1018474
RF breakdown studies in copper electron linac structures
The authors present a summary of RF breakdown-limited electric fields observed in experimental linac structures at SLAC (Stanford Linear Accelerator Center) and a discussion of how these experiments can be interpreted against the background of existing, yet incomplete, theories. The motivation of these studies, begun in 1984, is to determine the maximum accelerating field gradients that can be used safely in future e/sup +or-/ colliders, to contribute to the basic understanding of the RF breakdown mechanism, and to discover whether a special surface treatment can make it possible to supersede the field limits presently attainable room-temperature copper structures.<<ETX>>
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.
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.
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.
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.
102331940
Assembly history and structure of galactic cold dark matter haloes
We use the Aquarius simulation series to study the imprint of assembly history on the structure of Galaxy-mass cold dark matter haloes. Our results confirm earlier work regarding the influence of mergers on the mass density profile and the inside-out growth of haloes. The inner regions that contain the visible galaxies are stable since early times and are significantly affected only by major mergers. Particles accreted diffusely or in minor mergers are found predominantly in the outskirts of haloes. Our analysis reveals trends that run counter to current perceptions of hierarchical halo assembly. For example, major mergers (i.e. those with progenitor mass ratios greater than 1:10) contribute little to the total mass growth of a halo, on average less than 20 per cent for our six Aquarius haloes. The bulk is contributed roughly equally by minor mergers and by ‘diffuse’ material which is not resolved into individual objects. This is consistent with modelling based on excursion-set theory which suggests that about half of this diffuse material should not be part of a halo of any scale. The simulations themselves suggest that a significant fraction is not truly diffuse, since it was ejected from earlier haloes by mergers prior to their joining the main system. The Aquarius simulations resolve haloes to much lower mass scales than are expected to retain gas or form stars. Thus, the fraction of diffuse dark matter accreted by haloes represents a lower limit to the fraction of diffuse baryons accreted by galaxies. Our results thus confirm that most of the baryons from which visible galaxies form are accreted diffusely, rather than through mergers, and they suggest that only relatively rare major mergers will affect galaxy structure at later times.
14138217
Polarization of Quasars: Resonant Line Scattering in the Broad Absorption Line Region
Recent works showed that the absorbing material in broad absorption line (BAL) quasars is optically thick to major resonant absorption lines. This material may contribute significantly to the polarization in the absorption lines. In this paper, we present a detailed study of the resonant line scattering process using a Monte Carlo method to constrain the optical depth, the geometry, and the kinematics of the BAL region (BALR). By comparing our results with observed polarized spectra of BAL quasars, we find the following: (1) Resonant scattering can produce polarization up to 9% at the absorption trough for doublet transitions and up to 20% for singlet transitions in radially accelerated flows. To explain the large polarization degree in the C IV and N V absorption-line troughs detected in a small fraction of BAL QSOs, a nonmonotonic velocity distribution along the line of sight or/and additional contribution from the electron-scattering region is required. (2) The rotation of the flow can lead to the rotation of the polarization position angle (PA) in the line trough. A large extending angle of BALR is required to produce the observed large PA rotation in a few BAL QSOs. (3) A large extending angle of BALR is required to explain a subtrough in the polarized flux that was observed in a number of BAL QSOs. (4) The resonant scattering can contribute a significant part of N V emission line in some QSOs, and it may give rise to anomalous strong N V lines in these quasars. (5) The polarized flux and PA rotation produced by the resonant scattering regions is uniquely asymmetric, which may be used to test the presence of BALR in non-BAL QSOs.
118979006
Collisions among Clouds inside Dusty Tori in Active Galactic Nuclei: Observational Consequences
A geometrically thick dusty torus in NGC 1068 has been unambiguously resolved by an infrared interferometry telescope. This implies that clouds composing the dusty torus are undergoing supersonic collisions with each other. We show that the collisions form strong nonrelativistic shocks, which accelerate populations of relativistic electrons. The torus reprocesses emission from the accretion disk into an infrared band. We show that the energy density of the infrared photons inside the torus is much higher than that of the magnetic field in the clouds, and the seed photons of inverse Compton scattering are mainly from the infrared. The maximum energy of the relativistic electrons can reach a Lorentz factor of 105. We calculate the spectrum of the synchrotron and inverse Compton scattering radiation from the electrons in the torus. The relativistic electrons in the torus radiate nonthermal emission from radio to γ-ray, which isotropically diffuses in the region of the torus. We find that the most prominent character is a peak at ~0.5-1 GeV. We apply this model to NGC 1068 and find that the observed radio emission from the core component S1 can be explained by the synchrotron emission from the relativistic electrons. We predict that there is γ-ray emission with a luminosity of 1040 ergs s-1 peaking at ~1 GeV from the torus, which could be detected by the Gamma-Ray Large-Array Space Telescope in the future. This will provide a new clue to understanding the physics in the torus. The nonthermal radiation from the dusty torus may explain the radio emission from Seyfert galaxies. The cosmological implications of the nonthermal emission to the γ-ray background radiation are also discussed.
55420796
FABRICATION TECHNOLOGIES OF THE HIGH GRADIENT ACCELERATOR STRUCTURES AT 100MV/m RANGE
A CERN-SLAC-KEK collaboration on high gradient X-band structure research has been established in order to demonstrate the feasibility of the CLIC baseline design for the main linac stably operating at more than 100 MV/m loaded accelerating gradient. Several prototype CLIC structures were successfully fabricated and high power tested. They operated at 105 MV/m with a breakdown rate that meets the CLIC linear collider specifications of < 5 x 10{sup -7}/pulse/m. This paper summarizes the fabrication technologies including the mechanical design, precision machining, chemical cleaning, diffusion bonding as well as vacuum baking and all related assembly technologies. Also, the tolerances control, tuning and RF characterization will be discussed.
109333657
RF Design of X-Band RF Deflector for Femtosecond Diagnostics of LCLS Electron Beam
We designed a successful constant impedance traveling wave X-band rf deflector for electron beam diagnostics at the 14 GeV SLAC Linac Coherent Light Source (LCLS). This is the first practical deflector built with a waveguide coupler. The 1-meter rf deflector produces 24 MeV peak transverse kick when powered with 20 MW of 11.424 GHz rf. The design is based on our experience with high gradient X-band accelerating structures. Several deflectors of this design have been built at SLAC and are currently in use. Here we describe the design and distinguishing features of this device.