A fast edge charge exchange recombination spectroscopy system at the ASDEX Upgrade tokamak

In this work, a new type of high through-put Czerny-Turner spectrometer has been developed which allows us to acquire multiple channels simultaneously with a repetition time on the order of 10 μ s at different wavelengths. The spectrometer has been coupled to the edge charge exchange recom- bination system at ASDEX Upgrade which has been recently refurbished with new lines of sight. Construction features, calibration methods, and initial measurements obtained with the new setup will be presented.European Commission (EUROfusion 633053

Thermal and Chemical Equilibration in Relativistic Heavy Ion Collisions

We investigate the thermalization and the chemical equilibration of a parton plasma created from Au+Au collision at LHC and RHIC energies starting from the early moment when the particle momentum distributions in the central region become for the first time isotropic due to longitudinal cooling. Using the relaxation time approximation for the collision terms in the Boltzmann equations for gluons and for quarks and the real collision terms constructed from the simplest QCD interactions, we show that the collision times have the right behaviour for equilibration. The magnitude of the quark (antiquark) collision time remains bigger than the gluon collision time throughout the lifetime of the plasma so that gluons are equilibrating faster than quarks both chemically and kinetically. That is we have a two-stage equilibration scenario as has been pointed out already by Shuryak sometimes ago. Full kinetic equilibration is however slow and chemical equilibration cannot be completed before the onset of the deconfinement phase transition assumed to be at $T_c=200$ MeV. By comparing the collision entropy density rates of the different processes, we show explicitly that inelastic processes, and \emph{not} elastic processes as is commonly assumed, are dominant in the equilibration of the plasma and that gluon branching leads the other processes in entropy generation. We also show that, within perturbative QCD, processes with higher power in \alpha_s need not be less important for the purpose of equilibration than those with lower power. The state of equilibration of the system has also a role to play. We compare our results with those of the parton cascade model.Comment: 17 pages, revtex+psfig style with 14 embedded postscript figures, to appear in Phys. Rev.

Metal–Organic Framework Derived Copper Chalcogenides-Carbon Composites as High-Rate and Stable Storage Materials for Na Ions

Transition metal chalcogenides have been regarded as promising storage materials for sodium ions owing to their high theoretical capacity. Herein, copper-based metal–organic frameworks (Cu-BTC) are reported as precursors to fabrica copper chalcogenides-carbon composites, namely Cu$_{1.8}$S@C and Cu$_{2-x}$Se@C. The materials exhibit excellent electrochemical performance with high specific capacities (504 mAh g$^{-1}$ for Cu$_{1.8}$S@C and 317 mAh g$^{-1}$ for Cu$_{2-x}$Se@C at 0.1 A g$^{-1}$) and long-term cycling stability when used as anode materials in cells employing carbon-coated Na$_{3}$V$_{2}$(PO$_{4}$)$_{3}$ (NVP/C) positive electrodes. The Cu$_{2-x}$Se@C||NVP/C cell delivers a specific capacity of 73 mAh g$^{-1}$ at 1.2 A g$^{-1}$ (based on cathode mass) and excellent cycling stability (capacity retention of 85% after 500 cycles at 0.12 A g$^{-1}$) with Coulombic efficiency of ≈99.9%. Moreover, the Cu$_{2-x}$Se@C composite performs well as positive electrode storage material in a sodium-metal cell, offering a high reversible capacity of 216 mAh (per gram of Cu$_{2-x}$Se@C) after 1800 cycles at 2 A g$^{-1}$ and enabling high specific energy and power

On the Consistency of the Exact Renormalization Group Approach Applied to Gauge Theories in Algebraic Non-Covariant Gauges

