arXiv subjects

Z. Li

Publications and source records attributed to Z. Li.

284 records · Page 16Linked to original sources

arXiv2002

Manifold Damping Of Wakefields In High Phase Advance Linacs For The NLC

Earlier RDDS (Rounded Damped Detuned Structures) [1,2], designed, fabricated and tested at SLAC, in collaboration with KEK, have been shown to damp wakefields successfully. However, electrical breakdown has been found to occur in these structures and this makes them inoperable at the desired gradient. Recent results [3] indicate that lowering the group velocity of the accelerating mode reduces electrical breakdown events. In order to preserve the filling time of each structure a high synchronous phase advance (150 degrees as opposed to 120 used in previous NLC designs) has been chosen. Here, damping of the wakefield is analyzed. Manifold damping and interleaving of structure cell frequencies is discussed. These wakefields impose alignment tolerances on the cells and on the structure as a whole. Tolerance calculations are performed and these are compared with analytic estimations.

physics.acc-ph↗

arXiv2002

Spontaneous time reversal symmetry breaking in the pseudogap state of high-Tc superconductors

When matter undergoes a phase transition from one state to another, usually a change in symmetry is observed, as some of the symmetries exhibited are said to be spontaneously broken. The superconducting phase transition in the underdoped high-Tc superconductors is rather unusual, in that it is not a mean-field transition as other superconducting transitions are. Instead, it is observed that a pseudo-gap in the electronic excitation spectrum appears at temperatures T* higher than Tc, while phase coherence, and superconductivity, are established at Tc (Refs. 1, 2). One would then wish to understand if T* is just a crossover, controlled by fluctuations in order which will set in at the lower Tc (Refs. 3, 4), or whether some symmetry is spontaneously broken at T* (Refs. 5-10). Here, using angle-resolved photoemission with circularly polarized light, we find that, in the pseudogap state, left-circularly polarized photons give a different photocurrent than right-circularly polarized photons, and therefore the state below T* is rather unusual, in that it breaks time reversal symmetry11. This observation of a phase transition at T* provides the answer to a major mystery of the phase diagram of the cuprates. The appearance of the anomalies below T* must be related to the order parameter that sets in at this characteristic temperature .

cond-mat.str-el↗

arXiv2002

Search for new baryon resonances

Within a chiral constituent quark formalism, allowing the inclusion of all known resonances, a comprehensive study of the recent eta photoproduction data on the proton up to 2 GeV is performed. This study shows evidence for a new S11 resonances and indicates the presence of an additional missing P13 resonance.

nucl-th↗

arXiv2001

Dipole Wakefield Suppression In High Phase Advance Detuned Linear Accelerators For The JLC/NLC Designed To Minimise Electrical Breakdown And Cumulative BBU

Recent experiments at SLAC [1,2] and CERN [3] have revealed evidence of significant deformation in the form of "pitting" of the cells of the 1.8m series of structures DDS/RDDS (Damped Detuned Structure/Rounded Damped Detuned Structure). This pitting occurs in the high group velocity (vg /c = 0.012) end of the accelerating structure and little evidence of breakdown has been found in the lower group velocity end of the structure. Additional, albeit preliminary experimental evidence, suggests that shorter and lower group velocity structures have reduced breakdown events with increasing accelerating field strengths. Two designs are presented here, firstly a 90cm structure consisting of 83 cells with an initial vg/c = 0.0506 (known as H90VG5) and secondly, an even shorter structure of length 60cm consisting of 55 cells with an initial vg /c = 0.03 (known as H60VG3). The feasibility of using these structures to accelerate a charged beam over 10km is investigated. The particular issue focussed upon is suppression of the dipole wakefields via detuning of the cell frequencies and by locally damping individual cells in order to avoid BBU (Beam Break Up). Results are presented on beam-induced dipole wakefields and on the beam dynamics encountered on tracking the progress of the beam through several thousand accelerating structures. [1] C. Adolphsen, ROAA003, this conf. [2] R.H. Miller et al, FPAH062, this conf. [3] L. Groening et al, MPPH039, this conf

physics.acc-ph↗

arXiv2001

Quark Model Study of The eta Photoproduction: Evidence for a New S11 Resonance?

