RF Heating Research Articles

Iterative addition of finite Larmor radius effects to finite element models using wavelet decomposition

Modeling the propagation and damping of electromagnetic waves in a hot magnetized plasma is difficult due to spatial dispersion. In such media, the dielectric response becomes non-local and the wave equation an integro-differential equation. In the application of RF heating and current drive in tokamak plasmas, the finite Larmor radius (FLR) causes spatial dispersion, which gives rise to physical phenomena such as higher harmonic ion cyclotron damping and mode conversion to electrostatic waves. In this paper, a new numerical method based on an iterative wavelet finite element scheme is presented, which is suitable for adding non-local effects to the wave equation by iterations. To verify the method, we apply it to a case of one-dimensional fast wave heating at the second harmonic ion cyclotron resonance, and study mode conversion to ion Bernstein waves (IBW) in a toroidal plasma. Comparison with a local (truncated FLR) model showed good agreement in general. The observed difference is in the damping of the IBW, where the proposed method predicts stronger damping on the IBW.

MRI-Induced Heating of Coils for Microscopic Magnetic Stimulation at 1.5 Tesla: An Initial Study.

PurposeDeep brain stimulation (DBS) has proved to be effective in the treatment of movement disorders. However, the direct contact between the metal contacts of the DBS electrode and the brain can cause RF heating in magnetic resonance imaging (MRI) scanning, due to an increase of local specific absorption rate (SAR). Recently, micro coils (μMS) have demonstrated excitation of neuronal tissue through the electromagnetic induction both in vitro and in vivo experiments. In contrast to electrical stimulation, in μMS, there is no direct contact between the metal and the biological tissue.MethodsWe compared the heating of a μMS coil with a control case of a metal wire. The heating was induced by RF fields in a 1.5 T MRI head birdcage coil (often used for imaging patients with implants) at 64 MHz, and normalized results to 3.2 W/kg whole head average SAR.ResultsThe μMS coil or wire implants were placed inside an anatomically accurate head saline-gel filled phantom inserted in the RF coil, and we observed approximately 1°C initial temperature rise at the μMS coil, while the wire exhibited a 10°C temperature rise in the proximity of the exposed end. The numerical simulations showed a 32-times increase of local SAR induced at the tips of the metal wire compared to the μMS.ConclusionIn this work, we show with measurements and electromagnetic numerical simulations that the RF-induced increase in local SAR and induced heating during MRI scanning can be greatly reduced by using magnetic stimulation with the proposed μMS technology.

Development of Chinese yam/chicken semi-liquid paste for space foods

The semi-liquid paste made from various dietary fibers is very popular among Chinese community and is often used as a complementary food for increasing the appetite. The purpose of the study is to develop a yam/chicken semi-liquid with both dietary fiber and animal protein to facilitate consumers’ intake of multiple nutrients. Besides, the development of foods with Chinese characteristics not only enriches space recipes, but will also help meet the psychological needs of Chinese astronauts. Because the traditional thermal sterilization affects the sensory properties and nutritional value of the product, the study plans to use radio frequency heating in combination with natural preservative (chitosan/ε-polylysine) to reduce the negative effects. The results showed that at 6 kW, 27.12 MHz, 20 mm plate spacing, RF heating for more than 20 min could exert the thermoelectric effect better. Improved thickening and antiseptic effects were obtained with the chitosan concentration of 2 g/kg, and the ε-polylysine concentration of 0.2 g/kg. The heating parameters of 100 °C for 20 min or 110 °C for 20–30 min can be selected as the final sterilization conditions providing equivalent quality parameters of conventionally high-pressure steam (HPS, 121 °C, 30 min) product.

Ionospheric HF Heating Experiment With Frequency Ramping‐up Sweep: Approach for Artificial Ionization Layer Generation

