In this thesis, I will present the experimental results of the dynamics of superconducting single electron transistors (sSETs), under the influence of tunable dissipation. The sSET, consisting of two dc SQUIDs in series and the third gate electrode, is deposited onto a GaAs/AlGaAs heterostructure which contains a two dimensional electron gas plane 100nm beneath the substrate surface. The Josephson coupling energy, charging energy and dissipation related Hamiltonian can all be tuned in situ, while keeping others unchanged. We measured the switching current statistics and the transport properties, as a function of the dissipation and gate charge at different temperatures. If the sSET is in the classical regime where phase is a good quantum variable, we found that the switching current and corresponding Josephson energy decrease as dissipation increases. Our observation agrees qualitatively with the theoretical calculation of a single Josephson junction with dominant Josephson energy, in a frequency dependent dissipative environment where energy barrier decreases as dissipation increases in thermally activated escape regime. This dissipation dependence result can be understood as the consequence of a reduced quantum fluctuations in the charge numbers. Whereas in the charging regime, the switching current shows a 1e periodicity with respect to gate charge, indicating a pronounced charging effect. At a specific gate charge number, quantum fluctuations of the phase variable are compressed as dissipation increases, resulting in an enhanced switching current and Josephson energy. This result matches the theory of a sSET capacitively coupled to a dissipative environment qualitatively. The temperature dependence of the switching current histogram indicates the existence of both quantum and classical thermal phase diffusion. Moreover, quantum charge fluctuations are minimized at the degeneracy point, causing a sharp dip on the width of the switching current histogram. For a sSET with comparable Josephson energy and charging energy, quantum fluctuations of both phase and charge variables are significant. The influence of dissipation on the dynamics of the device is distinct in the classical and charging regimes. Dissipation compresses quantum phase fluctuations in the charging regime, whereas reduces the quantum charge fluctuations in the classical regime. The transition between these two regimes is found to be determined by the tunnel resistance of the SQUID. The competition between Josephson and charging energies, however, is not the intrinsic parameter of this transition. Our results imply that a detailed theoretical calculation of a sSET with comparable Josephson coupling energy and charging energy under the influence of dissipation is needed.

In this thesis, I will present the experimental results of the dynamics of superconducting single electron transistors (sSETs), under the influence of tunable dissipation. The sSET, consisting of two dc SQUIDs in series and the third gate electrode, is deposited onto a GaAs/AlGaAs heterostructure which contains a two dimensional electron gas plane 100nm beneath the substrate surface. The Josephson coupling energy, charging energy and dissipation related Hamiltonian can all be tuned in situ, while keeping others unchanged. We measured the switching current statistics and the transport properties, as a function of the dissipation and gate charge at different temperatures. If the sSET is in the classical regime where phase is a good quantum variable, we found that the switching current and corresponding Josephson energy decrease as dissipation increases. Our observation agrees qualitatively with the theoretical calculation of a single Josephson junction with dominant Josephson energy, in a frequency dependent dissipative environment where energy barrier decreases as dissipation increases in thermally activated escape regime. This dissipation dependence result can be understood as the consequence of a reduced quantum fluctuations in the charge numbers. Whereas in the charging regime, the switching current shows a 1e periodicity with respect to gate charge, indicating a pronounced charging effect. At a specific gate charge number, quantum fluctuations of the phase variable are compressed as dissipation increases, resulting in an enhanced switching current and Josephson energy. This result matches the theory of a sSET capacitively coupled to a dissipative environment qualitatively. The temperature dependence of the switching current histogram indicates the existence of both quantum and classical thermal phase diffusion. Moreover, quantum charge fluctuations are minimized at the degeneracy point, causing a sharp dip on the width of the switching current histogram. For a sSET with comparable Josephson energy and charging energy, quantum fluctuations of both phase and charge variables are significant. The influence of dissipation on the dynamics of the device is distinct in the classical and charging regimes. Dissipation compresses quantum phase fluctuations in the charging regime, whereas reduces the quantum charge fluctuations in the classical regime. The transition between these two regimes is found to be determined by the tunnel resistance of the SQUID. The competition between Josephson and charging energies, however, is not the intrinsic parameter of this transition. Our results imply that a detailed theoretical calculation of a sSET with comparable Josephson coupling energy and charging energy under the influence of dissipation is needed.

