply. Moreover, we optimized the vapor delivery into the plasma at a constant microwave power input. Consequently, we achieved intense U-ion beams, specifically 250 eμA for U35+, 36 eμA for U46+, and 10 eμA for U54+, at an injected microwave power of approximately 3000 W. This achievement stems from recent advancements, notably the optimization of the material consumption rates aimed at maximizing beam intensity, which we briefly review here.

Enhanced gas‐phase nucleation of diamond nanoparticles in a microplasma torch

AbstractIn this work, we present and characterize a microwave plasma microtorch that can support the formation of diamond nanoparticles (DNPs) in the gas phase when using hydrogen‐methane as a precursor gas. We first showed how the nonequilibrium character of the plasma obtained in this microtorch ensures enhanced H‐atom, , and production at moderate gas temperature levels that provides thermal stability of DNPs. The material characterization by TEM, Raman, and electron energy loss spectroscopy spectra confirm the presence of a substantial amount of diamond phase in the carbon dust particles collected from the plasma phase. Investigation of the effects of injected microwave power on the process characteristics showed a direct correlation between enhanced production and the appearance of the diamond phase, which points out the critical role of as far as gas‐phase nucleation of DNPs is concerned.

Development and validation of the numerical model of Electron Cyclotron Resonance Ion Sources

Processes of the secondary electron emission (SEE) from the walls are included into the Numerical Advanced Model of Electron Cyclotron Resonance Ion Sources (NAM-ECRIS). It is found that SEE strongly influences electron confinement time and ion production. With the modified model, we observe reactions of the source to changes in a gas flow into the source and in an injected microwave power. The source performance with scaling the hexapole magnetic field is investigated. The calculated tendencies are close to the experimental observations.

Modular Integration of a Compact Ku-Band Relativistic Triaxial Klystron Amplifier Packaged With Permanent Magnets for High-Power Microwave Generation

To achieve modular integration of relativistic triaxial klystron amplifier (TKA) devices and reduce the energy consumption of the whole high-power microwave (HPM) system, a compact Ku-band TKA packaged with permanent magnets is proposed and investigated in this paper. By optimizing the electromagnetic structure and utilizing TEM mode energy coupling, the length of the uniform magnetic field area required for the Ku-band TKA is reduced from 32 cm to 19.6 cm, with a reduction rate of 39%. By utilizing the NdFeB-N50M mainly magnetized in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}$ </tex-math></inline-formula> direction, a permanent magnet guiding system weighing 90 kg is proposed. Besides, the length of the uniform zone is 20 cm with a longitudinal magnetic field of 0.5 T. Verified by the PIC simulation software CHPIC, an HPM with power of 430 MW and frequency of 14.25 GHz is generated when the diode voltage, current, and the injected microwave power are 340 kV, 4.2 kA, and 15 kW, respectively. The phase jitter of the output microwave is controlled within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\pm } 5{^\circ }$ </tex-math></inline-formula> , which is beneficial to the coherent power combination of modular integrated TKA devices

Influence of the Shape of Composites Materials on the Stability of Microwave Heating Processes

This paper aims to present preliminary researches on the influence of the shape and dimensions of composites materials on the stability of microwave heating. The research was focused on cordierite composite material, which is used as ceramic substrate in selective catalytic reaction for reduction of nitrogen oxide emissions from combustion engines. For experimental program were used different shapes for cordierite material, such as cubic, cylindrical, sphere and rectangular. Based on previous researches where it was demonstrated that an injected microwave power between 600 W and 1200 W, the samples were heated in order to establish which shape is more suitable for fast heating, thermal stability in terms of thermal runaway as well as for avoiding the unwanted microwave plasma initiation.

High-Power Amplification Experiments on a Recirculating Planar Crossed-Field Amplifier

The recirculating planar crossed-field amplifier (RPCFA) was designed, constructed, and tested at the University of Michigan. The RPCFA was driven by a number of RF sources ranging in frequency from 2.40 to 3.05 GHz and powers of 1 to 800 kW. Pulsed voltage was delivered to the cathode by the Michigan electron long beam accelerator with ceramic insulator (MELBA-C) which was configured to supply pulses of −300 kV, 1–10 kA, with 0.3–1.0- $\mu \text{s}$ pulse lengths. The RPCFA demonstrated zero-drive stability and a bandwidth of 15%. Amplification of microwave signals, at the design frequency of 3 GHz, below 150 kW, was observed with a mean gain of 7.87 dB and high variability, $\sigma = 2.74$ dB. Filtering this data set to only include shots with identical voltage and current profiles yielded a gain of 6.6 ± 1.6 dB. The mean gain increased to 8.71 dB and the variability decreased to $\sigma =0.63$ dB when the injected microwave power increased beyond 150 kW. Peak output powers of nearly 6 MW were achieved with RF breakdown limiting the maximum output power of the device.