We study a class of Wilsonian formulations of non-Abelian gauge theories in algebraic non-covariant gauges where the Wilsonian infrared cutoff $\Lambda$ is inserted as a mass term for the propagating fields. In this way the Ward-Takahashi identities are preserved to all scales. Nevertheless BRST-invariance in broken and the theory is gauge-dependent and unphysical at $\Lambda\neq0$. Then we discuss the infrared limit $\Lambda\to0$. We show that the singularities of the axial gauge choice are avoided in planar gauge and light-cone gauge. In addition the issue of infrared divergences is addressed in some explicit example. Finally the rectangular Wilson loop of size $2L\times 2T$ is evaluated at lowest order in perturbation theory and a non commutativity between the limits $\Lambda\to0$ and $T\to\infty$ is pointed out.Comment: Latex2e, 49 pages, 2 EPS figures. Misprints corrected. Version to be published on IJMP

Lattice Gauge Description of Colliding Nuclei

We propose a novel formalism for simultaneously describing both, the hard and soft parton dynamics in ultrarelativistic collisions of nuclei. The emission of gluons from the initially coherent parton configurations of the colliding nuclei and low-$p_t$ color coherence effects are treated in the framework of a Yang-Mills transport equation on a coupled lattice-particle system. A collision term is added to the transport equation to account for the remaining intermediate and high-$p_t$ interactions in an infrared finite manner.Comment: 8 page

Time Dependence of Chemical Freeze-out in Relativistic Heavy Ion Collisions

We investigate chemical and thermal freeze-out time dependencies for strange particle production for CERN SPS heavy ion collisions in the framework of a dynamical hadronic transport code. We show that the Lambda yield changes considerably after hadronization in the case of Pb+Pb collisions, whereas for smaller system sizes (e.g. S+S) the direct particle production dominates over production from inelastic rescattering. Chemical freeze-out times for strange baryons in Pb+Pb are smaller than for non-strange baryons, but they are still sufficiently long for hadronic rescattering to contribute significantly to the final Lambda yield. Based on inelastic and elastic cross section estimates we expect the trend of shorter freeze-out times (chemical and kinetic), and thus less particle production after hadronization, to continue for multi-strange baryons.Comment: 10 pages, 7 postscript figure

Linking Dynamical and Thermal Models of Ultrarelativistic Nuclear Scattering

To analyse ultrarelativistic nuclear interactions, usually either dynamical models like the string model are employed, or a thermal treatment based on hadrons or quarks is applied. String models encounter problems due to high string densities, thermal approaches are too simplistic considering only average distributions, ignoring fluctuations. We propose a completely new approach, providing a link between the two treatments, and avoiding their main shortcomings: based on the string model, connected regions of high energy density are identified for single events, such regions referred to as quark matter droplets. Each individual droplet hadronizes instantaneously according to the available n-body phase space. Due to the huge number of possible hadron configurations, special Monte Carlo techniques have been developed to calculate this disintegration.Comment: Complete paper enclosed as postscript file (uuencoded

Elliptic Flow from a Transversally Thermalized Fireball

The agreement of elliptic flow data at RHIC at central rapidity with the hydrodynamic model has led to the conclusion of very rapid thermalization. This conclusion is based on the intuitive argument that hydrodynamics, which assumes instantaneous local thermalization, produces the largest possible elliptic flow values and that the data seem to saturate this limit. We here investigate the question whether incompletely thermalized viscous systems may actually produce more elliptic flow than ideal hydrodynamics. Motivated by the extremely fast primordial longitudinal expansion of the reaction zone, we investigate a toy model which exhibits thermalization only in the transverse directions but undergoes collisionless free-streaming expansion in the longitudinal direction. For collisions at RHIC energies, elliptic flow results from the model are compared with those from hydrodynamics. With the final particle yield and \kt-distribution fixed, the transversally thermalized model is shown not to be able to produce the measured amount of elliptic flow. This investigation provides further support for very rapid local kinetic equilibration at RHIC. It also yields interesting novel results for the elliptic flow of massless particles such as direct photons.Comment: revtex4, 15 pages + 10 embedded EPS figure