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Output power enhancement of ac Raman free-electron laser

Description: The ac free-electron laser (ac FEL) uses an ac (temporally oscillating but spatially uniform) field as the pump source with the hope that electromagnetic radiation will be produced with wavelength shorter than that can be generated by a conventional magnet FEL. The ac field can be provided by a superconducting cavity or by a plasma wave. In this note, the effects of the thermal motion of the electron beam on the linear gain and the nonlinear saturated output power in a Raman-regime ac FEL with the launch of an electromagnetic signal wave by particle-in-cell simulations are investigated. The output power enhancement of the ac FEL either by imposing an appropriate axial electric field at the time of radiation saturation or by properly tapering the strength or the frequency of the ac pump is discussed.
Date: January 1, 1987
Creator: Yan, Y.T.
Partner: UNT Libraries Government Documents Department

Measurement of oxygen disorder and nano-twin microstructure associated with columnar defects in YBCO.

Description: Studies of defects generated by high energy (>1 GeV) heavy ion irradiation in high-Tc superconductors have been performed by transmission electron microscopy (TEM). Our study shows that high dose irradiation leads to the formation of nano-twins, by which the columnar defects are connected. An analysis of the local Fourier components of the image intensity in [001] lattice images indicates that these new ''twin'' boundaries are much more diffuse than pre-existing twin boundaries in YBCO. The mechanism of the formation of nano-twin boundaries on {l_brace}110{r_brace} planes and their possible relation to superconducting properties are discussed.
Date: December 18, 1998
Creator: Yan, Y,
Partner: UNT Libraries Government Documents Department

Applications of differential algebra to single-particle dynamics in storage rings

Description: Recent developments in the use of differential algebra to study single-particle beam dynamics in charged-particle storage rings are the subject of this paper. Chapter 2 gives a brief review of storage rings. The concepts of betatron motion and synchrotron motion, and their associated resonances, are introduced. Also introduced are the concepts of imperfections, such as off-momentum, misalignment, and random and systematic errors, and their associated corrections. The chapter concludes with a discussion of numerical simulation principles and the concept of one-turn periodic maps. In Chapter 3, the discussion becomes more focused with the introduction of differential algebras. The most critical test for differential algebraic mapping techniques -- their application to long-term stability studies -- is discussed in Chapter 4. Chapter 5 presents a discussion of differential algebraic treatment of dispersed betatron motion. The paper concludes in Chapter 6 with a discussion of parameterization of high-order maps.
Date: September 1, 1991
Creator: Yan, Y.
Partner: UNT Libraries Government Documents Department

Understanding of Defect Physics in Polycrystalline Photovoltaic Materials: Preprint

Description: The performance of thin-film solar cells is influenced by the quality of interfaces and formation of defects such as point defects, stacking faults, twins, dislocations, and grain boundaries. It is important to understand the defect physics so that appropriate methods may be developed to suppress the formation of harmful defects. Here, we review our understanding of defect physics in thin-film photovoltaic (PV) materials such as Si, CdTe, Cu(In,Ga)Se2 (CIGS), Cu2ZnSnSe2 (CZTSe), and Cu2ZnSnS2 (CZTS) using the combination of nanoscale electron microscopy characterization and density-functional theory (DFT). Although these thin-film PV materials share the same basic structural feature - diamond structure based - the defect physics in them could be very different. Some defects, such as stacking faults and special twins, have similar electronic properties in these thin-film materials. However, some other defects, such as grain boundaries and interfaces, have very different electronic properties in these materials. For example, grain boundaries produce harmful deep levels in Si and CdTe, but they do not produce significant deep levels in CIGS, CZTSe, and CZTS. These explain why passivation is critical for Si and CdTe solar cells, but is less important in CIS and CZTS solar cells. We further provide understanding of the effects of interfaces on the performance of solar cells made of these PV materials.
Date: July 1, 2011
Creator: Yan, Y.
Partner: UNT Libraries Government Documents Department

Zlib: A numerical library for optimal design of truncated power series algebra and map parameterization routines

Description: A brief review of the Zlib development is given. Emphasized is the Zlib nerve system which uses the One-Step Index Pointers (OSIPs) for efficient computation and flexible use of the Truncated Power Series Algebra (TPSA). Also emphasized is the treatment of parameterized maps with an object-oriented language (e.g. C++). A parameterized map can be a Vector Power Series (Vps) or a Lie generator represented by an exponent of a Truncated Power Series (Tps) of which each coefficient is an object of truncated power series.
Date: November 1, 1996
Creator: Yan, Y.T.
Partner: UNT Libraries Government Documents Department

On the feasibility of tracking with differential-algebra maps in long-term stability studies for large hadron colliders

Description: A time-saving alternative to conventional element-by-element tracking in long-term stability studies is the use of truncated Taylor maps. This report discusses how the non-symplecticity of a moderately high-order truncated Taylor map affects its reliability when the map is used for tracking over several thousand turns. Various machines and two different map-constructing programs are compared. It is found that the discrepancies between the Taylor map results and those obtained by direct tracking grow with amplitude. Thus, such maps are not guaranteed to be sufficient for long-term tracking over millions of turns without suitable symplectification.
Date: January 1, 1992
Creator: Kleiss, R.; Schmidt, F. & Yan, Y.
Partner: UNT Libraries Government Documents Department

