SOURCE OF MICROBUNCHING AT BNL NSLS SOURCE DEVELOPMENT LABORATORY
We report experimental studies of the origins of electron beam microbunching instability at BNL Source Development Laboratory (SDL). We eliminated laser-induced microbunching by utilizing an ultra-short photocathode laser. The measurements of the resulting electron beam led us to conclude that, at SDL, microbunching arising from shot noise is not amplified to any significant level. Our results demonstrated that the only source of microbunching instability at SDL is the longitudinal modulation of the photocathode laser pulse. Our work shows that assuring a longitudinally smoothed photocathode laser pulse allows mitigating microbunching instability at a typical FEL injector with a moderate microbunching gain. In this paper we investigated the source of microbunching instability at the SDL. To distinguish microbunching induced by shot noise from that arising from the longitudinal modulation of the photocathode laser, we studied the beam created by a very short laser pulse, thus eliminating the possibility of laser-induced microbunching. While the measured energy spectra of compressed beam did reveal severe longitudinal fragmentation, an analysis of the beam dynamics proved this to be due to self-fields acting on a beam with an initially smooth longitudinal profile, and not due to microbunching instability. Such fragmentation only was possible with the very short bunch chosen for these studies, and is absent in routine SDL operations. Our experiment shows that in the absence of the initial laser-induced beam modulation, microbunching instability at the SDL is not observed, and must be well below the levels that would limit the FEL performance. This result agrees with assumption of previous SDL studies that (when present under different machine conditions) microbunching instability at the SDL was laser-induced. Microbunching instability gain at the SDL is moderate. This is mainly because the SDL utilizes a single stage bunch compressor as well as due to the small compression ratio. Since the design of the SDL injector is typical of the majority of FEL injectors, our experiment proves that one possible way to control microbunching instability in such machines (that by design have a moderate microbunching gain) is to maintain a sufficiently smooth longitudinal profile of the photo-cathode laser. We note that the general principles for designing a machine with a moderate microbunching instability gain are presented in . In conclusion, our experiment demonstrates that microbunching instability can be eliminated from a typical FEL injector with single stage bunch compressor (and operating without a laser heater) as long as the photocathode laser is longitudinally smooth. For machines with multi-stage bunch compressors, our results offer an important benchmark to establish a minimal laser heater power for instability-free operation.

title: SPECTRAL PHASE MODULATION AND CHIRPED PULSE AMPLIFICATION IN HIGH GAIN HARMONIC GENERATION
abstract: Amplitude and phase measurements are conducted on the 266 nm output of the DUVFEL laser at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. With the FEL operating in High Gain Harmonic Generation (HGHG) mode, we have studied narrow-bandwidth HGHG output, and observe pulses near the transform limit. We have also chirped both the electron bunch energy and the seed pulse wavelength, and successfully imposed a linear chirp on the HGHG output. These results demonstrate the potential to generate short harmonic pulses through chirped pulse amplification  or coherently shape FEL output.

title: Emittance studies of the BNL/SLAC/UCLA 1.6 cell photocathode RF gun
abstract: The symmetrized 1.6 cell S-band photocathode gun developed by the BNL/SLAC/UCLA collaboration is in operation at the Brookhaven Accelerator Test Facility (ATF). A novel emittance compensation solenoid magnet has also been designed, built and is in operation at the ATF. These two subsystems form an emittance compensated photoinjector used for beam dynamics, advanced acceleration and free electron laser experiments at the ATF. The highest acceleration field achieved on the copper cathode is 150 MV/m, and the guns normal operating field is 130 MV/m. The maximum rf pulse length is 3 /spl mu/s. The transverse emittance of the photoelectron beam were measured for various injection parameters. The 1 nC emittance results are presented along with electron bunch length measurements that indicated that at above the 400 pC, space charge bunch lengthening is occurring. The thermal emittance, /spl epsiv//sub 0/, of the copper cathode has been measured.

title: First experimental results of the BNL inverse free electron laser accelerator
abstract: A 40 MeV electron beam, using the inverse3e free-electron laser interaction, has been accelerated by {Delta}E/E = 2.5% over a distance of 0.47 m. The electrons interact with a 1--2 GW CO{sub 2} laser beam bounded by a 2.8 mm ID sapphire circular waveguide in the presence of a tapered wiggler with Bmax {approx} 1 T and a period 2.89 cm {le} {lambda}{sub w} {le} 3.14 cm. The experimental results of {Delta}E/E as a function of electron energy E, peak magnetic field Bw and laser power W{sub 1} compare well with analytical and 1-D numerical simulations and permit scaling to higher laser power and electron energy.

title: Micro-bunching diagnostics for inverse Cherenkov acceleration by coherent transition radiation
abstract: Here, we propose an effective method to detect micro-bunching effects (10 fs bunch length), produced by the inverse Cherenkov acceleration (ICA) interaction, by using the coherent transition radiation (CTR) spectrum. The pre-bunching of initially energy modulated electron beam passing through a Hydrogen gas cell is studied via a Monte Carlo simulation code (STI), as well as in a space-charge dominated region by a multi-particle time domain tracking code (PARMELA). The results show that even in a strong space-charge defocusing “washes out” the final bunching peak only by about 10% (FWHM). The longitudinal distribution of a micro-bunched beam is Fourier analyzed to find the dominant harmonics contributing to the CTR. The CTR spectrum is calculated analytically for the ICA situation. A schematic of the experimental set up is also proposed.

