15602690
Simulation of accelerating structures with large staggered tuning
One idea for controlling multibunch beam breakup in future linear colliders is to use accelerating structures that have a large ( approximately 10%) staggered tuning, that is, a large cell-to-cell spread in frequencies of the transverse dipole modes. Simulations of such structures using the programs TBCI and LINACBBU are compared. Issues relevant to the design and optimization of such structures, such as choice of cell frequency distribution and role of cell-to-cell coupling are discussed.<<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.
16400877
VECTOR MAGNETIC FIELDS OF SOLAR GRANULATION
Observations of the quiet Sun from the Solar Optical Telescope/Spectro-Polarimeter aboard the Hinode spacecraft reveal the magnetic characteristics of the solar photosphere. By making use of the deep-mode observations of three quiet regions, we have statistically studied the vector magnetic fields of solar granulation. More than 2000 normal granules are manually selected to form a sample. It is recognized that some granules are even darker than the mean photosphere in intensity, and there is a linear correlation between intensity and Doppler velocity in the granules. The distributions of longitudinal and transverse apparent magnetic flux densities, Doppler velocity, and continuum intensity of granules are obtained, and their unsigned magnetic flux measured. Two approaches are used in this study. First, we obtained the magnetic properties of granulation by averaging the measurements for all the sampling granules. Second, we reconstructed an average granular cell based on a subsample, and obtained the detailed distribution of apparent magnetic flux density within the model granular cell. All the results have been compared with those for intergranular lanes and a few typical abnormal granules. Our statistical analysis reveals the following results. (1) The unsigned magnetic flux of individual granules spans the range from 1.1 × 1015 Mx to 3.3 × 1018 Mx with a peak distribution at 1.6 × 1016 Mx. (2) The unsigned longitudinal apparent flux density of granules ranges from almost 0 to 212 Mx cm–2 with a mean longitudinal apparent flux density of 12 Mx cm–2, while the transverse apparent flux density of granules ranges from 4 to 218 Mx cm–2 with a mean transverse apparent flux density of 79 Mx cm–2. The longitudinal and transverse apparent magnetic flux densities of granules are positively correlated, and the longitudinal apparent flux density of granules is weaker than the corresponding transverse apparent flux density. (3) The magnetic inclination of granules with respect to the surface that is perpendicular to the line of sight falls in the range of 4.8-76.7 degrees with a peak distribution at 25 degrees. On average, the magnetic vectors in granules are more vertical than those in the intergranular lanes. (4) There is a strong preference that both the vertical and horizontal fields on the quiet Sun reside in the intergranular lanes. (5) The detailed distributions of apparent flux density, Doppler velocity, and continuum intensity within an average granular cell are presented. These distributions can be empirically formulated well.
55989671
Traveling wave structure optimization for the NLC
The JLC/NLC linac must accelerate multi-bunch beams in order to obtain a luminosity >3/spl times/10/sup 34/ cm/sup -2/ sec/sup -1/ at a center of mass energy of 1 TeV. It is essential for the structure design to minimize the long and short-range dipole wakefields to prevent emittance degradation and the beam breakup instability (BBU). In addition, the structures must operate at a high RF gradient to minimize the linac cost. High-power testing of prototype structures at SLAC has shown noticeable damage. The damage is largest in the front of the structure, where the group velocity is high, and there is minimal or no damage at the back end, where the group velocity is low. Theoretical analysis using a simple circuit model suggests using structures with a lower group velocity, on the order of a few percent, would be a way of avoiding damage. For the standard 2/spl pi//3 accelerating mode, it is difficult to lower the group velocity without losing efficiency or increasing the wakefields. With this in mind, we have taken the phase advance as an additional parameter in structure optimization. We found that a low group velocity structure at higher phase advance can maintain high RF efficiency and low wakefields. In this paper, we study the impact of phase advance on structure performance. We then optimize the NLC S-band and X-band structures to meet design requirements.
119176140
A general relativistic external Compton-scattering model for TeV emission from M87
M87 is the first detected non-blazar extragalactic Tera-Ele ctron-Volt (TeV) source with rapid variation and very flat spectrum in the TeV band. To explain the two-peaks in the spectral energy distribution (SED) of the nucleus of M87 which is similar to those of blazars, the most commonly adopted models are the synchrotron self-Compton scattering (SSC) models and the external inverse Compton scattering (EIC) models. Considering that there is no correlated variation in the soft band (from r adio to X-ray) matching the TeV variation, and the TeV sources should not suffer from the γγ absorption due to the flat TeV spectrum, the EIC models are advantageous in modeling the TeV emission from M87. In this paper, we propose a self-consistent EIC model to explain the flat TeV spectrum of M87 within the framework of fully general relativity, where the background soft photons are from the advection-dominated accretion flo w (ADAF) around the central black hole, and the high energy electrons are from the mini-jets which are powered by the magnetic reconnection in the main jet (Giannios et al. 2010). In our model, both the TeV flares obser ved in the years of 2005 and 2008 could be well explained: the γγ absorption for TeV photons is very low, even inside the region very close to the black hole 20Rg ∼ 50Rg; at the same region, the average EIC cooling time (∼ 10 2 ∼ 10 3 s) is short, which is consistent with the observed time scale of TeV variation. Furthermore, we also discuss the possibility that the accompanying X-ray flare in 2008 is due to the direct synchrotron radiation of the mini-jets. Subject headings:black hole physics — galaxies: individual (M87)
117434963
Dark Current Simulation for Linear Collider Accelerator Structures
The dynamics of field-emitted electrons in the traveling wave fields of a constant gradient (tapered) disk-loaded waveguide is followed numerically. Previous simulations have been limited to constant impedance (uniform) structures for sake of simplicity since only the fields in a unit cell is needed. Using a finite element field solver on a parallel computer, the fields in the tapered structure can now be readily generated. We will obtain the characteristics of the dark current emitted from both structure types and compare the two results with and without the effect of secondary electrons. The NLC and JLC detuned structures are considered to study if dark current may pose a problem for high gradient acceleration in the next generation of Linear Colliders.
119294737
The new model of fitting the spectral energy distributions of Mrk 421 and Mrk 501
The spectral energy distribution (SED) of TeV blazars has a double-humped shape that is usually interpreted as a Synchrotron Self Compton (SSC) model. The one zone SSC model is used broadly but cannot fit the high energy tail of SED very well. It needs a bulk Lorentz factor which is in conflict with the observation. Furthermore, a one zone SSC model cannot explain the entire spectrum. We propose a new model where the high energy emission is produced by the accelerated protons in the blob with a small size and high magnetic field, and the low energy radiation comes from the electrons in the expanded blob. Because the high and low energy photons are not produced at the same time, the requirement of a large Doppler factor from pair production is relaxed. We present the fitting results of the SEDs for Mrk 501 during April 1997 and Mrk 421 during March 2001.