Low-inductance Coil Research Articles

An Experimental Study of the Influence of the Longitudinal Magnetic-Field Distribution Profile on the Output Characteristics of a Gyrotron

The influence of the magnetic-field profile in the gyrotron interaction space on the generation efficiency and frequency was experimentally studied. A profiled magnetic field was created by a system of several low-inductance coils. It is shown that it is possible to increase the generation efficiency by ~1.3 times, which is comparable with the results that are obtained using single-stage recuperation of the residual electron-beam energy. In this case, the proposed method is easier to implement and can be used jointly with the recuperation. It is also demonstrated that it is possible to control the generation frequency within a range of at least twice the width of the radiation spectrum with characteristic times of the order of milliseconds. This allows one to believe that the gyrotron frequency-stabilization circuit can be successfully performed using small changes in the magnetic field.

Fast-switching magnet serving a spallation-driven ultracold neutron source

A fast-switching, high-repetition-rate magnet and power supply have been developed for and operated at TRIUMF, to deliver a proton beam to the new ultracold neutron (UCN) facility. The facility possesses unique operational requirements: a time-averaged beam current of 40~$\mu$A with the ability to switch the beam on or off for several minutes. These requirements are in conflict with the typical operation mode of the TRIUMF cyclotron which delivers nearly continuous beam to multiple users. To enable the creation of the UCN facility, a beam-sharing arrangement with another facility was made. The beam sharing is accomplished by the fast-switching (kicker) magnet which is ramped in 50~$\mu$s to a current of 193~A, held there for approximately 1~ms, then ramped down in the same short period of time. This achieves a 12~mrad deflection which is sufficient to switch the proton beam between the two facilities. The kicker magnet relies on a high-current, low-inductance coil connected to a fast-switching power supply that is based on insulated-gate bipolar transistors (IGBTs). The design and performance of the kicker magnet system and initial beam delivery results are reported.

EXPERIMENTAL RESEARCH ON MAGNETIC PULSE JOINING OF 3A21 ALUMINUM ALLOY-20 STEEL TUBES

Magnetic pulse joining (MPJ) is a solid state joining technology, by which mechanical or metallurgical joint is obtained. It can be used to assemble and join dissimilar metals. In this work, by means of MPJ setup with low inductance coil, the effects of discharge voltage, radial gap between tubes and lap length on the tensile and peel testing strength of 3A21 aluminum alloy-20 steel tubes joint were investigated experimentally. The microstructural observation and the energy spectrum analysis (by linear scan) of basic elements distribution within the MPJ joint interface were performed by SEM. The results showed that an increasing discharge voltage contributed to the change of MPJ joint from mechanical joint to metallurgical one. The metallurgical joint with the small wave joining interface was obtained under a voltage of 5.5 kV, a radial gap of 1.5 mm, and a lap length of 3/4 coil length, where the mutual diffusion of Al and Fe elements occurred. The severe plastic deformation with sharp strain gradient, and the distinct grain refinement occurred in the matrix metals near the joining interface.

Rapid rise time pulsed magnetic field circuit for pump-probe field effect studies

Here we describe an electronic circuit capable of producing rapidly switched dc magnetic fields of up to 20 mT with a rise time of 10 ns and a pulse length variable from 50 ns to more than 10 micros, suitable for use in the study of magnetic field effects on radical pair (RP) reactions. This corresponds to switching the field on a time scale short relative to the lifetime of typical RPs and maintaining it well beyond their lifetimes. Previous experiments have involved discharging a capacitor through a low inductance coil for a limited time using a switching circuit. These suffer from decaying field strength over the duration of the pulse given primarily by the ratio of the pulse width to the RC constant of the circuit. We describe here a simple yet elegant solution that completely eliminates this difficulty by employing a feedback loop. This allows a constant field to be maintained over the entire length of the pulse.

SQUID-measurements of relaxation time of Fe 3O 4 superparamagnetic nanoparticle ensembles

High- T C scanning SQUID magnetometer is applied for measuring the magnetization relaxation of the ensembles of spherical colloidal Fe 3O 4 nanoparticles dispersed in the rigid polymer matrix preventing nanoparticles from agglomeration. According to the transmission electron microscopy data, the size distribution of nanoparticles is well described by the Gaussian profile with the mean size and the dispersion of about 7.5 and 2.3 nm, respectively. High sensitivity of SQUID magnetometer allows us to study samples at low contents of the magnetic component (0.1–1.0 vol%) magnetized in low magnetic field (∼10 –4 T) produced by a low-inductance coil with short switching time (12 μs). A low content of the magnetic component provides the absence of interparticle dipolar interactions, thus simplifying significantly the theoretical description of the magnetization relaxation process. The measured magnetization relaxation curves are found to be in reasonable agreement with the numerical simulation data.

