Design and Characteristic Analysis of a Homopolar Synchronous Machine Using a NI HTS Field Coil

This paper deals with a homopolar synchronous machine (HSM) applying high-temperature superconducting (HTS) field coils. Superconductors, especially high-temperature superconductors, have high potential as advanced materials for next-generation electrical machines due to their high critical current density and excellent mechanical strength. However, coils made with high-temperature superconductors have a high risk of being damaged in the event of a quench due to the intrinsic low normal zone propagation velocity (NZPV). Therefore, the coil protection issue has been regarded as one of the most important research fields in HTS coil applications. Currently, the most actively studied method for quench protection of the HTS coils is the no-insulation (NI) winding technique. The NI winding technique is a method of winding an HTS coil without inserting an insulating material between turns. This method can automatically bypass the current to the adjacent turn when a local quench occurs inside the HTS coil, greatly improving the operating stability of the HTS coils. Accordingly, many institutions are conducting research to develop advanced electrical machines using NI HTS coils. However, the NI HTS coil has its intrinsic charge/discharge delay problem, which makes it difficult to successfully develop electrical machines using the NI HTS coil. In this study, we investigated how this charging/discharging problem appeared when the NI HTS coil was used in an HTS homopolar synchronous machine (HSM) which is one of the electrical machines with a high possibility of applying the HTS coil in the future because it has a stationary field coil structure. For this, the characteristic resistances of HTS coils were experimentally obtained and applied to the simulation model.

High Bandwidth Power Electronics and Magnetic Nanoparticles for Multichannel Magnetogenetic Neurostimulation

Objective: We present a power electronic system and magnetic nanoparticles for multiplexed magnetogenetic neurostimulation with three channels spanning a wide frequency range and rapid channel switching capability. This enables selective heating of magnetic nanoparticles with different coercivity using various frequency-amplitude combinations of the magnetic field. Such multiplexed operation could provide the technical means for selective magnetogenetic neurostimulation beyond its spatial focality limits. Approach: The electronic system uses a hybrid of silicon metal-oxide-semiconductor and gallium-nitride field-effect transistors, which generate the required high-amplitude current up to 500 A in the sub-MHz range and the high-frequency current in the MHz range, respectively. Via three discrete resonance capacitor banks, the system generates an alternating magnetic field in the same liquid-cooled field coil at three distinct frequency channels spanning 50 kHz to 4 MHz. Fast switching between channels is achieved with high-voltage contactors connecting the coil to different capacitor banks. We characterized the system by the output channels' frequencies, field strength, and switching time, as well as the system's overall operation stability. Three types of iron oxide nanoparticles with different coercivity are developed to form three magnetothermal channels. Specific absorption rate and infrared thermal imaging measurements are performed with the nanoparticles to characterize their heating and demonstrate selective actuation for all three channels. Main results: The system achieved the desired target field strengths for three frequency channels (70 kA/m at 50 kHz, 10 kA/m at 500 kHz, and 1 kA/m at more than 2 MHz), with rapid switching speed between channels on the order of milliseconds. The system can operate continuously for at least two hours at 30% duty cycle with 125 Arms load in the coil, corresponding to a stimulation protocol of cycling the three channels at target strength with 3 s pulses and 7 s interpulse intervals. The nanoparticles were heated with selectivity between 2.3 and 9 times for their respective frequency channel. The system's intended use was thus validated with three distinct channels available for magnetothermal heating. Significance: We describe the first combination of a power electronic system and magnetic nanoparticles that achieves three stimulation channels. Selective actuation of nanoparticles is demonstrated for each channel using the same field coil, including a novel composition responding to magnetic fields in the MHz range. This approach could improve the speed and flexibility of frequency-multiplexed magnetogenetic neural stimulation.