AbstractAn ionospheric high‐frequency (HF) heating experiment was conducted at the HAARP facility in Alaska by sweeping the heater frequency, near the third electron gyroharmonic, over a 145 kHz range with 4 min duration. A 446 MHz diagnostic radar was used to detect scattering from HF‐enhanced plasma waves corresponding to the HF‐enhanced ion lines (HFILs). Increasing frequency progressively moves the third gyroharmonic resonance layer downward, and this is evident in observations of both downgoing and upgoing HFILs dropping in height about 50 km. Artificial ionization was formed by the HF‐plasma interaction during this process and driven downward to about 150–155 km. Total electron content measurements oblique across the High Frequency Active Auroral Research Program transmit beam, above the altitude of primary ionization production, show variations that correspond to ionization production at lower heights. A theoretical interpretation of the experimental observations is presented to understand the HF‐plasma interaction process. It is shown that the double resonance condition enhances the excitation of the upper‐hybrid parametric decay instability (PDI). HF heater frequency ramping‐up sweep increases the spatial height span of the upper‐hybrid PDI generation region and the spectral width of generated upper‐hybrid waves, as well as provides a positive tuning frequency on the excited upper‐hybrid waves that keeps the continuous positive feedback on the Doppler‐shifted cyclotron harmonic resonance interaction. Resonant electrons gain energy from the upper‐hybrid waves and are generated in an extended spatial region; after a drift space, spatially distributed energetic electrons bunch together to build up a density structure.

Effect of magnetic field ripple on parallel electron transport during microwave plasma heating in the Proto-MPEX linear plasma device

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma device located at Oak Ridge National Laboratory to develop the plasma source concept for the Materials Plasma Exposure eXperiment (MPEX). Recent experiments have demonstrated the heating of electrons in a high-density deuterium plasma () via electron Bernstein waves and source plasma production via helicon waves in this linear configuration. Moreover, experimental observations suggest that the magnetic ripple adversely affects the parallel transport of the heated electrons toward the target where material samples are to be exposed to the plasma. To understand the transport process during microwave application, a test-particle Monte-Carlo (MC) code has been developed that incorporates the effects of Coulomb collisions, magnetic mirror adiabatic trapping and electron cyclotron interaction via a quasilinear RF heating operator. MC calculations indicate that the absence of 2nd turning points along the trajectory of cyclotron heated electrons significantly reduces adiabatic trapping and maximizes power transport to the Target surface. Thermalization of cyclotron heating electrons is analyzed with the MC code and it is found that for conditions relevant to Proto-MPEX, a significant fraction of the absorbed heating power is coupled to the Target surface via ballistic fast electrons; however, under certain conditions, up to 60% of the absorbed RF power can be dissipated via thermalization of the fast electrons on the background plasma. Results are compared with experimental measurements. Finally, the implications of the results are discussed in the context of the upcoming MPEX device.

Safety Study of Implant Wearers in MRI RF Field Environment

The complex electromagnetic field environments in magnetic resonance imaging system(MRI) can have a significant impact on patients carrying implants, the RF heating problems being particularly important. To ensure the safety of the patients, it is necessary to understand the distribution of tissue temperature in the MRI environment and its changes over time. Based on the analysis of tissue temperature rise in MRI, this paper constructs a bird cage coil for generating RF field in MRI system, and constructs ASTM standard/improved phantom and single-cavity pacemaker finite element models, use time-domain finite difference (FDTD) to simulate. Firstly, the correctness of the simulation software and simulation method was validated according to the method of ISO. Then the distribution of the electric field, SAR and temperature field and the temperature change with time were calculated in the environment of 64 MHz, 2 W/kg. The difference in temperature rise with blood heat exchange and no blood heat exchange (standard/improved phantom) was specifically compared. The simulation results show that there are electric field and SAR hotspots near the electrode tip, the wire tail and the case of pacemaker. There are high SAR values on both sides of the phantom, and the shorter the distance from the coil, the higher the SAR. The temperature field distribution is similar to the SAR distribution; the temperature is higher in the area around the end of the wire and the case of pacemaker because the heat accumulation is higher around this area. At the same time, blood heat exchange can reduce the temperature rise to a certain extent.

Chromosomal-level assembly of Juglans sigillata genome using Nanopore, BioNano, and Hi-C analysis.