Design of Shallow P-Type Dopants in ZnO: Preprint

Description: This paper describes approaches to lower the acceptor ionization energy in ZnO by codoping acceptors with donor or isovalent atoms and proposes a universal approach to overcome the doping polarity problem for wide-band-gap semiconductors.
Date: May 1, 2008
Creator: Wei, S.-H.; Li, J. & Yan, Y.
Partner: UNT Libraries Government Documents Department

Structure and Effects of Extended Defects in Polycrystalline Si Thin Films

Description: The structure and effects of extended defects in rapid deposited polycrystalline Si thin films were studied using a combination of high-resolution transmission electron microscopy and first-principles total energy calculations.
Date: January 1, 2000
Creator: Yan, Y.; Al-Jassim, M. M.; Wang, T. H. & Ciszek, T. F.
Partner: UNT Libraries Government Documents Department

Cladding embrittlement during postulated loss-of-coolant accidents.

Description: The effect of fuel burnup on the embrittlement of various cladding alloys was examined with laboratory tests conducted under conditions relevant to loss-of-coolant accidents (LOCAs). The cladding materials tested were Zircaloy-4, Zircaloy-2, ZIRLO, M5, and E110. Tests were performed with specimens sectioned from as-fabricated cladding, from prehydrided (surrogate for high-burnup) cladding, and from high-burnup fuel rods which had been irradiated in commercial reactors. The tests were designed to determine for each cladding material the ductile-to-brittle transition as a function of steam oxidation temperature, weight gain due to oxidation, hydrogen content, pre-transient cladding thickness, and pre-transient corrosion-layer thickness. For short, defueled cladding specimens oxidized at 1000-1200 C, ring compression tests were performed to determine post-quench ductility at {le} 135 C. The effect of breakaway oxidation on embrittlement was also examined for short specimens oxidized at 800-1000 C. Among other findings, embrittlement was found to be sensitive to fabrication processes--especially surface finish--but insensitive to alloy constituents for these dilute zirconium alloys used as cladding materials. It was also demonstrated that burnup effects on embrittlement are largely due to hydrogen that is absorbed in the cladding during normal operation. Some tests were also performed with longer, fueled-and-pressurized cladding segments subjected to LOCA-relevant heating and cooling rates. Recommendations are given for types of tests that would identify LOCA conditions under which embrittlement would occur.
Date: July 31, 2008
Creator: Billone, M.; Yan, Y.; Burtseva, T. & Daum, R.
Partner: UNT Libraries Government Documents Department

Swamp plots for dynamic aperture studies of PEP-II lattices

Description: With a newly developed algorithm using resonance basis Lie generators and their evaluation with action-angle Poisson bracket maps (nPB tracking) the authors have been able to perform fast tracking for dynamic aperture studies of PEP-II lattices as well as incorporate lattice nonlinearities in beam-beam studies. They have been able to better understand the relationship between dynamic apertures and the tune shift and resonance coefficients in the generators of the one-turn maps. To obtain swamp plots (dynamic aperture vs. working point) of the PEP-II lattices, they first compute a one-turn resonance basis map for a nominal working point and then perform nPB tracking by switching the working point while holding fixed all other terms in the map. Results have been spot-checked by comparing with element-by-element tracking.
Date: June 1, 1995
Creator: Yan, Y.T.; Irwin, J.; Cai, Y.; Chen, T. & Ritson, D.
Partner: UNT Libraries Government Documents Department

Nonlinear analyses of storage ring lattices using one-turn maps

Description: Using normalized one-turn resonance-basis Lie generators in conjunction with an action-angle tracking algorithm (nPB tracking), the authors have been able to better understand the relationship between the dynamic aperture and lattice nonlinearities. Tunes, tune shifts with amplitude and/or energy, and resonance strengths may be freely changed to probe their individual impact on the dynamic aperture. Fast beam-beam simulations can be performed with the inclusion of nonlinear lattice effects. Examples from studies of the PEP-II lattices are given.
Date: May 1, 1995
Creator: Yan, Y.T.; Irwin, J. & Chen, T.
Partner: UNT Libraries Government Documents Department

Atomic structures of inversion domain boundaries and dislocations in sintered AlN

Description: AlN is an attractive material because of its mechanical, thermal and electronic properties. However, these properties are often adversely altered by extended defects such as inversion domain boundaries (IDBs), and through impurities associated with the defects, especially oxygen. Thus to improve the properties of the material it is important to understand the structures of the defects. A number of studies have been carried out to characterize these defects using fringe contrast, weak-beam, convergent-beam electron diffraction and conventional high-resolution phase contrast imaging. These techniques all require the use of model structures for data interpretation. In this paper, the authors report a direct determination of the structures of IDBs and dislocation cores in AlN by high resolution Z-contrast imaging using a 300 kV VG HB603U scanning transmission electron microscope, with a probe size of 1.3 A.
Date: February 1, 1998
Creator: Yan, Y.; Chisholm, M.F. & Pennycook, S.J.
Partner: UNT Libraries Government Documents Department