Measurement of the energy distribution of electrons escaping confinement from an electron cyclotron resonance ion source

Production of high charge state ions in electron cyclotron resonance ion sources (ECRISs) is dependent on the electron energy distribution (EED) within the source plasma. In order to better understand the EED, a measurement of electrons escaping axially from an ECRIS device has been performed at the National Superconducting Cyclotron Laboratory. Electrons were measured escaping from the Superconducting Source for Ions, driven at 18 GHz. Dependencies of the observed EED on the confining magnetic field strength and injected microwave power are reported. This paper focuses on large peaks of electrons in the 400-1200 keV energy range. Measurements of the axial bremsstrahlung spectrum have been simultaneously carried out to provide a direct comparison between both techniques. A comparison between the energy associated with the peak of the electron distribution and the spectral temperature of the bremsstrahlung distribution is shown.

Spectroscopic study of ion temperature in minimum-B ECRIS plasma

Experimentally determined ion temperatures of different charge states and elements in minimum-B confined electron cyclotron resonance ion source (ECRIS) plasma are reported. It is demonstrated with optical emission spectroscopy, complemented by the energy spread measurements of the extracted ion beams, that the ion temperature in the JYFL 14 GHz ECRIS is 5–28 eV depending on the plasma species and charge state. The reported ion temperatures are an order of magnitude higher than previously deduced from indirect diagnostics and used in simulations, but agree with those reported for a quadrupole mirror fusion experiment. The diagnostics setup and data interpretation are discussed in detail to demonstrate adequate understanding of the line broadening mechanisms and instrumental effects, which justifies the use of the measured Doppler broadening of the emission lines to derive the corresponding ion temperature. The ion temperatures of the high charge state plasma of the minimum-B ECRIS are compared to those measured from low temperature singly charged microwave and arc discharge plasmas to exclude systematic errors. The ion temperatures in the minimum-B ECRIS plasmas are shown to increase with the charge state and injected microwave power whereas two-frequency heating and plasma instabilities have only a minor effect on them. Possible mechanisms causing the higher-than-expected ion temperatures are hypothesized and assessed based on the reported experimental data. Finally, the implications of the results are discussed, demonstrating the need for further experiments especially with charge breeder ECR ion sources, and experimental data on ion confinement times.

Assessment of ECH stray radiation levels at the W7-X Michelson Interferometer and Profile Reflectometer

Electron Cyclotron Heating and Electron Cyclotron Current Drive are key components for heating and control in magnetically confined fusion plasmas. The high power microwaves are not always completely absorbed leading to stray radiation [1], [2]. At W7-X, the total injected microwave power can be up to 7.5 MW @140 GHz while the entire Electron Cyclotron Emission picked-up by an observer at the edge of the plasma is a fraction of a mW. In the situation of a Michelson Interferometer, the principle measurement is the entire ECE spectrum. Thus, any stray radiation is bound to enter the spectrum. In this work initial stray radiation measurements without filters at the location of two microwave receivers -the Michelson Interferometer and the Profile Reflectometer -are discussed. The data is used to dimension a notch filter to be used with the broad band Michelson Interferometer.

The deteriorating effect of plasma density fluctuations on microwave beam quality

Turbulent plasma edge density fluctuations can broaden a traversing microwave beam degrading its quality. This can be a problem for scenarios relying on a high spatial localization of the deposition of injected microwave power, like controlling MHD instabilities. Here we present numerical estimations of the scattering of a microwave beam by density fluctuations over a large parameter range, including extrapolations to ITER. Two codes are used, the full-wave code IPF-FDMC and the wave kinetic equation solver WKBeam. A successful comparison between beam broadening obtained from DIII-D experiments and corresponding full-wave simulations is shown.

(Invited) Ultrapure Homoepitaxial Diamond Films Grown by Chemical Vapor Deposition