Background Juglans sigillata, or iron walnut, belonging to the order Juglandales, is an economically important tree species in Asia, especially in the Yunnan province of China. However, little research has been conducted on J. sigillata at the molecular level, which hinders understanding of its evolution, speciation, and synthesis of secondary metabolites, as well as its wide adaptability to its plateau environment. To address these issues, a high-quality reference genome of J. sigillata would be useful.FindingsTo construct a high-quality reference genome for J. sigillata, we first generated 38.0 Gb short reads and 66.31 Gb long reads using Illumina and Nanopore sequencing platforms, respectively. The sequencing data were assembled into a 536.50-Mb genome assembly with a contig N50 length of 4.31 Mb. Additionally, we applied BioNano technology to identify contacts among contigs, which were then used to assemble contigs into scaffolds, resulting in a genome assembly with scaffold N50 length of 16.43 Mb and contig N50 length of 4.34 Mb. To obtain a chromosome-level genome assembly, we constructed 1 Hi-C library and sequenced 79.97 Gb raw reads using the Illumina HiSeq platform. We anchored ∼93% of the scaffold sequences into 16 chromosomes and evaluated the quality of our assembly using the high contact frequency heat map. Repetitive elements account for 50.06% of the genome, and 30,387 protein-coding genes were predicted from the genome, of which 99.8% have been functionally annotated. The genome-wide phylogenetic tree indicated an estimated divergence time between J. sigillata and Juglans regia of 49 million years ago on the basis of single-copy orthologous genes.ConclusionsWe provide the first chromosome-level genome for J. sigillata. It will lay a valuable foundation for future research on the genetic improvement of J. sigillata.

Higher-order acoustic phonon oscillations in Au nanoparticles controlled by a sequence of ultrashort strain pulses generated by superdiffusive hot electrons

One of the quests in ultrafast plasmonics is the development of devices, based on surface plasmons and operating at high frequency and negligible heating conditions. Towards this goal, the authors use ultrashort strain pulses, generated by superdiffusive hot electrons to excite high-order spheroidal modes of Au nanoparticles. Their vibrations are accessed via periodic shifts of the surface plasmon band and further coherently controlled by a sequence of strain pulses. These results open new perspectives for ultrafast acoustoplasmonics towards the terahertz frequency regime.

Scaling mitigation in radio-frequency induction heated membrane distillation

Calcium sulfate (CaSO4) scaling is a limiting factor in membrane distillation as a result of concentration polarization and elevated temperatures. We assessed the influence of radio-frequency magnetic fields, used as a method of heating the membrane's surface, as part of a membrane distillation (RF-MD) process. The influence of RF heating on CaSO4 scaling is addressed in terms of distillate flux and crystal formation, and the results are compared to conventional MD systems. The RF-MD system included a thermally conducting dual-layer membrane containing a magnetic hydrophilic layer based on iron oxide-carbon nanotubes coated on a hydrophobic membrane. Heating the solution was mainly done directly at the membrane-water interface, and scaling was assessed at different feed flow velocities using feed solutions containing elevated concentrations of CaSO4 and NaCl and at ionic composition simulating RO brine. Results show only sporadic small CaSO4 crystals on the membrane surface following RF-MD, while high concentrations of small crystals were detected at the concentrate stream exiting the filtration module. Furthermore, the RF-MD system was able to produce enhanced distillate flux. The scaling mitigation mechanism is discussed in detail and hypothesized to be a result of the high-frequency movement and collision of the ions in the solution.

RF Heating of Implants in MRI: Electromagnetic Analysis and Solutions

RF Heating of Implants in MRI: Electromagnetic Analysis and Solutions

Exploring the regime of validity of global gyrokinetic simulations with spherical tokamak plasmas

Plasma turbulence is considered one of the main mechanisms for driving anomalous thermal transport in magnetic confinement fusion devices. Based on first-principle model, gradient-driven gyrokinetic simulations have often been used to explain turbulence-driven transport in present fusion devices, and in fact, many present predictive codes are based on the assumption that turbulence is gradient-driven. However, using the electrostatic global particle-in-cell gyrokinetic tokamak simulation (GTS) code (Wang et al 2010 Phys. Plasmas 17 072511), we will show that while global gradient-driven gyrokinetic simulations provide decent agreement in ion thermal transport with a set of NBI-heated NSTX (Ono et al 2000 Nucl. Fusion 40 557) H-mode plasmas, they are not able to explain the observed electron thermal transport variation in a set of RF-heated L-mode plasmas, where a factor of 2 decrease in electron heat flux is observed after the cessation of the RF heating. Thus, identifying the regime of validity of the gradient-driven assumption is essential for first-principle gyrokinetic simulation. This understanding will help us to more confidently predict the confinement performance of ITER and future magnetic confinement devices.

