Rf Current Source Research Articles

Self-consistent modeling of the electron–cyclotron maser interaction in lossy structures based on a full-wave Green's function approach

We develop a new self-consistent model for simulation of the electron–cyclotron maser interaction in cylindrical structures, where expansion of the fields in transverse eigenmodes cannot be directly applied. Instead of solving the nonhomogeneous equation for the fields as a differential equation, a different approach is followed. First, the Green's function for elementary azimuthal and radial RF current sources is analytically derived by expanding the fields in longitudinal modes. Then, the total generated field is calculated by representing the perturbed electron beam as a sum of elementary RF current sources along the axis with amplitude coefficients that are found from the kinematic quantities of the electrons. The self-consistent stationary solution is found by solving the equations of motion along with the field equation in an iterative procedure. The model is useful for the full-wave simulation of lossy structures, which are frequently found in gyro-devices, such as ceramic-loaded interaction circuits of gyro-traveling-wave tubes and beam tunnels of gyrotron oscillators.

Ionization instability induced striations in atmospheric pressure He/H2O RF and DC discharges

One-dimensional particle-in-cell (PIC) simulations of a 1 mm gap atmospheric pressure He/2% rf capacitive discharge showed standing striations in the bulk (Kawamura et al 2016 Plasma Sources Sci. Technol. 25 054009). We found that these striations were consistent with an ionization instability induced by non-local electron kinetics. We developed a theoretical instability criterion in good agreement with the numerical results which showed that discharges with larger bulk recombination rates tend to be more unstable. We also determined a critical wavelength such that shorter wavelengths are suppressed by diffusion while longer wavelengths may be restricted by the gap width. In this paper, we extend the gap size of the atmospheric pressure He/2% discharges in the PIC simulations to 2 and 4 mm and drive them by either dc or rf current sources. We compare the results to the 1 mm gap rf simulations and theoretical model in Kawamura et al (2016 Plasma Sources Sci. Technol. 25 054009). We find that wider gap discharges tend to be more unstable as they can accommodate a wider range of wavelengths. Furthermore, the mixture of the various excited modes in the wider gaps can lead to distinctly non-sinusoidal spatial oscillations.

Directional Point-Contact Josephson Junctions on Ba0.4K0.6(FeAs)2 Single Crystals

We investigated the Josephson effect between the conventional superconductor Pb0.7In0.3 and Ba0.4K0.6 (FeAs)2 single crystals by performing directional point-contact experiments. The junctions were irradiated with microwaves and the behavior of the Shapiro steps as a function of the square root of the microwave power was studied by means of the RSJ model extended to the nonautonomous case with an rf current-source term. The analysis reveals that, for current injection along the ab plane, the current-phase relation deviates from being simply described by the standard sin(ϕ) (as we indeed observed along the c axis) and a dominant sin(2ϕ) component is observed. The results are compared with a theory of the dc Josephson effect, formulated by using a method based on the calculation of temperature Green’s function in the junction within the tight-binding model (Burmistrova et al., Phys. Rev. B 91, 214501, 2015). The comparison shows that the experimental results here presented strongly favor an s ±-wave symmetry of the order parameter in this compound.

Analytical Determination of Collisional Sheath Properties for Triple Frequency Capacitively Coupled Plasma

A self-consistent analytical model for a time-independent collisional capacitively coupled plasma (CCP) sheath driven by a triple frequency (TF) RF current source is proposed. Sheath parameters are calculated using this model for some standard plasma parameters and are compared with those of a single frequency (SF) and a dual frequency (DF) capacitively coupled collisional sheath. This model estimates higher values of sheath width and potential with more oscillating behavior compared with SF and DF sheaths. By proper choice of source frequencies or phase differences in the source currents, it is possible to adjust the ion energy hitting the electrode. Use of TF source is found to facilitate better control upon sheath parameters for collisional CCP.

Injection-locked diode laser current modulation for Pound-Drever-Hall frequency stabilization using transfer cavities

A phase modulated RF current source is applied to an injection locked diode laser operating at $780\unit{nm}$. This produces tunable phase modulated sidebands of the laser suitable for stabilizing the length of an optical transfer cavity using the Pound-Drever-Hall technique. The Pound-Drever-Hall signal is anti-symmetric about the lock point, despite the presence of significant diode laser amplitude modulation. The stabilized optical transfer cavity is used to frequency stabilize a 776 nm external cavity diode laser. The stability and tunability of this transfer cavity locked laser is established by observation of the hyperfine components of the $^{87}$Rb $5P_{3/2}-5D_{5/2}$ transition in a vapor cell.

