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
125267189
Helium production from Kr-84- and Au-197-emulsion interactions at high energies
The properties of the relativistic helium fragments produced in interactions of Kr-84 at 1.8 A GeV and Au-197 at 1.7 A GeV in emulsion are investigated. The experimental results are compared with those obtained from various projectiles with emulsion collisions at different energies. It is found that the multiplicity distribution of helium projectile fragments (HPFs) is well described by the Koba-Nielsen-Olesen (KNO) scaling presentation. The second Mueller moment f(2) of the HPF multiplicity distribution is independent of the projectile energy for the same projectile, but it is dependent on the projectile mass number. The value of f(2) increases with the increase of projectile mass number A(p). The negative value of f(2), when A(p) 69, means that the emission of HPFs is anticorrelated, but positive value of f(2), when A(p) > 69, referes to that the emission of HPFs is corrleated. The non-zero f(2) moment in this experiment implies the strong correlation existing between the HPFs.
55717747
The drive laser system for dc-sc injector
PKU-SCAF has developed a photoinjector which adopt a 1+1/2 cell super conducting cavity and DC electron gun. We also developed a low cost drive laser system for the photocathode DC gun to provide high average beam current. This laser system include a commercial high repetition rate, ps, all solid-state laser, the home made SHG and FHG, Fourier relay optics and the uniform illumination optics. The test results shows the output power at 266 nm of the laser system is more than 1.2W and got more than 500 µA beam current from CsTe cathode from the DC gun.
127622395
Ward identity of two-loop diagrams in SUSY
122621857
Cold RF test and associated mechanical features correlation of a TESLA-style 9-cell superconducting niobium cavity built in China
The RF performance of a 1.3 GHz 9-cell superconducting niobium cavity was evaluated at cryogenic temperatures following surface processing by using the standard ILC-style recipe. The cavity is a TESLA-style 9-cell superconducting niobium cavity, with complete end group components including a higher order mode coupler, built in China for practical applications. An accelerating gradient of 28.6 MV/m was achieved at an unloaded quality factor of 4 × 109. The morphological property of mechanical features on the RF surface of this cavity was characterized through optical inspection. Correlation between the observed mechanical features and the RF performance of the cavity is attempted.
89654095
The First Nine-Cell TESLA Cavity Made in China
A totally home-made 9-cell TESLA type superconducting cavity is made at Peking University. The cavity fabrication is according to DESY specification. The cavity is made of high purity niobium from OTIC, Ningxia. The electron beam welding is carried out at Harbin Institute of Technology, Harbin. By the cooperation, the cavity is tested at Thomas Jefferson National Accelerator Facility, USA. The preliminary result shows the acceleration gradient Eacc is 23MV/m without quench and has potential for improvement.
18621135
Localization of RF Breakdowns in a Standing Wave Cavity
At SLAC, a 5-cell, normal-conducting, L-band (1.3 GHz), standing-wave (SW) cavity was built as a prototype positron capture accelerator for the ILC. The structure met the ILC gradient goal but required extensive rf processing. When rf breakdowns occurred, a large variation was observed in the decay rate of the stored energy in the cavity after the input power was shut off. It appeared that the breakdowns were isolating sections of the cavity, and that the trapped energy in those sections was then partitioned among its natural modes, producing a distinct beating pattern during the decay. To explore this phenomenon further, an equivalent circuit model of cavity was created that reproduces well its normal operating characteristics. The model was then used to compute the spectra of trapped energy for different numbers of isolated cells. The resulting modal patterns agree well with those of the breakdown data, and thus such a comparison appears to provide a means of identifying the irises on which the breakdowns occurred.
124365356
Current constraints on early dark energy and growth index using latest observations
Aims. In this paper we study observational constraints on early dark energy model proposed by Doran & Robbers and growth index using the latest Union2 type Ia supernovae (SNe Ia), the baryon acoustic oscillations (BAO) in the large-scale correlation function of the Sloan Digital Sky Survey and Two-degree Field Galaxy Redshift Survey, cosmic microwave background (CMB) from the Wilkinson Microwave Anisotropy Probe seven-year data, the linear growth factors data and gamma-ray bursts (GRBs). Methods. By using the χ 2 statics method, we constrain the early dark energy model and growth factor from the above datasets. When including the GRB data, we use the cosmographic parameters to calibrate the GRB luminosity relations using SNe Ia. Results. At 2σ confidence level, we find that the fractional dark energy density at early times is Ω e < 0.05 using SNe Ia, CMB and BAO. When we include high-redshift probes, such as measurements of the linear growth factors and GRBs, the constraint is tightened considerably and becomes Ω e < 0.03 at 2σ confidence level. We also discuss the growth rate index γ .W e f indγ = 0.661 +0.302 −0.203 (1σ) using the SNe Ia, CMB, BAO and linear growth factor data. After including high-redshift GRB data, the growth index is γ = 0.653 +0.372 −0.363 .
110770779
Planned waveguide electric field breakdown studies
This paper presents an experimental setup for X-band rf breakdown studies. The setup is composed of a section of WR90 waveguide with a tapered pin located at the middle of the waveguide E-plane. Another pin is used to rf match the waveguide so it operates in a travelling wave mode. By adjusting the penetration depth of the tapered pin, different surface electric field enhancements can be obtained. The setup will be used to study the rf breakdown rate dependence on power flow in the waveguide for a constant maximum surface electric field on the pin. Two groups of pins have been designed. The Q of one group is different and very low. The other has a similar Q. With the test of the two groups of pins, we should be able to discern how the net power flow and Q affect the breakdown. Furthermore, we will apply an electron beam treatment to the pins to study its effect on breakdown. Overall, these experiments should be very helpful in understanding rf breakdown phenomena and could significantly benefit the design of high gradient accelerator structures.
118854953
High-redshift star formation rate up to z~8.3 derived from gamma-ray bursts and influence of background cosmology
The high-redshift star formation rate (SFR) is difficult to measure directly even by modern approaches. Long-duration gamma-ray bursts (GRBs) can be detected to the edge of the visible universe because of their high luminosities. The collapsar model of long GRBs indicates that they may trace the star formation history. So, long GRBs may be a useful tool of measuring the high-redshift SFR. Observations show that long GRBs prefer to form in a low-metallicity environment. We study the high-redshift SFR up to z∼ 8.3 considering the Swift GRBs tracing the star formation history and the cosmic metallicity evolution in different background cosmological models including Λ cold dark matter (ΛCDM), quintessence, quintessence with a time-varying equation of state and brane-world model. We use latest Swift GRBs including two highest-z GRBs, GRB 080913 at z= 6.7 and GRB 090423 at z= 8.3. We find that the SFR at z > 4 shows a steep decay with a slope of ∼−5.0 in ΛCDM. In the other three models, the high-redshift SFR is slightly different from ΛCDM model and also shows a steep decay.
10678206
Two-Proton Correlations from Pb+Pb Central Collisions
The two-proton correlation function at midrapidity from Pb+Pb central collisions at 158 AGeV has been measured by the NA49 experiment. The preliminary results are compared to model predictions from proton source distributions of static thermal Gaussian sources and the transport models of rqmd and venus. We obtain an effective proton source size \( {\sigma _{{\mathbf{eff}}}} = 4.0 \pm 0.15({\mathbf{stat}}.)_{ - 0.18}^{ + 0.06}({\mathbf{syst}}.) \) fm. The rqmd model underpredicts the correlation function ( \( {\sigma _{{\mathbf{eff}}}} = 4.41 \) fm), while the venus model overpredicts the correlation function ( \( {\sigma _{{\mathbf{eff}}}} = 3.55 \) fm).