Local High-Frequency Heating in Electronics Technology

Modeling and investigation of HF electromagnetic heating in induction devices with unclosed magnetic circuit has allowed to optimize heating speed in local zones of formation of soldering connections and to improve their quality due to joint action of superficial effects and electromagnetic forces. For all magnetic materials is nonlinear decrease in heating power depending on frequency of HF. Installed the optimal parameters of HF heating for soldering electronics modules by inductor with open-ended magnetic conductor.

RF heating of deep brain stimulation implants in open-bore vertical MRI systems: A simulation study with realistic device configurations.

Patients with deep brain stimulation (DBS) implants benefit highly from MRI, however, access to MRI is restricted for these patients because of safety hazards associated with RF heating of the implant. To date, all MRI studies on RF heating of medical implants have been performed in horizontal closed-bore systems. Vertical MRI scanners have a fundamentally different distribution of electric and magnetic fields and are now available at 1.2T, capable of high-resolution structural and functional MRI. This work presents the first simulation study of RF heating of DBS implants in high-field vertical scanners. We performed finite element electromagnetic simulations to calculate specific absorption rate (SAR) at tips of DBS leads during MRI in a commercially available 1.2T vertical coil compared to a 1.5T horizontal scanner. Both isolated leads and fully implanted systems were included. We found 10- to 30-fold reduction in SAR implication at tips of isolated DBS leads, and up to 19-fold SAR reduction at tips of leads in fully implanted systems in vertical coils compared to horizontal birdcage coils. If confirmed in larger patient cohorts and verified experimentally, this result can open the door to plethora of structural and functional MRI applications to guide, interpret, and advance DBS therapy.

Improvement Scheme and Verification of High-Frequency Heating Uniformity of Wood

This study was based on the finite element method to construct a high-frequency heating temperature field simulation model. In the model, the bending length and angle of top plate were changed upwards. The heating uniformity of test material was then analyzed under different conditions (bending length and angle) to identify the best improvement scheme and carry out the experimental verification. The results showed that when the bending length and angle of top plate were 200mm and 45°, the heating uniformity was the best. Comparison of temperature distribution along the width direction of test material before and after improvement, it was concluded that the temperature distribution was more uniform after improvement. Comparison the temperature difference between the center of test material and its length, width and thickness direction measurement point before and after improvement, the temperature difference was reduced by 7.6℃, 1.7℃, 3.4℃, respectively, which effectively improved the heating uniformity. Comparison of the changes in the electromagnetic field distributions between plates before and after improvement revealed an increase in distribution uniformity from 0.631 to 0.811. This indicated that the electromagnetic field distribution after improvement was more uniform with ideal heating effect. Overall, the improvement in bending length and angle of top plate could change the distribution of electromagnetic fields between the plates and enhance temperature distribution uniformity of wood during high-frequency heating.

Effect of High Frequency Heat Treatment on the Microstructure and Wear Properties of Ni based Self Fluxing Composite Coating Layer Manufactured by HVOF Spray Process

Effect of High Frequency Heat Treatment on the Microstructure and Wear Properties of Ni based Self Fluxing Composite Coating Layer Manufactured by HVOF Spray Process

High-frequency wall heat flux measurement during wall impingement of a diffusion flame

The efficiency of internal combustion engines is limited by heat losses to the wall of the combustion chamber. A precise characterization of wall heat flux is therefore needed to optimize engine parameters. However, the existing measurements of wall heat fluxes have significant limitations; time resolution is often higher than the timescales of the physical phenomena of flame–wall interaction. Furthermore, few studies have investigated diesel flame conditions (as opposed to propagation flames). In this study, the heat flux generated by a diffusion flame impinging on a wall was measured with thin-junction thermocouple, with a time resolution of the whole acquisition chain better than 0.1 ms. The effects of variations in ambient gas temperature, injection pressure and injector–wall distance were investigated. Diesel spray impingement on the wall is shown to cause strong gas–wall thermal exchange, with convection coefficients of 6–12 kW/m2/K. Those results suggest the necessity of close-wall aerodynamic measurements to link macroscopic characteristics of the spray (injection pressure, impingement geometry) to turbulence values.