A Common-Gate Amplifier With Transconductance Nonlinearity Cancellation and Its High-Frequency Analysis Using the Volterra Series

A common-gate (CG) amplifier employing a transconductance nonlinearity cancellation technique is designed for transmitter circuitry. The major contributor to the third-order nonlinearity in the CG amplifier is the second derivative of the transconductor ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">''</sup> ), which is the same case with a common-source (CS) amplifier. The multiple gated transistor (MGTR) technique, which was developed in the CS amplifier for transconductor nonlinearity cancellation, is applied to a CG amplifier. However, in the CG amplifier, the input driving impedance of a CG amplifier comprises a voltage-current feedback loop. Thus, a second-order interaction with feedback components generates a third-order distortion that limits input-referred third-order intercept point (IIP3) enhancement. This feedback influence on IIP3 can be relaxed by eliminating harmonic feedback components. Based on high-frequency analysis on a CG amplifier using the Volterra series, an RF current source is proposed to replace the conventional current source in the CG amplifier to eliminate harmonic feedback components at 2omega and Deltaomega. By adapting the CG MGTR technique combined with the RF current source, a 2.4-GHz driver amplifier for Wibro/Wimax applications was implemented using a 0.18-mum 1P 6M CMOS process. Measurement results show a 9-dB output third-order intercept point improvement at an output power of -3 dBm.

A self-consistent hybrid model of a dual frequency sheath: Ion energy and angular distributions

This paper presents a self-consistent hybrid model including the fluid model which can describe the characteristics of collisional sheaths driven by dual radio-frequency (DF) sources and Monte Carlo (MC) method which can determine the ion energy and angular distributions incident onto the dual rf powered electrode. The charge-exchange collisions between ions and neutrals are included in the MC model in which a self-consistent instantaneous electric field obtained from the fluid model is adopted. In the simulation, the driven method we used is either the current-driven method or the voltage-driven method. In the current-driven method, the rf current sources are assumed to apply to an electrode, which is the so-called the equivalent circuit model and is used to self-consistently determine the relationship between the instantaneous sheath potential and the sheath thickness. In the voltage-driven method, however, the rf voltage sources are assumed to apply to an electrode. The dual rf sheath potential, sheath thickness, ion flux, ion energy distributions (IEDs), and ion angular distributions (IADs) are calculated for different parameters. The numerical solutions show that some external parameters such as the bias frequency and power of the lower-frequency source as well as gas pressure are crucial for determining the structure of collisional dual rf sheaths and the IEDs. The shapes of the IADs, however, are determined mainly by the gas pressure. Furthermore, it is found that the results from the different driven methods behave in the same way although there are some differences in some quantities.

Boundary Conditions and Particle Loading for the Modeling of a Semi-infinite Plasma

We present a method for the treatment of the boundary conditions and the particle loading in a self-consistent semi-infinite Particle-In-Cell simulation. We assume a nonionizing, collisional plasma in contact with an electrode. The simulation is planar and one-dimensional (1d-2v), and is driven by an rf and/or DC current source. The particle loading is done by appropriate drifting velocity distributions, both for ions and electrons. Limiting the simulation to the region immediately adjacent to the electrode dramatically improves run-times and diagnostic resolution and is ideal for studying the sheath and plasma-sheath interface independently from the rest of the plasma. The method can be generalized easily to model cylindrical and spherical geometries, and surface processes can also be included. The control parameters of the simulation are the values for the current drive (Irf, ωrf, IDC), the species temperatures in the bulk plasma Ts, and the values of the ion fluxes Γs.

Modeling of the stabilization of neoclassical tearing modes by localized radio frequency current drive

Numerical modeling of the stabilization of neoclassical tearing modes by localized radio frequency (rf) current drive is presented. The bootstrap current is self-consistently calculated from the pressure evolution equation, the rf current source is obtained from the ray-tracing code and the Fokker–Planck code, and the change of the driven current profile due to the change of the magnetic island width has also been taken into account by modeling the two-dimensional transport of the fast electrons induced by the rf wave. It is found that both parallel and the perpendicular transport of the fast electrons play important roles in the stabilization, and modulated and nonmodulated rf current drive have approximately the same stabilizing effect. The neoclassical tearing modes is shown to be stabilized by a continuous rf current drive. The simulation results essentially agree with experimental observations.

Analog simulation on enhanced AC Josephson effect for a junction driven by RF-current source

The ac Josephson effect for a current-driven Josephson junction enhanced by a non-monochromatic input signal is studied based on an analog simulation. The obtained rf-induced step-height in the current-voltage characteristic which is a measure of this effect suggest that the use of either a pulse train or a biharmonic consisting of fundamental and second harmonic sinusoidal functions is effective for the enhancement, if compared to a monochromatic signal drive. It is revealed that the individual phase locking between the rf-signal and the Josephson oscillation is much more stabilized by use of the pulse train-like signal and can be explained qualitatively by a pendulum motion, i.e., the relatively slow motion of the pendulum near the position of rest.

Current-voltage characteristics of superconducting field effect transistors with a 0.1 μm T-gate

Recently, we have used a simple resistively shunted junction (RSJ) model to analyze the current-voltage characteristics of the superconducting field effect transistor (SUFET) reported by Hatano and coworkers [J. Vac. Sci. Technol. B 7, 1333 (1989)]. In this paper, we compare the results of the simple RSJ model to the more complete SUFET circuit model of Glasser (which includes a magnetic flux-controlled gate voltage term arising from reciprocity arguments). We compare the simple RSJ model of a SUFET to Glasser's model and find the former to be accurate enough to predict the current-voltage characteristics of Hatano et al.'s SUFET, even in the presence of radiofrequency current sources. The magnetic-flux controlled gate voltage introduced by Glasser is calculated as a function of the applied gate voltage in the presence of DC and RF current sources and found to be negligible in Hatano's SUFET.