HTS SQUID Microscopy for Measuring the Magnetization Relaxation of Magnetic Nanoparticles

HTS SQUID microscopy is applied for measuring the magnetization relaxation of the ensembles of noninteracting Fe/sub 3/O/sub 4/ nanoparticles dispersed in rigid polymer matrix preventing nanoparticles from agglomeration. Transmission electron microscopy (TEM) is used to determine the size distribution of magnetic nanoparticles and to control the homogeneity of their spatial distribution in the matrix. High sensitivity of SQUID microscope allows us to study samples at low contents of the magnetic component (0.1-0.5 vol%) magnetized in low magnetic field (/spl sim/10/sup -4/ T) produced by a low-inductance coil with short switching time (20 /spl mu/s). A low content of the magnetic component provides the absence of interparticle dipolar interactions, thus simplifying significantly the theoretical description of the magnetization relaxation process. The detection of magnetization relaxation starts after the external magnetizing field is switched off and proceeds for several minutes. A series of samples with a mean size of nanoparticles of about 7 nm possessing relaxation times less than 5 min was measured at a temperature of 77 K. The calculation formula based on the Stoner-Wohlfarth model for single-domain particles with a size distribution as determined from the TEM images will enable to obtain reliable data on the anisotropy constant and the saturation magnetization of Fe/sub 3/O/sub 4/ nanoparticles studied.

A vector control scheme for EV induction motors with a series iron loss model

Electric vehicle (EV) motors are characterized by their low inductance and high current density, so that they run at high speed and produce a high starting torque. Due to the low inductance coil design, the current ripple caused by pulsewidth modulation (PWM) switching makes a significant amount of eddy-current loss and hysteresis loss, especially in high-speed operation. If one simply neglects the iron loss, the overall vector controller is detuned, resulting in an error in the torque control. The iron loss is modeled, in general, by a parallel resistor R/sub M/ to the magnetizing inductor L/sub M/. The authors propose a series R-L model that accounts for the effects of the iron loss. A major advantage of the series model is that it does not increase the number of state variables in developing a vector control. In this paper, they derive a rotor-flux-oriented flux error, orientation angle error, and torque error caused by iron loss. Finally, they demonstrate the effectiveness of the proposed control method through computer simulation and experimental results.

A novel design methodology for Nth order, shielded longitudinal coils for NMR

A new method is presented for the design of nuclear magnetic resonance (NMR) shim coils. The design methodology is the simulated annealing of the conductor positions, where the error function to minimize contains spherical harmonic field errors and other coil properties. This very flexible method allows pure, compact, low-inductance coils to be designed. Active magnetic shielding has been incorporated in the design process, enabling these coils to be used for rapid, pulsed experiments without the generation of deleterious eddy currents. The details for longitudinal coil designs are presented here. A shielded coil was built to test the design process and demonstrated good correlation with theoretical predictions.

Apparatus for complex AC magnetic susceptibility measurements in the radiofrequency range

Complex AC magnetic susceptibility measurement is one of the standard methods used to obtain information on the magnetic properties of HTSC materials. Usually the measurements are performed in the frequency range from a few tens of hertz to a few megahertz with the well known lock-in technique. Much less experimental information has been collected in the RF range. A dedicated instrument to measure the AC magnetic susceptibility ( chi ', chi '') and harmonic generations of high temperature superconductors in the frequency range 20-500 MHz is described. In the case of linear susceptibility complex values ( chi ', chi '') are measured in frequency bands centred around the values of (30+n*60) MHz while at harmonics the amplitudes are determined in the whole frequency range. A calibration procedure to obtain correct values of ( chi ', chi '') from the two components detected by lock-in of the RF signal picked up by a low inductance coil containing the sample is worked out. The dependences on frequency, temperature and static magnetic field of the complex AC susceptibility were measured for high temperature superconducting YBaCuO ceramics. Construction details, calibration methods and data evaluation are described.

Transverse Kerr magneto-optic measurements with a rotating analyzer ellipsometer

The use of a rotating analyzer ellipsometer is proposed to measure the spectral variation of the transverse Kerr magneto-optic effect. A low inductance coil is placed close to the sample in such a way that an alternating magnetic field is applied in the plane of the sample and perpendicular to the plane of incidence. First the quantities ψ and Δ are measured in the conventional way. For the magneto-optic effect the magnetization is varied at a high frequency. The analyzer and polarizer are fixed at specified angles while the magneto-optic signal is measured with a lock-in detector. The data are used to determine the real and imaginary parts of the magneto-optic coefficients and their relative sign. It is shown how to calculate from this measurement the value of the off-diagonal elements of the dielectric tensor.

Self excitation of iron core homopolar generators

In the interest of reducing homopolar generator (HPG) auxiliary requirements, a self-excited field coil for pulsed duty, iron-core HPG has been developed and tested at the Center for Electromechanics at The University of Texas at Austin (CEM-UT). In order to minimize rotor energy expended during excitation, a low-resistance, low-inductance coil was desired to allow field current to rise as rapidly as possible. A seven-turn field coil, having a nominal resistance of 500 µΩ was fabricated for the CEM-UT compact HPG system tester and subsequently tested. At 6,000 rpm, a field current rise time of 1 s was achieved and resulted in an average peak field density of 1.94 T. Only 300 kJ, about 13% of the 2.27 MJ stored in the rotor was required to fully excite the generator.