Nanocomposites exhibit promising performance in the application of dielectric capacitors due to their excellent dielectric properties. However, nanoparticles are easy to aggregate and difficult to be compatible with polymer matrices, thus requiring surface modification of the nanoparticles with organic ligands. Surface modification has been proposed as a useful method for regulating the surface properties of nanoparticles over a long history. In this Review, we have outlined some modifiers and their characteristics, including the reactive mode of each modifier and modification effect, as well as its characteristic advantages and limitations. The modifiers selected in this Review contain different functional groups, and they have also been widely used in the surface modification of nanoparticles. We have also summarized the characterization methods for surface composition, the properties of modified nanoparticles, the interfacial structure, and the performance of nanocomposites. At the same time, we have c...

Dielectric Nanomaterials for Power Energy Storage: Surface Modification and Characterization

The Gruneisen parameter, experimentally determined from the ratio of thermal expansion to specific heat, quantifies the pressure dependence of characteristic energy scales of matter. It is highly enhanced for Kondo lattice systems, whose properties are strongly dependent on the pressure sensitive antiferromagnetic exchange interaction between f- and conduction electrons. In this review, we focus on the divergence of the Gruneisen parameter and its magnetic analogue, the adiabatic magnetocaloric effect, for heavy-fermion metals near quantum critical points. We compare experimental results with current theoretical models, including the effect of strong geometrical frustration. We also discuss the possibility of using materials with the divergent magnetic Gruneisen parameter for adiabatic demagnetization cooling to very low temperatures.

https://iopscience.iop.org/article/10.1088/0034-4885/79/11/114502/pdf

Grüneisen parameter studies on heavy fermion quantum criticality.

The Gr\"uneisen parameter, experimentally determined from the ratio of thermal expansion to specific heat, quantifies the pressure dependence of characteristic energy scales of matter. It is highly enhanced for Kondo lattice systems, whose properties strongly dependent on the pressure sensitive antiferromagnetic exchange interaction between f- and conduction electrons. In this review, we focus on the divergence of the Gr\"uneisen parameter and its magnetic analogue, the adiabatic magnetocaloric effect, for heavy-fermion metals near quantum critical points. We compare experimental results with current theoretical models, including the effect of strong geometrical frustration. We also discuss the possibility to use materials with divergent magnetic Gr\"uneisen parameter for adiabatic demagnetization cooling to very low temperatures.

Gr\"uneisen parameter studies on heavy fermion quantum criticality

This paper presents the design and development of a repetitive Marx generator rated at 1 kJ, 300 kV, 12 kA, and 10 Hz and which is suitable to drive the load directly. Bipolar charging of a Marx generator scheme has been adopted to get the following: 1) faster rise time and 2) relatively low charging voltage. The faster rise time is due to the reduced number of spark gaps down to six for 12 in-series capacitors (0.15 μF and 50 kV) of a six-stage bipolar-charging Marx generator. Interstage inductive charging further enhances the efficiency of the overall system as compared to resistive charging. The generator is characterized by an aqueous resistive load for both polarities at the output voltage. Output voltages of the same magnitude have been achieved for both polarities. The matched impedance is found to be 25 Ω for which a suitable reflex triode is designed. Intense electron beams and high-power microwaves have been generated using this Marx generator and reflex triode system. Experimental results demonstrating the Marx generator and the reflex triode operation are presented and discussed.

Development and Characterization of Repetitive 1-kJ Marx-Generator-Driven Reflex Triode System for High-Power Microwave Generation

The most fundamental issues in virtual cathode oscillator (vircator)-based high-power microwave devices are efficiency improvement and oscillation frequency control. We present a study of the time-dependent behavior of the oscillation frequency of a reflex-triode vircator for various anode-cathode (AK) gaps. The typical electron-beam parameters were 200 kV, 4 kA, and 300 ns, with a current density of a few hundreds of amperes per square centimeter. Time-dependent frequency analysis is applied to the output signal of the vircator to investigate the time evolution of the emitted frequency spectrum for various AK gaps. We find that the dominant frequency peak is less affected by shot-to-shot fluctuations of diode voltage and current than the other frequency components. In addition, the time of occurrence of the dominant and secondary frequency peaks varies on a shot-to-shot basis. The highest microwave power is emitted when all the power is delivered into a single frequency with minimal mode hopping.