Z-contrast imaging of ordered structures in Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3} and Ba(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}

Description: Lead-based cubic perovskites such as Pb(B{sub 1/3}{sup 2+}B{sub 2/3}{sup 5+})O{sub 3} (B{sup 2+} = Mg, Co, Ni, Zn; B{sup 5+} = Nb, Ta) are relaxor ferroelectrics. Localized order and disorder often occur in materials of this type. In the Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3} (PMN) family, previous studies have proposed two models, space-charge and charge-balance models. In the first model, the ordered regions carry a net negative charge [Pb(Mg{sub 1/2}Nb{sub 1/2})O{sub 3}], while in the second model it does not carry a net charge [Pb((Mg{sub 2/3}Nb{sub 1/3}){sub 1/2}Nb{sub 1/2})O{sub 3}]. However, no direct evidence for these two models has appeared in the literature yet. In this paper the authors report the first direct observations of local ordering in undoped and La-doped Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}, using high-resolution Z-contrast imaging. Because the ordered structure in Ba(Mg{sub 1/3}Nb{sub 2/3})O{sub 3} is well known, the Z-contrast image from an ordered domain is used as a reference for this study.
Date: February 1, 1998
Creator: Yan, Y.; Pennycook, S.J.; Xu, Z. & Viehland, D.
Partner: UNT Libraries Government Documents Department

Raman Studies of Nanocrystalline CdS:O Film

Description: Oxygenated nanocrystalline CdS films show improved solar cell performance, but the physics and mechanism underlying this are not yet clearly understood. Raman study provides complementary information to the understanding obtained from other experimental investigations. A comprehensive analysis of the existing experimental data (including x-ray diffraction, transmission, transmission electron microscopy, and Raman) has led to the following conclusions: (1) The O-incorporation forms CdS1-xOx alloy nano-particles. (2) The observed evolution of the electronic structure is the result of the interplay between the alloy and quantum confinement effect. (3) The blue-shift of the LO phonon Raman peak is primarily due to the alloying effect. (4) Some oxygen atoms have taken the interstitial sites.
Date: January 1, 2005
Creator: Zhang, Y.; Wu, X.; Dhere, R.; Zhou, J.; Yan, Y. & Mascarenhas, A.
Partner: UNT Libraries Government Documents Department

Stability and Electronic Structures of CuxS Solar Cell Absorbers: Preprint

Description: Cu{sub x}S is one of the most promising solar cell absorber materials that has the potential to replace the leading thin-film solar cell material Cu(In,Ga)Se{sub 2} for high efficiency and low cost. In the past, solar cells based on Cu{sub x}S have reached efficiency as high as 10%, but it also suffers serious stability issues. To further improve its efficiency and especially the stability, it is important to understand the stability and electronic structure of Cu{sub x}S. However, due to the complexity of their crystal structures, no systematic theoretical studies have been carried out to understand the stability and electronic structure of the Cu{sub x}S systems. In this work, using first-principles method, we have systematically studied the crystal and electronic band structures of Cu{sub x}S (1.25 < x {le} 2). For Cu{sub 2}S, we find that all the three chalcocite phases, i.e., the low-chalcocite, the high-chalcocite, and the cubic-chalcocite phases, have direct bandgaps around 1.3-1.5 eV, with the low-chalcocite being the most stable one. However, Cu vacancies can form spontaneously in these compounds, causing instability of Cu{sub 2}S. We find that under Cu-rich condition, the anilite Cu{sub 1.75}S is the most stable structure. It has a predicted bandgap of 1.4 eV and could be a promising solar cell absorber.
Date: July 1, 2012
Creator: Wei, S. H.; Xu, Q.; Huang, B.; Zhao, Y.; Yan, Y. & Noufi, R.
Partner: UNT Libraries Government Documents Department

Atomic Structure and Electronic Properties of c-Si/a-Si:H Interfaces in Si Heterojunction Solar Cells

Description: The atomic structure and electronic properties of crystalline silicon/hydrogenated amorphous silicon (c-Si/a-Si:H) interfaces in silicon heterojunction (SHJ) solar cells are investigated by high-resolution transmission electron microscopy, atomic-resolution Z-contrast imaging, and electron energy loss spectroscopy. We find that all high-performance SHJ solar cells exhibit atomically abrupt and flat c-Si/a-Si:H interfaces and high disorder of the a-Si:H layers. These atomically abrupt and flat c-Si/a-Si:H interfaces can be realized by direct deposition of a-Si:H on c-Si substrates at a substrate temperature below 150 deg C by hot-wire chemical vapor deposition from pure silane.
Date: November 1, 2005
Creator: Yan, Y.; Page, M.; Wang, Q.; Branz, H. M.; Wang, T. H. & Al-Jassim, M. M.
Partner: UNT Libraries Government Documents Department