LCLS Injector Straight-Ahead Spectrometer

The spectrometer design was modified to allow the measurement of uncorrelated energy spread for the nominal lattice. One bunch from every 120 each second would be sent to the straight ahead spectrometer while the transverse cavity is on. The implementation of this 'stealing mode' will not be available for the LCLS commissioning and the early stage of operation. However, the spectrometer was redesigned to retain that option. The energy feedback relies independently on the beam position of the beam in the dispersive section of dogleg 1 (DL1). The main modification of the spectrometer design is the Pole face rotation of 7.5 degrees on both entrance and exit faces. The location and range of operation of the 3 quadrupoles remains unchanged relative to those of the earlier design

Wakefield Computations for the Injector (Part I)

In this document, we report on basic wakefield computations used to establish the impedance budget for the LCLS injector. Systematic comparisons between analytic formulae and results from ABCI are done. Finally, a comparison between 2D and 3D wakefield calculations are given for a cross. The three parts of the document are presented as follows: (1) ABCI computations for a few structures (Flange, Bellows...); (2) Comparison analytic with ABCI runs; and (3) Comparison Cross and Cavity using MAFIA

Wakefield Calculations for Radiation Stopper 1 (RST1)

The main result of this note is that no wakefield mitigation is required for the Radiation Stopper (RST1) in the LCLS injector. The RST1 geometry is not symmetric in the vertical direction, and we derive a slight modification to the diffraction model wake for a cylindrically symmetric (2D) cavity that can be used for this problem. Performing a full 3D MAFIA calculation for the nominal 1 mm (rms) long bunch, we show that the modified diffraction model well describes the wakefields generated in RST1. The results imply an on-axis emittance growth of 0.0075%, well below the 0.5% tolerance threshold. To reach the 0.5% threshold the beam would need to be mis-steered by a large amount - 7 mm - from the axis. One reason that the effect is small is that the beta functions at the RST1 are small

RF Design of the LCLS Gun

Final dimensions for the LCLS RF gun are described. This gun, referred to as the LCLS gun, is a modified version of the UCLA/BNL/SLAC 1.6 cell S-Band RF gun [1], referred to as the prototype gun. The changes include a larger mode separation (15 MHz for the LCLS gun vs. 3.5 MHz for the prototype gun), a larger radius at the iris between the 2 cells, a reduced surface field on the curvature of the iris between the two cells, Z power coupling, increased cooling channels for operation at 120 Hz, dual rf feed, deformation tuning of the full cell, and field probes in both cells. Temporal shaping of the klystron pulse, to reduce the average power dissipated in the gun, has also been adopted. By increasing the mode separation, the amplitude of the 0-mode electric field on the cathode decreases from 10% of the peak on axis field for the prototype gun to less than 3% for the LCLS gun for the steady state fields. Beam performance is improved as shown by the PARMELA simulations. The gun should be designed to accept a future load lock system. Modifications follow the recommendations of our RF review committee [2]. Files and reference documents are compiled in Section IV

Modifications of the LCLS Photoinjector Beamline

The LCLS Photoinjector beamline is now in the Design and Engineering stage. The fabrication and installation of this beamline is scheduled for the summer 2006. The Photoinjector will deliver 10 ps long electron bunches of 1nC with a normalized transverse emittance of less than 1 mm.mrad for 80% of the slices constituting the core of the bunch at 135 MeV. The calculations done to finalize the specifications of the photoinjector beamline components are described. Modifications include a new exit energy, additional focusing between the two linac modules, the insertion of a ''laser heater'', and a new geometry for the coupling cells of the RF structures. We also discuss two interesting tunings, one for the nominal charge of 1nC but using a longer laser pulse and the second one for a lower charge of 0.2nC. Sensitivity to field errors and misalignment for those two new configurations is compared to that of the nominal tuning

Commissioning the SPEAR3 Diagnostic Beamlines

SPEAR3 has two diagnostic beam lines: an x-ray pinhole camera and a visible/UV beam line. The pinhole camera images {approx}8 keV dipole synchrotron radiation (SR) incident on a phosphor screen. The visible beam line delivers conventional optical radiation to an array of cameras and diagnostic elements on a 1x3m optical bench. This paper briefly reviews the pinhole camera system while concentrating more on visible beam line power transmission calculations and streak camera data. Impedance estimates in the nominal optics and bunch length measurements in low-alpha optics are presented

Linac Coherent Light Source Electron Beam Collimation

This paper describes the design and simulation of the electron beam collimation system in the Linac Coherent Light Source (LCLS). Dark current is expected from the gun and some of the accelerating cavities. Particle tracking of the expected dark current through the entire LCLS linac, from gun through FEL undulator, is used to estimate final particle extent in the undulator as well as expected beam loss at each collimator or aperture restriction. A table of collimators and aperture restrictions is listed along with halo particle loss results, which includes an estimate of average continuous beam power lost. In addition, the transverse wakefield alignment tolerances are calculated for each collimator

Uncorrelated Energy Spread and Longitudinal Emittance of a Photo injector Beam

Longitudinal phase space properties of a photoinjector beam are important in many areas of high-brightness beam applications such as bunch compression, transverse-to-longitudinal emittance exchange, and high-gain free-electron lasers. In this paper, we discuss both the rf and the space charge contributions to the uncorrelated energy spread of the beam generated from a laser-driven rf gun. We compare analytical expressions for the uncorrelated energy spread and the longitudinal emittance with numerical simulations and recent experimental results

Measurement and Analysis of Field Emission Electrons in the LCLS Gun

The field emission was measured during the high-power testing of the LCLS photocathode RF gun. A careful study and analysis of the field emission electrons, or dark current is important in assessing the gun's internal surface quality in actual operation, especially those surfaces with high fields. The first indication of a good RF gun design and fabrication is short processing time to the required fields and low electron emission at high fields. The charge per 2 microsecond long RF pulse (the dark charge) was measured as a function of the peak cathode field for the 1.6 cell, 2.856GHz LCLS RF gun. Faraday cup data was taken for cathode peak RF fields up to 120MV/m producing a maximum of 0.6nC/RF pulse for a diamond-turned polycrystalline copper cathode installed in the gun. Digitized images of the dark charge were taken using a 100 micron thick YAG crystal for a range of solenoid fields to determine the location and angular distribution of the field emitters. The FN plots and emitter image analysis will be described in this paper

Results of the SLAC LCLS Gun High-Power RF Tests

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 1.6cell s-band (2856MHz) gun was chosen to be the best electron source for the LCLS, however a significant redesign 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 120Hz and a 140MV/m cathode field, or to an average power dissipation of 4kW. 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