55595696
The Design of a 5 MeV Accelerator Based on Multipactor Electron Gun
The Multipactor Electron Gun (MEG) can produce high current self-bunching electron beams. In this paper, the design of an S-band accelerator based on MEG and the result of low power test are presented. The mechanical structure was designed with ability of replacing secondary electron emitters. Pd-Ba alloy and Pt are used as the secondary electron emitters of the MEG. The distance between electrodes and the resonant frequency of the MEG can be adjusted separately by step motors. The parameters of the accelerator tube were optimized by using numerical simulation with the design output energy of 5MeV and an average current of 100mA. INTRODUCTION Multipactor electron gun is a new kind of cold cathode microwave electron gun. It has the advantages of short duration, high current electron beams, simple structure and long lifetime. Therefore it is very suitable for small industrial and medical accelerator or high power microwave generation. F. M. Mako designed Micro-Pulse Electron Gun (MPG) in 1993 [1]. The MPG (S-band) reached a high macro-pulse current density of 10A/cm [2, 3]. The Accelerator Lab of Tsinghua University has been doing research work on MEG since 1999. The research work includes Theoretical Analysis, simulation [4], design, manufacture, cold test and high power experiments and multipactor material et.al [5]. With CVD diamond cathode, a macro-pulse current density of 1.2 A / cm was reached [6]. This paper presents our work on the design of an S-band accelerator based on MEG and the result of preliminary experiments. PRINCIPLES We choose a standing wave accelerating tube works on S-band, 2.998GHz, and its output energy can reach 6MeV. For typical acceleration tube, the electron source is thermionic cathode electron gun, which uses a high voltage to pull the electron out with energy about more than 10KeV. MEG is a resonator in front of the accelerator cavity chains, and parts of the microwave power feed into MPG through coupling holes. The electron beam is produced, amplified and then bunched simultaneously in the cathode-grid gap. When the current in the cavity reaches a steady level by space charge and beam loading effect, the outgoing electron beams will be the saturation emission current. Finally the emission current comes into the accelerating cavity and is accelerated. The structure of the MEG accelerator is shown in Figure 1, it contains several parts: 1) adjusting device controlling the axis position of cathode and tuner. It uses two linear step motors and metal bellows to archive a minimal step of 3.2μm and vacuum sealing; 2) MEG resonate cavity with a microwave probe and replaceable grid, connected to the accelerating tube through flange; 3) accelerating tube, combined with resonate cavities, waveguide window, water cooling and Ti window. Figure 1: Structure of the MPG accelerator. MEG PARAMETER DESIGN A self-developed simulate software, SEEG, is used to simulate the electron emission behavior of MEG [4]. SEEG is a MEG beam dynamics simulating code , which can provide beam current destiny, beam energy and emittance under given conditions. Relativistic effect, RF field effect and space charge effect are considered. The secondary electron emission (SEE) model used in the program is that: SEE yield δ has a relation with primary electron energy Ei, δ = δmax (w •e ). δmax is the maximal value of SEE yield and w = Ei / Emax, Emax is the relevant primary electron energy, k = 0.62 while w 1. The injection angle is ignored and all SE come out vertically. The SEE velocity and emittance delay can be defined as constant. Table 1: Parameter of the MEG Parameter Design Value Cathode-grid distance d=0.25cm Frequency f=2.998GHz RF voltage amplitude Vrf=3.7kV
36756339
A MORE PRECISE EXTRACTION OF |Vcb| IN HQEFT OF QCD
The more precise extraction for the CKM matrix element |Vcb| in the heavy quark effective field theory (HQEFT) of QCD is studied from both exclusive and inclusive semileptonic B decays. The values of relevant nonperturbative parameters up to order $1/m^2_Q$ are estimated consistently in HQEFT of QCD. Using the most recent experimental data for B decay rates, |Vcb| is updated to be |Vcb|=0.0395±0.0011expt±0.0019theor from B→D*lν decay and |Vcb|=0.0434±0.0041expt±0.0020theor from B→Dlν decay as well as |Vcb|=0.0394±0.0010expt±0.0014theor from inclusive B→Xclν decay.
117848077
Could $Z_{b}(10610)$ be a $B^{*}\bar{B}$ molecular state?
Assuming the newly observed structure $Z_{b}(10610)$ as a bottomonium-like molecular state $B^{*}\bar{B}$, we calculate its mass in the framework of QCD sum rules. The numerical result is $10.54\pm0.22 GeV$ for $B^{*}\bar{B}$, which coincide with the mass of $Z_{b}(10610)$. This consolidates the statement made by Belle Collaboration that the $Z_{b}(10610)$ resonance could be a $B^{*}\bar{B}$ molecular state.
123052577
Unparticle effects on neutrinoless and neutrino double beta decay
The neutrinoless and neutrinos double beta decays are investigated in an effective theory where a scalar unparticle is introduced. We calculate the contributions of a virtual unparticle to the rates of 0ν2β and 2ν2β decays. The unparticle effect is negligibly small when the scaling dimension d of unparticle lies in the region 1.1-1.9. But it can be relatively large when d approaches to 1 or 2. We also study the neutrinoless double beta decay with a real unparticle in the final state which cannot be distinguished from 2ν2β decay in experiment. We consider the unparticle effect on extracting the neutrino mass parameter mββ from experimental data and find that it can be relatively large when d takes certain values.
9148058
Challenge to the a theorem in six dimensions.
The possibility of a strong a theorem in six dimensions is examined in multiflavor ϕ^{3} theory. Contrary to the case in two and four dimensions, we find that, in perturbation theory, the relevant quantity a[over ˜] increases monotonically along flows away from the trivial fixed point. a[over ˜] is a natural extension of the coefficient a of the Euler term in the trace anomaly, and it arises in any even spacetime dimension from an analysis based on Weyl consistency conditions. We also obtain the anomalous dimensions and beta functions of multiflavor ϕ^{3} theory to two loops. Our results suggest that some new intuition about the a theorem is in order.
118041104
Correction to the Running of Gauge Couplings out of Extra Dimensional Gravity
We study the gravitational correction to the gauge couplings in the extra dimension model where the gravity propagates in the (4+n)-dimensional bulk. We show by explicit calculation in the background field method that the one-loop correction coming from graviton Kaluza-Klein states is nontrivial and tends to make the theory anti-asymptotically free. For the theory characterized with asymptotic freedom, the correction will induce a nontrivial UV-stable fixed point. This may lead to an interesting possibility that the non-abelian gauge coupling constants $g_2$ and $g_3$ in the SM and that of the gravity $g_{\kappa}$ unify at the fixed point in the limit of high energy scale.
117276480
The Sun's Origin, Composition and Source of Energy
Composition: Diffusion enriches lighter elements and the lighter isotopes of each element at the solar surface. When corrected for mass fractionation, the most abundant nuclide that accreted on the Sun was shown to be Fe, the decay product of doubly-magic Ni; the next most abundant nuclide is the doublymagic O. These nuclides were recently observed in the ash of SN 1987A. The most abundant elements Fe, Ni, O, Si, S, Mg, and Ca are the seven, even-Z elements that Harkins found to comprise 99% of ordinary meteorites. The least abundant elements Li, Be and B have loosely bound nucleons, confirming a link between abundance and nuclear structure hidden beneath the Sun’s H-rich surface, with one conspicuous and important exception an excess of protons.