A resistive SQUID with two quantization loops

We introduce a SQUID (superconducting quantum interference device) consisting of two Josephson junctions incorporated into a partly resistive loop. An additional superconducting loop is connected to the junctions in parallel and inductively coupled to the tank circuit supplied by an rf current source. It is experimentally shown that this configuration can be used as a voltage-to-frequency converter. The voltage to be converted is applied to the resistive part of the first loop. In comparison to the known resistive SQUIDs, this device has the advantage of a larger output signal due to using two quantization loops. The design of the thin-film device and the experimental results are reported. Output signal values in order of up to 1 mV have been obtained.

A SQUID with two quantization loops working in a negative detuning mode

The SQUID (superconducting quantum interference device) consists of two Josephson junctions incorporated into a superconducting loop. We attached another superconducting loop in parallel to the junctions. The magnetic flux to be measured is introduced into the first loop. The second one is coupled inductively to the tank circuit supplied by an rf current source. Previously we have reported that this configuration allows to pick up a signal voltage from the tank circuit up to 1 mV when the tank circuit is driven by its resonant frequency. In this paper we show that this kind of SQUID also allows to work at a negative detuning from the resonant frequency. It is found that the negative detuning mode improves the output signal voltage and transconductance ratio. The values of these parameters obtained experimentally are 1.8 mV and 9 mV/ϕ O, respectively (ϕ O is the flux quantum).

Oscillation modes in the tunnel type and the weak-link type Josephson junctions

The present paper discusses in detail the oscillation modes and these dependency on system parameters in the tunnel type and weak-link type Josephson junctions driven by dc and rf current sources.

Prediction of transient temperature fields and cumulative tissue destruction for radio frequency heating of a tumor.

A therapeutic hyperthermia protocol using a radio frequency (rf) electrode placed adjacent to a bronchial wall tumor has been modeled using the finite element technique. Variable physical properties and variable blood perfusion have been assigned to the tumor and to the surrounding normal lung tissue. The Laplace equation was solved on a curvilinear grid for a single rf source electrode to determine the steady-state electric field, which in turn governs the energy deposition function. The heat generation in the tumor and in the lung tissue is then calculated from the energy deposition profile, and the bioheat equation is solved on the same finite element mesh to determine the transient temperature history. The temperatures are displayed as isothermal contours at designated times during the protocol and as temperature histories at selected points. In addition, an Arrhenius-type injury model has been implemented to predict thermally induced damage, from which equal total amounts of energy are deposited into the tissue using a constant power density for an appropriate time or using a cyclic heating pattern. The cyclic heating pattern consisted of a series of equal duration time periods during which the rf current source is alternately turned on and off (50% duty cycle). This study illustrates how a finite element model could be used to evaluate alternative protocols for heating a tumor of a specific geometry and to evaluate thermally induced damage to surrounding normal tissue.

Detailed measurements of the response of an rf SQUID in the regime LI&amp;lt;inf&amp;gt;c&amp;lt;/inf&amp;gt;&amp;lt; Φ&amp;lt;inf&amp;gt;0&amp;lt;/inf&amp;gt;/2π

Using a superconducting ring containing a Nb point contact inductively coupled to a resonant circuit fed with an rf current source, (typical rf SQUID configuration) we have measured the tank voltage as a function of frequency in the regime LI_{c} . For LI_{c} \ll \Phi_{0}/2\pi , the curves of tank voltage versus drive current for various values of external flux \Phi_{x} through the ring agree with the calculations of Hansma. For higher critical currents, there is an asymmetry in these curves for frequencies above and below ω 0 , the resonant frequency of the circuit. Calculations using an expanded theory including the effects of a conductance G_{o}(1+\alpha \cos \phi) and of screening flux give good agreement with the data. In principle α can be determined from these measurements, however the \cos \phi term and the screening flux produce qualitatively similar effects. A quantitative determination of α requires a precise knowledge of G o /I c . Using values of G o and I c from independent impedance measurements, the data yield a negative value of α in agreement with another experiment.

Conversion gain and noise in a Josephson mixer

Abstract : Fundamental mixing at 36 GHz in non-hysteretic Nb point contacts has been observed with overall conversion efficiencies in the range 0.33 < or = eta < or = 4 and mixer noise temperatures in the neighborhood of ( Taub M) approximately 200K. The observed conversion efficiencies are in good agreement with the Resistively Shunted model. The observed noise can be accounted for by assuming that the Johnson noise in the shunt resistor is the only source of noise driving the junction. Somewhat larger noise is seen from resonantly coupled mixers than is seen when the mixer is driven from an rf current source. Fundamental mixing in hysteretic junctions is limited to a narrow if bandwidth and gives much larger conversion gain but also larger noise. Parametric amplification at the if frequency is seen in hysteretic junctions. Fourth order harmonic mixing with omega sub (LO)/2 pi at 9GHz has been achieved with performance almost comparable to fundamental mixing. Cavity mode mixing appears less promising. (Author)