5947961
Meeting Tight Frequency Requirement of Rounded Damped Detuned Structure
Following successful design and fabrication of damped detuned structures, the R and 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.
36955336
Measurements of Ultimate Accelerating Gradients in the SLAC Disk-Loaded Structure
The work reported here describes measurements made to study the maximum attainable accelerating gradients in a conventional SLAC disk-loaded accelerator section of the constant-gradient type running at 2856 MHz. The objective was to reach an accelerating gradient of at least 100 MV/m. The accelerating gradient at which the SLAC disk-loaded waveguide runs routinely is approx. 9 MV/m (36 MW tubes without SLED) and approx. 12 MV/m with SLED I (2.5 ..mu..sec pulse). To reach 100 MV/m in a conventional 3 m constant-gradient section, one would need a klystron with a peak power output of 900 MW. since such a tube is not available, we decided to use a short standing-wave section in which the resonant fields would be allowed to build up. The design criteria for this section, the fabrication, matching and tuning, the experimental set-up and the results are described below.
55601369
50 MW X-BAND RF SYSTEM FOR A PHOTOINJECTOR TEST STATION AT LLNL
In support of X-band photoinjector development efforts at LLNL, a 50 MW test station is being constructed to investigate structure and photocathode optimization for future upgrades. A SLAC XL-4 klystron capable of generating 50 MW, 1.5 microsecond pulses will be the high power RF source for the system. Timing of the laser pulse on the photocathode with the applied RF field places very stringent requirements on phase jitter and drift. To achieve these requirements, the klystron will be powered by a state of the art, solid-state, high voltage modulator. The 50 MW will be divided between the photoinjector and a traveling wave accelerator section. A high power phase shifter is located between the photoinjector and accelerator section to adjust the phasing of the electron bunches with respect to the accelerating field. A variable attenuator is included on the input of the photoinjector. The distribution system including the various x-band components is being designed and constructed. In this paper, we will present the design, layout, and status of the RF system.
17263744
RF POWER SOURCES FOR LINEAR COLLIDERS
The next generation of linear colliders requires peak power sources of over 200 MW per meter at frequencies above 10 GHz at pulse widths of less than 100 nsec. Several power sources are under active development, including a conventional klystron with rf pulse compression, a relativistic klystron (RK) and a crossed-field amplifier. Power from one of these has energized a 0.5 meter two- section High Gradient Accelerator (HGA) and accelerated a beam at over 80 MeV meter. Results of tests with these experimental devices are presented here.
123848492
Modeling and design of an X-band rf photoinjector
A design for an X-band rf photoinjector that was developed jointly by SLAC National Accelerator Laboratory (SLAC) and Lawrence Livermore National Laboratory (LLNL) is presented. The photoinjector is based around a 5.59 cell rf gun that has state-of-the-art features including: elliptical contoured irises; improved mode separation; an optimized initial half cell length; a racetrack input coupler; and coupling that balances pulsed heating with cavity fill time. Radio-frequency and beam dynamics modeling have been done using a combination of codes including PARMELA, HFSS, IMPACT-T, ASTRA, and the ACE3P suite of codes developed at SLAC. The impact of lower gradient operation, magnet misalignment, solenoid multipole errors, beam offset, mode beating, wakefields, and beam line symmetry have been analyzed and are described. Fabrication and testing plans at both LLNL and SLAC are discussed.
45575016
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.
118535889
Quantifying the role of bars in the build-up of central mass concentrations in disc galaxies
We analyze the role of bars in the build-up of central mass concentrations in massive, disk galaxies. Our parent sample consists of 3757 face-on disk galaxies with redshifts between 0.01 and 0.05, selected from the seventh Data Release of the Sloan Digital Sky Survey. 1555 galaxies with bars are identified using position angle and el lipticity profiles of the i-band light. We compare the ratio of the specific star formation rate measu red in the 1-3 kpc central region of the galaxy to that measured for the whole galaxy. Galaxies with strong bars have centrally enhanced star formation; the degree of enhancement depends primarily on the ellipticity of the bar, and not on the size of the bar or on the mass or structure of the host galaxy. The fraction of galaxies with strong bars is highest at stellar m asses greater than 3×10 10 M⊙, stellar surface densities less than 3× 10 8 M⊙ and concentration indices less than 2.5. In this region of parameter space, galaxies with strong bars either have enhanced central star formation rates, or star formation that is suppressed compared to the mean. This suggests that bars may play a role in the eventual quenching of star formation in galaxies. Only 50% of galaxies with strongly concentrated star formation have strong bars, indicating that other processes such as galaxy interactions also induce central star-bursts. We al so find that the ratio of the size of the bar to that of the disk depends mainly on the colour of the galaxy, suggesting that the growth and destruction of bars are regulated by gas accretion, as su ggested by simulations.
118363385
Resummation Prediction on Top Quark Transverse Momentum Distribution at Large $$p_T$$
The top quark is one of the third-generation quarks, proposed to exist in 1973 by Kobayashi and Maskawa to explain the observed CP violations in kaon decay [1]. However, it has not been directly observed until 1995 by CDF and DO collaborations at the Tevatron [2, 3]. The reason is that the top quark is the heaviest particle discovered so far, with a mass close to the electroweak symmetry breaking scale.
209842197
Precise Mapping of the Magnetic Field in the CMS Barrel Yoke using Cosmic Rays
The CMS detector is designed around a large 4 T superconducting solenoid, enclosed in a 12000-tonne steel return yoke. A detailed map of the magnetic field is required for the accurate simulation and reconstruction of physics events in the CMS detector, not only in the inner tracking region inside the solenoid but also in the large and complex structure of the steel yoke, which is instrumented with muon chambers. Using a large sample of cosmic muon events collected by CMS in 2008, the field in the steel of the barrel yoke has been determined with a precision of 3 to 8% depending on the location.
54712812
Advances in X-Band TW Accelerator Structures Operating in the 100 MV/M Regime
A CERN-SLAC-KEK collaboration on high gradient X-band accelerator structure development for CLIC has been ongoing for three years. The major outcome has been the demonstration of stable 100 MV/m gradient operation of a number of CLIC prototype structures. These structures were fabricated using the technology developed from 1994 to 2004 for the GLC/NLC linear collider initiative. One of the goals has been to refine the essential parameters and fabrication procedures needed to realize such a high gradient routinely. Another goal has been to develop structures with stronger dipole mode damping than those for GLC/NLC. The latter requires that the surface temperature rise during the pulse be higher, which may increase the breakdown rate. One structure with heavy damping has been RF processed and another is nearly finished. The breakdown rates of these structures were found to be higher by two orders of magnitude compared to those with equivalent acceleration mode parameters but without the damping features. This paper presents these results together with some of the earlier results from non-damped structures.
17545642
Results from the CLIC X-Band Structure Test Program at NLCTA *
As part of a SLAC-CERN-KEK collaboration on high gradient X-band structure research, several prototype structures for the CLIC linear collider study have been tested using two of the high power (300 MW) X-band rf stations in the NLCTA facility at SLAC. These structures differ in terms of their fabrication (brazed disks and clamped quadrants), gradient profile (amount by which the gradient increases along the structure, which optimizes efficiency and maximizes sustainable gradient) and HOM damping (use of slots or waveguides to rapidly dissipate dipole mode energy). The CLIC goal in the next few years is to demonstrate the feasibility of a CLIC-ready baseline design and to investigate alternatives that could increase efficiency. This paper summarizes the high gradient test results from NLCTA in support of this effort.