In this decade, studies on semiconducting diamond for the power device application have intensively been conducted in several research groups. A use of diamond crystals with low defect density is crucial for obtaining high blocking voltage. High crystalline quality and high purity of diamond crystals are also requested in the field of research on quantum information using diamond. Although a lot of effort was devoted to improve the quality of diamond films, dislocations still remain in the crystals with the number density higher than 104 cm-2. In this study, we propose advanced diamond growth condition that removes crystalline defects effectively during the growth process of homoepitaxial diamond films. Homoepitaxial diamond films were deposited on Ib(100) substrates using the homebuilt microwave plasma-assisted chemical-vapor-deposition (MPCVD) apparatus [1]. In this study, effect of oxygen addition during diamond growth was investigated [2]. Typical growth condition was as follows; total gas pressure, microwave power, methane concentration (flow ratio of CH4 to the total gas flow), oxygen concentration (flow ratio of O2 to the total gas flow), and substrate temperature were 120‒140 Torr, 1.2‒1.4 kW, 4‒10%, 0‒2%, and 950‒1050 °C, respectively. Microwave power density estimated from the injected microwave power divided by the plasma volume was 110‒180 W cm-3. By optimizing growth conditions with higher oxygen concentration of 2%, high-purity homoepitaxial (100) diamond layers, typical nitrogen concentration of which was less than 1 ppb, were successfully grown. Cathodoluminescence mapping and 3D Raman measurements indicated a number density of dislocations in the homoepitaxial layer is typically 104-105 cm-2. Tungsten carbide Schottky electrodes fabricated on the non-doped diamond showed high blocking voltage of 2.2 kV. This diamond films growth technique is promising for designing high-performance quantum information devices. The author would like to thank Dr. T. Yamamoto, Dr. S. Koizumi, Dr. K. Watanabe, Dr. H. Umezawa, Dr. S. Onoda, Dr. O. Milikofu, Prof. F. Jelezko, and Prof. J. Isoya for helpful discussion and supporting characterization. [1] T. Teraji, T. Yamamoto, K. Watanabe, Y. Koide, J. Isoya, S. Onoda, T. Ohshima, L. J. Rogers, F. Jelezko, P. Neumann, J. Wrachtrup, and S. Koizumi, phys. stat. sol. (a) 212, 2365(2015). [2] T. Teraji, J. Appl. Phys. 118, 115304(2015).

Investigation of a large power water-cooled microwave resonance window for application with the ECR ion source

A large power water-cooled microwave resonance window used for the electron cyclotron resonance (ECR) ion source is investigated in this paper. The microwave characteristic simulation, thermal analysis, and structure design are deeply and successively carried out before fabrication. After the machining and welding of the components, the window is cold and hot tested. The application results demonstrate that when the input power is 2000 W, the reflected power is only 5 W. The vacuum is below 10−10 Pa, and the high power microwave operation can last 30 h continuously and reliably, which indicates that the design and assembling can achieve the high efficiency of the microwave transmission. Finally, the performance of the ECR ion source is enhanced by the improvement of the injected microwave power to the ECR plasma.

Nanocrystalline diamond films grown at very low substrate temperature using a distributed antenna array microwave process: Towards polymeric substrate coating

The growth of NanoCrystalline Diamond (NCD) films at very low substrate temperature on large area surface using a distributed antenna array (DAA) microwave reactor operating in H2/CH4/CO2 gas mixture is investigated. The estimated activation energy is in the range 1.3–3.2kcal·mol−1 depending on the injected microwave power and resulting substrate temperature range, which is comparable to values reported for other low-temperature NCD growth processes. The ability of the DAA reactor to deposit NCD films at a surface temperature down to 130°C is demonstrated. NCD films composed of nanometric grains of 4.5nm with a surface roughness of 27nm are thus obtained, with a growth rate of 5nm·h−1. The decrease of the deposition temperature is followed by an increase of the renucleation rate leading to a reduction of the grain size and to a subsequent promotion of non-diamond phases. At this temperature, the decrease of the CH4 percentage in the feed gas permits to improve the film purity but leads to a drastic decrease of the growth rate down to 2.5nm·h−1. Finally, a successful attempt of NCD film deposition on polytetrafluoroethylene (PTFE) substrate is shown aiming at exploring the coating of temperature-sensitive polymeric substrates employed for biomedical applications.

VUV irradiance measurement of a 2.45 GHz microwave-driven hydrogen discharge

Absolute values of VUV-emission of a 2.45 GHz microwave-driven hydrogen discharge are reported. The measurements were performed with a robust and straightforward method based on a photodiode and optical filters. It was found that the volumetric photon emission rate in the VUV-range (80–250 nm) is 1016–1017 cm−3 s−1, which corresponds to approximately 8% dissipation of injected microwave power by VUV photon emission. The volumetric emission of characteristic emission bands was utilized to diagnostics of molecular plasma processes including volumetric rates of ionization, dissociation and excitation to high vibrational levels and metastable states. The estimated reaction rates imply that each injected molecule experiences several inelastic electron impact collisions. The upper limit for the total density of metastable neutrals (2S atoms and molecules) was estimated to be approximately 0.5% of the neutral gas density.

