44994799
Internal shock model for the X-ray flares of Swift J1644+57
Swift J1644+57 is an unusual transient event, likely powered by the tidal disruption of a star by a massive black hole. Multiple short time-scale X-ray flares were seen over a span of several days. We propose that these flares could be produced by internal shocks. In the internal shock model, the forward and reverse shocks are produced by collisions between relativistic shells ejected from a central engine. The synchrotron emission from the forward and reverse shocks could dominate in two quite different energy bands; under some conditions, the relativistic reverse shock dominates the X-ray emission and the Newtonian forward shock dominates the infrared and optical emission. We show that the spectral energy distribution of Swift J1644+57 could be explained by an internal shock model.
119245048
An Evolving Stellar Initial Mass Function and the Gamma-ray Burst Redshift Distribution
Recent studies suggest that Swift gamma-ray bursts (GRBs) may not trace an ordinary star formation history (SFH). Here, we show that the GRB rate turns out to be consistent with the SFH with an evolving stellar initial mass function (IMF). We first show that the latest Swift sample of GRBs reveals an increasing evolution in the GRB rate relative to the ordinary star formation rate at high redshifts. We then assume only massive stars with masses greater than the critical value to produce GRBs and use an evolving stellar IMF suggested by Dave to fit the latest GRB redshift distribution. This evolving IMF would increase the relative number of massive stars, which could lead to more GRB explosions at high redshifts. We find that the evolving IMF can well reproduce the observed redshift distribution of Swift GRBs.
122814293
Constraints on generalized Chaplygin gas model including gamma-ray bursts
Generalized Chaplygin gas (whose equation of state is p GCG = − A/ρ α G CG ) was proposed as a candidate for uni fi cation of dark energy and dark matter. We investigate constraints on this model with the latest observed data. We test the model with type-Ia supernovae (SNe Ia), cosmic microwave background (CMB) anisotropy, X-ray gas mass fractions in clusters, and gamma-ray bursts (GRBs). We calibrate the GRB luminosity relations without assuming any cosmological models using SNe Ia. We show that GRBs can extend the Hubble diagram to higher redshifts ( z > 6 ). The GRB Hubble diagram is well behaved and delineates the shape of the Hubble diagram well. We measure A s ≡ A /ρ α +1 GCG , 0 = 0 . 68 +0 . 04 − 0 . 08 (where ρ GCG , 0 is the energy density today) and α = − 0 . 22 +0 . 15 − 0 . 13 at the 1 σ con fi dence level using all the datasets. Our results rule out the standard Chaplygin gas model ( α = 1 ) at the 3 σ con fi dence level. The Λ CDM is allowed at the 2 σ con fi dence level. We fi nd that acceleration could have started at a redshift of z ∼ 0 . 70 . The concordance of the generalized Chaplygin gas model with the age estimate of an old high redshift quasar is found. In addition, we show that GRBs can break the degeneracy between the generalized Chaplygin gas model and the XCDM model.
