2137940
IMPEDANCE MEASUREMENT SETUP FOR HIGHER-ORDER MODE STUDIES IN NLC ACCELERATING STRUCTURES WITH THE WIRE METHOD *
Dipole modes are the main cause of transverse emittance dilution in the Japanese Linear Collider / Next Linear Collider (JLC/NLC). A diagnostic setup has been built in order to investigate them. The method is based on using a coaxial wire to excite and measure electromagnetic modes of accelerating structures. This method can offer a more efficient and less expensive procedure than the ASSET facility. Initial measurements have been made and are presented in this paper.
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.
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.
110758806
FIELD EMISSION AND RF BREAKDOWN IN COPPER LINAC STRUCTURES
This paper presents a status report on rf field emission and high-gradient breakdown studies in linac structures at SLAC. The motivation behind these studies, begun in 1984, is to determine the maximum accelerating field gradients that could be used safely in future e/sup /plus minus// colliders, to contribute to the basic understanding of the rf breakdown mechanism, and to discover if special surface treatments might make it possible to shorten the time needed for rf processing and to supersede the field limits presently reachable in room temperature copper structures. Both theoretical ideas and experimental results are discussed. 8 refs., 3 figs., 1 tab.
118998253
Search for rare $B^0_{(s)}\rightarrow \mu^+ \mu^- \mu^+ \mu^-$ decays
A search for the decays $B^0_{s}\rightarrow \mu^+ \mu^- \mu^+ \mu^-$ and $B^0 \rightarrow \mu^+ \mu^- \mu^+ \mu^-$ is performed using data, corresponding to an integrated luminosity of 1.0\ensuremath{\{\,fb}^{-1}}\xspace, collected with the LHCb detector in 2011. The number of candidates observed is consistent with the expected background and, assuming phase-space models of the decays, limits on the branching fractions are set: \{${\ensuremath{\cal B}\xspace}(B^0_{s}\rightarrow \mu^+ \mu^- \mu^+ \mu^-) < 1.6 \ (1.2) \times 10^{-8}$} and \{${\ensuremath{\cal B}\xspace}(B^0 \rightarrow \mu^+ \mu^- \mu^+ \mu^-)< 6.6 \ (5.3) \times 10^{-9}$} at 95\,% (90\,%) confidence level. In addition, limits are set in the context of a supersymmetric model which allows for the $B^0_{(s)}$ meson to decay into a scalar ($S$) and pseudoscalar particle ($P$), where $S$ and $P$ have masses of 2.5 GeV and 214.3 MeV, respectively, both resonances decay into $\mu^+\mu^-$. The branching fraction limits for these decays are \{${\ensuremath{\cal B}\xspace}(\ensuremath{B^0_{s}\rightarrow SP}\xspace) < 1.6 \ (1.2) \times 10^{-8}$} and \{${\ensuremath{\cal B}\xspace}(\ensuremath{B^0\rightarrow SP}\xspace)< 6.3 \ (5.1) \times 10^{-9}$} at 95% (90%) confidence level.
964657
Beam loading compensation in the NLCTA
In the design of the Next Linear Collider (NLC), multibunch operation is employed to improve efficiency at the cost of substantial beam loading. The RF pulse that powers the accelerator structures will be shaped to compensate for the effect of the transient loading along the bunch train. This scheme has been implemented in the Next Linear Collider Test Accelerator (NLCTA), a facility built to test the key accelerator technology of the NLC. In this paper we describe the compensation method, the techniques used to measure the energy variation along the bunch train, and results from tests with NLC-like beam currents.
123461752
Neutron-deuteron differential cross sections at 35.0 and 46.3 MeV
Differential cross sections have been measured for $\mathrm{nd}$ and $\mathrm{pd}$ scattering at 35.0 and 46.3 MeV. Only cross sections for c.m. angles \ensuremath{\gtrsim} 90\ifmmode^\circ\else\textdegree\fi{} have been measured in this work, which is in part a higher-precision remeasurement of earlier $\mathrm{nd}$ cross sections and whose main purpose has been to check possible $\mathrm{nd}\ensuremath{-}\mathrm{pd}$ differences which appeared in the earlier measurements at 36.0 and 46.3 MeV. Absolute $\mathrm{nd}$ cross sections of $\ensuremath{\simeq}5%$ precision, obtained by normalization to precision $\mathrm{np}$ cross sections, are compared with absolute $\mathrm{pd}$ cross sections measured at the University of California at Los Angeles. The differences in this comparison and in the $\mathrm{nd}\ensuremath{-}\mathrm{pd}$ relative cross sections measured here are smaller than indicated by the previous measurements. However, some systematic differences do appear to exist at angles near the cross-section minima and beyond.
117803245
Unconventional Fermi surface in two-dimensional systems of Dirac fermions
At the low energy regime, the decay rate of two-dimensional massless Dirac fermions due to interactions can be written as $\mathrm{Im}\Sigma(\omega) \propto |\omega|^{x}$ at zero temperature. We find that the fermion system has: I) no sharp Fermi surface and no well-defined quasiparticle peak for $0 1$. In the presence of long-range gauge/Coulomb interaction or certain massless boson mode, the system exhibits unusual behavior belonging to class II.
110785158
Measurement of wakefields generated in accelerator test structures using the SLC
Research is underway at SLAC to develop accelerator structures for the next generation linear collider. An important feature of the design is a detuning of the dipole moddes -of the cells to suppress the long-range transverse wakefield bptwo orders of magnitude. This paper describes a facility, called ASSET, that will be incorporated into the SLAC Linear Collider (SLC) to test the long-range wakefield suppression and also to measure the other components of the wakefields generated in accelerator test structures. _- 1 INTRGDUCTION The-designs being considered at SLAC for the Next Linear Collider (NLC) employ multi-bunch operation in a high acceleration -gradient X-band linac. The limitations in this _ __ approach come in part from the transverse and longitudinal wakefields generated in the accelerator structures of the linac. A particularly troublesome problem is the long-range transverse wakefield, -which if not controlled, will produce a large growth in the transverse motion of the bunches. The .currently favored cure for this problem is to detune the dipole mode frequencies of the cells of the structures so the sum of the wakefields generated in each structure decohere (l). Since a two orders of magnitude reduction is required, and the theoretical calculations of the suppression (2) have some uncertainty, it is important that the performance of the structures be verified before they are produced for use in an accelerator. Thus far, measurements of the wakefield suppression have been made on short-length detuned structures at Argonne's Advanced Accelerator Test Facility (3). Although the results look promising, this facility currently does not have the measurement precision required for the NLC structures being developed, nor the capability of measuring the wakefields beyond a few ns compared to the few hundred ns needed. In the future, the plan is to do the measurements at SLAC with a dedicated facility called the Accelerator Structure SETup or ASSET (4). It exploits the "asset" of the-SLC in its capability to independently inject two low emittance bunches (electrons and positrons) into the linac with individual control of the bunch timing and intensity. The layout for. ASSET. is shown in figure 1. It is divided into two sect*3 that.will take the place of the first two 3