title: MIT microwiggler for free electron laser applications
abstract: A microwiggler-based FEL permits operation at shorter wavelengths with a reduction in the size and cost of the device. The MIT microwiggler is a pulsed ferromagnetic-core electromagnet with 70 periods of 8.8 mm each which generates an on-axis peak magnetic field of 4.2 kG. The pulse repetition rate is 0.5 Hz with FWHM 0.5 msec. The microwiggler is characterized by extensive tunability. We employed a novel tuning regimen through which the rms spread in peak amplitudes was reduced to 0.08%, the lowest ever achieved in a sub-cm period magnetic field, The microwiggler is a serviceable scientific apparatus: spontaneous emission has been observed for wavelengths of 700-800 nm using a 40 MeV beam from the Accelerator Test Facility LINAC at BNL.

title: Transverse Feedback System and Instability Analysis at HLS
abstract: In this paper, the BxB transverse feedback systems at Hefei Light Source (HLS) are introduced, which employ an analog system and a digital system. The experiment result of two systems. as well as the primary analysis of beam instability in HLS operation is also presented in this paper.  HLS is a synchrotron light source,injecting in the energy of 200MeV and operating in the 800MeV. The multi-cycle multi-turn injection system is used for current accumulation. The range of horizontal tune of HLS is from 3.54 to 3.56, and the range of vertical tune is from 2.58 to 2.60 for user operation. The injecting beam current usually cannot exceed 120mA without octupole magnet, and the injecting accumulating beam current is about 250mA with octupole magnet because the beam often occur sudden lose due to CB instabilities in storage ring to limit injection further increase. Therefore the constructing beam feedback system began from 2006 year. The analog feedback system has been built and commissioned at 2007[1-3], At the same time, the development of digital transverse BxB feedback system began at 2007 and has commissioned since 2008[4]. THE ANALOG TFB SYSTEM AND EXPERIMENT  The diagram of analog TFB system is shown as Figure 1. The analog feedback system is made up of: 3tap filter at centre frequency 612MHz (3*fRF) , hybrid for obtaining beam position oscillation signal, vector algorithm for two BPMs to obtain feedback signal of independent adjustable gain, multiple frequency circuit for centre frequency 612MHz (3*fRF) and 1.2GHz (6*fRF), PXI digitizer for data acquisition modular, the fiber correlator filter for eliminate DC and revolution frequencies, the power amplifier and kicker for exciting beam current. Commissioning Results of Analog TFB at HLS During the commissioning, the damping time and betatron sidebands have been measured by Turn-by-Turn (TBT) [5].and bunch-by-bunch (BXB) . measurement system at beam injecting is shown as Figure 1.  Overview of HLS analog TFB after update

title: Search for supersymmetry in pp collisions at $\sqrt{s} =7$ TeV in events with a single lepton, jets, and missing transverse momentum
abstract: Results are reported from a search for new physics processes in events containing a single isolated high-transverse-momentum lepton (electron or muon), energetic jets, and large missing transverse momentum. The analysis is based on a 4.98 fb−1 sample of proton–proton collisions at a center-of-mass energy of 7 TeV, obtained with the CMS detector at the LHC. Three separate background estimation methods, each relying primarily on control samples in the data, are applied to a range of signal regions, providing complementary approaches for estimating the background yields. The observed yields are consistent with the predicted standard model backgrounds. The results are interpreted in terms of limits on the parameter space for the constrained minimal supersymmetric extension of the standard model, as well as on cross sections for simplified models, which provide a generic description of the production and decay of new particles in specific, topology based final states.

title: Results of the SLAC LCLS gun high-power RF tests
abstract: The beam quality and operational requirements for the Linac coherent light source (LCLS) currently being constructed at SLAC are exceptional, requiring the design of a new RF photocathode gun for the electron source. Based on operational experience at SLAC's GTF and SDL and ATF at BNL as well as other laboratories, the 1.6 cell s-band (2856 MHz) gun was chosen to be the best electron source for the LCLS, however a significant re-design was necessary to achieve the challenging parameters. Detailed 3-D analysis and design was used to produce near-perfect rotationally symmetric rf fields to achieve the emittance requirement. In addition, the thermo-mechanical design allows the gun to operate at 120 Hz and a 140 MV/m cathode field, or to an average power dissipation of 4 kW. Both average and pulsed heating issues are addressed in the LCLS gun design. The first LCLS gun is now fabricated and has been operated with high-power RF. The results of these high-power tests are presented and discussed.

title: EFFECTIVE COSMOLOGICAL CONSTANT IN BRANE COSMOLOGY
abstract: We consider a brane-world of co-dimension one without reflection symmetry. Through it, we give a possible explanation of the great discrepancy between the vacuum energy and the observed cosmological constant without contradiction to the knowledge we have about our Universe. We also show the gravity observed will be standard four-dimensional gravity as long as the discrepancy of the bulk cosmological constant at different sides of the brane is small enough.

title: A compact RF power coupler for the NLC linac
abstract: A high power RF coupler connecting WR90 rectangular waveguide to the disc loaded waveguide accelerating structure of NLC is described. The coupler design is symmetric and free of beam deflecting field components. It makes an efficient electrical match between the rectangular waveguide power feed and the accelerator structure while keeping electrical surface field enhancement low. Electrical and mechanical design of the coupler are presented as well as quantitative comparisons between numerical simulation and low power cold test models.