An extensive and systematic study of the recent eta photoproduction data up to 1.2 GeV is presented within a chiral constituent quark model. A model embodying all known nucleonic resonances shows clear need for a yet undiscovered third S11 resonance in the second resonance region, for which we determine the mass (1.729 GeV) and the total width (183 MeV). Furthermore, we extract the configuration mixing angles, an important property of the quark-quark interaction in the quark model, for the resonances S11(1535) and S11(1650), as well as for the resonances D13(1520) and D13(1700). Our results agree well with the quark model predictions. In addition, the partial eta N decay widths and/or the photo-excitation helicity amplitudes for the nucleonic resonances S11(1535), S11(1650), P11(1710), P13(1720), D13(1520), D13(1700), D15(1675), and F15(1680) are also obtained in this approach.

nucl-th↗

arXiv2000

Numerical Studies on Locally Damped Structures

In the JLC/NLC X-band linear collider, it is essential to reduce the long-range dipole wakefields in the accelerator structure to prevent beam break up (BBU) and emittance degradation. The two methods of reducing the long-range wakefields are detuning and damping. Detuning reduces the wakefields rapidly as the dipole modes de-cohere but, with a finite number of modes, the wakefield will grow again as the modes re-cohere. In contrast, damping suppresses the wakefields at a longer distance. There are two principal damping schemes: synchronous damping using HOM manifolds such as that used in the RDDS1 structure and local damping similar to that used in the CLIC structure. In a locally damped scheme, one can obtain almost any Q value, however, the damping can have significant effects on the accelerating mode. In this paper, we present a medium local-damping scheme where the wakefields are controlled to meet the BBU requirement while minimizing the degradations of the fundamental rf parameters. We will address the load design and pulse heating issues associated with the medium damping scheme.

physics.acc-ph↗

arXiv2000

Design of the JLC/NLC RDDS Structure Using Parallel Eigensolver Omega3p

The complexity of the Round Damped Detuned Structue (RDDS) for the JLC/NLC main linac is driven by the considerations of rf efficiency and dipole wakefield suppression. As a time and cost saving measure for the JLC/NLC, the dimensions of the 3D RDDS cell are being determined through computer modeling to within fabrication precision so that no tuning may be needed once the structures are assembled. The tolerances on the frequency errors for the RDDS structure are about one MHz for the fundamental mode and a few MHz for the dipole modes. At the X-band frequency, these correspond to errors of a micron level on the major cell dimensions. Such a level of resolution requires highly accurate field solvers and vast amount of computer resources. A parallel finite-element eigensolver Omega3P was developed at SLAC that runs on massively parallel computers such as the Cray T3E at NERSC. The code was applied in the design of the RDDS cell dimensions that are accurate to within fabrication precision. We will present the numerical approach of using these codes to determine the RDDS dimensions and compare the numerical predictions with the cold test measurements on RDDS prototypes that are diamond-turned using these dimensions.

physics.acc-ph↗

arXiv2000

Design, fabrication and measurement of the first rounded damped detuned accelerator structure (RDDS1)

As a joint effort in the JLC/NLC research program, we have developed a new type of damped detuned accelerator structure with optimized round-shaped cavities (RDDS). This paper discusses some important R&D aspects of the first structure in this series (RDDS1). The design aspects covered are the cell design with sub-MHz precision, HOM detuning, coupling and damping technique and wakefield simulation. The fabrication issues covered are ultra-precision cell machining with micron accuracy, assembly and diffusion bonding technologies to satisfactorily meet bookshelf, straightness and cell rotational alignment requirements. The measurements described are the RF properties of single cavities and complete accelerator section, as well as wakefields from the ASSET tests at SLAC. Finally, future improvements are also discussed.

physics.acc-ph↗

arXiv2000

Higher Dipole Bands in the NLC Accelerating Structure

We show that scattering matrix calculations for dipole modes between 23-43 GHz for the 206 cell detuned structure (DS) are consistent with finite element calculations and results of the uncoupled model. In particular, the rms sum wake for these bands is comparable to that of the first dipole band. We also show that for RDDS1 uncoupled wakefield calculations for higher bands are consistent with measurements. In particular, a clear 26 GHz signal in the short range wake is found in both results.