185692670
Study on the High Order Modes of the 3.5cell Cavity at Peking University
As part of the updated DC-SC injector, a 3.5cell cavity has been fabricated at Peking University, which includes two Coaxial High Order Mode (HOM) couplers. The effect of the HOM couplers has been studied by numerical simulation and measurement. The results are highly uniform and show that the two couplers do effectively damp the HOMs. INTRODUCTION The 3.5cell cavity is part of the DC-SC injector, the feasibility of which is proved in 2004 [1] with beam done test. The updated 3.5cell cavity was designed and fabricated from large grain niobium, and the vertical test result (Eacc=23.4MV/m@Q0>10) at JLAB is exciting. The main RF parameters are listed in Table1. Table 1: RF Parameters of 3.5 Cell Cavity Type of accelerating structure Standing wave Accelerating mode TM010, pi mode fundamental frequency 1300MHz Quality factor ≥5x10 Active length 41.7cm Cell-to-cell coupling 1.8% Effective shunt impedance 418Ω Transit-time factor 0.74 Total longitudinal loss factor* 3.95(V/pC) transverse loss factor* 2.15(V/pC/cm) *the standard deviation length of the bunch is 2mm Among those RF parameters, the total longitudinal loss factor and transverse loss factor are from ABCI simulation; all the others are from Superfish. NUMERICAL CALCULATIONS OF HOMS The electron bunches passing through the cavity excite the eigenmodes of higher frequency which are HOMs. Here Microwave Studio is used to simulation the HOMs. To check whether there are trapped modes, two different boundary conditions at the end of beam pipes are used in two separate runs [2]. Large frequency shift of one mode indicates the mode depends on the boundary condition sensitivity; otherwise the mode could be trapped. The simulation shows TM110(2) and TM020(1)could be trapped. But they will not be excited by the beam as the beam bunch repetition frequency is 81.25MHz. All of the interested modes with ideal electric boundary condition below 3GHz are listed in Table 2. Table 2: HOMs of 3.5 Cell Cavity by Simulation Mode Mode f(MHz) *R/Q Mode1 TM010(1) 1278.69 9.75 Mode2 TM010(2) 1286.80 13.09 Mode3 TM010(3) 1296.83 16.63 Mode4 TM010(4) 1301.07 417.26 Mode5 TE111(1) 1634.35 0.07 Mode6 TE111(2) 1681.53 2.49 Mode7 TE111(3) 1747.03 10.07 Mode8 TM110(1) 1798.05 3.43 Mode9 TM110(2) 1849.61 3.47 Mode10 TM110(3) 1872.20 1.42 Mode11 TM110(4) 1883.84 0.77 Mode12 TM011(1) 2391.49 0.62 Mode13 TE111 (4) 2391.79 0.18 Mode14 TM011(2) 2421.67 12.65 Mode15 TM011(3) 2449.83 84.33 Mode16 TM020(1) 2658.30 23.97 Mode17 TM020(2) 2690.74 8.93 Mode18 TM020(3) 2728.82 4.26 Mode19 TM020(4) 2762.00 2.08 Mode20 TM111(1) 2819.80 0.71 Mode21 TM111(2) 2880.96 0.85 *unit of R/Q isΩ/cm; for monopole mode, n=0; dipole mode,n=2. The number next to the mode indicates the passband of the mode. For some modes, the electric field along the beam centre axis is almost zero, so the voltage is zero. Here we integrate the electric field along the axis 10 mm away from the beam centre and get the accelerating THPD009 Proceedings of IPAC’10, Kyoto, Japan 4296 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A08 Linear Accelerators voltage Vc , and the effective shunt impedance is ( ) ( ) U V Q R C ω / * / 2 * 0 = , where ω is the angular frequency of the mode, U is the stored energy. The effects of HOM couplers are simulated on a 2cell cavity also by CST Microwave Studio, which deduces the simulation time obviously [3]. The simulation shows Qe of HOM coupler on 2-cell TESLA cavity is 10 for fundamental mode, and 10~10 for HOMs. MEASUREMENTS OF THE RESONATE MODES In order to confirm the type of the modes, the electric field profile of the modes is measured by bead pulling. Two beads are used, one is a teflon cylinder with diameter of 4 mm and length of 4mm, the other is a copper cylinder with diameter of 5mm and length of 4mm. By comparing the measured electric fields with the simulated ones, 16 of all the 21 interested modes are obtained. The block diagram of the test bench is shown is Figure 1. Figure 1: Block diagram of the test bench. RF MEASUREMENTS OF THE HOM DAMPING ON THE CAVITY Before the HOM couplers are welded to the cavity, they are tested on a self-designed coaxial transmission line [4], to ensure