The Effect of RF Sputtering Conditions on the Physical Characteristics of Deposited GeGaN Thin Film

Ge0.07GaN films were successfully made on Si (100), SiO2/Si (100) substrates by a radio frequency reactive sputtering technique at various deposition conditions listed as a range of 100–400 °C and 90–150 W with a single ceramic target containing 7 at % dopant Ge. The results showed that different RF sputtering power and heating temperature conditions affected the structural, electrical and optical properties of the sputtered Ge0.07GaN films. The as-deposited Ge0.07GaN films had an structural polycrystalline. The GeGaN films had a distorted structure under different growth conditions. The deposited-150 W Ge0.07GaN film exhibited the lowest photoenergy of 2.96 eV, the highest electron concentration of 5.50 × 1019 cm−3, a carrier conductivity of 35.2 S∙cm−1 and mobility of 4 cm2·V−1∙s−1.

Equivalence ratio variation and combustion instability in hybrid rocket

In this paper, a series of combustion tests using PMMA/O2 was conducted to examine the initiating characteristics of low frequency instability (LFI) in relation to the changes in both flow and combustion conditions. By selecting parameters such as the oxidizer mass flow rate, fuel grain length, and mixing ratio of mixture oxidizer composed of oxygen and nitrogen; combustion tests were conducted to vary the equivalence ratio in the combustion. This study further validated instability characteristics in connection with coupling behavior between the high frequency pressure and heat release oscillations in 500 Hz band. Based on the results, a combustion stability map was plotted and it shows that the initiation of LFI is contingent upon the satisfaction of specific conditions of flow and combustion. The results also confirmed that the initiation of LFI is highly linked to an establishment of a positive coupling between high frequency pressure and heat release oscillations. In addition, equivalence ratio was found to be a critical factor in the transition of coupling status between the two high frequency oscillations. For a given flow condition, the sole increase in equivalence ratio above a certain threshold resulted in the phase shift between the two oscillations leading to a positive coupling or vice versa.

Polarization analysis of ELF/VLF waves generated by beating of two HF waves in the polar ionosphere

Modulated high frequency heating of the polar ionosphere in the presence of a polar electrojet current (PEJ) can generate extremely/very low frequency (ELF/VLF, 3 Hz–30 kHz). In order to overcome the dependence of the radiation intensity on the variable PEJ, researchers have focused on electrojet-independent approaches. One potential method is so-called “beat-wave” (BW) ELF/VLF generation invoking nonlinearity of the F-region ionospheric plasma, but this F-region source mechanism remains controversial. We perform polarization analysis of BW generated ELF/VLF waves using EISCAT heating facility near Tromsø. The results show that ELF/VLF waves can be decomposed into right- (R) and -left (L) handed circularly polarized waves with comparable strengths, and the polarization ellipse tends to be parallel to the well-known dominant background electric field, which is fully consistent with the recognized features of ELF/VLF waves generated by PEJ modulation. Hence, the results demonstrate that the BW generated current is located in the D/E region, and not the F region.

Validation of radio frequency assisted traditional thermal processing for pasteurization of powdered infant formula milk

This study validated the radio frequency assisted traditional thermal processing (RF assisted-TTP) for pasteurization of low-moisture foods. Powdered infant formula milk (PIFM) with water activity of 0.22 ± 0.01 was selected as a low-moisture food model. The cold spot during the RF heating of PIFM was found to be the center of top layer in a tray containing 312 g of PIFM. The PIFM was inoculated with Cronobacter sakazakii and 12 g inoculated sample packed in a plastic bag was placed at the pre-determined cold spot. The sample was treated until the cold spot reached 65 °C in a 6-kW, 27.12-MHz RF heating system and the sample tray was transferred to a hot air oven set at 65 °C for various holding times. Based on the microbial inactivation during holding time, D-value at 65 °C was determined. After 21 h oven holding, a 5-log reduction of C. sakazakii was achieved. The sample was also thermally treated in the same oven without any RF pre-heating. The quality analysis results indicated that both RF assisted-TTP and TTP had no significant effect on color, solubility, digestibility, and wettability, while significant changes were observed on water activity, moisture content, POV and TBArs after both treatments. Come-up time to heat up sample to 65 °C in RF system and oven was approximately 10 min and 5 h, respectively. RF assisted-TTP is more suitable for pasteurization of PIFM than TTP due to lower lipid oxidation and much shorter overall processing time. Thus, RF may provide a potential as an effective emerging technology for pasteurizing while maintaining the quality of PIFM.