physics.acc-ph↗

arXiv2000

Dipole Mode Detuning in the NLC Injector Linacs

The injector linacs of the JLC/NLC comprise the e+ booster, the e- drive linac, the e+ booster, and the two prelinacs. These four systems together transport the two types of beams from the 80 MeV point at the end of their respective bunchers up to the storage rings, and then from the storage rings to the 10 GeV point at the beginning of the bunch compressors. Nominally, the e+ booster structure will be L-band and the others S-band. In transport through these linacs it is important that the emittances and particle losses stay within specified tolerances. In particular, the effects of the dipole wakefield-especially the long-range dipole wakefield-need to be carefully considered. We present designs of the injector linac structures that employ simple mode detuning, without damping, and that allow us to satisfy our tolerance requirements, for both the nominal (2.8 ns) and alternate (1.4 ns) bunch spacing options.

physics.acc-ph↗

arXiv2000

Meeting Tight Frequency Requirement of Rounded Damped Detuned Structure

Following successful design and fabrication of damped detuned structures, the R&D for the accelerating structures of the NLC/JLC linear collider project proceeded to studies of Rounded Damped Detuned Structure with curved cross section of the cavity shape for increased shunt impedance. The important features of the structure are the accurately tuned accelerating mode frequency and the distribution of the first dipole modes smooth and close to the design distribution. These requirements were met based on the high-accuracy diamond turning with its capability to realize the periphery tolerance of two microns. The lowest dipole mode frequencies scattered by 0.6 MHz RMS. The error in the accelerating mode frequency averaged over a structure was 0.1 MHz by applying a feed-forward method.

hep-ex↗

arXiv1996

QED Penguin Contributions To Isospin Splittings of Heavy-Light Quark Systems

Recent experiments show that the isospin-violating mass splitting of the B mesons is very small, but the best fits with a QCD sum rule analysis give a splitting of at least 1.0 MeV. The isospin-violating mass splittings of the charmed mesons, on the other hand, are in agreement with experiment. In this letter we show that the inclusion of 2$^{nd}$ kind QED penguin diagrams can account for this discrepancy within the errors in the QCD sum rule method.

hep-ph↗

arXiv1995

Finite Size Scaling and Critical Exponents in Critical Relaxation

We simulate the critical relaxation process of the two-dimensional Ising model with the initial state both completely disordered or completely ordered. Results of a new method to measure both the dynamic and static critical exponents are reported, based on the finite size scaling for the dynamics at the early time. From the time-dependent Binder cumulant, the dynamical exponent $z$ is extracted independently, while the static exponents $β/ν$ and $ν$ are obtained from the time evolution of the magnetization and its higher moments.

cond-mat↗

arXiv1994

$I=0,1$ $ππ$ and $I=1/2$ $Kπ$ Scattering using Quark Born Diagrams

We extend the quark Born diagram formalism for hadron-hadron scattering to processes with valence $q\bar q$ annihilation, specifically $I=0$ and $I=1$ $ππ$ and $I=1/2$ $Kπ$ elastic scattering. This involves the $s$-channel hybrid annihilation process $q^2\bar q^2 \to q\bar q g \to q^2\bar q^2$ and conventional $s$-channel $q\bar q$ resonances, in addition to the $t$-channel gluon exchange treated previously using this formalism. The strength of the $t$-channel gluon amplitude is fixed by previous studies of $I=2$ $ππ$ and $I=3/2$ $Kπ$ scattering. The $s$-channel resonances $ρ(770)$, $K^*(892)$, $f_0(1400)$ and $K_0^*(1430)$ are incorporated as relativized Breit-Wigner amplitudes, with masses and energy-dependent widths fitted to experimental phase shifts. The strength of the $s$-channel gluon ``hybrid" annihilation diagrams is problematical since the perturbative massless-gluon energy denominator must be modified to account for the effect of confinement on the energy of the virtual hybrid state. Our naive expectation is that near threshold the hybrid diagrams are comparable in magnitude but opposite in sign to the contribution predicted using massless perturbative gluons. Fitting the strength of this amplitude to $I=0$ $ππ$ and $I=1/2$ $Kπ$ S-wave data gives a result consistent with this expectation. We find good agreement with experimental phase shifts from threshold to 0.9 GeV in $ππ$ and 1.6 GeV in $Kπ$ using this approach. We conclude that the most important contribution to low energy S-wave scattering in these channels arises from the nonresonant quark Born diagrams, but that the low-energy wings of the broad $s$-channel resonances $f_0(1400)$ and $K^*_0(1430)$ also give important contributions near threshold. The nonresonant contributions are found to be much smaller in $L\;

hep-ph↗