Oscillation Frequency of a Reflex-Triode Virtual Cathode Oscillator

An inductive energy storage pulse power system is being developed in BARC, India. Simple, compact, and robust opening switches, capable of generating hundreds of kV, are key elements in the development of inductive energy storage pulsed power sources. It employs an inductive energy storage and opening switch power conditioning techniques with high energy density capacitors as the primary energy store. The energy stored in the capacitor bank is transferred to an air cored storage inductor in 5.5 μs through wire fuses. By optimizing the exploding wire parameters, a compact, robust, high voltage pulse power system, capable of generating reproducibly 240 kV, is developed. This paper presents the full details of the system along with the experimental data.

Compact inductive energy storage pulse power system

This paper presents the experimental results of a repetitive Marx generator being developed at BARC. Effect of lead inductance, sparkgaps' alignment, charging inductor, ground inductor and shielding has been studied. Two types of configurations have been adopted in order to reduce the erected Marx inductance and results are compared. In this Marx generator plus-minus charging scheme was adopted for both test setups. Each setup comprised of 2-stage Marx with 4 series capacitors (each rated for 0.15 microfarad, 40nH, 50kV) while discharging giving 0.375 microfarads capacitance. In one of the scheme Type-I, all spark gaps were aligned in line of sight arrangements and successive discharge path were assembled in a zigzag manner so that the induced magnetic field gets cancelled out and effective inductance is reduced. This scheme was terminated to an aqueous load and critical matched condition was achieved at 12 ohm. This scheme had effective capacitance of 0.0375 microfarads, wave shape in lower voltage i.e. 10–15kV charging level. During experiments effect of load was also seen which gave half the open voltage to matched load [z=√(L/C)] and in critical damping condition voltage was 0.7 times of open circuit voltage. These data will be used for developing a 6-stage 1.2 kJ Marx generator for 20 pps burst output. Presently testing is limited to 2pps due to charging power supply limitations. Thus the results can be summarized as follows: (i) line of sight and ultra violet triggering is not effective for a distance of 200mm, (ii) cancellation of induced magnetic field is not effective at 5kA current peak at 100mm distance, (iii) longer length leads in smaller diameter is preferable over larger diameter with smaller length assembly with zigzag leads for giving shorter pulse, higher current and faster rise time.

Experimental results on design aspects of a compact repetitive Marx generator

The design, development and commissioning of 1 MV pulsed electron accelerator producing Flash X-Rays is described in this paper. This pulsed power system is based on bipolar MARX generator and Blumlein followed by Explosive electron emission diode assembly. The peak pulsed power is ~ 30 GW. The electron energies in the range of 400 keV to 1030 keV are produced and delivered to experimental load of Industrial diode. Electrons are emitted from a stainless steel ring at ground potential by explosive field emission and bombard the anode tungsten pin for flash X-rays generation. The relativistic electron beam has been simulated within the diode chamber and pattern shows the beam propagation. Imaging plates are used to characterize the source size and optimization has been reported.

https://iopscience.iop.org/article/10.1088/1748-0221/9/07/P07011/pdf

First results of KALI-30 GW with 1 MV flash X-rays generation and characterization by Imaging plate

Flash X-ray sources driven by pulsed power find applications in industrial radiography, and a portable X-ray source is ideal where the radiography needs to be taken at the test site. A compact and portable flash X-ray (FXR) system based on a Marx generator has been developed with the high voltage fed to the FXR tube via a cable feed-through arrangement. Hard bremsstrahlung X-rays of few tens of nanosecond duration are generated by impinging intense electron beams on an anode target of high Z material. An industrial X-ray source is developed with source size as low as 1 mm. The system can be operated from 150 kV to 450 kV peak voltages and a dose of 10 mR has been measured at 1 m distance from the source window. The modeling of the FXR source has been carried out using particle-in-cell and Monte Carlo simulations for the electron beam dynamics and X-ray generation, respectively. The angular dose profile of X-ray has been measured and compared with the simulation.

Development of cable fed flash X-ray (FXR) system

This paper is about the development of a copper sulphate based aqueous-electrolytic voltage divider for the measurement of high voltage pulses, 100 kV, with pulse widths of 1-2 ns and rise time <1 ns. Novel features are incorporated in the design of the divider, to meet the performance requirements for the application. Analytical calculations to justify design are described. Structural simulation of the divider is carried out using field wave simulation software to verify the effectiveness. A calibration procedure has been developed to calibrate the divider. Results obtained during calibration are subjected to statistical analysis to determine the confidence of measurement. Details of design, analysis, and simulation are described in this paper.

K. Senthil

Papers

Compact inductive energy storage pulse power system

Experimental results on design aspects of a compact repetitive Marx generator

First results of KALI-30 GW with 1 MV flash X-rays generation and characterization by Imaging plate

Development of cable fed flash X-ray (FXR) system

Development of a novel voltage divider for measurement of sub-nanosecond rise time high voltage pulses