Microwave experiments on Prairie View Rotamak

A 6 kW/2.45 GHz microwave system has been added on Prairie View Rotamak, and a series of experiments with microwave heating in both O-mode and X-mode configurations have been performed. Effective ionization of hydrogen in the two configurations is observed when filling pressure of the hydrogen gas is under pf=0.1 Pa. Clear oscillations in plasma current Ip and magnetic field BR are excited when microwaves are injected into plasma in the X-mode configuration. The higher the injected microwave power, the sooner the emergence of the magnetic oscillations in BR, which implies the microwave may have decreased the elongation of the plasma. In the experiments, the efficiency of the current drive mechanism due to the injected microwave is about 0.2 kA/kW.

A Microwave Reflection Readout Scheme for Hot Electron Bolometric Direct Detector

In this paper, we propose and present data from a fast THz detector based on the repurpose of hot electron bolometer mixers (HEB) fabricated from superconducting NbN thin film. This detector is essentially a traditional NbN bolometer element that operates under the influence of a microwave pump. The injected microwave power serves the dual purpose of enhancing the detector sensitivity and reading out the impedance changes of the device in response to incident THz radiation. We have measured an optical Noise Equivalent Power of 4 pW/ ${\surd{\hbox{Hz}}}$ for our detector at a bath temperature of 4.2 K. The measurement frequency was 0.83 THz and the modulation frequency was 1.48 kHz. The readout scheme is versatile and facilitates both high-speed operation as well as multi-pixel applications.

Resonant Microwave Absorption in Thermally Deposited Au Nanoparticle Films Near Percolation Coverage

We observe a resonant transition in the microwave absorption of thin thermally deposited Au nanoparticle films near the geometrical percolation transition pc where the films exhibit a 'fractal' heterogeneous geometry. Absorption of incident microwave radiation increases sharply near pc, consistent with effective medium theory predictions. Both the theory and our experiments indicate that the hierarchical structure of these films makes their absorption insensitive to the microwave radiation wavelength λ, so that this singular absorption of microwave radiation is observed over a broad frequency range between 100 MHz and 20 GHz. The interaction of electromagnetic radiation with randomly distributed conductive scattering particles gives rise to localized resonant modes, and our measurements indicate that this adsorption process is significantly enhanced for microwaves in comparison to ordinary light. In particular, above the percolation transition a portion of the injected microwave power is stored within the film until dissipated. Finally, we find that the measured surface conductivity can be quantitatively described at all Au concentrations by generalized effective medium theory, where the fitted conductivity percolation exponents and pc itself are consistent with known two-dimensional estimates. Our results demonstrate that microwave measurements provide a powerful means of remotely measuring the electromagnetic properties of highly heterogeneous conducting films, enabling purposeful engineering of the electromagnetic properties of thin films in the microwave frequency range through fabrication of 'disordered' films of conducting particles such as metal nanoparticles or carbon nanotubes.

ECR Heating System for the Gas Dynamic Trap

Physics and engineering aspects of a system for electron cyclotron resonance heating (ECRH) at the magnetic mirror device Gas Dynamic Trap (GDT, Budker Institute, Novosibirsk) are presented. This system based on two 450 kW/54.5 GHz gyrotrons is aimed at increasing the electron temperature up to the range 250-350 eV for improved energy confinement of hot ions. The basic physical issue of the GDT magnetic field topology is that conventional ECRH geometries are not accessible. The proposed solution is based on a peculiar effect of radiation trapping in inhomogeneous magnetized plasma. Under specific conditions, oblique launch of gyrotron radiation results in generation of right-hand-polarized (R) electromagnetic waves propagating with high N∥ in the vicinity of the cyclotron resonance layer, which leads to effective single-pass absorption of the injected microwave power. In the present paper, we investigate numerically an optimized ECRH scenario based on the proposed mechanism of wave propagation and discuss the design of the ECRH system, which is currently under construction at the Budker Institute.

Nonlinear induction detection of electron spin resonance

We present an approach to the induction detection of electron spin resonance (ESR) signals exploiting the nonlinear properties of a superconducting resonator. Our experiments employ a yttrium barium copper oxide superconducting stripline microwave (MW) resonator integrated with a microbridge. A strong nonlinear response of the resonator is thermally activated in the microbridge when exceeding a threshold in the injected MW power. The responsivity factor characterizing the ESR-induced change in the system’s output signal is about 100 times larger when operating the resonator near the instability threshold, compared to the value obtained in the linear regime of operation. Preliminary experimental results, together with a theoretical model of this phenomenon are presented. Under appropriate conditions, nonlinear induction detection of ESR can potentially improve upon the current capabilities of conventional linear induction detection ESR.