abstract: Sensing and controlling current flow is a fundamental requirement for many electronic systems, including power management (DC-DC converters and LDOs), battery chargers, electric vehicles, solenoid positioning, motor control, and power monitoring. Current Shunt Monitor (CSM) systems have various applications for precise current monitoring of those aforementioned applications. CSMs enable current measurement across an external sense resistor (RS) in series to current flow. Two different types of CSMs designed and characterized in this paper. First design used direct current reading method and the other design used indirect current reading method. Proposed CSM systems can sense power supply current ranging from 1mA to 200mA for the direct current reading topology and from 1mA to 500mA for the indirect current reading topology across a typical board Cu-trace resistance of 1 ohm with less than 10 µV input-referred offset, 0.3 µV/°C offset drift and 0.1% accuracy for both topologies. Proposed systems avoid using a costly zero-temperature coefficient (TC) sense resistor that is normally used in typical CSM systems. Instead, both of the designs used existing Cu-trace on the printed circuit board (PCB) in place of the costly resistor. The systems use chopper stabilization at the front-end amplifier signal path to suppress input-referred offset down to less than 10 µV. Switching current-mode (SI) FIR filtering technique is used at the instrumentation amplifier output to filter out the chopping ripple caused by input offset and flicker noise by averaging half of the phase 1 signal and the other half of the phase 2 signal. In addition, residual offset mainly caused by clock feed-through and charge injection of the chopper switches at the chopping frequency and its multiple frequencies notched out by the since response of the SI-FIR filter. A frequency domain Sigma Delta ADC which is used for the indirect current reading type design enables a digital interface to processor applications with minimally added circuitries to build a simple 1st order Sigma Delta ADC. The CSMs are fabricated on a 0.7µm CMOS process with 3 levels of metal, with maximum Vds tolerance of 8V and operates across a common mode range of 0 to 26V for the direct current reading type and of 0 to 30V for the indirect current reading type achieving less than 10nV/sqrtHz of flicker noise at 100 Hz for both approaches. By using a semi-digital SI-FIR filter, residual chopper offset is suppressed down to 0.5mVpp from a baseline of 8mVpp, which is equivalent to 25dB suppression. / Dissertation/Thesis / Ph.D. Electrical Engineering 2011

Switching of shunt reactors and capacitor banks is known to cause a very high rate of rise of transient recovery voltage across the circuit breaker contacts. With improvements in circuit breaker technology, modern SF6 puffer circuits have been designed with less interrupter per pole than previous generations of SF6 circuit breakers. This has caused modern circuit breakers to operate with higher voltage stress in the dielectric recovery region after current interruption. Catastrophic failures of modern SF6 circuit breakers have been reported during shunt reactor and capacitor bank de-energisation. In those cases, evidence of cumulative re-strikes has been found to be the main cause of interrupter failure. Monitoring of voltage waveforms during switching would provide information about the magnitude and frequency of small re-ignitions and re-strikes. However, measuring waveforms at a moderately high frequency require plant outages to connect equipment. In recent years, there have been increasing interests in using RF measurements in condition monitoring of switchgear. The RF measurement technique used for measuring circuit breaker inter-pole switching time during capacitor bank closing is of particular interest. In this thesis, research has been carried out to investigate switching transients produced during circuit breaker switching capacitor banks and shunt reactors using a non-intrusive measurement technique. The proposed technique measures the high frequency and low frequency voltage waveforms during switching operations without the need of an outage. The principles of this measurement technique are discussed and field measurements were carried out at shunt rector and capacitor bank installation in two 275 kV air insulated substations. Results of the measurements are presented and discussed in this thesis. The proposed technique shows that it is relatively easy to monitor circuit breaker switching transients and useful information on switching instances can be extracted from the measured waveforms. Further research works are discussed to realise the full potential of the measuring technique.