Collisionless shock acceleration of quasi-monoenergetic ions in ultra-relativistic regime

Collisionless shock acceleration of carbon ions (C$^{6+}$) is investigated in the ultra-relativistic regime of laser-plasma interaction by accounting for the radiation reaction force and the pair production in particle-in-cell simulations. Both radiation reaction force and pair plasma formation tend to slow down the shock velocity, reducing the energy of the accelerated ions, albeit extending the time scales of the acceleration process. Slab plasma target achieves lower energy spread while target with a tailored density profile yields higher ion acceleration energies.Comment: 10 pages,12 figure

Performance evaluation for ML sequence detection in ISI channels with Gauss Markov Noise

Inter-symbol interference (ISI) channels with data dependent Gauss Markov noise have been used to model read channels in magnetic recording and other data storage systems. The Viterbi algorithm can be adapted for performing maximum likelihood sequence detection in such channels. However, the problem of finding an analytical upper bound on the bit error rate of the Viterbi detector in this case has not been fully investigated. Current techniques rely on an exhaustive enumeration of short error events and determine the BER using a union bound. In this work, we consider a subset of the class of ISI channels with data dependent Gauss-Markov noise. We derive an upper bound on the pairwise error probability (PEP) between the transmitted bit sequence and the decoded bit sequence that can be expressed as a product of functions depending on current and previous states in the (incorrect) decoded sequence and the (correct) transmitted sequence. In general, the PEP is asymmetric. The average BER over all possible bit sequences is then determined using a pairwise state diagram. Simulations results which corroborate the analysis of upper bound, demonstrate that analytic bound on BER is tight in high SNR regime. In the high SNR regime, our proposed upper bound obviates the need for computationally expensive simulation.Comment: This paper will appear in GlobeCom 201

Diagnostics and Degradation Investigations of Li-Ion Battery Electrodes using Single Nanowire Electrochemical Cells

Portable energy storage devices, which drive advanced technological devices, are improving the productivity and quality of our everyday lives. In order to meet the growing needs for energy storage in transportation applications, the current lithium-ion (Li-ion) battery technology requires new electrode materials with performance improvements in multiple aspects: (1) energy and power densities, (2) safety, and (3) performance lifetime. While a number of interesting nanomaterials have been synthesized in recent years with promising performance, accurate capabilities to probe the intrinsic performance of these high-performance materials within a battery environment are lacking. Most studies on electrode nanomaterials have so far used traditional, bulk-scale techniques such as cyclic voltammetry, electrochemical impedance spectroscopy, and Raman spectroscopy. These approaches give an ensemble-average estimation of the electrochemical properties of a battery electrode and does not provide a true indication of the performance that is intrinsic to its material system. Thus, new techniques are essential to understand the changes happening at a single particle level during the operation of a battery. The results from this thesis solve this need and study the electrical, mechanical and size changes that take place in a battery electrode at a single particle level. Single nanowire lithium cells are built by depositing nanowires in carefully designed device regions of a silicon chip using Dielectrophoresis (DEP). This work has demonstrated the assembly of several NW cathode materials like LiFePO4, pristine and acid-leached α-MnO2, todorokite – MnO2, acid and nonacid-leached Na0.44MnO2. Within these materials, α-MnO2 was chosen as the model material system for electrochemical experiments. Electrochemical lithiation of pristine α-MnO2 was performed inside a glove box. The volume, elasticity and conductivity changes were measured at each state-of-charge (SOC) to understand the performance of the material system. The NW size changes due to lithiation were measured using an Atomic Force Microscope (AFM) in the tapping mode. Electronic conductivity changes as a function of lithiation was also studied in the model α-MnO2 NWs and was found to decrease substantially with lithium loading. In other measurements involving a comparison between the alpha and todorokite phases of this material system, it was observed that the rate capability of these materials is limited not by the electronic but, by the ionic conductivity. Mechanical degradation of a battery cathode represents an important failure mode, which results in an irreversible loss of capacity with cycling. To analyze and understand these degradation mechanisms, this thesis has tested the evolution of nanomechanical properties of a battery cathode. Specifically, contact-mode AFM measurements have focused on the SOC-dependent changes in the Young’s modulus and fracture strength of an α-MnO2 NW electrode, which are critical parameters that determine its mechanical stability. These changes have been studied at the end of the first discharge step, 1 full electrochemical cycle, and 20 cycles. The observations show an increase in Young’s modulus at low concentrations of lithium loading and this is attributed to the formation of new Li-O bonds within the tunnel-structured cathode. As the lithium loading increases further, the Young’s modulus was observed to reduce and this is hypothesized to occur due to the distortions of the crystal at high lithium concentrations. The experimental-to-theoretical fracture strength ratio, which points to the defect density in the crystal at a given stoichiometry, was observed to reduce with electrochemical lithium insertion / cycling. This capability has demonstrated lithiation-dependent mechanical property measurements for the first time and represents an important contribution since degradation models, which are currently in use for materials at any size scale, always assume constant values regardless of the change in stoichiometry