the tuning range of the resonate frequency. The antenna of the HOM coupler is match to the load and with a round tip to increase the coupling as shown in Figure 2. Figure 2: The shape of the antenna. The antenna is made of copper in a series of length. With these antennas amounted, the gap from the antenna to the HOM coupler is 0.5mm, 1mm, 1.5mm, 2mm, 2.3mm. During this change, the Qext does not change much. Finally 1mm is chosen and the result is shown in Table 3. Table 3: Qe of the HOMs by Measurements mode f (MHz) Qe TE111(1) 1652.406 9.55x10 TE111(2) 1693.484 3.82x10 TE111(3) 1757.300 peak overlaps TM110(1) 1797.265 1.66x10 6 TM110(2) 1864.493 1.89x10 7 TM110(3) 1876.990 1.06x10 7 TM110(4) 1882.496 3.45x10 6 TE111(4) 2374.582 4.69x10 6 TM011(1) 2412.774 7.46x10 5 TM011(2) 2429.367 2.27x10 5 TM011(3) 2462.537 7.26x10 5 TM020(1) 2678.580 1.57x10 5 TM020(2) 2703.662 1.60x10 4 TM020(3) 2732.066 1.50x10 4 TM020(4) 2769.828 8.93x10 4 During the measurements, one HOM coupler is used because the available vector network analyzer has only two ports. For TE111(3) pass band, the two polarization modes are too close to measure directly by the vector network analyzer. Table 3 only lays out one polarization mode of each passband, the Qe of the other one is much bigger. Obviously one coupler is not enough for all the modes. CONCLUSION The test results of the coaxial HOM coupler on the 3.5cell large grain niobium cavity at room temperature indicate that the coupler works efficiently after a careful adjustment. The effects at 2K with beam loading are looking forward. REFERENCES [1] J. Hao et al., NIM A 557 (2006) 138-141. [2] F. Marhauser, P. Huelsmann, H. Klein, Proceedings of the 1999 Particle Accelerator Conference, New York, 1999. 3405-3407. [3] X. Luo, CPC (HEP & NP), to be published. [4] E. Wang, CPC (HEP & NP), 2009, 33(5): 401-405 Proceedings of IPAC’10, Kyoto, Japan THPD009 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A08 Linear Accelerators 4297
124213992
High performance computation on beam dynamics problems in high intensity compact cyclotrons
This paper presents the research progress in the beam dynamics problems for future high intensity compact cyclotrons by utilizing the state-of-the-art high performance computation technology. A “Start-to-Stop” model, which includes both the interaction of the internal particles of a single bunch and the mutual interaction of neighboring multiple bunches in the radial direction, is established for compact cyclotrons with multi-turn extraction. This model is then implemented in OPAL-CYCL, which is a 3D object-oriented parallel code for large scale particle simulations in cyclotrons. In addition, to meet the running requirement of parallel computation, we have constructed a small scale HPC cluster system and tested its performance. Finally, the high intensity beam dynamics problems in the 100 MeV compact cyclotron, which is being constructed at CIAE, are studied using this code and some conclusions are drawn.
14838430
Progress in L-Band Power Distribution System R&D at SLAC
We report on the L-band RF power distribution system (PDS) developed at SLAC for Fermilab's NML superconducting test accelerator facility. The makeup of the system, which allows tailoring of the power distribution to cavities by pairs, is briefly described. Cold test measurements of the system and the results of high power processing are presented. We also investigate the feasibility of eliminating the expensive, lossy circulators from the PDS by pair-feeding cavities through custom 3-dB hybrids. A computational model is used to simulate the impact on cavity field stability due to the reduced cavity-to-cavity isolation.
13499411
Observation of cocurrent toroidal rotation in the EAST tokamak with lower-hybrid current drive.
Lower-hybrid waves have been shown to induce a cocurrent change in toroidal rotation of up to 40  km/s in the L-mode plasma core region and 20  km/s in the edge of the EAST tokamak. This modification of toroidal rotation develops on different time scales. For the edge, the time scale is no more than 100 ms, but for the core the time scale is around 1 s. A simple model based on turbulent equipartition and thermoelectric pinch predicts the experimental results.